Superelastic Response of Additively-Manufactured Nitinol Metamaterials

This thesis describes an investigation in the modeling and manufacturing of Nitinol (NiTi) superelastic metamaterials. To achieve an accurate agreement between model and experiment regarding superelasticity in Ni-rich NiTi metamaterials, the research focuses particularly on the martensitic transformation behavior. Each chapter contributes to a better understanding of the entire additive manufacturing process chain, including constitutive modeling, heat treatments and structural design, to develop NiTi-based metamaterials with tunable superelastic properties.
NiTi shape memory alloys exhibit unique shape memory effect (SME) and superelasticity due to reversible martensitic transformations. These properties make NiTi a suitable material for adaptive structures, biomedical devices, and aerospace components. Though computational models can be used to design NiTi structures and metamaterials with superelasticity and SME, the successful additive manufacturing of these designs remains challenging. Achieving full superelasticity in complex geometries produced via laser powder bed fusion (L-PBF) is particularly difficult. Thus, aiming for model-experiment consistency of superelastic metamaterials, the main research gap lies in establishing clear qualitative and quantitative relationships between material models, properties, mesoscopic structures and the resulting macroscopic responses.

In Chapter 3, the research started with the development of analytical expressions for the effective transformation stress and numerical models to evaluate the superelastic behavior and energy dissipation of truss-based metamaterials. NiTi truss-based metamaterials with body-centered cubic (BCC) and octet structures were selected to represent bending- and stretching-dominated architectures, respectively. A detailed parametric finite element analysis was performed to study the relationship between relative density and effective transformation criteria. Using L-PBF, crack-free BCC and octet samples were successfully fabricated from Ni-rich NiTi powder. However, the as-fabricated samples exhibited only partial superelasticity and premature fracture.
In Chapter 4, the study is focused on inhomogeneity of microstructural and functional properties and the underlying reasons for partial superelasticity. Through both numerical and experimental approaches, the study investigated how geometric factors, such as relative density, affect microstructural inhomogeneity and thermomechanical properties of NiTi in body-centered cubic (BCC) structures. Geometric effects on melt pool behavior lead to different solidification textures and inhomogeneous response to indentation. The numerical simulation shows that inhomogeneous transformation temperatures cause narrow stress hysteresis in the macroscopic response. This chapter reveals the interdependent relation between relative density, microstructure, localized properties of NiTi and the macroscopic response.

The focus in Chapter 5 is on understanding and mitigating premature fracture in Ni-rich NiTi metamaterials produced by L-PBF. To investigate the origins of fracture, a comparative analysis of two unit cell architectures, the a Gyroid network (bending-dominated) and a Diamond shell (stretching-dominated), was conducted. Due to the inherent tension-compression asymmetry of NiTi, the structural stability of these designs was found to be reversed compared to conventional elastic-plastic responses, leading to premature fracture and limited superelasticity in the as-fabricated samples. As large deformation can not be achieved through martensitic transformation or dislocation slip systems, partial superelasticity and low deformation recoverability were observed in the as-fabricated samples. Heat treatments were applied to address these issues and achieve qualitative agreement between experimental data and model predictions.

After the superelasticity was successfully achieved, the transformation stress-temperature relation and energy absorption in Ni-rich NiTi superelastic metamaterials were investigated in Chapter 6. Temperature dependence often restricts the practical use of superelasticity. In metallic metamaterials, energy absorption typically relies on the elastoplasticity of ductile metals; however, achieving energy absorption with recoverable deformation has not been fully explored. To address this, a numerical model of the Diamond shell structure was developed to predict temperature-dependent superelasticity and energy absorption. A heat treatment was applied to ensure agreement between the model and experimental results. The findings show that the transformation stress-temperature coefficient decreases from 9.5 MPa/°C for bulk samples to 0.9 MPa/°C for Diamond samples. Under uniaxial compression, the effective transformation stress can be controlled by relative density, with values of 41.8, 52.1, and 65.3 MPa for relative densities of 0.15, 0.2, and 0.25, respectively. A specific energy absorption of 3.5 J/g was achieved in cyclic compression tests with 15 cycles. The recoverable plateau-like response in the macroscopic stress-strain curves originated in a continuous transformation region forms along the macroscopic [100] direction under uniaxial compression. Post-yielding plasticity in the macroscopic stress-strain curves is related to a plastic shear band formed along the [110] direction.

In summary, this work successfully developed a model-manufacturing strategy for superelastic NiTi metamaterials. By addressing multiscale challenges such as microstructural inhomogeneity and tension-compression asymmetry, this study demonstrates that computation-based design and additive manufacturing can create functional NiTi structures with tunable thermomechanical properties. Multiscale issues often prevent computational designs from being fully realized in experiments. By identifying and controlling variable interdependencies across scales, this research achieves largely tunable superelasticity in experiments. This approach provides a foundation for practical applications of NiTi metamaterials in fields such as biomedical devices, aerospace, and civil engineering.