Freezing of a pure metal

3.1.1 Freezing of a pure metal

At some stage of production the majority of metals and alloys are melted and then allowed to solidify as

a casting. The latter may be an intermediate product, such as a large steel ingot suitable for hotworking, or a complex final shape, such as an engine cylinder block of cast iron or a single-crystal gas-turbine blade of superalloy. Solidification conditions determine the structure, homogeneity and soundness of cast products and the governing scientific principles find application over a wide range of fields. For instance, knowledge of the solidification process derived from the study of conventional metal casting is directly relevant to many fusionwelding processes, which may be regarded as ‘casting in miniature’, and to the fusion- casting of oxide refractories. The liquid/solid transition is obviously of great scientific and technological importance.

First, in order to illustrate some basic principles, we will consider the freezing behaviour of a melt of Figure 3.1 Cooling curve for a pure metal showing possible

undercooling .

like metal atoms. The thermal history of a slowly cooling metal is depicted in Figure 3.1; the plateau on the curve indicates the melting point (m.p.), which

conductivity rises and the diffusivity, or ability of the is pressure-dependent and specific to the metal. Its

atoms to migrate, falls.

value relates to the bond strength of the metal. Thus, Solidification is a classic example of a nucleation the drive to develop strong alloys for service at high

and growth process. In the general case of freezing temperatures has stimulated research into new and

within the bulk of pure molten metal, minute crys- improved ways of casting high-m.p. alloys based upon

talline nuclei form independently at random points. iron, nickel or cobalt.

After this homogeneous form of nucleation, contin- The transition from a highly-disordered liquid to an

ued removal of thermal energy from the system causes ordered solid is accompanied by a lowering in the

these small crystalline regions to grow independently energy state of the metal and the release of thermal

at the expense of the surrounding melt. Throughout energy (latent heat of solidification), forming the arrest

the freezing process, there is a tendency for bombard- on the cooling curve shown in Figure 3.1. This order-

ment by melt atoms to destroy embryonic crystals; ing has a marked and immediate effect upon other

only nuclei which exceed a critical size are able to structure-sensitive properties of the metal; for instance,

survive. Rapid cooling of a pure molten metal reduces the volume typically decreases by 1–6%, the electrical

the time available for nuclei formation and delays the

Structural phases: their formation and transitions 43 onset of freezing by a temperature interval of T.

This thermal undercooling (or supercooling), which is depicted in Figure 3.1, varies in extent, depending upon the metal and conditions, but can be as much as 0.1–0.3 T m , where T m is the absolute melting point. However, commercial melts usually contain suspended insoluble particles of foreign matter (e.g. from the refractory crucible or hearth) which act as seeding nuclei for so-called heterogeneous nucleation. Under- cooling is much less likely under these conditions; in fact, very pronounced undercooling is only obtainable when the melt is very pure and extremely small in volume. Homogeneous nucleation is not encountered in normal foundry practice.

The growing crystals steadily consume the melt and eventually impinge upon each other to form a struc-

Figure 3.2 Schematic diagram of three dendrites ture of equiaxed (equal-sized) grains (Figures 3.2 and

interlocking .

3.3). Heterogeneous nucleation, by providing a larger population of nuclei, produces a smaller final grain size than homogeneous nucleation. The resultant grain (crystal) boundaries are several atomic diameters wide. The angle of misorientation between adjacent grains is usually greater than 10–15 ° . Because of this mis- fit, such high-angle grain boundaries have a higher energy content than the bulk grains, and, on reheating, will tend to melt first. (During a grain-contrast etch of diamond-polished polycrystalline metal, the etchant attacks grain boundaries preferentially by an electro- chemical process, producing a broad ‘canyon’ which scatters vertically incident light during normal micro- scopical examination. The boundary then appears as a black line.)

During the freezing of many metals (and alloys), nucleated crystals grow preferentially in certain direc- tions, causing each growing crystal to assume a distinc-

Figure 3.3 Formation of grains from dendrites of

1 tive, non-faceted Figure 3.2 tree-like form, known as a dendrite . (Figure 3.2). In cubic crystals, the preferred axes of

growth are h1 0 0i directions. As each dendritic spike provide thermal pulses which cause dendritic branch grows, latent heat is transferred into the surrounding

tips to melt off and enter the main body of the melt liquid, preventing the formation of other spikes in its

where they act as ‘kindred nuclei’. Gentle stirring of immediate vicinity. The spacing of primary dendrites

the melt encourages this process, which is known as and of dendritic arms therefore tends to be regular.

dendrite multiplication, and can be used to produce a Ultimately, as the various crystals impinge upon each

fine-grained and equiaxed structure (e.g. electromag- other, it is necessary for the interstices of the dendrites

netic stirring of molten steel). Dendrite multiplication to be well-fed with melt if interdendritic shrinkage

is now recognised as an important source of crystals cavities are to be prevented from forming. Convection

in castings and ingots.

currents within the cooling melt are liable to disturb the delicate dendritic branches and produce slight angu- lar misalignments in the final solidified structure (e.g.

3.1.2 Plane-front and dendritic solidification

5–10 ° ). These low-angle boundaries form a lineage

at a cooled surface

(macromosaic) structure within the final grain, each surface of misfit being equivalent to an array of edge

The previous section describes random, multidirec- dislocations (Chapter 4). Convection currents can also

tional crystallization within a cooling volume of pure molten metal. In practice, freezing often commences at

the plane surface of a mould under more complex and Many metals and a few organic materials grow with

non-faceted dendritic morphology, e.g. transparent constrained conditions, with crystals growing counter succinonitrile-6% camphor has proved a valuable means of

to the general direction of heat flow. The morphology simulating dendrite growth on a hot-stage optical

of the interface, as well as the final grain structure of microscope. Most non-metals grow with a faceted

the casting, are then decided by thermal conditions at morphology.

the solid/liquid interface.

44 Modern Physical Metallurgy and Materials Engineering

Figure 3.4 Plane-front solidification (a) and dendritic solidification (b) of a pure metal, as determined by thermal conditions .

Figure 3.4a illustrates the case where all the latent as those discussed in the previous two sections. heat evolved at the interface flows into the solid and

When solidification commences at the flat surface of the temperature gradients in solid and liquid, G S and

a metallic ingot mould there is usually an extreme

G L , are positive. The solidification front, which moves undercooling or chilling action which leads to the at a velocity R, is stable, isothermal and planar. Any

heterogeneous nucleation of a thin layer of small, solid protuberance which chances to form on this front

randomly-oriented chill crystals (Figure 3.5). The size will project into increasingly hotter, superheated liquid

of these equiaxed crystals is strongly influenced by and will therefore quickly dissolve and be absorbed by

the texture of the mould surface. As the thickness of the advancing front. Planar-front solidification is char-

the zone of chill crystals increases, the temperature acterized by a high G L /R ratio (e.g. slow cooling). If

gradient G L becomes less steep and the rate of cooling the solid is polycrystalline, emerging grain boundaries

decreases. Crystal growth rather than the nucleation will form grooves in the stable planar front.

of new crystals now predominates and, in many In the alternative scenario (Figure 3.4b), for which

metals and alloys, certain favourably-oriented crystals

G L /R has relatively low values, latent heat flows into at the solid/liquid interface begin to grow into the both solid and liquid and G L becomes negative. A

melt. As in the case of the previously-described planar interface becomes unstable. Dendritic protu-

berances (spikes) grow rapidly into the undercooled liquid, which quickly absorbs their evolved latent heat. Thermal undercooling is thus an essential prerequi- site for dendritic growth; this form of growth becomes more and more likely as the degree of thermal under- cooling increases. Melts almost invariably undercool slightly before solidification so that dendritic mor- phologies are very common. (The ability of dilute alloy melts to produce a cellular morphology as a result of constitutional undercooling will be described in Section 3.2.4.3.)