Interactions of Phase Transformations and Plasticity: Session 6
Program Organizers: Valery Levitas, Iowa State University

Thursday 10:20 AM
July 13, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Elena Pereloma, University of Wollongong


10:20 AM  
Introduction of Nanoprecipitations into Dual Phase in Low Carbon Mn-TRIP Steels for Cryogenic Applications: Yu Li1; Wei Li1; Xuejun Jin1; 1Shanghai Jiao Tong University
    Two-step “Partition and Tempering (P&T)” heat treatment procedure has been applied in low carbon Mn-TRIP steels with designed NiAl and Cu-rich precipitates for good combined strength, plasticity and especially low temperature toughness. The designing mechanisms are based on synergistic effects of nanoscale precipitations in dual phases and enough volume of retained austenite, leading to effective improvement in both strength and elongation. The transmission electron microscopy correlated with 3D atom probe tomography observed the special constitutions of Cu-rich particles in austenite while effective NiAl nanoprecipitates in ferrite. In extraordinary, a high yield strength with unexpected total elongation was achieved at liquid nitrogen temperature. The austenite with intragranular nanoprecipitates experiences a deformation mechanism change from deformation twinning and stacking faults to deformation-induced martensite with deformation, which contributed to a simultaneous increase in strength and ductility for cryogenic applications.

10:40 AM  
Stability of Retained Austenite in Austempered Fe-Mn-Si Medium Carbon Steel: Tadashi Furuhara1; Takeshi Kaneshita1; Goro Miyamoto1; 1Tohoku University
    In further imporovement in mechanical properties of TRIP steels, it is essential to control stability of retained austenite in a more sophysticated manner. The present study has aimed to clarify natures of retained austenite (RA) formed in austempered Fe-2Mn-1Si medium carbon steel and to examine stability of RA during deformation from a viewpoint of crystallography. A transformation stasis of bainite appeared at 673K with carbon enrichment higher than the T0 composition and 15% volume fraction of RA. Lath-shaped RA was located between bainitic ferrite (BF) while blocky RA was distributed mainly at prior austenite grain boundaries or packet/block boundaries. Stress-induced martensitic transformation of RA occurred inhomogeneously depending on bainite packet orientation. In most cases, martensite variants shared the same close-packed plane parallel relationship with the surrounding BFs.

11:00 AM  
Mechanism of Reverse Transformation Induced by Shot-peening for Fe-33mass%Ni Alloy: Hisashi Sato1; Nagomi Tsuboi1; Tadachika Chiba1; Yoshimi Watanabe1; 1Nagoya Institute of Technology
    When subzero-treated Fe-33mass%Ni alloy containing large amount of martensite is shot-peened, reverse transformation from martensite to austenite occurs. However, mechanism of the reverse transformation induced by shot-peening for this Fe-Ni alloy is unclear. In this study, reverse transformation behavior induced by the shot-peening under several processing temperatures is investigated using the subzero-treated Fe-33mass%Ni alloy. When shot-peening is performed at less than –126 oC, deformation-induced martensitic transformation occurs. On the other hand, reverse transformation is taken place by shot-peening under more than -70 oC. With increasing the processing temperature, volume fraction of austenite in the shot-peened Fe-Ni alloy increases. Moreover, distribution of Ni around the peened surface is not changed by the shot-peening. This means that the reverse transformation induced by the shot-peening is undiffusional. From obtained results, mechanism of the reverse transformation induced by the shot-peening for the Fe-33mass%Ni alloy is discussed.

11:20 AM  
EBSD Analysis of Dual γ/ε Phase Microstructures in Tensile-deformed Fe-Mn-Si Shape Memory Alloy: Wataru Tasaki1; Takahiro Sawaguchi2; Koichi Tsuchiya1; 1University of Tsukuba; 2National Institute for Materials Science
    Crystallographic and microstructural features of a dual γ/ε phase developed in an Fe-28Mn-6Si-5Cr (mass-%) shape memory alloy under tensile deformation were analyzed using electron back-scattered diffraction. The observations were carried out on the specimens tensile deformed to 10%, 20% strain and to fracture, at just above Ms temperature (273 K) and just below Md temperature (423 K), respectively. It was suggested that the deformation temperature affects not only kinetics of deformation-induced martensitic transformation, but also further plastic deformation in the deformation-induced ε-phase and residual γ-phase. Due to the Shoji-Nishiyama and twinning relationships in the γ- and ε-phases, misorientation profiles in the ε-phase exhibited peaks at typical angles of 30°, 70.5°, 86°, and 94°. The misorientation angles deviated from the crystallographic relations when deformation temperature and tensile strain were increased, due to significant dislocation activity in the γ- and ε-phases. The formation mechanism of the ε-twins will also be discussed.

11:35 AM  
Effect of Bainite Formation on Martensitic Transformation and Cyclic Stability of Cu-Al-Mn Shape Memory Alloys: Nazim Babacan1; Kadri Can Atli2; Osman Selim Turkbas1; Ibrahim Karaman3; Benat Kockar4; 1Gazi University; 2Anadolu University; 3Texas A&M University; 4Hacettepe University
    Cyclic stability of Cu73Al16Mn11 (at.%) shape memory alloy during stress-free and isobaric heating-cooling experiments was investigated via heating the samples to different upper cycle temperatures (UCTs). No residual strain was observed up to 60 MPa in the isobaric heating-cooling experiments under incremental stress magnitudes for the UCT of 105˚C. Therefore, 60 MPa was chosen for further cyclic isobaric heating-cooling experiments to investigate the effect of UCTs on the cyclic stability. The results have shown that transformation temperatures increased with increasing UCT above 120C due to ordering of the parent phase. Bainitic transformation started with the further increase in the UCT and became more effective in preventing martensitic transformation with the increase in the number of cycles and the magnitude of the UCT. Bainite formation led to the inhibition of martensitic transformation by predominantly decorating the grain boundaries and thus, cyclic stability of the alloy was degraded.

11:50 AM  
Low Hysteresis in Strain-engineered Bulk Shape Memory Alloys via Containing Nanowires: Hongxiang Zong1; Xiangdong Ding1; Turab Lookman2; Jun Sun1; 1Xi’an Jiaotong University; 2Los Alamos National Lab
    The hysteresis accompanying a first-order martensitic transformation degrades the functional stability of shape memory alloys (SMAs) in devices that require high sensitivity, high durability and high energy efficiency. We demonstrate how superelasticity with low hysteresis and large recoverable strain can be achieved by essentially matching the transformation strain of bulk SMAs to the elasticity of metallic nanowires. We use molecular dynamics simulations to investigate a nanocomposite model, which contains nanowires embedded in an SMA matrix. Our results indicate that strain-matching generates an internal stress distribution or heterogeneously distributed spontaneous lattice distortion, which makes the martensitic transformation continuous. Simultaneously, the SMA matrix acts as a high-capacity sink for dislocations, enabling the embedded nanowires to retain their intrinsic large elastic strain.