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Title: Superelasticity by reversible variant reorientation in a Ni-Mn-Ga microwire with bamboo grains

The link between microstructure and mechanical properties is investigated for a superelastic Ni–Mn–Ga microwire with 226 μm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse γ precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load–unload cycles, reproducible stress fluctuations of ~3 MPa are measured on the loading superelastic stress plateau of ~50 MPa. During cycles at various temperatures spanning -70 to 55 °C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of ~13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic γ precipitates and the twinning martensite matrix. Lastly, the temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [2]
  1. Northwestern Univ., Evanston, IL (United States); Beijing Inst. of Technology, Beijing (China)
  2. Northwestern Univ., Evanston, IL (United States)
  3. Beijing Inst. of Technology, Beijing (China)
  4. Argonne National Lab. (ANL), Argonne, IL (United States)
  5. Univ. of Science and Technology, Beijing (China)
  6. Boise State Univ., ID (United States)
Publication Date:
OSTI Identifier:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Acta Materialia
Additional Journal Information:
Journal Volume: 99; Journal ID: ISSN 1359-6454
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
National Science Foundation (NSF); National Key Basic Research Program of China; USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
36 MATERIALS SCIENCE; Ni2MnGa; oligocrystalline materials; shape-memory effect; size effect; superelasticity; twinning