DEPENDENCE OF DEFORMATION BEHAVIOR AND MICRO-MECHANISM OF β MATRIX ON GRAIN SIZE AND ORIENTATION IN TI-7333

The strong interaction between deformation-induced structures (martensite, twinning) results in remarkable work hardening of the metastable β titanium alloy, which provides necessary conditions for plastic/toughness of high strength and toughness titanium alloy. The intrinsic relationship between the deformation behavior and micro-mechanism of β matrix in the fully-β Ti-7333 alloy (Ti-7Mo-3Cr-3A13Nb) and grain size as well as orientation was investigated in the present study, which is assisted by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and micropillar compression test. The results indicate that the predominant deformation mechanism for fully-β Ti-7333 alloy is stress-induced α″ martensitic phase transformation, resulting in the high work hardening rate exhibited by this alloy. The characteristics of tensile stress-strain curve and plastic deformation mechanism of Ti-7333 alloy with different β grain size (45~260 μm) hardly varies at room temperature, but the critical shear stress of stress-induced α″ martensite changed significantly, and the distribution and morphology of α″ martensite are also affected. The larger grain size can inhibit the stress-induced secondary martensitic transformation and the formation of martensite twinning in the initial stage of deformation. Ti-7333 alloy with β grain size of 84 μm not only produces higher work hardening platform in the second stage of plastic deformation, but also has the strongest ability to maintain work hardening. Moreover, the micro-mechanical behavior and micro-deformation mechanism of β matrix are strongly correlated with grain orientation. The stress-induced martensitic transformation is irreversible in [011]β grains, and the parallel martensitic lath can effectively hinder the passage of dislocations, resulting in smooth stress-strain curves and continuous plastic strain hardening. However, stress-induced α″ martensite reversibly transforms into β matrix in [001]β grains, resulting in double yield characteristics of stress-strain curve and super-elasticity under the condition of low strain (≤10%).