Stress relaxation in beryllia

Abstract

Beryllia beams were loaded at temperature in four-point bending to a deflection of 0.001 in. or 0.002 in. and the decrease in load necessary to maintain the deflection constant was measured as a function of time. The beryllia was fine-grained, in the range ∼ 1 to 7μm, and the porosity varied between 3 and 21%. The test conditions covered a temperature range of 850 to 1240°C, experiments took from 10 min to 8 h: the loads used produced initial maximum outer fibre stresses of the order 3000 to 6000 psi.

A method was developed for calculating the stress distribution in a beam at any time and this was used to analyse the results. The stress relaxation process was found to be stress-activated, and the conversion of elastic to plastic strain could be expressed as a creep law having the form

$$\dot \in = k_0 exp \left( {\frac{{ - Q}}{{RT}}} \right) exp \left( {\frac{{Cv\sigma }}{{RT}}} \right)$$

.

The activation energy was 100 ± 2 kcal mole⁻¹ and the activation volume was large, probably of the order of 10³ atoms. The rate constantk ₀ was approximately proportional to the fraction of intergranular porosity and inversely proportional to the cube of the grain diameter. It is suggested that the mechanism of grain boundary sliding is consistent with the observations.