Super Alloys

Super-Alloys

The label “super-alloy” refers to a family of nickel-based or cobalt-based alloys that are exceptionally heat resistant and are used, for example, in gas turbine engines in aircraft. A problem that arises from the repetitive cycling from ambient to high temperature operations causes the essential protective oxide skin on the metal surface to tend to spall off. Several commercial alloys use micro-Super Alloysadditions (≈0.05 wt%) of (pure) rare earth Cerium to the alloy to significantly improve oxidation resistance, provide creep resistance and confer a longer operating life.

The principle behind this property improvement involves the removal of trace unwanted sulfur impurities from the metal crystallite boundaries plus a modification to the diffusion mechanism for oxide skin growth. The oxide skin formed at high temperatures shows less tendency to spall off if cerium is present in the alloy.

Aluminum Alloys

Various aluminum alloy systems are under development for use at higher temperatures than is normally possible with aluminum. These alloys are made by rapid-solidification powder-metallurgy processes that produce novel compositions with additive element concentrations, such as cerium, beyond those possible with conventional ingot metallurgy.

One of the most promising of these new light weight alloys is an Al-8.31Fe-4.0Ce (wt %) material, with excellent properties in the range 230°C to 340°C. This alloy shows an attractive combination of creep resistance, elevated-temperature tensile-strength and corrosion resistance.

The technique of rapid-solidification enables relatively large amounts of insoluble metallic elements to be finely dispersed within atomized powders. Upon freezing very small intermetallic particles are formed. After the powders have been consolidated and fabricated into a wrought product, a high volume fraction of finely dispersed intermetallic particles exists within the aluminum matrix producing a dispersion-strengthened alloy. The intermetallic phases, or possibly oxidic species, responsible for the dispersion strengthening are probably binary Al-Fe and ternary Al-Fe-Ce compounds. In addition it is probable that the lanthanide, cerium, has acted as a scavenger for oxygen and that some dispersoids could be cerium oxide.

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