A microstructural investigation of Gd(5)(Si(x)Ge(1-x)(4) alloys produced by arc-melting.

The giant magnetocaloric effect (MCE) in Gd(5)Si(2)Ge(2) was first reported in 1997. This material is relatively complicated in terms of its structure and its metallurgy. The result of this is a wide variation in the values of the MCE reported by various researchers. These magnetocaloric properties are linked to the phase relationships in the Gd(5)Si(4)-Gd(5)Ge(4) pseudo-binary system and are very dependent on the purity of the materials, in particular the purity of the gadolinium, which can contain significant quantities of oxygen, carbon, nitrogen, and even hydrogen, and the method used to process them: different processing routes can produce very different microstructures. In this study the authors have looked at the techniques of arc melting and mechanical alloying. The resulting processed materials were assessed using x-ray diffractometry, scanning and transmission electron microscopy. Arc melting of 5 g buttons was found to be a problematic due to the presence of a coarse Widmanstatten structure. Mechanical alloying was hindered by the softness of the gadolinium. Arc melting of very small buttons was found to be the best method for producing the appropriate phases. The transmission electron microscope investigation revealed the presence of a twinned structure in the monoclinic phase without the need for low-temperature sample preparation.