【作者】 姚素娟；

【导师】 易丹青； 李旺兴；

【作者基本信息】 中南大学 ， 材料学， 2009， 博士

【摘要】 镁合金作为一种轻质结构材料,其应用领域不断扩展,部分镁合金零件要在一定高温环境下使用,但是目前大部分镁合金在使用温度超过120℃后,力学性能随温度的升高而降低,因此开发新的耐热合金系成为扩大镁合金应用领域的一个关键。论文以Mg-9Gd（5Gd）-3Y和Mg-5Gd-0.5Sc-0.5Mn为研究对象,运用高频耦合等离子发射光谱分析（ICP）,差示扫描热分析（DSC）,光学显微镜（OM）、扫描电子显微镜（SEM）、透射电镜（TEM）观察以及电子束微区分析（EDS）等手段和析方法,系统研究了合金的制备工艺、室温和高温性能以及合金在高温变形过程中的组织演变等。确定了铸造Mg-9Gd（5Gd）-3Y-0.3Zr合金的热处理工艺。研究发现两种合金均适合采用固溶+时效（T6）工艺进行热处理强化,固溶后在峰值时效状态下,均析出半共格的强化相β′,含Gd量高的Mg-9Gd-3Y-0.3Zr合金中β′相的体积分数和密度均高于Mg-5Gd-3Y-0.3Zr合金,25℃时Mg-9Gd-3Y-0.3Zr合金的抗拉强度和屈服强度分别达到314MPa和256MPa,高于Mg-5Gd-3Y-0.3Zr合金的230MPa和167MPa,但Mg-5Gd-3Y-0.3Zr合金室温下的伸长率为9.9%,比Mg-9Gd-3Y-0.3Zr合金的伸长率高了100%,具有更高的塑性,随着温度的升高,两种合金的强度下降缓慢,伸长率提高。研究了合金高温塑性变形过程中应力和应变的关系,结果显示Mg-9Gd-3Y-0.3Zr合金的真应力-真应变曲线属于动态再结晶型,随变形温度的提高,合金发生不同程度的动态再结晶,动态再结晶机制主要是应变诱发晶界迁动形核、孪晶形核和亚晶合并长大。挤压Mg-9Gd-3Y-0.3Zr合金和Mg-5Gd-3Y-0.3Zr合金都适宜采用直接时效（T5）工艺进行强化,时效后合金中均析出了片状过渡相β′,且析出相互成一定角度,交织成网状,β′相与α-Mg基体的关系为:[001]_β′∥[0001]_α,（100）_β′∥（2（?）0）_α,只是Mg-9Gd-3Y-0.3Zr合金中析出相的体积分数和密度高于Mg-5Gd-3Y-0.3Zr合金;挤压Mg-9Gd-3Y-0.3Zr-T5合金25℃时的抗拉强度和屈服强度分别为375MPa和335MPa,伸长率为5.1%,300℃时合金的强度分别为314MPa和284MPa,伸长率则达到了18%;相同温度下,Mg-5Gd-3Y-0.3Zr合金的强度低于Mg-9Gd-3Y-0.3Zr合金,但塑性更好。挤压Mg-9Gd-3Y-0.3Zr-T5合金室温条件下的断裂模式为混合断裂,200℃是延性断裂为主,脆性断裂为辅的混合断裂模式,250-300℃时则为微孔聚集型的韧性断裂;Mg-5Gd-3Y-0.3Zr-T5合金在200-300℃的断裂方式也是微孔聚集型的延性断裂。系统研究了挤压Mg-9Gd-3Y-0.3Zr-T5合金在200-300℃温度范围内的蠕变行为,表明在试验温度范围内合金的最小蠕变速率随温度升高和应力增大而增大,平均应力指数为3.62,蠕变激活能随温度升高而增加,在200-250℃范围内,激活能为85.6 KJ/mol,蠕变机制为伴随管扩散的位错蠕变;在250-300℃范围内,激活能为245.5 KJ/mol,蠕变机制为位错攀移和晶界滑移。蠕变过程中温度对析出相的转变有显著影响。在应力作用下,随着温度的升高,时效析出的过渡相β′粗化或颈缩,β₁在粗化的β′相中,或在β′相的颈缩处形核;当蠕变温度升高到300℃时,β′相和β₁相全部原位转化为平衡相β,并随蠕变时间的延长,β相粗化;应力和时间对第二相的影响比温度的影响小,随蠕变时间的延长和应力的增大,β′相逐渐粗化并有部分转化为β₁相;蠕变过程中在应力和温度的双重作用下,强化相出现动态析出。对铸造和挤压Mg-5Gd-0.5Sc-0.5Mn合金的性能进行了研究,表明铸造和挤压合金分别适合采用固溶+时效和直接时效工艺进行强化,两种状态下合金的强化相均为棒状的Mg和Gd化合物、圆盘状的Mn₂Sc相和β′相;铸造和挤压合金的抗拉强度和屈服强度在250℃以内随温度升高有上升趋势,温度升高到300℃时强度开始下降,伸长率随温度升高而增加,合金强度比Mg-5Gd-3Y-0.3Zr合金低,但塑性更好;蠕变温度为250℃时,随应力增加,挤压合金最小蠕变速率升高,应力低于70MPa条件下,蠕变速率比挤压Mg-5Gd-3Y-0.3Zr-T5合金低一个数量级,说明合金同样具有高温应用的潜力。更多还原

【Abstract】 Magnesium alloy is one kind of light structural metal material,and its application fields extend wider and wider.Some parts of the magnesium alloy are used in elevated temperature.But the service temperature of most magnesium products are not higher than 120℃,because the mechanical properties fall down rapidly when the temperature is increased.It is a key point to develop novel heat resistant magnesium alloy system to expand the application of the alloys.In this thesis,Mg-9Gd（5Gd）-3Y and Mg-5Gd-0.5Sc are investigated.ICP,DSC, OM,SEM,TEM and EDS etc are used to study the manufacture technology of the alloys.The properties of the alloys at room temperature and higher temperature and the structure evolution at the high temperature are studied.The heat treatment technology of cast Mg-9Gd（5Gd）-3Y-0.3Zr alloy is decided. It is found that the solid solution and aging（T6） technology is suitable for both alloys. After the aging and at the peak aging condition,half coherentβ’ phase precipitated in the two alloys.In high Gd content Mg-9Gd-3Y-0.3Zr alloy,the density and the volume fraction ofβ’ phase are higher than that in Mg-5Gd-3Y-0.3Zr alloy.At 25℃the tensile strength and yield strength for Mg-9Gd-3Y-0.3Zr alloy are 314MPa and 256MPa,respectively and higher than that 230MPa and 167MPa for Mg-5Gd-3Y-0.3Zr alloy.Mg-5Gd-3Y-0.3Zr alloy has better plasticity and the percentage elongation of Mg-5Gd-3Y-0.3Zr alloy at room temperature is 9.9%,100% higher than that of Mg-9Gd-3Y-0.3Zr alloy.While the temperature is increased,the strength decreased for both alloys,but the elongations are improved.From the curves true stress to true strain at elevated temperature,the Mg-9Gd-3Y-0.3Zr alloy shows a typical dynamic recrystallization type.The alloy experienced different scale dynamic recrystallizations when the temperature increased. The mechanisms of the dynamic recrystallization of the alloy are nucleation of grain boundary migration under the strain,nucleation of twins and mergence and growth of subgrains.Extruded Mg-9Gd-3Y-0.3Zr and Mg-5Gd-3Y-0.3Zr are all suitable for direct aging（T5） technology.After the aging,there are plate like metastableβ’ phase precipitates in both alloys and the precipitates have different directions forming a kind of net.The orientation relationship between the Mg matrix and theβ’ precipitate is as[001]_β’,//[0001]α,（100）_β’//（2（?）0）_α,But the density and volume fraction of theβ’ precipitates in Mg-9Gd-3Y-0.3Zr alloy are higher than that in Mg-5Gd-3Y-0.3Zr alloy.At 25℃the tensile strength and yield strength for the extruded Mg-9Gd-3Y-0.3Zr alloy are 375MPa and 335MPa,respectively and the elongation is 5.1%.At 300℃the tensile strength and yield strength for the extruded Mg-9Gd-3Y-0.3Zr alloy are 314MPa and 284MPa,respectively and the elongation reaches 18%.At the same temperature,the strength of Mg-5Gd-3Y-0.3Zr alloy is lower than that of Mg-9Gd-3Y-0.3Zr alloy,but the plasticity is better.The fracture mode of extruded Mg-9Gd-3Y-0.3Zr alloy with T5 aging condition is a kind of mixture fracture.At 200℃the plastic fracture is the main mode,but with ductile fracture together as mixture fracture behavior.Between 250 and 300℃the ductile fracture with micropore aggregation occurs.Between 200 and 300℃the fracture mode for Mg-5Gd-3Y-0.3Zr alloy（T5） is also the ductile fracture with micropore aggregation.The creep deformation of Mg-9Gd-3Y-0.3Zr alloy at a temperature range between 200 and 300℃was investigated.It showed that the creep rate of the alloy increased with the increasing of temperature and stress,and the average stress index is 3.62.The creep active energy increases when the temperature is raised.The creep active energy is 85.6 KJ/mol when the temperature between 200 and 250℃.The creep mechanism is dislocation creep associated with tube diffusion.The creep active energy is 245.5 KJ/mol when the temperature between 250 and 300℃.The creep mechanism is dislocation climb and grain boundary slip.In the creep deformation process,the temperature has significant influence on the precipitates transformation.Under the stress,the metastableβ’ becomes coarse or neck shrank.β₁ nucleates in coarsed or at the neck shrankedβ’ precipitates.When the temperature reached 300℃,β’ andβ₁ transformed in situ to the stableβphase.Theβphase becomes coarsen with the creep time becoming longer.The influence of creep stress and time on phase transformation is less than that of creep temperature during creep process.β’ becomes coarse and partially transforms toβ₁ phase.In the creep process,the dynamic precipitation occurred under the conditions of stress and temperature.更多还原