Ion-irradiation effect on strain rate sensitivity of nanoindentation hardness of W single crystal

The local strain rate (LSR) dependence of nanoindentation (NI) hardness was investigated by using standard constant strain rate (CSR) test method and strain rate jump (SRJ) test method for W single crystal with the surface orientation of {001} before and after 6.4 MeV Fe³⁺ irradiations (nominal damage level of 0.1, 1 and 2 dpa, 573 K). The effect of ion-irradiation on the LSR sensitivity of NI-hardness at room temperature (RT) was evaluated by changing LSR between 0.3 s^-1 and 0.01 s^-1 or 0.03 s^-1 and 0.001 s^-1. Under these experimental conditions, ion-irradiation increases NI-hardness and slightly decreases LSR sensitivity of NI-hardness for all damage levels. The effect is more pronounced with increasing damage level. The LSR sensitivity values are ranging between 0.015 and 0.04 in SRJ tests, and between 0.0425 and 0.06 in CSR tests, indicating that the deformation of bcc W{001} at RT is controlled by a high lattice friction stress. The decrease in LSR sensitivity by ion-irradiation could be attributed to the increase in the athermal stress caused by ion-irradiation induced defect structures, which is reflected to a decrease in the activation volume of dislocation motion in ion-irradiated W{001}.