Microstructure evolution during heat treatment of superalloys loaded with different amounts of carbon
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Zhang and Lin Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China (Received 23 January 2015; accepted 14 April 2015)
The effects of heat treatment for recovering microstructure of a Ni-based single crystal superalloy with carbon addition have been evaluated. The heat treatment resulted in increased levels of chemical homogeneity. All the samples experienced more c9 coarsening than as-cast samples. Significant changes to as-cast carbide morphologies were observed. Script-type, MC carbide networks transformed during heat treatment to smaller, spherical Ta-rich MC carbides. Heat treatment caused significant MC carbide decomposition and formation of Cr-rich secondary carbides on or near to decomposed carbides in all modifications. The size of carbides after heat treatment was less than that of cast alloy obviously, and the distribution of carbides became more and more dispersion than in cast alloy.
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.127
known to strongly determine the precipitation kinetics.14 Then, by aging heat-treatments, it is possible to achieve a controlled c9 phase re-precipitation, producing a uniform and isotropic material.7 As a matter of fact, the size of the precipitate increases from the cores of the dendrites to the interdendritic regions in the case of materials not fully homogenized. Their morphology also evolved from cuboidal in the cores to more irregular shapes in the interdendritic regions.15 Generally, a solution heat treatment is intended to dissolve the precipitating phase for subsequent re-precipitation in an optimized morphology and size. In addition, the solution heat treatment also results in elimination or reduction of the segregation to produce a more uniform, homogeneous microstructure.6,7 The c–c9 microstructure of nickel-base superalloys can encounter drastic change during high temperature heat treatment. According to previous research, the recent re-introduction of carbon into single crystals has been useful in reducing casting grain defects.16–18 Carbon additions are also helpful in reducing oxide inclusions and surface scale.19 Additions of B and N along with C altered carbide morphologies in the as-cast state with CMSX-4 alloy.20,21 Although a number of influences of heat treatment on the microstructure of superalloys have been reported, much of the alloys were directionally superalloy,9,22,23 the focus was the c9 precipitates, c/c9 eutectic phase, and TCP phase.10,14 The effect of heat treatment on the microstructure of carbon modified single crystal superalloys has not been fully characterized. In this work, four single-crystal superalloys with varied contents of C were prepared. The effect of heat treatment
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Materials Research Society 2015
I. INTRODUCTION
Ni-based single crystal superalloys with superior properties at high temperature are widely used as turbine blades in aircraft engines. In the p
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