As The strength of aluminium alloys is increased, other desirable properties are increasingly sacrificed and, as with other metallic materials, a major difficulty in developing better strong alloys is to assess these other properties in terms which are satisfactory indices of service performance. This difficulty becomes more acute as strength is increased and further progress demands that supplier and user are in the closest possible agreement on methods of assessing the properties involved and on where the line must be drawn between acceptable and unacceptable material behaviour. Discussion of this aspect of strong light alloy development is always opportune and much of what is said in this paper is intended to provoke constructive discussion between supplier and user on this subject.

The object of this paper is to present some of the user experience with the high strength aluminium alloys. The somewhat tarnished reputation of the Aluminium-Zinc-Magnesium alloys is undoubtedly due to their premature introduction; the very substantial weight savings offered by the relatively high static strength values proving too strong an attraction at the end of the war-time era dominated by short term considerations.

It will be appreciated that there is a wide range of alloys in this family. Table I lists the composition ranges of typical French, American and British specifications together with a proprietary alloy, of German origin, which is of particular interest. Manufacturers have their own favoured compositions within the permitted ranges. Fig. 1 shows diagrammatically the wide variation in percentage of the principal alloying elements zinc, magnesium and copper.

Conventional low-alloy steels at strengths above 100 ton/in2 have been successfully used in various aerospace applications, but laboratory test results, service-performance data and problems in production indicated a need for steels which possessed improved mechanical properties, and which were simpler to machine and fabricate. The 18% Ni-Co-Mo maraging steels were developed in 1961 with the aim of fulfilling this need, and since that date several thousand tons of the steels have been produced and used in all common shapes and forms. In comparison with low-alloy steels of equivalent strength, 18% nickel maraging steels have greater resistance to fracture at highly-stressed notches, defects or cracks, and their susceptibility to hydrogen embrittlement/stress-corrosion failure is generally far superior. The un-notched rotating bend fatigue properties of machined and mar aged specimens of 18% Ni-Co-Mo maraging steels tend to be superior in the low-cycle range and similar in the low-stress range to the properties of low-alloy steels of comparable strength. The fatigue properties of unnotched, in contrast to those of notched, specimens are affected by heat-treatment/machining procedure. Experience gained in production of components has proved that the 18% nickel maraging steels are extremely simple to heat treat, to machine, to cold form and to weld. Exploitation of these advantages can frequently offset the higher initial cost of maraging steel. For example, an analysis of production costs for a complex aircraft component has demonstrated that the use of maraging steel enabled the component to be made by forging and machining at 5-20% lower cost than with a conventional low-alloy steel, despite the fact that initial material costs were five times greater. 18% Ni-Co-Mo maraging steels retain high strength at temperatures up to 350-400°C, and have given good service in short-duration heating cycles at much higher temperatures. Maraging steels may be surface hardened by various methods, and gears and dies treated by simultaneous nitriding and maraging have given excellent performance.

The tremendous technological efforts which have been made in the past five years have yielded the majority of data and experience necessary to ensure successful use of the 18% Ni-Co-Mo maraging steels in the aerospace industry.

Mr. Haynes in his contribution to the Symposium, and also in previously published papers, has very adequately described the general development, advantages and characteristics of the maraging steels. However, as much of the information which he has given relates to the work done in the laboratories of International Nickel, and because we have only been able to find relatively little information available relating to production experience, we at Dowty Rotol set out to investigate the problems of steel making, forging, and resultant mechanical properties to be obtained under more normal production conditions. As landing gear manufacturers, our prime interest lies in the production of relatively large forgings of somewhat complex shape, and we were therefore particularly anxious to investigate first of all that our normal steel suppliers could produce material to the correct composition, that the material would forge as easily as conventional material, and that the subsequent forgings possessed adequate trans-verse properties, particularly insofar as ductility is concerned. In this latter connection, we had heard of certain difficulties encountered in the USA relating to inadequate ductility in large forgings, which they had concluded was related to the carbon and sulphur content of the steel: i.e. that it was essential to eliminate brittle intermetallic compounds such as titanium carbo-nitrides and titanium sulphides from the steel.

The intention in presenting this paper is to outline the main trends in recent alloy development and to indicate the current state of the art as far as the user is concerned, particularly on the behaviour of the new products in the problem areas so designated by the engineering metallurgists and materials specialists in the industry.

In reviewing recent alloy development it is convenient to identify two main lines in terms of the metallurgical structures involved, since these tend to define the more appropriate areas for design interest.

Beryllium is a metal possessing some attractive properties for aerospace applications, but its advantages are offset by the present inability to achieve acceptable isotropic ductility, by the high cost of the fabricated metal and by the special health precautions required for workshop treatments and handling.

Considerable research and development have been applied to the technology of beryllium during the past decade and comprehensive data are available in books and the proceedings of international conferences held in London, Gatlinburg, Philadelphia and Grenoble. Outstanding among the various problems discussed have been modes of deformation and fracture, the efficiency of various methods of purification and the influence of increased purification on the behaviour of the metal.

The refractory metals are usually referred to as metals which have melting points equal to, or higher than, chromium. A number of these metals, such as chromium, molybdenum, niobium, and tungsten are already being used quite extensively as alloying elements in both ferrous and non-ferrous alloys. Tungsten has been used in the lamp industry for many years, while molybdenum has been used for heating elements and niobium has been of interest in the atomic energy field.

The advancements in design and efficiency of aero engines has been associated with an increase in engine temperature. If this advancement is to continue, it will be necessary for a new family of alloys to be made available to the engineer. The current nickel-base high temperature alloys are now approaching their limit. It is difficult to be precise about these temperatures because the applications in terms of stress vary widely.

The strength of a composite material will depend sensitively on that of the fibre chosen for reinforcement. Metallic and non-metallic whiskers can be very strong and other less strong, but more practical, fibres such as carbon, silica and boron are now available in experimental quantities. Some of the non-metallic whiskers, and the last three fibres mentioned, will no doubt eventually be made on a production scale.

The strongest drawn wires yet available have tensile strengths of more than 0.5 X 106 psi which is still only one-third of that of iron or sapphire whiskers, and compares with boron in filament form, although the specific strength of the steel wire would only be one-third of the latter. On the basis of specific strength drawn beryllium wire would be only slightly better than the steel, but would have about 6 times its elastic modulus (E) which is comparable to that of boron.

We want structural materials to be as strong as possible, as light as possible and as stiff as possible. For all practical applications they must be tough or resilient. We might also want them to be of use at high temperatures, to have hard surfaces, to be resistant to corrosion and to be cheap and easy to fabricate.

With all orthodox structural metals strength and stiffness are roughly in proportion to the density of the material. There is therefore not a great deal of difference between them on the basis of specific strength and specific stiffness. This is illustrated in Table I.

There are a number of ceramic substances such as silicon carbide, aluminium oxide, beryllium oxide, boron and carbon which have very high elastic modulus values coupled with low densities and high melting points.

This paper represents the personal views of the author and the objectives do not necessarily form any part of current official thinking.

Sooner or later, Europe is going to accept that it must indulge in space exploration and subsequent exploitation if it is not to lose its position as a principal technical community. While it may be conceivable that exploration may be undertaken by “small scale” activities, it is certain that exploitation will require a substantially greater capacity to deploy men and materials in space. This paper demonstrates the approach we would make to the problem of transportation of such payloads, which will be a basic part of such a venture, and the philosophy which leads to this approach.

A Vehicle Millemile is a 1000-mile unit of flight production. A big jet may cost in total £1500 to fly 1000 statute miles or 1 VMM.

If 6 tons of passengers (or 60 passengers and their free baggage) earn £200/Paxton Ton-Millemile x 6=£1200, and the revenue from 3 tons of cargo is £100/Load Ton-Millemile x 3=£300, it is simple economics to see that this “mix” will provide breakeven with 9 tons load on board and a total revenue mix of £1200 + £300=£1500/VMM or per 1000 miles. A big jet flying 1600 VMMs per annum at £1500/VMM costs £2.4m and the above passenger and cargo ton factors would provide sufficient revenue to break even.

In a few seconds, therefore, we have ascertained the composite “mix” of load and revenue needed to meet the cost of flying the aircraft 1000 miles, and the answers are far more enlightening than saying that an average 60 passengers are carried at 4.8 d/PM and 2721 kg of cargo are carried at 24 d/LTM.

The fact that many medium size or small airlines have route stagelengths of less than one Vehicle Millemile at 500 miles or 200 miles makes no difference to the validity of ease of application of VMM Economics, but its great value Jies in the study of intercontinental big jet operations, SST analysis, sub-orbital flight and even interplanetary flight costing.