| الوصف: |
The creep and creep fatigue properties of a 14% Cr ferritic steel (MA 957) strengthened with a dispersion of Y2O3 particles have been determined using high precision constant stress creep machines. Creep testing was carried out under tension over a stress range of 250 to 420 MPa at 873 to 973K. In addition, compression creep tests were also performed over a stress range of 250 to 320 MPa at 948K. Normal creep curves were observed under all tensile conditions, i.e. after the initial strain on loading, the strain rate decreased to an apparently steady state before the onset of a tertiary stage leading to fracture. Curve shapes varied with testing conditions. At low temperatures, all creep curves were primary dominated. At higher temperatures, the creep curve shapes were primary dominated at low stresses but tertiary dominated at higher stresses. Like all oxide dispersion-strengthened alloys (ODS), the stress exponent (n) and the activation energy (Qc) values of MA 957 were found to be anomalously large; n varied from 25 to 52, whilst Qc varied from 498 to 856 KJmol-1 over the stress and temperature ranges studied. Despite much research on ODS alloys, power law creep behaviour of ODS alloys is still not well understood. Thus, the creep properties of MA 957 were analysed using the θ methodology. It was shown that entire creep curves can be constructed accurately when using the θ Projection Concept, allowing minimum creep rates and times to failure to be calculated for any stress and temperature. Moreover, this study indicated that the complex behaviour patterns observed when power law equations are used to describe the creep properties of MA 957 can be rationalised convincingly using the θ methodology. |
