ORIENTATION VARIANT SELECTION AND ITS EFFECTING FACTORS IN TITANIUM SHEET

By means of crystallite orientation distribution function (ODF) analysis, the influence on orientation variant selection of cold rolling reductions, recrystallization annealing and cooling rates during α→β→α cyclic phase transformation treatments in cold rolled titanium sheet has been investigated. The results show that, cold rolling reduction and annealing treatment both have a great effect on the formation of phase transformation textures.No variant selection occurs in cold rolled and recrystallization annealed titanium sheet which resulte in a more randomly distributed texture.The phase transformation texture formed α→β→α transformation is inherited after 3 cycles of α→β→α phase transformation. The cooling rates during phase transformation have little effect on orientation variant selection.

Variant selection map of external load during Ni_(4)Ti_(3) precipitation in nitinol:a theoretical and phase field study

The morphology of Ni_(4)Ti_(3) precipitates is important in tuning the martensitic transformation(MT)behavior and mechanical properties of nitinol.Constrained ageing is effective in engineering the morphology of Ni_(4)Ti_(3) precipitates due to the variant selection effect of external load which is still lacking.In this work,maps of variant selection effect of external load applied along all crystallographic directions are obtained by using a combination of theoretical analyses and phase field simulations.It is found that maps produced by uniaxial tension and uniaxial compression are quite different.The number and types of Ni_(4)Ti_(3) variants preferred by external load vary as the loading direction changes.Moreover,factors influencing the strength of variant selection effect are discovered.This work provides insights on understanding the Ni_(4)Ti_(3) precipitation process and sheds light on the engineering of morphology of Ni_(4)Ti_(3) precipitates for desired mechanical and functional properties.

Variant selection in laser powder bed fusion of non-spherical Ti-6Al-4V powder

The presence ofα/αon priorβ/βgrain boundaries directly impacts the final mechanical properties of the titanium alloys.Theβ/βgrain boundary variant selection of titanium alloys has been assumed to be unlikely owing to the high cooling rates in laser powder bed fusion(L-PBF).However,we hypothesize that powder characteristics such as morphology(non-spherical)and particle size(50–120μm)could affect the initial variant selection in L-PBF processed Ti-6Al-4V alloy by locally altering the cooling rates.Despite the high cooling rate found in L-PBF,results showed the presence ofβ/βgrain boundaryαlath growth inside two adjacent priorβgrains.Electron backscatter diffraction micrographs confirmed the presence ofβ/βgrain boundary variant selection,and synchrotron X-ray high-speed imaging observation revealed the role of the“shadowing effect”on the locally decreased cooling rate because of keyhole depth reduction and the consequentβ/βgrain boundaryαlath growth.The self-accommodation mechanism was the main variant selection driving force,and the most abundantα/αboundary variant was type 4(63.26°//[10553¯]).The dominance of Category IIαlath clusters associated with the type 4α/αboundary variant was validated using the phenomenological theory of martensite transformations and analytical calculations,from which the stress needed for theβ→αtransformation was calculated.

Variant selection of nanocrystalline a phase in metastableβTB8 titanium alloy via electric pulse treatment

In this paper,the metastableβTB8 titanium alloy with nanocrystallineαphase is achieved by electric pulse treatment.The morphology evolution and variant selection of nanocrystallineαphase in metastableβTB8titanium alloy were investigated by using scanning electron microscope(SEM),electron backscattered diffraction(EBSD)and transmission electron microscope(TEM)analysis.The results indicated that the morphologies of the nanocrystallineαphase were mainly triangular clusters and needle-like at the pressure of 0 MPa.With increasing pressure from 20 to 50 MPa,the volume fraction of needlelikeαphase decreased,and a large amount of V-shapedαphase formed in the interior ofβgrains.Based on the EBSD data,the parentβphase was reconstructed by MTEX software.In the interior of theβgrains,12 variants can form for the samples electric pulse treated at 0 and20 MPa,while only 3 and 6 variants can form for the samples electric pulse treated at 30 and 50 MPa.In the grain boundary of theβgrains,one or more grain boundaryαvariants can be generated for the samples electric pulse treated at different pressures as long as one of the neighborβgrains follows the Burgers orientation relationship.

SELECTION OF METHIONINE-RESISTANT VARIANT OF MEDICAGO SATIVA

A variant cell line of Medicago saliva L. resistant to methionine (100 mmol/L) was selected .from calli treated with 1.5 mmol/L sodium azide. Plants were regenerated from this selected cell line. After growing for 6 months on the medium devoid of selection pressure, the variant cell line still showed high level of resistance, which was 7.2-fold higher than that of the wild type. This indicated the stability of the resistance expression. The variant cell line also expressed a 3.3-fold higher cross-resistance to ethionine, than that of the wild type. The contents of methionine, lysine, threonine and isoleucine in the variant calli were respectively 2.0, 1.43, 1.71 and 1.57 times higher than those in the wild type. However, in the variant regenerated plantlets they respectively were 2.0, 1.33, 1.22 and 1.22 times higher than those in the wild type regenerants. Four new protein bands, viz. 65 kD, 23 kD, 20 kD and 18 kD, appeared in the SDS-PAGE pattern of soluble proteins in the variant calli. In the peroxidase isoenzyme electrophoresis pattern, two new bands were also observed in the variant calli.

Variant selection, coarsening behavior of α phase and associated tensile properties in an α+β titanium alloy

Effect of cooling rates,i.e.,air cooling and furnace cooling,after solution in α+β phase-field on variant selection,coarsening behavior of α phase and microstructure evolution were investigated in α+β TC21 alloy.The textures of primary α(α_(p))and lamellar α(α_(L))in β phase transformation microstructure(β_(t))were analysed separately,and the orientation relationship among α_(p),α_(L) and the parent β phase were studied.In addition,the influence of the microstructure characteristics on the tensile properties was investigated.The results showed that all parent β grains,despite their different orientations,produced 12 ideal α_(L) variants with the same texture components and interweave to form a basketweave β_(t) structure under the air-cooling condition.The α_(p) without Burgers orientation relationship(BOR)with β phase exhibited obviously texture component without overlapping theαL texture component.The volume fraction of α_(p) in the furnace-cooled sample(about 50%)was higher than that of the air-cooled sample(about 12%),while the size of it slightly increased with decreasing the cooling rate.In each β grain,the thick α_(L) in the same orientation formed anαcolony.A typical 3 variant colonies which were related to each other were observed.Consequently,the α_(L) spatial orientation distribution showed more heterogeneity.Moreover,the BOR between α_(p) and β and the same orientation of some α_(L) and the surrounding α_(p) grains resulting in the overlapping of α_(p) texture component and α_(L) texture component.At last,the relationship between microstructure and tensile properties was analysed.

Effect of cooling rate followingβforging on texture evolution and variant selection duringβ→αtransformation in Ti-55511 alloy

The control of the post-forging cooling rate has been a key issue in the industrial production process of titanium alloys. We investigated texture evolution and variant selection(VS) during β → α transformation through high-temperature compression experiments followed by quantitative control of varying cooling rates. Results show that post-forging cooling rates affect β grains, α variants, and α/β textures. The αprecipitation inhibits motions of β static recrystallization(β_(SRX)) grain boundaries and thus leads to grain refining from 0.1 ℃/s to 0.05 ℃/s. Further analysis reveals that lamellae grain boundary widmanstattenα(α_(WGB)) keeps growing rapidly within β-grain in an interface instability manner at 0.1–0.05 ℃/s. Most of α-phase with 50°–60°/<-12–10> is preferentially precipitated at β-medium angle GBs between 30°and 45° and strictly follows BOR with the side of of adjacent β-grain with the same or similar {110} or{111}. Moreover, the texture type transforms gradually from RGoss {110} <1–10> to Brass {110} <1–12>from 25 ℃/s to 1 ℃/s. βgrains exhibit(102) [-201] texture, while the corresponding α has textures of<0001>//Z and <11–20>//Y from 1 ℃/s to 0.05 ℃/s. Our findings lay a profound theoretical foundation in microstructure evolution of near-β titanium alloy for industrial production.

Variant selection in stationary shoulder friction stir welded Ti-6Al-4V alloy

Stationary shoulder friction stir welding of Ti-6Al-4V of 7 mm thickness was conducted with varying welding speeds and rotation speeds. Variant selection analysis was carried out based on the inherited α phase texture and the reconstructed 13 phase texture. The weld surfaces became significantly smoother with increasing welding speed and decreasing rotation speed. Heat input decreased greatly with increased welding speed and it decreased slightly with decreased rotation speed. The orientation relationship between the prior 13 grains was measured based on the reconstructed electron backscattered diffraction （EBSD） data. Weak Variant selection has occurred in all the welds because most of the prior 13 grains did not share { 110} poles. Strong links between crystal orientation of the prior 13 grains and hardness have been found.

Genes for the high life： New genetic variants point to positive selection for high altitude hypoxia in Tibetans

People living on the high plateaus of the world have long fascinated biological anthropologists and geneticists because they live in ＂thin air＂ and epitomize an extreme of human biological adaptation.

Tension-compression asymmetry and corresponding deformation mechanism in ZA21 magnesium bars with bimodal structure

We investigated the asymmetric tension-compression(T-C)behavior of ZA21 bars with bimodal and uniform structures through axial tension and compression tests.The results show that the yield strengths of bars having bimodal structure are 206.42 and 140.28 MPa under tension and compression,respectively,which are higher than those of bars having uniform structure with tensile and compressive yield strength of 183.71 and 102.86 MPa,respectively.Prismatic slip and extension twinning under tension and basal slip and extension twinning under compression dominate the yield behavior and induce the T-C asymmetry.However,due to the basal slip activated in fine grains under tension and the inhibition of extension twinning by fine grains under compression,the bimodal structure possesses a lower T-C asymmetry(0.68)compared to the uniform structure(0.56).Multiple extension twins occur during deformation,and the selection of twin variants depends on the Schmid factor of the six variants activated by parent grains.Furthermore,the strengthening effect of the bimodal structure depends on the grain size and the ratio of coarse and fine grains.

Implications of twinning on the microstructure development,crystallographic texture and mechanical performance of Mg alloys-a critical review

Deformation twinning is profusely activated in the Mg alloys due to lower critical resolved shear stress(CRSS) compared to the non-basal slip systems(prismatic and pyramidal ) and plays a significant role in texture reorientation, grain refinement and enhancement of mechanical performance. Twinning is a sequential process comprising twin nucleation, twin propagation and twin growth, hence several intrinsic and extrinsic parameters that facilitate or suppress the process have been critically reviewed. The dependence of twinning on the grain size, deformation temperature, favorable grain orientation and shear strain have been thoroughly discussed in the context of published literature and an attempt has been made to provide a benchmark conclusive finding based on the majority of works. Furthermore, the subsequent effect of twinning on the mechanical performance of Mg alloys, including ductility, formability and tension-compression asymmetry has been discussed in detail. Lastly, the stability of twins, including stress and thermal stability, is summarized and critical issues related to pertinent bottlenecks have been addressed.

Interaction among deformation, recrystallization and phase transformation of TA2 pure titanium during hot compression

TA2 pure titanium was chosen to research the interaction among deformation, recrystallization and phase transformation during hot compression. The samples were hot compressed by thermal simulation method with different processing parameters. Variant selection induced by stress during cooling after compression was found. The prismatical texture component which featured that the [0001] direction perpendicular to the compressing direction produced preferentially under the compressing stress. As a result, the transformedα phase possesses strong prismatical texture which is different with the basal texture of compressed αphase. The minimum elastic strain energy is demonstrated to be the main reason that causes the variant selection. Dynamic recrystallization behavior and microstructure evolution during hot compression were also studied.

Role of yttrium content in twinning behavior of extruded Mg−Y sheets under tension/compression

The influence of Y content on the grain-scale twinning behavior in extruded Mg−xY(x=0.5,1,5,wt.%)sheets under uniaxial tension and compression along the extruded direction was statistically investigated.An automatic twin variant analysis was employed,based on large data sets obtained by electron backscatter diffraction(EBSD),including 2691 grains with 977 twins.The{1012}tension twinning(TTW)dominance and prevailing anomalous twinning behavior(Schmid factor(m)<0)under both tension and compression were found.The anomalous twinning behavior was more pronounced as Y content increased under tensile loading,indicating a promoted stochasticity of twin variant selection for more concentrated Mg−Y alloys.However,the trend for the Y-content dependent anomalous twinning behavior was opposite in compression.The fractions of the anomalous TTWs were found to be well correlated with the maximum Schmid factor(m_(max))values of basal slip and prismatic slip in the corresponding parent grains for compression and tension,respectively,indicating that twinning and dislocation slip might be closely related in the present Mg−Y alloys.

Microstructure Evolution of Heat‑Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile–Brittle Transition Temperature

The comparative study of submerged arc welding(SAW)and laser hybrid welding(LHW)was carried out for a 690 MPa high strength steel with thickness of 20 mm.Microstructure and ductile–brittle transition temperature(DBTT)evolution in welded zone were elucidated from the aspect of crystallographic structure,particularly,digitization and visualization of 24 variants.The impact toughness of each micro zone in LHW joint is better than that of SAW,in which the DBTT of equivalent fusion line and heat-affected zone(HAZ)can reach−70 and−80℃,while that of SAW is only−50℃.LHW technology induces narrowing of the HAZ and refining of the microstructure obtained in weld metal and HAZ.Meanwhile,the austenite grain size and transformation driving force in the coarse grained heat-affected zone(CGHAZ)are reduced and increased,respectively.It makes variant selection mechanism occurring in CGHAZ of LHW dominate by close-packed plane grouping,which promotes lath bainite formation with high density of high angle grain boundary,especially block boundary dominated by V1/V2 pair.While for SAW,the lower transformation driving force inferred from the large amount of retained austenite in CGHAZ induces Bain grouping of variants,and thus triggers the brittle crack propagating straightly in granular bainite,resulting in lower impact toughness and higher DBTT.

Forging-microstructure-tensile properties correlation in a new nearβhigh-strength titanium alloy

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.

EBSD investigation of the crystallographic features of deformation-induced martensite in stainless steel

Variant selection during the martensitic transformation in steels may play an important role in determining the transformation kinetics and the resulting mechanical properties.In this study,the variant selection and crystallographic features of deformation-induced martensite were investigated by quasi in situ electron backscatter diffraction(EBSD) in grade SUS321 during tensile deformation.Significant differences in variant selection between austenite(γ)→hcp-martensite(ε)→bcc-martensite(α’) and γ→α’transformation routes were observed and reported in detail,which demonstrated that s-martensite plays an important role in the variant selection of α’.Variant selection at diffe rent deformation stages was also analysed and revealed that α’ variants with the highest priority and variant pairs were preferred at the initial and last deformation stages in the γ→ε→α’sequence,respectively.Meanwhile,the single α’ variant nucleated at the thin slip band keeps its crystallography feature upon further deformation in the γ→α’sequence.In addition,the strain work of the martensitic transformation for applied loads was quantitatively estimated to explain the variant selection and associated mechanism.When these calculations are compared to the experimental results it is found that they are not able to predict which α’ variant is forming pre ferentially during either during the γ→α’ or the ε→α’ sequences,while only accurate predictions are obtained for the γ→ε-transformation which indicates that the γ→α’ variant selection is more complex.

Relationship between martensite microstructure and ductility of H13 steel from aspect of crystallography

Based on the parent austenite orientation reconstruction method,it is aimed to reveal the origination of high angle grain boundaries(HAGBs)and its relationship with ductility of H13 steel.The orientation relationship between martensite and parent austenite of quenched H13 samples was(123.5°,9.3°,192.5°),which agreed with the Kurdumov–Sachs relationship.The variant distribution of quenched samples was dominated by close-packed plane group,and its high length fraction of V1/V2 inter-variant boundaries of calculated 62.6%was mainly contributed to HAGBs(>45°).When the quenched H13 samples underwent the pre-tempering treatment,their density of HAGBs(>45°)notably increased from 1.33 to 2.39μm^(−1),which improved its total elongation from 8.3%to 11.5%.Compared with the quenched H13 samples,the length fraction of V1/V2 inter-variant boundaries of H13 samples with pre-tempering for 5,10 and 60 min was reduced by 6.7%,7.0%and 7.5%,respectively.During pre-tempering treatment,V1/V3&V5 variant pairs,etc.,merged V1/V2 variant pair by strain-induced grain boundary migration,which decreased the length fraction of V1/V2 inter-variant boundaries by 7.0%.The pre-tempering treatment significantly increased HAGBs(>45°)of H13 samples by sub-grains coarsening and strain-induced grain boundary migration mechanism.

Multi-dimensional morphology of hydride diffusion layer and associated sequential twinning in commercial pure titanium

The dominant hydride precipitates have been well demonstrated to follow two types of orientation relationships(ORs)with Ti matrix:OR1 with{0001}//{001},<1210>//<110>and OR2 with{0001}//{111},<1210>//<110>.Within the grains with special orientations,the complicated interactions of different hydride variants inside Ti-hydride diffusion layer are characterized in this work.For OR1 hydride layer,the orientations of{1010}plane parallel to the sample surface and a-axis parallel to the normal direction prefer multiple OR1 variants.The orientations favorable for OR2 hydride layer are:{1013}plane parallel to sample surface corresponding to the layer with one OR2 variant dominated and c-axis parallel to the surface normal with multiple OR2 variant layer preferred.Furthermore,{1012}extension twins and{1122}contraction twins are activated to accommodate the OR2 hydride-induced surface expansion and local misfit strain.The stimulation of these two twins is also orientation-dependent:{1012}and{1122}twins are observed in the grains with c-axis parallel to and deviated from the surface normal,respectively.The further variant selection for each twin mode is performed through shear accommodation of hydride-twin pairs.