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Stress Induced Martensitic Transformation in NiTi at Elevated Temperatures: Martensite Variant Microstructures, Recoverable Strains and Plastic Strains

SSRN, ISSN: 1556-5068
2024
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Article Description

In order to shed light on the origin of the loss of functional properties of NiTi with increasing temperature, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by a special kind of thermomechanical testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains of deformed wires by transmission electron microscopy.It is found that the forward stress induced martensitic transformation is not completed at the end of the stress plateau and generates unrecoverable plastic strain in addition to recoverable transformation strain. The higher the test temperature, the larger is the plastic strain as well as the volume fraction of retained austenite. The reconstructed martensite variant microstructures in grains of the NiTi wire deformed up to the end of the stress plateau at 120 °C were found to be filled with single domains of (001) compound twin laminate, some grains were nearly detwinned and some grains contained (100) deformation twins. The recoverable transformation strain (~6%) generated by the forward martensitic transformation is nearly independent on the test temperature (plateau stress) because detwinning of (001) compound twins in the microstructure is prohibited by lateral constraint from the surrounding grains in nanocrystalline wire.It is concluded that the gradual loss of functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the lack of resistance of the stress induced martensite to the plastic deformation under increasing stress.

Bibliographic Details

Ondřej Tyc; Elizavieta Iaparova; Orsolya Molnárová; Luděk Heller; Petr Šittner

Elsevier BV

Multidisciplinary; shape memory alloy; NiTi; martensitic transformation; tensile test; TEM; martensite variant microstructure

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