Papers by Keyword: High-Temperature Creep

Abstract: A progressive increase of plant efficiency calls for new requirements of heat-resistantsteels used in the boiler and piping systems. In this paper, high-temperature creep behavior of T23and T24 steels were studied. Creep tests over a long period of time have been conducted for bothsteels at different temperatures. The creep mechanisms of the two steels have been clarified byanalyzing the minimum creep rate versus stress data. Besides, the creep rupture data from the creeptests were in good accordance with theoretical simulation on the basis of the CDM model over a longtime. Creep temperature has great effects on the rupture strength of the two steels. By creep ruptureexperiments and appropriate modelling, the high-temperature creep behavior can be well described.

Abstract: High temperature creep properties of Al₂O₃-SiO₂ ore (65% Al₂O₃ by mass percentage, abbreviation for Al₂O₃~65) was studied. The results show that the creep rates at 1300 °C × 50h, 1400 °C × 50h and 1500 °C × 50h were-0.89% ,- 1.75%, - 5.76%,respectively. At 1300 °C, ore has good creep resistance. As the temperature increases, the high temperature creep resistance of Al₂O₃ ~ 65 bauxite ore is significantly reduced. Creep process can be divided into two stages: when the time t <30h, the sample are in the densification process; after 30 hours, the creep properties show that the relationship between time and the creep rate is linear. At 1300 °C, the absolute value which is the slope of the linear relationship is smaller, exhibited an excellent high-temperature creep resistance. The loading softening-temperature of Al₂O₃ ~ 65 bauxite ore is 1343 °C. The Sample phase and microstructure are characterized by X-ray diffraction and scanning electron microscopy (SEM). The results show that mullite reticular is formed in sample at 1300 °C; at this point, TiO₂ mostly exist as Rutile. At 1400 °C and 1500 °C, TiO₂ mostly exist as Aluminum titanate, Corundum phase portion into mullite.

Abstract: In this study, three cooling rates, namely, Argon cooling, air cooling and furnace cooling after solution heat treatment of directionally solidified CM-247LC superalloy were proposed to explore the high temperature/low stress (982°C/200MPa) creep behavior. Standard heat treatment schemes of DS CM-247LC superalloy are solution treatment at 1260°C for 2 hour, then first aging at 1079°C for 4 hour and followed by second aging at 871°C for 20 hour. Results show that Argon cooling specimen provided the longest creep rupture life, which exceeds that of the air cooling specimen around 40 hour; whereas the creep rupture life of furnace cooling specimen was the shortest one of 100 hour, which is shorter than that of the air cooling specimen around 40 hour. Rupture strains of all three specimens were almost identical around 20%. Microstructural differences of gamma prime morphology were observed to explain the differences of creep rupture life.

Abstract: A new interpretation model, instead of classical Kohlrausch-Williams-Watt (KWW) equation, was applied to interpret stress relaxation behavior of Si3N4-Y2Si2O7 ceramics. Results revealed that the new model could obtain reasonable relaxation plastic viscosity and viscoelastic viscosity under testing temperature range of 1300°C~1575°C. From the plotted curve of viscosity vs 1/RT, an activation energy change occurred around 1500°C was found for both plastic viscosity and viscoelastic viscosity curves. This change, which indicated the microstructure change, was in good agreement with the significant decrease of high temperature strength retention property.

Abstract: Indirect method is used for determination of copper self-diffusion coefficient on the base of high-temperature creep data. These data are obtained by the use of the new measuring device and assuming that Nabarro-Herring mechanism acts.

Abstract: Mg-8Zn-4Al-xY base alloys containing an icosahedral quasicrystal phase (i-phase) as a main strengthening phase were prepared by casting into a copper mould at moderate cooling rates. The Y addition was effective for decreasing the size of i-phase and the more homogeneousness of its dispersed state. The mechanical properties at room temperature were much superior to those of AZ91 alloy. The creep tests indicated a promising high temperature creep resistance of the quasicrystal-reinforced Mg-Zn-Al-Y cast alloy. The dislocation characteristic in crept specimens which containing i-phase and no i-phase was analyzed and strengthening mechanism was discussed.

Abstract: The creep behavior and mechanisms of extruded NiAl-25Cr alloy at elevated temperatures have been studied in the paper. Analysis of the creep data over the temperature range 1073-1123 K reveals two distinct regions of creep behavior present in this material. At lower temperature, the creep characteristics are consistent with mobility-controlled deformation where viscous glide of dislocations controls creep. At higher temperature, the creep characteristics are consistent with a structure controlled creep process where some form of dislocation climb controls creep deformation.

Abstract: Crept microstructures in g-TiAl based alloys reveal a preponderance of 1/2[110]-type jogged-screw dislocations, suggesting that the rate of creep deformation is controlled by the glide of such dislocations. A creep model based on these microstructural observations has been recently developed. This leads to an excellent prediction of creep rates and stress exponents. In this paper, the framework of this model including the verification and validation of the functional dependencies of various microstructural model parameters is reviewed. It has also been observed that creep phenomenology is extremely sensitive to microstructure – fully lamellar g-based alloys exhibit lower creep rates and higher stress exponents even though the deformation microstructure is similar to that in equiaxed alloys. The modifications made to the model that account for the constrained nature of deformation in lamellar alloys are discussed. The applicability of the model is explored in materials systems, including a-Ti and a+b Ti alloys where similar creep exponents and deformation structures have been observed. Finally, the relevance, applicability and shortcomings of the model are critically analyzed.