Effect of annealing treatment on the microstructure and mechanical properties of warm-rolled Mg-Zn-Gd-Ca-Mn alloys

The basal texture of traditional magnesium alloy AZ31 is easy to form and exhibits poor plasticity at room temperature.To address these problems,a multi-micro-alloyed high-plasticity Mg-1.8Zn-0.8Gd-0.1Ca-0.2Mn(wt%)alloy was developed using the unique role of rare earth and Ca solute atoms.In addition,the influence of the annealing process on the grain size,second phase,texture,and mechanical properties of the warm-rolled sheet at room temperature was analyzed with the goal of developing high-plasticity mag-nesium alloy sheets and obtaining optimal thermal-mechanical treatment parameters.The results show that the annealing temperature has a significant effect on the microstructure and properties due to the low alloying content:there are small amounts of larger-sized block and long string phases along the rolling direction(RD),as well as several spherical and rodlike particle phases inside the grains.With increas-ing annealing temperature,the grain size decreases and then increases,and the morphology,number,and size of the second phase also change correspondingly.The particle phase within the grains vanishes at 450℃,and the grain size increases sharply.In the full recrystal-lization stage at 300-350℃,the optimum strength-plasticity comprehensive mechanical properties are presented,with yield strengths of 182.1 and 176.9 MPa,tensile strengths of 271.1 and 275.8 MPa in the RD and transverse direction(TD),and elongation values of 27.4%and 32.3%,respectively.Moreover,there are still some larger-sized phases in the alloy that influence its mechanical properties,which offers room for improvement.

Low-cycle fatigue behavior of solutionized and aged WE43 magnesium alloys at room temperature

The low-cycle fatigue behavior of solutionized(T4)and aged(T6)WE43 magnesium alloys was studied at room temperature.The total strain amplitudes(△ε_(t)/2)were 0.4%,0.5%,0.6%,0.7%and 1.0%.Detailed microstructure evolution was characterized by scanning electron microscope(SEM),electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM).The results showed that plastic strain amplitude decreased with the increasing cycle number in T4 alloy,which is due to the dense persistent slip bands(PSBs)and dynamic precipitates hinderingdislocation slip.In contrast,the plastic strain amplitude increases gradually in T6 alloy,which is attributed to the enhanced activation of pyramidal slip.The low-cycle fatigue life of T6 alloy with larger fatigue ductility coefficient is longer than that of T4 alloy.The Coffin-Manson model can accurately predict the fatigue life of T4 and T6 alloys compared to Jahed-Varvani(JV)energy model.For T4 alloy,the fatigue damage mechanism was dominated by basal slip.For T6 alloy,the enhanced pyramidal slip plays an important role to accommodate plastic deformation.

Tailoring the texture and mechanical properties of 3%Y_(2)O_(3)p/ZGK200 composites fabricated by unidirectional and cross rolling followed by annealing

3%Y_(2)O_(3)p/ZGK200 composites were subjected to unidirectional rolling(UR)and cross rolling(CR)at 400℃and 350℃followed by annealing at 300℃for 1 h.The microstructure,texture and mechanical properties of rolled and annealed composites were systematically studied.The rolled composites exhibited a heterogeneous microstructure,consisting of deformed grains elongated along rolling direction(RD)and Y_(2)O_(3)particles bands distributed along RD.After annealing,static recrystallization(SRX)occurred and most deformed grains transformed into equiaxed grains.A non-basal texture with two strong T-texture components was obtained after UR while a non-basal elliptical/circle texture with circle multi-peaks was obtained after CR,indicating that rolling path had great influences on texture of the composites.After annealing process,R-texture component disappeared or weakened,as results,a non-basal texture with double peaks tilting from normal direction(ND)to transverse direction(TD)and a more random non-basal texture with circle multi-peaks were obtained for UR and CR composites,respectively.The yield strength of rolled composites after UR showed obvious anisotropy along RD and TD while a low anisotropic yield strength was obtained after CR.Some Y_(2)O_(3)particles broke during rolling.The fracture of the composites was attributed to the existence of Y_(2)O_(3)clusters and interfacial debonding between particles and matrix during tension,as a result,the ductility was not as superior as matrix alloy.

Effect of Zr modification on solidification behavior and mechanical properties of Mg–Y–RE (WE54) alloy

Magnesium alloys containing rare earth elements (RE) have received considerable attention in recent years due to their high mechanical strength and good heat-resisting performance. Among them, Mg–5%Y–4%RE (WE54) magnesium alloy is a high strength sand casting magnesium alloy for use at temperatures up to 300 ℃, which is of great interest to engineers in the aerospace industry. In the present work, the solidification behavior of Zr-containing WE54 alloy and Zr-free alloy was investigated by computer-aided cooling curve analysis (CA-CCA) technique. And the solidification microstructure and mechanical properties of them were also investigated comparatively. It is found from the cooling curves and as-cast microstructure of WE54 alloy that the nucleation temperature of α-Mg in WE54 alloy increases after Zr addition, and the as-cast microstructure of the alloy is significantly refined by Zr. While the phase constitution of WE54 alloy is not changed after Zr addition. These phenomena indicate that Zr acts as heterogeneous nuclei during the solidification of WE54 alloy. Due to refined microstructure, the mechanical properties of Zr-containing WE54 alloy is much higher than Zr-free WE54 alloy.

Microstructure and mechanical properties of a high ductility Mg–Zn–Mn–Ce magnesium alloy

A high-ductility ZME200(Mg–2.3Zn–0.4Mn–0.2Ce^(1))alloy was newly developed for vehicle closure and structure applications,based on an earlier ZE20(Mg–2.0Zn–0.2Ce)alloy for extrusion applications.Previous study indicates that the hot deformation behavior of as-cast ZME200 alloy varies with processing parameters,namely temperature and strain rate.In this follow-up study,a conventional rolling process was optimized to obtain magnesium sheets with a very fine grain structure and high ductility.The microstructure,mechanical properties,and corrosion resistances of ZME200 alloy were investigated,and compared with those of commercial AZ31 magnesium alloy.It was demonstrated that the ZME200 alloy sheet exhibits extraordinarily higher ductility(36%in tensile elongation),much superior stretch formability(an Erichsen value of 9.5),lower anisotropy,comparable strength and corrosion resistance to AZ31 alloy.The unique RD–TD double split texture with remarkably reduced intensity and grain refinement gives rise to the significantly improved ductility and formability at room-temperature.

Effects of trace Gd concentration on texture and mechanical properties of hot-rolled Mg-2Zn-xGd sheets

The effects of Gd concentration(0.1,0.3,0.7 wt%)on the microstructure,texture and mechanical properties of rolled and annealed Mg-2Zn-xGd sheets have been investigated aiming to develop low cost and high ductile Mg-Zn-Gd sheets.Dynamic recrystallization,static recrystallization and grain growth during hot rolling process and annealing process were delayed with increase of Gd concentration,leading to fine grain microstructure.The rolled 0.1 wt%Gd sheet showed strong basal texture which remained stable after annealing process and exhibited medium elongation of about 25%.In contrast,the rolled 0.3 wt%Gd sheet had weak basal texture which transformed to non-basal texture with double peaks tilted about±48°to the transverse direction due to the static recrystallization during annealing process.Consequently,the annealed sheets exhibited higher elongation of 40%along the rolling direction and 50%along the transverse direction due to the existence of non-basal texture.It is suggested that the minimum effective concentration for texture randomization in the Mg-2Zn-xGd alloy is about 0.3 wt%.

Twin evolution in cast Mg-Gd-Y alloys and its dependence on aging heat treatment

Effects of rare-earth(RE)and precipitates on twin evolution in cast Mg-10Gd-3Y-0.5Zr(wt.%)(GW103)alloys of solid solution(T4)and aged(T6)states are investigated performing quasi-static room temperature compression tests and microstructural characterization.It is found that both{10–12}and{11–21}extension twins(ET1 and ET2)can appear in the T4 and T6 states but with different emergence sequences.As the aging heat treatment leads to consumption of RE solutes which could inhibit atomic shuffling required for nucleation of ET1 but not ET2,ET2 occurs prior to ET1 in the T4 state,and ET1 emerges before ET2 in the T6 state.The extension twins here mainly coordinate the plastic deformation through the non-Schmid effect.Our results shed light on the influence of RE elements on twin evolution in magnesium alloys and have implications in developing high-performance Mg-RE alloys.

Consideration of Castability and Formability for New Magnesium Alloys

A comprehensive consideration based on castability or plastic formability, as well as mechanical properties for development of either cast magnesium alloys or wrought magnesium alloys is a very important issue. To develop new magnesium alloy sheets with high formability at room temperature, the microstructure, texture, ductility and anisotropy of rolled Mg-Zn-Gd alloy sheets were investigated. The sheets exhibit an excellent ultimate elongation of nearly 50% and an uniform elongation greater than 30% with a very low planar anisotropy. The new sheet has a random basal texture and the basal pole is tilted by maximum 40° from the normal direction towards the transverse direction. The majority of grains in the tilted texture have an orientation favorable for both basal slip and tensile twining because of their high Schmid factor. The low planar anisotropy, the large uniform elongations and the high strain hardening rate observed in the Mg-Zn-Gd sheets result in excellent room temperature formability, the Erichsen values reach ~8, well comparable with the conventional aluminum alloys sheets at room temperature. The solidification pathways and phase equilibria of Mg-Al-Ca alloys have been profoundly investigated by using thermal analysis and thermodynamic calculations. The relationship between hot tearing tendency and alloy compositions were discussed in terms of strength of the mushy zone, solidification pathways and feeding mechanisms, et al. Thixoforming refers to as that metal components are formed in their semi-solid state. Criteria for thixoforming are summarized and then the thixoformability of Mg-Al-Ca based alloys (AC alloys) are evaluated using the thermodynamic calculations based on the consideration of metallurgical parameters.

The effect of precipitates on the fracture behavior and tensile properties of Mg-14Gd-0.5Zr(wt.%)alloy

The effect of precipitation aging on the fracture behavior of cast Mg-14.23Gd-0.45Zr(wt.%)alloy at room temperature has been studied in this work.Uniaxial tensile and three-point bending tests were conducted on samples peak-aged at 175,200,225,and 250 ℃.Notably,samples aged at 175 ℃ and 200 ℃ exhibited premature fracture during the uniaxial tensile test.Through fractographic observations of the tensile test samples and electron backscattered diffraction(EBSD)analysis on the samples sub-jected to three-point bending tests,a preferential formation of cleavage cracks in samples aged at 175 ℃ and 200 ℃ was identified as the reason for their premature fracture.The X-ray diffraction(XRD)results and transmission electron microscopy(TEM)observations of precipitates indicate that the dominant strengthening precipitates in all peak-aged samples are of theβ'phase,and their size significantly influences the formation of cleavage cracks.This phenomenon is attributed to the shearing mechanism of precipitates.Specifically,the smaller β'precipitates formed under the aging temperature of 175-200 ℃ are susceptible to dislocation shearing,leading to the formation of cleavage cracks.In contrast,the larger size of β'precipitates formed under the aging temperature of 225-250 ℃ provides resistance to shearing,resulting in the restrained formation of cleavage cracks and ultimately contributing to the enhancement of the ultimate tensile strength.

Formation mechanism of high-temperature oxidation film on WE43 magnesium alloy and its effect on corrosion performance

The oxidation behavior of WE43 magnesium alloy in dry air at three temperatures(225,440 and 525℃)and the corresponding corrosion performance of samples attached to oxide film in 3.5 wt%NaCl solution was investigated.The results show that the oxide films formed at all three temperatures are a complex MgO·RE_(2)O_(3)·ZrO film with different compositions.The film formed at 225℃is flat and dense,whose components are 3.2MgO·1.8RE_(2)O_(3)·1ZrO.The oxidation ridges begin to form,and then gradually grow into nodular oxides and form a loose and porous oxide layer as the temperature increases to 525℃.The oxide films formed at all three temperatures improve the corrosion resistance of the alloy due to the MgO·RE_(2)O_(3)·ZrO,with the protective properties of oxide films following the order of 225℃>440℃>525℃,because the dense MgO·RE_(2)O_(3)·ZrO film formed at 225℃can provide better protection to the substrate than the loose oxide film formed at higher temperatures.

Characterization and Prediction of Microporosity Defect in Sand Cast WE54 Alloy Castings

In order to study the effect of Zr modification and riser size on microporosity defect distributions in WE54 alloy sand castings, the microporosity volume percentage in Zr-free and Zr-containing WE54 alloy plate castings was determined by density measurement based on Archimedes＇ principle, and the microstructure of the microporosity defects was observed by optical microscopy and scanning electron microscopy. Then by using Procast software, the Niyama criterion was calculated in order to investigate the validity of Niyama criterion on prediction of microporosity defects in WE54 alloy sand castings. It is found from the density measurement results that Zr addition does not affect the microporosity distributions in WE54 alloy castings. While the distribution area of microporosity defect in the plate castings decreases significantly as the riser size increases. Based on the experimental results, a riser selection principle for production of compact WE54 alloy castings is proposed that the solidification modulus of the riser should be greater than that of the casting by 30%, simply mr ≥ 1.3mc. By comparing the experimental and simulating results, it is found that the predicted microporosity regions by Niyama criterion agrees well with experimental results, and a critical Niyama value of 0.4 ℃0.5 s0.5 mm-1 is suggested for prediction of microporosity formation in WE54 alloy sand castings.

Hot Tearing of Sand Cast Mg-5 wt.% Y-4 wt.% RE (WE54) Alloy

Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented ＂CRC＂ equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE （WE54） alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction （fs） in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.

Solidification of Mg-Zn-Zr Alloys:Grain Growth Restriction,Dendrite Coherency and Grain Size

The solidification characterization of Mg-xZn-0.5Zr(x=0,1,3,4,5 wt%)alloys has been extensively investigated through thermal analysis,microstructure characterization and thermodynamic calculations.The impact of Zn content on the grain growth restriction,dendrite coherency and thus the final grain size has been investigated and discussed.Increasing Zn content,the grain size of Mg-xZn-0.5Zr alloy was firstly refined and then coarsened with the finest grain size of^50μm for the Mg-3 Zn-0.5 Zr(ZK31)alloy.Significant effects of the grain size on the mechanical properties were observed in the investigated alloys.The combination of growth restriction factor theory and dendrite coherency point provides a reasonable explanation of the grain size results.It helps to further understand the mechanisms of grain refinement and grain coarsening related to solute content,providing reference for alloy design and grain size prediction.

Tailoring the Texture and Mechanical Anisotropy of Multi-cross Rolled Mg-Zn-Gd Alloy by Annealing

The unidirectional rolled Mg-Zn-Gd sheet usually exhibited non-basal texture with two peaks whose tilting angle were about 42°from normal direction to transverse direction(TD),which would cause the mechanical anisotropy.In this study,multi-cross rolling followed by annealing was used to tailor the texture and mechanical anisotropy for Mg-Zn-Gd alloy.With increasing annealing temperature,the rolled basal texture with two peaks gradually transformed into the circle texture with multi-peaks.In order to figure out different texture components evolution during annealing,the basal texture,R-texture and T-texture component were defined and studied.The results showed that the change of R-texture and T-texture component was asynchronous with increasing annealing temperature from 250 to 400℃.The tilting angle of R-texture component increased slightly,while the tilting angle of T-texture component increased obviously,and this phenomenon was attributed to the preferential nucleation at grain nucleation stage rather than preferential grain growth.The yield strength along TD was more sensitive to annealing temperature compared with that along rolling direction(RD),resulting in different descending slopes and yield strength anisotropy with increasing annealing temperature.Annealing at 300℃was the best annealing temperature due to low yield strength anisotropy,moderate strength and good elongation among these annealing temperatures.The Schmid factor for basal slip indicated that the activity of basal slip along RD increased slightly,while that along TD increased obviously with increasing annealing temperature from 250 to 400℃,which should be caused by the asynchronous change of R-texture component and T-texture component,consequently resulting in the transformation from isotropic yield strength to anisotropic yield strength.

Microstructure and Texture Evolution of Mg‑14Gd‑0.5Zr Alloy during Rolling and Annealing under Diferent Temperatures

Mg alloys containing high rare earth(RE)elements are hard to be rolled due to their low ductility and high strength at low temperatures.Therefore,rolling at high temperatures is necessary for these alloys.In this work,a Mg-14Gd-0.5Zr(wt%)alloy was rolled one pass with 40%reduction at high temperatures over 450°C.The efects of rolling temperature on the microstructure and dynamic recrystallization behavior were analyzed in detail by the electron backscattered difraction(EBSD)method.The results revealed that the alloy shows good rollability at high temperature due to the activity of nonbasal dislocations and twins.However,dynamic recrystallization is difcult to take place because of the easy activation of multi-slip system and thus more accumulated strain energy.However,Gd segregation was observed at the grain boundary and it is inferred that the segregation can partially enhance the dynamic recrystallization ratio.Statically recrystallized grains with large size took place after short-term inter-pass annealing treatment ranging from 450℃to 500°C,and formed basal texture.The result suggests that the formation of basal texture during short-term annealing treatment was attributed to the growth of dynamic recrystallized grains caused by weakened Gd segregation.