Theory and Methods for Martensite Design: Session 8
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Friday 10:20 AM
July 14, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Pedro Rivera-Diaz-del-Castillo, Lancaster University

10:20 AM  
In-situ Observation of Transformation Behavior of Lath Martensite in Steels with Various Strength of Austenite: Shoichi Nambu1; Takahiro Moriguchi1; Toshihiko Koseki1; 1The University of Tokyo
    The microstructure of lath martensite such as size, morphology and variant pairing which depend on the carbon content in steels should be controlled to achieve improved mechanical properties, so the understanding of the microstructure development during martensitic transformation is very important. In this study, transformation behavior including formation process and variant selection of lath martensite in steels with various chemical compositions was investigated by combining in-situ observation using a confocal laser microscopy and crystallographic analysis using a SEM/EBSD. The yield stresses of austenite and martensite start temperatures of steels were systematically varied by their chemical compositions. It was found that the microstructure such as block size becomes smaller with increase of yield stress but does not depend on martensite start temperature. The formation process and variant selection were also different according to the yield stress of austenite probably due to the different accommodation behavior.

10:40 AM  
Effect of Mo Content on Hardenability of Mo-B Combined Added low C Steels: Kyohei Ishikawa1; Hirofumi Nakamura1; Ryuichi Homma1; Masaaki Fujioka1; Manabu Hoshino1; 1Nippon Steel & Sumitomo Metal Corporation
    The limit of the Mo-B combined effect on hardenability was investigated in 0.15%C - B added steels containing 0 to 1.5%Mo. The hardenability of Mo-B steels increases up to Mo=0.75% suppressing the precipitation of Fe23(C,B)6. In contrast, the effect saturates in Mo>0.75% where Mo2FeB2 precipitates instead of Fe23(C,B)6. The precipitation temperature of Mo2FeB2 is assumed to increase with the increase in the Mo contents by Thermo-dynamical calculation. It is suggested that the precipitation of Mo2FeB2 during reheating determines the limit of the Mo-B combined effect.

11:00 AM  
An Interpretation on Kinetics of Martensitic Transformations in Some Shape Memory Alloys: Tomoyuki Kakeshita1; Takashi Fukuda1; 1Osaka University
    Our previous studies on kinetics of martensitic transformations using iron-based alloys revealed that an athermal martensitic transformation changes to an isothermal one by the application of hydrostatic pressure and vice versa by magnetic field. These results lead us to a new interpretation that all martensitic transformations possess isothermal nature. Recently, we have found the alloys exhibiting thermoelastic martensitic transformations also possess isothermal nature such as a Ti-Ni alloy and a Ni-Co-Mn-In magnetic shape memory alloy. In this presentation, we will show these results. Ti-Ni and Ni-Co-Mn-In alloys show a clear C-curve on the TTT diagram. By fitting the C-curve using a thermal activation model for martensitic transformations previously proposed, we have estimated the size of a nuclei of martensite. The estimated size of a nuclei is (2.2nm)^3 for a Ti-Ni alloy and (5nm)^3 for a Ni-Co-Mn-In alloy. These values are comparable with (1.5nm)^3 for Fe-Ni alloys previously studied.

11:35 AM  
Graph Theory Framework of Martensitic Crystallography: Yipeng Gao1; Yunzhi Wang1; 1The Ohio State University
    The classical phenomenological theory of martensitic crystallography is the first crystallography theory of structural phase transformations, which focuses on pattern formation during martensitic transformations. The theory includes two basic elements, one establishes a connection among multiple structural states (i.e., austenite states and martensite states) by crystallographically equivalent transformation pathways, and the other establishes a connection between morphological pattern formation and these structural states. We present a generalized mathematical framework capturing both of these elements through a combination of group theory, graph theory and continuum theory. By analyzing the pathway connectivity during transformation cycling, we investigate defect generation during martensitic transformations under either thermal or stress cycling in a number of typical systems (e.g., steels, NiTi alloys, Ti-based alloys, etc.) and show that dislocations and special grain boundaries are generated because of the broken symmetry during the transformations, which provides new insight into defect engineering through martensitic transformations.

11:50 AM  
Interaction of Martensitic Microstructures in Adjacent Grains: John Ball1; Carsten Carstensen2; 1University of Oxford; 2Humboldt Universitat
     It is often observed that martensitic microstructures in adjacent polycrystal grains are related. For example, micrographs of Arlt (J. Mat. Science 25 (1990) 2655-2666) exhibit propagation of layered structures across grain boundaries in the cubic-to-tetragonal phase transformation in BaTiO3. Such observations are related to requirements of compatibility of the deformation gradients at the grain boundary.This is explored using the nonlinear elasticity model of martensitic transformations for the case of a bicrystal with suitably oriented columnar geometry, in which the microstructure in both grains is assumed to involve just two martensitic variants, with either a planar or curved grain boundary. Generalizations of the Hadamard jump condition are used to express the compatibility at the grain boundary, leading to conditions on the shape of the boundary under which a zero-energy microstructure which is a pure phase (i.e. has constant deformation gradient) in one of the grains is possible or excluded.

12:10 PM Break