Effect of Basca Treatment on Multiscale Microstructures Evolution and Tensile Properties of High-Strength Near Β Ti Alloy

Abstract

The influences of cooling rates from β zone to target temperatures and the corresponding holding temperatures during BASCA heat treatment on multiscale microstructures evolution and tensile properties of near βTi alloy were investigated in this work. The results showed that amounts of α colonies consisting of lamellar α with large size and aspect ratio were formed during the slow cooling and isothermal holding stages, leading to great precipitation hardening effect. Compared with lamellar α phase, the fine secondary α precipitates formed during aging stage were more efficient to strengthen alloy, which could produce larger increment in hardness and strength of alloy. The increasing rate of hardness during heating process was closely dependent on the holding temperatures, and the difference in hardness and strength between various cooling rates was gradually reduced with the holding temperature raising. This was resulted by the phase transformation kinetics changed the evolution of multiscale microstructures features. With the holding temperature decreasing, the volume fraction of lamellar α phase rising and the fine secondary α precipitates dropping. When it was below 750 o C, typical basket microstructures could be obtained after aging treatment. During tensile deformation the lamellar α phase could accommodate larger degree of strain than those fine secondary α precipitates, resulting in higher fracture elongation of alloy. A balanced combination of strength and ductility was achieved after aging under the cooling rate of 10 o C/min and holding temperature of 800 o C condition, and the yield strength, ultimate tensile strength, elongation and area reduction were 1431 MPa, 1510 MPa, 5 % and 5.8 %, respectively.

Keywords: Titanium alloy, BASCA heat treatment, Microstructures evolution, Mechanical properties