Unravelling the dynamics of stress relaxation: a deep dive into activation volume analysis in martensitic steels

In this work, we investigate the stress relaxation behavior of 9% Cr-steel P91 through a comprehensive analysis of activation volumes using various evaluation methods. Experimental stress decay data were obtained from deformation dilatometer tests conducted at 650°C under a compressive stress of 200 MPa, with five repeated relaxation cycles. To determine activation volumes, we employed established evaluation methods (including the logarithmic relaxation law and the tangent method), and also developed a novel approach by combining the Orowan equation with Hooke’s law. Whereas traditional evaluation concepts partly lack a physical interpretation and oversee microstructural changes during relaxation, both a new variant of the tangent method and our new relaxation model (related to Hooke and Orowan) provide more accurate insights into the size of the activation volume and into dislocation evolution. The resulting activation volume turned out to be 45-55 times the atomic volume. In addition to producing simulated relaxation curves with high accuracy compared to experimental data, the new relaxation model revealed a gradual reduction in mobile dislocation density, accompanied by a less pronounced stress decay within each cycle. The relaxation model yielded an initial dislocation density of 10 13 to 10 14 m -2 and predicted an annihilation down to only 15-20% of the start value after five cycles of relaxation. Our findings underline the critical role of considering a material’s microstructural condition for its relaxation behavior.