AbstractsChemistry

The fatigue behaviour of three Galfenol steels, alloys predominantly composed of iron and gallium, was investigated. Gallium contents were 18.4 at.%, 14.9 at.%, and 12.4 at.% for F, C, and M series Galfenol, respectively. C series Galfenol also had 2.2 at.% tertiary chromium, additions known to improve corrosion resistance. For all three alloys, commercially pure gallium was alloyed with low carbon steels, resulting in less than 0.15 at.% carbon content. As-received rods were produced by Bridgman rod extrusion to enhance the preferential grain orientation required to maximize the sought-after magnetostrictive effect. The as-received rods measured 24 mm diameter and 125 mm long and each was machined into eight cylindrical dogbone fatigue specimens with 4.5 mm diameter gauge sections. Constant amplitude fully reversed tension compression (R = -1) fatigue tests were performed on the dogbone specimens. The initial fatigue results were highly scattered with one standard deviation on life being of one order of magnitude. The as-received material was also very porous, with macropores as large as 500 ????m. Hence, the primary objective of this study was to determine if reducing the porosity would improve fatigue behaviour. Hot iso-static pressing (HIP) at 310 MPa and 1250 ??C increased material densities by 1% ?? 0.1% and removed 86% ?? 6% of surface area porosity. More importantly, macroporosity was removed and the largest remaining pores after HIP had Feret mean diameters less than 30 ????m. After HIP, data scatter increased by 43% for the F series and was reduced by 8% for the C series. Although many of the specimens were longer lasting for the HIP F series, given the resulting high level of scatter, there was not enough statistical evidence to indicate that HIP improved its fatigue behaviour. On the contrary, mean fatigue lives for the HIP C series Galfenol was improved by 95%. Fatigue results of each of the three as-received Galfenol series were essentially the same, indicating that gallium contents between 12.4-18.4 at.% have no effect on fatigue behaviour of Galfenol. Given that fatigue behaviour of C and F series was not dramatically affected by HIP although the porosity was greatly reduced, HIP of M series was not pursued. It is suspected that the relatively large grain size and the weak boundary cohesion caused by gallium rich precipitates are responsible for the persistent large scatter in fatigue data, even after porosity reduction. La dur??e de vie en fatigue de trois alliages de Galfenol, un acier principalement compos?? de fer et de gallium, a ??t?? ??tudi??e. Les tenures en gallium des alliages ??taient respectivement de 18.4 at.%, 14.9 at.%, et 12.4 at.% pour les s??ries F, C, et M. La s??rie C contenait 2.2 at.% de chrome comme ??l??ment tertiaire, un ajout reconnu pour am??liorer la r??sistance ?? la corrosion des aciers. La puret?? du gallium employ?? dans les trois alliages et l'acier utilis?? resultat en une basse tenure en carbone, soit moins de 0.15 at.%. Les tiges de Galfenol de ce projet furent… Advisors/Committee Members: Royal Military College of Canada / Coll??ge militaire royal du Canada, DuQuesnay, David.