[Paper Review] Martensitic transformation induced by cooling NiTi wire under various tensile stresses: martensite variant microstructure, textures, recoverable strains and plastic strains

Ondřej Tyc, Xiaohui Bian+4 more|arXiv (Cornell University)|Jun 21, 2024

Shape Memory Alloy TransformationsMaterials Science3 citations

TL;DR

This study investigates how tensile stress during cooling induces martensitic transformation (MT) in nanocrystalline NiTi wire, revealing that stresses above 100 MPa promote single-domain (001) compound twins, maximize recoverable strain at ~5% near 200 MPa, and generate stress-dependent plastic strains via dislocation slip and kink-band formation. The findings clarify the role of external stress in reorienting martensite variants and controlling transformation mechanics in shape memory alloys.

ABSTRACT

To understand how martensitic transformation (MT) in polycrystalline NiTi shape memory alloy (SMA) proceeds under external stress, we evaluated recoverable transformation strains and plastic strains generated by the forward MT in nanocrystalline NiTi wire cooled under tensile stresses 0-600 MPa, determined textures in martensite and reconstructed martensite variant microstructures within the selected grains of the cooled wire. The obtained findings show that the forward MT proceeding under external stresses gives rise to characteristic recoverable transformation strains, plastic strains, martensite variant microstructures and martensite textures. MT occurring upon cooling under stresses exceeding 100 MPa creates martensite variant microstructures consisting of single domain of partially detwinned laminate of (001) compound twins filling entire grains. The recoverable transformation strain increases with increasing stress, reaches maximum ~5% at ~200 MPa stress, and remains constant with further increasing stress up to 600 MPa. Starting from 100 MPa stress, the forward MT generates also plastic strains, the magnitude of which also increases with increasing stress. We propose that, in the absence of external stress, the forward MT takes place via propagation of strain compatible habit plane interfaces between austenite and second order laminate of (001) compound twins. When the forward MT takes place under external tensile stress, it occurs equally, but the newly created martensite immediately reorients into single domains of (001) compound twins, partially detwins and deforms plastically under the action of the external stress. The plastic strain generated by the forward MT upon cooling under stress is attributed to the [100](001) dislocation slip in martensite at low stresses and plastic deformation of martensite by kwinking at high stresses.

Motivation & Objective

• To understand the influence of applied tensile stress on martensitic transformation (MT) in polycrystalline NiTi shape memory alloy (SMA) wire.
• To quantify recoverable transformation strains and plastic strains generated during forward MT under varying tensile stresses (0–600 MPa).
• To characterize martensite variant microstructures and textures formed during cooling under external stress.
• To elucidate the mechanism of martensite reorientation and plastic deformation under stress during MT.

Proposed method

• Cooling nanocrystalline NiTi wire under controlled tensile stresses from 0 to 600 MPa to induce forward martensitic transformation.
• Using electron backscatter diffraction (EBSD) to reconstruct martensite variant microstructures within individual grains.
• Analyzing texture development in the martensite phase via orientation mapping and crystallographic analysis.
• Measuring recoverable transformation strain and plastic strain from in-situ or ex-situ strain measurements during cooling.
• Correlating microstructural evolution with stress-dependent transformation behavior and deformation mechanisms.
• Proposing a mechanistic model for stress-induced martensite reorientation and plastic deformation based on observed microstructures and strain data.

Experimental results

Research questions

• RQ1 How does applied tensile stress during cooling affect the microstructure of martensite variants in NiTi wire?
• RQ2 What is the relationship between applied tensile stress and the magnitude of recoverable transformation strain in NiTi wire?
• RQ3 At what stress level does plastic strain begin to contribute significantly to the total strain during forward martensitic transformation?
• RQ4 What are the dominant deformation mechanisms responsible for plastic strain in martensite under tensile stress?
• RQ5 How does the texture of the martensite phase evolve under different tensile loading conditions during cooling?

Key findings

• Recoverable transformation strain increases with applied stress, peaks at approximately 5% near 200 MPa, and remains constant up to 600 MPa.
• Plastic strain begins to develop at tensile stresses exceeding 100 MPa and increases with further stress elevation.
• At stresses above 100 MPa, martensite forms single domains of (001) compound twins with partial detwinning, indicating stress-induced reorientation.
• The forward martensitic transformation under stress proceeds via immediate reorientation of newly formed martensite into single-domain (001) compound twins, unlike the strain-compatible habit plane interface mechanism in zero-stress conditions.
• Plastic deformation at low stresses (up to ~200 MPa) is attributed to [100](001) dislocation slip in martensite, while kink-band (kinking) mechanisms dominate at higher stresses.
• The microstructure evolution indicates that external stress suppresses twinning and promotes direct formation of single-domain martensite, altering the transformation pathway.