Microstructure-Resistivity Correlations in Controlled Waspaloy Microstructures
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Microstructure-Resistivity Correlations in Controlled Waspaloy Microstructures V. Siva Kumar G. Kelekanjeri and Rosario A Gerhardt Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Room 288, Love Bldg, Atlanta, GA, 30332
ABSTRACT Nickel-base superalloys are an important class of metallic ‘nanocomposite’ structural materials known for their good strength retention abilities at high homologous temperatures for long service times. Literature on electrical resistivity studies of age-hardening superalloys is limited. The current work is focused on developing microstructure-electrical resistivity correlations in controlled Waspaloy microstructures. The controlled Waspaloy microstructures were produced upon aging the initial homogenized alloy at nominal temperatures of 700°C, 800°C and 875°C for times up to 100 hrs. Resistivity measurements did not reveal a γ′ nucleation regime for the sampled aging intervals. The primary microstructural evolution mechanism contributing to the observed changes in resistivity was γ′ coarsening. Interestingly, the microstructures resulting from progressive aging at 700°C showed a slow transformation of etchpits from perfect polygonal shapes to more irregular shapes, which is observed simultaneously with γ′ coarsening. INTRODUCTION Nickel-base superalloys derive their superior high-temperature strength retention properties from the precipitation-hardening γ′ phase [1]. The overall mechanical behavior of the alloy during service is primarily determined by the size distribution of the γ′ phase. It is therefore important to have knowledge of the microstructural state of the alloy to assess the mechanical integrity of the components in service. Electrical resistivity measurements have been used by researchers for studying nucleation and growth phenomena in age-hardening metallic systems [2,3]. Some studies have reported resistivity in age-hardening nickel systems[4-6]. White et al. [4] report resistivity studies on aging of Nimonic alloy PE16 at various temperatures between 580°C and 780°C. They observe an initial increase in resistivity up to a maximum and then a monotonous decrease corresponding to the nucleation and growth of clusters. Al-Aql [5] reports that the measured resistivity increases initially with the aging time and eventually reaches saturation for isothermally aged Ni-Cr alloys. The initial increase was attributed to the formation of solute (Cr) clusters, which upon prolonged aging transformed into Ni3Cr precipitates or Ni2Cr superstructure. An anomalous behavior in resistivity, described by a sharp fall and then a sudden rise between 477°C and 620°C, was reported for Ni-25 at% Cr alloy. This behavior was attributed to the existence of an ordering reaction with the precipitation of Ni3Cr [5]. Rana et al. [6] also reported similar results for Ni-25 at% Cr alloy.
The microstructural evolution of Waspaloy, upon aging at 800°C after different initial solution treatments, was recently reported [7]. In the present paper, the microstruc
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