Quasimartensitic Modulations: Poster Session
Program Organizers: Avadh Saxena, Los Alamos National Laboratory

Tuesday 5:30 PM
July 11, 2017
Room: Crystal Ballroom A
Location: Hyatt Regency Chicago

P2-47: Commensurate-incommensurate Transition in Ni-rich Ti-Ni Alloys: Kodai Niitsu1; Yuta Kimura2; Yasukazu Murakami3; Daisuke Shindo2; Ryosuke Kainuma2; 1RIKEN; 2Tohoku University; 3Kyushu University
    Lattice modulation prior to the B19’ and/or R martensitic transformations in Ti-Ni-based systems is known as the commensurate-incommensurate (C-IC) transition but most of its physical character is still unknown. We herein reports on mechanical and thermodynamic properties of Ni-rich Ti-Ni alloys in terms of phase stability of the B2 parent phase under various temperatures and stresses. Specific heat measurements and TEM observations revealed that a wide hump arises in association with the C-IC transition in alloys whose Ni content is greater than 51.0 at.%. We successfully evaluated composition dependences of the associated entropy change ΔSC-IC and the C-IC transition temperature TC-IC. Isothermal holdings under an applied stress also revealed that the change in strain near the TC-IC is notably transitional, showing unique time and temperature-history dependences. From the findings in experiments, we deduce that the reorientation of modulated variants can occur under stresses with a strong isothermal nature.

P2-48: Integrated Molecular Statics and Phase Field Model for Strain Glass Transition: Chuanxin Liang1; 1Xi'an Jiaotong University
    Strain glass transition has been investigated for more than a decade. However its physical origin at the atomic scale has not be demonstrated. Here we investigate martensitic transformation (MT) and strain glass transition in NiTi and Fe-doped NiTi shape memory alloys using a combination of atomistic and continuum simulations. We show that a network of confinement to the MT caused by local stress field generated by point defects regulates the nucleation and growth of martensitic domains and turns the MT into a strain glass transition. In particular, we find a correlation between the critical doping concentration for strain glass transition and the type of doping atoms, which provide a useful guide to strain glass alloy design. Our simulation results in terms of both microstructure evolution and strain state diagram agree well with experimental observations.

P2-49: Superelastic Response of Annealed Ti49.8Ni40.3Cu9.9 Shape Memory Alloy Thin Films: Jing Wang1; Xianglong Meng1; 1Harbin Institute of Technology, China.
    The amorphous thin film was prepared by direct current magnetron sputtering deposition. The transformation behavior and superelaticity of the Ti49.8Ni40.3Cu9.9 shape memory alloy thin films annealed at 773 K for 1 h were studied. The crystallized thin film exhibits a two-step transformation and a relative large superelastic shape recovery strain up to 12%.

P2-50: Microstructure Evolution of TiNi-based Alloys after High-velocity Impact: Zhiyong Gao1; 1Harbin Institute of Technology
    Aiming at the urgent demand for reliability assessment of TiNi based alloy working in space, Ti50Ni50, aged Ti49Ni51 and Ti44Ni47Nb9 alloy are impacted at 1020 m/s using a powder gun. Microstructure evolution in alloys induced by impact is investigated through transmission electron microscope. In the Ti50Ni50 alloy, variants lose their self-accommodated morphology and the interfaces between variations migrate and bend under force, and many dislocations exist in the martensite plates. In the aged Ti49Ni51 alloy, the microstructure after impact emerge regionalization characteristics, which is attributed to the deformation of interface between Ti3Ni4 precipitate phases and matrix under the shock waves. In the Ti44Ni47Nb9 alloy, stress-induced martensite plates emerged in the matrix, and high-density dislocations were introduced into the matrix and β-Nb particles. The difference of microstructure evolution is attributed to the influence of the different second particles morphology on the deformation behavior of the alloys after high-velocity impact.

P2-51: The Martensitic Transformation and Shape Memory Effect of Ti-Nb-Al-Sc High Temperature Shape Memory Alloy: Bin Sun1; Xianglong Meng1; Zhiyong Gao1; Wei Cai1; 1Harbin Institute of Technology
    By controlling niobium content, the martensitic transformation start temperature (Ms) of Ti-Nb alloys can alter in a wide range, from about 173 K to 473 K. Ti-Nb alloys with low niobium content are promising candidates as high temperature shape memory alloys. However, the poor stability of martensitic transformation in Ti-Nb alloys limits their application. In this work, influence of aluminum and scandium on martensitic transformation behavior and shape memory effect (SME) of Ti-Nb alloys has been studied. The enthalpy of martensitic transformation increases significantly by addition of aluminum and scandium. With increasing aluminum and scandium content, the martensitic transformation temperatures rise, while the reverse transformation temperatures decline. The stability of martensitic transformation improves in Ti-Nb-Al-Sc alloys but it is still far from satisfaction. About 3% total recovery strain is obtained in Ti-Nb-Al-Sc alloys when 4% pre-strain is loaded.

P2-53: The Nanoscaled Precipitation Behaviors and Their Effects on the Mechanical and Recovery Characteristics of Ti44.5Ni44.5Nb9Co2 Alloys: Bo Cui1; Wei Cai1; 1Harbin Institute of Technology
    The precipitation behavior of new precipitates Ti2Co with the annealing temperatures and their effects on the mechanical and recovery properties of Ti44.5Ni44.5Nb9Co2 alloys are systematically investigated. It is noticeable that the GP zones of the Ti2Co precipitate are distributed dispersedly in the sample annealed at 550oC. When annealing at 650oC, spherical Ti2Co precipitates are homogeneously and densely observed in the TiNi matrix. The precipitates are grown up and get-together distributed with the annealing temperature increasing. Moreover, high densities GP zones of the Ti2Co precipitate cause coherency strain field to bring about the highest yield strength. Meanwhile, it can be expected that the recovery stress and ratio of Ti44.5Ni44.5Nb9Co2 alloys are greatly improved by precipitation strengthening.

P2-52: Taming Martensitic Transformation at Nanoscale: Jiaming Zhu1; 1Xi'an Jiaotong University
    Martensitic transformation is a typical strong first-order, sharp transition with autocatalysis in nucleation and accompanied by rapid growth. It usually takes place within a narrow temperature or stress range, making its utilization in a controllable manner difficult. We show how martensitic transformations can be tamed at nanoscale, which tunes the overall kinetic process of martensitic transformation. This suggests new ways to reduce or even eliminate the transformation hysteresis and produce giant quasi-linear elasticity. The low hysteresis is essential in enhancing the reversibility of martensitic transformations and the quasi-linear elasticity provides an effective means to reduce the apparent Young’s modulus of the material.