α phase growth and branching in titanium alloys

The morphology and spatial distribution of alpha (α) precipitates have been mapped as a function of Mo content in Ti-Mo binary alloys employing a combinatorial approach. Heat-treatments were carried out on compositionally graded Ti-<i>x</i>Mo samples processed using a rapid throughput laser engineered net shape (LENS) process. The composition space spans 1.5 at% to 6 at% Mo with ageing at 750°C, 650°C and 600°C following a β solution treatment. Three distinct regimes of α morphology and distribution were observed. These are colony-dominated microstructures originating from grain boundary α allotriomorphs, bundles of intragranular α laths, and homogeneously distributed individual fine-scale α laths. Branching of the α precipitates was observed in all these domains in a manner reminiscent of solid-state dendritic growth. The phenomenon is particularly apparent at low volume fractions of α. Similar features are present in a wide variety of alloy compositions. 3-dimensional features of such branched structures have been analysed. Simulation of the branching process by phase field methods incorporating anisotropy in the α/β interface energy and elasticity suggests that it can be initiated at growth ledges present at broad faces of the α laths, driven by the enhancement of the diffusion flux at these steps. The dependence of branching on various parameters such as supersaturation and diffusivity, and microstructural features like ledge height and distribution and the presence of adjacent α variants has been evaluated.本研究采用组合研究策略，系统表征了钛-钼（Ti-Mo）二元合金中α（alpha）析出相的形貌与空间分布随钼（Mo）含量的变化规律。研究采用高通量激光工程净成形（LENS）工艺制备了成分梯度Ti-xMo试样，并对其实施了热处理：该类试样的成分范围覆盖1.5 at.%至6 at.%的钼含量，所有试样均先经β相固溶处理，随后分别在750℃、650℃与600℃下进行时效处理。研究观察到三种截然不同的α相形貌与分布模式：一是以晶界α异形晶为起源的集簇型显微组织，二是晶内α板束结构，三是均匀分布的细小独立α板条。在上述三类显微结构区域中均观测到α析出相的分支现象，其形貌与固态枝晶生长高度相似，且在α相体积分数较低时该现象尤为显著。该类特征在多种合金成分体系中均有出现，研究人员已对这类分支结构的三维特征进行了分析。采用引入α/β界面能各向异性与弹性效应的相场法（Phase Field）对分支过程进行模拟，结果表明，该分支可由α板条宽面处的生长台阶引发，驱动力源于这些台阶处扩散通量的增强。本研究还评估了分支现象对过饱和度、扩散系数等参数，以及台阶高度与分布、相邻α变体存在性等显微组织特征的依赖关系。