镁稀土合金蠕变过程中析出相变体与晶界无析出区的选择性长大机制

Creep-resistant property is one of the most important service performance of the materials for application at high temperature, and it is determined to a large extent by the microstructure evolution and thermo-stability of the alloys during creep tests, which is always a key issue in material research fields. Mg-RE based alloys are new promising low-density materials with good creep-resistance at high temperature, however, their microstructure evolution mechanism during creep is still not clarified yet. With regard to the new phenomenon that we found in recent previous research, i.e., selective variant growth of initial precipitates and selective formation of grain-boundary precipitate free zone (GB-PFZ) during the creep of Mg-2.5Gd(at.%) alloy, we aims to, in this project, select three binary alloys, i.e., Mg-Nd，Mg-Gd and Mg-Y, as research objectives, to obtain three kinds of different initial precipitate morphologies by aging treatment. Then, these alloys will be done creep tests under 3 applied stresses at 250 and 300℃ respectively, and the crept microstructure of the samples with extending creep times will be systemically and deeply analyzed and characterized by TEM, EBSD,HAADF-STEM and 3DAP methods, so as to clarify the effect of composition, applied stress and temperatures on the selective variant growth of initial precipitates and selective formation of grain-boundary precipitate free zone. After that, by use of diffuse-interface phase-field model with the total energy equation coupling the applied stress field, eigenstrain energy and interfacial energy, we aim to reveal the physical mechanism of selective variant growth of initial precipitates and selective formation of GB-PFZ during Mg-RE based alloy’s creep.

抗蠕变性能是评价合金材料高温服役性能的重要指标，而微观组织在蠕变过程中的演化行为和机制对合金的蠕变性能几乎起着决定性的作用，在材料研究中占有重要地位。镁稀土合金是新型有潜力的轻质耐高温材料，但其微观组织在蠕变过程中的演变机制尚不清楚。本项目针对前期研究所发现的Mg-2.5Gd(at.%)合金存在析出相和晶界无析出区在蠕变过程中发生选择性生长或演化的现象，拟选择Mg-Nd，Mg-Gd和 Mg-Y系三种模型合金，获得不同的初始析出相形貌，进行250-300℃ 温度范围三种应力下的蠕变试验，并选择不同蠕变阶段的试样进行深入的微观特征分析和表征，探明成分、温度和应力对析出相和晶界无析出区选择性生长或演变的影响规律，然后采用基于“界面能-弹性应变能-外应力场”耦合的能量模型的扩散界面相场模拟等方法，揭示析出相变体和晶界面无析出区选择性演化的物理机制，为高温镁稀土合金开发提供理论指导。

蠕变性能是评价合金材料高温服役性能的重要指标，而微观组织在蠕变过程中的演化行为和机制对合金的蠕变性能几乎起着决定性的作用，具有极高的科学研究价值。镁稀土合金是新型有潜力的轻质耐高温材料，但其微观组织在蠕变过程中的演变机制以及对于蠕变性能的影响尚不清楚。.本项目针对前期研究所发现的MgGd合金存在析出相和晶界无析出区在蠕变过程中发生选择性生长或演化的现象，设计并制备了三种模型合金：Mg-2.5Gd-0.1Zr(at.%)、Mg-3.3Y-0.1Zr(at.%)、Mg-0.5Nd-0.1Zr(at.%)合金，得到各自的最优热处理制度，并对室温/高温拉伸以及蠕变性能进行了测试，对三种合金的基本性能有了较为全面的了解。.通过对蠕变后样品进行HAADF-STEM表征观察到MgGd、MgY合金中皆存在析出相变体选择生长的现象，结合计算模拟结果，认为其本质原因是外加应力的应变场可与特定位向的析出相变体应力场抵消，使其畸变能降低，而位错可使得这种变体优先形核，最终导致该种变体选择性生长。.通过准原位蠕变实验探明了不同类型无析出区的演化关系，提出了位错-扩散耦合作用的蠕变中PFZ演化模型：PFZ积累取向差并最终形成条带状亚晶是位错运动的结果，是一种释放晶界应变集中，协调变形的机制；而A型PFZ向B型PFZ的转变是空位/溶剂扩散导致的，会在蠕变后期引起孔洞导致裂纹萌生。.研究结果表明：开发耐热镁合金需要兼顾基体和晶界的高温强度，提升晶内析出相以及晶界结构的高温稳定性对于提升高温性能有至关重要的作用。