Overcoming the limits of strain-induced martensitic transformation in metastable face-centered cubic alloys

Abstract

Metastable phenomena are ubiquitous and have enabled substantial property enhancement for metallic alloys. Amongst them, martensitic phase transformations activated by plastic straining are considered as one of the most effective pathways to promote strength while preserving desirable ductility. The resultant transformation-induced plasticity effect has enabled great success in advancing steels, titanium alloys, and more recently complex concentrated alloys design. However, an intrinsic dilemma is still hindering the development of these metastable alloys: the limited plastic strain accommodation capability of the martensite often leads to early-stage damage nucleation.

This thesis builds upon the objective to overcome such an intrinsic dilemma and explores potential microstructural design guidance with the aids of in-situ experiments and theoretical calculations. Two categories of approaches, respectively focusing on phase transformations and plastic deformation micro-mechanisms are explored. Specifically, plastic strain-induced sequential martensitic transformation and thermally-driven martensite reversion are recognized to exhibit the potentials to further improve the mechanical properties of metastable alloys. In light of the atomistic processes of strain-induced face-centered cubic (FCC) to hexagonal close-packed (HCP) martensitic transformation, a plastic deformation-driven stacking fault formation concept is also assessed, which contributes to latent strain hardening while mitigating the formation of blocky HCP-martensite. Future suggestions for metastable alloy design are also proposed based on the current experimental and theoretical understandings.