Solidification behaviour and microstructural correlations in magnetocaloric Gd–Si–Ge–Nb alloys

Polycrystalline Gd5Si2−xGe2−xNb2x alloys (0 ≤ 2x ≤ 0.2) were investigated for microstructure and magnetocaloric effect. The 2x = 0.05 and 0.1 alloys were mainly composed of monoclinic Gd5Si2Ge2-type and orthorhombic Gd5Si4-type phases whereas other alloys exhibit monoclinic Gd5Si2Ge2-type phase at room temperature. The microstructural studies reveal that Nb addition results in the formation of Nb5(Si,Ge)3 and α-Nb phases. The former promotes the nucleation of Gd5Si3-type phase while the latter suppresses it. At low Nb addition (2x ≤ 0.1) the microstructural modification results in low volume fraction of detrimental Gd5Si3-type phase leading to enhanced magnetocaloric effect (ΔS)M value compared to that of the parent compound. A maximum (ΔS)M value of −9.6 J/kg K with refrigerant capacity of 98 J/kg was obtained for the 2x = 0.1 alloy for a field change of 2 T near room temperature. The magneto-structural transitions shifted to higher temperatures with increase in Nb content up to 2x = 0.1.