Nondestructive Evaluation of the Ductile to Brittle Transition Temperature Based on Magnetic Domain Wall Dynamics and Dislocation Dynamics

There has been an increasing interest in development of nondestructive evaluation (NDE) methods to detect the irradiation embrittlement of key components in nuclear reactor. In this paper, a NDE method was developed by testing the temperature (T) dependence of magnetization (M) and damping factor (tan δ) in three different alloys steels. The experimental results show a characteristic temperature in M-T curve and tan δ-T curve, respectively, which coincides well with the ductile–brittle transition temperature (DBTT) measured by Charpy impact test. In M-T curve, the phenomenon is closely related to magnetic domain wall (DW) motion state in the ductile and brittle regions, which is reflected by the motion of dislocations. To explain these observations, a simple model of magnetic DW state in the nanowire with a single defect was built and studied by employing micromagnetic simulation. Simulated results revealed the thermally activated DW depinning and displacement, which can be well characterized by T-dependent magnetization. Furthermore, an analytical model of stochastic dynamical DW was proposed to interpret the T-dependent displacements and magnetization of DW, with some formulae derived for these dependences in an analytical approach. The analytical results show consistence with the simulated data, suggesting that thermally activated DW depinning and motion is responsible for the temperature dependence of magnetization in martensitic steels. The present work is instructive to understand the underlying correlation between embrittlement and thermal dynamics of DW in martensitic steels, and also provides a promising NDE technique to estimate DBTT by tracking the temperature dependence of DW motion and magnetization.