Interactions of Phase Transformations and Plasticity: Session 4
Program Organizers: Valery Levitas, Iowa State University

Wednesday 10:20 AM
July 12, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Thomas Antretter, Montanuniversitaet Leoben

10:20 AM  
Coupling of Martensitic Transformation and Plasticity in Thermomechanically Loaded NiTi: Ludek Heller1; Lukas Kaderavek2; Pavel Sedmak2; Petr Sittner2; 1Institute of Physics of the Czech Academy of Sciences AND Nuclear Physics Institute of the Czech Academy of Sciences; 2Institute of Physics of the Czech Academy of Sciences
    Stress-strain-temperature thermomechanical behaviors due to martensitic transformation in NiTi are reversible if maximum stress is low, strain is below ~10% and temperature is below 100C. At higher temperatures, martensitic transformation proceeds alongside with plastic deformation due to slip and twinning. We have carried out systematic experiments on NiTi wires subjected to deformation followed by heating under external constraint up to ~400C. Evolution of tensile stress during the constrained heating/cooling tests was analyzed with the help in-situ electric resistance, DIC and synchrotron x-ray diffraction methods. Based on the obtained results, mechanism of the coupled martensitic transformation and plasticity was analyzed. It is claimed that, this deformation mechanism allows for shape setting of NiTi at temperatures as low as 100C-300C, it is responsible for the strain drift observed during the thermal cycling under stress and causes malfunction of embedded prestrained NiTi elements exposed to overheating.

11:00 AM  
Deformation Response and Cyclic Stability of Ni50Ti35Hf15 High Temperature Shape Memory Alloy Wires: Demircan Canadi1; W. Trehern2; H. Ozcan2; C. Hayrettin2; O. Karakoc2; I. Karaman2; 1Ko University; 2Texas A&M University
    The processing - phase transformation - deformation response relationships of Ni50Ti35Hf15 high temperature shape memory alloy were studied. Specifically, the initially as-cast and hot isostatic pressed cylindrical Ni50Ti35Hf15 bars were subjected to hot extrusion, and then wire drawing. As the material was hot extruded and further underwent wire drawing, the phase transformation temperatures decreased and the strength levels increased, yet the initial brittle response was replaced by near-perfect superelastic response above the austenite finish temperature. Moreover, the Ni50Ti35Hf15 wires exhibited a remarkably stable cyclic actuation response under 300 MPa, as compared to the poor cyclic stability of the hot extruded samples, which is beyond the operating stress levels for known shape memory actuators.

11:20 AM  
Nanoindentation of Biaxially Pre-strained NiTi (100) Single Crystals at Different Temperatures: Sandra Hahn1; Yuri Chumlyakov2; Martin Wagner1; 1TU Chemnitz, Institute of Materials Science and Engineering, Chemnitz, Germany; 2Tomsk State University Siberian Physical-Technical Institute, Tomsk, Russia
    This study considers the effect of biaxial pre-straining on the martensitic transformation in pseudoelastic NiTi single crystals. (100)-oriented single crystals were first deformed in biaxial compression (to strains of 0.5 or 1.0 % in two <100> directions) at room temperature. After pre-straining, the samples were cooled (and subsequently heated) through the transformation range; during this cooling-heating cycle, nanoindentation experiments were performed at different temperatures. A Berkovich indenter tip was used to determine hardness and Young’s modulus values both in the austenitic and in the martensitic states of the sample. The mechanical stress locally reduces the transformation barrier and helps to trigger the nucleation and growth of the martensite. Our experimental results allow to characterize the martensitic transformation and its interaction with plastic strain gradients during heating and cooling while subjected to local mechanical loads, and to correlate the mechanical behavior to the stabilization effect of the biaxial pre-straining.

11:50 AM  
Atomistic Simulation of Carbon Ordering in Martensitic Steels under External Stress: Pavel Chirkov1; Alexander Mirzoev1; 1South Ural State University
     The Zener ordering in Fe-C solid solution under external stress has been studied by statistical analysis and molecular dynamic (MD) simulation. Simulations of martensite were carried out using EAM interatomic potential for Fe-C system. It is shown that external stress has strong influence on carbon order at octahedral sites. When the compression stress along tetragonality (z) are applied the redistridution of carbon atoms occurs and the direction of ordered state changes from z to another axis.This phenomenon occurs when threshold stress value is achieved. Dependences of critical stress on temperature and carbon content were obtained. When temperature is increased at fixed carbon content the critical stress is decreased to zero and is increased with concentration rise. The computer simulation results are consistent with theoretical data.