Theory and Methods for Martensite Design: Session 7
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Friday 8:00 AM
July 14, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Wenzheng Zhang, Tsinghua University

8:00 AM  Invited
A Unified Theory for Dislocation Glide, Twinning, Trip and Martensite Transformation Processes: Pedro Rivera-Diaz-del-Castillo1; Enrique Galindo-Nava1; Isaac Toda-Caraballo1; 1University of Cambrdige
    A single framework describing the conditions for dislocation glide, transformation induced plasticity and twinning is presented. The work takes as a starting point Olson and Cohen’s theory for martensite nucleation, developing it further to quantify the energy formation of micron-scale epsilon and twin bands. The work naturally incorporates the effects of composition, grain size, temperature and strain rate. Critical input to the model is the stacking fault energy, a discussion of several approaches to obtain this from the literature is presented. The plasticity behaviour of a range of stainless and TWIP steels is well described by the new theory, which can be used to design alloys of tailored stress/strain response.

8:40 AM  
Design of a Core-Shell Structure Carbide for Enhancing Toughness of UHS Steels: Wei Xiong1; Ye Tian2; Oleg Kontsevoi2; Greg Olson2; 1University of Pittsburgh; 2Northwestern University
    Low fraction Zener pinning particles of nano-scale size are often employed for grain refinement after homogenization. However, at ambient temperature service, these particles act as microvoid nucleation sites, participating in the process of ductile fracture. In order to reduce their microvoid softening effects, it is desirable to strengthen the interface bonding between matrix and these MC carbides. We employ the DFT-FLAPW method to compute the interfacial adhesion between matrix bcc Fe and different MC carbide phases. The calculated separation energy clearly demonstrates the sequence of cohesion energy by substituting different species in the metal sublattice. Since TiC-VC shows the strongest adhesion and high thermodynamic stability, a two-step processing was designed employing a CALPHAD database, which leads to a successful demonstration of Zener pinning particles with TiC-VC core-shell structures. This work is a successful demonstration of the Materials by Design technique using a combined approach of DFT, CALPHAD and focused experiments.

9:00 AM  
Simulation and Theory Reveal the Austenite/Martensite Interface Structure and Glissile Transformation to Provide Guidelines for Steel Design: Francesco Maresca1; William Curtin1; 1Ecole Polytechnique Federale de Lausanne (EPFL)
    The austenite/martensite (fcc/bcc) interface dominates a broad class of high-strength steels. Decades of research left unclear both its defect structure and mechanism of motion. We have used atomistic simulations to create a glissile fcc-bcc iron interface with a defect structure completely consistent with experiments, namely [-101]fcc screws gliding on (111)fcc, as envisioned by previous theories, and [1-11]bcc edges gliding on (-101)bcc, which was never envisioned before. Based on this newly discovered interface structure, we have proposed a revised, parameter-free double-shear theory of lath martensite. Predictions of the theory are in very good agreement with experiments in Fe-Ni-Mn and Fe-C, and show that the dominant factor controlling the transformation strain, and hence toughening, is the lattice parameter ratio. This new understanding of this interface can provide guidelines for the design of new high strength steels.

9:20 AM  
Cofactor Conditions - the Super Compatibility Conditions for Low-hysteresis Martensitic Materials: Xian Chen1; Richard James2; 1The Hong Kong University of Science and Technology; 2University of Minnesota
    Martensitic materials have been exploited for many emerging applications such as biomedical devices, MEMS and solid-state cooling systems. The functionalities of these materials usually degrade significantly after only few transformation cycles. The origin of such degradation comes from the formation of microstructure consisting stressed- transition layers due to lattice mismatch at interfaces. It has been proven that when the lattice parameters satisfy a set of kinematic conditions of compatibility, i.e. Cofactor Conditions, both hysteresis and reversibility can be optimized, thus long life-time is achieved for these applications. In this talk, we will present the formulation of Cofactor Conditions and derive the microstructure of materials satisfying these conditions. We will also introduce the algorithm for determination of the transformation stretch tensor of phase transformation, and the experimental approaches to quantitatively examine the cofactor conditions, formation of microstructure and the micromechanical behaviors of a real material closely satisfying the Cofactor Conditions.

9:40 AM  
Modeling NiTi Superelasticity in Presence of Coherent Precipitate: Piyas Chowdhury1; Guowu Ren2; Huseyin Sehitoglu1; Luca Patriarca3; 1University of Illinois at Urbana-Champaign; 2China Academy of Engineering Physics; 3Politecnico di Milano
    Atomistic simulations are performed to examine the role of a coherent Ni4Ti3 precipitate on the pseudoelasticity of austenitic NiTi alloys using molecular dynamics. A lens-shaped precipitate of rhombohedral lattice structure is embedded in a B2 austenite, and energetically stabilized. A local stress distribution at the matrix-precipitate periphery is generated as a direct consequence of: (a) the inter-lattice atomic misfit and (b) the moduli mismatch. Due to the presence of the local fields, the preference for activating different martensitic variants, given the uni-directionality thereof, is influenced substantially. Constitutive response is thus observed to undergo an adjustment in the form of reduced transformation stress, strain and hysteresis. Mechanistic implications of the model output are thoroughly investigated in the light of experimental behaviors. Results are relevant for understanding how martensite evolves during deformation, a knowledge which could prove critical in novel alloy design.

10:00 AM Break