Modification of Olson–Cohen model for predicting stress-state dependency of martensitic transformation

Abstract

The well-known Olson–Cohen model is modified by incorporating the effect of stress state on the transformation kinetics in metastable austenitic stainless steels. By assuming isothermal condition, the relationships between the parameters of Olson–Cohen model, stress triaxiality and absolute value of Lode angle parameter have been established. The proposed model predicts that the saturation level and slope of transformation curve increase with increasing the stress triaxiality in addition to the absolute value of Lode angle parameter. Uniaxial and plane strain tension tests have been conducted on the two types of austenitic stainless steels AISI 304 and AISI 201 to evaluate the validity of the model. The results show that the predicted transformation curves are in good agreement with the experimental data.

Influence of Strain State on the Kinetics of Martensitic Transformation Induced Plasticity (TRIP) in AISI 304 Stainless Steel

“…The effect of strain state on the martensitic phase transformation. The points denote experimental data and lines are predicted by the modified Olson‐Cohen model …”

“…The change in transformation curves resulting from variation of strain state can arise from the change in the both Lode angle parameter and stress triaxiality η . Figure shows these parameters as a function of strain ratio ρ .…”

“…This is based on the assumption that for a given material and strain rate, n in Equation is constant and β and α only depend on the temperature. However, by regarding the modified Olson‐Cohen model that considers α as a stress state independent parameter, the rate of phase transformation is controlled by two stress state dependent parameters β and n . The fitting of Equations to the obtained experimental data yields = 15, A = 7, β 0 = 0.19, B = 0.237, C = 0.06.…”

“…As mentioned earlier in the paper, the volume fraction of strain induced martensite as a function of strain can be calculated by using Olson‐Cohen model as follows: where n is constant and α and β are parameters depending on strain rate, temperature, and composition. However, Olson‐Cohen model is not sensitive to the stress state, and as such, it has been modified to incorporate the effect of stress state on the kinetics of martensite phase transformation. According to the modified Olson‐Cohen model, quite contrary to the α , two parameters of β and n are functions of stress state, as can be seen in the following equations: where , A , , B and C are constant, is the absolute value of normalized Lode angle parameter and is stress triaxiality.…”

“…However, Kim et al suggested that the influences of both Lode angle parameter and stress triaxiality on the kinetics of martensitic phase transformation are nonlinear. It is mentioned that the lode angle parameter is a measure of magnitude of intermediate principal stress with respect to the major and minor principal stresses . As mentioned above, the influence of this component of stress state on the kinetics of martensitic phase transformation is controversial.…”