The spray-deposition was used to produce billets of Mg-4Al-1.5Zn-3Ca-1Nd(A alloy)and Mg-13Al-3Zn-3Ca-1Nd(B alloy),and evolution of deformation substructure and Mg_(x)Zn_(y)Ca_(z)metastable phase in fine-grained(3μm)M...The spray-deposition was used to produce billets of Mg-4Al-1.5Zn-3Ca-1Nd(A alloy)and Mg-13Al-3Zn-3Ca-1Nd(B alloy),and evolution of deformation substructure and Mg_(x)Zn_(y)Ca_(z)metastable phase in fine-grained(3μm)Mg alloys was investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and electron backscattered diffraction(EBSD).It was found that different dislocation configurations were formed in A and B alloys.Redundant free dislocations(RFDs)and dislocation tangles were the ways to form deformation substructure in A alloy,no RFDs except dislocation tangles were found in B alloy.The interaction between nano-scale second phase particles(nano-scale C15 andβ-Mg_(17)(Al,Zn)_(12)phase)and different dislocation configurations had a significant effect on the deformation substructures formation.The mass transfer of Mg_(x)Zn_(y)Ca_(z)metastable phases and the stacking order of stacking faults were conducive to the Mg-Nd-Zn typed long period stacking ordered(LPSO)phases formation.Nano-scale C15 phases,Mg-Nd-Zn typed LPSO phases,c/a ratio,β-Mg_(17)(Al,Zn)_(12)phases were the key factors influencing the formation of textures.Different textures and grain boundary features(GB features)had a significant effect on k-value.The non-basal textures were the main factor affecting k-value in A alloy,while the high-angle grain boundary(HAGB)was the main factor affecting k-value in B alloy.展开更多
The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg2Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy （TEM）. Th...The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg2Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy （TEM）. The precipitates of the peak aged alloy include both β＂ and if, but the amount ratio of β＂ to β＂ varies with the aging temperature and time increasing. The precipitates during aging at 175 ℃ are dominated by needle-like β＂ phases （including pre-β＂ phase）, the size of which increases with the time prolonging, but does not increase substantially after further aging. The evolution of electrical conductivity is directly related to such microstructural evolution. However, the hardness of the alloy stays at the peak value for a long term. When the alloy is aged at 195 ℃, the ratio of β＂ to β＇ becomes the main factor to influence relative resistivity （Ap） value. The higher the temperature is, the smaller the ratio is, and the faster the Ap value decreases. Moreover, the hardness peak drops with the decrease of the ratio. With the size and distribution parameters measured from TEM images, a semi-quantitative relationship between precipitates and the electrical resistivity was established.展开更多
In this paper,a comprehensive understanding of stable and metastable phase equilibria in binary Mg-Gd system was conducted with an aid of the CALculation of PHAse Diagram(CALPHAD)modeling.Firstly,thermodynamic descrip...In this paper,a comprehensive understanding of stable and metastable phase equilibria in binary Mg-Gd system was conducted with an aid of the CALculation of PHAse Diagram(CALPHAD)modeling.Firstly,thermodynamic descriptions of all the stable phases in the Mg-Gd system were re-assessed by considering all the experimental data in the literature.The discrepancy between the phase equilibria and thermochemical properties existing in the previous assessments was eliminated,and the better agreement with the experimental data was achieved in the present assessment.Secondly,the Gibbs energies for metastable β"-Mg3Gd and β"-Mg7Gd were constructed based on the first-principles and CALPHAD computed results as well as their correlation,and then incorporated into the CALPHAD descriptions.The model-predicted solvuses of(Mg)in equilibrium with the metastable β"-Mg7Gd and β'-Mg7Gd compounds showed very good agreement with the limited experimental data.Finally,the presently obtained thermodynamic descriptions of both stable and metastable phases in the binary Mg-Gd system were further validated by realizing the quantitative Scheil-Gulliver solidification simulations of 5 as-cast Mg-Gd alloys,and the successful prediction of the precipitation sequences in Mg-15Gd and Mg-12Gd alloys during the aging process.展开更多
The metastable liquid phase separation and rapid solidification behaviors of Co_(40) Fe_(40) Cu_(20) alloy were investigated by using differential thermal analysis(DTA) in combination with glass fluxing and electromag...The metastable liquid phase separation and rapid solidification behaviors of Co_(40) Fe_(40) Cu_(20) alloy were investigated by using differential thermal analysis(DTA) in combination with glass fluxing and electromagnetic levitation(EML) techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the(Fe,Co) dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.展开更多
The non-equilibrium microstructure and a new metastable phase of Al-9.6wt%Mg alloy solidified at 6 GPa were studied by optical microscope,differential scanning calorimetry,X-ray diffraction and transmission electron m...The non-equilibrium microstructure and a new metastable phase of Al-9.6wt%Mg alloy solidified at 6 GPa were studied by optical microscope,differential scanning calorimetry,X-ray diffraction and transmission electron microscope.The results showed that dendrite microstructure was refined,and the solid solubility of Mg inα-Al phase increased greatly.Correspondingly,the lattice parameter ofα-Al phase increased.Al3Mg2 phases disappeared under high pressure solidification.In particular,a metastable phase with small size(20 nm or so)was produced in the alloy,its melting temperature range was 464~518.2℃,which was higher than that of Al3Mg2 phase(453~465℃)under normal pressure.These metastable phases located in the interdendritic position.It was the first time that the metastable phase was found in Al-Mg alloy at a high pressure of 6 GPa.The formation mechanism of the metastable phases was discussed.展开更多
The radiation of material surface by high intensity laser beams is used to produce the uhrafine powder of pure Fe and ZrO_2.The morphology,size and phases of the powder were examined by X-ray diffraetometer,scanning e...The radiation of material surface by high intensity laser beams is used to produce the uhrafine powder of pure Fe and ZrO_2.The morphology,size and phases of the powder were examined by X-ray diffraetometer,scanning electron microscopy as well as transmission electron microscopy.In pure Fe a considerable quantity of γ-phase was found in the powder.In ZrO_2 powder,instead of stable phase,two metastable phases appeared.展开更多
Variable-composition evolutionary structure searches are used to explore stable stoichiometries for the Zn-O system below 300 GPa. Our results confirm the previous structural phase transition sequence of pressurised Z...Variable-composition evolutionary structure searches are used to explore stable stoichiometries for the Zn-O system below 300 GPa. Our results confirm the previous structural phase transition sequence of pressurised ZnO. ZnO is thermodynamically stable up to 300GPa and zinc peroxide （Zn02, space group Pa3） is metastable under lower pressure. Insulating I4/mcm-Zn02 is thermodynamically stable between 128.3-300 GPa. Insulated metastable P3121-Zn02, controlling the pressure range of 51.5-128.3 GPa, has a wide band gap compared to the Pa3-Zn02 and I4/mem-Zn02. Phonon and elastic constant calculations conclude the dynamical and mechanical stability for the explored thermodynamically stable or metastable structures.展开更多
Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase,designated asβ’’phase,induced to form by electron beam irradiation in a Mg-9.8 wt.%Sn alloy.This phase ...Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase,designated asβ’’phase,induced to form by electron beam irradiation in a Mg-9.8 wt.%Sn alloy.This phase is spherical in three dimensions,having a D019 structure with the lattice parameters of a=0.642 nm,c=0.521 nm and space group of P63/mmc.Its chemical formula is Mg_(3)Sn,like theβ’metastable precipitate phase.The orientation relationship between theβ’’phase and theα-Mg matrix is such that■_(β)’’//■_(α)and(0001)_(β)’’//(0001)_(α).Its formation involves solely the ordering of Sn atoms in the solid solution magnesium matrix.First-principles calculations indicate that the formation of theβ’’phase is energetically favored.展开更多
The powder mixture of Al, Ti and graphite has been mechanically alloyed in a planetary ball mill.The structural evolution of as-milled powder sample has been characterized by XRD, DTA. The results show that the amorph...The powder mixture of Al, Ti and graphite has been mechanically alloyed in a planetary ball mill.The structural evolution of as-milled powder sample has been characterized by XRD, DTA. The results show that the amorphous phase is formed first at an early milling stage, then crystallization occurs during further milling, leading to formation of a nanocrystalline fcc metastable phase. In contrast, during annealing the amorphous phase is crystallized to the equilibrium phase instead of the fcc phase. This indicates that crystallization during ball milling is different from that induced by annealing展开更多
The metastable liquid phase separation and rapid solidification behaviours of Co61.8Cu38.2 alloy were investigated by using differential thermal analysis （DTA） in combination with glass fluxing, electromagnetic levi...The metastable liquid phase separation and rapid solidification behaviours of Co61.8Cu38.2 alloy were investigated by using differential thermal analysis （DTA） in combination with glass fluxing, electromagnetic levitation （EML） and drop tube techniques. It is found that the liquid phase separation process and the solidification microstructures intensively depend on the experimental processing parameters, such as undercooling level, cooling rate, gravity level, liquid surface tension and the wetting state of crucible. Large undercooling and surface tension difference of the two liquids tend to facilitate further separation and cause severe macrosegregation. On the other hand, rapid cooling and low gravity effectively suppress the coalescence of the minority phase, Severe maerosegregation patterns are formed in the bulk samples processed by both DTA and EML. In contrast, disperse structures with fine spherical Cu-rich spheres homogeneously distributed in the matrix of Co-rich phase have been obtained in drop tube.展开更多
The splat foils of Al-Sm alloys with 0.04-0.06mm in thickness were made by hammeranvil technique. The estimated cooling rute was 106K/ order of magnitude. The extended solid solubility of Sm in α-Al reached 0.5at. % ...The splat foils of Al-Sm alloys with 0.04-0.06mm in thickness were made by hammeranvil technique. The estimated cooling rute was 106K/ order of magnitude. The extended solid solubility of Sm in α-Al reached 0.5at. % Sm. The intermediate phase in the rapidlysolidified Al-Sm alloys is Al11Sm3 phase, which is stable only at temperatures above 1339K in equilibrium state and retained to ambient temperature as a metastable phase, whereas the equlibrium intermediate phase, Al3Sm, was restricted to occur.展开更多
1 Introduction The Pingluoba brine,which characterized as high concentration with sodium,potassium,boron,lithium,and rubidium,possess great development value.The main composition of the brine can be summarized to the
Laser processing of Al-Ge eutectic alloys is used to produce two different metastable intermetallic-matrix nano-scale lamellar composite microstructures:(ⅰ)β_(1)(monoclinic)Al_(6)Ge_(5)(60 vol%)-α(FCC)Al(40 vol%),a...Laser processing of Al-Ge eutectic alloys is used to produce two different metastable intermetallic-matrix nano-scale lamellar composite microstructures:(ⅰ)β_(1)(monoclinic)Al_(6)Ge_(5)(60 vol%)-α(FCC)Al(40 vol%),and(ⅱ)β_(2)(monoclinic)AIGe(67 vol%)-α(FCC)Al(33 vol%).Nanoindentation and micropillar compres-sion tests were performed to characterize mechanical behavior and compare with the equilibrium struc-ture of as-cast diamond cubic Ge(42 vol%)-FCC Al(58 vol%)micrometer-scale eutectic composite.The as-processed and deformed microstructures were characterized by scanning/transmission electron mi-croscopy.Bothβ_(1)-αandβ_(2)-αeutectics exhibit high compressive flow strengths of≈1 GPa and 1.2 GPa respectively,whereas the maximum compressive flow strength of Al-Ge eutectic is about 450 MPa.In spite of complex monoclinic structures and higher volume fraction of intermetallic phase as compared to metallic,bothβ_(1)-αandβ_(2)-αcomposite microstructures have higher level of plastic deformability than as-cast Al-Ge.The microstructure with the highest strength,β_(2)-α,also exhibits the highest plastic strain to failure attributed to a gradual strain softening behavior due to interaction of micro shear cracks with nano-twins in theβ_(2)(monoclinic)AlGe phase,whereas theβ_(1)(monoclinic)Al_(6)Ge_(5) phase without the nano-twins exhibits sudden fracture by a sharp crack.In the Al-richαphases in bothβ_(1)-αandβ_(2)-αmicrostructures,nanoscale Ge-rich clusters were observed that led to profuse stacking faults post defor-mation.Density functional theory calculations suggest that Ge solutes can lower stacking fault energy of FCC Al,thereby promoting partial dislocation glide in Al.This study highlights unusual mechanisms that impart plastic deformability at ultra-high yield strengths in intermetallic-metal composites with a low-symmetry intermetallic matrix phase.展开更多
Planar films of pure and Ti^(4+)-dopedβ-Fe_(2)O_(3)were prepared by a spray pyrolysis method.X-ray diffraction patterns and Raman spectra of the metastableβ-Fe_(2)O_(3)film showed that its thermal stability was sign...Planar films of pure and Ti^(4+)-dopedβ-Fe_(2)O_(3)were prepared by a spray pyrolysis method.X-ray diffraction patterns and Raman spectra of the metastableβ-Fe_(2)O_(3)film showed that its thermal stability was significantly improved because of covalent bonds in the interfaces between the film and substrate,while only weak Van der Waals bonds existed at the interfaces within the particle-assembledβ-Fe_(2)O_(3)film prepared by electrophoretic deposition.The as-prepared planar films were thus able to withstand higher annealing temperature and stronger laser irradiation power in comparison with theβ-Fe_(2)O_(3)particle-assembly.Ti^(4+)doping was used to increase the concentration of carriers in the metastableβ-Fe_(2)O_(3)film.Compared with pureβ-Fe_(2)O_(3)photoanodes,the highest saturated photocurrent for water splitting over the Ti^(4+)-dopedβ-Fe_(2)O_(3)photoanode was increased by a factor of approximately three.Theβ-Fe_(2)O_(3)photoanode exhibited photochemical stability for water splitting for a duration exceeding 100 h,which indicates its important potential application in solar energy conversion.展开更多
Exploration of metastable phases holds profound implications for functional materials.Herein,we engineer the metastable phase to enhance the thermo-electric performance of germanium selenide(GeSe)through tailoring the...Exploration of metastable phases holds profound implications for functional materials.Herein,we engineer the metastable phase to enhance the thermo-electric performance of germanium selenide(GeSe)through tailoring the chemical bonding mechanism.Initially,AgInTe2 alloying fosters a transition from stable orthorhombic to metastable rhombohedral phase in GeSe by substantially promoting p-state electron bonding to form metavalent bonding(MVB).Besides,extra Pb is employed to prevent a transition into a stable hexagonal phase at elevated temperatures by moderately enhancing the degree of MVB.The stabilization of the metastable rhombohedral phase generates an optimized bandgap,sharpened valence band edge,and stimulative band convergence compared to stable phases.This leads to decent carrier concentra-tion,improved carrier mobility,and enhanced density-of-state effective mass,culminating in a superior power factor.Moreover,lattice thermal conductivity is suppressed by pronounced lattice anharmonicity,low sound velocity,and strong phonon scattering induced by multiple defects.Consequently,a maximum zT of 1.0 at 773 K is achieved in(Ge_(0.98)Pb_(0.02)Se)_(0.875)(AgInTe_(2))_(0.125),resulting in a maximum energy conversion efficiency of 4.90%under the temperature difference of 500 K.This work underscores the significance of regulating MVB to stabilize metastable phases in chalcogenides.展开更多
The solubilities and densities of the aqueous metastable ternary systems(NaCl-MgCl_(2)-H_(2)O)and(KCl-MgCl_(2)-H_(2)O)at 308.15 K were determined by the isothermal evaporation method.On the basis of the experimental r...The solubilities and densities of the aqueous metastable ternary systems(NaCl-MgCl_(2)-H_(2)O)and(KCl-MgCl_(2)-H_(2)O)at 308.15 K were determined by the isothermal evaporation method.On the basis of the experimental results,the phase diagrams for those systems were plotted.It was found that the former system belongs to the hydrate-I type with one invariant point of(NaCl+MgCl_(2)·6H_(2)O),two univariant curves,and two crystal-lization regions corresponding to halite(NaCl)and bischofite(MgCl_(2)·6H_(2)O);and the latter system belongs to the type of incongruent-double salts with two invariant points of(KCl+KCl·MgCl_(2)·6H_(2)O)and(MgCl_(2)·6H_(2)O+KCl·MgCl_(2)·6H_(2)O),three univariant curves,and three crystallization regions corresponding to potassium chlor-ide(KCl),carnallite(KCl·MgCl_(2)·6H_(2)O)and bischofite(MgCl_(2)·6H_(2).No solid solutions were found in both systems.展开更多
Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects,however,little information has been known on its nucleation,growth and phase transformation.In this paper,a 90...Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects,however,little information has been known on its nucleation,growth and phase transformation.In this paper,a 900 nm-thick tungsten film with double-layer structure(α-W underlayer andβ-W above it)was produced on SiO_(2)/Si substrate by high vacuum magnetron sputtering at room temperature.The structural properties ofβ-W were systemically investigated by X-ray diffraction,transmission electron microscopy,thermodynamic calculation,first-principle and phase-field simulations.It is found that theβ-W nucleation is energetically favoured on the SiO_(2)surface compared to theα-W one.As the film thickening proceeds,β-W[211]turns to be preferred direction of growth owing to the elastic strain energy minimization,which is verified by phase-field simulations.Moreover,theβ→αphase transformation takes place near the film-substrate interface while the rest of the film keeps theβ-W phase,leading to a doublelayer structure.This localized phase transition is induced by lower Gibbs free energy ofα-W phase at larger grain sizes,which can be confirmed by thermodynamic calculation.Further in-situ heating TEM analysis of the as-deposited film reveals that theβ→αphase transformation is fulfilled byα/βinterface propagation rather than local atomic rearrangements.Our findings offer valuable insights into the intrinsic properties of metastable phase in tungsten.展开更多
This study investigates the development of novel high-entropy alloys(HEAs)with enhanced mechanical properties through an innovative fabrication method of direct energy deposition(DED).The focus is on the creation of m...This study investigates the development of novel high-entropy alloys(HEAs)with enhanced mechanical properties through an innovative fabrication method of direct energy deposition(DED).The focus is on the creation of metastable core-shell precipitation-strengthened HEAs that exhibit a unique multi-stage terrace-like slip wave toughening mechanism,a novel approach to improving both strength and ductility simultaneously.Mechanical testing reveals that the developed HEAs exhibit superior mechanical proper-ties,including high yield strength,ultimate tensile strength,and exceptional ductility.The improvement in these properties is attributed to the multi-stage terrace-like slip wave toughening mechanism activated by the unique microstructural features.This toughening mechanism involves the sequential activation of slip systems,facilitated by the stress concentration around the core-shell precipitates and the subsequent propagation of slip waves across the material.The terrace-like pattern of these slip waves enhances the material's ability to deform plastically,providing a significant toughening effect while maintaining high strength levels.Furthermore,the study delves into the fundamental interactions between the microstruc-tural elements and the deformation mechanisms.It elucidates how the core-shell precipitates and the matrix cooperate to distribute stress uniformly,delay the onset of necking,and prevent premature failure.This synergistic interaction between the microstructural features and the slip wave toughening mecha-nism is central to the remarkable balance of strength and ductility achieved in the HEAs.The introduction of a multi-stage terrace-like slip wave toughening mechanism offers a new pathway to designing HEAs with an exceptional amalgamation of strength and ductility.展开更多
High-pressure β-Sn germanium may transform into diverse metastable allotropes with distinctive nanostructures and unique physical properties via multiple pathways under decompression.However,the mechanism and transit...High-pressure β-Sn germanium may transform into diverse metastable allotropes with distinctive nanostructures and unique physical properties via multiple pathways under decompression.However,the mechanism and transition kinetics remain poorly understood.Here,we investigate the formation of metastable phases and nanostructures in germanium via controllable transition pathways of β-Sn Ge under rapid decompression at different rates.High-resolution transmission electron microscopy reveals three distinct metastable phases with the distinctive nanostructures:an almost perfect st12 Ge crystal,nanosized bc8/r8 structures with amorphous boundaries,and amorphous Ge with nanosized clusters (0.8–2.5 nm).Fast in situ x-ray diffraction and x-ray absorption measurements indicate that these nanostructured products form in certain pressure regions via distinct kinetic pathways and are strongly correlated with nucleation rates and electronic transitions mediated by compression rate,temperature,and stress.This work provides deep insight into the controllable synthesis of metastable materials with unique crystal symmetries and nanostructures for potential applications.展开更多
TiAl alloys with the(α2+γ)lamellar structure are highly valued for their excellent high-temperature strength and creep resistance.Understanding the formation mechanism of the lamellar structure is crucial for tuning...TiAl alloys with the(α2+γ)lamellar structure are highly valued for their excellent high-temperature strength and creep resistance.Understanding the formation mechanism of the lamellar structure is crucial for tuning the microstructure and properties.This work investigates the formation of lamellar structure in Ti-48AI-7Nb-2.5V-1Cr alloy,revealing the presence of hcp-based long-period superstructure(hcp-LPS)as a metastable phase during lamellar formation.The identification of hcp-LPS demonstrates that the necessary solute enrichment for the formation ofγlamellae occurs on the hexagonalαmatrix,implying that phase separation ofα→Al-richαlamellae+Al-depletedαlamellae is the first step of lamellar formation.Once phase separation is completed,all subsequent phase transitions occur within the Al-richαlamellae.Additionally,the formation of twin lamellae is further discussed.The formation of the twin lamellae occurs sequentially.Pre-existing lamella promotes the formation of later lamella by inducing so-lute enrichment in its surrounding region,and then the successive slip of Shockley partial dislocations with opposite Burgers vectors ensures special stacking of later lamellae.These findings not only con-tribute to the fundamental understanding of spinodal mechanisms in hexagonal crystals,but also provide novel insights into the formation of twin lamellae.展开更多
基金financial support by the National Natural Science Foundation of China(No.51364032)the Inner Mongolia Natural Science Foundation(No.2022MS05028)。
文摘The spray-deposition was used to produce billets of Mg-4Al-1.5Zn-3Ca-1Nd(A alloy)and Mg-13Al-3Zn-3Ca-1Nd(B alloy),and evolution of deformation substructure and Mg_(x)Zn_(y)Ca_(z)metastable phase in fine-grained(3μm)Mg alloys was investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and electron backscattered diffraction(EBSD).It was found that different dislocation configurations were formed in A and B alloys.Redundant free dislocations(RFDs)and dislocation tangles were the ways to form deformation substructure in A alloy,no RFDs except dislocation tangles were found in B alloy.The interaction between nano-scale second phase particles(nano-scale C15 andβ-Mg_(17)(Al,Zn)_(12)phase)and different dislocation configurations had a significant effect on the deformation substructures formation.The mass transfer of Mg_(x)Zn_(y)Ca_(z)metastable phases and the stacking order of stacking faults were conducive to the Mg-Nd-Zn typed long period stacking ordered(LPSO)phases formation.Nano-scale C15 phases,Mg-Nd-Zn typed LPSO phases,c/a ratio,β-Mg_(17)(Al,Zn)_(12)phases were the key factors influencing the formation of textures.Different textures and grain boundary features(GB features)had a significant effect on k-value.The non-basal textures were the main factor affecting k-value in A alloy,while the high-angle grain boundary(HAGB)was the main factor affecting k-value in B alloy.
基金Project(51105139)supported by the National Natural Science Foundation of ChinaProject(2010CB731706)supported by the National Basic Research Program of China
文摘The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg2Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy （TEM）. The precipitates of the peak aged alloy include both β＂ and if, but the amount ratio of β＂ to β＂ varies with the aging temperature and time increasing. The precipitates during aging at 175 ℃ are dominated by needle-like β＂ phases （including pre-β＂ phase）, the size of which increases with the time prolonging, but does not increase substantially after further aging. The evolution of electrical conductivity is directly related to such microstructural evolution. However, the hardness of the alloy stays at the peak value for a long term. When the alloy is aged at 195 ℃, the ratio of β＂ to β＇ becomes the main factor to influence relative resistivity （Ap） value. The higher the temperature is, the smaller the ratio is, and the faster the Ap value decreases. Moreover, the hardness peak drops with the decrease of the ratio. With the size and distribution parameters measured from TEM images, a semi-quantitative relationship between precipitates and the electrical resistivity was established.
基金The financial support from the National Key Research and Development Program of China(Grant no.2016YFB0301101)the Hunan Provincial Science and Technology Program of China(Grant no.2017RS3002)-Huxiang Youth Talent Plan+2 种基金the Youth Talent Project of Innovation-driven Plan at Central South University(Grant no.2019XZ027)the Hebei Provincial Science and Technology Program of China(Grant no.BJ2018026)-Outstanding Young Talents Plan is acknowledgedYing Tang acknowledges the financial support from the Yuanguang fellowship released by Hebei University of Technology.
文摘In this paper,a comprehensive understanding of stable and metastable phase equilibria in binary Mg-Gd system was conducted with an aid of the CALculation of PHAse Diagram(CALPHAD)modeling.Firstly,thermodynamic descriptions of all the stable phases in the Mg-Gd system were re-assessed by considering all the experimental data in the literature.The discrepancy between the phase equilibria and thermochemical properties existing in the previous assessments was eliminated,and the better agreement with the experimental data was achieved in the present assessment.Secondly,the Gibbs energies for metastable β"-Mg3Gd and β"-Mg7Gd were constructed based on the first-principles and CALPHAD computed results as well as their correlation,and then incorporated into the CALPHAD descriptions.The model-predicted solvuses of(Mg)in equilibrium with the metastable β"-Mg7Gd and β'-Mg7Gd compounds showed very good agreement with the limited experimental data.Finally,the presently obtained thermodynamic descriptions of both stable and metastable phases in the binary Mg-Gd system were further validated by realizing the quantitative Scheil-Gulliver solidification simulations of 5 as-cast Mg-Gd alloys,and the successful prediction of the precipitation sequences in Mg-15Gd and Mg-12Gd alloys during the aging process.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFB1100101)the National Natural Science Foundation of China(Grant No.51471135)+2 种基金Shenzhen Science and Technology Program,China(Grant No.JCYJ20170815162201821)Shaanxi Provincial Key R&D Program,China(Grant No.2017KW-ZD-07)the Fundamental Research Funds for the Central Universities,China(Grant No.31020170QD102)
文摘The metastable liquid phase separation and rapid solidification behaviors of Co_(40) Fe_(40) Cu_(20) alloy were investigated by using differential thermal analysis(DTA) in combination with glass fluxing and electromagnetic levitation(EML) techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the(Fe,Co) dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.
基金This work is financially supported by the Science Foundation of Harbin Institute of Technology(No HIT200229)
文摘The non-equilibrium microstructure and a new metastable phase of Al-9.6wt%Mg alloy solidified at 6 GPa were studied by optical microscope,differential scanning calorimetry,X-ray diffraction and transmission electron microscope.The results showed that dendrite microstructure was refined,and the solid solubility of Mg inα-Al phase increased greatly.Correspondingly,the lattice parameter ofα-Al phase increased.Al3Mg2 phases disappeared under high pressure solidification.In particular,a metastable phase with small size(20 nm or so)was produced in the alloy,its melting temperature range was 464~518.2℃,which was higher than that of Al3Mg2 phase(453~465℃)under normal pressure.These metastable phases located in the interdendritic position.It was the first time that the metastable phase was found in Al-Mg alloy at a high pressure of 6 GPa.The formation mechanism of the metastable phases was discussed.
文摘The radiation of material surface by high intensity laser beams is used to produce the uhrafine powder of pure Fe and ZrO_2.The morphology,size and phases of the powder were examined by X-ray diffraetometer,scanning electron microscopy as well as transmission electron microscopy.In pure Fe a considerable quantity of γ-phase was found in the powder.In ZrO_2 powder,instead of stable phase,two metastable phases appeared.
基金Supported by the National Natural Science Foundation of China under Grant No 11347007the Qing Lan Project+1 种基金the Colleges and Universities in Jiangsu Province Natural Science Research Project under Grant No 14KJB460013the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘Variable-composition evolutionary structure searches are used to explore stable stoichiometries for the Zn-O system below 300 GPa. Our results confirm the previous structural phase transition sequence of pressurised ZnO. ZnO is thermodynamically stable up to 300GPa and zinc peroxide （Zn02, space group Pa3） is metastable under lower pressure. Insulating I4/mcm-Zn02 is thermodynamically stable between 128.3-300 GPa. Insulated metastable P3121-Zn02, controlling the pressure range of 51.5-128.3 GPa, has a wide band gap compared to the Pa3-Zn02 and I4/mem-Zn02. Phonon and elastic constant calculations conclude the dynamical and mechanical stability for the explored thermodynamically stable or metastable structures.
基金financially supported by National Natural Science Foundation of China(51771036 and 51421001)。
文摘Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase,designated asβ’’phase,induced to form by electron beam irradiation in a Mg-9.8 wt.%Sn alloy.This phase is spherical in three dimensions,having a D019 structure with the lattice parameters of a=0.642 nm,c=0.521 nm and space group of P63/mmc.Its chemical formula is Mg_(3)Sn,like theβ’metastable precipitate phase.The orientation relationship between theβ’’phase and theα-Mg matrix is such that■_(β)’’//■_(α)and(0001)_(β)’’//(0001)_(α).Its formation involves solely the ordering of Sn atoms in the solid solution magnesium matrix.First-principles calculations indicate that the formation of theβ’’phase is energetically favored.
文摘The powder mixture of Al, Ti and graphite has been mechanically alloyed in a planetary ball mill.The structural evolution of as-milled powder sample has been characterized by XRD, DTA. The results show that the amorphous phase is formed first at an early milling stage, then crystallization occurs during further milling, leading to formation of a nanocrystalline fcc metastable phase. In contrast, during annealing the amorphous phase is crystallized to the equilibrium phase instead of the fcc phase. This indicates that crystallization during ball milling is different from that induced by annealing
文摘The metastable liquid phase separation and rapid solidification behaviours of Co61.8Cu38.2 alloy were investigated by using differential thermal analysis （DTA） in combination with glass fluxing, electromagnetic levitation （EML） and drop tube techniques. It is found that the liquid phase separation process and the solidification microstructures intensively depend on the experimental processing parameters, such as undercooling level, cooling rate, gravity level, liquid surface tension and the wetting state of crucible. Large undercooling and surface tension difference of the two liquids tend to facilitate further separation and cause severe macrosegregation. On the other hand, rapid cooling and low gravity effectively suppress the coalescence of the minority phase, Severe maerosegregation patterns are formed in the bulk samples processed by both DTA and EML. In contrast, disperse structures with fine spherical Cu-rich spheres homogeneously distributed in the matrix of Co-rich phase have been obtained in drop tube.
文摘The splat foils of Al-Sm alloys with 0.04-0.06mm in thickness were made by hammeranvil technique. The estimated cooling rute was 106K/ order of magnitude. The extended solid solubility of Sm in α-Al reached 0.5at. % Sm. The intermediate phase in the rapidlysolidified Al-Sm alloys is Al11Sm3 phase, which is stable only at temperatures above 1339K in equilibrium state and retained to ambient temperature as a metastable phase, whereas the equlibrium intermediate phase, Al3Sm, was restricted to occur.
基金Project supported by National High Technology Research and Development Program of China (2012AA061704)China National Nature Science Foundation (No. 41173071)+2 种基金the Research Fund for the Doctoral Program of Higher Education from the Ministry of Education of China (20115122110001)the Sichuan Youth Science and Technology Innovation Research Team Funding Scheme (2013TD0005)Innovation Team of CDUT(KYTD201405)
文摘1 Introduction The Pingluoba brine,which characterized as high concentration with sodium,potassium,boron,lithium,and rubidium,possess great development value.The main composition of the brine can be summarized to the
基金funded by DOE,Office of Science,Office of Basic Energy Sciences with the grant number of DE-SC0016808.
文摘Laser processing of Al-Ge eutectic alloys is used to produce two different metastable intermetallic-matrix nano-scale lamellar composite microstructures:(ⅰ)β_(1)(monoclinic)Al_(6)Ge_(5)(60 vol%)-α(FCC)Al(40 vol%),and(ⅱ)β_(2)(monoclinic)AIGe(67 vol%)-α(FCC)Al(33 vol%).Nanoindentation and micropillar compres-sion tests were performed to characterize mechanical behavior and compare with the equilibrium struc-ture of as-cast diamond cubic Ge(42 vol%)-FCC Al(58 vol%)micrometer-scale eutectic composite.The as-processed and deformed microstructures were characterized by scanning/transmission electron mi-croscopy.Bothβ_(1)-αandβ_(2)-αeutectics exhibit high compressive flow strengths of≈1 GPa and 1.2 GPa respectively,whereas the maximum compressive flow strength of Al-Ge eutectic is about 450 MPa.In spite of complex monoclinic structures and higher volume fraction of intermetallic phase as compared to metallic,bothβ_(1)-αandβ_(2)-αcomposite microstructures have higher level of plastic deformability than as-cast Al-Ge.The microstructure with the highest strength,β_(2)-α,also exhibits the highest plastic strain to failure attributed to a gradual strain softening behavior due to interaction of micro shear cracks with nano-twins in theβ_(2)(monoclinic)AlGe phase,whereas theβ_(1)(monoclinic)Al_(6)Ge_(5) phase without the nano-twins exhibits sudden fracture by a sharp crack.In the Al-richαphases in bothβ_(1)-αandβ_(2)-αmicrostructures,nanoscale Ge-rich clusters were observed that led to profuse stacking faults post defor-mation.Density functional theory calculations suggest that Ge solutes can lower stacking fault energy of FCC Al,thereby promoting partial dislocation glide in Al.This study highlights unusual mechanisms that impart plastic deformability at ultra-high yield strengths in intermetallic-metal composites with a low-symmetry intermetallic matrix phase.
文摘Planar films of pure and Ti^(4+)-dopedβ-Fe_(2)O_(3)were prepared by a spray pyrolysis method.X-ray diffraction patterns and Raman spectra of the metastableβ-Fe_(2)O_(3)film showed that its thermal stability was significantly improved because of covalent bonds in the interfaces between the film and substrate,while only weak Van der Waals bonds existed at the interfaces within the particle-assembledβ-Fe_(2)O_(3)film prepared by electrophoretic deposition.The as-prepared planar films were thus able to withstand higher annealing temperature and stronger laser irradiation power in comparison with theβ-Fe_(2)O_(3)particle-assembly.Ti^(4+)doping was used to increase the concentration of carriers in the metastableβ-Fe_(2)O_(3)film.Compared with pureβ-Fe_(2)O_(3)photoanodes,the highest saturated photocurrent for water splitting over the Ti^(4+)-dopedβ-Fe_(2)O_(3)photoanode was increased by a factor of approximately three.Theβ-Fe_(2)O_(3)photoanode exhibited photochemical stability for water splitting for a duration exceeding 100 h,which indicates its important potential application in solar energy conversion.
基金National Key R&D Program of China,Grant/Award Number:2021YFB1507403National Natural Science Foundation of China,Grant/Award Number:52071218+1 种基金China Postdoctoral Science Foundation,Grant/Award Number:2022M722170Shenzhen University 2035 Program for Excellent Research,Grant/Award Number:00000218。
文摘Exploration of metastable phases holds profound implications for functional materials.Herein,we engineer the metastable phase to enhance the thermo-electric performance of germanium selenide(GeSe)through tailoring the chemical bonding mechanism.Initially,AgInTe2 alloying fosters a transition from stable orthorhombic to metastable rhombohedral phase in GeSe by substantially promoting p-state electron bonding to form metavalent bonding(MVB).Besides,extra Pb is employed to prevent a transition into a stable hexagonal phase at elevated temperatures by moderately enhancing the degree of MVB.The stabilization of the metastable rhombohedral phase generates an optimized bandgap,sharpened valence band edge,and stimulative band convergence compared to stable phases.This leads to decent carrier concentra-tion,improved carrier mobility,and enhanced density-of-state effective mass,culminating in a superior power factor.Moreover,lattice thermal conductivity is suppressed by pronounced lattice anharmonicity,low sound velocity,and strong phonon scattering induced by multiple defects.Consequently,a maximum zT of 1.0 at 773 K is achieved in(Ge_(0.98)Pb_(0.02)Se)_(0.875)(AgInTe_(2))_(0.125),resulting in a maximum energy conversion efficiency of 4.90%under the temperature difference of 500 K.This work underscores the significance of regulating MVB to stabilize metastable phases in chalcogenides.
基金Financial support from the State Key Program of National Natural Science of China(No.20836009)the National Natural Science Foundation of China(Grant Nos.40573044,40773045)+1 种基金the“A Hundred Talents Program”of the Chinese Academy of Sciences(No.0560051057)the Specialized Research Fund for the Doctoral Program of Chinese Higher Education(No.20060616004)is greatly acknowledged.
文摘The solubilities and densities of the aqueous metastable ternary systems(NaCl-MgCl_(2)-H_(2)O)and(KCl-MgCl_(2)-H_(2)O)at 308.15 K were determined by the isothermal evaporation method.On the basis of the experimental results,the phase diagrams for those systems were plotted.It was found that the former system belongs to the hydrate-I type with one invariant point of(NaCl+MgCl_(2)·6H_(2)O),two univariant curves,and two crystal-lization regions corresponding to halite(NaCl)and bischofite(MgCl_(2)·6H_(2)O);and the latter system belongs to the type of incongruent-double salts with two invariant points of(KCl+KCl·MgCl_(2)·6H_(2)O)and(MgCl_(2)·6H_(2)O+KCl·MgCl_(2)·6H_(2)O),three univariant curves,and three crystallization regions corresponding to potassium chlor-ide(KCl),carnallite(KCl·MgCl_(2)·6H_(2)O)and bischofite(MgCl_(2)·6H_(2).No solid solutions were found in both systems.
基金financially supported by the National Natural Science Foundation of China(Nos.51904015 and 51534009)the General Program of Science and Technology Development Project of Beijing Municipal Education Commission(No.KM202010005008)+1 种基金the National Natural Science Fund for Innovative Research Groups(No.51621003)the Beijing Municipal Great Wall Scholar Training Plan Project(No.CIT&TCD20190307)。
文摘Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects,however,little information has been known on its nucleation,growth and phase transformation.In this paper,a 900 nm-thick tungsten film with double-layer structure(α-W underlayer andβ-W above it)was produced on SiO_(2)/Si substrate by high vacuum magnetron sputtering at room temperature.The structural properties ofβ-W were systemically investigated by X-ray diffraction,transmission electron microscopy,thermodynamic calculation,first-principle and phase-field simulations.It is found that theβ-W nucleation is energetically favoured on the SiO_(2)surface compared to theα-W one.As the film thickening proceeds,β-W[211]turns to be preferred direction of growth owing to the elastic strain energy minimization,which is verified by phase-field simulations.Moreover,theβ→αphase transformation takes place near the film-substrate interface while the rest of the film keeps theβ-W phase,leading to a doublelayer structure.This localized phase transition is induced by lower Gibbs free energy ofα-W phase at larger grain sizes,which can be confirmed by thermodynamic calculation.Further in-situ heating TEM analysis of the as-deposited film reveals that theβ→αphase transformation is fulfilled byα/βinterface propagation rather than local atomic rearrangements.Our findings offer valuable insights into the intrinsic properties of metastable phase in tungsten.
文摘This study investigates the development of novel high-entropy alloys(HEAs)with enhanced mechanical properties through an innovative fabrication method of direct energy deposition(DED).The focus is on the creation of metastable core-shell precipitation-strengthened HEAs that exhibit a unique multi-stage terrace-like slip wave toughening mechanism,a novel approach to improving both strength and ductility simultaneously.Mechanical testing reveals that the developed HEAs exhibit superior mechanical proper-ties,including high yield strength,ultimate tensile strength,and exceptional ductility.The improvement in these properties is attributed to the multi-stage terrace-like slip wave toughening mechanism activated by the unique microstructural features.This toughening mechanism involves the sequential activation of slip systems,facilitated by the stress concentration around the core-shell precipitates and the subsequent propagation of slip waves across the material.The terrace-like pattern of these slip waves enhances the material's ability to deform plastically,providing a significant toughening effect while maintaining high strength levels.Furthermore,the study delves into the fundamental interactions between the microstruc-tural elements and the deformation mechanisms.It elucidates how the core-shell precipitates and the matrix cooperate to distribute stress uniformly,delay the onset of necking,and prevent premature failure.This synergistic interaction between the microstructural features and the slip wave toughening mecha-nism is central to the remarkable balance of strength and ductility achieved in the HEAs.The introduction of a multi-stage terrace-like slip wave toughening mechanism offers a new pathway to designing HEAs with an exceptional amalgamation of strength and ductility.
基金supported by the National Nature Science Foundation of China(NSFC)(Grant No.11974033)Xuqiang Liu acknowledges support from the National Postdoctoral Foundation Project of China under Grant No.GZC20230215+2 种基金the National Nature Science Foundation of China under Grants No.12404001The XRD measurements at room and high temperatures were performed at the 4W2 HPStation of the Beijing Synchrotron Radiation Facility(BSRF)and beamline 15U1 of the Shanghai Synchrotron Radiation Facility(SSRF)In situ high-pressure,low-temperature XRD measurements were conducted at sector 16 ID-B,HPCAT of the Advanced Photon Source,and were supported by DOE-NNSA under Award No.DE-NA0001974.
文摘High-pressure β-Sn germanium may transform into diverse metastable allotropes with distinctive nanostructures and unique physical properties via multiple pathways under decompression.However,the mechanism and transition kinetics remain poorly understood.Here,we investigate the formation of metastable phases and nanostructures in germanium via controllable transition pathways of β-Sn Ge under rapid decompression at different rates.High-resolution transmission electron microscopy reveals three distinct metastable phases with the distinctive nanostructures:an almost perfect st12 Ge crystal,nanosized bc8/r8 structures with amorphous boundaries,and amorphous Ge with nanosized clusters (0.8–2.5 nm).Fast in situ x-ray diffraction and x-ray absorption measurements indicate that these nanostructured products form in certain pressure regions via distinct kinetic pathways and are strongly correlated with nucleation rates and electronic transitions mediated by compression rate,temperature,and stress.This work provides deep insight into the controllable synthesis of metastable materials with unique crystal symmetries and nanostructures for potential applications.
基金supported by the National Science and Technology Major Project of China(No.J2019-VI-0011-0125)ND Basic Research Funds of China(No.G2022WD)Shaanxi Province Innovation Capability Support Program(No.2023-CX-TD-47).
文摘TiAl alloys with the(α2+γ)lamellar structure are highly valued for their excellent high-temperature strength and creep resistance.Understanding the formation mechanism of the lamellar structure is crucial for tuning the microstructure and properties.This work investigates the formation of lamellar structure in Ti-48AI-7Nb-2.5V-1Cr alloy,revealing the presence of hcp-based long-period superstructure(hcp-LPS)as a metastable phase during lamellar formation.The identification of hcp-LPS demonstrates that the necessary solute enrichment for the formation ofγlamellae occurs on the hexagonalαmatrix,implying that phase separation ofα→Al-richαlamellae+Al-depletedαlamellae is the first step of lamellar formation.Once phase separation is completed,all subsequent phase transitions occur within the Al-richαlamellae.Additionally,the formation of twin lamellae is further discussed.The formation of the twin lamellae occurs sequentially.Pre-existing lamella promotes the formation of later lamella by inducing so-lute enrichment in its surrounding region,and then the successive slip of Shockley partial dislocations with opposite Burgers vectors ensures special stacking of later lamellae.These findings not only con-tribute to the fundamental understanding of spinodal mechanisms in hexagonal crystals,but also provide novel insights into the formation of twin lamellae.
