The authors report on a relatively new alloy, Ni54Ti45Hf1, that exhibits strengths more than 40% greater than those of conventional NiTi-based shape memory alloys − 2.5 GPa in compression and 1.9 GPa in torsion − and retains those strengths during cycling. Furthermore, the superelastic hysteresis is very small and stable with cycling. Aging treatments are used to induce a very high density of Ni4Ti3 precipitates, which impede plasticity during cycling yet do not impart substantial dissipation to the reversibility of the phase transformation. Pairing compression testing with high-energy synchrotron X-ray diffraction and aberration-corrected electron microscopy provides an in-depth look at the structure-property relationships of this alloy. Specifically, it is found that a combination of small, untwinned retained martensite laths, and dislocations on the austenite-martensite interfaces primarily strengthen the alloy as opposed to dislocation networks. Furthermore, some combination of nanoprecipitation and interface dislocations is responsible for the remarkably low mechanical hysteresis exhibited by this material.
Bibliographical noteFunding Information:
ANB and APS acknowledge support of this work from NASA contract NNC14VB99P, which supported the crystallographic calculations. ANB also acknowledges support from NSF Fellowship DGE-1057607, which supported her efforts writing her results; APS and GMH NSF-CAREER Award #1454668 from CMMI-MoMS, which supported the X-ray and mechanical testing measurements and analyses and writing of this article. This work has benefitted from the use of CHESS, which was funded under NSF grant DMR-0936384; specifically, the X-ray measurements and time and support from M.O. and D.C.P. where made possible by the CHESS facility. Support for processing the materials for this effort came from the NASA Fundamental Aeronautics Program and the Aeronautical Sciences and Transformational Tool & Technologies Projects (technical lead Dale Hopkins). We also thank Drs. Jacob Ruff and Abbas Ebnonnasir for assistance with the synchrotron experiment. LC and MJM acknowledge support from the Mechanical Behavior and Radiation Effects Program Award No. DE-SC0001258 for the electron microscopy characterizations and writing of this article, Department of Energy, Office of Basic Sciences (Dr. John Vetrano, Program Manager). We thank Dr. Anita Garg of University of Toledo/NASA Glenn for discussions about the microstructures and early electron microscopy work on this material system.
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- In situ x-ray diffraction
- Scanning transmission electron microscopy
- Shape memory alloy