Effect of Hydrostatic Pressure on LPSO Kinking and Microstructure Evolution of Mg-11Gd-4Y-2Zn-O.5Zr Alloy

Two different kinds of hot compressions,namely normal-compression and can-compression,were performed on the Mg-11Gd-4Y-2Zn-0.5Zr alloy,featured with long period stacking ordered (LPSO) phase.The kinking behavior of LPSO phase and microstructure evolution was investigated to clarify the effect of levels of imposed hydrostatic pressure.The results suggest that the LPSO phases including both the intragranular 14H-LPSO phase and intergranular 18R-LPSO phase suffer severe kinking behavior under higher hydrostatic pressure induced by can-compression,which is firstly characterized with more kinking times and smaller relative kinking width.The main reason for such enhanced LPSO kinking during cancompression may be mainly ascribed to the higher dislocation density under a higher level of hydrostatic pressure.Meanwhile,a competitive relationship between the kink behaviors of intergranular 18R-LPSO phase and intragranular 14H-LPSO phase was observed.That is,the intergranular 18R-LPSO phase only kinks obviously on the condition that the surrounded intragranular 14H-LPSO phase scarcely kinks.In contrast to the distinctive kinking of LPSO phase,the dynamic recrystallization (DRX) mechanism shows less dependence on the hydrostatic pressure.Resultantly,similar DRX fractions and crystallographic texture were attained for two compression processes owing to the similar operation of deformation mode.     

Effect of Hydrostatic Pressure on LPSO Kinking and Microstructure Evolution of Mg–11Gd–4Y–2Zn–0.5Zr Alloy

Two different kinds of hot compressions, namely normal-compression and can-compression, were performed on the Mg–11 Gd–4 Y–2 Zn–0.5 Zr alloy, featured with long period stacking ordered(LPSO) phase. The kinking behavior of LPSO phase and microstructure evolution was investigated to clarify the effect of levels of imposed hydrostatic pressure. The results suggest that the LPSO phases including both the intragranular 14 H-LPSO phase and intergranular 18 R-LPSO phase suffer severe kinking behavior under higher hydrostatic pressure induced by can-compression, which is firstly characterized with more kinking times and smaller relative kinking width. The main reason for such enhanced LPSO kinking during cancompression may be mainly ascribed to the higher dislocation density under a higher level of hydrostatic pressure. Meanwhile, a competitive relationship between the kink behaviors of intergranular 18 R-LPSO phase and intragranular 14 H-LPSO phase was observed. That is, the intergranular 18 R-LPSO phase only kinks obviously on the condition that the surrounded intragranular 14 H-LPSO phase scarcely kinks. In contrast to the distinctive kinking of LPSO phase, the dynamic recrystallization(DRX) mechanism shows less dependence on the hydrostatic pressure. Resultantly, similar DRX fractions and crystallographic texture were attained for two compression processes owing to the similar operation of deformation mode.     

固溶处理对压铸GZ142合金显微组织的影响

借助XRD、SEM、EDX和TEM检测手段,对比了固溶处理前后压铸GZ142合金的显微组织。结果表明,固溶处理前压铸GZ142合金由α-Mg基体和(Mg, Zn)₃Gd共晶次生相组成,在400 ℃下固溶处理1 h后,晶内生成了大量层状14H型的长周期堆垛有序结构（LPSO）,同时,部分(Mg, Zn)₃Gd共晶次生相转变成同样具有长周期堆垛有序结构的X相。     

挤压比及不同Mn含量对Mg-10Gd-6Y-1.6Zn-xMn镁合金组织性能的影响

通过模铸法制备了Mg-10Gd-6Y-1.6Zn-xMn (x=0.4, 0.8, 1.2, 1.6, 2.0, wt.%)系列镁合金,研究了挤压比及Mn含量对Mg-10Gd-6Y-1.6Zn-xMn镁合金显微组织及室温力学性能的影响。研究结果表明：铸态Mg-10Gd-6Y-1.6Zn-xMn合金经热挤压后,合金中的长周期堆垛有序(LPSO)结构由亚稳的18R结构转变为稳定的14H结构。大挤压比能够显著提高合金的室温力学性能,当Mn含量为0.8%时,未时效态抗拉强度达到386MPa,断后延伸率约为10%。     

Mg-11Gd-4Y-2Zn-0.4Zr合金热扭转过程组织演化规律及变形机理

针对固溶态Mg-11Gd-4Y-2Zn-0.4Zr合金,利用金相显微镜(OM)、扫描电子显微镜(SEM)和电子背散射衍射(EBSD)技术结合剪应力等效施密特因子计算,系统研究该合金在350~450 ℃热扭转过程中组织演化规律及变形机理,为该合金剪切变形工艺的开发提供理论支撑。结果表明：扭转变形后,该合金原始等轴晶粒沿着剪切方向被拉长,同时,形成具有剪切特征的变形织构。350 ℃变形时,该合金变形机制以基面 滑移为主,部分晶粒发生拉伸孪晶,孪晶变体的选择满足施密特定律,且当基面 滑移和拉伸孪晶受抑制时,发生LPSO相扭折变形协调应变;400 ℃变形时,部分晶粒出现二次孪晶,并在孪晶界和扭折界面发生动态再结晶;450 ℃变形时,形成变形晶粒和再结晶晶粒的“双模”组织,再结晶织构为随机织构,可以显著弱化变形织构。     

Zn含量对Mg-6Gd-4Y变形镁合金显微组织与力学性能的影响

研究了Mg-6Gd-4Y(wt.%)合金与添加1％Zn的Mg-6Gd-4Y-1Zn合金的显微组织与力学性能。结果表明：Mg-6Gd-4Y合金的铸态组织由?-Mg基体和Mg24(GdY)5两相组成。而含有Zn的Mg-6Gd-4Y-1Zn合金的铸态组织则主要由α-Mg,Mg24(GdY)5和具有18R-LPSO结构的Mg12Y1Zn1相组成。挤压后,在含锌合金中发现了14H-LPSO相,分布于条状分布的Mg12Y1Zn1之间。14H-LPSO相的形成机理为沉淀析出,反应可表示为α-Mg′→α-Mg + 14H。Zn含量对β系列沉淀物没有明显的影响。在Mg-6Gd-4Y合金和Mg-6Gd-4Y-1Zn合金上进行的时效（T6和T5）处理均引起β\"析出相的形成。T6处理后的Mg-6Gd-4Y-1Zn合金具有高拉伸强度和良好的延展性,屈服强度（YS）,抗拉强度（UTS）和延伸率分别为309MPa,438MPa和6.8％。这是18R-LPSO相与细小弥散分布的14H-LPSO相和β\"沉淀相共同作用的结果。     

Precipitate evolution in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). After under-ageing for 2 h, the precipitates in the alloy are ordered solute clusters with rare earth atomic columns exhibiting hexagonal ring structure, zigzag GP zones and β' in its early formation. After peak-ageing for 18 h, the precipitates are mainly β' and new rod-like β'_p accompanied with β'. After over-ageing for 100 h, the precipitates are β', β₁, long-period stacking ordered (LPSO) building block known as γ′ and 14H-LPSO. β' has the three-dimensional shape of convex lens with smaller length-to-width ratio viewed along 〈0001〉_α than that in the EW75 alloy. The excellent thermal stability of this alloy can be attributed to the γ' and 14H-LPSO retarding the growth of β' and β₁, low diffusion rate of rare earth atoms and physical character of β' and β₁.     

热处理对挤压态Mg-6Gd-5Y-1Zn合金组织与性能的影响

通过对Mg-6Gd-5Y-1Zn(质量分数,%)合金在固溶和时效处理状态下显微组织和力学性能的研究发现,α-Mg基体、沿挤压方向分布的条状18R-LPSO相、少量的Mg₂₄(GdYZn)₅ 相以及细层片状的14H-LPSO相构成了挤压态合金的组成相。挤压态合金经固溶(T4)处理后,一部分18R-LPSO相溶入基体,并且基体中的14H-LPSO相伸长同时粗化。挤压态合金经过固溶加时效(T6)处理后,大量β′相从α-Mg基体中析出。T6态合金的室温力学性能最好,其屈服强度、抗拉强度及伸长率分别为272 MPa、406 MPa和6.1%。β′相沉淀也发生在挤压态合金的直接人工时效(T5)处理过程,但相比于T6处理,14H-LPSO相和β′相在基体中的体积分数均偏低。     

具有长周期结构的Mg–Gd–Zn–Zr合金的微结构和摩擦学行为

利用传统的熔铸法制备Mg-14.28Gd-2.44Zn-0.54Zr合金,研究铸态和固溶态合金的微结构。利用销-盘装置研究铸态和固溶态合金的室温润滑滑动摩擦磨损行为研究。在外载荷为40 N,滑动速度为30-300 mm/s以及滑行路程为5000 m情况下,测量磨损率和摩擦因数。研究结果表明：铸态合金主要由α-Mg固溶体、分布在基体内的层片状的14H型长周期结构（LPSO）和β-[（Mg,Zn）3Gd]相组成。经过温度为773 K固溶处理35 h后,大量的β相转变成具有14H型X相LPSO结构。由于固溶处理后大量β相转变为热稳定的韧性X-Mg12Gd Zn长周期结构相,固溶合金呈现较低的抗磨损能力。     

Evolution of precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy with fine plate-like 14H-LPSO structures aged at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr (wt.%) alloy with fine plate-like 14H-LPSO structures aged at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Fine plate-like 14H-LPSO structures precipitate after heat treatment at 500 °C for 2 h, and β-type phases precipitate after the alloy is aged at 240 °C. The long-period atomic stacking sequence of 14H-LPSO structures along the [0001]_α direction is ABABCACACACBABA. After being aged at 240 °C for 2 h, the β-type phases are the ordered solution clusters, zig-zag GP zones, and a small number of β′ phases. The peak hardness is obtained at 240 °C for 18 h with a Brinell hardness of 112, the β-type phases are β’ phases and local RE-rich structures. After being aged at 240 °C for 100 h, the β-type phases are β’, β₁ and β’_F phases. β′ phases nucleate from the zig-zag GP zones directly without β″ phases, and then transform into β₁ phase by β’→β’_F→β₁ transformations. The Zn not only can form LPSO structure, but also is the constituent element of β₁ phases. LPSO structures have a certain hindrance to the coarsening of β’ and β₁ along 〈0001〉_α.     

Microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr alloys

The microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr (x=1, 2, 3, 4; normal mass fraction in %) alloys were investigated. In low Zn content, aside from the major second phase of Mg₂₄(Er, Y, Zn)₅, there are a few lamellar phases that grow parallel with each other from the grain boundaries to the grain interior. With Zn content increasing, the Mg₂₄(Er,Y,Zn)₅ phase decreases, but the Mg₁₂Zn(Y, Er) phase and lamellar phases continuously increase. When Zn content reaches 4% (normal mass fraction), the Mg₁₂Zn(Y,Er) phase mainly exists as large bulks, and some α-Mg grains are thoroughly penetrated by the lamellar phases. Moreover, the crystallography structures of the Mg₁₂Zn(Y,Er) and Mg₂₄(Er,Y,Zn)₅ phases are confirmed as 18R-type long-period stacking ordered structure and body-centred cubic structure, respectively.     

Microstructure and mechanical properties of a compound reinforced Mg95Y2.5Zn2.5 alloy with long period stacking ordered phase and W phase

The microstructure evolution of Mg100-2xYxZnx(x=2, 2.5, 3, 3.5) alloys was investigated. Results show that the Mg100-2xYxZnx alloys are composed of α-Mg, long period stacking ordered(LPSO) phase and eutectic structure phase(W phase), and the Mg95Y2.5Zn2.5 alloy has the best comprehensive mechanical properties. Subsequently, the microstructure evolution of the optimized alloy Mg95Y2.5Zn2.5 during solidification and heat treatment processes was analyzed and discussed by means of OM, SEM, TEM, XRD and DTA. After heat treatment, the lamellar phase 14H-LPSO precipitated in α-Mg and W phase transforms into particle phase(MgYZn2). Due to the compound reinforcement effect of the particle phase and LPSO phase(18R+14H), the mechanical properties of the alloy are enhanced. The tensile strength and elongation of the Mg95Y2.5Zn2.5 alloy is improved by 9.1% and 31.3% to 215 MPa and 10.5%, respectively, after solid-solution treatment.     

Dual phases strengthening behavior of Mg–10Gd–1Er–1Zn–0.6Zr alloy

The microstructure evolution and strengthening mechanisms of Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy were focused in the view of the size parameters and volume fraction (f_p) of dual phases (long period stacking ordered (LPSO) structures and β′ precipitates). Results show that two types of LPSO phases with different morphologies formed, and the morphology and size of both LPSO phases varied with the solution conditions. However, the volume fraction decreased monotonously with increasing solution temperature, which in turn raised the volume fraction of β′ phase during aging. The alloy exhibited an ultimate tensile strength of 352 MPa, a yield strength of 271 MPa, and an elongation of 3.5% after solution treatment at 500 °C for 12 h and aging at 200 °C for 114 h. In contrast to the LPSO phase, the β′ phase seems to play a more important role in enhancing the yield strength, and consequently, a decreased f_LPSO/f_β′ ratio results in an increased yield strength.