Microstructural Response During Isothermal and Isobaric Loading of a Precipitation-Strengthened Ni-29.7Ti-20Hf High-Temperature Shape Memory Alloy

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 43; no. 12; pp. 4539 - 4552
• Main Authors: Benafan, O., Noebe, R. D., Padula, S. A., Vaidyanathan, R.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.12.2012; Springer; Springer Nature B.V
• Subjects: Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Exact sciences and technology; Materials Science; Metallic Materials; Metals. Metallurgy; Microstructure; Nanotechnology; Nickel alloys; Precipitation hardening; Structural Materials; Surfaces and Interfaces; Thin Films; Neutron Diffraction; Martensite; Austenite Phase; Austenite; Thermal Cycle; Precipitation; Temperature; High temperature; Shape memory alloy; Microstructure; Precipitation hardening
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-012-1297-z

A stable Ni-rich Ni-29.7Ti-20Hf (at. pct) shape memory alloy, with relatively high transformation temperatures, was shown to exhibit promising properties at lower raw material cost when compared to typical NiTi-X (X = Pt, Pd, Au) high-temperature shape memory alloys (HTSMAs). The excellent dimensional stability and high work output for this alloy were attributed to a coherent, nanometer size precipitate phase observed using transmission electron microscopy. To establish an understanding of the role of these precipitates on the microstructure and ensuing stability of the NiTiHf alloy, a detailed study of the micromechanical and microstructural behaviors was performed. In-situ neutron diffraction at stress and temperature was used to obtain quantitative information on phase-specific internal strain, texture, and phase volume fractions during both isothermal and isobaric testing of the alloy. During isothermal testing, the alloy exhibited low isothermal strains due to limited detwinning, consistent with direct measurements of the bulk texture through neutron diffraction. This limited detwinning was attributed to the pinning of twin and variant boundaries by the dispersion of fine precipitates. During isobaric thermal cycling at 400 MPa, the high work output and near-perfect dimensional stability was attributed to the presence of the precipitates that act as homogeneous sources for the nucleation of martensite throughout the material, while providing resistance to irrecoverable processes such as plastic deformation.