A MESOSCALE STUDY ON THE THERMODYNAMIC EFFECT OF STRESS ON MARTENSITIC-TRANSFORMATION

The effect of stress on martensitic transformation (MT) is addressed with special emphasis on the mechanical driving force (MDF) for triaxial stress states. The mechanical driving force appears to be additional to the chemical driving force in thermodynamic transformation conditions derived from a Gibbs free energy formulation for stressed solids undergoing MT. The thermodynamic criterion of Patel and Cohen predicts the change in martensite start temperature if MT occurs in a stress field. This criterion is extended from uniaxial to triaxial stress states and is discussed in the light of emerging microstresses. As a source of microstress, the elastic anisotropy of single crystals is taken into account. Its influence on the martensite start temperature is investigated by mesomechanical finite-element modeling. The critical stress for the start of transformation occurring in a stress field is calculated from a transformation condition and compared with results based on statistical theories for stress-assisted nucleation. In the context of martensitic transformation and mechanical effects, the MDF accounts for the Magee effect. The range of temperature for which the Magee effect has an influence on the macroscopic deformation behavior of a specimen is determined in dependence of the level of uniaxial applied stress. Finally, a constitutive equation in incremental form for transformation-induced plasticity (TRIP) derived within the continuum-thermodynamics framework is suggested.