Asymmetry of microstructure and mechanical characteristics in metastable Ti–10V–2Fe–3Al alloy under tension and compression

The microstructure evolution of stress-induced martensite (SIM) \({\alpha }^{\prime\prime}\) and mechanical twins in metastable \(\beta\) Ti-1023 alloy under different strains during tensile and compressive deformation was investigated by the help of X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results demonstrate that there is no obvious SIM \({\alpha }^{\prime\prime}\) stress platform in the compressive stress–strain curve; however, it exhibits a higher yield stress and strain hardening capability than tensile deformation although SIM \({\alpha }^{\prime\prime}\) and \({\alpha }^{\prime\prime}\) martensite twinning are the major deformation products that appeared in metastable \(\beta\) Ti-1023 alloy under both tensile and compressive loading. In fact, SIM \({\alpha }^{\prime\prime}\) is preferentially activated in well-oriented \(\beta\) grains at the beginning of deformation, and those activated variants have the largest phase transformation strain along the loading direction affected by the loading stress state. As the accumulated deformation strain increases, SIM \({\alpha }^{\prime\prime}\) is reoriented to form a martensite co-deformation region with twin structures through {111}_α″ type I and <211>_α″ type II twinning systems. Then, the primary lath \({\alpha }^{\prime\prime}\) martensite is consumed, resulting in the development of the {130} <310>_α″ and {110} \(<\!110\!>_{{\alpha }^{\prime\prime}}\) deformation twinning inside the martensite. In comparison with tensile deformation, compressive deformation produces a higher volume fraction of SIM \({\alpha }^{\prime\prime}\) and \({\alpha }^{\prime\prime}\) martensite twins in \(\beta\) grains, along with a higher density of dislocations in the \({\alpha }^{\prime\prime}\) martensite and the coordinated deformation region of the martensite. This results in a higher work hardening ability of the alloy under compression and a marked asymmetry of mechanical characteristics compared to tensile deformation.