Orientation-dependent irradiation hardening in pure Zr studied by nanoindentation, electron microscopies, and crystal plasticity finite element modeling

Orientation-dependent irradiation hardening phenomena in pure Zr have been studied, and the corresponding mechanisms investigated based on the combined use of SEM, TEM, and CPFEM. Nanoindentation was conducted on ten grains at room temperature before and after ion irradiation (0.3 dpa, 250 degrees C). The hardness decreases as the declination angle (the angle between c-axis and sample surface normal) increases, both before and after irradiation. However, the irradiation induced maximum absolute and relative increment of hardness occur when the declination angle is about 55 degrees and 60 degrees, respectively. The hardness increment was found to be proportional to the relative activation of the < a > type slip with increase of declination angle, at low angles. However for angles higher than 55-60 degrees the hardness increment was found to be reduced by the activation of deformation twinning and cross slip. A peak increment of irradiation hardening is therefore observed due to the orientation-dependent combination of different deformation mechanisms. CRSS values of all slip systems and for {10-12} < -1011 > tension twinning before and after irradiation were obtained by fitting to a crystal plasticity finite element model (CPFEM). The obstacle strength of heavy ion induced < a > type dislocation loops generated at 250 degrees C on subsequent room temperature slip was thus calculated to be approximately 0.29 +/- 0.05.