Mechanisms of martensite nucleation at grain boundaries

Abstract

Martensitic transformation is a nondiffusive phase transition, in which strain plays the role of the order parameter [1, 2]. In many materials such as steels and iron-based alloys, martensitic transformations are phase transitions of the first order, which are characterized by a small activation barrier and, in the limiting case, can even be athermal. In some alloy systems, the martensitic transformation is a phase transition of the first order that is close to the second-order process, and is accompanied by a decrease in the elastic modulus [2, 3]. It was suggested that certain precursor states (significantly reducing the barrier for nucleation) in the lattice could play the role of subcritical nuclei for such transformations, while defects could be the centers for the local loss of stability of the lattice [3]. The question concerning the character of nucleation of martensite crystals has not yet been given a final answer. Most researchers believe that the martensite phase nucleates via a heterogeneous mechanism. It was suggested that dislocations, stacking faults, grain boundaries, and other structural defects are preferential sites for the nucleation of martensite. Elastic stresses related to the structural defects favor the nucleation process. In addition, dislocations can serve as boundaries of the growing crystalline phase. Numerous investigations were devoted to the nucleation of martensite on separate dislocations, stacking faults, dislocation loops, and various dislocation ensembles [4‐9]. However, experiments [10] did not reveal any influence of dislocations on the nucleation of martensite, while showing evidence for a facilitated nucleation at grain boundaries of certain types. Ueda et al. [11] studied the martensitic transformation in Fe‐Ni bicrystals, established that twinned martensite crystals could nucleate via a self-accommodation mechanism at the tilt boundaries, and calculated the elastic energy of such crystals with accommodated transformation strains. A molecular-dynamics simulation of the bcc-to-hcp martensite transformation in zirconium [12] also showed the possibility of martensite nucleation at the grain boundary, which was accompanied by the appearance of anomalies in the phonon spectrum. This paper is devoted to an analysis of the two possible mechanisms of martensite nucleation at grain boundaries in the alloy systems (such as iron-based alloys) without a decrease in the elastic moduli. Thermodynamics of the martensite nucleation via selfaccommodation across grain boundaries and the nucleation at nonequilibrium boundaries and their contacts with disclination stress fields are considered. Homogeneous and heterogeneous nucleation of martensite. It is assumed that a martensite nucleus has the same platelike shape as a macroscopic crystal, with a small ratio of thickness h to the lateral dimensions l and L , and is typically oriented along the transformation invariant plane with the unit normal vector n . In the absence of strain sources, the free energy of such a martensite plate is described by the following expression