Influence of thermal treatments on the martensitic transformation in NiMnSb ferromagnetic shape memory alloys. Its relation to the magnetocaloric effect.Thursday (27.09.2018) 10:15 - 10:30 S1/03 - 223 Part of:
NiMn-based alloys are appreciated in ferromagnetic shape memory alloys group (FSMA) for technological applications because these alloys can promote a temperature change when a martensitic transformation (MT) is developed. In fact, the MT induced by applying an external magnetic field due to the change of entropy during the transformation leads to a temperature change in the sample, called magnetocaloric effect (MEC). If the entropy change is negative (ΔS < 0), the sample get warmed when an external magnetic field is applied, this effect is called direct MCE. Whereas if ΔS > 0, the sample get cool down when an external magnetic field is applied and then the effect is called inverse magnetocaloric effect.
In particular, NiMnSb near the composition Ni50Mn37Sb13 presents an inverse MCE during the MT, positioning to this alloy as very attractive for refrigeration applications. It is well know that the MT is a diffusionless phase transition obtained by quenching, therefore the appearance of high density of quenched-in-defects in the sample is an intrinsic characteristic in these alloys. Consequently, the appearance of both quenched-in-defects and defects promoted by thermomechanical treatments should have an important influence on the MT in FSMA as well as, on the mechanical and magnetic properties. Moreover, the MCE is usually studied through indirect methods involving the measure of the change in entropy as a function of the magnetic field, from the usual classical thermodynamic relationships.
In this work, alloys of composition near to the stoichiomeric composition, Ni50Mn37.5Sb12.5, Ni50Mn39Sb11, Ni50Mn36Sb14, produced by arc melting under argon atmosphere are studied by means of mechanical spectroscopy, X-ray diffraction, electron microscopy and also by means of differential thermal analysis under magnetic field. The behaviour of the MT on those alloys viewed from the above experimental techniques as a function of thermal treatments at several temperatures is shown. In addition, the behaviour of the MT related to the effectiveness of the MCE determined from differential thermal analysis under magnetic field, i.e. without an indirect determination, is discussed.