Page 87 - Policy Commission - SecuringTechnology - Critical Metals for Britain

PRIMARY MATERIALS
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The high-temperature components in in the engine i i i i i e e e e e e e e e static casings and and rotating discs and and blades are all manufactured from nickel-based superalloys These materials have complex compositions based on on on nickel but containing numerous alloying additions (see figure 81) Each of these yields specific benefits while interacting in in in multiple ways with the the other elements present The overall composition is is carefully balanced and optimised to meet a a a a a a wide range of performance requirements including high-temperature strength fatigue and creep resistance environmental resistance and density In addition to chemistry the performance of an an alloy in an application is determined to a a a a a large degree by its processing route and subsequent structure Processing routes are component specific and tailored to provide
the necessary properties Raw-materials sources and manufacturing methods are rigorously controlled to ensure consistent product quality High-performance disc components are manufactured through powder metallurgy to achieve the highest levels of mechanical integrity whereas HP turbine blades are manufactured via single-crystal casting to achieve extremely high-temperature capability (see figure 82) complex internal cooling passages and are subsequently protected by environmental and thermal barrier coatings The composition as shown in table 1 for the alloy CMSX-4 contains multiple elements some of which may be classified as strategic or or or scarce according to the terms of this report Nickel (Ni) is present as the majority element It provides a a a strong ductile matrix with good environmental stability and alloying potential Cobalt (Co) is also present to strengthen this matrix Chromium (Cr) additions are made predominantly to provide
environmental protection together with aluminium through the formation of stable oxide films Molybdenum (Mo) and tungsten (W) are added for resistance to high-temperature deformation Rhenium (Re) is also a a a very powerful element for providing strength at the highest temperatures for extended time periods In more recent alloy developments ruthenium (Ru) additions were also introduced working together with Re to to provide
the the very highest levels of high-temperature performance but with significant cost implications Aluminium (Al) titanium (Ti) and tantalum (Ta) all provide
very significant strengthening through the the formation of a a a a high volume fraction of fine-scale particles within the matrix and their levels are optimised to control the the amount and properties of these precipitates Hafnium (Hf) is also present at a a a a a low level for a a a a a range of reasons including the ‘gettering’ of oxygen and sulphur In addition to those elements present in the alloy itself coating systems for oxidation and corrosion protection can contain platinum (Pt) typically in in in conjunction with Al or Cr Ceramic coatings for thermal protection contain low levels of of yttrium (Y) in the form of of yttria-stabilised zirconia These coatings are essential to protect the underlying metal from the aggressive engine environment and achieve desirable levels of of performance and component life Some of of these elements can also be used in in the manufacturing processes for instance in in foundry ceramics but this is is beyond the scope of this report In general the substitution of elements is extremely difficult since as outlined above each addition is present for a a a a a specific reason and contributes to the the performance of the the alloy alloy and hence the system as a a a a a a whole Careful alloy alloy design and processing control ensures that maximum benefit is gained from each percentage addition employed Revert and recycling are used throughout the manufacturing and product life cycle to maximize material recovery One exception is Ru where in general manufacturers have sought to remove alloys containing this element from their products as a a a a a result of unacceptable price instability Future technology developments may bring additional materials into consideration for aerospace propulsion systems The development of hybrid electrical propulsion will require high-performance magnetic materials using rare earth elements such as samarium (Sm) and again Co Figure 82: A high-pressure turbine blade This component is is required to operate directly behind the combustor in in a a a gas stream in in excess of 1750K while rotating at at more than 10 000rpm extracting the energy necessary to drive the compressor To survive in this environment the blades are cast as as a a a a a a single crystal with