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Modeling and Simulation of Shape Memory Alloys
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Proceedings Papers
SMST2024, SMST 2024: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 76-77, May 6–10, 2024,
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NiTi alloy stands out as a promising material, particularly in biomedical, aerospace, and heat pump applications, owing to its biocompatibility and distinctive properties, including exceptional shape memory (SME) and pseudoelasticity. These properties arise from the alloy's capacity to undergo phase transformations during heating/cooling and loading/unloading cycles. The specific phase transformation temperatures and pseudoelastic behavior are heavily influenced by the composition of the alloy. The introduction of transition elements such as V, Cr, Mn, Fe, and Co in place of Ni and Ti can lower the martensite start (Ms) temperature, while substitution with Hf, Zr, Ag, and Au for Ni and Sc, Y, Hf, and Zr for Ti can elevate the Ms temperature. Consequently, it is imperative to comprehensively comprehend the effects of third alloying elements on the properties of these alloys from an atomic perspective. Molecular dynamics (MD) simulations, operating at the atomic level, offer a valuable means to explore the impact of various compositions, including the addition of a third element, on the alloy properties, thereby enhancing their performance. However, a significant challenge in MD simulations lies in selecting reliable interatomic potentials between the elements. Developing new potentials poses challenges, prompting the evaluation of existing potentials for ternary systems in this study, which will be compared with experimental results. While several interatomic potentials have been proposed for binary NiTi SMAs, limitations arise when extending to tertiary alloys. Existing studies have reported ternary interatomic potentials for NiTiV, NiTiNb, and NiTiHf, but these are insufficient for capturing the phase transformation of these tertiary systems. To address this, a hybrid model will be employed, combining different types of interatomic potentials such as modified embedded atom method (MEAM), embedded atom method (EAM), Lennard-Jones (LJ), and Morse potentials. This approach aims to capture the phase transformation of ternary alloys doped with elements like Cu, Hf, Pd, Pt, Sc, Ta, Mn, Zr, Y, and Au, which can increase transformation temperatures such as the martensite start temperature (Ms), martensite finish temperature (Mf), austenite start temperature (As) and Austenite finish temperature (Af), during cooling and heating processes. These alloys, known as high-temperature shape memory alloys (HT-SMAs), hold significant potential for diverse applications, including actuators, owing to their unique properties and enhanced transformation behaviours.
Proceedings Papers
Molecular Dynamics Simulations of Microstructural Effects on Austenite-Martensite Interfaces in NiTi
SMST2024, SMST 2024: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 78-79, May 6–10, 2024,
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The formation and migration of austenite-martensite interfaces plays a key role in the reversible martensitic transformations of shape memory alloys (SMAs). How these interfaces interact with the SMA microstructure is a primary determining factor in important functional properties such as hysteresis and transformation span. Therefore, successful microstructural engineering of SMAs requires in-depth knowledge of interface behavior. The rapid nature of martensitic transformations makes experimental observations of moving interfaces challenging. Molecular dynamics (MD) simulation is a unique tool which can probe the atomic-scale details of austenite-martensite interfaces as they migrate and interact with different microstructural features. While MD simulations allow access to atomic-scale mechanisms, they are limited in time scale, typically to nanoseconds. This limitation creates problems when focusing on the entire transformation process in SMAs, specifically nucleation of new phases. To trigger nucleation on the nanosecond time scale, MD simulations must be performed so far from equilibrium that their relevance to experiment becomes questionable. Here, we demonstrate new MD simulation techniques to generate energetically preferred austenite-martensite interfaces in NiTi under near-equilibrium conditions. We then take advantage of this approach to probe interface behavior under conditions relevant to experiments. Our results demonstrate how austenite-martensite interfaces behave with dramatic differences in single crystals compared to more realistic microstructures containing features such as grain boundaries and precipitates. We identify trends in interface behavior which can be utilized to inform microstructural engineering approaches for SMAs.
Proceedings Papers
SMST2024, SMST 2024: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 80-81, May 6–10, 2024,
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Morphing shape memory alloy hybrid composites (SMAHC) offer a great potential for various engineering applications due to their lightweight properties and their ability to undergo significant deformations. They fundamentally consist of two primary components: a stiff substrate and a shape memory alloy (SMA) layer, enabling external two-way shape memory capability. During actuation the metallic SMA undergoes a contraction of several percent when subjected to electric current. As the SMA contracts, the substrate retains its original length, causing a bending movement in the composite. A third component to be mentioned is the interlayer, positioned between the rigid substrate and the SMA layer, opens up a greater design flexibility. By adjusting the distance between these two components fine-tuning of the relationship between deflection and bending moment in response to an external load can be achieved. (compare Fig. 1).
Proceedings Papers
SMST2024, SMST 2024: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 82-83, May 6–10, 2024,
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Great attention has been recently paid to the investigation of plastic deformation in NiTi. Experimental investigations of the mechanical response of NiTi polycrystalline samples within a broad stress-strain-temperature state space have revealed a complex response involving martensitic transformation, reorientation, and plastic deformation processes. The interactions between them result in complex coupled phenomena, such as transformation-induced plasticity, martensite stabilization through plastic deformation, and micro-strain heterogeneity induced by plasticity. Plastic deformation in NiTi not only generates irrecoverable strain at the macroscale, but it also induces substantial strain heterogeneity in the microstructure. This heterogeneity significantly affects the functional properties and may open up new technology pathways for designing sophisticated products. Tailored constitutive models that can reproduce the response in complex loading scenarios can be extremely beneficial.
Proceedings Papers
SMST2024, SMST 2024: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 84-85, May 6–10, 2024,
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Shape memory alloys (SMAs) are a novel solution to reducing the actuating system weight and mechanical complexities which consequentially comes with integration challenges. Some of these challenges are due to limited production accessibility and often need reengineering. A materials model turned into analytical routines would allow empirical data to predict the structural behavior of SMAs. An SMA design tool would bridge material science and engineering practices by guiding the users through an iterative design approach to improve integration efficiency for actuating devices. A user-friendly first-order approximation model was developed by the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) to capture the macroscopic effects of forward and reverse transformation for various actuator types such as wires, springs, and torque tubes. This tool allows quick changes and iterations throughout the design process, which is particularly helpful in comparing actuator types. Although it is convenient to use, the ability to capture the SMA nonlinear and complex inelastic response is limited. Phenomenological-based models have been developed by incorporating classical theory of plasticity and by relating martensite volume fraction to the laws of thermodynamics. The phase transformation can be characterized as a rate-independent plasticity using the volume of martensite to govern the change in material properties between austenite and martensite. The development of the model itself incorporates other characteristics such as the degradation of performance due to plastic strain and the change in transformation strain during partial cycling. The challenge with these models is the complexity for the user. The starting point to create a user-friendly SMA design tool involved combining a phenomenological-based model with Finite Element Models (FEM). Integrating a complex 3D constitutive model with ABAQUS Finite Element Analysis (FEA) encompasses both conventional material parameters and unique shape memory alloy parameters.