Phase Transformations and Microstructural Evolution: Martensitic Transformation
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Rongpei Shi, Harbin Institute of Technology; Yipeng Gao, Jilin University; Fadi Abdeljawad, Lehigh University; Bharat Gwalani, North Carolina State Universtiy; Qi An, Iowa State University; Eric Lass, University of Tennessee-Knoxville; Huajing Song, Los Alamos National Laboratory
Monday 2:00 PM
March 15, 2021
Room: RM 57
Location: TMS2021 Virtual
Session Chair: Yipeng Gao, Idaho National Laboratory
2:00 PM
Monte Carlo Simulation and Three-dimensional Diffuse Scattering Study of Martensitic Transformation: Xiaoxu Guo1; Yongmei Jin1; Yu Wang1; Yang Ren2; 1Michigan Technological University; 2Argonne National Laboratory
A quasi-spin Ising model of ferroelastic phase transition is developed and employed to perform Monte Carlo simulation of thermoelastic martensitic transformation. The quasi-spin variable associated with the lattice sites characterizes the local unit cells of the orientation variants of the ground-state martensite phase, which interact with each other through long-range elastic interactions. The simulation study focuses on the intrinsic behaviors of a defect-free crystal that undergoes cubic-to-tetragonal martensitic transformation. Diffuse scattering is calculated from the simulated microstructures during the transformation. It is shown that the diffuse scattering in the pre-martensitic austenite state results from the spatial correlation of the atomic-scale heterogeneous lattice displacements and manifests the displacement short-range ordering. The effects of temperature, elastic anisotropy, and shear modulus softening on diffuse scattering, displacement short-range ordering, and martensitic transformation are investigated. The simulated diffuse scattering is compared and agrees with the complementary synchrotron X-ray single-crystal diffuse scattering experiment of Ni-Mn-Ga alloys.
2:20 PM
Size Effects and Microstructural Evolution of Shape Memory Ceramics during Cyclic Phase Transformations: Isabel Crystal1; Christopher Schuh1; 1Massachusetts Institute Of Technology
Bulk shape memory ceramics (SMCs) exhibit transformation-induced cracking due to mismatch stresses arising at the grain boundaries. Current strategies for mitigating cracking in SMCs involve moving towards smaller volume structures, which feature high surface area for stress relaxation and fewer grain boundaries to minimize transformation stresses. While this approach has proven successful, it typically limits SMCs to sample sizes at the micrometer-scale. Here we explore size effects in cyclic martensitic transformations in millimeter-scale polycrystalline structures to study cracking-induced disaggregation as a function of grain size from ~0.6 to 7.9 microns, and we compare these results to that of a large single crystal. The number of thermal cycles until the polycrystal fully disaggregates is inversely related to grain size while the single crystal sustains no macroscopic damage after >100 cycles. Quantitative calorimetry analysis captures the evolution in heat lost to cracking as the polycrystals gradually disaggregate into loose grains with cycling.
2:40 PM
Super-critical Elasticity: A Challenge to Martensitic Transformation Theory: Haiyang Chen1; Yan-Dong Wang1; Yang Ren2; 1University of Science and Technology Beijing; 2Argonne National Laboratory
We recently discovered that the first-order stress-induced martensitic transformation (SIMT) of shape-memory-alloys can be completely suppressed by bringing the transformation to a supercritical region [Chen et al., Nature Materials, (2020). https://doi.org/10.1038/s41563-020-0645-4]. In this talk, we will show a supercritical elasticity (SCE) in NiCoFeGa single crystals, which exhibit a large elasticity up to 15.2% strain, with non-hysteretic mechanical responses, a small temperature dependence and high-energy-storage capability and cyclic stability over a wide temperature and composition range. In-situ synchrotron X-ray diffraction shows that the SCE is correlated with a stress-induced continuous variation of lattice parameter accompanied by structural fluctuation. Neutron diffraction and electron microscopy observations reveal an unprecedented microstructure consisting of atomic-level entanglement of ordered and disordered crystal structures, which can be manipulated to tune the SCE. We propose to describe this phenomenon by a modified Landau-theory-based model and show that the SCE is a manifestation of a double-well elastic energy landscape.
3:00 PM
Uncovering the Role of Nanoscale Precipitates on Martensitic Transformation and Superelasticity: Shivam Tripathi1; Karthik Guda Vishnu1; Michael Titus1; Alejandro Strachan1; 1Purdue University
We characterize the role of nanoscale spherical Ni50Al50 precipitates on temperature and stress-induced martensitic phase transformation in polycrystalline Ni63Al37 shape memory alloys using multi-million-atoms molecular dynamics simulations. We studied two types of precipitates: one where grain boundaries cut through precipitates (polycrystal precipitates) and a second case where precipitates are single crystals. The non-transforming precipitates result in a reduction of both thermal and stress-strain hysteresis, but the effect is more prominent in the case of single-crystal precipitates. Single crystal precipitates also result in lower remnant strain. A detailed analysis of the trajectories reveals that grain boundaries slide is the dominant mechanism over intra-grain plasticity and is mainly responsible for irrecoverable strain after loading-unloading cycles. We also characterize the role of grain orientation and size on martensitic transformation to provide detailed insight into the origins of experimentally reported characteristics of phase transformation.