Relaxation and crystallization in MnCo_0.8Fe_0.2Ge₁₋_ySi_y alloys: Implications for phase control

Controlling phase stability and transformation kinetics in MnCoGe-based alloys is critical for optimizing their functional performance, particularly in magnetocaloric and related applications. In this work, the devitrification behavior of mechanically amorphized MnCo_0.8Fe_0.2Ge₁₋_ySi_y alloys (y = 0.2–0.6) is investigated to establish composition–processing–microstructure relationships enabling such control.

Fully amorphous states are achieved only for Si-rich compositions (y ≥ 0.4), providing a suitable starting point for designing homogeneous microstructures. Upon thermal treatment, crystallization pathways are strongly composition-dependent: alloys with y ≤ 0.4 develop single-phase austenite, whereas higher Si content (y = 0.6) promotes austenite–martensite coexistence, which is particularly relevant for tuning functional responses linked to phase transformations.

A key practical challenge identified is the overlap between structural relaxation and crystallization during DSC analysis, which can lead to significant errors in kinetic parameter estimation. This issue is effectively mitigated through a tailored pre-treatment that suppresses relaxation without altering the structural state, enabling robust kinetic characterization.

Activation energies obtained from Kissinger and Gao–Wang approaches reveal a maximum at intermediate Si content (y = 0.4), indicating an optimal composition for thermal stability. KJMA analysis shows that crystallization proceeds via three-dimensional diffusion-controlled growth with constant nucleation, providing a predictive framework for microstructure design. Deviations from ideal behavior are linked to residual transformed fractions at low Si and refined crystal size at high Si.

These results demonstrate that Si content is a key parameter for tuning crystallization pathways and phase constitution, offering practical guidelines for designing MnCo-based alloys with controlled microstructures and improved functional performance.