arXiv subjects

Won-Seok Ko

Publications and source records attributed to Won-Seok Ko.

3 recordsLinked to original sources

arXiv2026

Finely Tunable Thermal Expansion of NiTi by Stress-Induced Martensitic Transformation and Thermomechanical Training

Tailoring the thermal expansion of martensitic materials by crystallographic texture and anisotropic variation of lattice parameters is a promising route to a flexible design of thermally stable systems. NiTi alloys are prototype materials in this respect, with shape-memory and superelastic properties owing to their thermoelastic martensitic transformations. Here, we propose a method to realize finely tunable coefficients of thermal expansion (CTE) for the NiTi alloy based upon a special combination of mechanical and thermal training. We achieve a near-zero in-plane CTE that is smaller in value than that of the FeNi-based Invar alloy. Atomistic simulations and theoretical calculations guide the method design and clarify the underlying mechanisms of the relationship between the processing conditions, the microstructural evolution, and the thermal expansion behavior. The directions for further, finer adjustments of the CTE without constraints on the shape of the materials are indicated.

cond-mat.mtrl-sci↗

arXiv2022

Atomistic simulations of the deformation behavior of an Nb nanowire embedded in a NiTi shape memory alloy

The influence of pre-strain and temperature on the superior properties exhibited by an Nb nanowire embedded in a NiTi shape memory alloy (SMA) are investigated via molecular dynamics simulations. To this end, a new Nb-Ni-Ti ternary interatomic potential based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) is developed and employed. The origin of the unique phenomena of quasi-linear elasticity, slim hysteresis, and reduction in Young's modulus observed for pre-strained nanowire-SMA composites is uncovered. The results demonstrate the importance of plastic deformation in the embedded Nb nanowires and reveal how the deformation facilitates the just-mentioned, unprecedented phenomena. A simple and straightforwardly obtainable descriptor to correlate and monitor Young's modulus evolution during pre-straining is proposed. Furthermore, our simulations suggest that the desired Young's modulus can be obtained for a wide range of application temperatures through appropriate pre-straining.

cond-mat.mtrl-sci↗

arXiv2021

Dissecting functional degradation in NiTi shape-memory-alloys containing amorphous regions via atomistic simulations

Molecular dynamics simulations are performed to provide a detailed understanding of the functional degradation of shape memory alloys at small scale. The origin of the experimentally reported accumulation of plastic deformation and the anomalous sudden increase of the residual strain under cyclic mechanical loading are explained by detailed insights into the relevant atomic scale processes. Our work reveals that the mechanical response of shape-memory-alloy pillars under cyclic compression is significantly influenced by the presence of an amorphous-like surface region as experimentally induced by focused ion beam milling. The main factor responsible for the observed degradation of superelasticity under cyclic loading is the accumulated plastic deformation and the resultant retained martensite originating from a synergetic contribution of the amorphous and crystalline shape-memory-alloy regions. We show that the reported sudden diminishment of the stress plateaus and hysteresis under cyclic loading is caused by the increased stability of the martensite phase due to the presence of the amorphous phase. Based on the identified mechanism responsible for the degradation, we validate reported methods of recovering the superelasticity and propose a new method to prohibit the synergetic contribution of the amorphous and crystalline regions, such as to achieve a sustainable operation of shape memory alloys at small scale.

cond-mat.mtrl-sci↗