Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σIS, for the unirradiated material. It was also shown that σIS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, DC, because at higher doses the material shows necking at yield. The DC values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a DC value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.