Role of Carbide Precipitates and Process Parameters on Achieving Grain Boundary Engineered Microstructure in a Ni-Based Superalloy

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Web End = Role of Carbide Precipitates and Process Parameters on Achieving Grain Boundary Engineered Microstructure in a Ni-Based Superalloy

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Web End = SHYAM SWAROOP KATNAGALLU, SUMANTRA MANDAL, ATHREYA CHEEKUR NAGARAJA, BERND DE BOER, and SUBRAMANYA SARMA VADLAMANI

Thermo-mechanical processing (one-step and iterative) comprising strain (5, 10, and 15 pct cold rolling) and annealing [at 1273 K, 1323 K, and 1373 K (1000 C, 1050 C, and 1100 C) for dierent times of 30 minutes, 1 and 2 hours] were employed to realize a grain boundary engineered (GBE) microstructure in alloy 617. Among the single-step routes, the process employing 15 pct cold reduction and annealing at 1373 K (1100 C) for 1 hour was found to be eective in increasing the fraction of R3 boundaries; however, it also induced partial recrystallization. The iterative processing employing lower reductions and higher annealing temperatures failed to realize GBE microstructure. The second-phase carbides in this material eectively pin the boundaries thus requiring higher pre-strain to initiate the boundary migration and subsequent multiple twinning events. The iterative processing designed based on the outcomes of the single step route resulted in GBE microstructure by signicantly increasing the R3 fraction and substantially disrupting the random high-angle grain boundaries connectivity. The newly added R3 boundaries in the GBE microstructure predominantly terminated on (111) plane indicating that they have low-energy conguration. The GBE specimen has shown remarkable resistance to intergranular corrosion as compared to the as-received condition.

DOI: 10.1007/s11661-015-3064-4 The Minerals, Metals & Materials Society and ASM International 2015

I. INTRODUCTION

ENHANCEMENTS in the properties and performance of materials without modifying the chemical composition have always been the quest of the manufacturing industry. In this context, grain boundary engineering (GBE) has emerged as a promising approach to improve bulk polycrystalline properties such as the resistance against oxidation,[1] segregation,[2] embrittlement,[3,4] weld decay,[5] corrosion,[6,7] creep,[8] fatigue,[9] and fracture.[10,11] The basic philosophy of GBE is to optimize the grain boundary character distribution (GBCD) by increasing the proportion of the so-called special boundaries (SBs). SBs are those that have relatively better properties when compared...