Design of Heusler Precipitation Strengthened NiTi- and PdTi-Base SMAs for Cyclic Performance

Abstract

For a wide range of actuation applications, the performance of NiTi-based shape memory alloys is limited by cyclic instability associated with accommodation slip. For medical applications, low-Ni compositions are also desirable. Increasing yield strength via precipitation of a coherent nanoscale Ni 2 TiAl-type Heusler phase from a supersaturated B2 matrix is an effective approach for eliminating slip in order to improve the stability of the functional response and increase the structural fatigue life. Quaternary additions that partition into the L2 1 Heusler phase, such as Zr or Pd, are favorable for reducing interphase misfit and maintaining coherency during aging. Phase relations and precipitation kinetics in quaternary Ni(TiZrAl), low-Ni (PdNi)(TiAl), and Ni-free (PdFe)(TiAl) systems are summarized from TEM and atom probe tomography data in the literature. Strengthening behavior during isothermal aging is compared in the NiTiZrAl and PdNiTiAl systems, and recent work characterizing a high-strength, low-Ni ''Hybrid'' (PdNi)(TiZrAl) alloy is presented. A systems design approach is taken in which an optimal microstructure for peak strengthening is identified while other property objectives such as transformation temperature, misfit, radiopacity, and biocompatibility are satisfied.

Composition, Compatibility, and the Functional Performances of Ternary NiTiX High-Temperature Shape Memory Alloys

“…While significant developments have been made in models of chemistry-microstructure-property relationships (e.g., [13][14][15][16]), predicting the performances of new SMAs solely through computations remains an open challenge. Thus, establishing an efficient procedure to analyze experimentally measured process-structure-property trends of SMA systems is a critical tool toward predicting and designing new SMAs.…”

Computational Thermodynamics and Kinetics-Based ICME Framework for High-Temperature Shape Memory Alloys

“…This is followed by a discussion of the use of computational kinetics approaches to predict the microstructural evolution in SMA systems, particularly those modified through nanoprecipitation [7,17]. We then briefly discuss some very recent attempts to combine computational thermodynamics and kinetics into a design framework for precipitate-engineered HTSMAs [18]. The paper then concludes by outlining some of the major challenges that need to be overcome for ICME approaches to become widely used in the HTSMA community.…”