Accelerated Mn-Al τ-phase formation induced by electric current assisted annealing
The use of electric current assisted processing has shown various effects in metallic materials, including changing the kinetics in solid state phase transformations, formation of precipitates and inducing plasticity during mechanic deformation (electroplastic effect) [1-2]. By exploring these processes, this new degree of freedom can be used for engineering microstructures besides the classical metallurgical variables of temperature, time, pressure and atmosphere. Even though different studies have been reported on the use of electric current assisted processes, the cause and the observed behaviors are dependent on the material system and the understanding of the underlying mechanisms is still not fully understood .
In this work, we perform electric current assisted annealing of the Mn-Al high temperature non-magnetic ε-phase (hexagonal structure) and study the solid state transformation to the ferromagnetic metastable τ-phase (L10-tetragonal structure). For this purpose, a specific setup was developed, which allows for application of electric current and to measure the sample temperature and the electrical resistance in-situ during the annealing process. Through these simultaneous measurements, the shift of phase transition temperature was evaluated by exploiting different electric current densities in DC mode. It was found that the onset temperature for ε → τ transition is shifted to lower temperatures when a certain threshold of electric current density is surpassed. A maximum temperature shift of 120°C was achieved using 75 A/mm2 current density. Higher current densities led to overheat of the sample and to a decomposition of the metastable ferromagnetic phase.
Even though a noticeable change was observed in the transition temperature when an electric current is applied during annealing, magnetic properties were very similar compared to the annealing process without electric current. The phase transition evolution was confirmed by XRD analysis, SEM and TEM were used to evaluate the microstructure. Moreover, DFT calculations of migration barriers in an external electric field were performed, and the relevant formalism for computing current induced forces in L10 phases.