High temperature creep of Ni-20Cr-2ThO 2 single crystals

Abstract

The high temperature creep properties of single crystals of Ni-20Cr-2ThO 2 have been investigated in the temperature range 650–1300°C. Single crystal tensile specimens were cut from plates having a large elongated grain structure and constant stress vacuum creep tests were conducted at various stresses and temperatures. By including the temperature dependence of the elastic modulus in the analysis of the data, the creep activation energy was found to be very nearly equal to that for self diffusion of Ni-20Cr. The stress dependence of the creep rate was observed to be large and variable, with power law exponents ranging from 9 to 75 over the temperature range studied. The ductility of these crystals as measured by percentage reduction of area was very high, typically ranging from 40 to 60%. The percentage elongation varied inversely with the stress exponent and ranged from 1 to 28%. The creep properties of these dispersion strengthened crystals can be described accurately by considering the creep strength to be given by a simple sum of the creep strength of unthoriated polycrystalline Ni-20Cr and the Orowan strength, as calculated from the measured ThO 2 particle distribution. This simple model describes the measured creep rates to within a factor of three at all stresses and temperatures.