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

JOURNAL OF ALLOYS AND COMPOUNDS

卷:643

Article

Benafan, O.¹  Garg, A.^1,2  Noebe, R. D.¹  Bigelow, G. S.¹  Padula, S. A., II¹  Gaydosh, D. J.^1,3  Vaidyanathan, R.⁴  Clausen, B.⁵  Vogel, S. C.⁵ 

[1] NASA Glenn Res Ctr, Struct & Mat Div, Cleveland, OH 44135 USA

[2] Univ Toledo, Toledo, OH 43606 USA

[3] Ohio Aerosp Inst, Cleveland, OH 44142 USA

[4] Univ Cent Florida, Mat Sci & Engn Dept, Adv Mat Proc & Anal Ctr, Orlando, FL 32816 USA

[5] Los Alamos Natl Lab, Div Mat Sci & Technol, Los Alamos, NM 87545 USA

关键词: High temperature shape memory alloy;    Neutron diffraction;    NiTiPd;    Actuation;    Two-way shape memory effect;   

DOI  :  10.1016/j.jallcom.2015.04.081

来源: Elsevier

【 摘 要 】

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 was investigated. Aged tensile specimens (400 degrees 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. Texture developed in the martensite phase was composed mainly of those martensitic tensile variants observed during thermomechanical cycling. Presence of a 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 similar to 2%, at relatively high actuation temperatures. Published by Elsevier B.V.

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