Correlation Between Irradiation Defects and Transition Dimension for TEM In Situ Mechanical Testing

The objective of this study is to establish a relationship between specimen size effect transition dimension and defect number density. Transmission electron microscopic (TEM) in situ mechanical testing enables one to directly observe fundamental plastic phenomena simultaneous to quantitative measurement of applied load and displacement, in specimens having dimensions on the order of nanometers to micrometers. Thus, these techniques hold great promise for enhancing our scientific understanding of deformation mechanisms in volume-limited materials, such as shallow ion irradiated layers. The 'specimen size effect' is a major detractor, however, of any miniaturized mechanical testing technique. This effect is the direct result of low-volume specimens containing too few dislocations, such that plasticity is governed by the stress required to introduce more dislocations to the specimen. The consequence is an inflated yield stress measurement relative to the 'bulk' yield stress. As the specimen size increases, so too does the number of dislocations present, and plasticity becomes controlled by the stress needed to move these dislocations. Yield stress measurements decrease to their bulk values. The minimum specimen dimension for which plastic yield is controlled by dislocation motion rather than dislocation introduction, is what we herein refer to as the 'transition dimension'. However, the overall yield stress is governed by the superposition of dislocations, grain boundaries, and dispersed obstacles of varying morphologies. It is theorized that the higher the number density of these dispersed obstacles, the lower the transition dimension. Since it is well known that irradiation introduces a high density of defects, we hypothesize that the transition dimension is sufficiently low in irradiated materials so as to enable meaningful quantitative values of yield stress through TEM in situ mechanical tests