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Optimizing Actuation Fatigue Performance of NiTiHf High Temperature Shape Memory Alloys
Abstract
Since their first applications in the 1960s, shape memory alloys have been more frequently employed in industrial applications. Initially they had very specialized applications, but ongoing research has led to a better understanding of these materials allowing for broader applications. Currently the most versatile use of these alloys is for actuators due to their unparalleled energy density for their light weight. This makes them even more attractive to aerospace and automotive industries where added weight comes at a premium. Within the realm of shape memory alloys, NiTiHf is proving to be the most promising alloy for actuation application due to its high transformation temperatures, high actuation work output, and high functional stability.
The main issue facing ready application of NiTiHf actuators is the relatively low understanding of the underlying actuation fatigue failure mechanisms. This area of shape memory alloy research is still young and the added complexity of phase transformation reduces the number of independent variables that can be controlled in a research environment. This work strives to uncover the main factors affecting actuation fatigue so that they may be better designed during manufacturing, processing, and implementation to optimize the resulting actuation performance.
The effects of composition, impurity content, H-phase precipitation, and degree of transformation on the actuation fatigue performance in Ni-rich NiTiHf high temperature shape memory alloys are presented. Higher compositional Ni contents enhance the cyclic actuation stability during fatigue, but there is a limit above which Ni-richness ceases to provide any improvement to stability and only reduces the fatigue life. Impurities were found to not affect cyclic stability but greatly reduce the fatigue lifetime. More ductile compositions are more resilient to higher impurity contents. Repeated partial austenitic transformation increases the fatigue lifetime by an order of magnitude, however, repeated partial martensitic transformation provides no significant increase to fatigue life.
Citation
Demblon, Alexander Robert (2023). Optimizing Actuation Fatigue Performance of NiTiHf High Temperature Shape Memory Alloys. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /199190.