Computational and Experimental Design of Novel CoNiGa High Temperature Shape Memory Alloys

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

TECHNICAL: Shape memory alloys (SMAs) are an important class of smart materials that can produce large recoverable shape changes as a result of reversible martensitic phase transformations, which can be triggered by changes in temperature, stress or magnetic field. Currently, practical uses for SMAs are limited to temperatures below 1000 C, i.e. the transformation temperature (TT) limits of NiTi binary and Cu?]based systems, the most successful SMAs to date. Recently, SMAs based on the CoNiAl/Ga systems have shown promising characteristics, such as broad range of TT control through compositional adjustments, cheaper constituents than Pd, Pt, and Au used in NiTiX high temperature SMAs (HTSMAs), higher thermal stability, and less affinity to oxygen/carbon. Moreover, the fact that a third, hard, ordered phase is often at equilibrium with the transformable phases, offers the possibility of forming nanocomposites to improve the cyclic stability, creep susceptibility, and shape memory (SM) properties of such alloys. The ultimate goal of the current work is to develop CoNiGa HTSMAs through a combined computational/experimental approach. The specific objectives are to: 1) maximize TTs through computational/experimental composition optimization of the high symmetry transformable phase; 2) determine the phase equilibria to find optimal compositions and temperatures at which three?]phase systems can be found; 3) design heat treatment schedules through predicted/experimentally determined kinetics of the precipitation/dissolution of secondary phases; 4) fabricate and characterize the SM and PE responses of single crystals as a function of distribution and amount of secondary phases and crystallographic orientation; 5) explore the selection of hard nanoprecipitate variants through constrained aging and their effects on cyclic stability and creep of SM properties; and 6) demonstrate improved formability of polycrystalline CoNiGa HTSMAs in the presence of ductile secondary phases. The scientific merits of this transformative research are: (a) the potential of revolutionizing the field of HTSMAs, by moving away from current expensive, unstable and brittle materials to HTSMAs possessing high thermal stability, low susceptibility to creep, aging and cyclic damage as well as good ductility to improve the formability of polycrystals for transportation, aerospace, and public utility applications.; (b) transformation of the design process for new HTSMAs through the synergistic experimental computational approach; (c) the creation of an entirely new family of SMAs, i.e. shape memory superalloys, with very high temperature (500?]7000C) SM and PE properties. NON-TECHNICAL: The broader impacts of the activities are reflected in the following areas: (a) maintaining US leading role in active materials research despite the vast amount of recent works at overseas on SMAs; (b) development of teaching modules for incorporation into undergraduate courses; (c) helping K-12 students in developing science projects with SMAs; (d) enriched graduate and undergraduate research experiences in coordination with the new IGERT and Nanomaterials certificate programs; (e) development of a graduate course in computational materials science; (e) involvement of underrepresented groups; (f) disseminating the knowledge generated to both academia and industry through the presentations, publications, and a website, and (g) close collaboration with industry.