AN ABSTRACT OF THE THESIS OFABHIYAN OLI, for the Master of Science degree in Applied Physics, presented on August 10, 2023 at Southern Illinois University Carbondale. TITLE: STUDY OF PHASE TRANSITION AND MAGNNETOCALORIC EFFECT FOR THE SYSTEM Ni-Mn-In-Bi MAJOR PROFESSOR: Dr. Saikat Talapatra We experimentally investigate the Heusler alloys Ni50Mn35In12Bi3 and Ni47Mn35In15Bi3 on their different magnetic properties: structural, magnetic, magnetocaloric and magnetotransport properties by using room-temperature X-ray diffraction (XRD), and magnetization measurements in the temperature interval of 10 -380K and field up to 5T. This alloys shows both high temperature austenite phase (AP) and martensite phase (MP). The alloy Ni47Bi3Mn35In15 crystallize in primitive Cubic structure with space group Fm-3m and Ni50Mn35In12Bi3 with the crystal structure of Tetragonal L21 type with space group I4-3m. Alloy Ni47Bi3Mn35In15 show two phase transition FOPT from Ferrimagnetic/AFM to FM and SOPT from FM to PM towards higher temperature and its result will be discussed here mainly. The martensitic transition (TM) takes place around 200K and Curie temperature (TC) 313K in presence of 100Oe field. The saturation magnetization (Ms) at 10K was found to be increasing at lower field and stabilized at higher field indicating ferromagnetic behavior. The Ni47Bi3Mn35In15 shows high magnetocaloric effects (ΔSM = -47.36 Jkg-1K-1) and Relative Cooling Power (RCP = 222.12 J/Kg) in the vicinity of its Curie temperature (TC =313K). Magnetotransport measurement is done by using a standard four-probe method from 10-380 K temperature in presence of zero field and 50 kOe field.

The processes leading to the retention of small quantities of austenite following the bainite and martensite phase transformations have been examined, together with the influence of retained austenite on the properties of low alloy steels. It was found that the upper and lower bainite transformations are separate reactions, although both involve a displacive transformation mode, Growth seems to occur by the repeated nucleation of martensitic sub-units, and this leads to an apparently slow growth rate, The partitioning of carbon from bainitic ferrite into residual austenite was thermodynamically proven to occur subsequent to transformation, and was shown to be directly responsible for the 'incomplete reaction phenomenon'. The nature of sympathetic nucleation and of the limited size of bainitic sub-units was rationalised in terms of the relatively low driving force available for bainite transformations. It was shown that the retention, stability and morphology of austenite could be directly derived from the basic transformation mechanism. Under certain circumstances, the bainitic retained austenite conferred exceptional strength/toughness properties to silicon steels; these were· shown to be superior to the properties associated with tempered martensite microstructures, Using thermodynamics, a model was established which could predict the toughness behaviour of silicon steel bainites simply from a knowledge of the composition. The tempered martensite embrittlement phenomenon was not found to be directly linked to the decomposition of retained austenite films, but to the coarsening of inter- or intra-lath carbides. In dislocated martensites, it was found that the distribution and quantity of retained austenite could be rationalised in terms of the degree of accommodation between adjacent martensite variants. The incipient twins generally observed in lath martensites were shown to be accommodation defects such that the extent of twinning was the greatest when adjacent martensite units had twin-related lattices. The thermodynamics of dislocated martensites have been briefly examined, The inhomogeneous deformation behaviour of dual-phase steels has been analysed in terms of available models.

The present thesis is devoted to the investigation of modulated structures as well as the direct measurement of magnetocaloric effect (MCE) in Ni-Mn based magnetic shape memory (MSM) Heusler compounds in pulsed magnetic fields after analyzing isothermal entropy data taken in static magnetic fields. The emphasis is on the modulated structure of MSM Heusler compounds because of lower twinning stress which facilitates the easy transformation from austenite to martensite structure. Synchrotron x-ray powder diffraction (SXRPD) was carried out to study the modulated structure and NPD for antisite disorder as Ni and Mn have easily the same atomic scattering factor. Direct measurement of the adiabatic temperature change ΔTad was done in pulsed magnetic fields, because of fast response of ~10 to 100 ms to the sample temperature on magnetic field, providing adiabatic conditions. It also gives an opportunity of very high magnetic fields up to 70 T because of short pulse duration during the measurement. The modulated structure has been studied for the off-stoichiometric Ni2Mn1.4In0.6 and Ni1.9Pt0.1MnGa MSM Heusler compounds from SXRPD and NPD. Ni2Mn1.4In0.6 exhibits martensitic transition at TM ~ 295 K and Curie temperature TC ~ 315 K. Rietveld refinement reveals uniform atomic displacement in the modulated structure of martensite phase and the absence of premartensite phase and phason broadening of the satellite peaks which was further confirmed by HRTEM study. Therefore, the structural modulation in Ni2Mn1.4In0.6 can be successfully explained in term of the adaptive phase model. Whereas, Ni1.9Pt0.1MnGa shows the premartensite phase in addition to the martensite and austenite phases and follows the soft phonon model. The temperature dependent ac-susceptibility shows the change in slope at different temperatures 365, 265, 230 and 220 K corresponding to the Curie temperature TC, first premartensite T1, second premartensite T2 and martensite temperature TM, respectively. Temperature-dependent high resolution SXRPD data analysis shows first, a nearly 3M modulated premartensite phase with an average cubic-like feature i.e. negligible Bain distortion of the elementary L21 unit cell results from the austenite phase. This phase then undergoes an isostructural phase transition 3M like premartensite phase with robust Bain distortion in the temperature range from 220 to 195 K. Below 195 K, the martensite phase appears which results from the larger Bain-distorted premartensite phase. In this work, the magnetocaloric properties of Ni2.2Mn0.8Ga and Ni1.8Mn1.8In0.4 magnetic shape memory (MSM) Heusler compounds were studied. Ni2.2Mn0.8Ga exhibits the reversible conventional MCE, measured from isothermal entropy change ΔSM and adiabatic temperature change ΔTad because of the geometric compatibility condition (GCC) for cubic austenite phase to tetragonal martensite phase as a consequence of low thermal hysteresis of the martensite phase transition. The reversible MCE has been confirmed by applying more than one pulse in the hysteresis region at 317 K. Ni1.8Mn1.8In0.4 possess improved reversible behavior of inverse MCE due to the closely satisfying of GCC from cubic austenite to modulated monoclinic martensite structure. The maximum value of ΔSM has been found to the same for both heating and cooling curves measured from isothermal magnetization M(T) curves until a magnetic field of 5 T. The adiabatic temperature change ΔTad results in a value of -10 K by applying a magnetic field of 20 T in a pulsed magnetic field. Furthermore, reversible magnetostriction of 0.3% was observed near the first-order martensite phase transition temperatures 265, 270 and 280 K. A reduction of thermal hysteresis has been found in MSM Heusler compounds Ni2Mn1.4In0.6 and Ni1.8Co0.2Mn1.4In0.6 with the application of hydrostatic pressure followed by GCC from pressure dependent x-ray diffraction in both austenite and martensite phase. By increasing pressure, the lattice parameters of both phases change in such a way that they increasingly satisfy the GCC. The approach of GCC for different kind of martensite structures (tetragonal, orthorhombic and monoclinic) will help to design new MSM Heusler compounds taking advantage of first-order martensite phase transition.