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Abstract Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 m in a Summit, Greenland (72°35′N, 38°25′W) firn core were subjected to creep tests at −14°C and 0.21 MPa compressive stress to strains of 7%, 12%, 18%, and 29%. Microstructural analyses using thin‐section imaging and microcomputed x‐ray tomography (micro‐CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain‐induced boundary migration (SIBM) and nucleation and growth starting at ∼7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth. 

Rare-earth-free permanent magnet materials based on Mn show great promise for applications in electric motors and devices. The metastable ferromagnetic τ phase of the Mn-Al system has magnetic properties between those of the high-performance Nd-Fe-B magnets and the lower-performance ferrite magnets. However, the hybrid displacive-diffusional pathway of τ formation, from the parent ε phase through the intermediary ε’ phase, is still not fully understood. This phase transformation progression was studied in-situ using diffractive, calorimetric, and magnetometric techniques to show that the progression from ε to τ in Mn54Al46 at <450 ◦C involves the ordering of ε into ε’. Density functional theory calculations were performed on each phase and confirmed the experimental observation that the ε to ε’ to τ pathway is energetically favorable. Isothermal annealing of quenched-in ε at 350 ◦C demonstrated that ε’ is ferromagnetic, also in agreement with theoretical results, with a moderate coercivity of at least 50 kA/m. The τ phase was observed to nucleate along the prior ε phase grain boundaries and grow into the ε’ phase regions. A boundary front of ε’ was observed between the τ and ε phases. Both Kissinger and Flynn-Wall-Ozawa methods were used to determine the activation energies for the ε’ and τ phase transformations with values of ~140 kJ/mol obtained for both phases. Therefore, the ordering transformation to ε’ and the hybrid displacive-diffusional transformation to τ were shown to overcome the same magnitude energy barrier. Both activation energies were less than previous τ phase activation energies measured on Mn55Al45 in the absence of a significant ε’ ordering exotherm, providing a kinetic benefit to the ε to ε’ to τ pathway at 350 ◦C. The results of this study give insight into the phase transformation of L10 binary materials as well as materials that undergo a disorder–order transformation followed by displacive shear.