Download Free Microstructural Evolution And Fracture In Creep Of An Xd Tial Based Alloy Book in PDF and EPUB Free Download. You can read online Microstructural Evolution And Fracture In Creep Of An Xd Tial Based Alloy and write the review.

By the late 1940s, and since then, the continuous development of dislocation theories have provided the basis for correlating the macroscopic time-dependent deformation of metals and alloys—known as creep—to the time-dependent processes taking place within the metals and alloys. High-temperature deformation and stress relaxation effects have also been explained and modeled on similar bases. The knowledge of high-temperature deformation as well as its modeling in conventional or unconventional situations is becoming clearer year by year, with new contemporary and better performing high-temperature materials being constantly produced and investigated. This book includes recent contributions covering relevant topics and materials in the field in an innovative way. In the first section, contributions are related to the general description of creep deformation, damage, and ductility, while in the second section, innovative testing techniques of creep deformation are presented. The third section deals with creep in the presence of complex loading/temperature changes and environmental effects, while the last section focuses on material microstructure–creep correlations for specific material classes. The quality and potential of specific materials and microstructures, testing conditions, and modeling as addressed by specific contributions will surely inspire scientists and technicians in their own innovative approaches and studies on creep and high-temperature deformation.
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.
Using the purpose built arbitrary strain path rig, hot deformation testing has been carried out on a commercial Ti-6A1-4V alloy. Initially, simple reverse torsion tests were performed, followed by tests combining torsion and uniaxial deformation. Optical and scanning electron microscopy have been used to assess the initial, heat treated and deformed structures. Bulk texture analysis has been carried out on the initial material, Electron Beam Backscattering Diffraction (EBSD) was used to assess micro-texture changes in heat-treated and deformed material.