Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 73
depends upon the rate of nucleation of solid crystals within the melt. Equation
[1.6] shows that the critical nucleus size decreases with increasing degree of
supercooling of the liquid, so that rapidly cooled liquids will form solids of
finer grain size than slowly cooled liquids. For example, when large bronze
sand castings are made (e.g. a ship’s propeller), the cooling rate and degree
of supercooling of the melt are low, so the product has a coarse grain-size –
typically tens of millimetres in dimension. Conversely, when die-cast objects
are formed by injecting molten metal into a water-cooled metal die, the high
supercooling leads to a high nucleation-rate and, thus, a fine grain-size –
typically tens of micrometres in dimension.
Fine-grained castings can be produced if an inoculant is added to the
molten metal before it is introduced into the mould. For example, the addition
of a small quantity of zirconium to molten magnesium alloys results in a
dramatic refinement of the grain size in the casting.
In recent years, ultrafine (sub-micrometre) grain-size material has been
produced by rapid-solidification processing (RSP), whereby molten metal is
sprayed in fine droplet form onto a water-cooled substrate, thus achieving
extremely high supercoolings. The high cost of this process limits its wide
application, however.
Solid state phase changes
In the case of those materials which undergo a phase change in the solid
state, it is possible to refine the grain size by thermal treatment. The most
important example of this process is that of normalizing steel, whereby a
fine-grained microstructure can be developed in (say) a coarse-grained steel
casting by subjecting it to an appropriate heating and cooling cycle.
Pure iron (α-iron, or ‘ferrite’) undergoes a change in crystal structure
when heated above 910 °C, forming γ-iron, or ‘austenite’. Most ferritic steels
exhibit a similar transformation and, if such a steel is heated just above its γ
transformation temperature, a new (γ) grain structure will form. When the
material is allowed to undergo normal air-cooling, the γ structure will transform
to new α-grains by a process of nucleation and growth. Air-cooling permits
sufficient supercooling to encourage prolific nucleation of the α phase, so
the original grain structure is refined. If the normalizing cycle is repeated, an
even finer grain-size can be achieved. This effect is illustrated in Fig. 3.1 for
a steel specimen of initial grain size 50 µm (ASTM 6). If the specimen is
immersed in a molten lead bath at 815 °C and then allowed to air cool to
room temperature, a structure of grain size 11 µm (ASTM 10) is formed. If
the cycle is repeated, grain sizes of 8 µm (ASTM 11) and 5 µm (ASTM 12)
are formed.