Processing and creep resistance of nickel/yttria composites

In this study, pure nickel and yttria (Y2O3) were selected as a model syste m to investigate the feasibility of processing metal matrix composites (MMC s) through a powder metallurgy approach for the in-situ formation of a cont inuous three-dimensional reinforcement network or the in-situ formation of discrete reinforcements with certain degrees of interconnected clusters. Co mposites with a volume fraction of Y2O3 ranging from 20 to 50% were prepare d through hot pressing. The density, microstructure and creep resistance of these composites were evaluated as a function of the yttria volume fractio n. It was found that a continuous Y2O3 network was formed in composites wit h 40 and 50 vol % Y2O3, while yttria was discrete with some degrees of inte rconnected clusters in composites with 20 and 30 vol % Y2O3 The creep rate was reduced by two to three orders of magnitude with the addition of 20 to 30 vol % Y2O3, and it continued to decrease with increasing the volume frac tion of yttria to 50%. The analysis indicated that the load transfer to iso lated yttria particles could not account for the improved creep resistance of composites with 20 and 30 vol % Y2O3, while the load transfer to a conti nuous yttria network in composites with 40 and 50 vol % Y2O3 could not be a pproximated by the model of the load transfer to continuous fibres. The dis crepancies are believed to be related to the presence of interconnected ytt ria clusters, the low relative density of the yttria phase in the composite , and the low load-carrying capability through a three-dimensional network in comparison with the load-carrying capability through continuous fibres. It is suggested that the density of the yttria phase and hence the creep re sistance of the composite can be further improved over what have been obtai ned in this study by densifying the composite at high temperatures and pres sures. (C) 1998 Kluwer Academic Publishers.