INTERACTION BETWEEN V_4C_3 PARTICLES AND DEFORMATION TWINS

By using Moire′ pattern formed by the double diffraction of precipitated V_4C_3 and matrix of γ-phase,the nature of the structure of the interface between precipitated V_4C_3 and matrix as well as the retarding effect of V4C3 particles on the development of deformation twins have been investigated.It is shown that partially coherent interfaces exist between V_4C_3 and ma- trix.Twinning dislocations may transform into interfacial dislocations under external stress, and may move continuously through bypassing or cross-sliping.

Exploring the Formation Mechanism of Deformation Twins in CrMnFeCoNi High Entropy Alloy

The deformation twins initiated in CrMnFeCoNi high entropy alloy at cryogenic temperature are experimentally studied. Under the external loading, a three-dimensional shear stress concentration originating from dislocation tangling at both the grain boundaries and twin boundaries could be formed, which promotes emission of partial dislocations from the planar defects and is thus considered to be the key factor for twin formation. A sympathetic nucleation mechanism is proposed to describe the nucleation behaviors of twins.

STRAIN INDUCED STRUCTURAL TRANSITION OF INTERFACES AND TWINS IN A HOT－DEFORMED DUAL－PHASE TIAL ALLOY

The structure characteristics of a2/γinterfaces and the features of deformation twins in a quasi-isothermal forged Ti-45Al-10Nb alloy were studied by highresolution transmission electron microscopy. Three types of strain induced a2/γinterfaces and two types of strain induced twin boundaries were identified The most,important features are high density of ledges and the existence of I/3[111] Frank partial dislocation. Mechanisms for the formation these interfaces were proposed Two types of deformation twins were observed These deformation twins always start from the ledges it seems that ledges at interfaces are important features of interfacial structure for the mechanical behavior of alloys.

Effects of twin and stacking faults on the deformation behaviors of Al nanowires under tension loading

The effects of twin spacing and temperature on the deformation behavior of nanotwinned Al under tensile loading are investigated using a molecular dynamic（MD） simulation method.The result shows that the yield strength of nanotwinned Al decreases with the increase of twin spacing,which is related to the repulsive force between twin boundary and the dislocation.The result also shows that there is no strain-hardening at the yield point.On the contrary,the stress is raised by strain hardening in the plastic stage.In addition,we also investigate the effects of stacking fault thickness and temperature on the yield strength of the Al nanowire.The simulation results indicate that the stacking fault may strengthen the Al nanowire when the thickness of the stacking fault is below a critical value.

Effect of deformation twinning on the evolution of texture in TWIP steels during cold-rolling

Texture evolution of high-manganese twining-induced plasticity (TWIP) steels (Fe-16Mn-0.6C) during cold-rolling is studied by means of quantitative orientation distribution function (ODF)analysis.Thickness reductions of the specimens during cold-rolling are 10%,20%,30%,50% and 65%,respectively.Evolution of texture is of the Brass type,which is typical for low-stacking fault energy (SFE) materials.The contribution of deformation twinning to the development of texture is clearly illustrated by the monotonic increase of the twinned Cu component.In the present study,the deformation twinning was identified as significantly contributing to deformation up to the maximum reduction applied.These results are useful for the prediction and control of the texture in TWIP steels.

AN IN SITU STUDY ON DEFORMATION TWINNING NEAR CRACK TIP IN TWO－PHASE TiAl－BASE ALIOY

An in situ study of twinning at crack tip in a TiAl-base alloy has been performed.The result shows that twinning with long shear vector(2/6) [112] can generate on(111) plane, even though usually it is very difficult to occur because of the high energy barrier. It was further shown that (1/6) [112](111) twinning is considerably easier to generate. Furthermore,(1/2)<110) ordinary dislocations were very active and dominated nearly the whole plastic zone, in spite of low Schmid factors. On the other hand, however <101) and (1/2) <112] superdislocations with higher Schmid factors can hardly be observed.

THE α_2/γ INTERFACIAL STRUCTURE CHANGES IN A DUAL-PHASE γ-TiAl ALLOY AFTER HOT DEFORMATION AND SUBSEQUENT SHORT-TIME ANNEALING

The nonequilibrium α2/γ interfacial structures have been studied using the conventional and high-resolution transmission electron microscope. The nonequilibrium γ/γ and α2/γ interfaces are important microstructure features of hot-forged Ti-45Al-8Nb2.5Mn-0.05B alloys. The characteristics of these nonequilibrium interfacial boundaries include the frequent interfacial ledges and the deviation from the conventional orientation relationship {111}γ.

An effective rolling process of magnesium alloys for suppressing edge cracks: Width-limited rolling

To suppress the edge crack of the magnesium alloy sheet during the ordinary rolling process, a new rolling process named width-limited rolling was proposed in this paper. Width-limited rolling is a rolling method in which the width of the alloy sheet is limited by modifying the shape of the rollers, allowing a compressive stress field to form at the edge portion of the alloy sheet during rolling, resulting in the reduction of edge cracks. At present work, magnesium alloy sheets were separately subjected to ordinary rolling and width-limited rolling. The microstructure evolution and mechanical properties of the rolled sheets were investigated by EBSD, TEM, and XRD. The results exhibited that under the same rolling conditions, the sheet after ordinary rolling exhibited obvious edge cracks while no crack was found at the edge of the sheet after width-limited rolling. The edge crack suppressing effect was attributed to the reduction of the tensile stress along rolling direction during WLR, promoting the synchronous extension of the edge and center regions to suppress edge crack tendency. Microstructure observation showed that the compressive twins formed in the sheet after ordinary rolling usually exhibited as thin plates and cannot continue to fully develop due to the premature generation of the edge cracks. However, the compressive twins developed maturely in some of which double twins formed and various slip systems with different dislocation Burgers vectors occurred in the rolled sheet after WLR. More twin intersections and shear bands, providing more potential recrystallization nucleation sites, which are beneficial to weaken basal texture. With the cooperation of twinning and dislocation slip, the texture of the sheet after the width-limited rolling is weakened and the mechanical properties are improved.

Strengthening and deformation mechanism of high-strength CrMnFeCoNi high entropy alloy prepared by powder metallurgy

Multiphase CrMnFeCoNi high-entropy alloys(HEAs)were prepared by a powder metallurgy process com-bining mechanical alloying(MA)and vacuum hot-pressing sintering(HPS).The single-phase face-centered cubic(FCC)HEA powder prepared by MA was sintered into a bulk HEA specimen containing FCC phase matrix along with precipitated M 23 C 6 phase and nanoscaleσphase particles.When the sintering temper-ature was 1223 K,the ultimate strength reaches 1300±11.6 MPa,and the elongation exceeds 4%±0.6%.Microstructural characterization reveals that the formation of nanoscale particles and deformation twins play critical roles in improving the strain hardening(SH)ability.Prolonging the MA time promoted the formation of the precipitated phase and enhanced the SH ability by increasing the number of precip-itated particles.The SH capacity increases significantly with increasing sintering temperature,which is attributed to a significant enhancement in the twinning capacity due to grain growth and the reduced number ofσphase particles.Through systematic studies,the planar glide of dislocations was found to be the main mode of deformation,while deformation twinning appeared as an auxiliary deformation mode when the twinning stress was reached.Although the formation of precipitates leads to grain bound-ary and precipitation strengthening effects,crack initiation is more prominent owing to increased grain boundary brittleness around the precipitated M 23 C 6 phase.The prominence of crack initiation is a contra-diction that must be reconciled with regard to precipitation strengthening.This work serves as a useful reference for the preparation of high-strength HEA parts by powder metallurgy.

GRAIN SIZE AND ACTIVATION ENERGY OF SERRATED YIELDING

The apparent activation energy of serrated yielding was measured by two different methods for a commercial brass H68.The results showed that the apparent activation energy of serrat- ed yielding measured by a method involving(m+β)values increases with the grain size.In contrast,those measured by another method without involving(m+β)values are all the same for specimens with different grain sizes.Combining with the observation of the microstructures,the above phenomena have been explained.The method to measure the true activation energy of serrated yielding has also been proposed.

Effect of Frequency on Fatigue Lifetime of Extruded Mg-3%Al-1%Zn Alloy

Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1 Hz and 10 Hz in air. The microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）.When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10 Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at these two frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.

Orientation-dependent ductility and deformation mechanisms in body-centered cubic molybdenum nanocrystals

The knowledge regarding anisotropic mechanical behaviors in nanoscale body-centered cubic (bcc) metals remains obscure. Herein, we report the orientation-dependent ductility in bcc Mo nanocrystals (NCs), which exhibit poor ductility along [110] direction but possess relatively better ductility along the [001] and [112] orientations. The origin of different deformability can be traced down to the distinct deformation mechanisms: the unexpected crack nucleation and propagation induce premature fractures in [110]-oriented NCs;in contrast, deformation twinning could contribute to the enhanced ductility in [001]-oriented NCs;interestingly, we find the activation of multiple dislocation slips in [112]-oriented NCs with the highest ductility. Further molecular dynamics simulations provide deeper insights into the defect dynamics that are closely interlinked with experimental observations. Our findings advance the basic understanding of orientation-dependent mechanical properties and help to guide endeavors to architecture the microstructures of bcc metals with enhanced ductility.

Positive Strain Rate Sensitivity and Deformation Behavior of a Fe–29Mn–3Al–3Si TWIP Steel

The Fe-29 Mn-3 Al-3 Si twin-induced plasticity(TWIP)steel is used to conduct quasi-static compression and dynamic impact deformation with strain rates ranging from 8.3×10^(-4) to 3800 s^(-1).The microstructures and properties of deformed samples under different strain rates were investigated comparatively.These results show that positive strain rate sensitivity was observed with the increase in strain rates and that there was a significant difference in strain rate sensitivity factor(m)between quasi-static compression(m=0.029)and dynamic impact deformation(m=0.190).Compared to the quasi-static compression,the dynamic impact deformation exhibited higher yield strength.Microstructural examination reveals that the primary twins were frequently found during the quasi-static compression process,and the secondary twins were rarely observed.However,the secondary and multi-fold deformation twins were florescent in the dynamic impact samples.At the initial stage of dynamic impact deformation,partial dislocations and staking faults on multiple conjugate{111}planes were simultaneously activated and produced a large number of Lomer-Cottrell dislocations,resulting in a large increase in yield strength during dynamic impact.

Effects of deformation twinning on the mechanical properties of biodegradable Zn-Mg alloys

To satisfy the property requirements for biodegradable medical implants,Zn alloyed with low levels of Mg(≤0.8 wt%)has attracted increased research interest.In the present study,deformation twinning was observed in tensile tests and twinning appears to have an adverse impact on ductility.The profuse twinning in the as-cast Zn-Mg alloys accelerated crack growth in tension due to twinning impingement which caused local stress concentrations and initiates cracking.As-rolled Zn-Mg alloys have better ductility than their as-cast counterparts due to the inhibition of twinning by the refined Mg2Zn11 intermetallic phase and the finer grain size.

Microstructure Characteristics of an Fe-Mn-C TWIP Steel After Deformation

The microstructure characteristics of an Fe-Mn-C TWIP steel after deformation are investigated. The results show that the hot-rolled, cold-rolled and then annealed sample of the Fe-Mn-C TWIP steel has excellent mechanical properties, and the true stress-true strain curve from tension tests exhibits repeated serrations. The deformed microstructure exhibits the typical planar glide characteristics such as no cell formation, dislocation pile-ups on a single slip plane, mechanical twins and stacking faults. There are equiaxial and deep dimple structures in the fractograph, indicative of a ductile fracture. Microcracks initiate from inclusions and twin-twin intersections. Deformation and fracture processes are the formation, growth and coalescence of microvoids.

Effect of the Deformation Twin Density on the Corrosion Behavior in TWIP Steel

Based on its excellent tensile strength-ductility property combination,twinning-induced plasticity (TWIP) steel shows great potential in applications for structural components in automobile industry.The aim of this research is to investigate the corrosion resistance properties and corrosion mechanism under room temperature in TWIP steel.The influence of the deformation twin density on corrosion property was primarily considered by salt spray test.The specimens used in the investigation are as-annealed and as-deformed respectively.The microstructure and corrosion resistance property were characterized by scanning electron microscope (SEM),optical microscope (OM) and so on.There are some annealing twins distributed randomly in austenitic grains in the as-annealed specimen.After the specimen was subjected to tensile experiment,the density of the deformation twins increased sharply,which are different from the annealing twins in size and morphology.It was found that the corrosion potential of the as-annealed is lower than that of the as-deformed and the corrosion current density behaves contrarily.After immersed in 5% NaCl solution salt spray for 48h,the as-deformed showed a bit better than the as-annealed in corrosion resistance.With the time prolonged,the gap between the two specimens in corrosion resistance increased rapidly.The corrosion morphologies varied in color and shape.Further investigation,carried out by SEM and EDS,indicated that as-annealed and the as-deformed followed pitting corrosion and uniform corrosion mechanism respectively.The reason for the difference in corrosion mechanism is possibly the presence of the deformation twins.The deformation twins formed during the tensile test refine grains by way of segmentation.The twin boundaries largely belong to the coincidence site lattice (CSL),which is on lower energy state.It suggests that the twins not only play a role in strengthening,but also improve effectively the corrosion resistance in TWIP steel.

In Situ TEM Observation on Martensitic Transformation during Tensile Deformation of SUS304 Metastable Austenitic Stainless Steel

Through in situ transmission electron microscopy observation on SUS304 metastable austenitic stainless steel during stretching at room temperature,it is found that e martensite plates were induced preferentially from the sites of dislocation pile-ups.With increasing deformation,some of ε thin martensite platelets disappear and reversibly transform toγ austenite without heating treatment,which is different from the previous result that ε martensite can entirely transform toα＇martensite.Then,some of deformation twins appear and grow along the vertical direction of ε martensite due to（111）_γ⊥（1010）_ε.Moreover,it is directly observed that multiple transformation mechanisms via γ→ε→γ,γ→ε,γ→α′,γ→ε→α′,γ→ deformation twins →α′ can co-exist.

Effect of Pre-cold Rolling-Induced Twins and Subsequent Precipitated x-Phase on Mechanical Properties in a β-Type Ti–Mo Alloy

This paper reported an effectiveness of pre-cold rolling-induced{332}／113[twins combined with subsequent isothermal x-phase formation for enhancement of uniform elongation in a b-type Ti–15Mo alloy with high yield strength level.Mechanical{332}／113[twins were induced by cold rolling with an thickness reduction of 5%,which had little effect on x-phase precipitation after aging at 573 K for 3.6 ks.Twinning after the cold rolling was further activated during tensile deformation,even with the presence of isothermal x-phase.This combination of twins and x-phase enhanced uniform elongation from 0 to 9%at yield strength level of 890 MPa.The high yield strength was mainly dominated by dislocation slip due to the isothermal x-phase formation,and early onset of plastic instability after yielding was hindered due to the pre-cold rolling-induced twins.Dynamic microstructural refinement was induced by further twinning activation during deformation,which resulted in high work hardening rate corresponding enhancement of uniform elongation.

Deformation Behavior of Fe-36Ni Steel during Cryogenic( 123-173 K) Rolling

Microstructural evolution and mechanical properties of cryogenic rolled Fe-36Ni steel were investigated. The annealed Fe-36Ni steel was rolled at cryogenic temperature（ 123-173 K） with 20%- 90% rolling reduction in thickness.The deformation process was accompanied by twinning at cryogenic temperature,and the mean thickness of deformation twins was about 200 nm with 20% rolling reduction. When the rolling reduction was above 40%,twinning was suppressed due to the stress concentration in the tested steel. Deformation microstructure of Fe-36Ni steel consisted of both twin boundaries and dislocations by cryogenic rolling（ CR）,while it only contained dislocations after rolling at room temperature（ RT）. The tensile strength of Fe-36Ni steel was improved to 930 MPa after 90% reduction at cryogenic temperature,while the tensile strength after 90% reduction at RT was only 760 MPa. More dislocations could be produced as the nucleation sites of recrystallization during CR process.

Probing deformation mechanisms of gradient nanostructured CrCoNi medium entropy alloy

The gradient nanostructured medium entropy alloys(MEAs) exhibit a good yielding strength and great plasticity. Here, the mechanical properties, microstructure, and strain gradient in the gradient nanostructured MEA CrCoNi are studied by atomic simulations. The strong gradient stress and strain always occur in the deformed gradient nanograined MEA CrCoNi. The origin of improving strength is attributed to the formation of the 9 R phase, deformation twinning, as well as the fcc to hcp phase transformation, which prevent strain localization. A microstructure-based predictive model reveals that the lattice distortion dependent solid-solution strengthening and grain-boundary strengthening dominate the yield strength,and the dislocation strengthening governs the strain hardening. The present result provides a fundamental understanding of the gradient nanograined structure and plastic deformation in the gradient nanograined MEA, which gives insights for the design of MEAs with higher strengths.