Steady State Creep Stress Research Articles

Steady state creep of ZrNb alloys in a temperature interval 350 to 550°C

Creep of three Zr-Nb alloys (0.5, 2.5 and 4.5 wt% Nb) was investigated in a temperature interval 350 to 550°C using the isothermal creep test and transmission electron microscopy techniques. Relations between steady-state creep rate and applied stress were determined; the parameter of applied stress sensitivity of steady-state creep rate increases with the applied stress, reaching values sometimes greater than 30 at 350°C. The apparent activation energy of creep was determined and the mean effective stress in steady-state creep measured. Possible creep-rate controlling mechanisms were discussed, with the conclusion that the creep is most probably recovery-controlled. Relations between steady state flow stress and temperature suggest a significant contribution of an athermal deformation mechanism to the measured steady-state strain rate at low temperatures and high applied stresses. The creep-strengthening effect of niobium increases linearly with niobium concentration at high steady-state creep rates, while at low steady-state creep rates the optimum niobium concentration does not exceed 2.5%. This, together with the temperature sensitivity of the strengthening effect of niobium, was explained by structural stability decreasing as niobium concentration increases from 2.5 to 4.5%. Creep-controlling mechanisms for very low creep rates are discussed.

Effective stress in steady-state creep in alpha Cu–Zn solid solutions

Abstract Effective stress in steady-state creep in alpha Cu–10% Zn and Cu–30% Zn alloys in a temperature interval 723 to 973 K was measured using the strain transient dip test technique. The measured effective stress was found to be higher at higher zinc concentration than at any given applied stress. The apparent internal stress [sgrave] = [sgrave] – [sgrave]∗ is related rather to dislocation distribution than to dislocation density as compared in both alloys investigated. The apparent activation energy obtained, where is the steady-state creep rate, represents a purely phenomenological quantity which cannot be compared with the activation energy of dislocation glide. The same applies to the activation area determined on the assumption that the moving dislocation density is proportional to the total dislocation density. It is concluded that high-temperature creep in the alloys investigated is controlled by recovery due to dislocation climb and annihilation, and that the recovery is not driven by the appa...

Effective stress in steady state creep in an Al-5.5 at.% Mg solid solution

Using the strain transient dip test technique the mean effective stress σ ∗ was measured in steady state creep in an Al-5.5 at.% Mg solid solution. The mean effective stress increases slightly with increasing temperature. The σ ∗ σ ratio (σ is the applied stress) reaches values as low as about 0.4 at both low applied stresses and temperatures and as high as 0.94 at both high temperatures and applied stresses. The parameter m ∗ characterizing effective stress sensitivity of dislocation glide velocity was found to be close to one at lower effective stresses, while at higher effective stresses it increases linearly with σ ∗ . The apparent activation energy of dislocation glide was found to increase slightly with the effective stress; to σ ∗ = 0 a value of 23.7 kcal mol −1 corresponds. Dislocation core drag of solute atoms was suggested as a process controlling dislocation glide velocity in creep of the alloy investigated.

An interpretation of steady state creep

Abstract Effective stress in steady state creep in aluminum was measured by the strain transient dip test technique with the aim to analyse the creep in terms of two separate kinetic processes driven by different components of the applied stress [sgrave]: the effective stress [sgrave]∗ and the internal stress [sgrave]i (Ahlquist, Gasca-Neri and Nix 1970). The analysis has led to values of’ the apparent activation energy of slip ‘, Q∗, and ‘the apparent activation energy of recovery’, Qi, which generally differ from the apparent activation energy of creep and, therefore, also from the activation enthalpy of lattice self-diffusion, and cannot be correlated with the process controlling dislocation glide velocity and recovery rate, respectively. This supports the contention that if [sgrave]∗ is measured by a dip test technique [sgrave]i, obtained as [sgrave]—[sgrave]∗, cannot be identified with the internal stress taking part in processes other than the dislocation glide. Assuming that the recovery is driven by the applied stress the creep in aluminum can be described in terms of recovery due to dislocation climb as a rate-controlling process.

Steady-State Creep of a Thick-Walled Cylinder Under Combined Axial Load and Internal Pressure

Equations are developed for prediction of steady-state creep stresses and strain rates in a thick-walled tube under combined internal pressure and axial load. An analytical solution of the equations, in closed form, is not possible but numerical solutions to the equations are tabulated for various values of the variables. For values of the variables, other than those tabulated, interpolation is usually adequate. If this is not the case, a trial method of calculation is presented which uses the tabulated solutions as a starting point. Simple approximate solutions, which apply for the cases in which the additional axial load is small or large relative to the axial load due to internal pressure, are also given.