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Timothy Brown1 Patrick Shamberger1

1, Texas A&M University, College Station, Texas, United States

Hysteresis associated with the non-diffusive phase transformation in magnetocaloric (Mn,Fe)2(P,Si) alloys creates substantial energy dissipation undesirable for cooling applications. Although reduced hysteresis has been achieved in this system through tuning of Mn/Fe and P/Si site occupancies, a deeper understanding of the underlying mechanisms necessary for designing low-hysteresis materials across a wide range of critical temperatures has proved elusive. A successful and general mechanistic theory relating hysteresis to lattice matching has been developed for similar transformations in thermoelastic martensites, dependent on both the change in lattice parameters and the orientation relationships between the parent and daughter crystal lattices. Despite the theory’s success and generality, the parent-daughter orientation relationships in (Mn,Fe)2(P,Si) have not yet been established, and the lattice matching theory has not yet been tested against hysteresis in this system. In this work, we establish the orientation relationships for basal and prismatic planes in hexagonal (Mn,Fe)2(P,Si) alloys through two independent experiments: (1) comparison of temperature-dependent x-ray diffraction pole figures below and above the phase transition and (2) electron backscatter diffraction orientation mapping of adjacent grains of coexisting magnetic parent and non-magnetic daughter phases. Afterwards, we combine these orientation relationships with experimental measurements of lattice parameter discontinuities in order to calculate the lattice mismatch parameter λ2 for several representative alloys in the (Mn,Fe)2(P,Si) system. Finally, the calculated mismatch is correlated with the samples’ hysteresis as measured from calorimetry experiments and compared with the predictions of the theory, thereby establishing whether lattice matching effects may also be used to control hysteresis in (Mn,Fe)2(P,Si) alloys’ phase transitions.

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