Effect of pre-induced twinning on microstructure and tensile ductility in GW92K magnesium alloy during multi-direction forging at decreasing temperature

Effect of pre-induced twinning on the microstructure evolution and mechanical properties of extruded Mg-9.26Gd-2.08Y-0.36Zr(GW92K)alloy have been investigated during multi-direction forging with large strains at decreasing temperature from 400℃ to 300℃.The results showed that,whether there pre-induced twinning existing in the initial microstructure by pre-deformation or not,a mixed microstructure of residual coarse grains and notably refined grains formed under both conditions,combing with some residual coarse grains with less deformation inside grains and lots of dispersed nano-precipitates distributed along refined grain boundaries.However,a significant improvement with the tensile ductility was obtained by the pre-induced twinning in the former alloy.It was suggested that,the pre-induced twinning largely contributed to the grain refinement and lead to an increase in the ratio of fine grain structure which would be responsible for the better properties.Furthermore,during subsequent forging deformation in the pre-deformed sample,the grain refinement mechanism by gradual grain orientation rotation around the surface section in residual coarse-grains was a little different from that by the macro-shear deformation in the as-extruded condition.

Effect of heat treatment on the microstructures and mechanical properties of the sand-cast Mg-2.7Nd-0.6Zn-0.5Zr alloy

The tensile testing bars of the Mg-2.7Nd-0.6Zn-0.5Zr(wt.%)alloy were prepared by sand casting.The effect of solution temperature and aging time on the microstructures and mechanical properties were investigated.The as-cast alloy was composed ofαmagnesium matrix and Mg12Nd eutectic compounds.After solution treatment at 500℃ for 18 h,the volume fraction of eutectic compounds decreased from∼7.8%to∼2.3%,and some small Zr-containing particles were observed to precipitate at grain interiors.As the solution temperature increased to 525℃ for 14 h,most of the eutectic compounds dissolved into the matrix.Peak-aged at 200℃ for 12 h,fineβ″particles was the dominant strengthening phase.The yield strength,ultimate tensile strength and elongation in the peak-aged condition were 191 MPa,258 MPa and 4.2%,respectively.Moreover,the Mg-2.7Nd-0.6Zn-0.5Zr alloys under different heat treatment conditions exhibited different tensile fracture modes.

The preferential growth and related textural evolution during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The grain growth process plays an important role in the texture formation in magnesium alloys.The microstructural and micro-textural evolution of a cold-rolled Mg-Zn-Gd alloy during annealing at 350℃for 60-190 min were tracked by quasi-in-situ electron backscatter diffraction method.The results show that grain growth takes place gradually with the annealing time increasing.Moreover,the TD-split texture maintains the texture type but alters in three aspects-the increased tilting angle,the decreased pole intensity and the widened distribution of high-intensity area.Grains with their c-axis tilting 45-70°from normal direction show preferential growth which is closely associated with the texture changes.The original grain size advantage is one of the important factors leading to the growth advantage,some grain boundaries,such as 50-60°[1^(-)21^(-)0],50-60°[2750],60-70°[1^(-)21^(-)0](18b),and 70-80°[1^(-)01^(-)0](10)are also considered to be related to this preferential growth.

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor(Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107 cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to−1 may also result in a bad fatigue property.The flux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the flux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

Ductility enhancement of EW75 alloy by multi-directional forging

In this study,the Mg-7Gd-5Y-1Nd-0.5Zr alloy can reach a high ductility by the process of multi-directional forging,and the evolution of the microstructure,texture and the mechanical properties were discussed systematically.The results show that after the solutionized sample was multi-forged at 500℃,its grain size can be refined from 292 um to 58 um.As the forging temperature decreased,fine particles precipitated in the matrix.The volume fraction of the particles increased with the forging temperature decreasing,so the nucleation and growth of crystallization were strongly restricted.There was no recrystallization as the forging temperature fell to 410℃,and the severe deformed grains distributed as streamlines perpendicular to the final compression axis.The texture intensity decreased with increasing forging passes.The sample with best ductility was obtained after compressed at 470℃,with an elongation to failure of 21%at room temperature,which is increased by 200%,in comparison with that of the samples in solutionized condition.EBSD results revealed that the mean grain size was 15 um.Refined grains as well as the weakened texture were the key factors to its high ductility.

Quasi-in-situ observing the rare earth texture evolution in an extruded Mg-Zn-Gd alloy with bimodal microstructure

The static recrystallization and associated texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure based on a quasi-in-situ electron back-scatter diffraction(EBSD)method.The typical rare earth(RE)texture formed during annealing,evolving from the bimodal microstructure with[1010]basal fiber texture that consisted of fine recrystallized(RXed)grains and coarse unrecrystallized(un RXed)grains elongated along the extrusion direction.In both RXed and un RXed regions,the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation had more intensive growth ability than those with basal fiber orientation,thereby leading to the preferred selection of RE texture orientation during grain growth.The relationships between stored strain energy,solute drag,grain growth and texture evolution are discussed in detail.This study provided direct evidence of the RE texture evolution in an extruded Mg-RE alloy,which assists in understanding the formation mechanisms for RE texture during extrusion and better developing wrought Mg alloys with improved formability.

Microstructural evolution of Mg-7Al-2Sn Mg alloy during multi-directional impact forging

Multi-directional impact forging(MDIF)was applied to a Mg-7Al-2Sn(wt.%)Mg alloy to investigate its effect on the microstructural evolution.MDIF process exhibited high grain refinement efficiency.After MDIF 200 passes,the grain size drastically decreased to 20µm from the initial coarse grains of~500µm due to dynamic recrystallization(DRX).Meanwhile,original grain boundaries remained during MDIF and large numbers of fine sphericalβ-Mg_(17)Al_(12) particles dynamically precipitated along the original grain boundaries with high Al concentration,acting as effective pinning obstacles for the suppression of DRXed grain growth.Besides,micro-cracks nucleated during MDIF and propagated along the interface between the remained globular or cubic Al-Mn particles and Mg matrix.

Effect of Solution Treatment on the Microstructure and Mechanical Properties of Sand-Cast Mg–9Gd–4Y–0.5Zr Alloy

Sand-cast Mg–9Gd–4Y–0.5 Zr（wt%） alloy was solution-treated at 500–565 ℃ in the time range of 0.5–30 h in air or vacuum to investigate its microstructure evolution and mechanical properties. The results showed that solution treatment temperature had a significant influence on the dissolving rate of eutectic phase and grain growth. Taken both of them into consideration, 510–520 ℃ was considered to be the optimum solution treatment temperature range for this alloy.It should be noted that the trace（0.4–0.9 vol%） and insoluble cuboid-shaped phase precipitated during solution treatment was identified to be YH2, of which the hydrogen was thought to come from both the melting and solution heating process.In addition, the 3D morphology and dissolving process of Mg24（Gd,Y）5 eutectic phases in the as-cast alloy were also discussed via in-situ observation under X-ray tomography.

Effects of Zinc and Calcium Concentration on the Microstructure and Mechanical Properties of Hot-Rolled Mg–Zn–Ca Sheets

Four kinds of Mg alloys with different Zn and Ca concentration were selected to analyze the effect of Zn and Ca concentration on the microstructure and the mechanical properties of Mg-Zn-Ca alloys. It was found that Zn and Ca concentration has a great influence on the volume fraction, the morphology and the size of second phase. The Mg-1.95Zn- 0.75Ca （wt%） alloy with the highest volume fraction, continuous network and largest size of Ca2Mg6Zn3 phase showed the lowest elongation to failure of about 7%, while the Mg-0.73Zn-0.12Ca （wt%） alloy with the lowest volume fraction and smallest size of Ca2Mg6Zn3 phase showed the highest elongation to failure of about 37%. It was suggested that uniform elongations of the Mg-Zn-Ca alloys were sensitive to the volume fraction of the Ca2Mg6Zn3 phases, especially the network Ca2Mg6Zn3 phases; post-uniform elongations were dependent on the size of the Ca2Mg6Zn3 phase, especially the size of network Ca2Mg6Zn3 phase. Reduction in Zn and Ca concentration was an effective way to improve the room- temperature ductility of weak textured Mg-Zn-Ca alloys.

Creep Behavior and Microstructure Evolution of Sand–Cast Mg–4Y–2.3Nd–1Gd–0.6Zr Alloy Crept at 523–573 K

High temperature tensile-creep behavior of Mg-4Y-2.3Nd-IGd-O.6Zr （wt%, WE43（T6）） alloy at 523- 573 K was investigated. The creep stress exponent is equal to 4.6, suggesting the underlying dislocation creep mechanism. The activation energy is （199 _＋ 23） kJ/mol, which is higher than that for self- diffusion in Mg and is believed to be associated with precipitates coarsening or cross slip. The creep mechanism is further suggested to be dislocation climb at 523 K, while a cross slip at 573 K is possible. The metastable 13＇ and ~]1 phases in the WE43（T6） alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb, which contributed to its excellent creep resistance. However, at 573 K it readily transforms into equilibrium/3e phase and coarsens within two hours, thereby causing a decrease of creep resistance. In addition, precipitate free zones approximately normal to applied stress direction （directional PFZs） developed during the creep deformation, especially at 573 K. Those zones became preferential sites to nucleate, extend and connect microcracks and cavities, which lead to the intergranular creep fracture. Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.

Dynamic Recrystallization and Texture Evolution of GW94 Mg Alloy During Multi-and Unidirectional Impact Forging

Multi- and unidirectional impact forgings were successfully applied to a （GW94） Mg-RE alloy. The microstructure and texture evolution were investigated systematically. The obtained results indicated that during unidirectional impact forging, a bimodal chain deform microstructure was sustained till last forging pass, whereas { 10-12} extension twins-assisted continuous dynamic recrystallization took place during the multidirectional impact forging （MDIF）. The coalescence and intersection of {10-12} extension twins during MDIF efficiently refined the original coarse grains and led to an almost recrystallized homogeneous microstructure. The texture analysis demonstrated that unidirectional impact forging yielded out the strong basal texture; however, MDIF resulted in non-basal texture, which was attributed to the cooperative effects of continuous DRX, twinning, and MDIF itself during the deformation process.

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.

Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy

Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect, was studied in Mg-3Nd-Zn alloy by tensile test at 250℃ with a strain rate of l x 10^-3 s^-1 Microstructure and tensile property of the Mg-3Nd-Zn alloy in solution and aging conditions were also investigated. Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei.The results show that orientation distribution of nuclei is associated with nucleation mechanism.The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation.Differently,discontinuous static recrystallization nuclei formed within the parent grains(TD-45~0 orientation) show random orientations and a variety of misorientation angles but preferred axes <5273> or <5270>.Interestingly,a special oriented nucleation is found.Discontinuous static recrystallization nuclei originated from boundaries of the parent grain(TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70°<1120>(∑18 b).It is speculated that these two special misorientation relationships are related to the dislocation type.

Microstructure and texture optimization by static recrystallization originating from {10-12} extension twins in a Mg-Gd-Y alloy

During the deformation of Mg alloys,{10-12} extension twin often contributes to the formation of basal texture but rarely assists the nucleation of recrystallization,i.e.,effective grain refinement,therefore it seems to make against the improvement of formability and mechanical properties.In this work,{10-12} extension twin has been creatively utilized as a preference nucleation site for static recrystallization(SRX),achieving grain refinement and orientation randomization in a Mg-Gd-Y alloy using multi-directional impact forging(MDIF)and subsequent annealing treatment.Effect of {10-12} extension twin on SRX behavior has been investigated by annealing treatment at 450℃ using quasi-in-situ optical microscopy(OM)and quasi-in-situ electron back-scattering diffraction(EBSD).The microstructural evolution during annealing shows that several SRX gains can nucleate from the grain boundary of untwinned grains,but they only have few influences on the final microstructure due to their limited volume faction and sluggish growth.In contrast,a large number of SRX gains can initiate from {10-12} extension twin and grow up without the confine of twin boundaries.Finally,they consume their parent grains and make the main contribution to grain refinement.This should be attributed to those pinned {10-12} twin boundary,by interacting with various dislocation slips during the MDIF process,which can operate like grain boundary,store enough strain energy,and promote the nucleation of SRX during annealing.On the other hand,SRX grains usually keep initial random orientation and further randomize the forging texture during annealing treatment.

Enhanced tensile properties in a Mg-6Gd-3Y-0.5Zr alloy due to hot isostatic pressing(HIP)

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.