[en] Creep in copper dispersion strengthened with fine alumina particles (ODS copper) and reinforced with alumina short fibres--an ODS copper matrix composite--is investigated in two temperature intervals, namely lower temperature interval (LTI) 673-773 K and higher temperature interval (HTI) 923-1023 K. In both intervals, creep is associated with true threshold stresses decreasing with increasing temperature more strongly than the shear modulus of copper. The true threshold stress in the composite is higher than that in its matrix by a factor Λ congruent with 2 in both LTI and HTI, by which the flow stress in the matrix is reduced due to load transfer. The minimum creep strain rate is dislocation core diffusion controlled in LTI and lattice diffusion controlled in HTI. High values of the apparent activation energies are fully explained in terms of the temperature dependence of the true threshold stress in LTI as well as in HTI. Also the high values of the apparent stress exponents are explained in terms of the true threshold stresses. The true threshold stress is identified with the detachment stress, i.e. the stress necessary to detach a dislocation from a fine interacting particle without assistance of thermal activation, σ_d=σ_OB√(1-k²_R). Its nonproportionality to the Orowan stress σ_OB is explained by the relaxation factor k_R that represents the relative strength of the attractive dislocation/particle interaction, increasing with temperature

[en] In zirconium alloys for applications in nuclear reactor technology the applied stress sensitivity of steady-state creep rate characterized by the parameter m'=[delta ln(d epsilonsub(s)/dt)/delta ln sigma]sub(T) (d epsilonsub(s)/dt is steady-state creep rate, sigma applied stress and T absolute temperature) decreases with decreasing applied stress. The value of m' approaches 1 at steady-state creep rates 10^-11 to 10^-12 s^-1. This strongly suggests that Nabarro-Herring or Coble creep mechanisms operate at the above creep rates. Some alloying elements as niobium and molybdenum improve creep strength of zirconium alloys at stresses at m' approximately equal to 1, and m'>>1. This cannot be explained by simply applying the Nabarro-Herring or the Coble creep models in their original form. The said models must be modified with regard to the fact that emission and absorption of vacancies at grain boundaries are a rate controlling process, due to the restriction of grain boundary dislocation motion caused by secondary phase particles situated in grain boundaries. According to models modified along this line by Ashby, creep rate linearly depends on stress and decreases with decreasing mean interparticle spacing. This is in qualitative agreement with the experiment. Another factor that may influence the creep strength at m' approximately equal to 1 stresses is the mean effective grain diameter. The possible presence of secondary phase particles affecting the effective grain diameter is also considered. (author)

[en] Creep in three Zr-Mo alloys (0.1, 0.8 and 2.0 wt.% Mo) was investigated in a temperature interval of 350⁰ to 600⁰C. 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 350⁰C 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 350⁰C and as high as about 130 kcal/mol at 600⁰C 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. (Auth.)

No abstract available

[en] Stress dependences of time to intergranular fracture in creep of copper, a low alloy CrMoV steel and an austenitic CrNi stainless steel were analysed with the aim of identifying the cavity growth mechanisms. The analysis was based on the theories of diffusional cavity growth and coupled diffusion and power-law creep cavity growth proposed by Edward and Ashby. Both the above processes were found to take place under certain conditions. Pure power-law creep cavity growth has not been observed, in contrast to pure grain boundary sliding growth. Also the intragranular fracture was observed under conditions when the intergranular fracture due to the power-law creep cavity growth would be expected. The agreement of stress dependences of time to fracture derived from the theory with those determined experimentally makes it possible to predict the time to fracture. However, successful prediction depends critically on the correct choice of a number of parameters characterising the properties of the material under consideration, as well as a number of parameters characterising the intergranular damage. The proper choice of the latter parameters is discussed in detail. (author)

[en] Zr-Sn based experimental alloys were prepared by triple melting in a vacuum furnace of iodinated zirconium and tin of a purity of 99.99%. The alloy chemical composition was Zr-Sn (0.8 to 6% Sn). Creep tests were conducted in pure argon. The shape of creep curves was typical for polycrystalline metal materials. Stress and temperature dependences of the stationary creep rate, effective stress corresponding to stationary creep, deformation stress, fracture time, the relationship between the fracture time and the stationary creep rate, and the effect of tin and molybdenum contents on the above parameters were studied. A comparison of the values of creep activation energies of the alloys with those of activation energies of Zr-Sn-Mo ternary alloys show that the effect of molybdenum was not significant. Fracture occurring at precipitation-reinforced zirconium alloys does not show characteristic signs of a creep fracture; no presence of intercrystalline cavities or cracks was observed in the vicinity of a fracture surface. The results also showed that reducing the stationary creep rate resulted in extending the time prior to fracture and that the optimum tin concentration in the Zr-Sn-Mo alloy should not exceed 4.5%. (J.P.)

[en] Some characteristics of fracture in creep of five Zr-Sn-Mo alloys and a Zr-Sn-Mo-Nb alloy at temperatures between 673 and 823 K, and in a broad applied stress interval are summarized and discussed. Applied stress and temperature dependences of time to fracture were obtained, as well as relations between creep strain to fracture and time to fracture, and relations between creep strain to fracture and applied stress. Further, relations between 'mean' creep rate, defined as the ratio of creep strain to fracture and time to fracture, and steady state creep were determined. The results are discussed considering temperature and applied stress dependences of steady state creep rate. It is suggested that fracture in creep is controlled by the same mechanism as the creep itself. The influence of alloy composition on time to fracture was evaluated and the strengthening effect of tin, molybdenum and niobium is discussed. (Auth.)

[en] Harper-Dorn (H-D) creep in zirconium-alpha has been investigated at homologous temperatOres 0.35 to 0.48 (773 to 1023 K) and stresses ranging from 4x10^-6 to 9x10^-5 G (G is the shear modulus) by the helicoid specimen technique. It has been shown that H-D creep takes place at intercept grain sizes larger than about 125 μm, while at smaller grain sizes Coble creep operates under the same external conditions. The H-D creep is most probably dislocation core diffusion controlled. A threshold stress for steady state creep has been detected increasing with decreasing temperature. The existence of threshold stress has been qualitatively accounted for by high dislocation density (approx. 10¹² m^-2) in the specimens tested. The nonconservative motion of jogs on screw dislocations dependent on dislocation core diffusion has been suggested as the creep rate controlling mechanism. The energy of jog formation has been estimated to wh ion has been estimated to which a mean distance between jogs approx. 5b corresponds at 1000 K, where b is the Burgers vector. Also the observed transient creep has been briefly discussed. The transient component of the creep strain cannot be accounted for exclusively by inelastic bowing out of links of dislocation network. (author)

No abstract available

[en] The results of an investigation of creep in Zr-ALPHA at stresses ranging at temperatures ranging from 748 to 973 K and with linear intercept grain sizes less than 120 microm are presented. It is shown that the steady state creep rate is inversely proportional to the third power of intercept grain size, increases with the stress linearly and is grain boundary diffusion controlled. For steady state creep a threshold stress is characteristic. The results are correlated with the model of non-uniform diffusional flow due to Ashby and Verrall. The measured threshold stress is accounted for on the assumption that it represents the stress opposing the motion of grain boundary dislocations. From the results of the present work together with those of previous work on power law creep and Harper-Dorn creep in Zr-ALPHA creep mechanism maps were constructed. (orig./WE)