Driving force for discontinuous coarsening in a Ni-Al-Mo base superalloy
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INTRODUCTION
DISCONTINUOUS coarsening (DC) involves precipitate coarsening at a moving grain boundary to form a lamellar structure of lower interphase interfacial area.l'2 For conventional nickel base superalloys, DC of the 3/' strengthening phase has been reported only as a consequence of recrystallization wherein reduction of the stored energy of plastic deformation provides the major portion of the driving force for grain boundary (interface) motion. For experimental Ni-A1-Mo base superalloys, DC of 3/' occurs in the absence of recrystallization. Interface velocity is controlled by grain boundary diffusion and therefore increases with temperature and grain boundary misorientation. 3 The Ni-A1-Mo base superalloys are also characterized by a high y ' volume fraction, a large negative y/3/' lattice misfit, and the precipitation of the Mo-rich bcc a phaseY '6 In a previous study of DC in a Ni-A1-Mo base superalloy, grain boundary diffusivities were calculated using measured DC interface velocities and available analytical solutions. 3 It was assumed that reduced y ' interfacial energy was the sole DC driving force and a value of 3/' surface energy (3 • 10-2 j/m2), which had been reported for 3/' in Ni-A1 binary alloys, 7 was used. The calculated diffusivities were more than an order of magnitude greater than previously reported grain boundary self diffusivities for Ni. When considering the apparent discrepancy between the calculated values and measured nickel self diffusivities, it was pointed out that the rate controlling diffusing species in Ni-A1-Mo alloys is unknown and further that additional contributions to the driving force for DC (which were not considered in the calculation) may exist in Ni-A1-Mo base superattoys. Omission of the additional driving force would have caused calculation of grain boundary diffusivities which were too large. Additional DC driving force in Ni-A1-Mo base superalloys could be contributed by: (1) 3/' interfacial energies larger than 3 • 10-2 J/m 2 due to the large negative 3//3/' lattice misfit in these alloys, (2) coherency strain associated with the large 3//3/' lattice misfit which is relaxed by DC, A.W. FUNKENBUSCH, T.A. STEPHENSON, and G. McCARTHY are Research Metallurgists with United Technologies Research Center, East Hartford, CT 06108. H. FRASER is Associate Professor with The University of Illinois, Champaign, IL. Manuscript submitted May 4, 1984. METALLURGICALTRANSACTIONS A
(3) changes in phase chemistries and volume fraction which occur during DC and produce a more equilibrium microstructure, and (4) reduced surface area per unit volume of the bcc a phase during DC. The objective of the present study was to provide additional information on the driving force and mechanism for DC in Ni-A1-Mo base superalloys. The study was motivated by the possible DC induced degradation of the outstanding high temperature properties of Ni-A1-Mo base superalloys. The microstructural and microchemical changes which occurred during DC were investigated using a variety of techniques.
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