Strain hardening of titanium: Role of deformation twinning

The quasi-static mechanical response of high purity, polycrystalline α-titanium is investigated in terms of the underlying deformation mechanisms that govern its macroscopic behavior at room temperature. Constant strain rate tests were conducted on this material in simple compression, plane-strain compression and simple shear, and the true stress (σ)-true strain (ϵ) responses were documented. From the measured data, the strain hardening rates were numerically computed, normalized with shear modulus (G), and plotted against both normalized stress. These normalized strain hardening plots exhibited three distinct stages of strain hardening, Stage A with falling strain hardening rate, followed by stage B with increasing strain hardening rate, and finally stage C characterized by a falling strain hardening rate.; The microstructure evolution has been characterized using both optical microscopy and orientation imaging microscopy at different strain levels. It was found that the onset of deformation twinning correlated with an increase in strain hardening rate in compression tests. In shear tests, a much lower rate of strain hardening was found, at all strains, and this correlated with a lower density of deformation twinning.; The increase in the strain-hardening rate was found to match quantitatively with Hall-Petch hardening mechanism. The new twin boundaries appear to act as barriers to dislocations and reduce the effective slip length in the matrix. Moreover, Microhardness measurements indicated that twinned regions are harder than the untwinned areas, enhancing the increase in the strain hardening rate, stage B. On the other hand, the observed increase of twin volume fraction at low and moderate strain levels was observed to saturate at high strain levels explaining the falling strain hardening rate in stage C. The saturated twin volume fraction is believed to be a result of reorienting twinned regions into new orientations, favorable for slip, by twin transformation.; Conveying the insights from the experiments, new functions for slip hardening and twin hardening were introduced to a Taylor type crystal plasticity model. The model was able to predict the anisotropic strain hardening behavior and texture evolution. The predictions were in agreement with the experimental measurements.