Publications Details

Model for the Softening Factor within Stages of Work Hardening

Jankowski, Alan F.

A formulation for a dimensionless coefficient c_b is derived that represents a scale of microstructural softening for alloys which follow Kocks-Mecking (K-M) work hardening behavior. The variation of the true plastic strain ε_p between the proportional limit σ_y and the strength σ_u at the instability is determined using the Considère criterion. Parameterization of the model is limited to variables expressly measured within tensile experiments. Further development is now made for the softening factor c_bi through the individual and sequential stages 3 and 4 of plastic deformation during K-M work hardening Q behavior. Application is shown for tensile test results of Ti-6Al-4V made by different additively manufactured (AM) processes. It is found that the variation in plastic strain as a function of c_bi produces a continuous curve representative of the alloy system. The results of data analysis indicate that Θ_o3 and c_b3 increase, while Θ_o4 and c_b4 decrease, as the total plastic strain ε_p increases. Furthermore, formulations derived for K-M stages 3 and 4 enable the evaluation of other material parameters such as the activation volume ν* for the onset of plastic deformation. This activation volume is found to be near constant, at a ν*-value of 0.353±0.036 nm³ as computed using a strain-rate sensitivity of strength exponent m of 0.014, irrespective of the AM method used to produce the Ti-6Al-4V alloy.