不同预时效挤压态Mg−Gd−Y−Zn−Zr合金的再结晶行为和强化机制

通过控制预时效时间制备3种不同状态的试样,研究不同预时效状态对挤压态Mg−9.5Gd−4Y−2.2Zn−0.5Zr(质量分数,%)合金的动态再结晶行为(DRX)和性能的影响。结果表明,欠时效挤压(UAE)样品的细晶体积分数为17.4%,而峰时效挤压(PAE)和过时效挤压(OAE)样品的细晶体积分数分别达到89.7%和50.4%。在晶粒内部和晶界处分布的致密、细小的β颗粒相通过粒子激发形核机制显著提高了形核位点和位错密度。然而,致密针状γ'相抑制位错滑移,延迟DRX形核。PEA和OAE样品中细小晶粒的差异归因于原始颗粒相的数量和尺寸的不同,而其拉伸性能的差异归因于不同的显微组织。由于晶界强化和析出强化机制的贡献更大,PAE样品具有更优异的拉伸性能。

含不同初始织构的Mg−Gd−Y−Zn−Zr合金热轧板材的显微组织和力学性能

通过光学显微镜、扫描电子显微镜、电子背散射衍射、透射电子显微镜和拉伸试验研究含不同初始织构的Mg−4.7Gd−3.4Y−1.2Zn−0.5Zr(质量分数,%)合金热轧板材的显微组织和力学性能。结果表明,初始织构为0001//RD(板材轧向)的板材获得相对较粗的晶粒和双峰基面织构;而初始织构为0001//TD(板材横向)的板材动态再结晶程度更高,因此,获得更细的晶粒尺寸和较强的0001//ND基面织构。此外,在两种板材中均发现沿RD拉长的不规则形状LPSO相,这种LPSO相导致各向异性的承载强化行为,并进一步引起板材的屈服各向异性。对强化机理的分析表明,晶界强化对轧制板材的强化效果最好。由于具有较小的晶粒尺寸和LPSO相带来的承载强化效果,初始织构为0001//TD的板材在RD方向上获得最高的屈服强度。

Gd对冷旋锻制备纳米晶镁合金的影响

通过显微硬度测试、拉伸试验和透射电镜手段研究Gd元素对冷旋锻过程镁合金纳米化的影响。结果表明,由于纯镁中很难形成位错阵列,旋锻不能实现纯镁纳米化。相比之下,单独添加Gd,旋锻后镁合金晶粒可得到显著细化。在旋锻早期阶段,纯镁和Mg−Gd合金中都形成孪晶片层。随着增加旋锻变形的继续,相比纯镁,更多的非基面位错在Mg−Gd合金中形成,这促进了Mg−Gd合金中位错阵列的形成,导致合金纳米化。随着Gd含量的增加,Mg−Gd合金晶粒细化效果显著提升,晶粒细化至100 nm以下的临界Gd含量约为5.4%(质量分数)。

Microstructures,corrosion and mechanical properties of as-cast Mg-Zn-Y-(Gd)alloys

Long period stacking ordered（LPSO） structure phases were prepared by conventional solidification method in Mg（94）Zn3YxGd（3-x）（x=3,2,1.5,1,mole fraction） alloys,the microstructures,corrosion and compressive mechanical properties of which were investigated,separately.The results reveal that the microstructures of the as-cast Mg（94）Zn3YxGd（3-x） alloys,with n（Zn）/n（Y＋Gd）=1：1,consist of α（Mg） phase,Mg3Zn3RE2（W） phase,Mg（12）ZnRE（14H-LPSO） phase and a few bright cube-shaped Mg-Y-Gd phases.The formation and the distribution of LPSO-phase in the alloys can be influenced by the content of Gd.The volume fraction of 14H-LPSO phase increases first and then decreases with the increase of the Gd content.For the electrochemical impedance spectroscopy（EIS） measurement,a R（Q（R（QR））） model was used to fit the test results in 3.5%（mass fraction） NaCl solution at room temperature.The corrosion current densities of all samples are about 10-（-5） A/cm-2.When x（Gd）≤1%,Mg-Zn-Y-（Gd）alloy shows good corrosion resistance,which is better than that of the commercial AZ91 D magnesium alloy.The corrosion rate increases when the Gd content is higher than 1.5%.At room temperature,the compressive properties of Mg-Zn-Y-（Gd） alloys increase remarkably with the increase of the volume fraction of LPSO phase.In addition,the pinning effect of W-phase and dispersive cube-shaped Mg-Y-Gd phase is beneficial to improving the mechanical properties of as-cast Mg（94）Zn3YxGd（3-x） alloy in deformation process.

Electrochemical codeposition of Mg-Li-Gd alloys from LiCl-KCl-MgCl_2-Gd_2O_3 melts

Mg-Li-Gd alloys were prepared by electrochemical codeposition from LiCl-KCl-MgCl 2 -Gd 2 O 3 melts on molybdenum electrode with constant current density at 823 and 973 K. The microstructure of the Mg-Li-Gd alloys was analyzed by X-ray diffraction （XRD）, optical microscopy （OM） and scanning electron microscopy （SEM）. The results show that magnesium and gadolinium deposit mainly in the first 30 min, and the alloy obtained contains 96.53% Mg, 0.27% Li and 3.20% Gd （mass fraction）. Then, the reduction of lithium ions occurs quickly. The composition of alloy can be adjusted by controlling electrolysis time or Gd 2 O 3 concentration in LiCl-KCl melts. With the addition of Gd into Mg-Li alloys, the corrosion resistance of the alloys is enhanced. XRD results suggest that Mg 3 Gd and Mg 2 Gd can be formed in Mg-Li-Gd alloys. The distribution of Gd element in Mg-Li-Gd alloys indicates that Gd element mainly distributes at the grain boundaries of Mg-Li-Gd alloys.

Enhancement of ductility in high strength Mg-Gd-Y-Zr alloy

A high strength GW94 alloy with fully recrystallized microstructure and equiaxed ultrafine grains of submicron size was produced by multiaxial forging and ageing. The alloy exhibits an ultimate tensile strength of 377 MPa, proof stress of 295 MPa and elongation to failure of 21.7%. The ductility is improved in comparison with that of the conventional extrusion processing. Superplastic ductility is achieved in tensile testing at 573 K with a maximum elongation of 450%. These high ductility and high strength are attributed to the coexistence of fully recrystallized grains and nanoscale Mg 5 （Gd, Y） particles dynamically precipitated at grain boundaries.

Microstructure evolution of Mg-9Gd-2Er-0.4Zr alloy during solid solution treatment

Microstructure evolution of the cast Mg-9Gd-2Er-0.4Zr alloy during solid solution treatment at temperature of 460-520 ℃ for 3-12 h was investigated by using optical microscope（OM）,scanning electron microscope（SEM） and transmission electron microscope（TEM）.The results indicated that the grain size and the shape of second phase were obviously changed with time and/or temperature going on.At 460 ℃ for 3 h,the morphology of the Mg5（GdEr） phase was changed into fragmentized island morphology and the volume faction of the phase decreased.After solution solid treatment at 460 ℃ for 6 h,the Mg5（GdEr） phase was already completely dissolved,but some cuboid-shaped RE-rich phase precipitated.As the temperature increased,the morphology of the Mg5（GdEr） phase was transformed into the same morphology as that at 460 ℃ for 6 h.It was suggested that the microstructure evolution of the alloy during the solid solution treatment was concluded as follows：Mg5（GdEr） eutectic phase→Gd/Er atom diffusing into matrix→spheroidic Mg5（GdEr） phase→cuboid-shaped RE-rich phase→grain boundary immigration.

Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy cast by metal mould and lost foam casting

The microstructure and mechanical properties of Mg-10.1Gd-3.74Y-0.25Zr （mass fraction, %） alloy （GW104 alloy） cast by metal mould casting （MMC） and lost foam casting （LFC） were evaluated, respectively. It is revealed that different forming modes do not influence the phase composition of as-cast alloy. In the as-cast specimens, the microstructures are similar and composed of α-Mg solid solution, eutectic compound of α-Mg＋Mg 24 （Gd, Y） 5 and cuboid-shaped Mg 5 （Gd, Y） phase; whereas the average grain size of the alloy produced by metal mould casting is smaller than that by lost foam casting. The eutectic compound of the alloy is completely dissolved after solution treatment at 525 ℃for 6 h, while the Mg 5 （Gd, Y） phase still exists after solution treatment. After peak-ageing, the lost foam cast alloy exhibits the maximum ultimate tensile strength of 285 MPa, and metal mould cast specimen 325 MPa at room temperature, while the tensile yield strengths of them are comparable. It can be concluded that GW104 alloy cast by lost foam casting possesses similar microstructure and evidently lower mechanical strength compared with metal mould cast alloy, due to slow solidification rate and proneness to form shrinkage porosities during lost foam casting process.

Effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-Gd-Y-Zr alloy

The effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-10Gd-3Y-0.6Zr alloy were investigated.The results indicate that more precipitation cores can be provided by the crystal defects caused by the plastic deformation,as well as increasing the amount of β＇ phases,and the formation of precipitations at grain boundaries and interfaces between the twins and matrix.Because of an increase in precipitations,the dislocation slipping during deformation process is effectively hindered and the matrix is strengthened,especially for the 2% deformed alloy which can achieve a good combination of strength and ductility.With increasing the plastic deformation,the microcracks occur at the interface between grain boundary precipitations and matrix,and then propagate intergranularly.When intergranular fracture combines with the formation of smoothing facets on the fracture surface,the tensile properties decrease.

Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy processed by equal channel angular pressing

Microstructure evolution and texture development and their effects on mechanical properties of a Mg-Gd-Y-Zr alloy during equal channel angular pressing（ECAP） were investigated.It is found that the microstructure is still inhomogeneous after four passes,and two zones,namely the fine grain zone（FGZ） and the coarse grain zone（CGZ） are formed.The grain refinement occurs mainly by particle-stimulated nucleation（PSN） mechanism,which led to a more random texture after four passes of ECAP.In the ECAP-processed alloy,the strength did not increase while the ductility was enhanced dramatically compared with the as-received condition.The change of ductility of this alloy was discussed in terms of texture and second phase particles.

Dry sliding wear behavior of cast Mg-11Y-5Gd-2Zn magnesium alloy

Dry sliding wear tests on as-cast and cast＋T6 Mg-11Y-5Gd-2Zn magnesium alloys were performed using a ball-on-plate configuration. The wear rates were measured within a load range of 3-15 N, sliding speed range of 0.03-0.24 m/s, test temperature range of 25-200 °C and at a constant sliding distance of 400 m. The wear tracks, worn surfaces and wear debris of the alloys were analyzed using scanning electron microscope （SEM）. The results show that the wear rate of the alloys increases almost linearly with increasing applied load and decreases with increasing sliding speed. The wear rate of the as-cast alloy is higher than that of the cast＋T6 alloy. The amount of Mg12Y1Zn1 phase, surface oxidation and retained wear debris affect the wear rate. The dominant wear mechanisms under the test condition are abrasion and plastic deformation.

Effect of purification treatment on corrosion resistance of Mg-Gd-Y-Zr alloy

Mg-Gd-Y-Zr alloys were purified by filtering purification with and without vacuum. The type, morphology, size distribution and volume fraction of inclusion were analyzed with OM and SEM. The effect of inclusion in Mg-Gd-Y-Zr alloys on anticorrosion ability was investigated with salt spray test and electrochemical test. The results show that the inclusions in the alloy can be removed effectively by filtering purification. The average size of inclusions in the alloys is decreased from 12.7 μm to 2.0 μm and the volume fraction of inclusions is reduced from 0.30% to 0.04%. With the decrease of the size of inclusions in the alloys, the corrosion rate of the alloys decreases dramatically from 38.8 g/（m 2 ·d） to 2.4 g/（m 2 ·d） in the salt spray test. The corrosion potential increases while the corrosion current decreases and the polarization resistance increases in the electrochemical tests, which indicates that the anticorrosion ability is improved.

Effects of Ce, Y and Gd additions on as-cast microstructure and mechanical properties of Mg-3Sn-2Sr magnesium alloy

The effects of Ce, Y and Gd additions on the as-cast microstructure and mechanical properties of Mg-3Sn-2Sr alloy were investigated and compared by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry analysis, tensile and creep tests. The results indicate that the Mg-3Sn-2Sr ternary alloy is mainly composed ofα-Mg, primary and eutectic SrMgSn, and Mg2Sn phases. After the additions of 1.0%Ce, 1.0%Y and 1.0%Gd to the Mg-3Sn-2Sr alloy, the Mg12Ce, YMgSn, GdMgSn and/or Mg17Sr2 phases are formed, respectively. At the same time, the formation of the primary SrMgSn phase is suppressed and the coarse needle-like primary SrMgSn phase is modified and refined. In addition, the additions of 1.0% Ce, 1.0% Y and 1.0% Gd to the Mg-3Sn-2Sr alloy can simultaneously improve the tensile and creep properties of the alloy. Among the Ce-, Y-and Gd-containing alloys, the tensile properties of the Ce-containing alloy are relatively higher than those of the Y-and Gd-containing alloys.

Mechanical properties of Mg-6Gd-1Y-0.5Zr alloy processed by low temperature thermo-mechanical treatment

An as-solution treated Mg-6Gd-1Y-0.4Zr alloy was processed by low temperature thermo-mechanical treatments （LT-TMT）, including cold tension with various strains followed by aging at 200 °C to peak hardness. The results show that the precipitation kinetics of the alloy experienced LT-TMT is greatly accelerated and the aging time to peak hardness is greatly decreased with increasing tensile strain. The tensile yield strength, ultimate tensile strength and elongation at room temperature of the alloy after cold tension with strain of 10% and peak aging at 200 °C are 251 MPa, 296 MPa and 8%, respectively, which are superior to the commercial heat-resistant WE54 alloy, although the latter has a higher rare earth element content.

Microstructure and texture evolution during high-strain-rate superplastic deformation of coarse-grained Mg-Gd-Y-Zr rolled sheet

Microstructure and texture evolution during high-strain-rate superplastic deformation of the rolled Mg-Gd-Y-Zr sheet were investigated.The tensile tests at the strain rate of 0.01 s-1 achieved the elongations of 180%-266% in the deformation temperature range of 400-500 ℃.Post-deforming microstructures were characterized by optical microscopy,scanning electron microscopy and transmission electron microscopy,while crystallographic orientation information was obtained from macro-texture analysis.The results show that the high strain-rate superplasticity was attributed to class-I dislocation creep accommodated by dynamic recrystallization （DRX）.During preheating at 435 ℃ for 600 s,twinning-induced recrystallization occurred.The initial strain of 80% made original grains fragmented and produced homogenous DRX grains.The interaction between dynamic recrystallization and dynamic precipitation yielded out such a phenomenon that finer DRX grains were often accompanied by denser particles.The macro-texture evolution exhibited some characteristics of the crystal rotation arising from basal slip and prismatic slip despite the occurrence of DRX.

Wear behavior of Mg-10Y-4Gd-1.5Zn-0.4Zr alloy

Dry sliding wear tests were performed on a Mg-10Y-4Gd-1.5Zn-0.4Zr alloy using a Ball-on-Flat type wear apparatus against an AISI 52100 type bearing steel ball counterface. The wear rates were measured within a load range of 3-25 N, a sliding speed range of 0.03-0.3 m/s and a sliding temperature range of 25-200 ℃ at a constant sliding distance of 400 m. The morphologies of the worn surfaces and wear debris were studied by scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy （EDS）. Comparatively, the wear properties of a hypereutectic Al-Si aluminium alloy under the same condition were measured. The results indicate that the wear rates of Mg-10Y-4Gd-1.5Zn-0.4Zr alloy are lower than that of cast＋T6 AC9B aluminium alloy. The dominant mechanism of cast＋T6 Mg-10Y-4Gd-1.5Zn-0.4Zr alloy is abrasion wear mixed with other wear mechanisms, which tends to be an abrasion and plastic deformation wear at high normal load such as 10-25 N, abrasion and plastic deformation wears with small participation of delamination and oxidative wears at high sliding speed such as 0.12-0.3 m/s, and an oxidative and abrasion wear at high test temperature such as 100-200 ℃. The Mg12Y1Zn1 phase in Mg-10Y-4Gd-1.5Zn-0.4Zr alloy plays an important role in the wear rate.

Microstructure and mechanical properties of extruded Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr alloy

The microstructure, age hardening behavior and mechanical properties of an Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr alloy prepared by casting and hot extrusion techniques were investigated. The solution-treated （T4 temper） alloys were extruded at 400, 450 and 500 °C with an extrusion ratio of 10：1, respectively. Optimized mechanical properties were obtained by extrusion at 400 °C followed by T5 treatment under the combined effects of grain refinement and precipitation strengthening. The alloy exhibits a grain size of about 5.0 μm, initial and peak microhardness of HV 109 and HV 129, respectively. The tensile yield strength, ultimate tensile strength and elongation at room temperature are 391 MPa, 430 MPa and 5.2%, respectively.

Microstructure and mechanical properties of Mg-6Gd-3Y-0.5Zr alloy processed by high-vacuum die-casting

GW63K （Mg-6Gd-3Y-0.5Zr） magnesium alloys were prepared successfully by high-vacuum die-casting. Effects of fast shot speed and vacuum level on the grain size and mechanical properties of this alloy were studied. Microstructure of the alloys was analyzed by SEM, EDX and optical microscope （OM）. The effect of heat treatment on high vacuum die-casting （HVDC） GW63K alloy was also studied. The results indicate that with the increase of fast velocity, the tensile yield strength hardly changes, but the elongation first increases, then decreases. The optimum heat treatment process is solution treatment at 748 K for 2 h and aging at 473 K for 80 h. Under this condition, GW63K magnesium alloy exhibits a maximum tensile strength and elongation of 308 MPa and 9.45%. There is significant correlation between ductility and the presence of external solidified cells （ESCs）. The as-cast GW63K alloy consists ofα-Mg and Mg24（Gd,Y）5 particles. After heat treatment, Gd and Y atoms dissolve intoα-Mg matrix.

Isothermal section of Mg-Nd-Gd ternary system at 723 K

An isothermal section of the Mg-Nd-Gd ternary system at 723 K was established by diffusion triple technique and electron probe microanalysis （EPMA）. Mg3Gd and Mg3Nd form a continuous solid solution （Gd,Nd）3Mg, and a continuous solid solution （Gd,Nd）Mg is also formed between MgGd and MgNd. Mg7Gd, Mg5Gd, Mg2Gd, Mg41Nd5, （Gd,Nd）3Mg and （Gd,Nd）Mg are found in the ternary system. In these intermetallic phases, Mg7Gd has been reported to be a metastable phase in previous literatures. The solubilities of Mg, Gd and Nd in all the phases were detected. Furthermore, four three-phase equilibria, α（Mg）＋Mg7Gd＋Mg41Nd5, Mg7Gd＋Mg5Gd＋Mg41Nd5, Mg5Gd＋Mg41Nd5＋（Gd,Nd）3Mg and （Gd,Nd）3Mg＋（Gd,Nd）Mg＋Mg2Gd, were identified in the isothermal section.

Mechanical properties of Mg-Gd-Zr alloy by Nd addition combined with hot extrusion

The effect of Nd addition on the microstructure and mechanical properties of as-extruded Mg-9Gd-0.5Zr(wt.%) alloy was investigated. The Mg-9Gd-0.5Zr and Mg-9Gd-2Nd-0.5Zr alloys were extruded at 673 K. The elongated non-dynamic recrystallized(un-DRXed) grains disappear after adding Nd, and uniformly distributed dynamic recrystallized grains with a grain size of 1.68 μm were obtained in the alloy. In addition, numerous nano-Mg5(Gd,Nd)particles were found to precipitate dynamically in the Mg-9Gd-2Nd-0.5Zr alloy, which gave rise to the dynamic recrystallization process via providing nucleation energy through hindering the release of deformation energy and promoting an increase in the strength through the Orowan strengthening mechanism. Moreover, the dynamically recrystallized(DRXed) grains have a weak texture, which plays a significant role in improving the ductility. Therefore,the Nd addition favors the improvement of strength and elongation for the as-extruded Mg-9Gd-0.5Zr alloy,simultaneously.