1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

Below 1000°C the oxidation of nickel cannot be controlled by the diffusion of ions through the bulk crystal lattice of the pure oxide, because the measured oxidation rates are several orders of magnitude faster than would be predicted on this basis. Short-circuit diffusion through oxide grain boundaries or dislocations has usually been held responsible, but there has hitherto been no proper quantitative confirmation of this mechanism. We report measurements of the oxide scale thickness and oxide grain size as a function of time during the oxidation of high-purity nickel in the temperature range 500–800°C. All the oxidation experiments were carried out in pure oxygen at a pressure of one atmosphere. The measured parabolic oxidation rate constants have been compared with those calculated from grain boundary diffusion data obtained in our previous work, using a grain boundary diffusion model for the oxidation process. The quantitative agreement between measured and calculated oxidation rates shows c...

A quantitative demonstration of the grain boundary diffusion mechanism for the oxidation of metals

The addition of reactive elements can have a significant effect on the oxidation behavior of alumina- and chromia-forming alloys. A model has been developed to explain the effects associated with the addition of reactive elements that is based on the segregation of reactive-element ions to scale grain boundaries and the metal-oxide interface. Reactive-element ions use these interaces as pathways for diffusion from the metal substrate to the gas interface of the scale. The driving force for this outward diffusion is the oxygen potential gradient across the scale. Doping of the scale grain boundaries results in scale growth primarily by inward oxygen diffusion, while doping at the metal-oxide interface slows the growth of interfacial voids and thus improves scale adhesion.

Experimental observations in support of the dynamic-segregation theory to explain the reactive-element effect

On the mechanics of delamination and spalling in compressed films

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The influence of alloying elements on the development and maintenance of protective scales

Alloys of Ni-20 wt. % Cr containing 3 vol. % of a dispersed oxide phase have been prepared by a mechanical alloying method and oxidized in oxygen at 100 Torr in the temperature range of 900 to 1200°C. It appears that the dispersed oxide has four distinct effects on the oxidation: (1) the selective oxidation of chromium to form a continuous protective Cr2O3scale is promoted; (2) the rate of growth of Cr2O3is reduced compared with particle-free alloys; (3) the adhesion of the Cr2O3is greatly improved; and (4)the scale-forming reaction appears to be at the scale-metal interface in alloys containing a dispersion, but at the scale-oxygen interface in alloys without a dispersion. It appears that the nature of the dispersed oxide is not important, since very similar effects can be obtained with ThO2,Y2O3,and CeO2dispersions. It is demonstrated that a logical deduction from this evidence is that the growth of Cr2O3scales on dispersion-free systems must involve short-circuit diffusion of chromium through the scale, and that it seems probable that an effect of the dispersion must be to retard or eliminate this short-circuit process. It is suggested that the oxide particles act as nucleation centers for the oxide, thus reducing the oxide grain size; and it is shown that this simple hypothesis is sufficient to explain a number of the experimental observations.

The high-temperature oxidation of nickel-20 wt. % chromium alloys containing dispersed oxide phases

Stress generation and relief in growingoxide films

The effect of small amounts of yttrium (up to 1 wt. %) and hafnium (up to 1.5 wt.%) on the oxidation behavior of Co-Cr-Al alloys in the temperature range 1000–1200°C for times up to 1000 hr in air has been studied. The major portion of the study has been concerned with Co-10Cr-11Al base alloys. Both isothermal and cyclic tests have been carried out; the cycle used consisted of 20 hr at temperature, followed by cooling to room temperature. Both additions reduce the overall oxidation, Hf somewhat more so than Y. In part, this is due to the improved adhesion between scale and alloy reducing scale spallation at temperature, and in part due to possible modification of the Al2O3 grain size. The former factor is far more critical under thermal cycling conditions. Under isothermal conditions the oxidation rate increases with increasing Hf content with all but the 1.5 wt.% alloy oxidizing more slowly than the Hf-free alloy; increase in Y content has the reverse effect. Under thermal cycling conditions the 0.3 and 1.0 wt.% Hf alloys show the lowest overall weight gain. Metallographic evidence suggests that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al2O3 into the alloy. However, if the intrusions are too large, they can act as initiators of scale failure.

The oxidation behavior of CoCrAI systems containing active element additions

Alloys of Co-21 wt. % Cr-3 vol. % Y2O3have been prepared by a mechanical alloying method, and oxidized in oxygen at 100 Torr in the temperature range 900–1200°C. The general effects of the dispersed oxide phase are similar to those reported for nickel-base alloys: the selective oxidation of chromium to form a continuous protective Cr2O3scale is promoted; the rate of growth of Cr2O3is reduced compared to dispersoid-free alloys; the adhesion of the Cr2O3is greatly improved; and the scale-forming reaction is probably at the scale-metal interface in the alloys containing the dispersoid, whereas it is at the scale-oxygen interface in dispersoid-free alloys. This last point has not been positively demonstrated. The improvement in adhesion is of particular significance, since the scales on cobalt-base alloys are prone to spallation, and it has been possible to study the mechanism of adhesion in more detail. It appears that in dispersoid-free material the metal recedes from the scale-metal interface, leaving the scale supported on the tops of metal “peaks” but this does not happen in the alloy containing the dispersoid, either because the growth direction of the scale has been changed, or because of changes in the substrate grain size. In general, the observations support the model proposed in an early study for the oxidation of Ni-20 wt.% Cr alloys containing oxide dispersions.

John Stringer

Papers

The high-temperature oxidation of nickel-20 wt. % chromium alloys containing dispersed oxide phases

Stress generation and relief in growingoxide films

The oxidation behavior of CoCrAI systems containing active element additions

The high-temperature oxidation of cobalt-21 wt.% chromium-3 vol.% Y2O3 alloys

Oxidation and hot corrosion of nickel-based alloys containing molybdenum