Improving Fatigue Resistance in Shape Memory Alloys for Biomedical Applications

Due to their unique thermo-mechanical properties, Shape Memory Alloys (SMAs) have been used in a wide range of applications. A promising new biomedical application for SMAs is NiTi (Nitinol)-based artificial heart valve frames, which can be inserted via a minimally invasive procedure. However, fatigue cracking and degradation of mechanical/thermal cyclic stability due to cyclic applied stresses limit the reliability and lifetime of these devices. My research focuses on improving fatigue resistance in the ultrahigh cycle fatigue (UHCF) regime using a two-pronged approach: (1) Increasing material strength via controlled precipitation of nanoscale aluminide Heusler phase precipitates and (2) Characterizing, understanding, and controlling micron-scale non-metallic inclusion particles that form during solidification and can act as potent fatigue crack nucleation sites. Both of these areas of research deal with fabrication and manufacturability issues introduced by commercial melt processes. Previous research into the quaternary NiTiZrAl system shows increased precipitation strengthening capability over the ternary NiTiAl system, but scaled-up buttons of these alloys have fractured during hot rolling due to the presence of a brittle and low-melting NiTiZr laves phase, resulting in major manufacturability issues in the scale-up of this promising SMA. Additionally, thermodynamic simulations of the non-metallic inclusion phases that form in NiTi (Ti(C,O)- type oxycarbides and Ti4Ni2Ox-type oxides) during commercial melt processes such as Vacuum Arc Remelting (VAR) and Vacuum Induction Melting (VIM) indicate solution temperatures of 1350°C and 1100°C for carbides and oxides respectively. The switch from melt processing to low-temperature solid state powder metallurgy techniques for fabrication could help bypass high-temperature equilibrium solidification pathways and possibly suppress or minimize the formation of these harmful inclusions in NiTi or the brittle laves phase in NiTiZrAl. A unique powder metallurgy method known as cold sintering is a promising processing technique that could address these issues by avoiding high-temperature processing routes. Cold sintering is a near-net-shape processing technique in which metal powders are compressed at very high pressures and near-ambient temperatures to produce a consolidated form that can reach near-theoretical density. The use of the cold sintering facilities supervised by Professor Elazar Gutmanas at Technion – Israel Institute of Technology is discussed in this proposal. In this collaborative research effort, I would visit Professor Gutmanas’s lab at the Technion for approximately 8 weeks. In that period, I will receive training from current graduate students and lab technicians and fabricate three prototype alloys using the cold sintering technique: (1) a NiTi alloy with a composition similar to commercial alloys (2) The high-strength NiTiZrAl alloy characterized in previous research and (3) a novel Ni-free PdFeTiAl-based SMA. Additionally, equal channel angular pressing (ECAP) facilities will be utilized to investigate the effect of high-strain deformation on reducing the size and potency of non-metallic inclusions in these NiTi-based systems. I will investigate the resulting microstructure and inclusion morphology in these prototypes using the electron microscopy facilities at the Technion. The fabrication, processing, and analysis facilities available at the Technion and the expertise of Professor Gutmanas in these techniques will be very valuable in addressing my graduate proje