The elastic behavior of Ti–40 wt % Nb single crystal near martensite start (Ms) temperature was studied using a resonant ultrasound spectroscopy measurements and the first principle calculation. The experimental results showed that both c′ and cs shear moduli tended to soften as the temperature decreased from room temperature down to and across the Ms temperature of 267 K, suggesting that both the {112}⟨111⟩ and {110}⟨110⟩ shears were operating during martensitic transformations of disordered bcc to orthorhombic structure. The consideration of detailed atomic mechanisms involved in the shear sequences and the calculation of the energy of intermediate state suggested that the {110}⟨110⟩ shear was operating as primary shear and the {112}⟨111⟩ shear operating as a secondary shear to complete the transformation. The measurement further showed that the anisotropy constant, A, was rather increasing at the transition temperature approaching Ms, similarly to the transformation of ordered bcc (B2) to orthorhombic (B19) martensite.

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