α phase growth and branching in titanium alloys

ABSTRACT

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-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°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.

KEYWORDS:

• Phase field modelling
• 3d metallography
• solid state branching

Acknowledgements

We - acknowledge support from NSF-DMR-1905844 and NSF-DMREF-1435611 grants. The authors also gratefully acknowledge the use of equipment in Materials Research Facility (MRF) at the University of North Texas. One of the authors, DB, acknowledges the JC Bose Fellowship of the Department of Science and Technology, India and the Raja Ramanna Fellowship. A substantial portion of this work was carried during the visits of DB to the University of North Texas. Part of the work (R. Shi) was supported Laboratory Directed Research and Development programme at LLNL under the project tracking code 18-SI-003 and 20-SI-004. This phase field simulations were also supported in part by the allocation of computing time from the Ohio Supercomputer Center. The authors acknowledge the support of Tushar Borkar, Soumya Nag and Pavani Kami at the University of North Texas and Shanoob Balachandran at the Indian Institute of Science in various aspects of this study. Discussions with Abhik Choudhury of the Materials Engineering Department of the Indian Institute of Science are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work has been supported by the ISES contract awarded to the University of North Texas by the U.S. Air Force Research Laboratory, AFRL contract number FA8650-08-C-5226; Division of Materials Research; Lawrence Livermore National Laboratory, USA.