The Physical and Mechanical Properties of Metals and Alloys

Of well over one hundred elements—if we include the increasing number of man-made ones—only eighteen have definite non-metallic properties. Six are usually classed as metalloids —elements like silicon, germanium and arsenic—in which physical and chemical properties are generally intermediate between those of metals and non-metals, but the remainder have clearly defined metallic properties. Metals are generally characterised by their lustrous, opaque appearance and, in respect of other physical properties, metals and non-metals contrast strongly. As we have seen (1.76) a metal consists of an orderly array of ions surrounded by and held together by a cloud of electrons. This is reflected in many of the physical properties of metals.
Melting point All metals (except mercury) are solids at ambient temperatures and have relatively high melting points (see Table 1.1) which vary between 234K (-390C) for mercury and 3683K (34100C) for tungsten. Non-metals include gases, a liquid (bromine) and solids. Their melting points vary much more widely: between IK (-2720C) for helium and approximately 5300K (50000C) for carbon.
Density The relative density (formerly specific gravity) of a material is defined as
the weight of a given volume of the material the weight of an equal volume of water.
Metals generally have higher relative densities (Table 1.1) than nonmetals. Values vary between lithium (0.534) which will float in water and osmium (22.5) which is almost twice the density of lead, which suggests that the simile as heavy as lead needs revision.
Electrical conductivity Non-metals are generally very poor conductors of electricity, indeed those where the bonding is entirely covalent will be insulators since all valance electrons are held captive in individual bonds and can move only in restricted orbits. By comparison in metals electrical conductivity arises from the presence of a sea or cloud of mobile electrons permeating the static array of ions. The electrons are able to flow through the ion framework when a potential difference is applied across the ends of the metal—which may be many miles apart as in the electric grid system. As indicated in Table 2.1 the electrical and thermal conductivities of metals follow roughly the same order. This is to be expected since both the flow of electricity and heat depend upon the ability of electrons to move freely within the metallic structure. For purposes of simple comparison Table 2.1 relates electrical and thermal conductivities of some important metals to those of silver (100). Although silver is marginally superior in terms of electrical conductivity to copper, the latter is used industrially because of relative costs. In fact for power transmission through the national grid aluminium lines are generally used for reasons given later (17.13). Electrical conductivity is reduced by alloying and the presence of impurities (16.21) as well as by mechanical straining.