Detection of Compositional Fluctuations in High Temperature Exposed Waspaloy
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Detection of Compositional Fluctuations in High Temperature Exposed Waspaloy Xiaodong Zou, Tariq Makram and Rosario A. Gerhardt School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245 ABSTRACT Waspaloy is a nickel base super-alloy used in aircraft engines. When this alloy is placed in service, it is subjected to long term exposure at high temperatures, which can cause the reinforcing gamma prime precipitate population to fluctuate and thus affect its structural integrity. The population fluctuates as a result of coarsening, dissolution or reprecipitation. Samples exposed to 1200°F for times ranging from 0 to 12626 hours were characterized using impedance spectroscopy, microhardness measurements, x-ray diffraction and quantitative stereology. Two important parameters were derived from the impedance measurements: (1) the imaginary admittance peak magnitude (Y”max) and (2) the associated relaxation frequency (fmax). As the distribution, shape and size of the precipitates change with exposure time, these parameters were also found to vary. In addition to the changes in precipitate geometry, lattice constant changes detected by analyzing x-ray diffraction data suggest that there are compositional shifts in the matrix as well as the gamma prime precipitates. Furthermore, the preferred orientation of the precipitates can also be seen to change with exposure time. These changes in composition, size and shape as a function of thermal exposure time are accompanied by changes in the volume fractions of primary and secondary gamma prime particles present. Using effective medium models, it is possible to predict that the measured properties are related to the gamma prime population. The grain boundary carbides do not appear to play any role at the conditions presented. INTRODUCTION Nickel base super-alloys have been used in high temperature applications for a number of years because their microstructure contains coherent precipitates of intermetallics such as Ni3Ti or Ni3Al that provide stability under applied loads and thermal cycling[1]. However, long term exposure at high temperatures can result in coarsening of the precipitates as well as dissolution and re-precipitation. Understanding the evolution of the precipitate population may provide the key to understanding the mechanical behavior of components that utilize these types of materials. Waspaloy is a commercial alloy that is used to fabricate turbine blades used in aircraft engines. It is therefore desirable to develop a methodology for detecting changes in the precipitate population in a non-destructive way so that life prediction models could be modified to account for these changes. An earlier account of this work is available[2]. EXPERIMENTAL PROCEDURE A single forged bar of Waspaloy with square cross-section (shown in Figure 1) was used to conduct the thermal exposure experiments at 1200°F. Individual slices were cut after each exposure time and the bar was placed back in the furnace for additional R2.6.1
times. Each sli
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