Powder metallurgy synthesis of ferromagnetic Heusler alloys: Feasibility and microstructural characterization studies

Abstract

Ni-Mn-X (X – Ga, Sn, In, Sb) based ferromagnetic Heusler alloys are deemed to be multiferroic materials with the multiferroicity realized by engineering reversibility through composition tuning, of an inherent first-order diffusionless martensitic phase transformation and a second-order magnetic transition accompanying the first with the application of an electromagnetic field. Martensitic phase transformation transforms high-temperature austenite phase to low-temperature martensite phase while magnetic transition changes the electromagnetic ordering in the phases. The engineered reversibility of the magnetostructural transformations is brought with remarkable functional changes such as change in entropy, saturation magnetization, strain recovery, etc. that offer great scope for applications. There are examples in literature of such alloys with great potential for magnetic refrigeration by virtue of a giant entropy change, or with potential for magnetic shape memory effect because of giant magnetic field-induced strains. However, the most interesting characteristics of these alloys is their ability in the conversion of waste heat into electricity on account of the sudden thermally induced surge in saturation magnetization around the phase transformation, suitably utilized to generate electricity. A majority of these ferromagnetic Heusler alloys have been synthesised by liquid processing method of arc/induction-melting of high purity elements in a controlled environment usually followed by annealing. Synthesis procedures which are a sequel to arc/induction melting such as rapid solidification by melt-spinning, directional solidification etc. are also prevalent. Solid processing through powder metallurgy is sparsely employed on pre-alloyed powders of these alloys and not elemental powders even though use of elemental powders is advantageous for reasons such as good compaction characteristics and ease of obtaining new alloy compositions through precise control of stoichiometry. Ferromagnetic Heusler alloys of compositions discussed herein are meant to transform martensitically and vice versa, irrespective of the way they are synthesized. Do these alloys necessarily have to be synthesized through liquid processing? The question is broader in concept and the lack of literature on solid processed ferromagnetic shape memory Heusler alloys provided scope for investigation of the feasibility of adopting conventional pressure-less sintering process as a cost-effective alternative to casting. This research therefore endeavoured to explore the feasibility of solid processing of ferromagnetic Heusler alloys through conventional sintering using elemental powders. The starting composition was Ni45Co5Mn40Sn10. The choice of composition was the singular magnetostructural behaviour at phase transformation of the cast composition that drove its potential as an energy material similar to or even better than thermoelectric materials. Two sets of alloys were prepared – a quinary Ni45Co5Mn40(Sn,Cu)10 and quaternary Ni45Co5Mn40Sn10 using the conventional press and sinter procedure. The quinary compacts were prepared at a compaction load of 70 KN (184 MPa) and sintered at two different temperatures of 950°C and 1050°C for 24 h in order to first study the feasibility of powder processing in fabricating these alloys. The addition of Cu in the quinary composition was to understand the effect of compositional change on the transformation temperatures. With the results of the quinary alloy turning out to be favourable, quaternary alloys were then synthesized in order to understand the effect of process parameters such as compaction pressure, temperature and time of sintering on the magnetostructural characteristics of these alloys. The quaternary compacts were prepared at two different compaction loads of 70 KN (184 MPa) and 80 KN (210 MPa). Sintering on them was carried out at two different temperatures of 950 °C and 1050 °C at sintering times of 12, 24, 72 and 144 h. With the use of standard characterization techniques of differential scanning calorimetry, optical/electron microscopy including transmission electron microscopy, Xray diffraction and magnetization testing various issues concerning synthesis and characterization of ferromagnetic Heusler alloys using powder metallurgy are reported and discussed.