【作者】 郑开云；

【导师】 丁文江； 曾小勤； 董杰；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2008， 博士

【摘要】 镁-稀土合金是一类新型的高强耐热镁合金,在航空、航天、国防和汽车工业中具有重要应用前景。本文通过向镁中添加轻、重两种稀土元素,设计了名义成分为Mg–xGd- 2Nd-Zr（x=6,8,11wt.%）的三种镁-稀土合金（以下分别简称GN62K、GN82K、GN112K）。以此为研究对象考察了合金在经历热处理和形变热处理后组织和力学性能,重点探讨了合金组织与力学性能的关系。文中综合运用了各种材料研究方法,包括光学金相分析、扫描和透射电子显微分析、X射线衍射分析、热分析和硬度、拉伸、蠕变力学试验,系统地开展了包括合金中第二相表征、组织演变规律、合金高强度的来源、提高强度和耐热性能的工艺方法、影响力学行为（屈服、变形、断裂、高温蠕变）的组织因素等方面的研究工作,旨在为新型高强耐热镁-稀土合金的开发设计提供必要的实验依据。合金的组织分析主要针对铸造态组织、固溶态组织和时效析出组织。铸造合金中的组织由初生α-Mg固溶体、骨胳状共晶相、小方块相、Zr团簇和晶界棒状相组成,其中共晶相可视为α-Mg+β-Mg5（Gd,Nd）,共晶β相被鉴定为Mg5Gd型FCC结构,晶格常数a～2.22nm。固溶处理时,铸造组织的演变包括三个过程:共晶相溶解、方块相生长和晶粒粗化,其中,方块相具有FCC结构,晶格参数a～0.55nm,是GdH2型化合物。经适当的固溶处理,合金组织中几乎无共晶相残留同时保持合金均匀细晶结构。合金时效析出组织的分析以GN112K合金为例,时效析出过程中过饱和固溶体分解过程分为四个阶段:α-Mg过饱和固溶体→β″(D0₁₉,a～0.64nm,c～0.52nm, {2110}α惯习面薄板)→β′（CBCO,a～0.64nm,b～2.2nm,c～0.52nm,C轴长椭球）→β1（FCC,a～0.74nm,{ 0110}α惯习面板条,与Mg₃Nd同构）→β（FCC,a～2.22nm, {0110}α惯习面板条,与Mg5Gd同构）,四种析出相中,β′相是主要的强化相,对合金强度贡献最为关键。时效析出过程中还伴随晶界结构的演变:β平衡相在晶界的析出和晶界两侧无析出带的形成,这对材料的力学性能十分不利。铸造合金的主要强化组织为共格析出相,通过一系列的试验确定,铸造合金的最佳热处理（固溶+时效）条件分别为: GN62K—500℃/6h+200℃/24h、GN82K—515℃/4h+225℃/12h、GN112K—525℃/4h+250℃/2h。对应的室温拉伸性能分别为:GN62K—TYS～182MPa,UTS～342MPa,E～7.9%;GN82K—TYS～200MPa,UTS～342MPa,E～5.0%;GN112K—TYS～224MPa,UTS～353MPa,E～3.7%。合金在上述热处理状态下还具有优越的高温力学性能,但在温度高于200℃时强度开始大幅下降,高于250℃下降速度进一步加剧。形变热处理可使合金获得多样的强化组织,适当的处理工艺还将进一步提高合金的力学性能。固溶态合金在时效前作室温预变形将在合金中产生大量的位错和孪晶,促进β1析出相形核,因而加速合金时效析出动力学。得益于位错亚结构和孪晶强化,预应变时效态合金的屈服强度远高于未变形时效态合金（增强超过50MPa）,并且少量的预变形不会引起延伸率的明显损伤。合金经热挤压发生再结晶,晶粒细化到10μm或以下。典型热挤压时效态合金的室温拉伸力学性能为:GN62K（350℃挤压+200℃/24h时效）—TYS～273MPa,UTS～381MPa,E～17.6%;GN82K（350℃挤压+200℃/24h时效）—TYS～304MPa,UTS～397MPa,E～10.6%;GN112K（350℃挤压+200℃/24h时效）—TYS～328MPa,UTS～422MPa,E～4.3%。退火态GN62K板材,在450℃轧制可细化组织,加工强化合金,由于轧制后合金中形成基面板织构,轧板力学性能呈现各向异性。终轧GN62K板材时效态（70%压下量+225℃/2h时效）力学性能达到:TYS=330～356MPa,UTS=385～394MPa,E=5～10%。高温蠕变行为是合金的耐热性能的重要标志,本文主要观测了合金在250～300℃温度区间和50～100MPa应力区间的蠕变行为,探讨了合金蠕变机制、动态析出、强化相和蠕变断裂问题。稳态蠕变速率的温度和应力相关性研究表明,合金蠕变表观激活能为110～280kJ/mol,合金蠕变应力指数为～4。合金的蠕变机制是位错攀移+滑移机制。高温蠕变过程中的动态析出造成包括淬火态、欠时效态、峰值时效态和部分过时效态合金蠕变曲线中出现介于初始和稳态蠕变阶段的S形蠕变阶段。稳态蠕变阶段合金中的主要强化相是β平衡相,过时效态合金得益于初始组织中均匀分布的β相板条的强化其蠕变抗力较佳。三叉晶界处楔形微裂纹的形核和长大、三叉晶界和晶界面微孔的形核和长大、晶内微裂纹的形核和长大是合金常见的蠕变断裂机制,其中晶界断裂是最主要的断裂形式。垂直外应力方向的晶界上出现的无析出带的形成促进了合金蠕变时的晶间断裂。更多还原

【Abstract】 Magnesium-rare earth alloys are considered to be a kind of novel high strength and heat resistant magnesium alloys with promising applications in industry. In this paper, Magnesium alloys added with a mixture of heavy and light rare earth elements were designed with nominal composition of Mg-xGd-2Nd-Zr（x=6, 8, 11, wt.%）. The alloys were subjected to heat and thermo-mechanical treatment in order to strengthen them by various structures. The microstructures and their evolution, strengthening mechanism, processing for the performance improvement, microstructure dependence of performance were studied by using optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, thermal analysis and mechanical test of hardness, tension and creep. The primary attempt was to gain knowledge on the relationship between structure and performance of the alloys and provide instructive information for the development of new high performance magnesium-rare earth alloys.The as-cast microstructure of the Mg-Gd-Nd-Zr alloys consists of primaryα-Mg solid solution, skeleton-like eutectic structures, small cuboid-shaped phases and Zr-rich clusters. Eutectic structures can be identified asα-Mg+β-Mg₅RE, and the eutectic phase was characterised to have Mg5Gd-type FCC crystal structure with a～2.22nm. Microstructural evolution of the alloys during homogenisation involves three coinstantaneous processes: dissolving of eutectic phases, growth of cuboid-shaped phases and coarsening ofα-Mg grains. Cuboid-shaped phase in the alloys has a FCC crystal structure with a～0.55nm and is a GdH2-type compound with partial replacement of Gd atoms by Nd and Mg atoms. The alloys could be substantially homogenised by the optimised homogenisation treatment leaving little eutectic phases and relatively fine grains.The decomposition of supersaturated solid solution of GN112K alloy during isothermal aging at 250℃consists of four stages of precipitation transformation as follows: supersaturatedα-Mg solid solution→β″(D0₁₉, a～0.64nm, c～0.52nm, {2110}αplate)→β′（CBCO, a～0.64nm, b～2.2nm, c～0.52nm, ellipsoidal）→β1（FCC, a～0.74nm, {0110}αplate, isomorphous with Mg₃Nd）→β（FCC, a～2.22nm, {0110}αplate, isomorphous with Mg₅Gd）。The evolution of grain boundary structure during precipitation aging is characterized by the formation of GBPs and PFZs, and they grow with prolonged aging time. Among the four types of precipitate phases, theβ′phase acts as the key contributor to the high strength of the GN112K alloy. The existence of GBPs and PFZs deteriorates the mechanical properties and promotes the occurrence of intergranular fracture of the alloy.The cast alloys were primarily strengthened by the precipitates in the microstructure. The optimum heat treatment condition for the cast alloys are as follows: GN62K—500℃/6h + 200℃/24h, GN82K—515℃/4h + 225℃/12h, GN112K—525℃/4h + 250℃/2h, corresponding to tensile properties as follows: GN62K—TYS～182MPa, UTS～342MPa, E～7.9%; GN82K—TYS～200MPa, UTS～342MPa, E～5.0%; GN112K—TYS～224MPa, UTS～353MPa, E～3.7%. The strength lowers quickly as temperature goes above 200℃, which gets more drastic above 250℃, whilst the elongation goes up.Thermo-mechanical treatment was used to produce a variety of microstructures in the alloys, and proper treatment can further improve the mechanical properties. Cold stretching deformation between solution treatment and aging at 200℃creates high density of dislocations and twins. Pre-deformation accelerates the overaging process of the alloy due to heterogeneous precipitation ofβ1 phase at dislocations. The improvement of strength with pre-deformation is attributed to the increased dislocation density, the presence of twins and the heterogeneous precipitation ofβ1 phase in the pre-deformed specimens. Small pre-deformation can improve the yield strength of the alloys with slight damage of the ductility. The grains can be refined to below 10μm by hot extrusion. The typical tensile properties of hot extruded and aged alloys are as follows: GN62K（350℃extrusion+200℃/24h aging）—TYS～273MPa, UTS～381MPa, E～17.6%; GN82K（350℃extrusion +200℃/24h aging）—TYS～304MPa, UTS～397MPa, E～10.6%; GN112K（350℃extrusion +200℃/24h aging）—TYS～328MPa, UTS～422MPa, E～4.3%. Hot rolling can be used to refine the grains of homogenized GN62K plate and strengthening the alloy by work hardening at the same time. The hot rolled plates show isotropic tensile properties. The tensile properties of the rolled plate are as follows: TYS=330～356MPa, UTS=385～394MPa, E=5～10%.The creep behavior of the Mg-Gd-Nd-Zr alloys was studied in a temperature range of 250～300℃and a stress range of 50～100MPa. The activation energies for creep of the alloy were found to be 110～280KJ/mol at temperature range of 250～300℃. The stress exponents for creep were measured to be～4, indicating a dislocation creep mechanism. A sigmoidal creep stage appears between primary and steady-state creep stage in the creep curves for specimens in unaged, underaged, peak aged and slightly overaged conditions. On contrary, there is no evidence of such a creep stage for overaged specimens. The sigmoidal creep stage is induced by the dynamic precipitation during creep. The thermally stable equilibriumβ-phase is regarded as the primary contributor to the creep resistance of the alloy. The substantially overaged alloys show better creep resistance. Creep failure of the alloy is mainly initiated by means of the nucleation and growth of triple point wedge cracking. Cavity nucleation and growth at triple point and cracking along grain boundary facets are also the fracture mechanism. Precipitate free zones at the grain boundaries perpendicular to the applied stress can facilitate the intergranular fracture of the specimens during creep.更多还原