Thermomechanical Instabilities in M-Lattices with Application to Shape Memory Materials

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

THERMOMECHANICAL INSTABILITIES IN M-LATTICES WITH APPLICATIONS TO SHAPE MEMORY ALLOYS Nicolas Triantafyllidis and John A. Shaw Department of Aerospace Engineering The University of Michigan 1. Project Summary Shape memory alloys (SMAs) are technologically important materials, because of two useful features: the shape memory effect and the pseudoelastic effect. Their interesting thermomechanical response makes them attractive as low frequency robust actuators and passive devices for a variety of novel applications. The thermo-mechanical behavior of SMAs is due to displacive, first-order phase transformations (or martensitic transformations), which produce complex, finescale microstructures. This feature is at the heart of modeling difficulties encountered in the continuum descriptions of these materials. Existing work on continuum modeling of SMAs is based on finite thermoelasticity theory and requires the use of phenomenological strain energy density functions. Solutions of boundary value problems based on these functions are successful in predicting many features of the experimentally observed fine scale microstructures. This approach, however, leaves a host of other problems unanswered, such as the lattice-level origin of the instabilities, the number of coexisting phases and the characteristic wavelengths of the observed microstructures. The novel feature of the proposed investigation is the use of atomic lattice simulations of SMAs, and in particular for NiTi, the most technologically promising material in its class. Our goal is to generate realistic energy densities and to simulate and understand the stability phenomena at the nanometer scale. Specifically, we intend to find all equilibrium phases for temperature and stress and their corresponding microstructures (twinning and crystal interfaces) and study their stability with the ultimate objective to bridge the scales between the nano-(lattice) phenomena and micro- (single crystal) behavior. Besides advancing a fundamental understanding of SMAs at the nanoscale, the work has the potential to help answer why certain intermetallic alloys exhibit shape memory phenomena and other do not, and this work could eventually be used to guide the search for new SMAs. The research team consists of two investigators, Professors Nicolas Triantafyllidis and John Shaw, who bring complementary expertise to the project. The requested funds will fund one doctoral student over a period of three years plus a one year post-doctoral appointment for Mr. Ryan Elliott, who is working with the PI's on this area. Collaboration with the Theoretical Physics group of Los Alamos National Labs is underway through hosting of the graduate students during the summers. Results of this research will be used in a new graduate level, cross-disciplinary course on atomistic modeling in solid mechanics. Broader Impact of the proposed work: This work fits well within the current nanomechanics initiative at NSF, since it pertains to the study of underlying lattice stability phenomena of m-crystals from atomistic considerations. Understanding these phenomena will eventually be useful in the conception of novel materials with shape memory effects. The proposed work not only addresses a fundamental problem in mechanics, but it also requires input from other disciplines, i.e. condensed matter physics and applied mathematics. The existing collaboration with Los Alamos National Labs provides a natural framework for the interaction with these disciplines and for the dissemination of the knowledge to be created.