Microstructural Investigation of Grain Growth in Cryomilled Inconel 625 Powder
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Microstructural Investigation of Grain Growth in Cryomilled Inconel 625 Powder Kyung H. Chung, Jongsang Lee, Rodolfo Rodriguez and Enrique J. Lavernia Dept. of Chem., Biochem. Eng. and Mater. Sci., University of California at Irvine, Irvine, CA 92697-2575, USA ABSTRACT The grain growth behavior of cryomilled nanocrystalline Inconel 625 powders was investigated during isothermal heat treatment at 600 ~ 900 °C for 1 ~ 4 hours. The grain size of cryomilled Inconel 625 remained under 250 nm following heat treatment in the temperature range of 600 ~ 900 °C up to 4 hours, which represents an improved grain stability compared to that of conventional Inconel 625 and cryomilled pure Ni. Microstructural studies, using TEM, revealed the existence of oxide particles after cryomilling. The pinning effects on grain stability by oxide particles were quantitatively analyzed. The grain stability of cryomilled Inconel 625 powders at 900 °C was noted to be better than that at lower temperatures. This behavior was attributed to the presence of two types of precipitates found at this temperature, which were identified as spherical NbC and rod shaped Ni3Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain boundary pinning mechanism. The preferred nucleation sites of those precipitates was noted to be at grain boundaries, thereby augmenting the grain boundary pinning effect. INTRODUCTION Recently, much research efforts have been driven by the need to manufacture two (e.g., thin film or coating) or three (e.g., bulk material) dimensional nanocrystalline structures that can retain the attributes of the individual nanostructured particles[1]. In the case of nanocrystalline materials, it is important to understand the stability of the structure during thermal exposure, as consolidation typically involves some type of deformation / temperature cycle. It is not surprising that a number of studies of the grain growth behavior of many types of materials[2,3] fabricated using varying approaches[4] are available. In recent studies, the grain growth behavior of nanocrystalline materials was reviewed by Malow et al.[4] and Gleiter[5]. A number of phenomena were proposed in an effort to account for the observed grain stability, namely: grain boundary segregation[6], solute drag[7] and second phase drag[8]. The present study was undertaken in an effort to provide insight into the fundamental mechanisms that govern the thermal stability and grain growth behavior of nanocrystalline Inconel 625 produced by cryomilling. EXPERIMENTAL PROCEDURE The commercial atomized Inconel 625 powders were chosen for this study. The powders were mechanically milled using a Szegvari attritor model B in stainless steel tank with stainless steel balls. The diameter of the balls were 6.4 mm and the ball to powder ratio was 20:1. The milling of the powders was conducted under a liquid nitrogen environment at a temperature of 180 ± 5 °C for 8 hours, and the impeller was operated at 180 rpm. The heat treatme
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