Abstract
Creep studies over the range 700–1100° C have been made on four Ni–1 vol.-% ThO2-base alloys, which contained 0, 13·5, 22·6, and 33·7 wt.-% Cr. The phenomenological creep expression that describes the stress- and temperature-dependence of the steady-state creep rate, έ s , is έ s = K(σ/E) n exp − (Qc /RT). The stress exponent was n ≃ 7 for each alloy, and the activation energy for creep, Qc , increased from 64 kcal/mole for the 0% Cr alloy to 78 kcal/mole for the 33·7% Cr alloy. The constant K was dependent on the matrix stacking-fault energy. At a given stress and temperature, increasing the Cr concentration reduced the steady-state creep rate. This is attributed to a combination of three possible effects: (a) decreasing the matrix stacking-fault energy, γ, by Cr additions; (b) short-range ordering (SRO), especially in the 22·6 and 33·7% Cr alloys; (c) a slightly lower volume self-diffusivity in Cr-containing alloys, assuming that the rate-determining creep process is diffusion-controlled. Using the Barrett–Sherby analysis it was possible to show that έ s ∝ γ m , where m was 2·6–3·0. It was not possible to assess the magnitude of the SRO strengthening effect. The presence of 1% ThO2 reduced έ s by ∼ 102 by comparison with previous results on polycrystalline single-phase Ni and Ni–Cr alloys This “creep-strengthening” is attributed both to direct strengthening by particles blocking dislocation motion during creep and to indirect strengthening caused by ThO2 particles helping to develop and pin a fine, stable, pre-creep substructure during alloy fabrication.
Get full access to this article
View all access options for this article.