【作者】 袁明；

【导师】 郑子樵；

【作者基本信息】 中南大学 ， 材料学， 2014， 博士

【摘要】 镁合金中添加稀土元素是最有效的强化元素,开展Mg-RE合金的开发,提高合金的室温和高温力学性能,有利于镁合金得到广泛的应用。WE系稀土耐热镁合金(Mg-Y-Nd-Zr)由于其优良的室温高温力学性能、高温抗蠕变性能和耐腐蚀性能在航空航天和汽车工业上得到越来越广泛的应用。Sm和Gd在镁中的最大固溶度和析出强化效果分别比同组的Nd和Y大,用Sm和Gd分别替换WE系镁合金中的Nd和Y元素可能有更优良的综合力学性能。本文研究同时添加元素Sm、Gd和Zn到镁基体中,开发出一种新型耐热镁合金,研究了热处理工艺对新开发设计的Mg-Sm系耐热镁合金组织和性能的影响。采用光学显微镜、X射线衍射仪、扫描电子显微镜、透射电子显微镜和电子背散射衍射等分析手段,以及硬度测试和拉伸试验,研究了不同Zn和Gd含量对铸造Mg-3Sm-xGd(x=0.5,1.5)-yZn(y=0,0.3,0.6)-0.5Zr合金组织与力学性能的影响,得到优化的合金化学成分：Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金。合金铸态组织由镁基体和晶界处共晶相(Mg,Zn)3(Sm,Gd)i组成,(Mg,Zn)3(Sm,Gd)i相为FCC结构,晶格参数为a=0.727nm。合金经T6处理后,屈服强度达到185MPa,抗拉强度为282MPa,延伸率为6.1%。合金表现出优异的力学性能是由于合金中的析出相产生的析出强化效应,合金时效后为在棱柱面析出了片状的强化相,相的长轴垂直于基面,能形成对位错基面滑移的阻碍,依靠析出相与基体的共格强化和Orowan机制的联合作用使合金的强度提高。Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金在200℃的析出序列可描述为：Mgssss→β"(D019)→β’(bct)→β1(fcc)→β(fcc)。β”相(D019超点阵六方结构),晶格常数a=2aMg=0.64nm, c=CMg=0.52nm,其惯习面为{1010},与基体的取向关系：[2110] β"//[2110]α,(0110)β"//(0110) α; β’相具有底心正交结构,晶格常数为a=0.64nm, b=2.2nm, c=0.52nm。β’相与基体的取向关系为：[001]β’‖[0001]α (020)β’‖(1010)α; β1相为面心立方结构,晶格常数为a=0.74nm,它与基体的取向关系为：(220)β1//(0002)α,(111)β1//(2110)α。平衡相β也是面心立方结构,只是晶格常数不同,a=2.23nm；在时效过程中有一种基面沉淀相析出,作者定义为γ相,通过晶体结构分析,Y相为密排六方结构,晶格常数a=0.55nm,c=0.52nm,与基体的取向关系为(1100)γ//(1120)α,[0001]γ//[0001]α。在时效初期,β”相最先析出,随时效时间的延长,β”相减少,β’开始形核并长大引起合金的硬度值增大并达到峰值。在过时效阶段,基体中析出平衡相p。Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金通过热模拟变形研究,合金在高温等温压缩过程的真应力-真应变曲线属于动态再结晶型,当变形温度一定时,随着应变速率的加快,合金的峰值流变应力和它对应的应变值均提高；当应变速率一定时,变形温度的升高能降低峰值流变应力。Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金的材料常数结果为：变形激活能Q=246.2987KJ/mol,应力指数n=6.189,应力水平参数为α=0.01309MPa-1,结构因子A=4.486x1016s-1. Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金在热变形条件下流变应力σ、应变速率ε、变形温度T与参数Z满足下面的关系式：ε=4.486x1016[sinh(0.01309σ)]6.189exp(-246.2987x103/RT) Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金挤压变形后的组织为完全再结晶组织,形成的动态再结晶晶粒尺寸细小。合金的强度和塑性得到明显的提高,室温屈服强度、抗拉强度和延伸率分别为225MPa、302MPa和30.8%。挤压态合金直接在200℃时效有较强的时效硬化现象,峰时效时合金中析出了强化相p’,合金的力学性能有所提高,屈服强度达到267MPa,比挤压态合金提高了18.7%。Mg-3Sm-1.5Gd-0.3Zn-0.5Zr合金具有优良的耐蚀性能和抗蠕变性能,在5%氯化钠水溶液浸泡72h后的腐蚀失重速率为0.32mg/cm2day。在200℃,100MPa条件下具有优良的抗蠕变性,其稳定蠕变速率为2.63×10-9s-1,100h后的蠕变量小于0.1%。更多还原

【Abstract】 Rare earth elements(RE) is the most effective strengthening elements in magnesium alloy, develop the Mg-RE series alloys, improve the mechanical properties at room temperature and high temperature can make the magnesium alloy widely used. The WE series alloys(Mg-Y-Nd-Zr) become more and more attractive for aerospace and automotive industries because of their high room temperature mechanical properties, excellent high temperature creep resistance and good corrosion resistance. Sm and Gd maximum solubility and precipitation hardening effect in magnesium are stronger compared with the same group of Nd and Y, thus replace Nd and Y by Sm and Gd in WE series alloy may have better mechanical properties. In this paper, adding elements Sm, Gd and Zn to the magnesium matrix, developed a new heat-resistant magnesium alloy, study the effect of heat treatment on Mg-Sm alloy microstructure and properties.Study the effect of Zn and Gd on microstructure and mechanical properties of Mg-3Sm-xGd(x=0.5,1.5)-yZn(y=0,0.3,0.6)-0.5Zr alloy-0.5Zr) alloy by Using optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction equipment. The optimized alloy chemical composition ratio is Mg-3Sm-1.5Gd-0.3Zn-0.5Zr. The alloy was mainly composed α-Mg and (Mg,Zn)3(Sm,Gd)1eutectic phase in as-cast state.(Mg,Zn)3(Sm,Gd)1has a FCC structure with lattice parameters a=0.727nm. After the solution treatment at510℃for4h and subsequent peak aged at200℃, alloy exhibited remarkable mechanical properties. The ultimate tensile strength, yield strength and elongation of the alloy were282MPa,185MPa and6.1%at room temperature. These good mechanical properties were mainly attributed to the fine microstructure and fine metastable phase precipitates in the matrix during aging process.The decomposition of a-Mg supersaturated solid solution in Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy during isothermal aging at200℃is as follows: Mgssss→β"(D019)→β’(bct)→β1(fcc)→β(fcc).At the beginning of aging process, β" precipitates, with the aging time prolonged, β’appears and reached the peak hardness.During hot deformation, the true stress-strain curves of alloy is belong to dynamic recrystallization. Flow stress decreases with the temperature increase and strain rate decrease. The material constants of alloy are figured out as:Q=246.2987KJ/mol, n=6.189, α=0.01309MPa-1, A=4.486×1016s-1. The following relationship between the flow stress, strain rate, deformation temperature and Z parameters was derived:ε=4.486×1016[sinh(0.01309σ)]6.189exp(-246.2987×103/RT)Extruded Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy is fully recrystallized structure, grain size are small. The strength and ductility of the extruded alloy has been significantly improved, the yield strength, ultimate tensile strength and elongation are225MPa,302MPa and30.8%at room temperature, respectively. the extruded alloy directly at200℃aging, there is a strong age hardening phenomenon, precipitates strengthening phase β’, thus increased the mechanical properties of the alloy.the yield strength reached267MPa, more than18.7%higher than as-extruded alloy.β" phase has D019structure (a=2aMg=0.64nm and c=cMg=0.52nm) with an orientation relationship of [2110]β"//[2110]α,(0110)β"//(0110)α.β’phase is known to have a base-centred orthorhombic unit cell a=0.64nm, b=2.223nm, c=0.521nm, an orientation relationship with the matrix in this paper was [001]β’,||[0001]α,(020)β’,||(1010)α.β1phase has a fcc structure with lattice parameters a=0.74nm. β phase also has a fcc structure, lattice parameters a=2.23nm. γ phase habiting on the basal plane of the Mg matrix, hexagonal, a=0.55nm, c=0.52nm and orientation relationship is (1100)γ’//(1120)α,[0001]γ’//[0001]α.Mg-3Sm-1.5Gd-0.3Zn-0.5Zr alloy has excellent corrosion resistance and creep resistance, at a5%sodium chloride solution after72h immersion corrosion weight loss rate is0.32mg/cm2day. The steady creep rate is2.63×10-9s-1at200℃,100MPa conditions, the creep amount is less than0.1%after100h.更多还原