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The present work investigates fracture toughness, and actuation and mechanical fatigue crack growth responses of Ni50.3Ti29.7Hf20 HTSMAs across martensitic transformation with two different microstructures, one with H-phase nanoprecipitates and one without. H-phase precipitation is known to stabilize the actuation cycling response of NiTiHf HTSMAs and notably impacts transformation-induced plasticity. The fracture toughness tests performed reveal that precipitate-free NiTiHf has a higher fracture toughness and undergoes significantly more inelastic deformation than the one with the precipitates resulting in toughness enhancement, i.e., stable crack advance during fracture toughness experiments, which is not observed in the precipitated NiTiHf for the crack configuration and loading conditions tested. Furthermore, the precipitate free NiTiHf has higher actuation and mechanical fatigue crack growth resistance than the precipitation-hardened microstructure. This is attributed to plasticity buildup, which exacerbates the manifestation of retained martensite upon repeated transformations. The fatigue crack growth rates obtained from both actuation and mechanical fatigue experiments align to a single Paris Law Curve for the precipitation-hardened NiTiHf. This work aims to determine if unified Paris Law curves can be generated from mechanical and actuation fatigue experiments, irrespective of composition and microstructure, to estimate actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.
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Property and microstructure of Ni50.3Ti29.7Hf20 high-temperature shape memory alloys with different aging conditions
NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications.
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- Award ID(s):
- 2004752
- PAR ID:
- 10503850
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Acta Materialia
- Volume:
- 265
- Issue:
- C
- ISSN:
- 1359-6454
- Page Range / eLocation ID:
- 119642
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.actamat.2023.119642
