alpha phase growth and branching in titanium alloys

The morphology and spatial distribution of alpha (alpha) 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-xMo 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 degrees C, 650 degrees C and 600 degrees C following a beta solution treatment. Three distinct regimes of alpha morphology and distribution were observed. These are colony-dominated microstructures originating from grain boundary alpha allotriomorphs, bundles of intragranular alpha laths, and homogeneously distributed individual fine-scale alpha laths. Branching of the alpha 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 alpha. 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 alpha/beta interface energy and elasticity suggests that it can be initiated at growth ledges present at broad faces of the alpha 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 alpha variants has been evaluated.