The Effect of Fe Substitutions on the Microstructure of Gd5Si2Ge2

Abstract

Gd₅Si₂Ge₂ exhibits the so-called giant magnetocaloric effect, making it a potential material for next-generation near-room-temperature permanent-magnet-based refrigeration devices. In the as-cast form this material is relatively complicated in terms of its structure and its metallurgy.

Recent reports have suggested that small additions of iron can substantially improve the problems associated with hysteresis losses, so enhancing the material’s potential for real-world applications. We have looked at iron substitutions in the range X=0 to X=1 for Gd₅Si_2-XGe₂ to investigate the macrostructures and microstructures of cast and homogenized materials, with particular emphasis on the amount of iron substituting for silicon in the Gd₅(SiGe)₄ phase and the effect of the substituting iron on the Ge/Si ratio. The larger iron substitutions were found to eliminate the unusual macrostructures, and conventional ingots with smooth surfaces were the result. The iron was also found to dissolve at levels of 1-3% in the Gd₅(SiGe)₄ phase and encourage the formation of the Gd₅(SiGe)₃ phase at the expense of the magnetocaloric Gd₅(SiGe)₄ phase. A substitution of X=1 was found to be sufficient to produce as-cast microstructures consisting almost entirely of the Gd5(SiGe)3 phase.

With a scanning electron microscope (Jeol 840A) and an analytical transmission electron microscope (Jeol 2010 F with a field-emission gun) and an EDXS system from Oxford Instruments (Link Isis 300) we performed a detailed microstructural investigation of the processed materials. Special emphasis was placed on the intergrain regions and an assessment of the amount of Fe substituting in the main magnetocaloric phase.