Microstructural Control Through Precipitation Engineering in Fe-Pd-Ga Ferromagnetic Shape Memory Ribbons: Martensitic Transformation Behavior, Magnetoelastic and Magnetic Response

Melt-spun Fe70−xPd30Gax ribbons (x = 1 and 3 at.% Ga) were heat-treated at 1223 K for 1 h and 2 h and characterized by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, magnetometry, and magnetoelastic measurements. Increasing Ga content decreases thermodynamic equilibrium temperature from 292.0 K (1 at.% Ga) to 283.5 K (3 at.% Ga) in as-prepared ribbons. Extended heat treatment then shifts it to 288.0 K and 264.5 K, respectively, and promotes Fe-rich precipitation. Fine precipitates at 1 h preserve a large transformable matrix fraction and introduce microstructural heterogeneity that governs variant mobility and domain-wall pinning; prolonged annealing triggers coalescence, depleting the matrix and reducing both the transformation heat and the magnetoelastic response. Kissinger analysis yields apparent activation energies of 338 kJmol−1 (1 at.% Ga) and 228 kJmol−1 (3 at.% Ga), confirming that higher Ga content lowers the transformation energy barrier. The magnetostrictive response depends on annealing: 1 h-annealed samples exhibit field-induced variant reorientation and saturation magnetostriction of ~60 ppm at 200 K, whereas 2 h-annealed samples approach volume-conserving behavior. Coercivity scales with precipitate density, with Ga3-2h showing anomalously soft magnetic behavior following coalescence. Thermally induced precipitation thus emerges as a route to simultaneously control microstructure, transformation kinetics, magnetoelastic response, and magnetic behavior in ferromagnetic shape memory alloys.