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The newly developed FeMnAlNiTi shape memory alloy (SMA) holds significant promise due to its desirable properties including ease of processing, room temperature superelasticity, a wide superelastic window of operation, and high transformation stress levels. In this study, we report single crystals with tensile axis near h123i exhibiting transformation strains of 9% with a high trans- formation stress of 700 MPa. The functional performance revealed excellent recovery of 98% of the applied strain in an incremental strain test for each of the 40 applied cycles. Concomitantly, the total residual strain increased after each cycle. Accumulation of residual martensite is observed possibly due to pinning of austenite/martensite (A/M) interface. Subsequently, under structural fatigue loading with a constant strain amplitude of 1%, the recoverable strains saturate around 1.15% in local residual martensite domains. Intermittent enhancement of recoverable strains is observed due to transformation triggered in previously untransformed domains. Eventually, fatigue failure occur- red after 2046 cycles and the dominant mechanism for failure was microcrack initiation and coalescence along the A/M interface. Thus, it is concluded that interfacial dislo- cations, which play a crucial role in the superelastic (SE) functionality, invariably affect the structural fatigue per- formance by acting as the weakest link in the microstructure.
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Shape Memory Alloys – Frontier Developments
This article is mainly concerned with summarizing the recent discoveries in the deformation physics of shape memory materials with special emphasis on (1) the need to develop fundamental understanding of twinning, slip, shear/shuffle at atomic scales, martensite twin boundary topologies, atomic stacking and stable/metastable fault structures in shape memory materials, and (2) factors that result in performance degradation and accumulation of residual strains leading to fatigue and fracture. The fundamental understanding has benefitted from ab-initio modeling of atomic-electronic structures and the fault energetics of stable/metastable crystal structures. Especially, the slip phenomenon during nucleation of martensite needs further elaboration, as it exercises a strong influence on hysteresis, functional and mechanical degradation of the shape memory alloys Finally, we discourse on studies of fatigue and fracture of shape memory alloys from the literature and outline efforts to explain the complex experimental trends with respect to fatigue thresholds and nucleation. It is in the fatigue area where advancing the understanding of cycle by cycle accumulation of the irreversibilities, the strong orientation dependence of slip resistance (i.e., non-Schmid behavior), complex evolution of elastic moduli, strain sensitive evolution of interfaces with terrace-disconnection energy minimal nanostructures, and asymmetric stress-strain response will lead to the development of a comprehensive model. In the case of fracture, continuum models employ LEFM concepts and the results need corrections for martensite-induced tractions which are rather complex with localized variants. Overall, deeper scientific activities, with potential use of lattice scale theories and ab-initio based empirical atomic potentials, are paramount to advance the field to the next level.
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- Award ID(s):
- 2104971
- PAR ID:
- 10587490
- Editor(s):
- Aliabadi, Ferri MH; Soboyejo, Winston Wole
- Publisher / Repository:
- Elsevier
- Date Published:
- ISBN:
- 9780323919456
- Page Range / eLocation ID:
- 610 to 679
- Subject(s) / Keyword(s):
- shape memory, superelasticity, NiTi
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Book Chapter:
- https://doi.org/10.1016/B978-0-12-822944-6.00058-X
