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Materials for engineering, 3rd Edition - (Malestrom)

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84<br />

<strong>Materials</strong> <strong>for</strong> <strong>engineering</strong><br />

an important example of this approach. The machine-finished components<br />

are produced from a steel containing about 1% aluminium, and then<br />

nitrogen is allowed to diffuse into the surface at about 500 °C. A hard,<br />

shallow layer is <strong>for</strong>med, whose depth (usually less than 1 mm) is dependent<br />

on the time of treatment. The layer contains fine particles of aluminium<br />

nitride, whose <strong>for</strong>mation introduces highly localized surface compressive<br />

stresses, resulting in significant improvement in fatigue resistance.<br />

3.2 The families of <strong>engineering</strong> alloys<br />

We will now survey the structure–property relationships of the important<br />

<strong>engineering</strong> alloys. We will consider them in three groups, namely:<br />

1. The light alloys, based on aluminium, magnesium and titanium.<br />

2. The ‘heavy’ non-ferrous alloys based on copper, lead, zinc and nickel.<br />

3. The ferrous alloys, namely steels and cast irons.<br />

A reading list is included at the end of this chapter, suggesting books<br />

giving fuller details of the physical metallurgy of the various families of<br />

alloys. Tables are included which present the nomenclature of industrially<br />

important alloys and designations employed to describe their metallurgical<br />

condition or temper.<br />

3.2.1 Aluminium alloys<br />

Cast aluminium alloys<br />

About 20% of the world production of aluminium is used <strong>for</strong> cast products.<br />

Aluminium alloys have a relatively low melting temperature, but exhibit a<br />

high shrinkage during solidification. Shrinkage of between 3.5 and 8.5%<br />

may occur and allowance has to be made <strong>for</strong> this in mould design in order to<br />

achieve dimensional accuracy in the product.<br />

Aluminium–silicon alloys are the most important of the aluminium casting<br />

alloys, and the relevant phase diagram is shown in Fig. 3.9, which is seen to<br />

be of simple eutectic <strong>for</strong>m. Slow solidification produces a very coarse eutectic<br />

structure consisting of large plates or needles of silicon in an aluminium<br />

matrix. The silicon particles are brittle, so castings with this coarse eutectic<br />

exhibit low ductility. Refinement of the eutectic improves the mechanical<br />

properties of the casting and this can be brought about by rapid cooling, as<br />

in permanent mould casting, or by modification, which involves adding<br />

sodium salts or metallic sodium to the melt be<strong>for</strong>e pouring. Improved tensile<br />

strength and improved ductility is the result of this microstructural change,<br />

a combination that is rarely encountered in physical metallurgy.<br />

If the silicon content is below 8%, modification is unnecessary, because<br />

the primary aluminium phase is present in sufficient quantity to confer adequate

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