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

On the basis of the -Mo-Ti-Mo- linear unit along the specific <111>(beta), <110>(beta), and <100>(beta) directions, the cluster structures of alpha ''-martensite and omega-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}(beta) plane along <110>(beta) direction decreased, leading to change in the crystal structure of martensite from hexagonal close-packed to orthorhombic structure; the displacive collapse degree of {112}(beta) plane along <111>(beta) direction decreased, indicating that the crystal structure of omega-phase transited from hexagonal to trigonal structure. The softening effect of tetragonal shear elastic constant (C') and Young's modulus (E-100) was favorable for the shuffle and shear components of alpha ''-martensite, respectively, whereas that of shear modulus (G(111)) was beneficial to the collapse component of omega-phase. The competition among C', E-100, and G(111) affected the phase transformation following the sequence of hexagonal close-packed alpha'-martensite, orthorhombic alpha ''-martensite, hexagonal omega-phase, and trigonal omega-phase in metastable titanium alloys. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.