Novel insight into the formation of α″-martensite and ω-phase with cluster structure in metastable Ti-Mo alloys

Abstract

On the basis of the “-Mo-Ti-Mo-” linear unit along the specific <111>β, <110>β, and <100>β directions, the cluster structures of α″-martensite and ω-phase were constructed in metastable Ti-Mo alloys to examine phase stability, elastic property, and crystal structure evolution by first-principles calculations combined with experimental analyses. With the increase in Mo content, the orthorhombicity and shuffle magnitude of {110}β plane along <110>β direction decreased, leading to change in the crystal structure of martensite from hexagonal close-packed to orthorhombic structure; the displacive collapse degree of {112}β plane along <111>β direction decreased, indicating that the crystal structure of ω-phase transited from hexagonal to trigonal structure. The softening effect of tetragonal shear elastic constant (C′) and Young's modulus (E100) was favorable for the shuffle and shear components of α″-martensite, respectively, whereas that of shear modulus (G111) was beneficial to the collapse component of ω-phase. The competition among C′, E100, and G111 affected the phase transformation following the sequence of hexagonal close-packed α′-martensite, orthorhombic α″-martensite, hexagonal ω-phase, and trigonal ω-phase in metastable titanium alloys.