The Broadening of Crystallographic Orientation Distribution in Crept Ni-base Superalloys
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0980-II05-28
The Broadening of Crystallographic Orientation Distribution in Crept Ni-Base Superalloys Toru Inoue, Katsushi Tanaka, Hiroki Adachi, Kyosuke Kishida, and Haruyuki Inui Materials Science and Engineering, Kyoto University, Yoshidahon-machi, Sakyo-ku, Kyoto, 606-8501, Japan ABSTRACT The crystallographic orientation distribution, and its change accompanied with tilting γ / γ’ boundaries in Ni-based single crystal superalloys have been investigated by a theoretical elastic-plastic calculation, X-ray diffractometry and SEM-EBSD analysis. The distribution of the crystallographic orientation has significantly broadened by creep deformations. The broadening can be explained by an unbalance of the amount of creep dislocations of each slip system, which agrees with the result of elastic-plastic calculations. Creep strain of superalloys crept at a condition forming the raft structure can be estimated by the measurement of the width of rocking curve of a diffraction peak. INTRODUCTION Improvement of the efficiency of aircraft engines and gas turbine generators requires to increase the gas temperature and consequently the operating temperature of the turbine blades. Since modern nickel base single-crystal alloys exhibit superior performance at high temperatures, the alloys are widely used for turbine blades operated at the temperature up to about 1350 K. When the alloys are crept under a low tensile stress at a high temperature (around 137MPa, 1373K), the microstructure changes into a lamellar structure consisting of γ and γ’ plates alternately stacked along the tensile stress direction [1-14]. This microstructure is so called “raft structure”. The raft structure suppresses creep deformations because creep dislocations can not penetrate into the γ’ phase and stacked at γ / γ’ interfaces; the elastic field caused by interfacial dislocations suppresses a further creep deformation. In a later creep stage, the raft structure cannot remain stable, and the structure gradually collapses and creep deformation starts to accelerate as a result. It is important to understand the formation/collapse mechanism of the raft structure to improve further life time of the alloys. We discuss the stability of the raft structure in the accompanied paper [15], in which we report that the raft structure is elastically unstable at the end of primary creep stage and the instability leads the raft structure to titled or wavy ones. In actual, these partially collapsed raft structures are observed in crept specimens, however, the effect of lamellar boundary tilting from the (001) plane on the properties of the alloys has not been clarified yet. It is expected that tilted lamellar boundary loses the balance of the amount of creep dislocations of each slip system because the symmetry of the elastic stress field may not be a tetragonal. Since an unbalance of the amount of creep dislocations leads a local crystallographic rotation in the alloys, the unbalance can be detected as a broadening of crystallographic orientation distribution that can be d
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