Role of Carbide Precipitates and Process Parameters on Achieving Grain Boundary Engineered Microstructure in a Ni-Based
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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 to the random high-angle grain boundaries (HAGBs) and are often described in terms of the coincidence site SHYAM SWAROOP KATNAGALLU, formerly Post-Graduate Student with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India, is now Lecturer with the Rajiv Gandhi University of Knowledge Technologies, Idupulapaya, 516329, India. SUMANTRA MANDAL, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. ATHREYA CHEEKUR NAGARAJA, Graduate Student, and SUBRAMANYA SARMA VADLAMANI, Associate Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras. Contact e-mail: [email protected] BERND DE BOER, Senior Manager, is with the VDM Metals GmbH, Plettenberger Strasse, 258791 Werdohl, Germany. Manuscript submitted March 4, 2015. Article published online July 14, 2015 4740—VOLUME 46A, OCTOBER 2015
lattice (CSL) model. Although SBs and low R (R £ 29) CSL boundaries are often used synonymously, there is increasing evidence that only a subset of CSLs are special.[12,13] A more complete description of SB is the one which terminates on low-index planes.[14] In practice, GBE has been extensively applied to low-tomedium stacking fault energy (SFE) materials as these exhibit prolific twinning during annealing and majority of CSL boundaries in GBE microstructure in these materials are essentially R3 and its variants (viz. R9 and R27).[15–17] Since R3 boundaries are generally associated with special properties (as they preferentially terminate on low-index planes[18]), the main thrust of twinningrelated GBE in low SFE materials is to enhance the fraction of R3 boundaries.[19] Unlike R3 boundaries, the higher order R3 boundaries (i.e., R9 and R27) generally do not show any clear preference to terminate on the low-index plane and whether or not they are ‘special’ depends on their terminating on the low-index planes.[20,21] However, the main role of R9 and R27 boundaries is rather geometric i.e., they take part in the reconfiguration of the existing grain boundary network that eventually breaks down the random HAGBs connectivity.[22,23] GBE microstructure has been realized through various processing approaches such as application of magnetic field[24] o
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