The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The result...The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.展开更多
With the invention of coronary stent, promising clinical outcomes appeared. However, the long-term success of stent has been beaten by significant in-stent restenosis and consequently stents fractures (SF). Cardiologi...With the invention of coronary stent, promising clinical outcomes appeared. However, the long-term success of stent has been beaten by significant in-stent restenosis and consequently stents fractures (SF). Cardiologists have been looking on SF as a threat to patients’ life because it is associated with short- and possibly long-term morbidity rate. In this review, stent materials and properties from the perspective of materials engineering and clinical drivers are discussed. The review also outlines how stent materials and design have evolved with time. Opinions are given as to the merit and direction of various on-going and future developments.展开更多
Deformation twinning is known to be important in the acquisition of plasticity for hexagonal close-packed crystal structures,of great implication to the design and development of novel high-strength Mg alloys with enh...Deformation twinning is known to be important in the acquisition of plasticity for hexagonal close-packed crystal structures,of great implication to the design and development of novel high-strength Mg alloys with enhanced formability.Accurate understanding of deformation twinning necessitates critical mechanistic knowledge of the activation and selection of twins at nanoscale.In this work,considering polycrystalline Mg,we performed comprehensive molecular dynamics simulations to investigate deformation twinning under uniaxial tension and compression loading.An algorithm has been developed and implemented to identify the active twin variants of three operating twin modes during deformation.Sharp contrast between tension and compression loading in terms of twin patterns and twin growth was observed,attributed to difference in twin variant activation and twin-twin interaction under the two loading conditions.Furthermore,the critical role of Schmid factor in twin variant activation and selection has been elucidated,in good agreement with experimental observations.This study contributes critical insights towards advancing our understanding of the complex behaviors of deformation twinning in polycrystalline Mg.展开更多
A newly proposed rapid fracture test in four-point bending was used to evaluate the effect of tempering on the hydrogen embrittlement(HE)susceptibility of an AISI 4135 steel,where it was tempered to four different str...A newly proposed rapid fracture test in four-point bending was used to evaluate the effect of tempering on the hydrogen embrittlement(HE)susceptibility of an AISI 4135 steel,where it was tempered to four different strength(or hardness)levels.It was observed that HE susceptibility increases with the increase in hardness.It was shown that there will be minimal impact of hydrogen(H)on the fracture of materials with hardness 37 HRC and below,even if they are completely saturated with H.On the other hand,H will have similar detrimental effect on fracture properties of quench and tempered(Q and T)steels having hardness higher than 45 HRC.Ductile to brittle transition behavior was observed for a critical hardness(or strength)range as well as for a critical concentration level of H.Additionally,a critical H concentration was observed to exist for each of the strength levels.Fractography was performed in addition to microstructural characterization using transmission electron microscopy(TEM).A very good correlation was observed between the fast fracture test results and fractography.The fast fracture test was further compared with a conventional incremental step load(ISL)test for the evaluation of HE susceptibility.The ISL test results and fracture surface characteristics corroborate very well with the observations from the fast fracture test.This study successfully establishes the fast fracture test as a novel technique to study HE susceptibility and mechanism(s).展开更多
The present work aimed to investigate the effect of coiling process conditions on microstructure development in a low-Si content TRIP （transformation-induced plasticity）-assisted steel after thermomechanical process...The present work aimed to investigate the effect of coiling process conditions on microstructure development in a low-Si content TRIP （transformation-induced plasticity）-assisted steel after thermomechanical processing. In this framework, compression samples which were deformed above Tnr and then intercritically annealed were held isothermally for different durations at temperatures below bainite transformation start temperature. Microstructure of samples were characterized by optical and electron microscopy, XRD （X-ray diffraction） and M6ssbauer spectroscopy. The results indicated that due to low-silicon content of the present steel, the incomplete bainite reaction phenomena was not observed and, hence, the maximum carbon enrichment of residual austenite was achieved in the samples which held for short durations. It was also shown that the maximum carbon enrichment and volume fraction of residual austenite were achieved at intermediate bainite hold temperature of 450 ℃ as the result of competing phenomena, such as microstructural refinement, dislocation density, carbide precipitation and growth.展开更多
Cd coating is used in aerospace industries from last five decades due to its sacrificial protection and lubrication properties. Although Cd coating is primarily used due to its sacrificial corrosion protection when ap...Cd coating is used in aerospace industries from last five decades due to its sacrificial protection and lubrication properties. Although Cd coating is primarily used due to its sacrificial corrosion protection when applied on steel substrate, the added benefit of modifying the tribological behavior by acting as a lubricious layer gives it a leading-edge than other coatings. Often the measurement of friction coefficient (CoF) is reported as a value generated after full sliding cycle. This measurement of average CoF generally limits the study of local variation in CoF occurring within one sliding cycle, which can be significantly different with change in spatial position due to change in third body morphology. In this study, a linearly reciprocating sliding test is used to measure the CoF at a sampling rate of 800 Hz along the track length to generate triboscopic image with steel countersphere. The instantaneous CoF obtained with triboscopy is correlated with the wear track morphology using scanning electron microscope (SEM) and optical profilometer for variation in contact conditions. Tribological test performed in dry atmosphere shows an average CoF of 0.4 till the end of the test whereas with increase in relative humidity to 60%, the average CoF changes from 0.4 to 0.8 at the end of the test due to change in contact conditions. Soft Cd coating on low carbon steel substrate is used to study these variations in third body morphology.展开更多
基金This research is financially supported by the China Scholarship Council under Grant No.201806935054the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi,China,under Grant No.201802035.
文摘The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.
文摘With the invention of coronary stent, promising clinical outcomes appeared. However, the long-term success of stent has been beaten by significant in-stent restenosis and consequently stents fractures (SF). Cardiologists have been looking on SF as a threat to patients’ life because it is associated with short- and possibly long-term morbidity rate. In this review, stent materials and properties from the perspective of materials engineering and clinical drivers are discussed. The review also outlines how stent materials and design have evolved with time. Opinions are given as to the merit and direction of various on-going and future developments.
基金financial support from Natural Sciences and Engineering Research Council of Canada(NSERC RGPIN-2023–03628)H.C.also thanks the financial support from China Scholarship Council(201706050149)McGill Engineering Doctoral Award for financial support.
文摘Deformation twinning is known to be important in the acquisition of plasticity for hexagonal close-packed crystal structures,of great implication to the design and development of novel high-strength Mg alloys with enhanced formability.Accurate understanding of deformation twinning necessitates critical mechanistic knowledge of the activation and selection of twins at nanoscale.In this work,considering polycrystalline Mg,we performed comprehensive molecular dynamics simulations to investigate deformation twinning under uniaxial tension and compression loading.An algorithm has been developed and implemented to identify the active twin variants of three operating twin modes during deformation.Sharp contrast between tension and compression loading in terms of twin patterns and twin growth was observed,attributed to difference in twin variant activation and twin-twin interaction under the two loading conditions.Furthermore,the critical role of Schmid factor in twin variant activation and selection has been elucidated,in good agreement with experimental observations.This study contributes critical insights towards advancing our understanding of the complex behaviors of deformation twinning in polycrystalline Mg.
基金This work was financially supported by the Government of Canada through Natural Sciences and Engineering Research Council(NSERC),and the industrial collaborators led by Industrial Fasteners Institute(USA),Canadian Fasteners Institute(CFI),Boeing Company(USA),Infasco(Canada)and the Research Council on Structural Connections(RCSC).
文摘A newly proposed rapid fracture test in four-point bending was used to evaluate the effect of tempering on the hydrogen embrittlement(HE)susceptibility of an AISI 4135 steel,where it was tempered to four different strength(or hardness)levels.It was observed that HE susceptibility increases with the increase in hardness.It was shown that there will be minimal impact of hydrogen(H)on the fracture of materials with hardness 37 HRC and below,even if they are completely saturated with H.On the other hand,H will have similar detrimental effect on fracture properties of quench and tempered(Q and T)steels having hardness higher than 45 HRC.Ductile to brittle transition behavior was observed for a critical hardness(or strength)range as well as for a critical concentration level of H.Additionally,a critical H concentration was observed to exist for each of the strength levels.Fractography was performed in addition to microstructural characterization using transmission electron microscopy(TEM).A very good correlation was observed between the fast fracture test results and fractography.The fast fracture test was further compared with a conventional incremental step load(ISL)test for the evaluation of HE susceptibility.The ISL test results and fracture surface characteristics corroborate very well with the observations from the fast fracture test.This study successfully establishes the fast fracture test as a novel technique to study HE susceptibility and mechanism(s).
文摘The present work aimed to investigate the effect of coiling process conditions on microstructure development in a low-Si content TRIP （transformation-induced plasticity）-assisted steel after thermomechanical processing. In this framework, compression samples which were deformed above Tnr and then intercritically annealed were held isothermally for different durations at temperatures below bainite transformation start temperature. Microstructure of samples were characterized by optical and electron microscopy, XRD （X-ray diffraction） and M6ssbauer spectroscopy. The results indicated that due to low-silicon content of the present steel, the incomplete bainite reaction phenomena was not observed and, hence, the maximum carbon enrichment of residual austenite was achieved in the samples which held for short durations. It was also shown that the maximum carbon enrichment and volume fraction of residual austenite were achieved at intermediate bainite hold temperature of 450 ℃ as the result of competing phenomena, such as microstructural refinement, dislocation density, carbide precipitation and growth.
基金Natural Science and Engineering Research Council Canada(NSERC)Boeing Research and Technology+2 种基金Pratt and Whitney CanadaHéroux Devtek,Canadian Fastener InstituteMessier-Bugatti-Dowty
文摘Cd coating is used in aerospace industries from last five decades due to its sacrificial protection and lubrication properties. Although Cd coating is primarily used due to its sacrificial corrosion protection when applied on steel substrate, the added benefit of modifying the tribological behavior by acting as a lubricious layer gives it a leading-edge than other coatings. Often the measurement of friction coefficient (CoF) is reported as a value generated after full sliding cycle. This measurement of average CoF generally limits the study of local variation in CoF occurring within one sliding cycle, which can be significantly different with change in spatial position due to change in third body morphology. In this study, a linearly reciprocating sliding test is used to measure the CoF at a sampling rate of 800 Hz along the track length to generate triboscopic image with steel countersphere. The instantaneous CoF obtained with triboscopy is correlated with the wear track morphology using scanning electron microscope (SEM) and optical profilometer for variation in contact conditions. Tribological test performed in dry atmosphere shows an average CoF of 0.4 till the end of the test whereas with increase in relative humidity to 60%, the average CoF changes from 0.4 to 0.8 at the end of the test due to change in contact conditions. Soft Cd coating on low carbon steel substrate is used to study these variations in third body morphology.
