Limit of cold deformation in a near β-Ti alloy: role of α′ phases on the failure of β phase during cold rolling

Recently, severe plastic deformation of alloys under cold conditions has been experimentally investigated to develop refined microstructures. The present study proposed to demonstrate the feasibility of applying cold deformation to a β-solution treated near β-Ti alloy. To be specific, the evolution of microstructure and mechanical properties of a β-solution treated near β-Ti alloy subjected to unidirectional 20% cold rolling were investigated. Results showed that long cracks formed in the 20% cold rolled sample. Meanwhile, two types of deformation-inducted α′ martensite have been observed in cold-rolled β matrix, namely, acicular α′ martensite with an orientation relationship of (12¯10)α′//(111)β with β matrix, and deformed (irregular shape) α′ martensite without coordinating orientation relationship. The formation of cracks was mainly related to the pre-existing α′ plate, and the micro-crack was prone to occur in α′ plates and propagate to β matrix. The microstructure characteristics result in poor ductility and low ultimate strength in the cold-rolled sample. Lastly, the lattice distortion and {101¯2}1¯011 α′-twinning were also characterized in the deformed α′ plates.