Interactions of Phase Transformations and Plasticity: Session 2
Program Organizers: Valery Levitas, Iowa State University

Tuesday 10:20 AM
July 11, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Alexander Zhilyaev, Institute for Metals Superplasticity Problems, Russian Academy of Science

10:20 AM  
Deformation Induced ε-martensite vs. High Pressure Induced ε-iron in Metastable Austenitic Steel: Stefan Martin1; Stephanie Ackermann1; Anja Weidner1; Marcus Schwarz1; David Rafaja1; Horst Biermann1; 1TU Bergakademie Freiberg
    During plastic deformation of metastable CrMnNi steel the martensitic fcc (austenite) → hcp (ε-martensite) → bcc (α’-martensite) transformation in addition to dislocation slip enhances ductility and strength tremendously. Formation of the intermediate ε-martensite is facilitated via generation of a high stacking fault density in the deformation bands of austenite. This phase transformation is strain dependent and can be observed at moderate shear stresses of several hundred MPa. To compare this phenomenon with the high pressure fcc → hcp transformation of iron, quasi-hydrostatic experiments were performed by using multi-anvil press at room temperature up to 18 GPa employing in situ synchrotron X-ray diffraction. The evolution of the lattice parameters (e.g. c/a-ratio) distinguishes both transformation paths (strain induced ε-martensite and high pressure ε-iron) and from the peak broadening information about the growth mechanisms can be retrieved. Post-mortem EBSD investigations reveal their crystallographic and microscopic appearance and underline their difference in nature.

10:40 AM  
Crystallographic Characteristics of Low-cycle Fatigue Fracture Surface on Fe-Mn-Si-based Alloys: Takahiro Sawaguchi1; Ilya Nikulin1; Nobuo Nagashima1; Susumu Takamori1; 1National Institute for Materials Science
    Low-cycle fatigue fracture surfaces on Fe-Mn-Si-based alloys exhibit crystallographic characteristics associated with deformation-induced γ → ε martensitic transformation. Fractographic and microstructural observations were carried out on a Fe-33Mn-6Si shape memory alloy (SMA) and a Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy (SDA), subjected to axial strain-controlled push-pull type low-cycle fatigue testing, by means of SEM-EBSD and TEM. Quasi-cleavage fracture surfaces on the crystallographic planes of {111}γ , {0001}ε, {1-100}ε , {1-102}ε were observed in the SMA, which are interrelated with typical angular differences reflecting their crystallographic orientation relationships, because of the crack propagation on those crystallographic planes. The SDA showed a rather ductile nature on the fracture surface patterns, whereas their significantly rubbed surface and irregularly waved striation-like patterns are quite distinct from the conventional striations accounting for the so-called mode I crack propagation. The result suggests an underlying crack propagation mechanism responsible for the superior fatigue life of the SDA.

11:00 AM  
Modeling of Strain-Induced Phase Transformations under High Pressure and Shear : Mehdi Kamrani1; Biao Feng1; Valery Levitas1; 1Iowa State University
    The strain-induced α→ω phase transformation (PT) in a zirconium sample under compression and torsion under fixed load is investigated using finite-element method (FEM), and results are compared to those for a sample in a diamond anvil cell (DAC) and a rotational diamond anvil cell (RDAC). Highly heterogeneous fields of stresses, strains, and concentration of the high-pressure phase are presented and analyzed. Some experimentally observed effects are analyzed and interpreted.

11:20 AM  
Phase-Field Model of Ductile Fracturing: Alexander Umantsev1; 1Fayetteville State University
    Application of cyclic stress to ductile materials may cause fatigue, which leads to failure. In this process, the material passes through the stages of elasticity, plastic flow, and fracturing. We consider the plasticity and fracturing in ductile materials as a cascade of the second-order phase transitions and construct a phase-field model where order parameters describe the structural state of the material. The dynamic stress-strain curve of cyclic loading describes transformations of the viscoplastic medium including the work hardening, Baushinger effect, and creep. The model is used to analyze material performance in the conditions of cyclic loading.

11:35 AM  
Modeling The Microstructure Evolutions of NiTi Thin Film During Tension: S. Ehsan Esfahani1; Iman Ghamarian2; Valery Levitas1; Peter Collins1; 1Iowa State University; 2University of Michigan
    A microscale phase field model for the multivariant martensitic phase transformation is advanced and utilized for studying the pseudoelastic behavior of a thin film of equiatomic single crystal NiTi under tensile loading. The thermomechanical model includes the strain softening as a mechanism leading to strain (transformation) localization and discrete microstructure formation. To avoid a small scale limitation, gradient term is dropped. Numerical solutions have shown a negligible mesh sensitivity for different element shapes and densities, which is due to rate dependent kinetic equations for phase transformation. Microstructure evolution and corresponding stress-strain curves are presented for several cases. Obtained stress-strain curves, band-like martensitic microstructure, a sudden drop in the stress at the beginning of the martensitic transformation, residual austenite, and multiple stress oscillations due to nucleation events are qualitatively similar to those in known experiments.

11:50 AM  
Deformation Induced Martensitic Transformation in Ultra-fine Grained SUS304 Stainless Steel Fabricated by Simple Cold Rolling and Annealing Processes: Si Gao1; Ruixiao Zheng1; Wenqi Mao1; Yu Bai1; Akinobu Shibata1; Nobuhiro Tsuji1; 1Kyoto Univesity
    Commercial SUS304 stainless steel having fully recrystallized,ultra-fine grained (UFG) microstructures with a minimum average grain size of 0.8 micronmeter was successfully fabricated by simple cold rolling and subsequent annealing processes.Tensile test at room temperature revealed excellent combinations of tensile strength and uniform elongation in the UFG specimens,which were attributed to the grain refinement strengthening of austenite and the deformation induced martensitic transformation.On the other hand,the UFG specimens exhibited yield point phenomena with Lüders-band-type deformation while their coarse grain sized counterparts showed continuous yielding.The yield point phenomena became more prominent as the grain sizes decreased,accompanying with dramatically increasing of the Lüders strain which was measured by the digital image correlation (DIC) technique.SEM-EBSD and nano-indentation were employed to characterize the microstructures and hardness distribution in the Lüders band.The prominent Lüders deformation was explained in terms of the grain size effect on necking of austenite and the occurrence of deformation induced martensitic transformation.