Thermally induced phase transformation in NiTi shape memory alloys (SMA) shows strong size and shape, collectively termed length scale effects, at the nano to micrometer scales, and that has important implications for the design and use of devices and structures at such scales. This paper, based on a recently developed multiscale model that utilizes molecular dynamics (MD) simulations at small scales and MD-verified phase field (PhF) simulations at larger scales, reports results on specific length scale effects, i.e. length scale effects in martensite phase fraction evolution, transformation temperatures (martensite and austenite start and finish) and in the thermally cyclic transformation between austenitic and martensitic phase. The multiscale study identifies saturation points for length scale effects and studies, for the first time, the length scale effect on the kinetics (i.e. developed internal strains) in the B19 phase during phase transformation. The major part of the work addresses small scale single crystals in specific orientations. However, the multiscale method is used in a unique and novel way to indirectly study length scale and grain size effects on evolution kinetics in polycrystalline NiTi, and to compare the simulation results to experiments. The interplay of the grain size and the length scale effect on the thermally induced martensite phase fraction (MPF) evolution is also shown in this present study. Finally, the multiscale coupling results are employed to improve phenomenological material models for NiTi SMA.

Martensitic phase transformation in NiTi shape memory alloys (SMA) occurs over a hierarchy of spatial scales, as evidenced from observed multiscale patterns of the martensitic phase fraction, which depend on the material microstructure and on the size of the SMA specimen. This paper presents a methodology for the multiscale tracking of the thermally induced martensitic phase transformation process in NiTi SMA. Fine scale stochastic phase field simulations are coupled to macroscale experimental measurements through the compound wavelet matrix method (CWM). A novel process for obtaining CWM fine scale wavelet coefficients is used that enhances the effectiveness of the method in transferring uncertainties from fine to coarse scales, and also ensures the preservation of spatial correlations in the phase fraction pattern. Size effects, well-documented in the literature, play an important role in designing the multiscale tracking methodology. Molecular dynamics (MD) simulations are employed to verify the phase field simulations in terms of different statistical measures and to demonstrate size effects at the nanometer scale. The effects of thermally induced martensite phase fraction uncertainties on the constitutive response of NiTi SMA is demonstrated.

Many industries including the medical, aerospace, and automobile industries have increasingly adopted the use of shape memory alloys (SMAs) for a plethora of applications due to their unique thermomechanical properties. From the commercially available SMAs in the market, binary NiTi SMAs have shown the most desirable properties. However, SMA properties can be significantly affected by the fabrication process. One of the most familiar applications of NiTi SMAs is in the design of actuating devices where the shape memory effect properties are highly advantageous. Spring NiTi SMA actuators are among the most commonly used and are generally made by torsion loading a straight wire. Consequently, stress concentrations are formed causing a reduction in recovery force. Other methods for producing springs and other NiTi SMA components is the fast emerging manufacturing method of additive manufacturing (AM). AM often uses metal powders to produce the near-net shape components. A major challenge for SMAs, in particular, is their well-known composition sensitivity. Therefore, it is critical to control composition in NiTi SMAs. In this thesis, a novel method for processing NiTi SMAs for pre-alloyed NiTi SMA powders and springs is presented. A low pressure and low temperature hydriding-pulverization-dehydriding method is used for preparing the pre-alloyed NiTi SMA powders with well-controlled compositions, size, and size distributions from wires. By hydrogen charging as-drawn martensitic NiTi SMA wires in a heated H3PO4 solution, pulverizing, and dehydriding, pre-alloyed NiTi powders of various well-controlled sizes are produced. In addition, a low pressure and low temperature hydriding-dehydriding method is used for producing NiTi SMA helixes from wires. The helix pattern in the pre-alloyed NiTi SMA wires was obtained by hydrogen charging NiTi SMA 500 μm diameter wires at different time intervals, followed by dehydriding to remove the hydrogen. The wires, powders, and resulting helixes were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD). The relationship between the wire diameter, powder particle size, and helix geometry as a function of hydrogen charging time is investigated. Lastly, the recovery behavior due to the shape memory effect is also investigated after dehydriding.

The main focus of this research is the transformation induced fatigue behavior of shape memory alloy (SMA) actuators undergoing thermally induced martensitic phase transformation. The recent development of aerospace applications employing shape memory alloys (SMAs) has expanded the need for fatigue life characterization and modeling. Lightweight, compact and with a great work output, SMAs are ideal materials for actuated structural components. However, fatigue life becomes a key factor in applications such as commercial airplanes. Therefore, it is necessary to not only perform fatigue testing but also to investigate the causes of fatigue failure. As a new class of materials, SMAs have unique characteristics and require novel test methodologies to conduct repeatable and reliable fatigue testing. For this research, two materials are being investigated: TiNiCu and Ni-rich NiTi. The experiments performed on the first selected alloy, i.e. TiNiCu SMA, explore three major parameters: the applied stress level, the amount of actuation, and the corrosive nature of the environment. Experimental results show that SMAs undergoing transformation induced fatigue exhibit a low-cycle fatigue behavior and the measurement of the accumulated plastic strain at failure is associated to a Manson-Coffin type failure criterion. Investigations conducted on the post-mortem microstructure showed evidence of a multiphysical coupling between corrosion and cyclic phase transformation, from which a novel cyclic damage mechanism is proposed and explained using the micromechanical shear lag model accounting for actuation and accumulated plastic strains. Thereafter, based upon the identified failure mechanism and considering damage accumulation through crack formation, a stress renormalization procedure is proposed in combination with the Miner’s rule to predict the reduction of number of cycles to failure due to cyclic phase transformation and corrosion. A direct method is first presented and the predictions show good agreement with experimental results. However, both corrosion and corrosion-free fatigue data are required. Therefore, a new approach is proposed: the inverse Miner’s rule, which requires corrosion fatigue data only to predict corrosion-free life. The new and attractive properties of the selected second alloy, i.e. Ni-rich NiTi SMA, have revived the motivation of the aerospace industry to design SMA actuators. One particular property is cyclic stability generated by precipitation hardening mechanism using precipitates. However, are also precipitates due to high Nickel content (60 wt.% or 55 at.%). Parameters such as processing, heat treatments, size effects, surface quality and environment are investigated. Thermomechanical response and fatigue life are discussed and the greatest impact is found to come from specimen surface quality. Finally, a detailed fractography presents the different microstructural aspects of the fatigue damage and concludes to a precipitation driven fatigue failure mechanism cause by precipitates.