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Texture Evolution during Isothermal, Isostrain and Isobaric Loading of Polycrystalline Shape Memory NiTi
D E Nicholson1, S A Padula2, O Benafan2
1Advanced Materials Processing and Analysis Center (AMPAC); Materials Science and Engineering Department; Mechanical and Aerospace Engineering Department; University of Central Florida, 12760 Pegasus Drive, Orlando, FL 32816, USA.
Applied Physics Letters
|December 15, 2018
Summary
Shape memory NiTi variant microstructures depend on strain, not stress. This finding allows for optimized training paths to improve alloy stability and fatigue life in applications.
Area of Science:
- Materials Science
- Metallurgy
- Crystallography
Background:
- Shape memory alloys (SMAs) like nickel-titanium (NiTi) exhibit unique properties due to reversible martensitic transformations.
- Understanding the evolution of martensite variant microstructures under various loading conditions is crucial for their application.
Purpose of the Study:
- To investigate the influence of isothermal, isobaric, and isostrain loading on martensite variant microstructures in NiTi.
- To determine the correlation between loading conditions (strain and stress) and the resulting microstructure.
- To assess the impact of thermomechanical loading history on microstructure reversibility and stability.
Main Methods:
- Utilizing *in situ* neutron diffraction to observe martensite variant microstructures in real-time.
- Applying controlled isothermal, isobaric, and isostrain loading conditions to NiTi samples.
- Analyzing microstructure evolution and reversibility under different thermomechanical cycles.
Main Results:
- Martensite variant microstructures were found to be equivalent for corresponding strain levels, irrespective of the loading path (isothermal, isobaric, or isostrain).
- Reversibility and equivalency of microstructures were observed even after reorientation following different loading sequences.
- The resulting variant microstructure correlated directly with applied strain, showing no significant correlation with stress.
- Microstructures formed isothermally were stable during subsequent thermal cycling under constant strain.
Conclusions:
- The martensite variant microstructure in NiTi is primarily dictated by the applied strain, not the stress.
- This strain-controlled behavior allows for flexibility in selecting thermomechanical loading paths for training SMAs.
- Optimized training sequences can minimize cycles, enhancing the stability and fatigue life of NiTi in actuators and medical devices.
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