A new β-Ti alloy with good tensile properties by the mixed microstructure characteristics of α phase

Abstract

In this study, the mechanical properties, microstructural characteristics and deformation behavior of a novel high-strength metastable β alloy Ti-6.5Cr-4.5Sn-4.5Mo (wt %) were investigated. By obtaining α phase mixed microstructure with different size, morphology and orientation combinations in β matrix, a good combination of tensile strength σb~1305 MPa and total elongation ɛf~13.4 % was achieved. The microstructural characteristics of α phase consisting of discontinuous thin grain boundary α phase (αGB), lamellar α colonies and acicular αs precipitates were obtained by the solution treatment above β transus for a short time plus subsequent aging treatment after the heavy cold rolling. Detailed TEM analysis shows that it is difficult to deform for acicular αs precipitates due to their fine-scale, leading to a large number of α/β interfaces acting as dislocation barriers, which may ultimately result in an improvement on strength of the alloy. Compared with αs, the αGB and lamellar α colonies are soft, which could produce the large enough deformation and allow dislocations to generate and accumulate within them, so that it is beneficial to improve the mechanical properties of the alloy. Stress-relaxation experiments show that the sample with a uniform and finer distribution of α precipitate has a larger value of mobile dislocation ρm/ρm0 after relaxation, implying that most of the mobile dislocations remains in the sample, which results in the ultrahigh strength meanwhile accepted ductility in the specimen.