Tensile brittleness and ductility improvement in a novel metastable β titanium alloy with lamella structure

Abstract

The microstructure evolution, mechanical property and deformation behavior of α lamella structure in a novel metastable β alloy Ti–5Al–4Zr–8Mo–7V were studied in this paper. Via different heat treatments, large differences in size, morphology and volume fraction of α lamella were obtained. Tensile tests show that high strength with low ductility is represented after low temperature aging. Tensile brittleness at low temperature aging is originated from precipitation of high fraction nano-scaled α lamella, which induced intensive stress concentration along α/β interface where local shear and plastic instability takes place. The alloy exhibits improved strength-ductility combination after BASCA (β annealing slow cooling plus aging) treatment and high temperature aging. High resolution-TEM results suggest that highly defected substructure and local distortion of (101‾0) planes are exhibited in α lamella. Both the highly defected substructure and finely dispersed α lamella could effectively strengthen the alloy. Improvement of ductility is on account of the increase of free slip length in β matrix where a large amount of dislocation accumulated and compatible deformation between coarse α lamella and β matrix. In addition, deformation twining is generated in coarse α lamella which will increase the ductility as well.