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Abstract: Despite the large body of work done in the area of high-temperature creep, the present understanding of creep mechanisms and the effect of alloying and microstructure is limited. The first part of this investigation concentrates on the creep behavior in the equiaxed microstructure of gamma-TiAl alloys, to understand the mechanisms and develop a physically-based model for creep in the gamma phase. A modification of the classic jogged-screw model has been previously adopted to explain observations of 1/2[110]-type jogged-screw dislocations in equiaxed Ti-48Al under creep conditions. The goal of this study is to verify and validate the parameters and functional dependencies that have been assumed in that model. The original solution has been reformulated with the aid of analytical modeling, numerical simulations and Transmission Electron Microscopy. Both experiment and theory lead to an excellent prediction of creep rates and stress exponents. In the second part of this study creep behavior of the fully lamellar TiAl alloys is investigated. Dislocation structures similar to those observed in the equiaxed alloys suggests that the jogged-screw model can be adapted for lamellar alloys. The aim of the model is to predict the unique creep phenomenology of fully lamellar alloys. The strengths and shortcomings of the model are discussed. Probable low stress creep mechanisms are also suggested. The origin of fully lamellar alloys' superior creep properties stems from the constrained nature of deformation in the lamellae. The results from stress drop experiments are analysed to explore the origin of the large back stresses associated with the fully lamellar alloys. Reduction of the lamellar spacing is proposed as the best way to lower creep rates. In the final part of this study, the microstructural stability of lamellar alloys during exposure to creep conditions is investigated. A detailed investigation of the creep behavior of the aged (stabilized) and unaged (unstabilized) alloys was carried out and subsequent TEM studies were done to characterize the microstructural changes during creep. Continued loss of metastable alpha-2 is proposed as the reason for the larger strain rates in all stages of creep for the alloy in the unaged condition. Dynamic recrystallization and the formation of equiaxed gamma grains are discussed. These results suggest that microstructural stability is critically important in order to achieve the highest possible creep strengths. The effect of microalloying and the role of precipitation hardening in creep are investigated. The probable mechanisms of primary, secondary and tertiary creep are discussed and methods for improving the creep properties are suggested.
This volume reviews and updates the creep behavior of advanced materials, including ceramic and metal-matrix composites, laminated materials, nanostructured materials, advanced intermetallics, and advanced dispersion strengthened materials. This collection from the 1999 TMS Annual Meeting & Exhibition also covers the influence of microstructure on various aspects of creep and how this knowledge can be used to design more creep resistant materials.
Creep refers to the slow, permanent deformation of materials under external loads, or stresses. It explains the creep strength or resistance to this extension. This book is for experts in the field of strength of metals, alloys and ceramics. It explains creep behavior at the atomic or “dislocation defect level. This book has many illustrations and many references. The figure formats are uniform and consistently labeled for increased readability. This book is the second edition that updates and improves the earlier edition. Numerous line drawings with consistent format and units allow easy comparison of the behavior of a very wide range of materials Transmission electron micrographs provide direct insight into the basic microstructure of metals deforming at high temperatures Extensive literature review of about 1000 references provides an excellent overview of the field
These proceedings contain 48 innovative papers consolidating the development of creep research since 1990 and discussing the new horizons in this fundamental field of applied mechanics in the coming century. This volume is useful for researchers and graduate course students in the relevant fields.
Proceedings from: EPRI’s 9th International Conference on Advances in Materials Technology for Fossil Power Plants and the 2nd International 123HiMAT Conference on High-Temperature Materials