Coarsening Kinetics and Evolution of Precipitate Morphology in Ni-Base Gamma/Gamma Prime Alloys

在选定的二元镍合金（Ni-Al, Ni-Ga, Ni-Ge， Ni-Si和Ni-Ti）中，研究了粗化行为与γ素质析出相体积分数的异常依赖关系和析出相的形貌演变。选择这些合金的基本原理是，晶格失配从Ni-Si合金的- 0.00226低到Ni-Ga合金的0.01005高不等。晶格失配的大小对析出相的形貌和排列趋势起着重要的作用。在这些合金中，平衡γ质数体积分数可以变化一个数量级。本研究主要有两个重点：(1)研究大范围沉淀体积分数下的粗化动力学；(2)定量表征γ素质析出相的形貌随粒径、晶格失配和体积分数的变化。实验表征方法包括暗场透射电镜、扫描TEM负片的图像分析软件和测量溶质浓度变化的磁分析。在选定的合金中考察了异常粗化与晶格失配之间的关系。先前对镍铝合金的研究表明，当γ′相的体积分数小于0.05时，单个大相的界面凹化。本研究表征了γ素数析出形态的演化，以确定凹立方形态是否为热力学平衡形态。这对科学理解很重要，因为最近的理论分析表明，凹立方体形状是亚稳态的

9900714ArdellThe anomalous dependence of coarsening behavior on gamma prime precipitate volume fraction and the evolution of the morphology of the precipitates are investigated in selected binary nickel alloys (Ni-Al, Ni-Ga, Ni-Ge, Ni-Si, and Ni-Ti). The rationale for selecting these alloys is that the lattice mismatch varies from a low of - 0.00226 in the Ni-Si alloys to a high of 0.01005 in the Ni-Ga alloys. The magnitude of lattice mismatch plays an important role in the precipitate morphology and the tendency of the precipitates to align. The equilibrium gamma prime volume fractions can be varied by an order of magnitude in these alloys. The research has two principal thrusts: (1) investigation of coarsening kinetics over a wide range of precipitate volume fraction; and (2) quantitative characterization of the morphologies of gamma prime precipitates as functions of particle size, lattice mismatch, and volume fraction. Experimental characterization methods include dark-field transmission electron microscopy, image analyzing software applied to scanned TEM negatives, and magnetic analysis to measure the variation of solute concentration. The relationship between anomalous coarsening and lattice mismatch is examined in the selected alloys.%%%Prior research on nickel-aluminum alloys has shown that when the volume fraction of gamma prime precipitate is below 0.05, the interfaces of individual large precipitates become concave. This research characterizes the evolution of gamma prime precipitate shape to determine whether the concave cuboidal shape is a thermodynamic equilibrium shape. This is important for scientific understanding because recent theoretical analyses suggest that concave cuboidal shapes are metastable.***