The plastic flow behavior and model of b.c.c. polycrystalline metals with application to pure Ta, Mo, Nb and V

To understand and model the thermomechanical response of b.c.c. polycrystalline metals, a lot of uniaxial compression tests for polycrystalline metals Ta, Mo, Nb and V are systematically investaged. These test results have a wide range of strain rates from 0.001/s to about 8000/s, the temperatures from 77K to 1000K and true strains exceeding 40%. Experimental data display that a very remarkable dynamic strain aging effects occur under quasi-static strain rates below 10 1/s. This implies that plastic flow mechanism is not completely the same as that under higher strain rates. The concept of the kinetics and kinematics of dislocation motion is introduced to obtain the general expression of flow stress. The motion of dislocations and the barriers that they must overcome in their motion is used as an underlying motivation. Following Nemat-Nasser's approach, these parameters can be evaluated by direct comparison with experimental data. Finally, a physically-based general model is introduced into the plastic flow behavior of these bcc materials, but excluding the flow stress under lower strain rates, which there is dynamic strain aging effects. The model predictions are compared with the experimental results. The good agreement between the theoretical predictions and experimental results is obtained.