Feature Article A Reflection On Gas Turbine Alloys by Dr. David Ford T
he gas turbine industry, be it electrical or aero power generation has, in the last year, gone from hero to zero in the opinion of those supporting the
‘green lobby’. There is no doubt that these industries have contributed to the generation of greenhouse warming gases and must accept some of the consequences, albeit unfair since alternative technologies have only recently been considered necessary. Although engineers have greatly increased the efficiency of engines to reduce fuel burn, the heyday of the industry has probably passed, and with it, a necessity for future adjustment in the market for the products produced by our industry.
Manufacturing for gas turbines could be considered
the investment casting industry’s ‘middle period’ where we excelled in solutions to produce materials and components capable of operating well above their melting point for many thousands of hours. With this engineering excellence the gas turbine industry was able to provide consistent electrical power, and to enable passengers to travel the world in a few hours and for a few dollars. In hindsight we should be proud of our contribution to society but continue to look for opportunities to exploit our precious technology.
During the gas turbine period there have been outstanding advances in metallurgy and manufacturing engineering, to name just a couple from many. Supporting professions have not only contributed to the development of the industry, but in so doing, have gained considerable experience to vastly increase their own specialized knowledge. In this short article I hope to reflect on the contribution from the metallurgical profession and hope that it will encourage young professionals to stay with the industry and support its contribution to society. The development of superalloys has been well publicised and since these represent the major metallurgical advances since the second world war they have received more than their share of scientific investigations and academic thesis. However, it is worth recording the principal successes and occasional pitfalls which give credit to those whose lifelong passion has helped give the world the fruits of the gas turbine engine. The story commences with the Austenal Laboratories use of the cobalt chrome alloy vitallium for investment cast dental prosthetics which in turn gave the aerospace industry the process to produce turbine aerofoils. The early success of the UK jet engines towards the end of the second world war was the availability of high temperature nickel base nimonic alloys. These alloys were
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a derivative of the Ni/Cr ‘brite’ alloy heating elements and formed the foundation of nearly all future superalloys. Nickel as an element is not resistant to elevated
temperature oxidation or corrosion but has the advantage, given by nature, of solidifying and retaining a face centred cubic crystal structure, which is compatible with a number of important elements which can dissolve and harden the base nickel. As a wonderful bonus, nickel will also combine with aluminium to form a hard coherent compound which both strengthens the alloy and affords a degree of oxidation resistance. These relatively simple alloys formed the basis of generations of what became known as ‘superalloys’ which could be formulated for very high temperature applications or hostile environmental conditions. In a simple nutshell, superalloys owe their strength and
high temperature resistance to a solid solution (referred to as gamma) of elements including Ni, Cr, Mo, Co, W etc and a hard precipitate (referred to as gamma prime) of Ni, Al, Ti, Ta, etc. also present are C, Zr and B which strengthen the grain boundaries. To maximise their strength the alloys are heat treated to optimise and refine the gamma prime. Many of the early alloys were developed by the nickel
development laboratories and the early UK industry owes much to the International Nickel Company (INCO) laboratories, Jessop Saville, Henry Wiggins and Martin Marietta. Later alloys were developed by OEMs and due to the commercial sensitivity of the single crystal process, their properties are retained exclusively by the OEM owner. A notable exception are the DS and SX alloys developed by Cannon Muskegon which are used not only by OEMs but are used by academic researchers as benchmark alloys. The chart, Figure 1, shows the development of alloys
over the last 70 years which has culminated in alloys which, when solidified as a single crystal, represent the ultimate in high temperature resistance (Figure 2). Justice though demands that record should be made of some of the pitfalls which befell the introduction of these alloys or their processing.
Over Alloying Alloy development during the 50s and 60s followed an empirical process where potential elements were added on a ‘suck it and see’ basis (never admitted of course). The result in many cases was an unusable alloy often only discovered later in development. The principal culprit was the formation of brittle compounds which formed in service as a result of a combination of stress and particular temperatures. These were complex compounds such as sigma phase (Cr,Mo)x(Ni,Co)y or mu phase
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