Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
an important example of this approach. The machine-finished components
are produced from a steel containing about 1% aluminium, and then
nitrogen is allowed to diffuse into the surface at about 500 °C. A hard,
shallow layer is formed, whose depth (usually less than 1 mm) is dependent
on the time of treatment. The layer contains fine particles of aluminium
nitride, whose formation introduces highly localized surface compressive
stresses, resulting in significant improvement in fatigue resistance.
3.2 The families of engineering alloys
We will now survey the structure–property relationships of the important
engineering alloys. We will consider them in three groups, namely:
1. The light alloys, based on aluminium, magnesium and titanium.
2. The ‘heavy’ non-ferrous alloys based on copper, lead, zinc and nickel.
3. The ferrous alloys, namely steels and cast irons.
A reading list is included at the end of this chapter, suggesting books
giving fuller details of the physical metallurgy of the various families of
alloys. Tables are included which present the nomenclature of industrially
important alloys and designations employed to describe their metallurgical
condition or temper.
3.2.1 Aluminium alloys
Cast aluminium alloys
About 20% of the world production of aluminium is used for cast products.
Aluminium alloys have a relatively low melting temperature, but exhibit a
high shrinkage during solidification. Shrinkage of between 3.5 and 8.5%
may occur and allowance has to be made for this in mould design in order to
achieve dimensional accuracy in the product.
Aluminium–silicon alloys are the most important of the aluminium casting
alloys, and the relevant phase diagram is shown in Fig. 3.9, which is seen to
be of simple eutectic form. Slow solidification produces a very coarse eutectic
structure consisting of large plates or needles of silicon in an aluminium
matrix. The silicon particles are brittle, so castings with this coarse eutectic
exhibit low ductility. Refinement of the eutectic improves the mechanical
properties of the casting and this can be brought about by rapid cooling, as
in permanent mould casting, or by modification, which involves adding
sodium salts or metallic sodium to the melt before pouring. Improved tensile
strength and improved ductility is the result of this microstructural change,
a combination that is rarely encountered in physical metallurgy.
If the silicon content is below 8%, modification is unnecessary, because
the primary aluminium phase is present in sufficient quantity to confer adequate