Interactions of Phase Transformations and Plasticity: Session 7
Program Organizers: Valery Levitas, Iowa State University

Thursday 2:00 PM
July 13, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Harshad Paranjape, Colorado School of Mines

2:00 PM  
Effect of Phase Transformation on Dynamic Plasticity of Plain Carbon Steels at High Temperature: Steven Mates1; Sindhura Gangireddy1; Mark Stoudt1; Sheng Yen Li1; Greta Lindwall1; 1NIST
    Dynamic plastic deformation of plain carbon steels (AISI 1018, 1045 and 1075) at high temperatures is of interest to understand chip formation in high speed machining processes. When the workpiece and/or chip temperatures exceed austenite forming temperatures, the transformation can significantly influence dynamic plasticity as the ferrite-cementite structure dissolves into the much weaker austenite phase. We measure the influence of the austenite transformation on dynamic plasticity using a pulse-heated Kolsky bar, including the effect of heating time and temperature on the amount of transformation and resulting effect on flow stress. Post-test microstructures are examined for evidence of transformation, which can include complex microstructures consisting of both un-transformed phases and phases resulting from quenched austenite of varying carbon content, including martensite and bainite. We further perform pulse-heated experiments without mechanical deformation to determine the influence of plastic strain on the amount of transformation observed at high temperatures.

2:20 PM  
Effect of Strain on Martensite Formation in High Mn Steel under Room Temperature Plane Strain Conditions: Sudipta Pramanik1; Ahmed Saleh1; Azdiar Gazder1; Elena Pereloma1; 1University of Wollongong
    A 52% hot rolled Fe-17Mn-3Al-2Si-1Ni-0.06C (wt.%) steel was subjected to plain strain compression from 5% to 88% thickness reduction. The microstructures were characterised using electron back-scattering diffraction and high resolution scanning transmission electron microscopy (STEM). The hot rolled microstructure consists of coarse γ-austenite grains with high fraction of annealing twins and ε- and α′- martensite that formed on quenching. Both thermally (upon quenching after hot rolling) and deformation-induced (upon cold deformation) α′-martensite were found to form directly from γ-austenite and via ε-martensite formation. STEM revealed the presence of intrinsic stacking faults in ε-martensite after deformation, signifying the operation of basal slip by partial dislocations. Lastly, analysis of the crystallography of α′ and ε martensite was carried out.

2:40 PM  
The Energetics of the α − ω Phase Transformation Coupled to Plasticity in Ti and Zr: A First-principles Study : Anil Kumar1; Curt Bronkhorst1; Turab Lookman1; 1Los Alamos National Laboratory
    Titanium and Zirconium undergo a structural phase transformation from the ambient pressure hcp α-phase to a high pressure hexagonal ω-phase. Our understanding of how plastic deformation modes of the individual phases affect the phase transformation process is very much in its infancy. In this work, we use first-principles density functional theory calculations to understand the energetics of how deformation on dominant slip systems affects the α to ω phase transformation. In particular, we study the energy barriers accompanying the shear process across coexisting α and ω phases and distill the key physics and mechanisms controlling the interplay of plasticity and phase transformation in these metals.

3:00 PM  
Influence of Transformation Induced Plasticity on Failure of Shape Memory Alloy Actuators: SAMEER JAPE1; THEOCHARIS BAXEVANIS2; DIMITRIS LAGOUDAS1; 1Texas A&M University; 2University of Houston
    Shape memory alloys (SMAs) are gaining popularity as solid-state actuators due to their large, reversible strains from austenite-to-martensite phase transformations. When SMAs undergo cyclic transformations, permanent microstructural changes in the material lead to substantial plastic strains, known as transformation induced plasticity (TRIP). Design of high-performance and durable SMA actuators requires an understanding of their fracture response in the presence of phase transformation and TRIP. Thus, prototype problem of an infinite-medium center-cracked SMA subjected to thermal variations under plane-strain mode-I tensile load in the presence of TRIP, is solved using FEA. Driving force for crack growth is identified as the crack-tip energy release rate(ERR) and is calculated using VCCT in Abaqus. Global phase transformation and TRIP leads to stress redistribution at the crack-tip, increasing the ERR during cooling. TRIP is seen to have a significant effect on crack-tip mechanical fields in stationary and growing cracks and on fracture toughness of SMA.

3:15 PM  
Fatigue of Superelastic NiTi Wires under Uniaxial Loading: Sebastián Jaureguizahar1; Mirco Chapetti1; Alejandro Yawny2; 1INTEMA (CONICET-Universidad de Mar del Plata, FI); 2División Física de Metales, Centro Atómico Bariloche, CNEA – CONICET
    Fatigue is an important issue in applications based on the pseudoelastic behavior of NiTi shape memory alloys where the stress induced transformation is induced repeatedly. There is a lack in the literature concerning the appropriate characterization of the intrinsic uniaxial fatigue life of commercial superelastic NiTi wires with ultrafine grain size. Therefore, in the present work an alternative method for accurate evaluation of the intrinsic fatigue properties of NiTi wires under uniaxial tensile loading conditions was developed. The proposed test methodology was applied to characterize the fatigue life under uniaxial tensile loading conditions of commercial ultrafine grained (40-50 nm) superelastic Ni rich NiTi wires (50.9 at.% Ni). In addition, and for comparison purposes, strain controlled experiments were performed in wires in fully austenite and fully martensite states. The obtained results indicate a deleterious effect of the stress induced martensic transformation on the fatigue life of superelastic NiTi wires.

3:30 PM Break