Irradiation creep under proton bombardment

An apparatus has been developed to study the creep of thin metal specimens under tensile stress during bombardment by 4 MeV protons from the Harwell Van de Graaff accelerator. The specimen is held in a helium atmosphere and the proton beam reaches it through a thin metal window at the end of the accelerator beam line. The proton beam passes through the thin (25 μM) specimen, losing approximately 1.5 MeV in the process (most of which contributes to heating the specimen) and creating almost uniform radiation damage at the rate of 1-10 x 10-7 displacements per atom per second (dpa s-1). The specimen temperature is monitored by infrared pyrometry and controlled to +- 0.20C by additional DC heating via the infrared pyrometer output to compensate for ion-beam fluctuations. The irradiation creep strain of the specimen is continuously measured with a sensitivity of 5 x 10-6 by a linear variable differential transformer. Irradiation times up to about 100h with reasonable beam stability are possible. In preliminary experiments the irradiation creep behaviour of annealed nickel in the temperature range 400-6000C under tensile stresses in the range 20-100 MPa has been investigated. The creep exhibits a logarithmic dose dependence and the creep rate has been found to consist of two components, irradiation-enhanced creep and irradiation hardening, both of which are a function of dose, dose rate and temperature. Electron microscopy of specimens irradiated at 450-5000C has revealed a 1-5 x 1014 cm-3 concentration of small dislocation loops with long bowed dislocations between them. In specimens bombarded at 100 MPa and 5500C a linear dependence of creep rate on damage rate in the range 2-10 x 10-7 dpa s-1 was detected. (author)