Aerospace Materials and Material Technologies

This chapter gives an overview of magnesium alloys with the emphasis on aerospace applications. The strengthening mechanisms, physical metallurgy principles, effects of alloying elements, conventional processing techniques, recent advancements in alloy development and processing are briefly discussed in the following sections. The mechanical properties, corrosion behaviour of aerospace castings and wrought alloys are presented and commented upon in detail. Recent trends in corrosion protection techniques and applications in national and international aerospace projects are presented at the end.

This chapter starts with a brief overview of the historical development of aerospace aluminium alloys. This is followed by a listing of a range of current alloys with a description of the alloy classification system and the wide range of tempers in which Al alloys are used. A description is given of the alloying and precipitation hardening behaviour, which is the principal strengthening mechanism for Al alloys. A survey of the mechanical properties, fatigue behaviour and corrosion resistance of Al alloys is followed by a listing of some of the typical aerospace applications of Al alloys. The Indian scenario with respect to production of primary aluminium and some aerospace alloys, and the Type Certification process of Al alloys for aerospace applications are described. Finally there is a critical review of some of the gaps in existing aerospace Al alloy technologies.

This chapter summarises the development and limitations of the first and second generation Al–Li alloys, and then discusses the recent developments leading to the third generation alloys. Emphasis is placed on the physical metallurgy of Al–Li alloys, progressive development of the three generations of these alloys, and finally the strategies for obtaining improved property combinations via various microstructural modifications closely linked to multistage processing. The way forward for Indian development of Al–Li alloys is also briefly discussed.

Titanium sponge is widely produced employing the Kroll process of high-temperature reduction of titanium tetrachloride by magnesium. The technological developments over the last few decades have focused on cost/energy savings in addition to introducing sophisticated systems in the manufacturing technology. This chapter concerns Indian efforts to develop ‘state-of-the-art’ Kroll technology for producing titanium sponge in industrial scale batches. While covering various features of the combined process technology developed at DMRL, the chapter also discusses advanced quality evaluation and sponge processing practice as developed at DMRL and implemented at the KMML sponge plant (which was established with the DMRL technology). Extensive measures that have been implemented to obtain high purity metal and assured quality of the product are discussed.

Titanium alloys are the principal replacements, and in many cases also prime candidate materials to replace (i) aerospace special and advanced steels, owing to their higher specific strength properties, (ii) aluminium alloys, due to their better elevated temperature properties and (iii) nickel-base superalloys for high pressure compressors of modern engines, owing to their superior intermediate temperature (up to 600 °C) creep strength and excellent oxidation and corrosion resistance and good damage tolerant properties. This chapter summarily presents the chemical compositions, properties, aerospace applications and briefly covers (a) the physical metallurgy of titanium alloys, (b) Primary (Melting and Casting) and Secondary (Processing) processes and (c) alloy development (commercially pure Ti, α, near-α, α + β and β alloys). Towards the end, the Indian scenario is presented in terms of available production facilities and some of the indigenous alloys.

Titanium alloys are the principal replacements, and in many cases also prime candidate materials to replace (i) aerospace special and advanced steels, owing to their significantly higher usable specific strength properties, (ii) aluminium alloys due to their better elevated temperature properties and (iii) nickel-base superalloys for much of the high pressure compressors (HPCs) of modern engines, owing to their superior medium temperature (up to 550 °C) creep strength and acceptable oxidation and corrosion resistances. This chapter summarizes the chemical compositions, properties and applications of commercially pure α-titanium, near-α, α + β and β titanium alloys.

This chapter demonstrates the objectives of adding alloying elements to steel through their effects on microstructure and consequent improvements in mechanical properties. Classifications and designations followed by different international standards are briefly outlined. The development of medium carbon low alloy steels used for aerospace applications is described, including their compositions and mechanical properties. Salient aspects of the physical metallurgy including heat treatment and surface hardening methods are brought out. The engineering properties of ultra high strength steels are briefly mentioned. The efforts towards indigenous development and manufacture of some aero steels are also presented.

Highly alloyed steels (total alloying element content more than 8 wt%) are generally classified as secondary hardening steels, maraging steels and precipitation hardening (stainless) steels. The effects of alloying elements on microstructure and mechanical properties are briefly described in this chapter. The manufacturing procedures and optimum processing parameters are discussed, and the heat treatment schedules and achievable properties are tabulated. Details on machining and weldability of these steels are also provided.

Nickel-based superalloys are an exceptional class of structural materials for high temperature applications, particularly in the challenging environment of the turbine sections of aircraft engines. Continued improvements in the properties of these materials have been possible through close control of chemistry and microstructure as well as the introduction of advanced processing technologies. Surface modification by application of coating technology concurrent with the introduction of directional structures and then single crystals, has extended the useful temperature range of superalloys. Further improvements are likely with the development and implementation of tools for alloy design, microstructure-process evolution, and mechanical-property modelling. To date, six generations of single crystal (SC) nickel-based superalloys have been developed with improved creep properties and phase stability. Therefore it appears that the evolution of advanced nickel-based superalloys is a never ending process, and their replacement in turbine engine applications seems to be impossible at least for a few more decades. The present chapter is a brief review of various aspects pertaining to chemical composition, heat treatment, microstructure, properties and applications of both cast, and wrought alloys as well as the evolution of advanced cast nickel-based superalloys.

Development of materials for structural applications at elevated temperatures in aeroengines has encouraged research on intermetallic alloys. A select group of aluminides and silicides has shown significant promise for high temperature structural applications owing to their high melting temperatures, as well as their ability to retain strength and oxidation resistance at elevated temperatures. In recent years the focus is on multiphase multicomponent intermetallic alloys with significant volume fractions of ductile constituents to achieve an optimum combination of toughness and elevated temperature strength. The engineering properties and actual or potential aerospace applications of the currently most important structural intermetallics, the nickel, iron, and titanium aluminides, are concisely discussed.

This chapter provides an overview of various bronzes with special attention to aerospace applications. Bronzes consist of several families of alloys but only those with specific properties applicable to the aircraft industries are discussed here. Bronze alloys like tin bronzes (C5100–C54400), beryllium bronzes, manganese bronzes, high leaded tin bronzes, oil impregnated bronzes, aluminium bronzes and silicon bronzes are some of the bronzes which have found their place in aircraft applications. This chapter briefly discusses these bronzes, highlighting their physical metallurgy, processing and properties. A specific part of this chapter has been dedicated to discussing the indigenous development of two types of bronzes, Al-bronzes and Si-bronzes, mainly for aerospace applications. These two bronze types have been specifically developed for fabrication of anti-friction bearing cages for aircraft and have to undergo rigorous quality assurance during production type certification.

Refractory metal alloys based on Nb, Mo, Ta, W, and Re find applications in the aerospace industries because of their high melting points and high temperature strengths. They are generally produced by powder metallurgy technique due to their very high melting points. However, when refining is desired, melting under high vacuum becomes necessary, for which nuggets or powder based electrodes are employed. Niobium is the lightest refractory metal with density close to that of nickel, and exhibits good thermal conductivity. Niobium can be alloyed to improve high temperature strength and oxidation resistance. Applications in nuclear, aerospace, and defence sectors have been reported. The goal of current research in Nb alloys is to simultaneously achieve high strength and workability, and provide protection from oxidation for long-term operation. There is strong research interest in intermetallics also. This chapter will discuss the salient features of refractory metals and alloys in general, and Nb-based alloys in particular.

This chapter surveys the applications and properties of the fibre metal laminate (FML) concept GLARE. This concept includes a family of FMLs that can be tailored to specific requirements for aerospace applications, including resistance to fatigue crack growth, fracture, impact and fire and blast damage.

This chapter concisely surveys the applications and properties of polymer matrix structural composites (PMCs), concentrating on carbon fibre reinforced composites. These are the most widely used composite materials, notably in aerospace, and are commonly called carbon fibre reinforced plastics (CFRP). A major source for this chapter is Baker et al. (Composite materials for aerospace structures. American Institute of Aeronautics and Astronautics, Inc., Reston, Virginia, USA, 2nd edn, 2004).

This chapter deals with different aspects of the carbon fibre-reinforced carbon composites (C/C) and carbon fibre-reinforced silicon carbide composites (C/SiC), especially for aerospace applications. The reinforcement and matrix materials and the process technologies developed for these composites are discussed. Typical mechanical and thermal properties at room and high temperatures are also presented, together with some actual and potential aerospace applications. Some products developed in India are also included.

Ceramic materials have excellent properties, but are brittle and the strengths are highly variable. Particulate reinforcements give isotropic properties, but only marginal improvement in toughness. Continuous reinforcements improve the ceramic materials both in terms of fracture toughness as well as strength variability. The processing of ceramic matrix composites and improving the required properties with the available reinforcements is an emerging technology that is finding new critical applications.

This chapter briefly summarizes the types of nanocomposites, their fabrication and properties. Emphasis is placed on the strengthening mechanisms for metal, polymer and ceramic matrix nanocomposites. A brief introduction to the types of reinforcement and matrix materials is given, and finally the current developments and future trends of nanocomposites are discussed.

Following a brief introduction to monolithic structural ceramics and their mechanical properties and micromechanisms of toughening, the materials development and salient features of various high- and ultrahigh-temperature (UHT) ceramics are discussed. The discussion includes alumina, zirconia, silicon nitride, silicon carbide, molybdenum disilicide and carbon-based ceramics. Subsequently, emerging ceramics such as the titanium- and zirconium-boride ceramics are introduced and discussed. Finally, the Indian scenario on the development and production of these materials is described.

This chapter discusses the significance of nano-enabled multifunctional materials for aerospace applications. Several studies of these materials report research breakthroughs on the in situ formation of nanostructures and hierarchical structures, and their effects on the improvement of both functional and structural properties for space and aircraft applications such as the EMI shielding, thermal, electrical and opto-magnetic properties, fracture toughness and strength. The materials discussed here relate mostly to polymers.

This chapter summarizes the new class of carbides and nitrides discovered in recent decades. These materials serve as a bridge between metals and ceramics, having advantages from both classes of materials. Due to this reason, MAX phases fit into a wide range of applications from electronic to structural materials. The synthesis methods have been discussed, with the scope for improvement, in order to discover newer phases as well.

Shape memory alloys (SMAs) have the ability to ‘memorise’ or recover their previous form when subjected to thermal, thermomechanical or magnetic variations. This ability has resulted in a new class of materials for engineering applications in the aerospace, medical, automotive and home appliance sectors. This chapter surveys SMAs and the developments for aerospace applications.

This chapter presents a concise overview of detonation spray technology and the associated principles and applications for the aerospace industry. The most popular feedstock powders for obtaining a wide variety of coatings with varying composition and properties are emphasized. The strategies for obtaining improved structure–property combinations via spray process optimization are discussed, and also the utilization of novel powders for enhanced protection. The typical microstructural features as a key to achieving the required mechanical, tribological and corrosion properties are briefly illustrated with specific examples.

Piezoelectric materials produce electric charges on application of mechanical stress, or change their dimensions when subjected to an electric field. Lead zirconate titanate (PZT) is a synthetic piezoceramic material with high piezoelectric properties. In aerospace PZT is used widely as sensors and actuators for vibration control of structures, health monitoring, development of smart aeroplane wings/morphing structures, energy harvesting and self-powering applications in micro aerial vehicles (MAVs), unmanned aerial vehicles (UAVs), as precision fuel injectors in propulsion systems, etc. This chapter reviews the preparation of piezoceramic materials, e.g. PZT, PZT–PMN and PMN–PT, and the fabrication and characterization of multilayer (ML) stacked devices and their applications in aerospace.

“Stealth” normally signifies “radar stealth”, but it actually means suppression of all the following signatures: visual, radar, infrared, electromagnetic and sound. After a brief historical introduction, this chapter summarizes the basic stealth requirements for military assets, particularly airborne systems. Special sections are devoted to radar-absorbing materials and structures, plasma stealth (a means of active stealth), acoustic stealth and counter stealth.

Paints are important for aerospace applications in view of appearance, surface protection, and as stealth coatings. Paints are generally applied as a scheme involving primer-coats and top-coats for achieving ultimate protection. The satisfactory performance of paint coatings throughout the service life of an aircraft depends on many aspects. The present chapter discusses these paint aspects under the following topics: importance for aerospace applications; selection for aerospace applications; application areas in military aircraft; special functional paints; the properties, testing and analysis of paints; ageing of paints; airworthiness certification of paints; volatile organic compounds regulations; and paint monitoring. Some important new developments of paints are also discussed.

This chapter deals with the varieties and characteristics of elastomers and adhesives used in the aerospace industry. The key terms, various grades, structure and properties of each elastomer are discussed. An outline of rubber compounding and vulcanisation is presented. Significant elastomer properties for aerospace applications are highlighted. Further, an adhesives section includes the varieties of adhesives, mechanism of adhesive bonding, surface preparation, and joint designs for some loading conditions. Applications of elastomers and adhesives in the aerospace field are also surveyed.