Magnetostructural phase transformations in magnetic shape memory Heusler alloy Ni-Mn-In

Blankenau, Brian J.

Permalink

https://hdl.handle.net/2142/127223 Copy

Description

Title

Magnetostructural phase transformations in magnetic shape memory Heusler alloy Ni-Mn-In

Author(s)

Blankenau, Brian J.

Issue Date

2024-12-06

Director of Research (if dissertation) or Advisor (if thesis)

Ertekin, Elif

Doctoral Committee Chair(s)

Ertekin, Elif

Committee Member(s)

• Johnson, Harley T
• Sehitoglu, Huseyin
• Shoemaker, Daniel P

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Shape Memory Alloys
• Magnetism
• Lattice Model
• Phase Transformation
• First-Principles Methods, High Pressure
• Heusler Alloys
• Cluster Expansion
• Ising Model
• Diamond Anvil Cell
• Ni2MnIn

Abstract

When considering the structural properties of materials, the effects of magnetism may usually be treated as an afterthought. Indeed, due to its typically smaller energy scale magnetism usually plays a subordinate role, if any at all, when compared to electronic and vibrational effects as well as microstructure. However, in certain cases magnetism plays a critical role in determining structural and micro-structural material properties. A particular example in which this is the case is the magnetic Heusler alloy Ni50 Mn50−x Inx at compositions around x = 15. Heusler structure Ni50 Mn50−x Inx is a shape memory alloy where magnetic exchange interactions strongly couple to competing structural phases. The favorablility of each phase, and therefore the observed properties of Ni50 Mn50−x Inx are sensitive to changes in composition on the order of a few percent In. This sensitivity results in a complex magnetostructural phase diagram. This work focuses on the investigation of the magnetostructural phase transformation of Ni50 Mn50−x Inx using first-principles and experimental techniques. Due to the sensitivity of the Ni50 Mn50−x Inx to small changes in composition, a computational approach is well suited to studying the phase transformations in this material. In particular lattice models, such as the Ising, Potts, cluster expansion, and magnetic cluster expansion models, have been proven to be effective tools for studying complex magnetic alloys and compounds. Existing lattice model codes proved insufficient for accurate descriptions of Ni50 Mn50−x Inx across wide compositions x, therefore it was necessary to develop GenMC-MA, a generalized Monte Carlo code for magnetic alloys. The code is comprised of three main components. The first component is GenMC Preprocess, which converts data sets consisting of density functional theory simulations into a compact format. The second component, GenMC Fit, uses the compact data set to parameterize a lattice model, selected from a set of available options. The third component is GenMC Run, which is a Monte Carlo solver for generating ensembles of configurations, accounting for both magnetic and alloy configurational entropies, at different temperatures. These ensembles and their analysis can be used for simulating phase transformations and constructing phase diagrams. The code can also be used for generating special quasi-random structures and structures with user-defined short-range order. Using GenMC-MA in conjunction with a first-principles treatment of vibrational entropy, a magnetostructural phase diagram for Ni50 Mn50−x Inx was created relying solely on first principles inputs. The magnetic contributions to the free energy were determined from spin lattice models using GenMC-MA and data sets obtained from density functional theory simulations. The data sets consisted of the austenite and martensite phases of Ni50 Mn50−x Inx for compositions from x = 0 to x = 25. A simulated annealing approach, implemented in GenMC-MA, was then used to determine the magnetic free energy of each phase across the composition range from x = 0 to x = 25. In order to capture the effects of vibrational entropy on the magnetostructural phase transformation, the quasi-harmonic approximation was employed. The quasi-harmonic approximation was applied to calculate the Gibbs free energy for the ground state at each composition in the density functional theory data set. No structural phase transformation is observed when considering vibrational contributions alone. The combined analysis of magnetic and vibrational Gibbs free energy surfaces provides a quantitative description of many features of the phase diagram. For instance, it successfully reproduces all experimentally observed phases and predicts the critical composition at which the stable martensitic phase vanishes to be within 0.5% of the experimental value. Additionally, the model accurately captures the Curie temperature of the austenite phase (280 K predicted vs. 310 K experimental), and its relatively constant behavior across a composition range of 16% to 25% In. There are also aspects of the predicted phase diagram that can be improved by further refinements to the model adopted here. While the model correctly captures the phenomenon of re-entrant ferromagnetism, it predicts this effect to occur at higher compositions than observed experimentally (22% In vs. 15% In). And, in the low In concentration range, the stability of the martensite phase is overestimated in the transition from martensite to austenite. In addition to the computational work, the effects of hydrostatic pressure on the magneto-structural phase transformations in Ni50 Mn50−x Inx were also investigated experimentally. Whitebeam Laue and powder X-ray diffraction were used to study the isobaric temperature-induced and isothermal pressure-induced transformations. Hydrostatic pressure was applied using diamond anvil cells. A transformation from cubic austenite to a modulated 6M monoclinic martensite phase was observed between 4.25 and 4.5 GPa. We observed the nucleation of the temperature-induced martensitic transformation using spatially resolved Laue data at a pressure of 4.5 GPa. A hysteresis of 5.5 K was observed between heating and cooling cycles. This observation is consistent with previous measurements of hysteresis in similar alloys. Powder XRD was used to search for additional phase transformations at higher pressures. Between 15 and 20 GPa, the 6M structure was observed to transform into a 14M structure. No additional phase transformations were observed below 40 GPa. This result could not be replicated by first-principal calculations which showed the 6M structure to be the most stable of the two phases from 4.5 GPa to 40 GPa. However, the energy difference between 6M and 14M structures was small, only 3 meV/atom. Additional contributions, such as vibrational and magnetic effects, internal strain, or short range order could be the source of this inconsistency.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127223

Copyright and License Information

Copyright 2024 Brian Blankenau

Owning Collections