Conditions of Invar-Type Effect Origin in Martensitic Titanium Alloy

Negative 1D  thermal expansion or Invar-type effect was observed in a number of materials Cu–Zn(Mn)–Al, Ni-Ti alloys and Gum metals (Ti-alloys). It is mostly related to the shape memory effect wherein reverse transformation of textured orthorhombic martensite takes place or to “strain glass” formation during the cold working with significant reduction ratios. However, since the Gum metals are near β alloys and the volume fraction of strain induced (SI) martensite after deformation is small, its effect on temperature coefficient of linear expansion (TCLE) is rather negligible. Herein, we investigate commercial martensitic alloy Ti-3Al-5Mo-5V(mass.%) rods with strong axial [110]_β  texture quenched from critical temperature to α+β_metastable structure.  Following cold tensile strain with reduction ratios of 0; 0,7; 1; 2; 3; 4 and 8% resulted in SI  β / α′′ transformation with [010]_α′′//[011]_β lattice correspondence. The volume fraction of martensite textured along [010]_α′′ depends on reduction ratio. Due to the special aspect of lattice constant b_α′′ particularly in extension during cooling, which was shown earlier in orthorhombic metals, we can obtain desired TCLE from 8*10^-6 K^-1 to -2*10^-6 K^-1 in a temperature range -140…+70°C. Although yield strength varies from 150 MPa for quenched rod up to 900 MPa for strained rod with reduction ratio of 8%, the tensile strength exceeds 950 MPa for all presented treatments. The data on XRD, EDBS analysis, metallography and crystallographic interpretation of phenomenon are presented.