Crystallography and asymmetry of tensile and compressive stress-induced martensitic transformation in metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al

The early stage (strain ranges from 3% to 7%) microstructure evolution of a newly developed metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al under both tensile and compressive deformation at room temperature was investigated contrastively. Although the true strain-true stress curves and the corresponding strain hardening rate (SHR) curves of the samples under tensile/compressive deformation seems to distinct from each other, the dominate deformation product in all the processes proves to be Stress-Induced Martensite (SIM) with an orthorhombic structure. Strong variant selection of SIM happens under both tensile and compressive deformation. And lattice strain formed during β→α’’ phase transformation plays a key role to determine the variant selection process. In the 3% deformed samples (both tensile and compressive samples), the lath-shaped SIM with or without twinned structure is the main deformation product. With the increase of deformation strain, lath-shaped SIM remains to be the dominate deformation induced feature in the compressive samples, while the SIM configuration changes dramatically in the tensile samples. Morphology of SIM varies from grain to grain in the samples with 7% tensile strain, and the ratio of grains which have zigzag SIM with a 30° included angle increases obviously. Zigzag SIM with an approximately 150° included angle was found in the 7% compressive sample in the region near β grain boundaries. The features of these two kind of zigzag SIM were characterized in detail with the help of electron backscatter diffraction (EBSD) technology. Concrete analyses were conducted by employing the phenomenological theory of martensite crystallography (PTMC).