Influence of thermal treatment on the microstructure and martensitic transformation in an as-cast Ni45Mn36In14Co5 alloy

The Ni-Mn-In-Co alloys exhibit a high magnetocaloric effect. However, their practical application is hindered by low cyclic stability of functional properties, resulting in sample destruction during cycling through martensitic transformation. It is known that thermo-mechanical treatment may enable the production of a material with an optimal combination of mechanical and functional properties. However, in order to perform processing, it is first necessary to obtain a specified structure within the material. This paper will discuss the procedure for preparing a specified structure through heat treatment. The impact of heat treatment on the microstructure and martensitic transformation of as-cast alloy Ni45Mn36In14Co5 is presented. There are four treated states: 1) after argon arc melting; 2) after homogenization annealing at 900ºC for 24 h followed by quenching in water; 3) after homogenization annealing at 900ºC for 24 h followed by slow cooling in a furnace; and 4) after additional annealing of the quenched sample at 700ºC for 1 h and slow cooling in a furnace. The alloy is two-phase in all structural states, with the main phase surrounded by a secondary phase enriched in Co. The differences between the states are the morphology and volume fraction of the secondary phase. After melting, it takes the form of a chain along the crystals of the matrix phase. In the other three homogenized states the secondary phase is partially dissolved. Simultaneously, it takes the form of particles that are 5 µm thick and up to 10 µm long. Of the four states, the quenched state has the highest enthalpy of martensitic transformation (∆HM = −5.6395 J/g) and the lowest hysteresis (AF-MF = 24ºC). Only this state has specified structure for the subsequent thermomechanical treatment and it is able to exhibit high values of functional effect.