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  
A Coupled Modeling and Experimental Study of the Interaction Between Phase Transformation and Slip in Shape Memory Alloys: Peter Anderson1; Harshad Paranjape2; Sivom Manchiraju1; 1The Ohio State University; 2Colorado School of Mines
    Numerous experimental studies show that plastic deformation accompanies the austenite-martensite transformation in shape memory alloys (SMAs). This contributes to functional fatigue and structural fatigue, thus reducing material performance. This interaction between phase transformation and plasticity is explored using a unique phase field-crystal plasticity (PF-CP) modeling approach that predicts the evolution of martensite microstructure at the correspondence variant scale and slip activity on the austenite slip systems. The implementation utilizing finite element approach captures the large rotation and deformation of material elements and allows complex loading and boundary conditions to be imposed. We present three outcomes from a coupled study utilizing the PF-CP approach and experiments of NiTi micropillar compression. Distinct martensite microstructure forms in various compression cycles. Slip activity is localized in bands spatially corresponding with the martensite-martensite interfaces formed under stress. Different austenite slip systems dominate in distinct locations near the austenite-martensite interface.

11:00 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.

11:20 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.

11:35 AM  
In-situ Study on the Effect of Pre-deformation on Phase Transformation Path of AISI A8-Mod Martensitic Tool Steel: Hadi Ghasemi Nanesa1; Mohammad Jahazi1; Ali Vedaei Sabegh1; Tom Levasseur2; 1Ecole de technologie superieure; 2DK SPEC
    Ledeburitic tool steels such as AISI A8-Mod are considered as hard to deform materials. Their microstructure is composed of soft ferrite and hard carbides. In this study, by applying 10% cold rolling on AISI A8-Mod, 60HV enhancement in the hardness was obtained prior to the quench-hardening cycle. In the next step using TA-DIL 805 A/D dilatometer, the phase transformation path during the quench-hardening cycle was studied for un-deformed and 10% cold rolled samples. It was found that during heating, before austenitization temperature, carbide dissolution rate has been accelerated in cold rolled samples. The analysis of dilatometry results during the cooling cycle indicated that prior deformation did not modify the martensite start temperature. However, a splitting phenomenon was observed for both conditions at the start of martensite formation which was associated to the influence of deformation in modifying the carbon content of austenite by affecting the kinetics of carbides dissolution.

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.