Influences of grain size on the deformation behavior of a twinning-induced plasticity metastable β titanium alloy

The influences of grain size on the deformation behavior of a metastable β-type Ti–12Mo (wt%) alloy has been investigated by using uniaxial tensile testing, scanning electron microscopy and electron backscatter diffraction. The results reveal that the twinning-induced plasticity (TWIP) effect mediated by {332}<113>_β twins occurs in this alloy when its mean grain size (d₀) ranges from 49 to 104 µm, leading to pronounced work hardening behavior and a large uniform elongation (> 25%). Furthermore, there is a Hall-Petch relationship between the yield strength and d₀ for this d₀ range, with a Hall-Petch coefficient higher than that expected for pure slip deformation. However, when this alloy is in a state with heterogeneous grain structures and smaller grain sizes (d₀ ∼ 30 µm), only a very limited volume fraction (< 1.8%) of {332}<113>_β twins occurs and these twins prefer to nucleate in large grains (> ∼40 µm), but they can propagate into rather small grains (< ∼25 µm). In such a state, the Ti−12Mo alloy has a high yield strength (1025 MPa) but does not exhibit any sign of work hardening, i.e., the TWIP effect is absent. Based on the α"-assisted {332}<113>_β twinning mechanism, the effects of grain size on the {332}<113>_β twinning and yield strength are discussed in terms of its role in affecting the stress-induced β→α" martensitic transformation.