The effect of strain rate and orientation on He ion irradiated Ni single crystals - An in situ micro-tensile study

The strain rate response of He2+ ion-irradiated Ni single crystal samples along < 100 > and < 110 > crystallographic directions is studied here under two different irradiation conditions at room temperature. For one set of samples, a single energy (SE) He2+ ion beam of 5 MeV was used to irradiate the foil to a fluence of 1.7 x 10(17) ions/cm(2) while the other set of samples were irradiated to 4.4 x 10(17).ions/cm(2) using an energy degrader wheel, thus creating an approximately uniform dispersion of energies (DE). Micro-tensile samples were fabricated from the irradiated specimens via Focused Ion Beam (FIB) milling and tested at nominal displacement rates of 5 nm/s and 500 nm/s. These corresponded to nominal strain rates of the order of 1.7 x 10(-4) and 1.7 x 10(-2)/s, respectively. Electron back scatter diffraction (EBSD) analysis was performed on the samples pre- and post-tensile testing to elucidate the deformation behaviour. A significant finding of this study is that the strain rate sensitivity of the samples depended strongly on the dose and distribution of the radiation damage, and that irradiation caused a suppression of the strain rate sensitivity. The degree of strengthening and embrittlement is particularly pronounced for the DE irradiated samples as its strength increased fourfold and its plasticity was reduced to almost zero. The critical resolved shear stress (CRSS) was derived for different irradiation conditions and a linear relationship was determined between the CRSS and the irradiation damage experienced. The significance of the results in comparison to alternative micro-mechanical testing methods as well as the effect of irradiation on different crystal orientations is further discussed in this work.