Samuel Hemery

Metallic materials experience irreversible deformation with increasing applied stress, manifested in localized slip events that result in fatigue failure upon repeated cycling. We discerned the physical origins of fatigue strength in a large set of face-centered cubic, hexagonal close-packed, and body-centered cubic metallic materials by considering cyclic deformation processes at nanometer resolution over large volumes of individual materials at the earliest stages of cycling. We identified quantitative relations between the yield strength and the ultimate tensile strength, fatigue strength, and physical characteristics of early slip localization events. The fatigue strength of metallic alloys that deform by slip could be predicted by the amplitude of slip localization during the first cycle of loading. Our observations provide a physical basis for well-known empirical fatigue laws and enable a rapid method of predicting fatigue strength as reflected by measurement of slip localization amplitude.

Although nickel-based superalloys are widely used in the industry, their response to shock loading is still rarely investigated. Here, the impacts of laser-launched flyers were used to study the dynamic behavior of a Rene 65 superalloy under shock pressures of about 10 GPa at very high strain rates of about 106 s−1. Three types of samples, wrought or additively manufactured (laser powder bed fusion) and subjected to different heat treatment conditions, were investigated. These experiments allowed the measurement of the Hugoniot elastic limit (compressive yield strength) and the spall (tensile) strength, both in upper ranges compared with the most common metals. Post-recovery analyses involving various techniques provided insight into dynamic failure with a combination of transgranular ductile fracture and intergranular cracks with preferential nucleation sites, strongly dependent on the different microstructures related to fabrication routes and thermal treatments.