Micromechanism of High-Temperature Tensile Deformation Behavior of a Directionally Solidified Nickel Base Superalloy
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Micromechanism of High-Temperature Tensile Deformation Behavior of a Directionally Solidified Nickel Base Superalloy R.K. Rai, J.K. Sahu, P.S.M. Jena, S.K. Das, N. Paulose, and D. Chandru Fernando (Submitted October 2, 2017; in revised form December 18, 2017) High-temperature tensile deformation behavior of directionally solidified nickel base superalloy CM 247 DS is studied by conducting tensile tests in temperature range RT-955 C employing a constant strain rate of 1023 s21 and carrying out extensive electron microscopic examinations to understand the concomitant substructural evolution. The alloy exhibits yield strength anomaly (YSA) behavior like many other superalloys, and the yield strength maxima occur at 750 C. However, unlike in most of the superalloys, ductility continuously increases with temperature. The deformation behavior of the alloy changes significantly with temperature. Transmission electron microscopic examination confirmed that at lower temperature ( £ 750 C), c¢ shearing is the dominant deformation mechanism; whereas at temperatures above 750 C, thermally activated dislocation looping around c¢ precipitate is dominant. Substructures evolved during deformation at 750 C consists mainly of superlattice stacking faults (SSFs) inside c¢ precipitates, whereas at 850 C uniform dislocation tangles are observed in c matrix. Superlattice stacking faults result from shearing of c¢ precipitate by a/3Æ112æ dislocations, which arise from the decomposition of a/2Æ110æ matrix dislocations. YSA in this alloy is attributed to dislocation interactions inside c¢; however, the enhanced ductility even at 750 C is due to formation of SSFs. Keywords
advanced characterization, electron microscopy, superalloys, superlattice stacking faults, tensile test
1. Introduction CM 247 DS is a directionally solidified (DS) nickel base superalloy used for manufacturing of turbine blades and vanes due to its adequate high-temperature mechanical properties, oxidation and corrosion resistance (Ref 1-5). The alloy has multiphase microstructure consisting of c¢ precipitates embedded in c matrix, carbides, and c–c¢ eutectics. The volume fraction of c¢ precipitates in this alloy is about 65%. Although tensile properties of CM 247 DS are known at application temperature, a detailed study on the tensile deformation behavior across a broader spectrum of temperature with emphasis on substructural evolution is scarce (Ref 1-5). As nickel base superalloys are undergoing continuous evolution, these kinds of studies with emphasis on deformation micromechanisms provide impetus for future alloy development. The deformation micromechanisms accounting for yield strength anomaly (YSA) behavior in a few other nickel base superalloys is discussed in the ensuing paragraph. For example, Sajjadi et al. (Ref 6) studied the tensile deformation behavior of alloy GTD-111 in the temperature R.K. Rai, CSIR-National Metallurgical Laboratory, AcSIR, Jamshedpur 831007, India; J.K. Sahu, P.S.M. Jena, and S.K. Das, CSIR-Nat
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