Introduction

Metal alloys used in aircraft structures and engines must have high mechanical properties to ensure weight-efficiency. It is essential that metals used in aircraft have sufficient strength to avoid permanent deformation and damage under the structural, aerodynamic and other loads experienced during flight. The mechanisms by which aerospace metals achieve their high strength are complex. Various mechanisms occur at the atomic, nanometre, microstructural and millimetre scales and these control the strength properties, as shown in Fig. 4.1 for the aluminium alloys used in an aircraft wing. Without these strengthening mechanisms occurring at different length scales, metals would be too soft and susceptible to plastic deformation to use in highly loaded structures and engine components. The mechanisms, when used in combination, provide aerospace engineers with weight-efficient metals with the capacity to withstand the extremely high loads experienced by modern aircraft.

4.1 Strengthening processes used to maximise the strength of aluminium alloys in aircraft wings.

The strength properties of aerospace metals, which include proof strength, ultimate strength, fatigue strength and creep strength, are controlled by a multitude of factors. The main factors are:

• alloy composition;

• arrangement and bonding of the atoms;

• type, size and concentration of precipitates and second-phase particles;

• types and concentration of imperfections and defects;

• metal casting processes, forming techniques and manufacturing methods.

The describes the main factors that control the strength properties of metal alloys. The explains the fundamental engineering science behind the development of high-strength metals for use in weight-efficient aircraft structural components. To understand the strengthening of metals, it is necessary to have a basic understanding of the arrangement of atoms in metals. A brief description of the crystal structures of the metals used in aircraft is provided. Following this is an overview of the various imperfections in the crystal structure which affect the strength of metals. The mechanisms by which these imperfections increase the strength of metals are outlined. The strengthening of metals by dislocations is explained. Dislocations are central to all the main strengthening mechanisms in metals, and what they are and how they increase strength is described. The strengthening of metals by precipitation hardening, intermetallic compounds, and control of the grain structure is explained. The information in this provides a basic understanding of the various ways that aerospace engineers can increase the strength of metal alloys for structural applications.