SPM-based Electrical Characterization of Aged Waspaloy Microstructures
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SPM-based Electrical Characterization of Aged Waspaloy Microstructures V. Siva Kumar G. Kelekanjeri, and Rosario A Gerhardt School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Room 288, Love Bldg., Atlanta, GA, 30332-0245 ABSTRACT The results of Scanning Probe Microscopy (SPM) based localized electrical examination of precipitation-hardened Waspaloy microstructures using Electrostatic Force Microscopy (EFM) and Current-Atomic Force Microscopy (I-AFM) are reported herein. Measurements were conducted on two differently etched specimens with the same initial microstructure. Selective etching by preferentially removing the γ′ or the γ phase resulted in non-uniform surface topography leaving the less reactive phase standing in relief relative to the depressed phase. The surface roughness introduced upon etching affected the measured EFM response over and above the chemical work function effects because of a different atomic arrangement relative to a polished surface. Regions of high current found near γ-γ′ interphase boundaries using I-AFM were explained to be an artifact of etch topography and the feedback mechanism of the SPM. INTRODUCTION Nickel-base superalloys derive their superior strength-retention capability from the presence of nanometer sized Ni3(Al,Ti) phase (referred to as γ′), which precipitates coherently within the nickel-rich matrix phase (γ). The microstructural variations in these alloys induced by thermal aging treatments have been studied using macro electrical techniques such as DC fourpoint probe resistivity [1, 2]. However, the measured resistivity in this case is an effective quantity that is averaged over a much larger volume relative to the size of the precipitates. To characterize the electrical properties of γ and γ′ phases separately, even on a qualitative basis, requires a localized examination of these microstructures at the sub-grain level. Research on localized electrical investigations of microstructures of multiphase metallic alloys is virtually non-existent in the literature. Goken et al. [3] reported on SPM-based nanomechanical characterization of CMSX-6 superalloy and Waspaloy microstructures. The local mechanical properties of γ and γ′ phases in these alloys were determined to a lateral resolution of 50nm. In the present article, we report on localized SPM-based electrical characterization of γ-γ′ microstructures in Waspaloy. The microstructures were investigated via EFM and I-AFM. EFM is based on probing the capacitive forces between the tip and the surface features, which depend on the local work function (for conductive surfaces) as well as the capacitive gradient (see equation (1)) [4, 5]. In the I-AFM mode, a current map is generated by a conductive tip that scans across a region of the specimen with locally varying resistivity in contact mode. EXPERIMENTAL PROCEDURE The Waspaloy specimens used for SPM-based electrical experiments were synthesized using controlled aging-treatments [2]. The γ′ precipitates present in the he
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