【作者】 李大全；

【导师】 丁文江； 王渠东；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2008， 博士

【摘要】 WE系列的WE43和WE54镁合金由于其优异的室温力学性能、高温抗蠕变性能和耐腐蚀性能在航天航空和汽车工业中得到越来越广泛的应用。WE系列合金的主要成分为Mg-Y-Nd-Zr,其中Zr为晶粒细化剂。而该系列合金主要通过稀土元素Y和Nd的时效析出强化提高合金性能。在镁合金中Sm具有比Nd更高的固溶强化和析出强化效果。但是关于Sm元素在镁合金中应用的研究很少,对Sm元素在镁合金中的强化机理还不清楚。因此,本文用Sm元素代替WE系列镁合金中的Nd元素,设计出Mg-4Y-xSm-0.5Zr(x=1,4,8)系列镁合金,系统研究了Sm元素对该系列镁合金组织结构、室温和高温力学性能的影响规律。并在此基础上重点研究了Mg-4Y-4Sm-0.5Zr合金的热处理和挤压成形工艺,研究Mg-4Y-4Sm-0.5Zr合金在热处理和挤压成形过程中的时效析出相变化和强化机制。文章最后还研究了挤压态Mg-4Y-4Sm-0.5Zr合金的高温压缩和压缩蠕变变形行为,对比分析了合金不同温度和不同应变速率下的高温压缩变形机制。实验设计Mg-4Y-1Sm-0.5Zr、Mg-4Y-4Sm-0.5Zr和Mg-4Y-8Sm-0.5Zr三种合金成分,研究了不同Sm含量对Mg-Y-Sm-Zr系镁合金在热处理和挤压成形过程中的组织性能的影响规律。研究结果表明铸态Mg-4Y-xSm-0.5Zr系列镁合金的组织主要为Mg基体+Mg5(Sm,Y)共晶化合物。不同合金中共晶化合物的稀土元素Y、Sm含量虽然稍有差异,但这些共晶化合物都具有与Mg5Sm相同的面心立方晶体结构(a=2.246nm)。另外,在铸态Mg-4Y-xSm-0.5Zr系列镁合金的晶界附近还存在一定量的富集稀土元素Y和Sm的面心立方结构(a=0.5581nm)方块相。经过固溶热处理,铸态Mg-4Y-1Sm-0.5Zr和Mg-4Y-4Sm-0.5Zr合金中的共晶相固溶入基体。而Mg-4Y-8Sm-0.5Zr合金由于Sm元素含量超过其在镁基体中的最大固溶度,固溶不完全,固溶处理后,面心立方晶体结构的Mg5(Sm,Y)共晶化合物转变为体心四方结构的Mg41Sm5,并有很少量Y元素置换Sm元素固溶其中。Mg-4Y-8Sm-0.5Zr合金,由于大量脆性相的存在,塑性低,强度差;Mg-4Y-1Sm-0.5Zr合金塑性好,但是由于Sm元素含量低,强化效果发挥不充分;Mg-4Y-4Sm-0.5Zr合金具有最佳的综合力学性能。研究Mg-4Y-4Sm-0.5Zr合金的固溶热处理工艺,结果表明合金在525℃固溶处理8小时后,得到最佳的固溶效果。合金经过固溶热处理,稀土元素Y和Sm完全固溶入基体并且均匀分布,起到一定程度的固溶强化效果,但主要作用还是得到均匀的过饱和固溶体,为随后的时效热处理提供最佳条件。进一步研究合金在不同温度和不同时间条件下的时效热处理工艺,结果表明Mg-4Y-4Sm-0.5Zr合金在200℃欠时效状态(16h),析出相在晶内和晶界上弥散分布,有效强化合金,合金力学性能得到大幅度提高,抗拉强度达到348MPa,屈服强度达到217MPa,延伸率为6.9%,表现出优异的力学性能。析出强化是Mg-4Y-4Sm-0.5Zr合金的主要强化机制。主要通过β′析出相与镁基体之间的应力场强化合金。通过对Mg-4Y-4Sm-0.5Zr合金时效析出相的进一步研究表明:合金在不同温度下的时效析出相包括β′相,β1相和β相。合金在200℃时效,晶内只有β′一种相。该相在200℃温度下非常稳定。欠时效状态,β′相在晶内大量弥散均匀析出,有效强化合金,对应合金的最佳力学性能;随时效时间的增加,β′相逐渐合并长大呈盘状沿{2110 }惯习面析出。由于析出相的粗化和弥散度的降低合金力学性能相应有所降低。合金在较高温度250℃时效48小时,晶内沿{1010}惯习面析出盘状面心立方晶体结构(a=0.74nm)的β1相。合金在更高温度300℃时效处理13小时,大量面心立方晶体结构(a=2.223nm)平衡相(β相)在晶内析出。该平衡相的惯习面与β1相相同,形貌也与β1相相似,是由β1相原位转变而来。Mg-4Y-4Sm-0.5Zr合金高温(350℃及以上)挤压,挤压过程中发生动态再结晶,晶粒细化,起到显著的细晶强化作用。合金挤压后直接进行时效热处理,通过析出强化进一步提高合金强度,且析出强化效果随挤压温度的升高稍有提高。经过挤压和时效热处理,Mg-4Y-4Sm-0.5Zr合金最高抗拉强度达到400MPa,屈服强度超过300MPa,延伸率达到7%,表现出比WE系列合金更加优异的室温力学性能。析出强化和细晶强化都是合金有效的强化手段。文章最后还系统研究了挤压态Mg-4Y-4Sm-0.5Zr合金的高温压缩和压缩蠕变变形行为。合金在450℃,应变速率2.5×10-4s-1条件下高温压缩,应变速率敏感系数m=0.4,压缩变形激活能Q=92.1KJ/mol,压缩过程中合金晶粒长大但基本保持等轴,说明晶界滑移是合金在该条件下压缩的主要变形机制。合金在200℃,180MPa条件下具有较好的抗压缩蠕变性能,计算得到压缩蠕变应力指数n=4.1,蠕变激活能Q=156.7KJ/mol,结合微观组织观察表明,位错攀移是合金在该条件下压缩蠕变变形速率控制机制。更多还原

【Abstract】 The WE series alloys, based on the Mg-Y-Nd-Zr system, identified as WE54 (5wt.%Y, 3.3wt%RE, 0.5wt.%Zr) and WE43 (4wt.%Y, 3.3wt.%RE, 0.5wt.%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. These alloys are strengthened essentially by precipitation hardening. Samarium belongs to the same subgroup as neodymium, and the maximum solubility of samarium in solid magnesium is higher than neodymium. Therefore it is reasonable to assume that the precipitation hardening effect of Mg-Y-Sm-Zr alloys is higher than that of WE series alloys based on Mg-Y-Nd-Zr system, which may result in a higher strength. But there is little research on strengthening effects and strengthening mechanisms of Mg-Y-Sm-Zr alloys. It is the purpose of present study to investigate the effects of Sm on microstructure and mechanical properties of Mg-Y-Sm-Zr alloys. Then the heat treatments and plastic deformation of Mg-4Y-4Sm-0.5Zr alloy are investigated. And the precipitation sequence and the precipitation strengthening mechanism in Mg-4Y-4Sm-0.5Zr alloy are emphasized. Finally, the high temperature compression and compression creep behavior of extruded Mg-4Y-4Sm-0.5Zr alloy are contrastively studied.Thermo-mechanical treatments containing solution, artificial ageing and extrusion are carried out on Mg-4Y-xSm-0.5Zr (x=1, 4, 8) alloys. Effects of samarium on microstructure and mechanical properties of Mg-Y-Sm-Zr alloys during thermo-mechanical treatments are investigated. The microstructure of as-cast alloys involves Mg solid solution + eutectic compounds. The eutectic phase has the same fcc crystal structure (a=2.246nm) and similar composition to Mg5Sm. But some Y elements are dissolved in it. And there are also some small RE-enriched quadrate phases which have the fcc crystal structure (a=0.5581nm) in the as-cast Mg-Y-Sm-Zr alloys. The eutectic phases in Mg-4Y-1Sm-0.5Zr and Mg-4Y-4Sm-0.5Zr alloys are solutionized into the Mg-matrix after solutionized at 525℃for 8h. There is still large amount of the second phases remained at grain boundaries in the solutionized Mg-4Y-8Sm-0.5Zr alloy for the incomplete dissolution of the samarium. The fcc crystal structure Mg5(Sm,Y) eutectic phase is transformed to the body-centred tetragonal crystal structure Mg41Sm5 phase with little Y element dissolved in it. The strength and elongation of Mg-4Y-8Sm-0.5Zr alloy are low because of the large amount undissolved fragile compounds; the precipitation strengthening effect of Mg-4Y-1Sm-0.5Zr alloy is not evident because of the low Sm content; as a result, the Mg-4Y-4Sm-0.5Zr alloy has the best mechanical properties.The eutectic phases in Mg-4Y-4Sm-0.5Zr alloy dissolve into the matrix and the alloying elements Y and Sm homogenously distributes through out the grains after solutionized at 525℃for 8h. Elongation (EL) greatly increase for the dissolution of eutectic phase and homogenous distribution of alloying elements, and ultimate tensile strength (UTS) increases a little for the solution strengthening effect of Y and Sm. The alloy peak-aged at 225℃has the highest EL. But the UTS and yield strength (YS) are relatively low. The alloy peak-aged at 175℃has the highest hardness, but the ageing time is long, and the UTS, YS and EL are not the highest. And the highest UTS and YS are reached when the alloy peak-aged at 200℃. Very fine scale precipitates form inside the grains and along the grain boundaries when under-aged at 200℃for 16h, resulting of the highest UTS and EL. With increasing the ageing time, The EL of alloy greatly decreases for increasing amount and coalescence of the precipitates along the grain boundaries, and the UTS decreases respectively. The optimal ageing parameter 200℃for 16 hours is chosen for Mg-4Y-4Sm-0.5Zr alloy. The precipitation strengthening of the alloying elements Y and Sm is the main strengthening mechanism in this Mg-4Y-4Sm-0.5Zr alloy. The mechanical properties of the alloy greatly increase after the solution-plus-ageing heat treatment.The UTS of 348MPa, YS of 217MPa and EL of 6.9% are attained after this solution-plus-ageing heat treatment.The precipitation sequence containingβ′,β1 andβis determined in the Mg-4Y-4Sm-0.5Zr alloy during ageing at different temperatures. Theβ′precipitates are formed within grains when the alloy is aged at 200℃for up to 3000h. This intermediate phase is thermally stable at 200℃, and there is no phase transformation occurred at this temperature. The globular shapeβ′precipitates are formed in the under aged state, which is corresponding to the highest strength of the alloy. The precipitates coalesce, and the plate shapeβ′precipitates are formed lying in the {2110 } habit planes with increasing ageing time. Theβ1 precipitates are formed within the grains when the alloy is aged at 250℃for 48h. These plate shapeβ1 precipitates with fcc crystal structure (a=0.74nm) lie in the {1010 } habit planes and in contact withβ′precipitates. Large numbers of plate shape equilibriumβphases with fcc crystal structure (a=2.223nm) precipitate along the {1010 } habit planes when the alloy is aged at 300℃for 13h. These equilibriumβprecipitates are transformed in situ from theβ1 precipitates.The grains of Mg-4Y-4Sm-0.5Zr alloy are evidently refined for the occurrence of DRX (dynamic re-crystallization) during high temperature (350℃and above) extrusion process. The mechanical properties of the alloy greatly increase due to the grain refining strengthening effect. The ageing heat treatment can directly carry out on the extruded alloy, and the precipitation strengthening effect in extruded alloy is a little increased with increasing extrusion temperatures. The UTS of 400MPa, YS of more than 300MPa and EL of 7% are attained after thermo-mechanical treatments.The high temperature compression and compression creep behavior of extruded Mg-4Y-4Sm-0.5Zr alloy are investigated. The strain rate sensitivity m is 0.4 and the activation energy Q is 92.1KJmol-1 when the alloy compressed at a strain rate of 2.5x10-4s-1 and at a temperature of 450℃, indicating that grain boundary sliding (GBS) is the main compression mechanism. The extruded Mg-4Y-4Sm-0.5Zr alloy has good creep resistance at temperature 200℃and at stress 180MPa. The creep activation energy Q is 156.7KJ/mol and the stress exponent n is 4.1 indicating that dislocation climb is the main creep rate controlling mechanism.更多还原