Ultrafine composite microstructure in a bulk Ti alloy for high strength, strain hardening and tensile ductility

Ultrafine-grained (UFG) or nanostructured alloys usually lack the strain hardening capability needed to sustain uniform (tensile) deformation under high stresses. To circumvent this problem, we have designed a multi-phase composite microstructure in a Ti-based UFG alloy. The multi-component composition (Ti ₆₀Cu₁₄Ni₁₂Sn₄Nb₁₀) was chosen such that upon chill casting of the alloy the liquid underwent a metastable eutectic reaction, forming an in situ composite made of a micrometer-sized dendritic Ti-based solid solution intermixed with a UFG eutectic matrix. Such a microstructure imparts a high strength in excess of those of commercial Ti alloys, and, more importantly, allows strain hardening at relatively high rates. As a result, uniform elongation in tensile deformation was observed at high flow stresses. We present extensive microscopy results to illustrate the dislocation pile-ups and the origin of the high strength, as well as the extensive dislocation interactions and interface crossing responsible for the obvious strain hardening sustained to large plastic strains.