Strain rate and mechanical stability in determining deformation behavior of beta Ti alloys

The effect of strain rate on flow behaviors of three beta Ti alloys with different beta phase stability was studied. The alloys were subjected to compression deformation at four strain rates ranging from 7 x 10-5 to 7 x 10-2 s1 at room temperature. Microstructure evolution was characterized using laser scanning microscopy and electron backscattered diffraction methods. The effect of strain rate on the yield strength was found to vary between the alloys increasing with increased stability. The microstructural observations showed that the alloys exhibited different deformation mechanisms: deformation induced martensite formation, twinning and slip band forma-tion. The strain hardening exponent (n) depended on the deformation mechanism, i.e., the stability. In the low and intermediate stability alloys showing martensite formation and combination of the deformation mechanisms respectively, n was about 0.07 independent of strain rate. In the alloy with the highest stability it was only 0.01 when the deformation mechanism was dislocation slip at low strain rates but increased significantly to approximate to 0.13 at high strain rates when also twinning occurred. The strain rate sensitivity factor (m) was about 0.014 and 0.017 for the low and intermediate stability alloy respectively independent of strain, but very low approximate to 0.002 for the highest stability alloy though increasing to approximate to 0.011 with increasing strain. Based on the experiments, the stability-strain rate-mechanism diagram was introduced to predict the effect of phase stability and strain rate on the deformation mechanism of beta Ti alloys.