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.

Metastable austenitic and duplex stainless steels are widely used materials in industrial anddomestic applications, owing to their attractive characteristics such as good corrosion resistanceand favorable mechanical properties. Both types of steel experience enhanced mechanicalproperties during plastic deformation due to the formation of the martensite phase from theparent austenite phase, this is called deformation-induced martensitic transformation (DIMT).It is therefore of technical interest to study the transformation mechanism and its impact onmechanical properties for a better understanding and ultimately for developing new materialswith improved performance in certain applications. In the present thesis, two austenitic stainless steels (201Cu, HyTens® 301) and two duplexstainless steels (FDX25®, FDX27®) were investigated. Samples were tensile tested during insitusynchrotron radiation experiments performed at the Cornell High Energy SynchrotronSource (CHESS), Ithaca, USA. Tests were performed at both room temperature and at elevatedtemperatures. The collected diffraction data were then processed by software such as Fit2D andMATLAB. Quantitative phase fraction analysis based on the direct comparison method wasperformed successfully. Microstructural analysis of samples before deformation and after thefull tensile testing was also performed using electron microscopy. The deformation induced martensitic transformation took place in HyTens 301, FDX25 andFDX27, but in 201Cu the austenite was stable during the tensile tests conducted here. The a’-martensite formed in a significantly higher fraction than the ε-martensite in all alloys. At roomtemperature, the critical stress levels for martensitic transformation were 490 MPa, 700 MPaand 700MPa for HyTens 301, FDX25 and FDX27, respectively.

This thesis concerns the mathematical modelling of phase transformations with a special emphasis on martensitic phase transformations and their application to the modelling of steels. In Chapter 1, we develop a framework that determines the optimal transformation strain between any two Bravais lattices and use it to give a rigorous proof of a conjecture by E.C. Bain in 1924 on the optimality of the so-called Bain strain. In Chapter 2, we review the Ball-James model and related concepts. We present some simplification of existing results. In Chapter 3, we pose a conjecture for the explicit form of the quasiconvex hull of the three tetragonal wells, known as the three-well problem. We present a new approach to finding inner and outer bounds. In Chapter 4, we focus on highly compatible, so called self-accommodating, martensitic structures and present new results on their fine properties such as estimates on their minimum complexity and bounds on the relative proportion of each martensitic variant in them. In Chapter 5, we investigate the contrary situation when self-accommodating microstructures do not exist. We determine, whether in this situation, it is still energetically favourable to nucleate martensite within austenite. By constructing different types of inclusions, we find that the optimal shape of an inclusion is flat and thin which is in agreement with experimental observation. In Chapter 6, we introduce a mechanism that identifies transformation strains with orientation relationships. This mechanism allows us to develop a simpler, strain-based approach to phase transformation models in steels. One novelty of this approach is the derivation of an explicit dependence of the orientation relationships on the ratio of tetragonality of the product phase. In Chapter 7, we establish a correspondence between common phenomenological models for steels and the Ball-James model. This correspondence is then used to develop a new theory for the (5 5 7) lath transformation in low-carbon steels. Compared to existing theories, this new approach requires a significantly smaller number of input parameters. Furthermore, it predicts a microstructure morphology which differs from what is conventionally believed.

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.