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Title: Thermomechanical behavior and microstructural evolution of a Ni(Pd)-rich Ni24.3Ti49.7Pd26 high temperature shape memory alloy

We investigated the effect of thermomechanical cycling on a slightly Ni(Pd)-rich Ni24.3Ti49.7Pd26 (near stochiometric Ni–Ti basis with Pd replacing Ni) high temperature shape memory alloy. Furthermore, aged tensile specimens (400 °C/24 h/furnace cooled) were subjected to constant-stress thermal cycling in conjunction with microstructural assessment via in situ neutron diffraction and transmission electron microscopy (TEM), before and after testing. It was shown that in spite of the slightly Ni(Pd)-rich composition and heat treatment used to precipitation harden the alloy, the material exhibited dimensional instabilities with residual strain accumulation reaching 1.5% over 10 thermomechanical cycles. This was attributed to insufficient strengthening of the material (insufficient volume fraction of precipitate phase) to prevent plasticity from occurring concomitant with the martensitic transformation. In situ neutron diffraction revealed the presence of retained martensite while cycling under 300 MPa stress, which was also confirmed by transmission electron microscopy of post-cycled samples. Neutron diffraction analysis of the post-thermally-cycled samples under no-load revealed residual lattice strains in the martensite and austenite phases, remnant texture in the martensite phase, and peak broadening of the austenite phase. The texture we developed in the martensite phase was composed mainly of those martensitic tensile variants observed during thermomechanical cycling. Presence of amore » high density of dislocations, deformation twins, and retained martensite was revealed in the austenite state via in-situ TEM in the post-cycled material, providing an explanation for the observed peak broadening in the neutron diffraction spectra. Despite the dimensional instabilities, this alloy exhibited a biased transformation strain on the order of 3% and a two-way shape memory effect (TWSME) strain of ~2%, at relatively high actuation temperatures.« less
 [1] ;  [2] ;  [1] ;  [1] ;  [1] ;  [3] ;  [4] ;  [5] ;  [5]
  1. NASA Glenn Research Center, Cleveland, OH (United States). Structures and Materials Division
  2. NASA Glenn Research Center, Cleveland, OH (United States). Structures and Materials Division; Univ. of Toledo, OH (United States)
  3. NASA Glenn Research Center, Cleveland, OH (United States). Structures and Materials Division; Ohio Aerospace Inst., Cleveland, OH (United States)
  4. Univ. of Central Florida, Orlando, FL (United States). Advanced materials Processing and Analysis Center
  5. Los Alamos National Lab. (LANL), Los Alamos, NM (United States). Materials Science and Technology Division
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 0925-8388
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of Alloys and Compounds
Additional Journal Information:
Journal Volume: 643; Journal Issue: C; Journal ID: ISSN 0925-8388
Research Org:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
36 MATERIALS SCIENCE High temperature shape memory alloy; Neutron diffraction; NiTiPd; Actuation; Two-way shape memory effect