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This article is written from the viewpoint of a structures engineer who has to make the best use of the materials available, rather than that of the metallurgist who aids their production or development.

THE use of metals at temperatures in excess of 1,200 deg. F. and up to temperatures in the vicinity of their melting points is a challenging and fascinating portion of the fight to pass the heat barrier in the design and performance of aircraft and their power plants. The materials available for service in this temperature range are restricted. The considerations of designing structural components involve many more problems than the old criteria of strength to weight ratio and fabrication costs. Such properties as thermal expansion, heat conductivity, surface emissivity and scaling resistance are as important in determining which metal should be used for a given application as are the various measurements of strength heretofore the primary considerations in material selection.

THE previous articles in this series, concerning the titanium, magnesium and aluminium alloys, followed a very similar form, in that in each case consideration of the aircraft engineering applications was preceded by a metallurgical appreciation of the alloy systems under review. In the case of steels, a comprehensive article on similar lines would be nothing less than a monograph, and if steels are to be discussed within the space of a single article, then a quite different approach must be adopted. This review will not, then, examine steels generally in any great metallurgical detail, but will rather consider their special merits in aircraft engineering, particularly in the context of supersonic aircraft.

A review is made of existing and likely future aircraft materials and their choice for use on airframes is discussed in relation to the problems of advanced aircraft design. Both technological and production problems arc included and it is finally suggested that urgent governmental action is required to remedy Great Britain's grave lack of suitable capital equipment.

The corrosion behaviour of low alloy steel type AISI 4130 (before and after nitriding) and austenitic stainless steel type AISI 304L were studied in tap water +3.5 per cent NaCl. A liquid nitriding process had been applied on the low alloy steel.

The tests that were carried out in this study were anodic polarization, rotating bending fatigue and axial fatigue using compact tension (CT). For determining the corrosion potential and pitting potential (breakdown potential) for the alloys, anodic polarization curves were established using the potentiodynamic technique. Rotating bending fatigue tests were used to calculate the fatigue strength and damage ratio. Using linear elastic fracture mechanics, the CT specimens were prepared for determining the threshold stress intensity factor, fatigue crack growth rate and fracture toughness in air and in the solution.

The results showed that nitrided specimens showed higher fatigue strength in air compared to stainless steel. However, the corrosion fatigue limit for both these samples were approximately equal, while this limit for non‐nitrided sample was less. Moreover, the non‐nitrided steel had lower corrosion and pitting potentials than did the stainless steel. In addition, the CT tests showed that the nitrided specimens had a lower resistance to crack initiation in air and the solution compared to the non‐nitrided sample and the stainless steel.

These results can be attributed to the chemical and mechanical behaviour of the nitrided layer constituents, mainly FeN and CrN, which were recognized by X‐ray diffraction. Since, these components consist of very hard particles, they act to increase the hardness and fatigue limit. Moreover, due to the low conductivity of these nitrides, the corrosion and pitting potential of the nitrided steel becomes very high. However, the high breakdown potential does not help to increase the corrosion fatigue or damage ratio values due to the porous nature of the nitrided layer.

Although the nitrided steel had very high fatigue strength and pitting potential, this did not reflect in its corrosion fatigue and/or damage ratio improvement because of its surface roughness and the porous nature of the nitrided layer.

The full development of marine technologies for the industrial exploitation of deep‐sea resources requires the availability of exhaustive engineering data on the degradation of constructional materials when immersed at great length in ocean environments. An overall review of behavioural figures from reliable sources allows to point out major weaknesses and uncertainties with candidate alloys and consequent demands for ameliorative and innovative investigation. Prominent objects of research can be envisaged, accordingly, in: (i) formulating low‐alloy steels whose surface is chemically convertible (by suitable pretreatments or free corrosion itself) so as to abate the rate of oxygen cathodic reduction i.e. the current density required for cathodic protection; (ii) singling out high‐strength steels whose resistance to corrosion‐fatigue and hydrogen embrittlement is in proportion to the strength level; (iii) developing cheaper alternatives to high‐molybdenum stainless alloys resistant to localised corrosion; (iii) combining (in optimal composites) the outstanding resistance of titanium to saltwater corrosion and the low‐cost good mechanical properties of steels.

This study aims to review the recent advancements in high entropy alloys (HEAs) called high entropy materials, including high entropy superalloys which are current potential alternatives to nickel superalloys for gas turbine applications. Understandings of the laser surface modification techniques of the HEA are discussed whilst future recommendations and remedies to manufacturing challenges via laser are outlined.

Materials used for high-pressure gas turbine engine applications must be able to withstand severe environmentally induced degradation, mechanical, thermal loads and general extreme conditions caused by hot corrosive gases, high-temperature oxidation and stress. Over the years, Nickel-based superalloys with elevated temperature rupture and creep resistance, excellent lifetime expectancy and solution strengthening L12 and γ´ precipitate used for turbine engine applications. However, the superalloy’s density, low creep strength, poor thermal conductivity, difficulty in machining and low fatigue resistance demands the innovation of new advanced materials.

HEAs is one of the most frequently investigated advanced materials, attributed to their configurational complexity and properties reported to exceed conventional materials. Thus, owing to their characteristic feature of the high entropy effect, several other materials have emerged to become potential solutions for several functional and structural applications in the aerospace industry. In a previous study, research contributions show that defects are associated with conventional manufacturing processes of HEAs; therefore, this study investigates new advances in the laser-based manufacturing and surface modification techniques of HEA.

The AlxCoCrCuFeNi HEA system, particularly the Al0.5CoCrCuFeNi HEA has been extensively studied, attributed to its mechanical and physical properties exceeding that of pure metals for aerospace turbine engine applications and the advances in the fabrication and surface modification processes of the alloy was outlined to show the latest developments focusing only on laser-based manufacturing processing due to its many advantages.

It is evident that high entropy materials are a potential innovative alternative to conventional superalloys for turbine engine applications via laser additive manufacturing.

TITANIUM is a new metal but not a rare one. It is new in the sense that although its existence has been known since 1791, it is only within the last decade that it has become a product of metallurgical industry. It was not until 1925 that it was made by van Arkel, on a small experimental scale, in a state of sufficient purity for an assessment to be made of its properties and of its potential value as an engineering material. So far, it has not been possible to translate into a large scale and economically attractive extraction process the van Arkel technique and it was, in fact, left to W. J. Kroll to devise the first industrial process for the production of ductile titanium, which he described in 1940.

Geigy Co. Ltd. Stand 75. Diversified application of benzotriazole as a corrosion inhibitor specifically for copper and its alloys is the main theme of Geigy's stand.

A very wide variety of alloy types are available for selection to combat the potential corrosion problems posed in a diverse range of industries. Although in today's climate cost reduction is an important goal, the price of unexpected failure of equipment is often measured as risk to human life, and materials selection must always be given a prime place in design, engineering and construction. Material selection should not be based simply on low installed cost of equipment — the need to maintain safety standards and effective long‐term utilization of a production asset, with minimum costly maintenance and downtime, mandate the selection of materials which can be justified on the basis of life‐cycle cost and risk analysis. The material chosen should provide the lowest cost viable, and if possible, “fit and forget” solution. In the Offshore Oil and Gas industry in the North Sea the solution adopted would need to address the CRINE cost reduction strategy.