Heat treatment plays an important role in tailoring the mechanical properties of powder-metallurgy(PM)titanium alloys.However,only limited work about the rapid heat treatment(RHT)of PM titanium alloys has been reporte...Heat treatment plays an important role in tailoring the mechanical properties of powder-metallurgy(PM)titanium alloys.However,only limited work about the rapid heat treatment(RHT)of PM titanium alloys has been reported.In this work,RHT was applied to PM Ti-5Al-5Mo-5V-1Cr-1Fe alloy after hot rolling to study the evolution of its mechanical properties and the influence of residual pores on its properties.Through hot rolling and annealing,a fine and uniformα+βstructure with few residual pores is ob-tained.During RHT,the primaryαdissolves gradually and completes in theβregion,and theβgrains then grow,resulting in the continuous decrease in elongation after aging.Moreover,the tensile strength first increases and then decreases with increasing RHT temperature,owing to the increase in volume fraction of secondaryαinα+βregion and theβgrain growth inβregion.In contrast to the RHT of cast-and-wrought titanium,the negative influence of residual pores lowers the RHT temperature for obtaining the highest tensile strength to a temperature below theβ-transus temperature.Despite the negative influence of the residual pores,retained primaryαand fineβgrains with fine secondaryαinside contribute to achieving a good strength/ductility balance(1570 MPa and 6.1%,respectively).Addi-tionally,although at higher cycles to failure,the negative influence of residual pores increases as it affects the crack initiation zone at the subsurface,the good resistance of secondaryαto fatigue crack propaga-tion still enhances the fatigue strength considerably(about 300 MPa).This work suggests a cost-effective strategy to produce titanium alloys with high performance.展开更多
The hot deformation characteristics of as-forged Ti−3.5Al−5Mo−6V−3Cr−2Sn−0.5Fe−0.1B−0.1C alloy within a temperature range from 750 to 910℃and a strain rate range from 0.001 to 1 s^(-1) were investigated by hot compre...The hot deformation characteristics of as-forged Ti−3.5Al−5Mo−6V−3Cr−2Sn−0.5Fe−0.1B−0.1C alloy within a temperature range from 750 to 910℃and a strain rate range from 0.001 to 1 s^(-1) were investigated by hot compression tests.The stress−strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate.The microstructure is sensitive to deformation parameters.The dynamic recrystallization(DRX)grains appear while the temperature reaches 790℃at a constant strain rate of 0.001 s^(-1) and strain rate is not higher than 0.1 s^(-1) at a constant temperature of 910℃.The work-hardening rateθis calculated and it is found that DRX prefers to happen at high temperature and low strain rate.The constitutive equation and processing map were obtained.The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map,which indicates excellent hot workability.At the strain rate of 0.1 s^(-1),the stress−strain curves show an abnormal shape where there are two stress peaks simultaneously.This can be attributed to the alternation of hardening effect,which results from the continuous dynamic recrystallization(CDRX)and the rotation of DRX grains,and dynamic softening mechanism.展开更多
The effects of sub-transus(α+β)annealing treatment(ST),followed by single aging(SA)or duplex aging(DA)on the microstructural evolution and mechanical properties of near-βTi-4Al-1Sn-2Zr-5Mo-8V-2.5Cr(mass fraction,%)...The effects of sub-transus(α+β)annealing treatment(ST),followed by single aging(SA)or duplex aging(DA)on the microstructural evolution and mechanical properties of near-βTi-4Al-1Sn-2Zr-5Mo-8V-2.5Cr(mass fraction,%)alloy were investigated using optical microscopy,scanning electron microscopy,and transmission electron microscopy.The results show that the finer secondaryαphase precipitates in the alloy after DA than SA(e.g.,149 nm for SA and 69 nm for DA,both after ST at 720℃).The main reason is that the pre-aging step(300℃)in the DA process leads to the formation of intermediateωphase nanoparticles,which assist in the nucleation of the acicular secondaryαphase precipitates.In addition,the strength of the alloy after DA is higher than that of SA at the specific ST temperature.A good combination is achieved in the alloy subjected to ST at 750℃,followed by DA(UTS:1450 MPa,EL:3.87%),which is due to the precipitation of nanoscale secondaryαphase by DA.In conclusion,DA is a feasible process for this new near-βtitanium alloy.展开更多
Profound and comprehensive investigations on the morphology characteristics ofαprecipitates are essential for the microstructural control of metastableβtitanium alloys.At the very beginning of aging treatment,intrag...Profound and comprehensive investigations on the morphology characteristics ofαprecipitates are essential for the microstructural control of metastableβtitanium alloys.At the very beginning of aging treatment,intragranularαprecipitates with a dot-like morphology begin to generate nearby the dislocations,then those dot-likeαprecipitates with the same crystallographic orientation tend to connect with each other to develop a lath-like morphology.With the progress of aging treatment,the orientated lath-likeαprecipitates gradually combine with each other to form the V-shaped clusters or the triangular ones.The dislocations of{110}_(β)<111>βedge type are evidenced within theβgrains,and it is found that variant selection ofαprecipitates induced by the transformation strain and the interplay betweenαvariants and the dislocations are confirmed as the key factors for the formation of the V-shaped or triangular clusters.The results of this work could provide underlying knowledge on the morphology characteristics of intraguranularαprecipitates related to the crystal defects and the strain accommodation ofαvariants in metastableβtitanium alloys.展开更多
This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bi...This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bimodal structure)through fatigue crack propagation rate tests and fatigue threshold value tests.The resistance of the alloy to fatigue crack propagation was found to be closely correlated with the morphology and distribution ofαparticles,as evidenced by microscopic examination of fracture surfaces and analysis of crack propagation paths.The primaryαparticles demonstrated superior resistance to crack propagation compared to the secondaryαparticles.The basketweave structure showed exceptional resistance to fatigue crack propagation at all stages.The lamellar structure mainly resists long crack propagation during rapid propagation,and its threshold value is the lowest,which makes it easy to produce microcrack propagation.On the contrary,the bimodal structure has the highest threshold value among the three,so its resistance to short crack growth is more excellent,but it has the highest crack growth rate in the higher stress intensity factor range.Theαparticles in the three microstructures also undergo rotational motion relative to the force axis during fatigue crack propagation,thereby adjusting the uneven stress distribution betweenα/βphases through slip behavior and further coordinating deformation.展开更多
This study investigated the effect of Si addition on the microstructure and the silicide precipitation behavior in a novel near-βtitanium alloy.The results show that coarse and continuous silicides were preferentiall...This study investigated the effect of Si addition on the microstructure and the silicide precipitation behavior in a novel near-βtitanium alloy.The results show that coarse and continuous silicides were preferentially precipitated at the grain boundary during the solidification process,and theβgrain size of the as-cast alloy was refined.Dynamic recrystallization occurs under isothermal compression,and the silicide could inhibit the growth of recrystallized grains.The element redistribution and dislocation accumulation during hot deformation promote the dynamic precipitation of silicide,resulting in a discontinuous distribution of silicides at the grain boundaries.This work provides insight into how silicide dynamic precipitation will affect the microstructure and plastic deformation behavior of metal alloys.展开更多
The microstructures of a Ti−5Al−5Mo−5V−1Cr−1Fe alloy with a strain gradient from 0.1 to 1 were obtained by double-cone compression experiments.The deformation microstructures were analyzed by EBSD and TEM.The results ...The microstructures of a Ti−5Al−5Mo−5V−1Cr−1Fe alloy with a strain gradient from 0.1 to 1 were obtained by double-cone compression experiments.The deformation microstructures were analyzed by EBSD and TEM.The results show that the deformation mechanism is dynamic recovery when the strain is less than 0.42.In the strain range of 0.42−0.88,the deformation mechanism is dynamic recrystallization.When the strain exceeds 0.88,the deformation bands appear.The deformation mechanism map combined with the homogenization degree of the microstructure was constructed.The more homogeneous deformed microstructure was obtained at the strain of 0.6 and the temperature of 860°C.The dynamic recrystallization forms new grains and reduces the size difference between the grains,which improves the homogenization degree of the microstructure.展开更多
To overcome the strength-plasticity trade-offin the structural titanium alloys,a novel metastableβti-tanium alloy Ti-5Mo-4Cr-1V-1Zr(Ti-5411)with high strength and high plasticity was designed by the d-electrons theor...To overcome the strength-plasticity trade-offin the structural titanium alloys,a novel metastableβti-tanium alloy Ti-5Mo-4Cr-1V-1Zr(Ti-5411)with high strength and high plasticity was designed by the d-electrons theory,average electron-to-atom ratio(e/α^(-))and atomic radius difference(Δr^(-))theory.Com-bined in-situ scanning electron microscope(SEM)and electron backscatter diffraction(EBSD),the defor-mation mechanisms of the novel Ti-5411 metastableβtitanium alloy were systematically investigated.The results show that the Ti-5411 alloy exhibits excellent yield strength(∼689 MPa),tensile strength(∼930 MPa)and total elongation(∼39%).The in-situ tension indicates that slip activities,crystal rota-tion,stress induced martensite(SIM)α''transformation and{332}<113>deformation twin are the major deformation mechanisms of Ti-5411 alloy.Besides,with the increase of strain degree(0-0.5 mm displace-ment),deformation twins increase,widen and interlace.At 0.35 mm tensile displacement,the orientation of theβgrains rotates∼6.65°to accommodate the increased macrostrain.Additionally,martensiteα''also assists the nucleation of twins.Some{332}<113>twins grow and merge by consuming martensiteα''during deformation,and the residual martensiteα''remains in the merged twins.展开更多
To study the relationship between the microstructure and tensile properties of the novel metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr,a heat treatment process of ABFCA(solid solution in α+βregion with subse...To study the relationship between the microstructure and tensile properties of the novel metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr,a heat treatment process of ABFCA(solid solution in α+βregion with subsequent furnace cooling followed by aging treatment finally)was designed,by which α phases of different sizes can be precipitated in the β matrix.The results show that the microstructure obtained by this heat treatment process is composed of primary α(α_(p))phase,submicro rod-like α(α_(r))phase and secondary α(α_(s))phase.The alloy with multi-scale α phase has an excellent balance between strength and ductility.The elongation is about 18.3% at the ultimate tensile strength of 1125.4 MPa.The relationship between the strength of the alloy and the α phase was established.The strength of the alloy is proportional to the power of‒1/2 of the average spacing and width of α phase.The α_(s) phase with a smaller size and phase spacing can greatly improve the strength of the alloy by hindering dislocation slip.The transmission electron microscope analysis shows that there is a large amount of dislocation accumulation at the α/β interfaces,and many deformation twins are found in the α_(p) phase after tensile deformation.When the dislocation slip is hindered,twins occur at the stress concentration location,and twins can initiate some dislocations that are difficult to slip.Meanwhile,the plastic strain is distributed uniformly among the α_(p),α_(r),α_(s) phases and β matrix,thereby enhancing the ductility of the alloy.展开更多
Laser beam welding was used to join a near-β titanium alloy(Ti-3Al-6Mo-2Fe-2Zr),followed by aging treatments.The relations among aging temperature,microstructure,and tensile properties of joints were revealed.For as-...Laser beam welding was used to join a near-β titanium alloy(Ti-3Al-6Mo-2Fe-2Zr),followed by aging treatments.The relations among aging temperature,microstructure,and tensile properties of joints were revealed.For as-welded joints,the fusion zone features primarily single β phase.It is attributed to the high Mo equivalency of this alloy and the fast cooling rate in laser beam welding.After aging treatments,many α precipitates form in the fusion zone and heat affected zone.The rising aging temperature coarsens α precipitates and reduces the volume fraction of α precipitates.Compared with the as-welded joints,the aging treated joints'tensile strength and elongation are improved.The increasing aging temperature weakens the strengthening effect because of the decreasing volume fraction of α precipitates.After the aging treatment at 500℃ for 8 h,the joints obtain the optimal match between strength and plasticity.The fracture mode of joints changes from quasi-cleavage fracture in as-welded condition to microvoid coalescence fracture after heat treatments.展开更多
High strength β titanium alloys are widely used in large load bearing components in the aerospace field. At present, large parts are generally formed by die forging. Different initial microstructures and deformation ...High strength β titanium alloys are widely used in large load bearing components in the aerospace field. At present, large parts are generally formed by die forging. Different initial microstructures and deformation process parameters will significantly affect the flow behavior. To precisely control the microstructures, researchers have conducted many studies to analyze the microstructure evolution law and deformation mechanism during hot compression. This review focuses on the microstructure evolution of high strength β titanium alloys during hot deformation, including dynamic recrystallization and dynamic recovery in the single-phase region and the dynamic evolution of the α phase in the two-phase region. Furthermore, the optimal hot processing regions, instability regions,and the relationship between the efficiency of power dissipation and the deformation mechanism in the hot processing map are summarized. Finally, the problems and development direction of using hot processing maps to optimize process parameters are also emphasized.展开更多
It is well accepted that grain-boundary phases in metallic alloys greatly deteriorate the mechanical properties.In our work,we report on a novel strategy to prepare high strength-ductility β-type(Ti69.71 Nb23.72Zr4.8...It is well accepted that grain-boundary phases in metallic alloys greatly deteriorate the mechanical properties.In our work,we report on a novel strategy to prepare high strength-ductility β-type(Ti69.71 Nb23.72Zr4.83Ta1.74)97Si3(at.%)(TNZTS) alloys by tailoring grain-boundary metastable Si-containing phase.Specifically,the thin shell-shaped metastable S1 phase surrounding the columnar β-Ti grain was intercepted successfully via nonequilibrium rapid solidification achieved by selective laser melting(SLM).Subsequently,the thin shell-shaped metastable(Ti,Nb,Zr)5 Si3(called S1) phase was transformed into globular(Ti,Nb,Zr)2 Si(called S2) phase by the solution heat treatment.Interestingly,the globular S2 phases reinforced TNZTS alloy exhibits ultrahigh yield strength of 978 MPa,ultimate strength of 1010 MPa and large elongation of 10.4 %,overcoming the strength-ductility trade-off of TNZTS alloys by various methods.Especially,the reported yield strength herein is 55 % higher than that of conventionally forged TNZT alloys.Dynamic analysis indicates the globularization process of the metastable S1 phase is controlled by the model of termination migration.The quantitative analysis on strengthening mechanism demonstrates that the increase in yield strength of the heat-treated alloys is mainly ascribed to the strengthening of the precipitated silicide and the dislocations induced by high cooling rate.The obtained results provide some basis guidelines for designing and fabricating β-titanium alloys with excellent mechanical properties,and pave the way for biomedical application of TNZTS alloy by SLM.展开更多
High-strength β titanium alloys represented by near β titanium alloy and metastable β titanium alloy are preferred materials for large-scale load-carrying structures.In order to achieve the precise regulation of mi...High-strength β titanium alloys represented by near β titanium alloy and metastable β titanium alloy are preferred materials for large-scale load-carrying structures.In order to achieve the precise regulation of microstructure in the deformation process, massive efforts have been made to study the flow behavior and microstructure evolution of βtitanium alloy in the hot deformation process. This paper reviews the flow behavior of high-strength titanium alloy,including the effects of initial microstructure, deformation process parameters, work hardening, and dynamic softening on flow stress. Furthermore, the effects of deformation process parameters on the apparent activation energy for deformation and strain rate sensitivity coefficient are analyzed. The discontinuous yield phenomenon is discussed,and the constitutive models of flow stress are summarized.Furthermore, some microstructural evolution models are reviewed. Finally, the development direction and difficulties of the flow behavior and constitutive model are pointed out.展开更多
The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat ...The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.展开更多
β titanium alloys with bi-modal structure which exhibit improved strength-ductility combination and fatigue property are widely used in aviation and aerospace industry.However,owing to the small size of primary α(α...β titanium alloys with bi-modal structure which exhibit improved strength-ductility combination and fatigue property are widely used in aviation and aerospace industry.However,owing to the small size of primary α(αp) and nano-scaled multi variant distribution of secondary α platelets(αs),investigating the deformation behavior is really a challenging work.In this work,by applying transmission electron microscopy(TEM),the slip behavior in αp and transformed β matrix with different tensile strain was studied.After α/β solution treatment,the initial dislocation slips on {110} plane with <1 1 1> direction in β matrix.During furtherdeformation,(110),(101) and(1 1 2) multi slip is generated which shows a long straight cro s sing configuration.Dislocation cell is exhibited in β matrix at tensile strain above 20 %.Diffe rent from the solid solution treated sample,high density wavy dislocations are generated in transformedβ matrix.High fraction fine as hinders dislocation motion in β matrix effectively which in turn dominates the strength of the alloy.In primary α phase(αp),a core-shell structure is formed during deformation.Both pyramidal a+c slip and prismatic/basal a slip are generated in the shell layer.In core region,plastic deformation is governed by prismatic/basal a slip.Formation of the core-shell structure is the physical origin of the improved ductility.On one hand,the work hardening layer(shell) improves the strength of αp,which could deform compatibly with the hard transformed β matrix.Meanwhile,the center area(core) deforms homogeneously which will sustain plastic strain effectively and increase the ductility.This paper studies the slip behavior and reveals the origin of the improved strength-ductility combination in Bi-modal structure on a microscopic way,which will give theoretical advises for developing the next generation high strength β titanium alloys.展开更多
Thermomechanical processing(TMP)is especially crucial for metastableβtitanium alloys,which has received significant attention in the community for a long time.In this contribution,the processing-responding behaviour ...Thermomechanical processing(TMP)is especially crucial for metastableβtitanium alloys,which has received significant attention in the community for a long time.In this contribution,the processing-responding behaviour including microstructure evolution process,texture variation mechanism,and un-derlying deformation process of powder metallurgy Ti-5553 alloy in a wide processing parameter range was comprehensively investigated.Thermal physical simulation was performed on the alloy at temper-atures ranging from 800℃ to 1100℃,and strain rates between 0.001 s^(−1) and 10 s^(−1),to varied defor-mation degrees of 20%-80%height reduction.It was found that the processing parameters(i.e.temper-ature,strain rate,and deformation degree)are influential on the deformation process and resultant mi-crostructure.Varied microstructural evolution processes forβphase including flow localization,dynamic recovery,dynamic recrystallization,and grain coarsening are activated in different processing domains,while different evolution mechanisms forαphase including dynamic precipitation,phase separation,dy-namic coarsening,and mechanical shearing also play their roles under different processing conditions.In particular,four exceptional evolution mechanisms ofαprecipitation which have not been previously reported in titanium alloys were discovered and clearly demonstrated,more specifically,they are multi-interior twinning,internal compositing,layered coarsening and selective diffusion-actuated separation.After the establishment of comprehensive microstructural evolution mechanism maps,the guidance for precise processing and the knowledge reserve extension for deformation process of metastableβtita-nium alloys can be effectively achieved.展开更多
Microstructures and tensile properties of a newβhigh-strength titanium alloy Ti-5321(Ti-5 Al-3 Mo-3 V-2 Zr-2 Cr-1 Nb-1 Fe)were investigated in this study.Four microstructures,including equiaxed microstructure(EM),bim...Microstructures and tensile properties of a newβhigh-strength titanium alloy Ti-5321(Ti-5 Al-3 Mo-3 V-2 Zr-2 Cr-1 Nb-1 Fe)were investigated in this study.Four microstructures,including equiaxed microstructure(EM),bimodal microstructure(BM),basket-weave microstructure(WM)and lamellar microstructure(LM),were tailored by changing the forging process,and the influences of different microstructures on tensile properties were also analyzed.The results indicated that Ti-5321 exhibits a better combination of strength and ductility,compared to Ti-5553,Ti-1023,BT22 and Ti15-3.The ultimate tensile strength,total elongation and reduction in area could be achieved in a range of 1200-1300 MPa,10%-15%and40%-60%,respectively.The influences of variant selection on the tensile properties in Ti-5321 alloy were also analyzed.Afterβforging and solution treatment,αphase maintained strictly Burgers orientation relation with adjacentβphase.Morphological features of the fractography in BM and LM were also explored to further explain the tensile properties and the fracture mode of Ti-5321 alloy.展开更多
In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using ...In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1.Results show that the fatigue strength is closely related to the microstructure features,especially theα_(p) percentage.The Ti-7333 alloy with a lowerα_(p) percentage exhibits a higher scatter in fatigue data.The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lowerα_(p) percentage.Crack initiation region shows the compoundα_(p)/βfacets.Facetedα_(p) particles show crystallographic orientation and morphology dependence characteristics.Crack-initiation was accompanied by faceting process across elongatedα_(p) particles or multiple adjacentα_(p) particles.These particles generally oriented for basal slip result in near basal facets.Fatigue crack can also initiate at elongatedα_(p) particle well oriented for prismatic slip.Theβfacet is in close correspondence to{110}or{112}plane with high Schmid factor.Based on the fracture observation and FIB-CS analysis,three classes of fatigue-critical microstructural configurations are deduced.A phenomenological model for the formation ofα_(p) facet in the bimodal microstructure is proposed.This work provides an insight into the fatigue damage process of theβprecipitate strengthened metastableβtitanium alloys.展开更多
文摘Heat treatment plays an important role in tailoring the mechanical properties of powder-metallurgy(PM)titanium alloys.However,only limited work about the rapid heat treatment(RHT)of PM titanium alloys has been reported.In this work,RHT was applied to PM Ti-5Al-5Mo-5V-1Cr-1Fe alloy after hot rolling to study the evolution of its mechanical properties and the influence of residual pores on its properties.Through hot rolling and annealing,a fine and uniformα+βstructure with few residual pores is ob-tained.During RHT,the primaryαdissolves gradually and completes in theβregion,and theβgrains then grow,resulting in the continuous decrease in elongation after aging.Moreover,the tensile strength first increases and then decreases with increasing RHT temperature,owing to the increase in volume fraction of secondaryαinα+βregion and theβgrain growth inβregion.In contrast to the RHT of cast-and-wrought titanium,the negative influence of residual pores lowers the RHT temperature for obtaining the highest tensile strength to a temperature below theβ-transus temperature.Despite the negative influence of the residual pores,retained primaryαand fineβgrains with fine secondaryαinside contribute to achieving a good strength/ductility balance(1570 MPa and 6.1%,respectively).Addi-tionally,although at higher cycles to failure,the negative influence of residual pores increases as it affects the crack initiation zone at the subsurface,the good resistance of secondaryαto fatigue crack propaga-tion still enhances the fatigue strength considerably(about 300 MPa).This work suggests a cost-effective strategy to produce titanium alloys with high performance.
文摘The hot deformation characteristics of as-forged Ti−3.5Al−5Mo−6V−3Cr−2Sn−0.5Fe−0.1B−0.1C alloy within a temperature range from 750 to 910℃and a strain rate range from 0.001 to 1 s^(-1) were investigated by hot compression tests.The stress−strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate.The microstructure is sensitive to deformation parameters.The dynamic recrystallization(DRX)grains appear while the temperature reaches 790℃at a constant strain rate of 0.001 s^(-1) and strain rate is not higher than 0.1 s^(-1) at a constant temperature of 910℃.The work-hardening rateθis calculated and it is found that DRX prefers to happen at high temperature and low strain rate.The constitutive equation and processing map were obtained.The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map,which indicates excellent hot workability.At the strain rate of 0.1 s^(-1),the stress−strain curves show an abnormal shape where there are two stress peaks simultaneously.This can be attributed to the alternation of hardening effect,which results from the continuous dynamic recrystallization(CDRX)and the rotation of DRX grains,and dynamic softening mechanism.
基金the financial supports from the Key Research and Development Program of Shanxi Province,China(Nos.201903D421084,201903D121056)the National Natural Science Foundation of China(Nos.52171122,52071228,51901151)。
文摘The effects of sub-transus(α+β)annealing treatment(ST),followed by single aging(SA)or duplex aging(DA)on the microstructural evolution and mechanical properties of near-βTi-4Al-1Sn-2Zr-5Mo-8V-2.5Cr(mass fraction,%)alloy were investigated using optical microscopy,scanning electron microscopy,and transmission electron microscopy.The results show that the finer secondaryαphase precipitates in the alloy after DA than SA(e.g.,149 nm for SA and 69 nm for DA,both after ST at 720℃).The main reason is that the pre-aging step(300℃)in the DA process leads to the formation of intermediateωphase nanoparticles,which assist in the nucleation of the acicular secondaryαphase precipitates.In addition,the strength of the alloy after DA is higher than that of SA at the specific ST temperature.A good combination is achieved in the alloy subjected to ST at 750℃,followed by DA(UTS:1450 MPa,EL:3.87%),which is due to the precipitation of nanoscale secondaryαphase by DA.In conclusion,DA is a feasible process for this new near-βtitanium alloy.
基金the Science and Technology Major Project of Shanxi Province(Nos.20191102008 and 20181101014)the Applied Basic Research Foundation of Shanxi Province(Nos.201901D211255 and 201901D211256)+4 种基金Platform and Talent Project of Shanxi Province(No.201805D211036)Guiding Local Science and Technology Development Projects by the Central Government(No.YDZX20191400002796)Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi(No.2020L0304)Key projects of Equipment Pre-research Foundation(No.61409230407)the Natural Science Research Project of NUC(No.XJJ201916)。
文摘Profound and comprehensive investigations on the morphology characteristics ofαprecipitates are essential for the microstructural control of metastableβtitanium alloys.At the very beginning of aging treatment,intragranularαprecipitates with a dot-like morphology begin to generate nearby the dislocations,then those dot-likeαprecipitates with the same crystallographic orientation tend to connect with each other to develop a lath-like morphology.With the progress of aging treatment,the orientated lath-likeαprecipitates gradually combine with each other to form the V-shaped clusters or the triangular ones.The dislocations of{110}_(β)<111>βedge type are evidenced within theβgrains,and it is found that variant selection ofαprecipitates induced by the transformation strain and the interplay betweenαvariants and the dislocations are confirmed as the key factors for the formation of the V-shaped or triangular clusters.The results of this work could provide underlying knowledge on the morphology characteristics of intraguranularαprecipitates related to the crystal defects and the strain accommodation ofαvariants in metastableβtitanium alloys.
基金financial support from the Equipment Pre-research Project(51312030507).
文摘This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bimodal structure)through fatigue crack propagation rate tests and fatigue threshold value tests.The resistance of the alloy to fatigue crack propagation was found to be closely correlated with the morphology and distribution ofαparticles,as evidenced by microscopic examination of fracture surfaces and analysis of crack propagation paths.The primaryαparticles demonstrated superior resistance to crack propagation compared to the secondaryαparticles.The basketweave structure showed exceptional resistance to fatigue crack propagation at all stages.The lamellar structure mainly resists long crack propagation during rapid propagation,and its threshold value is the lowest,which makes it easy to produce microcrack propagation.On the contrary,the bimodal structure has the highest threshold value among the three,so its resistance to short crack growth is more excellent,but it has the highest crack growth rate in the higher stress intensity factor range.Theαparticles in the three microstructures also undergo rotational motion relative to the force axis during fatigue crack propagation,thereby adjusting the uneven stress distribution betweenα/βphases through slip behavior and further coordinating deformation.
基金funded by the National Natural Science Foundation of China(Nos.52371117,52171122,52275362)the Central Government Guides the Special Fund Projects of Local Scientific and Technological Development,China(Nos.YDZJSX2021A016,YDZX-20191400002149)+1 种基金the Key Project of Natural Science Foundation of Ningxia,China(No.2022AAC02077)the Natural Science Foundation of Shanxi Province,China(No.20210302124077)。
文摘This study investigated the effect of Si addition on the microstructure and the silicide precipitation behavior in a novel near-βtitanium alloy.The results show that coarse and continuous silicides were preferentially precipitated at the grain boundary during the solidification process,and theβgrain size of the as-cast alloy was refined.Dynamic recrystallization occurs under isothermal compression,and the silicide could inhibit the growth of recrystallized grains.The element redistribution and dislocation accumulation during hot deformation promote the dynamic precipitation of silicide,resulting in a discontinuous distribution of silicides at the grain boundaries.This work provides insight into how silicide dynamic precipitation will affect the microstructure and plastic deformation behavior of metal alloys.
基金supported by the National Natural Science Foundation of China(Nos.52441410,52020105013)the State Key Laboratory for Powder Metallurgy,China(No.2022)。
文摘The microstructures of a Ti−5Al−5Mo−5V−1Cr−1Fe alloy with a strain gradient from 0.1 to 1 were obtained by double-cone compression experiments.The deformation microstructures were analyzed by EBSD and TEM.The results show that the deformation mechanism is dynamic recovery when the strain is less than 0.42.In the strain range of 0.42−0.88,the deformation mechanism is dynamic recrystallization.When the strain exceeds 0.88,the deformation bands appear.The deformation mechanism map combined with the homogenization degree of the microstructure was constructed.The more homogeneous deformed microstructure was obtained at the strain of 0.6 and the temperature of 860°C.The dynamic recrystallization forms new grains and reduces the size difference between the grains,which improves the homogenization degree of the microstructure.
基金supported by the National Natural Science Foundation of China(Nos.52104372,52374332)the Postdoctoral Research Foundation of China(Nos.2019M651129,2019TQ0053)the Fundamental Research Funds for the Central Universities(No.N2324003-02).
文摘To overcome the strength-plasticity trade-offin the structural titanium alloys,a novel metastableβti-tanium alloy Ti-5Mo-4Cr-1V-1Zr(Ti-5411)with high strength and high plasticity was designed by the d-electrons theory,average electron-to-atom ratio(e/α^(-))and atomic radius difference(Δr^(-))theory.Com-bined in-situ scanning electron microscope(SEM)and electron backscatter diffraction(EBSD),the defor-mation mechanisms of the novel Ti-5411 metastableβtitanium alloy were systematically investigated.The results show that the Ti-5411 alloy exhibits excellent yield strength(∼689 MPa),tensile strength(∼930 MPa)and total elongation(∼39%).The in-situ tension indicates that slip activities,crystal rota-tion,stress induced martensite(SIM)α''transformation and{332}<113>deformation twin are the major deformation mechanisms of Ti-5411 alloy.Besides,with the increase of strain degree(0-0.5 mm displace-ment),deformation twins increase,widen and interlace.At 0.35 mm tensile displacement,the orientation of theβgrains rotates∼6.65°to accommodate the increased macrostrain.Additionally,martensiteα''also assists the nucleation of twins.Some{332}<113>twins grow and merge by consuming martensiteα''during deformation,and the residual martensiteα''remains in the merged twins.
基金National Natural Science Foundation of China(52104379,U21A20117,52071219,52271249)。
文摘To study the relationship between the microstructure and tensile properties of the novel metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr,a heat treatment process of ABFCA(solid solution in α+βregion with subsequent furnace cooling followed by aging treatment finally)was designed,by which α phases of different sizes can be precipitated in the β matrix.The results show that the microstructure obtained by this heat treatment process is composed of primary α(α_(p))phase,submicro rod-like α(α_(r))phase and secondary α(α_(s))phase.The alloy with multi-scale α phase has an excellent balance between strength and ductility.The elongation is about 18.3% at the ultimate tensile strength of 1125.4 MPa.The relationship between the strength of the alloy and the α phase was established.The strength of the alloy is proportional to the power of‒1/2 of the average spacing and width of α phase.The α_(s) phase with a smaller size and phase spacing can greatly improve the strength of the alloy by hindering dislocation slip.The transmission electron microscope analysis shows that there is a large amount of dislocation accumulation at the α/β interfaces,and many deformation twins are found in the α_(p) phase after tensile deformation.When the dislocation slip is hindered,twins occur at the stress concentration location,and twins can initiate some dislocations that are difficult to slip.Meanwhile,the plastic strain is distributed uniformly among the α_(p),α_(r),α_(s) phases and β matrix,thereby enhancing the ductility of the alloy.
基金National Natural Science Foundation of China(51804097)Fundamental Research Funds for the Central Universities of China(B220202026)Changzhou Sci&Tech Program(CJ20220074)。
文摘Laser beam welding was used to join a near-β titanium alloy(Ti-3Al-6Mo-2Fe-2Zr),followed by aging treatments.The relations among aging temperature,microstructure,and tensile properties of joints were revealed.For as-welded joints,the fusion zone features primarily single β phase.It is attributed to the high Mo equivalency of this alloy and the fast cooling rate in laser beam welding.After aging treatments,many α precipitates form in the fusion zone and heat affected zone.The rising aging temperature coarsens α precipitates and reduces the volume fraction of α precipitates.Compared with the as-welded joints,the aging treated joints'tensile strength and elongation are improved.The increasing aging temperature weakens the strengthening effect because of the decreasing volume fraction of α precipitates.After the aging treatment at 500℃ for 8 h,the joints obtain the optimal match between strength and plasticity.The fracture mode of joints changes from quasi-cleavage fracture in as-welded condition to microvoid coalescence fracture after heat treatments.
基金supported by the Project of National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, China (No. 6142909190207)Shaanxi Key Laboratory of High-performance Precision Forming Technology and Equipment (SKL-HPFTE), China (No. PETE-2019-KF-01)。
文摘High strength β titanium alloys are widely used in large load bearing components in the aerospace field. At present, large parts are generally formed by die forging. Different initial microstructures and deformation process parameters will significantly affect the flow behavior. To precisely control the microstructures, researchers have conducted many studies to analyze the microstructure evolution law and deformation mechanism during hot compression. This review focuses on the microstructure evolution of high strength β titanium alloys during hot deformation, including dynamic recrystallization and dynamic recovery in the single-phase region and the dynamic evolution of the α phase in the two-phase region. Furthermore, the optimal hot processing regions, instability regions,and the relationship between the efficiency of power dissipation and the deformation mechanism in the hot processing map are summarized. Finally, the problems and development direction of using hot processing maps to optimize process parameters are also emphasized.
基金supported financially by the National Natural Science Foundation of China (Nos.U19A2085 and 51627805)the Key-Area Research and Development Program of Guangdong Province (No.2020B090923001)+2 种基金the Key Basic and Applied Research Program of Guangdong Province (No.2019B030302010)the Optical Valley Science Research Project,WEHDZ (No.2019001)financial support from the China Postdoctoral Science Foundation (Nos.2019TQ0099 and 2019M662908)。
文摘It is well accepted that grain-boundary phases in metallic alloys greatly deteriorate the mechanical properties.In our work,we report on a novel strategy to prepare high strength-ductility β-type(Ti69.71 Nb23.72Zr4.83Ta1.74)97Si3(at.%)(TNZTS) alloys by tailoring grain-boundary metastable Si-containing phase.Specifically,the thin shell-shaped metastable S1 phase surrounding the columnar β-Ti grain was intercepted successfully via nonequilibrium rapid solidification achieved by selective laser melting(SLM).Subsequently,the thin shell-shaped metastable(Ti,Nb,Zr)5 Si3(called S1) phase was transformed into globular(Ti,Nb,Zr)2 Si(called S2) phase by the solution heat treatment.Interestingly,the globular S2 phases reinforced TNZTS alloy exhibits ultrahigh yield strength of 978 MPa,ultimate strength of 1010 MPa and large elongation of 10.4 %,overcoming the strength-ductility trade-off of TNZTS alloys by various methods.Especially,the reported yield strength herein is 55 % higher than that of conventionally forged TNZT alloys.Dynamic analysis indicates the globularization process of the metastable S1 phase is controlled by the model of termination migration.The quantitative analysis on strengthening mechanism demonstrates that the increase in yield strength of the heat-treated alloys is mainly ascribed to the strengthening of the precipitated silicide and the dislocations induced by high cooling rate.The obtained results provide some basis guidelines for designing and fabricating β-titanium alloys with excellent mechanical properties,and pave the way for biomedical application of TNZTS alloy by SLM.
基金the project of National Key Laboratory for Precision Hot Processing of Metals,Harbin Institute of Technology(No.6142909190207)Shaanxi Key Laboratory of High-Performance Precision Forming Technology and Equipment(NSKL-HPFTE)(No.PETE-2019-KF01)。
文摘High-strength β titanium alloys represented by near β titanium alloy and metastable β titanium alloy are preferred materials for large-scale load-carrying structures.In order to achieve the precise regulation of microstructure in the deformation process, massive efforts have been made to study the flow behavior and microstructure evolution of βtitanium alloy in the hot deformation process. This paper reviews the flow behavior of high-strength titanium alloy,including the effects of initial microstructure, deformation process parameters, work hardening, and dynamic softening on flow stress. Furthermore, the effects of deformation process parameters on the apparent activation energy for deformation and strain rate sensitivity coefficient are analyzed. The discontinuous yield phenomenon is discussed,and the constitutive models of flow stress are summarized.Furthermore, some microstructural evolution models are reviewed. Finally, the development direction and difficulties of the flow behavior and constitutive model are pointed out.
基金the financial support from Baosteel Australia Joint Research Centre(BA16003)ARC Research Hub for Computational Particle Technology(IH140100035)funded by Australian Research Council grant LE0882821。
文摘The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.
基金supported financially by the National Natural Science Foundation of China (Nos. 51671158 and 51621063)the National Program on Key Basic Research Project (No. 2014CB644003)the Programme of Introducing Talents of Discipline to Universities (No. PB2018008)。
文摘β titanium alloys with bi-modal structure which exhibit improved strength-ductility combination and fatigue property are widely used in aviation and aerospace industry.However,owing to the small size of primary α(αp) and nano-scaled multi variant distribution of secondary α platelets(αs),investigating the deformation behavior is really a challenging work.In this work,by applying transmission electron microscopy(TEM),the slip behavior in αp and transformed β matrix with different tensile strain was studied.After α/β solution treatment,the initial dislocation slips on {110} plane with <1 1 1> direction in β matrix.During furtherdeformation,(110),(101) and(1 1 2) multi slip is generated which shows a long straight cro s sing configuration.Dislocation cell is exhibited in β matrix at tensile strain above 20 %.Diffe rent from the solid solution treated sample,high density wavy dislocations are generated in transformedβ matrix.High fraction fine as hinders dislocation motion in β matrix effectively which in turn dominates the strength of the alloy.In primary α phase(αp),a core-shell structure is formed during deformation.Both pyramidal a+c slip and prismatic/basal a slip are generated in the shell layer.In core region,plastic deformation is governed by prismatic/basal a slip.Formation of the core-shell structure is the physical origin of the improved ductility.On one hand,the work hardening layer(shell) improves the strength of αp,which could deform compatibly with the hard transformed β matrix.Meanwhile,the center area(core) deforms homogeneously which will sustain plastic strain effectively and increase the ductility.This paper studies the slip behavior and reveals the origin of the improved strength-ductility combination in Bi-modal structure on a microscopic way,which will give theoretical advises for developing the next generation high strength β titanium alloys.
基金support from National Natural Science Foundation of China(No.52101122).Q.Zhao,R.Torrens,F.Yang and L.Bolzoni would like to gratefully acknowledge the support from New Zealand Ministry of Business,Innovation and Employment(No.UOWX1402)supported by Science and Technology Major Project of Shaanxi Province(No.2020-zdzx04-01-02)+1 种基金Key Research Plan of Shaanxi Province(No.2021KW-18)Young Talents Promotion Project of China Association for Science and Technology(No.YESS20200335).
文摘Thermomechanical processing(TMP)is especially crucial for metastableβtitanium alloys,which has received significant attention in the community for a long time.In this contribution,the processing-responding behaviour including microstructure evolution process,texture variation mechanism,and un-derlying deformation process of powder metallurgy Ti-5553 alloy in a wide processing parameter range was comprehensively investigated.Thermal physical simulation was performed on the alloy at temper-atures ranging from 800℃ to 1100℃,and strain rates between 0.001 s^(−1) and 10 s^(−1),to varied defor-mation degrees of 20%-80%height reduction.It was found that the processing parameters(i.e.temper-ature,strain rate,and deformation degree)are influential on the deformation process and resultant mi-crostructure.Varied microstructural evolution processes forβphase including flow localization,dynamic recovery,dynamic recrystallization,and grain coarsening are activated in different processing domains,while different evolution mechanisms forαphase including dynamic precipitation,phase separation,dy-namic coarsening,and mechanical shearing also play their roles under different processing conditions.In particular,four exceptional evolution mechanisms ofαprecipitation which have not been previously reported in titanium alloys were discovered and clearly demonstrated,more specifically,they are multi-interior twinning,internal compositing,layered coarsening and selective diffusion-actuated separation.After the establishment of comprehensive microstructural evolution mechanism maps,the guidance for precise processing and the knowledge reserve extension for deformation process of metastableβtita-nium alloys can be effectively achieved.
基金financially supported by the International Science and Technology Cooperation Program of China(No.2015DFA51430)the National Natural Science Foundation of China(No.51471136)。
文摘Microstructures and tensile properties of a newβhigh-strength titanium alloy Ti-5321(Ti-5 Al-3 Mo-3 V-2 Zr-2 Cr-1 Nb-1 Fe)were investigated in this study.Four microstructures,including equiaxed microstructure(EM),bimodal microstructure(BM),basket-weave microstructure(WM)and lamellar microstructure(LM),were tailored by changing the forging process,and the influences of different microstructures on tensile properties were also analyzed.The results indicated that Ti-5321 exhibits a better combination of strength and ductility,compared to Ti-5553,Ti-1023,BT22 and Ti15-3.The ultimate tensile strength,total elongation and reduction in area could be achieved in a range of 1200-1300 MPa,10%-15%and40%-60%,respectively.The influences of variant selection on the tensile properties in Ti-5321 alloy were also analyzed.Afterβforging and solution treatment,αphase maintained strictly Burgers orientation relation with adjacentβphase.Morphological features of the fractography in BM and LM were also explored to further explain the tensile properties and the fracture mode of Ti-5321 alloy.
基金financially supported by the Major State Research Development Program of China(No.2016YFB0701303)the National Natural Science Foundation of China(No.51801156)the Natural Science Basic Research Plan in Shaanxi Province of China(No.2019JM-584)。
文摘In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1.Results show that the fatigue strength is closely related to the microstructure features,especially theα_(p) percentage.The Ti-7333 alloy with a lowerα_(p) percentage exhibits a higher scatter in fatigue data.The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lowerα_(p) percentage.Crack initiation region shows the compoundα_(p)/βfacets.Facetedα_(p) particles show crystallographic orientation and morphology dependence characteristics.Crack-initiation was accompanied by faceting process across elongatedα_(p) particles or multiple adjacentα_(p) particles.These particles generally oriented for basal slip result in near basal facets.Fatigue crack can also initiate at elongatedα_(p) particle well oriented for prismatic slip.Theβfacet is in close correspondence to{110}or{112}plane with high Schmid factor.Based on the fracture observation and FIB-CS analysis,three classes of fatigue-critical microstructural configurations are deduced.A phenomenological model for the formation ofα_(p) facet in the bimodal microstructure is proposed.This work provides an insight into the fatigue damage process of theβprecipitate strengthened metastableβtitanium alloys.
