A micromechanical model for the grain size dependent super-elasticity degeneration of NiTi shape memory alloys

Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.

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Experimental and FEM Analysis of the Fracture Behavior in NiTi Shape Memory Alloys

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.324-325.919 ◽

2006 ◽

Vol 324-325 ◽

pp. 919-922 ◽

Cited By ~ 2

Author(s):

Xin Mei Wang ◽

Zhu Feng Yue

Keyword(s):

Fracture Toughness ◽

Phase Transformation ◽

Shape Memory Alloys ◽

Shape Memory ◽

Fracture Behavior ◽

Micromechanical Model ◽

Crack Tip ◽

Fem Analysis ◽

Transformation Behavior ◽

Niti Shape Memory Alloys

In the present work, the fracture toughness of a NiTi pseudoelastic alloy has been obtained by experiments on CT specimens, which is KIC =39.38MPa·m1/2. Then the stress induced phase transformation behavior in front of the crack tip of the CT specimen is simulated by a micromechanical model considering the different elastic properties between martensite and austenite. The results show that the pre-crack promotes phase transformation at the crack tip. And the phase transformation is localised near the crack tip. It is also shown that phase transformation reduces the Mises stress around the crack tip.

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A micromechanics-based constitutive model for nanocrystalline shape memory alloys incorporating grain size effects

Journal of Intelligent Material Systems and Structures ◽

10.1177/1045389x211028294 ◽

2021 ◽

pp. 1045389X2110282

Author(s):

Xiang Zhu ◽

Guansuo Dui ◽

Yicong Zheng

Keyword(s):

Grain Size ◽

Shape Memory Alloys ◽

Shape Memory ◽

Size Effects ◽

Boundary Phase ◽

Length Scale ◽

Internal Length ◽

Length Scales ◽

Size Dependent ◽

Grain Size Effects

A micromechanics-based model is developed to capture the grain-size dependent superelasticity of nanocrystalline shape memory alloys (SMAs). Grain-size effects are incorporated in the proposed model through definition of dissipative length scale and energetic length scale parameters. In this paper, nanocrystalline SMAs are considered as two-phase composites consisting of the grain-core phase and the grain-boundary phase. Based on the Gibbs free energy including the spatial gradient of the martensite volume fraction, a new transformation function determining the evolution law for transformation strain is derived. Using micromechanical averaging techniques, the grain-size-dependent superelastic behavior of nanocrystalline SMAs can be described. The internal length scales are calibrated using experimental results from published literature. In addition, model validation is performed by comparing the model predictions with the corresponding experimental data on nanostructured NiTi polycrystalline SMA. Finally, effects of the internal length scales on the critical stresses for forward and reverse transformations, the hysteresis loop area (transformation dissipation energy), and the strain hardening are investigated.

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