Some basic creep characteristics of ZrSn&.z.sbnd;Mo and ZrSnMoNb alloys part I. Steady state creep

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The steady state creep rate, dot ϵ s , of five ZrSnMo alloys and a ZrSnMoNb alloy was measured within a temperature interval of 623–823 K, and a broad, applied stress interval. The stress sensitivity parameter, m′ = (∂ ln dot ϵ s /∂ ln σ) T , where σ and T are applied stress and temperature, respectively, was found to increase with applied stress from about 3 at dot ϵ s ⋍ 10 −9 s −1 to eventually more than 20 at dot ϵ s ∼- 10 −5 s −1 . The apparent activation energy of creep, Q, increases with temperature from values lower to values considerably higher than the expected value of activation enthalpy of lattice diffusion, ΔH . Correction of Q for the temperature dependence of the elastic modulus does not lead to the value of the activation energy of creep which is temperature independent and close to ΔH . Nevertheless, it is suggested that creep in the alloys investigated is controlled by recovery at medium applied stresses, while at low applied stresses the modified Nabarro-Herring or Coble mechanism is probably the rate controlling one. At high applied stresses (and lower temperatures) the measured steady state creep rates are significantly contributed to by an athermal deformation mechanism, at least at the lowest tin concentration studied, i.e., 3 wt.%. Creep in the alloys investigated cannot be interpreted in terms of measured effective stress. The influence of alloy composition on steady state creep rate and steady state flow stress, respectively, was evaluated and the creep strengthening effect of tin, molybdenum and niobium is discussed.