From Nano-precipitates to Macroscale Composites: How Inclusion-Matrix Interactions Influence the Behaviors of Shape Memory Alloys and Structures.

摘要

Shape memory alloys (SMAs) and composites present multiple new challenges in terms of understanding, prediction, design, and optimization of inclusion effects. This dissertation covers three specific projects on inclusion effects across the spatial length scales from nm to cm. The 1st project simulates the TRansformation-Induced Dislocation (TRID) effect in single crystal NiTi micropillar compression, and compares the simulation results with scanning TEM observations. The close correlation between experimental and numerical results provides further mechanistic insight into this intriguing phenomenon. The 2nd project focuses on the link in Ni-Ti-Hf high temperature SMAs between aging-induced H-phase precipitates and thermo-mechanical properties. By analyzing the chemical and mechanical effects of the H-phase, a qualitative explanation of the dependence of DSC behavior on aging time is proposed. A microstructural finite element (MFE) model that incorporates the chemical and diffusion kinetic effects was able to capture the nucleation and growth process of martensite and to provide insights into the origin of optimal-aging behaviors. The 3rd project studies the behavior of NiTi/Al composites fabricated through an emerging joining technique -- Ultrasonic Additive Manufacturing (UAM). A combined experimental-simulation approach is used to develop and validate the MFE model, enabling it to closely reproduce the macroscopic strain vs. temperature cyclic response, including initial transient effects in the first cycle. The simulation identified crystallographic orientation of the NiTi fiber for optimal performance, and suggests that the UAM process may reduce hysteresis in embedded SMA wires.