Effect of Molybdenum on some basic creep characteristics of alpha zirconium in a temperature interval of 350° to 600°C

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Creep in three ZrMo alloys (0.1, 0.8 and 2.0 wt.% Mo) was investigated in a temperature interval of 350° to 600°C. The parameter of applied stress sensitivity of steady-state creep rate was found to increase with applied stress, reaching values as high as 50 at 350°C and high applied stresses. Apparent activation energy of creep Q is applied stress and temperature independent for the alloy Zr - 0.1 wt.% Mo only. For the alloys of higher molybdenum concentrations it depends on both applied stress and, especially, temperature. Values of Q as low as about 35 kcal/mol at 350°C and as high as about 130 kcal/mol at 600°C were found for Zr - 2.0 wt.% Mo alloy. Relations between steady-state flow stress and temperature indicate a significant contribution of an athermal deformation mechanism at low temperatures and high steady-state strain rates. The mean effective stress measured by strain transient dip test technique increases with increasing temperature and the effective stress to applied stress ratio increases with applied stress reaching values as high as 0.5. Possible creep rate controlling mechanisms are discussed. It is suggested that the creep rate is controlled by recovery though the current recovery creep theories of dispersion strengthened systems are not in satisfactory agreement with the results obtained. Creep strengthening effect of molybdenum is compared with that of niobium with the conclusion that it is significantly stronger, especially at higher testing temperatures.