Thermal-expansion anisotropy of orthorhombic martensite in the two-phase (α + β) titanium alloy

Anisotropy of the thermal expansion coefficient (TEC) has been revealed along the axes of the crystal lattice of the α″ titanium martensite in the two-phase (α + β) titanium alloy of grade VT16 (Ti–3Al–5V–4.5Mo, wt %). It has been established by the method of in situ X-ray diffraction analysis that the lattice parameter b of the orthorhombic martensite obtained by quenching from different temperatures decreases upon heating. The TECs along the axes of the crystal lattice of the martensite obtained by quenching from different temperatures have been calculated. It has been shown that the uniaxial extension of bars of the VT16 alloy quenched for the metastable β phase with relative deformations of 0.7, 1, 2, 3, 4, 5, 6, and 8% leads to the formation of the deformation-induced martensite with an axial texture along the b direction of the martensite lattice. In the course of dilatometric studies of the deformed bars, it has been established that there are two temperature intervals (from–100 to +70°C and from 150 to 300°C) with a low TEC. In the first interval, the value of the TEC varies from–2 × 10 –6 to +8 × 10 –6 K –1 and is determined by the volume fraction of the oriented α″ martensite. This Invar effect is one-dimensional and is manifested along the b axis of the martensite.