Effect of carbide formation on phase equilibria and compositional modulation of transformation properties in (Mn,Fe)2(P,Si) alloys

Abstract Practical implementation of efficient magnetocaloric effect-based refrigeration and energy conversion applications requires the design of whole alloy families with narrow tolerances on the transformation properties, and therefore composition, of the component alloys. In the promising class of magnetocaloric (Mn,Fe)2(P,Si) alloys, this task of compositional tuning is made especially difficult by the observed co-existence of several P-depleted impurity phases, which for example, increases both the P content and transformation hysteresis losses of the main quaternary phase. In this work, we study the mechanisms that induce impurity phase formation in the Mn–Fe–P–Si system, and investigate the impact of sequential carbide formation on observed phase microstructure along with its effect on the composition and transformation properties of the (Mn,Fe)2(P,Si) phase. Using quantitative analyses to measure the composition within main and impurity phases in samples throughout the bulk alloy space, we establish that (1) repeated processing steps increase the content of a (Mn,Fe)9Si2 carbide phase, resulting in (2) deviations in the magnetocaloric (Mn,Fe)2(P,Si) phase from bulk nominal composition of up to ∼4 at. %. Finally, we map out (3) the dependence of transformation critical temperature, hysteresis, and enthalpy on the composition of the (Mn,Fe)2(P,Si) phase of interest. Together, these results suggest carbon impurities can have a critical impact on magnetocaloric transformation properties in (Mn,Fe)2(P,Si) alloys, since 0.3 wt % carbon content can cause critical temperatures and hystereses to deviate by more than 95 K and 8 K from desired design values.