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Abstract BackgroundSuperelastic shape memory alloys (SMAs) such as nickel-titanium, also known as Nitinol, recover large deformations via a reversible, stress-induced martensitic transformation. ObjectivePartitioning the deformation into the contributions from superelasticity and plasticity and quantifying the interaction between these mechanisms is key to modeling their fatigue behavior. MethodsWe capture these microscopic interactions across many grains using a combination of scanning electron microscopy digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD). Modeling our data as a statistical distribution, we employ a Gaussian Mixture Model (GMM) soft clustering framework to understand how these mechanisms interact and evolve as a function of global strain. ResultsOur findings show that, under globally-applied uniaxial tensile loading, plasticity bridges deformation in regions where competing positive and negative martensitic shear bands intersect. Early stage transformation-induced plasticity is concentrated at these intersections and forms concurrently with the Lüders-like martensitic transformation front, often appearing with a zig-zag pattern that is linked to compound twinning at the martensite-martensite interface. At higher strains, austenite slip is activated as a second mechanism of plastic deformation. ConclusionsWe propose that this plastic bridging mechanism underpins the prestrain effects previously reported in the literature, where higher prestrains can enhance the fatigue strength of superelastic materials within a given loading mode.
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Fatigue Crack Initiation in the Iron-Based Shape Memory Alloy FeMnAlNiTi
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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- Award ID(s):
- 1709515
- PAR ID:
- 10180317
- Date Published:
- Journal Name:
- Shape Memory and Superelasticity
- ISSN:
- 2199-384X
- 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
- Journal Article:
- https://doi.org/10.1007/s40830-020-00293-z
