Effect of Grain Boundaries and Indentation Load on the Electrical Properties of Nickel Base Super-alloys
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Effect of Grain Boundaries and Indentation Load on the Electrical Properties of Nickel Base Super-alloys Kimberly Pinkos, Celestina Laboy and Rosario A. Gerhardt School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332–0245 ABSTRACT We have used impedance spectroscopy to evaluate the effect of the presence of grain boundaries and plastic deformation, as caused by hardness indentation, on the electrical response of several commercial nickel base super-alloys at room temperature. These alloys consist of a mostly nickel matrix that contains small precipitates of intermetallic phases such as Ni3Al or Ni3Ti(often referred to as gamma prime) as a reinforcing phase. Measurements were made as point contacts so that data could be tracked from point to point. Results indicate that the grain boundaries tend to have higher conductivities than the individual grains in most cases. It is speculated that this is due to the boundary regions having a different compositional profile than the center of the grains(as determined by gamma prime size, shape and distribution). Hardness indentation, on the other hand, had a more dramatic effect, by causing the magnitude of the imaginary impedance to change in size as well as position. Complementary microscopy results are included as supporting evidence for the effects discussed. INTRODUCTION Non-destructive testing methods are important for detecting and monitoring the effect that defects can have on a material’s performance. When materials are used in such applications as turbine blades in power plants or in aircraft engines, it is very crucial that the deformation in the material be carefully monitored. The failure of one of these blades can cause the power plant or the aircraft engine to shut down completely. The existence of a weak grain boundary or the presence of extended deformation in a gas turbine blade presents the hazard of failure at lower temperatures than expected. Material and service costs for replacement of a part are expensive and therefore, it is preferred that the blades and vanes be cast as single crystals. However, it is not always possible to grow single crystals large enough for all applications or to prevent extended plastic deformation. Resistivity and eddy current measurements have previously been used to monitor crack formation in metallic materials[1]. Both of these methods tend to be done at a fixed frequency or in a dc mode. Impedance Spectroscopy (IS), on the other hand, is an ac method that has the potential of being used as a non-destructive method for monitoring the microstructural state of a blade or vane because the impedance of a material, Z*=Z'jZ", gives a more complex measure of the material’s resistance to the flow of electricity. Impedance measurements include not only the resistive but also the capacitive and inductive elements present[2]. If a material undergoes deformation, then the structure of the material and the flow of electricity through the material will change in accordance to changes in those same cir
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