Fine-grained Ti-Cu microstructures by solid state thermal cycling

Ti-Cu alloys built by additive manufacturing (AM) exhibit equiaxed, fine grains thought to originate from constitutional supercooling during solidification and desirable mechanical properties. But, the observed grain refinement may also be due to phase transformations during solid state thermal cycling experienced during AM. Here we show that solid state thermal cycling, which is inherent to layer-by-layer build processes during AM, results in grain refinement in Ti-Cu alloys, much like that observed in the thermal cycling of steels with eutectoid (i.e., pearlitic) microstructures. If a Ti-Cu alloy's pearlitic or martensitic microstructure is heated above the eutectoid temperature in the solid state, new parent beta-Ti grains will nucleate and grow, resulting in grain refinement and fresh martensite or eutectoid product upon cooling. Thus, solid state thermal cycling is a viable pathway to produce grain refinement in Ti-Cu alloys under AM conditions; it also constitutes a strategy to develop alloys matched to AM processes that achieve grain refinement in the solid state, rather than relying upon grain refinement achieved during solidification.