Revealing the role of pyramidal slip in the high ductility of Mg-Li alloy

The high ductility of Mg-Li alloy has been mainly ascribed to a high activity of pyramidal<c+a>slip to accommodate plastic strain.In the present study,however,a quantitative analysis reveals that Li-addition can only slightly stimulate the activation of pyramidal<c+a>slip under compression along the normal direction of a hot-rolled Mg-4.5 wt.%Li plate,with a relative activity of approximately 18%.Although the limited activity of pyramidal<c+a>slip alone cannot accommodate a large plastic strain,it effectively reduces the number of{10.11}−{10.12}double twins,which are believed to be favorable sites for crack initiation.The evidently reduced activity of double twins leads to a lower cracking tendency,and therefore improves ductility.

Mg-3Al-1Zn alloy deformed along different strain paths: Role of latent hardening

The mechanical properties of magnesium alloy AZ31 were investigated experimentally with visco-plastic self-consistent modeling. Tension,compression and plane strain compression(PSC) tests were performed along 3 directions of a hot rolled plate, and the material parameters input in the model were fitted with the uniaxial stress-strain curves. The critical resolved shear stress(CRSS) for tension twinning was modeled with a modified Voce hardening law first decreasing, and then increasing with strain, that could reproduce better the flow stress for twin-predominant deformation. Such CRSS evolution may better model twin nucleation, propagation and growth. Firstly simulations were carried out assuming latent hardening coefficients for slip by other slip systems equal to self-hardening. Then different heterogeneous latent hardening were used, whose values were based on dislocation dynamics simulations from the literature. This study shows that equal self and latent hardening can reproduce the stress strain curves and plastic anisotropy as well as heterogeneous mode on mode latent hardening.Discrepancies between simulations and experimental results from PSC are explained by an under-estimation of twinning for some PSC strain paths.

Thermal stability of different texture components in extruded Mg-3Al-1Zn alloy

Grain growth can modify the texture orientation and the fraction of different texture component.The thermal stability of two texture component in an extruded magnesium AZ31 alloy was investigated.Three types samples with different texture distribution were prepared.The results show that normal grain growth takes place in the magnesium AZ31 alloy during annealing at 300℃ and 450℃.But the grain growth does not lead to the strengthening of either texture component.Both the<0002>⊥ED texture and<0002>//ED texture components show good thermal stability,without influence of the texture volume fraction.The two different texture component possess comparable boundary migration ability,so grains of the two texture component consume indifferently the other grains,or are equally consumed during annealing.

Deformation mechanisms and differential work hardening behavior of AZ31 magnesium alloy during biaxial deformation

Magnesium alloys are frequently subjected to biaxial stress during manufacturing process,however,the work hardening behavior under such circumstance are not well understood.In this study,the deformation mechanisms and differential work hardening behavior of rolled AZ31 magnesium alloy sheets under biaxial loading are investigated.The change of plastic work contours with increasing plastic strain indicates the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state,resulting in higher macroscopic work hardening rates of biaxial loading than uniaxial loading,with the elastic-plastic transition part of work hardening extended and stage Ⅲ hardly emerged.Electron backscatter diffraction and Schmid factor analysis confirm the low activation of non-basalslip during biaxial loading tests.While the thickness strain is primarily accommodated by pyramidal<c+a>slip at the initial stage of biaxial deformation,{10–11}contraction twinning is activated at larger plastic strain.The low activation of non-basalslip also retards the dynamic recovery and cross-slip of basal and prismaticslips,leading to the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state.

Quantitative study on the tension-compression yield asymmetry of a Mg-3Al-1Zn alloy with bimodal texture components

This study demonstrates the yield asymmetry in Mg-3Al-1Zn alloy containing both ND-texture(c-axis//ND(Normal direction))and TD-texture(c-axis//TD(Transverse direction))in a quantitative view.The results showed that the yield asymmetry is strongly dependent on the distribution of bimodal texture components,on the basis of the successful establishment of the quantified relationship between pre-deformation parameters and texture components distribution.It’s meaningful for providing key reference to texture design.Mechanical behavior of bimodal textured Mg alloy under tension and compression was tested.CYS/TYS(compressive yield stress/tensile yield stress)equal to 1 is obtained,implying that the yield asymmetry is eliminated when two textures distribute at specific fractions.The corresponding mechanism for the texture-dependence of tension-compression yield asymmetry is revealed by the analysis of slip/twinning activities and a compound use of the activation stress difference of slip/twinning(ΔStress)and geometrical compatibility factor(m′)between neighboring grains.Balanced activity of{10■2}twinning and a quite similar boundary obstacle effect against slip/twinning transfer under tension and compression accounts for such good symmetry performance.

On the rule of mixtures for bimetal composites without bonding

In the present study,three types of bimetal composites,Al 6082 sleeve/Al 6082 core,Mg AZ31 sleeve/Mg AZ31 core and Al 6082 sleeve/Mg AZ31 core,were fabricated by drilling and assembling.The rule of mixtures(ROM)for the flow curves and yield strengths during compressive test were addressed.Our results show that the ROM can predict well the experimental flow curves and yield strengths of bimetal composites without bonding,irrespectively of the different strain hardening behavior between the two components.

Microstructure and texture in an extruded Mg-Al-Ca-Mn flat-oval tube

In the present study,mechanical property,microstructure and texture of an extruded Mg–1.0Al–1.5Ca–1.0Mn(wt.%)flat-oval tube in both the flat and the oval regions were systematically studied.Our results show that there exists a great difference in microstructure and texture between the flat and the oval regions.Both the flat and the oval regions have fine DRXed grains of 3–4μm,while more row stacked grains are observed in the flat region.A large number of fine particles with a size of about 0.5μm are dispersed in both the grain boundary and grain interior.The particles exhibit a uniform distribution in the oval region,while they tend to aggregate into bands along the extrusion direction(ED)in the flat region.The texture in the oval region is similar to that in extruded Mg rods with a preferred distribution of prismatic planes and random distribution of(0002)poles around the ED,while that in the flat region is close to that in extruded sheets containing two texture components,<0002>//normal direction(ND)and<0002>//transverse direction(TD).A high fraction of<0002>//TD component,about 52–62%,is observed in the flat region.The flat region has a tensile yield strength of 254 MPa,an ultimate tensile strength of 290 MPa and an elongation to failure of 4.3%.

Refining 18R-LPSO phase into sub-micron range by pre-kinking design and its prominent strengthening effect on Mg_(97)Y_(2)Zn_(1)alloy

In this study,a composite deformation strategy of pre-kinking(equal channel angular pressing(ECAP))followed by large-ratio hot extrusion(HE)was designed to refine the 18R long period stacking ordered(LPSO)phase into sub-micron range in a Mg_(97)Y_(2)Zn_(1)(at.%)alloy.After the composite processing,the mechanical properties of the alloy are significantly enhanced,superior to the majority of reported Mg_(97)Y_(2)Zn_(1) and other LPSO-containing Mg alloys.Among the composite deformed alloys,the 16P-HE alloy exhibits the best mechanical properties with tensile yield strength(TYS)of 475 MPa,ultimate tensile strength(UTS)of 526 MPa,and fracture elongation(FE)of 14.5%.Quantitative analysis of 18R phase indicates that increasing ECAP pass from 1 to 16 gradually decreases the average size of 18R phase from 5.1μm to 2.3μm.After HE,the 18R phase is further refined with a corresponding decrease in the average size in the descending order of 1P-HE(4.3μm),4P-HE(3.2μm),and 16P-HE(1.4μm)alloys.Calculation of the strengthening contributions confirms that the superior mechanical properties of 16P-HE alloy are mainly due to its strongest interface strengthening(145 MPa)and grain boundary strengthening(189 MPa)from the sub-micron 18R phase andα-Mg grains.Moreover,the strengthening effect of 18R phase decreases gradually with their morphology changing from particles to fibers,and to blocks.The obtained results further deepen and broaden the strengthening-toughening theory of 18R phase.

Detwinning and Anneal-Hardening Behaviors of Pre-Twinned AZ31 Alloys under Cryogenic Loading

Detwinning behavior of a pre-twinned magnesium alloy AZ31 at cryogenic temperature was investigated,also with a focus on the annealing hardening behavior of samples with different fractions of pre-twins.Pre-compression along the transverse direction with strains of 1.7%,3.0%,and 6.0% was applied to generated[1012]twins.Mechanical behavior,microstructure,and texture evolution during subsequent tension were examined.Our results show that low temperature did not change the fact that detwinning still pre-dominated in the pre-twinned samples under reverse loading.However,a relatively harder migration of twin boundaries was found at cryogenic temperature.An annealing hardening of 27-40 MPa was observed in the pre-twinned samples,and such a hardening effect shows a close relation with the fraction of pre-twins or the level of pre-strains.The annealing hardening effect disappeared if the matrix was consumed by twins along with the increased pre-compression strains.The corresponding reasons for the annealing hardening behavior were discussed.

Understanding the Mechanism for the In-Plane Yielding Anisotropy of a Hot-Rolled Zirconium Plate

Previously,the in-plane mechanical anisotropy of Zr hot-rolled plates is ascribed mainly to the different activities of the deformation modes activated when loading along different directions.In this work,a quantitative study on the deformation behavior of a pure Zr hot-rolled plate under tension along the rolling direction(RD)and transverse direction(TD)reveals that both the activities of deformation modes and the anisotropy of grain boundary strengthening account for a tensile yield strength anisotropy along the TD and RD.Crystal plasticity simulations using viso-plastic self-consistent model show that prismatic slip is the predominant deformation mode for tension along the RD(RD-tension),while prismatic slip and basal slip are co-dominant deformation modes under tension along the TD(TD-tension).A low fraction of■under TD-tension,while hardly activated under RD-tension.The activation of basal slip with a much higher critical resolve shear stress under TD-tension contributes to a higher yield strength along the TD than along the RD.The grain boundary strengthening effect under tension along the TD and RD were compared by calculating the activation stress difference(△Stress)and the geometric compatibility factor(m′)between neighboring grains.The results indicate a higher grain boundary strengthening for TD-tension than that for RD-tension,which will lead to a higher yield strength along the TD.That is,the anisotropy of grain boundary strengthening between TD-tension and RD-tension also plays an important role in the in-plane anisotropy along the RD and TD.Afterward,the reasons for why there is a grain-boundary-strengthening anisotropy along the TD and RD were discussed.

On the modeling of deformation mechanisms in a Mg-3Al-1Zn alloy under biaxial tension

Although the{10-12}twinning behavior of Mg alloys under uniaxial tension and compression has been extensively investigated,the simulations of{10-12}twinning behavior under biaxial tension have rarely been reported.In this work,the EVPSC-TDT model is first employed to systematically investigate the deformation behavior of a Mg alloy AZ31 plate under biaxial tension in the RD-TD and ND-TD planes.The RD,TD and ND refer to the rolling direction,transverse direction,and normal direction of the hot rolled plate.The measured stress-strain curves and texture evolutions are well predicted and the con-tours of plastic work under biaxial tension are also constructed for comparison with experiments.The plastic response has been interpreted in terms of relative activities of various deformation modes.For bi-axial tension in the RD-TD plane,basal and pyramidal slips mainly contribute to the plastic deformation for stress ratios ofσRD:σTD=1:2 to 2:1.Prismatic slip becomes more active forσRD:σTD=1:4 and 4:1.Compression twinning could be activated and so cause texture reorientation at large strains,especially forσRD:σTD=1:1.The six-fold feature of{10-10}pole figure could still be observed forσRD:σTD=1:4 and 4:1 at large strain.For biaxial tension in the ND-TD plane,tensile twinning plays an important role forσND:σTD≥1:2,while prismatic slip contributes to plastic deformation for the other cases.With the in-crease of stress ratio fromσND:σTD≥1:1 to 1:0,the predicted twin volume fractions(VFs)at a specific strain along the ND,εND,almost linearly decrease,however,it is seen that the experimental ones at given strains along the ND do not follow such a trend with the measured twin VFs within the range of stress ratios,2:1≤σND:σTD≤6:1,clearly being overestimated,and the difference between experiments and simulations becomes most obvious at the relatively small strain ofεND=0.015.The possible reasons for the observed difference are discussed.

A weak texture dependence of Hall–Petch relation in a rare-earth containing magnesium alloy

Hall–Petch slope(k),an important parameter in Hall–Petch relation,describes the efficiency of strengthening effect by grain boundaries.Previously,a highly texture dependent k for Mg alloys is frequently reported,but,in the present study,we report a weak texture dependence of k in a rare-earth containing Mg-2Zn-1Gd plate with two peaks of(0002)poles inclining approximately±30°away from the ND toward the TD.Although there is a strong mechanical anisotropy between tension along the TD and RD,the k for TD-tension(280 MPaμm^(1/2))is quite similar to that for RD-tension(276 MPaμm^(1/2)).Here,RD,TD and ND refer to the rolling direction,transverse direction and normal direction of the plate,respectively.The weak texture dependence of k is well predicted by the compound use of the activation stress difference between neighboring grains(ΔStress)and the geometric compatibility factor(m').By analyzing how the texture affects the values for ΔStress and m,the mechanism for this texture independence of k is ascribed to the activation of a high fraction of additional deformation mode,besides the predominant one for both RD-tension and TD-tension,namely,prismatic slip accompanied by a high fraction of basal slip for RD-tension and basal slip accompanied by a high fraction of prismatic slip for TD-tension.This will lead to multiple deformation transfer modes and,consequently,the effect of texture on the ease of deformation transfer across grain boundaries is weakened.As a result,there is a similar k for TD-tension and RD-tension.

Hall-Petch relationship in Mg alloys: A review

Grain refinement could effectively enhance yield strength of Mg alloys according to the well-known Hall-Petch theory. For decades, many studies have been devoted to the factors influencing the Hall- Petch slope （k） in Mg alloys. Understanding the factors influencing k and their mechanisms could offer guidance to design and produce high-strength Mg alloys through effective grain refinement hardening. A review and comments of the past work on the factors influencing k in Mg alloys are presented. Results of these previous investigations demonstrate that the value of k in Mg alloys varies with texture, grain size, temperature and stain. The influence of texture and grain size on k is found to be an essential result of the variation of deformation mode on k value. Without the variation of deformation modes, it is revealed that texture could also impose a significant effect on k and this is also summarized and discussed in this paper. The reason for texture effect on k is analyzed based on the mechanism of Hall-Petch relationship. In addition, it is found in face-centered cubic （fcc） or body-centered cubic （bcc） metals that boundary parameters （boundary coherence, boundary energy and boundary diffusivity） could strengthen twinning or slips to a different extent. Therefore, the role of boundary parameters is also extended into the k values in Mg alloys and discussed in this paper. In the end, we discuss the future research perspective of Hall-Petch relationship in Mg alloys.

Microstructure and texture evolution of the β-Mg17A12 phase in a Mg alloy with an ultra-high Al content

In the present study,the effects of equal channel angular pressing(ECAP)on the microstructure and mechanical property of the Mg-20Al alloy were systematically investigated.For the first time,the texture of Mg17Al12 phase and its evolution with ECAP conditions were reported.The results show that increasing the processing temperature and passes generates more uniform distribution and finer size ofβ-Mg17Al12 phases.The large pieces ofβ-Mg17A12 phases are composed of many fine grains with different crystallographic orientations.For theβ-Mg17A12 phase,a preferred distribution of(001)appears at 523 K and 573 K,and hardly varies with temperature.Nevertheless,a random texture is observed at 623 K.The(0002)poles exhibit a preferred distribution at 473 K,but this preferred distribution varies with temperature.A random distribution of(0002)poles is observed when processed at 623 K.Many types of crystallographic planar relationship betweenβ-Mg17A12 phase andα-Mg matrix are observed and the relationships of{11-23}//{100}or{110}or{111}and{1-211}//{100}or//{110}or{111}have a relatively higher frequency than others.The texture ofα-Mg matrix is much different from that of the ECAPed Mg alloys with a relative low Al content,in which a texture with basal poles inclining approximately 45°away from the extrusion direction often develops.The mechanical properties of Mg-20Al alloy are closely related to the temperature and passes of ECAP.A higher temperature often decreases the yield strength,but hardly alters the maximum strength.There is a low plasticity for all the samples and increasing processing temperature slightly enhances the plasticity.The corresponding mechanisms were deeply discussed.

On the texture memory effect of a cross-rolled Mg-2Zn-2Gd plate after unidirectional rolling

A Mg-2Zn-2Gd alloy subjected to a cross rolling and annealing often has a texture with c-axes of grains evenly distributing on a circle approximately 40°away from the normal direction(ND),which can completely remove the planar mechanical anisotropy.The texture memory effect and planar mechanical anisotropy of such a cross-rolled plate after an unidirectional rolling and subsequent annealing were systematically investigated.The results show that the circle-shaped texture is partially retained after the unidirectional rolling and annealing,with basal poles evenly distributing on an ellipse approximately 30°–40°away from the ND.This small texture difference will lead to quite different mechanical anisotropies between the two plates,namely,a quite low anisotropy of yield strength between the rolling direction(RD)and transverse direction(TD)before the unidirectional rolling,in contrast to an obvious mechanical anisotropy after unidirectional rolling.A quantitative calculation shows that tension along the RD of the plate after the unidirectional rolling will activate a much higher fraction of prismatic slip than tension along the TD.The much higher critical resolved shear stress for prismatic slip than that for basal slip or{10■2}twinning accounts for the much higher yield strength of TD tension than that of RDtension.The mechanisms for texture evolution during the unidirectional rolling and annealing were also discussed.

Achieving ultra-high strength using densely ultra-fine LPSO phase

Long period stacking ordered(LPSO) structures are an effective strengthening phase in Mg alloys.However,the coarse LPSO phases in as-cast alloys are very difficult to refine,and they dramatically reduce the strengthening effect.In the present study,friction stir processing(FSP) was employed to refine the structure of an Mg-12.8 Y-4.7 Zn(wt%) alloy with a very high content of coarse LPSO phases.An optimized FSP regime refined the coarse LPSO phases into densely ultrafine blocks with an average width of ~200 nm and an average length of~1 μm.An ultrahigh yield strength of ~800 MPa was achieved under compression of the FSP region.Theoretical calculations indicated that the strengthening by the densely ultrafine LPSO phases was up to approximately 630 MPa.

Tailoring the microstructure and mechanical properties of the final Al-Mn foils by different intermediate annealing process

In this work, two different intermediate annealing processes, single-step annealing（SSA, 530?C/15 h）and two-step annealing（TSA, 450?C/5 h + 530?C/15 h）, were used to tailor microstructure before coldrolling and annealing of the final twin-roll cast Al-Mn foils. The recrystallization behavior and mechanical properties during annealing of severely cold-rolled foils were systematically studied. Our results show that discontinuous recrystallization occurs in SSA-foils during annealing at 150–310?C, in contrast with continuous recrystallization in TSA-foils. The continuous recrystallization develops much finer grains（;.35 μm） in the TSA-foils than those by discontinuous recrystallization in the SSA-foils（;4.7 μm）. The texture components in cold-rolled TSA-foils hardly change（retained rolling-textures） after continuous recrystallization, while those in the cold-rolled SSA-foils mainly transform into a strong cube component{001} <100> after discontinuous recrystallization. Finally, a maximized elongation to fracture of;3.9%was achieved in the TSA-foil, much higher than that of the SSA-counterparts,;.3%. The relationships between the microstructure and mechanical properties were discussed.

Deformation mechanism of commercially pure titanium under biaxial loading at ambient and elevated temperatures

Commercially pure(CP)titanium is thermally processed and subjected to biaxial stress.However,the evolution of the microstructural deformation mechanisms under such circumstances is not adequately understood.In this study,the mechanical responses and microstructural deformation mechanisms of TA2 CP titanium sheets under equi-biaxial loading at room temperature(RT),300℃,and 400℃were studied.The activated slip and twinning systems were investigated by transmission electron microscopy(TEM)after polished cruciform specimens were biaxially tensile-tested at RT and elevated temperatures.The results show that{11¯22}contraction twinning and{10¯12}extension twinning are the main deforma-tion mechanisms of RT biaxial deformation,while dislocation glide is predominant in biaxial deformation at 300℃and 400℃.This difference yields varied work-hardening behaviors at RT and elevated tem-peratures.In biaxial deformation at 400℃,the main slip trace type is multiple slip.The interaction of different slip systems in multiple slip created shear deformation concentration areas and further induced cross-slip.However,in biaxial deformation at 300℃,the amounts of simplex and multiple slip were significantly reduced compared to those at 400℃because the lower temperature increased the critical resolved shear stress and insufficient activated slip systems were available for grain deformation.There-fore,several stress-concentration areas were generated with the activation of cross-slip.

Effect of Twin Boundary–Dislocation–Solute Interaction on Detwinning in a Mg–3Al–1Zn Alloy

In the present study,the influence of solute atoms together with dislocations at {101^-2} twin boundary（TB） on mechanical behavior of a detwinning predominant deformation in a Mg alloy AZ31 plate was systematically studied.The results show that a large number of {101^-2} twins disappear during recompression along the normal direction.Both the TB-dislocation interaction and TB-solute-dislocation interaction can greatly enhance the yield stress of the recompression along the normal direction（ND）.However,the solute segregation at {1012} TBs with an intensive interaction with 〈a〉 dislocations cannot further enhance the yield stress of ND recompression.The samples with TB-dislocation interaction show a similar working hardening performance with that subjected to a TB-solute-dislocation interaction.Both the TB-dislocation interaction and TB-solute-dislocation interaction greatly reduce the value of work hardening peaks during a detwinning predominant deformation.

A Quantitative Study on the Effect of Heat Treatment on the Microstructure and Orientation of Titanium Hydride in Electrochemically Hydrogenated Pure Titanium

The electrochemical hydrogen charging of pure titanium and its alloys has been investigated previously,while how a subsequent annealing treatment aff ects the type of hydride and its orientation relationship with matrix is not clear.In the present study,a quantitative study on the microstructure and orientation of titanium hydrides during electrochemical hydrogen charging and subsequent annealing treatment was carried out using scanning electron microscopy,transmission electron microscopy and electron backscatter diff raction.The results show thatδ-hydride is the main in both the electrochemically hydrogenated sample and the subsequent annealing treated sample.After electrochemical hydrogen charging for 48 h,the surface is mainly composed of denseδ-hydride with a thickness of approximately 42μm,the orientation relationship betweenα-matrix andδ-hydride follows only the orientation relationship of OR2,{0001}_(α)//{111}_(δ),<1210>_(α)//<110>_(δ) and an interface plane{1013}_(a)//{110}_(δ).Besides OR2,a part of hydrides show an orientation relationship of OR1 with the matrix after annealing,{0001}a//{001}_(δ),<1210>_(a)//{110}_(δ) and an interface plane of {1010}_(δ)//{110}_(δ).It is further found that the relative frequency of OR1and OR2 is closely related to annealing duration.Under an argon atmosphere at 450℃,the frequencies of OR1 and OR2 are nearly balance with an annealing time of 12 h,while OR1 becomes to be the predominant one with a relative frequency of 96.5%after annealing for 96 h.The mechanism for the evolution of orientation relationship of hydrides with annealing time was discussed.