Microstructural analysis and molecular dynamics modeling of shape memory alloys

Yang, Chia Wei; Tsou, Nien‐Ti

doi:10.1016/j.commatsci.2017.02.011

Cited by 7 publications

(10 citation statements)

References 26 publications

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“…The current work adopts the crystal variant identification method developed by Wu et al [8] and Yang and Tsou [13] to analyze the microstructural evolution of superelasticity in NiTi SMAs. There are many crystal systems which can be observed in NiTi, such as austenite (A), trigonal (R), orthorhombic (O), monoclinic (M), and body-centered orthorhombic (BCO) phases, consisting of 1, 4, 6, 12, and 12 crystal variants respectively.…”

Section: Crystal Variant Identification Methodsmentioning

confidence: 99%

“…This is also applied to other crystal systems, such as the trigonal, orthorhombic, etc. In the work of Yang and Tsou [13], 15 conditions that can distinguish variants among the A, R, O, and M/BCO phases were proposed. By checking these conditions, the similarity between the determined transformation matrix and the ideal one for each variant can be obtained.…”

Section: Crystal Variant Identification Methodsmentioning

confidence: 99%

“…For example, the first variant of the trigonal phase is expressed as R1 and the fifth variant of the body-centered orthorhombic is expressed as BCO5. More detail for the crystal variant identification method can also be found in Yang and Tsou [13].…”

Section: Crystal Variant Identification Methodsmentioning

confidence: 99%

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Microstructural Evolution of Superelasticity in Shape Memory Alloys

Lai

Tsou

2019

Multiscale Sci. Eng.

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Superelasticity in shape memory alloys (SMAs) is an important feature which caused by martensitic transformation and microstructural evolution. The composition of crystal variants and microstructures during the superelasticity process are simulated by molecular dynamics in the current work. Then, a computational post-processing scheme, which can identify variants type and interface orientation are proposed. This method reveals that the crystal adopts a multi-phase mixture during the superelsticity process, including many crystal systems, such as orthorhombic (O), R-phase (R), monoclinic (M) and body-centered-orthorhombic (BCO). In addition, the interface normal vectors between two different variants are examined by the compatibility equation, and the result has good agreement. The stress-strain curve, volume fraction for present variants and total energy diagram are illustrated. The microstructural evolution shows that R phase can serve as the transitional region between pairs of BCO variants. The variants O, BCO, M phase coherent between each other with specific pattern to form twinned arrangement. Furthermore, different random seeds for the initial velocity of the atoms, are used in the simulation to obtain equivalent microstructural evolution paths. The corresponding macroscopic properties are analyzed. The microstructural evolution path can have different energy barrier of initialization, which also leads to different present variant in the crystal. The results discovered in the current work are expected to provides design guidelines for the applications in SMAs.

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Section: Crystal Variant Identification Methodsmentioning

confidence: 99%

Section: Crystal Variant Identification Methodsmentioning

confidence: 99%

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Microstructural Evolution of Superelasticity in Shape Memory Alloys

Lai

Tsou

2019

Multiscale Sci. Eng.

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“…Such distortion reduces the symmetry of the lattice, resulting in the distinct variants in the crystal systems, e.g., 12 variants in the monoclinic system and six variants in the orthorhombic system. Yang and Tsou [17] developed a post-processing method for MD simulations that can identify multiple crystal variants and phases by examining the transformation matrix of each lattice. Then, the detailed microstructures, twin patterns, and volume fraction of each variant of the crystals under applied boundary conditions simulated by MD can be determined.…”

Section: Introductionmentioning

confidence: 99%

“…In the current work, a standard MD simulation package (LAMMPS) and the post-processing method proposed by Yang and Tsou [17] were used to examine the superelasticity and the corresponding microstructural evolution in the equiatomic NiTi alloys, subjected to the three loading processes, i.e. tensile, compression, and shearing.…”

Section: Introductionmentioning

confidence: 99%

The Analysis of Superelasticity and Microstructural Evolution in NiTi Single Crystals by Molecular Dynamics

Chen

Tsou

2018

Materials

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Superelasticity in shape memory alloys is an important feature for actuators and medical devices. However, the function of the devices is typically limited by mechanical bandwidth and fatigue, which are dominated by the microstructures. Thus, in order to correlate the mechanical response and the microstructures, the microstructural evolution in NiTi single crystals under the compression, tensile, and shearing tests is simulated by molecular dynamics (MD) in the current study. Then, the martensite variant identification method, which identifies the crystal variants/phases of each lattice based on the transformation matrix, is used to post-process the MD results. The results with the detailed information of variants and phases reveal many features that have good agreement with those reported in the literature, such as X-interfaces and the transitional orthorhombic phase between the austenite and monoclinic phases. A new twin structure consisting of diamond and wedge-shaped patterns is also discovered. The macroscopic behavior, such as stress-strain curves and the total energy profile, is linked with the distribution of dislocation and twin patterns. The results suggest that the loading cases of shear and compression allow a low critical strain for the onset of martensitic transformation and a better superelasticity behavior. Therefore, the two loading cases are suitable to apply to the NiTi actuators. The current work is expected to provide insight into the mechanical responses and design guideline for NiTi shape memory alloy actuators.

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