Dynamic mechanical properties and microstructure of Ti-6Al-7Nb biomedical alloy as function of strain rate

The deformation behaviour and microstructural evolution of Ti-6Al-7Nb biomedical alloy under high strain rate conditions are investigated using a compressive split Hopkinson pressure bar. Dynamic compression tests are performed at room temperature under strain rates ranging from 10₃ to 3 × 10³ s^-1. Experimental results show that the flow stress, workhardening coefficient and strain rate sensitivity of the Ti-6Al-7Nb alloy all increase with increasing strain rate. However, the activation volume decreases with increasing strain and strain rate. Moreover, it is shown that the high strain rate deformation behaviour of the Ti-6Al-7Nb alloy can be adequately described using the combined Johnson-Cook and Zerilli-Armstrong constitutive equation. The undeformed and deformed microstructures of the Ti-6Al-7Nb samples are found to consist of equiaxed primary a phase and transformed β phase. The size and volume fraction of the a phase increase with increasing strain rate. Moreover, the dislocation density also increases with increasing strain rate, thereby resulting in a significant strengthening effect.