Influence of Rare Earth Elements and Minor Additions on Properties and performance of Magnesium- Yttrium Alloys in Critical Aerospace Applications

摘要

The most significant factor in maintaining the use of high performance magnesium alloys in ever more demanding aerospace applications over the last 20 years has been the development of alloys in the Magnesium-Yttrium-Rare Earth-Zirconium alloy system by Magnesium Elektron. These alloys represented a significant advance on previously available magnesium alloys in terms of ambient temperature mechanical properties, short term and long term high temperature performance and corrosion resistance. Two alloys ELEKTRON WE54 (Mg-5 percent Y-4 percent Combined RE-Zr) and ELEKTRON WE43 (Mg-4 percent Y-3 percent Combined RE-Zr) have been comercialised and ELEKTRON WE43 particularly is now widely specified in new aerospace applications, notably for helicopter transmission components and aeroengine gearbox and related applications. ELEKTRON WE alloys are relatively complex systems by conventional commercial magnesium alloy standards. In addition to Yttrium and Zirconium, they contain a complex mixture of Rare Earth elements, which individually have specific effects on the final properties and performance of the alloy. In the course of the development of the alloys, the specific role of individual elements has been investigated and compositions optimised to meet the demanding requirements of critical aerospace components. Further insight into these effects has been gained during the design and development phase of specific aerospace components. ELEKTRON WE alloys are used in the fully heat treated (T6) condition. Again, there is an inter-relationship between composition and heat treatment parameters, which can affect the ultimate performance characteristics of the cast components. In this paper, some of the key interactions between composition, heat treatment and production parameters are reviewed in relation to their effects on: Tensile properties at ambient and elevated temperatures Creep and fatigue performance Long term stability of properties up to 10,000 hrs Corrosion performance Knowledge of these effects is essential in order to be able to specify acceptable parameter envelopes for alloy production, casting and component production. ELEKTRON WE alloy castings being used in aerospace applications today have a proven pedigree, which has resulted from an extensive optimisation programme. This has also resulted in a relatively complex and expensive alloy. There is a strong desire to significantly reduce this cost, e.g. by simplification of the alloy, or use of lower cost input materials. These areas are being investigated, but are of necessity longer term because of the need to check the effect of even relatively minor changes on all aspects of the performance envelope to maintain user confidence.