Microstructure and tensile ductility in a β heat treated titanium alloy

Abstract

The nature of tensile failure in a β-treated titanium alloy, Ti-6Al-l.6Zr-3.3Mo-0.3Si, is demonstrated to be strongly dependent on cooling rate following β treatment. A characterization of microstructure, the fracture surface of tensile tested samples, and the associated slip behavior shows the fracture mode to be determined by the microstructure as well as composition of the α laths. A martensitic matrix with fine grain boundary α particles leads to intergranular failure. At slower cooling rates, α laths form as side plates from grain boundary allotriomorphs, and an fcc interface phase is present at the α lath/retained β interfaces. This microstructure results in transgranular fracture caused by ductile failure at the α lath/β interfaces within the grains rather than at grain boundary α. If the composition is equilibrated across the α laths by a long-term anneal following β treatment, the slip mode becomes extremely planar leading to slip band related cracking across the grains.