Gas porosity and segregation

3.1.4 Gas porosity and segregation

So far we have tended to concentrate upon the behaviour of pure metals. It is now appropriate to consider the general behaviour of dissimilar types of atoms which, broadly speaking, fall into two main categories: those that have been deliberately added for

a specific purpose (i.e. alloying) and those that are accidentally present as undesirable impurities. Most metallic melts, when exposed to a furnace atmosphere, will readily absorb gases (e.g. oxygen, nitrogen, hydrogen). The solubility of gas in liquid metal can

be expressed by Sievert’s relation, which states that the concentration of dissolved gas is proportional to the square root of the partial pressure of the gas in the contacting atmosphere. Thus, for hydrogen, which is one of the most troublesome gases:

[H solution ] D Kfp⊲H 2 ⊳ g 1/2

(3.1) The constant K is temperature-dependent. The solu-

bility of gases decreases during the course of freezing, usually quite abruptly, and they are rejected in the form of gas bubbles which may become entrapped within and between the crystals, forming weakening blow- holes. It follows from Sievert’s relation that reducing the pressure of the contacting atmosphere will reduce the gas content of the melt; this principle is the basis of vacuum melting and vacuum degassing. Similarly, the passage of numerous bubbles of an inert, low-solubility gas through the melt will also favour gas removal (e.g. scavenging treatment of molten aluminium with chlo- rine). Conversely, freezing under high applied pres- sure, as in the die-casting process for light alloys, suppresses the precipitation of dissolved gas and pro- duces a cast shape of high density.

Dissolved gas may precipitate as simple gas bubbles but may, like oxygen, react with melt constituents to form either bubbles of compound gas (e.g. CO 2 , CO, SO 2 ,H 2 O vap ) or insoluble non-metallic particles. The latter are potential inoculants. Although their presence may be accidental, as indicated previously, their delib- erate formation is sometimes sought. Thus, a specific addition of aluminium, an element with a high chem- ical affinity for oxygen, is used to deoxidize molten steel in the ladle prior to casting; the resultant par- ticles of alumina subsequently act as heterogeneous nucleants, refining the grain size.

Segregation almost invariably occurs during solid- ification; unfortunately, its complete elimination is impossible. Segregation, in its various forms, can seri- ously impair the physical, chemical and mechanical properties of a cast material. In normal segregation, atoms different to those which are crystallizing can

be rejected into the melt as the solid/liquid interface advances. These atoms may be impurities or, as in the case of a solid solution alloy, solute atoms. Insolu- ble particles can also be pushed ahead of the interface.

46 Modern Physical Metallurgy and Materials Engineering Eventually, pronounced macro-segregation can be pro-

duced in the final regions to solidify, particularly if the volume of the cast mass is large. On a finer scale, micro-segregation can occur interdendritically within both equiaxed and columnar grains (coring) and at the surfaces of low- and high-angle grain boundaries. The modern analytical technique of Auger electron spec- troscopy (AES) is capable of detecting monolayers of impurity atoms at grain boundary surfaces and has made it possible to study their very significant effect upon properties such as ductility and corrosion resis- tance (Chapter 5).

In the other main form of separation process, 1 which

is known as inverse segregation, thermal contraction of the solidified outer shell forces a residual melt of low melting point outwards along intergranular channels until it freezes on the outside of the casting (e.g. ‘tin sweat’ on bronzes, ‘phosphide sweat’ on grey cast iron). The direction of this remarkable migration thus coincides with that of heat flow, in direct contrast to normal macro-segregation. Inverse segregation can

be prevented by unidirectional solidification. Later, in Section 3.2.4.4, it will be shown how the process of zone-refining, as used in the production of high-purity materials for the electronics industry, takes positive advantage of segregation.