Grain boundary segregation of boron in INCONEL 718
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I.
INTRODUCTION
The alloy INCONEL* 718 is widely used in manufac*INCONEL is a trademark of INCO Alloys International, Huntington, WV.
turing discs for gas turbine engines. It possesses excellent corrosion and oxidation resistance in addition to superior tensile and creep properties at elevated temperatures. While the alloy is easy to weld due to its sluggish precipitation kinetics, it suffers from heat-affected zone (HAZ) liquation cracking (microfissuring).[1] This cracking, although generally believed to be caused by Nb-rich phases,[1–5] has been also found to be caused by B segregation at grain boundaries.[6,7] The segregation of B at grain boundaries, in general, occurs by two mechanisms—equilibrium segregation[8,9] and nonequilibrium segregation.[10–15] In equilibrium segregation, boron atoms that diffuse to grain boundaries are actually bound to grain boundary sites, presumably those of high distortion. The amount of this segregation increases with decreasing annealing temperature and increasing solute concentration in the matrix. Nonequilibrium segregation occurs during cooling from high temperatures[10,11,12] and during annealing following plastic deformation.[16,17] This type of segregation requires a formation of solute-vacancy complexes and a gradient of these complexes between the grain interiors and grain boundaries. The complexes migrate to the grain boundaries, where the vacancies are absorbed by the grain boundaries, and then move along the grain boundaries toward the free surface of the crystal. The annihilation of vacancies at the grain boundary results in a concentration gradient of solute-vacancy complexes, which causes the complexes to diffuse from the grain interior to
W. CHEN, formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba, is Research Scientist with Nova Chemicals, Calgary, AB, Canada T2E 7K7. M.C. CHATURVEDI, NSERC Industrial Research Professor, is with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. N.L. RICHARDS, Manager, Materials and Processes/Facilities, is with Bristol Aerospace Ltd., Winnipeg, MB, Canada R3C 2S4. G. McMAHON, Scientist, is with the Metals and Materials Technology Laboratories, CANMET, Ottawa, ON, Canada K1A 0G1. Manuscript submitted April 25, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
grain boundaries. This diffusion causes an excessive amount of solute atoms to concentrate in the vicinity of grain boundaries, resulting in nonequilibrium segregation. The degree of nonequilibrium segregation has been found to depend on the concentration of solute-vacancy complexes and the diffusion time available during cooling or isothermal annealing.[10–15] The concentration of complexes, Cc, at temperature T is given by Cc 5 kccn ci exp (Eb /kT)
[1]
where Eb is the energy of formation of the recombined complex, k is Boltzmann’s constant and kc is a geometric constant, ci is the concentration of solute atoms, cv is the equilibrium concentration of free v
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