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 (Ti60Cu14Ni12Sn4Nb10) 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 inicrometer-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. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.