The Kinetics of the Precipitation of Co from Supersaturated Cu-Co Alloy
- PDF / 547,726 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 37 Downloads / 166 Views
ODUCTION
KNOWLEDGE of the nucleation and growth processes involved in a solid-solid phase transformation resulting in a microstructure with specific properties, e.g., mechanical, electric, or magnetic, is of great interest both from a fundamental scientific point of view and with regard to practical applications. In order to follow the progress of the transformation reaction, a global, macroscopic parameter as the degree of transformation f (0 £ f £ 1) can be determined experimentally as a function of time and temperature. However, it is no easy task to extract from such experimental data quantitative information on the operating modes of nucleation, growth, and impingement. To this end, a modular phase transformation model[1] was developed recently that allows separate determination of kinetic data for nucleation, growth, and impingement. This model has until now been successfully applied to a variety of phase transformations: crystallization of amorphous metal alloys,[2–7] the austenite-ferrite transformation in Fe-based alloys,[8–10] and the polytypic transformations of Laves phases.[11] To explore the applicability of this general description of transformation kinetics to the precipitation of a second, product phase in a supersaturated parent, matrix phase, the precipitation of Co from an initially R. BAUER, formerly Ph.D. Student, Institute for Materials Science, University of Stuttgart, 70569 Stuttgart, Germany, is Quality Engineer with the Evobus GmbH, 68301 Mannheim, Germany. B.F. RHEINGANS, Ph.D. Student, is with the Max Planck Institute for Metals Research. Contact e-mail: [email protected] E.J. MITTEMEIJER, Professor, is with the Institute for Materials Science, University of Stuttgart and is also with the Max Planck Institute for Metals Research. Manuscript submitted July 15, 2010. Article published online January 5, 2011 1750—VOLUME 42A, JULY 2011
supersaturated dilute solid solution of Cu-0.95 at. pct Co was investigated as a model system. At lower temperatures, Cu and Co show only very small mutual solubility.[12] Upon annealing of supersaturated Cu-rich Cu-Co alloys, Co-rich precipitates of fcc structure form within the fcc Cu matrix. During the early stages of the precipitation reaction, these particles show full coherency with the Cu matrix and are of spheroidal shape.[13] Pure Co exhibits an allotropic reaction at the equilibrium temperature Ta = 690 ± 7 K (416.85 C) (at 1 atm),[14] with the hcp modification as low-temperature phase and the fcc modification as high-temperature phase. Yet, the coherent Co precipitates developing upon precipitation in Curich Cu-Co alloys are generally of fcc structure both above and below Ta. With conventional transmission electron microscopy (TEM), the small Co-rich precipitates are only indirectly visible due to the strain contrast resulting from the lattice misfit, d, between the lattices of fcc Co and fcc (d = (aCo – aCu) / aCu Æ 100 pct –1.9 pct; Cu[13,15] with the lattice parameter of Co, aCo = 0.35447 nm, and of Cu, aCu = 0.36146 nm, Reference 16). In the
Data Loading...
