Fig. 1.25. Cooling curves for a pure metal

 

At very low rates, the degree of supercooling is small and solidification proceeds at a temperature near to the equilibrium one (curve v₃). The horizontal portion found on the cooling curves at the solidification temperature (representing a stop in the fall of temperature) is due to the evolution of the latent heat of solidification, even when heat is dissipated in cooling. The degree of supercooling increases with the cooling rate and the solidification process proceeds at temperatures below the equilibrium temperature of solidification.

The rate of solidification and the structure of the solidified metal depend on the rate of nucleation RN (number of crystals appearing in unit time and in unit volume) and on the rate of growth RG of the nuclei, i.e. on the linear rate of crystal growth. The higher the rate of nucleation and growth of the nuclei is, the more rapid the process of solidification will be. It is usually assumed that with an increase in the degree of supercooling, the rate of nucleation and, consequently, the number of nuclei, increases faster than the rate of crystal growth. The greater the rate of nucleation and the lower the rate of crystal growth, the smaller the crystals (grains) finally will be, and the finer the grain structure of metal will be obtained. The size of obtained grains S is related to the number of nuclei (nucleation rate) RN and to the rate of crystal growth RG by the equation:

 

                                        (1.18)

 

The grain size of metal strongly affects its mechanical properties. These properties, especially, ductility and plasticity, are higher for fine-grained metal.

Nucleation in a liquid metal by the described mechanism is considered to be spontaneous. Spontaneous nucleation, i.e. the formation of nuclei, based on phase and energy fluctuations may take place only in highly pure liquid metals. Commercial, or engineering, metals always contain a large amount of impurities of various kinds (oxides, sulphides, nitrides, etc.), which under definite conditions facilitate the formation of nuclei. These conditions are:

- the impurity should have a higher melting point than the base metal;

- the crystal lattice of the impurity and crystal lattice of base metal should differ only to the minimum extend (principle of structural and dimensional conformity).

Inoculation is the use of impurities, or admixtures, called inoculants, which are added to the liquid metal to obtain fine grain according to the mechanism described above. These inoculants are added in such small amounts (0.001...0.15%) that practically they do not change the chemical composition of the metal. But they enable a fine grain to be obtained and thereby improve the mechanical properties of the metal.

 

1.4.2. Macro- and Microstructure of Metals and Alloys

 

In their ordinary structural state pure metals are of low strength and do not possess, in many cases, the required physicochemical, mechanical and technological properties. Consequently, they are comparatively seldom used in engineering practice. The majority of metals employed are alloys. Alloys are produced by melting or sintering of two or more metals, or metals and nonmetals, together. The chemical elements that make up an alloy are called its components.

The conceptions of phase and structure, widely used in physical metallurgy, have been introduced to deal with the constitution, transformations and properties of metals and alloys.

A phase is a homogeneous portion of an alloy, having the same composition and the same state of aggregation throughout its volume, and separated from the other portions of system by interfaces. For instance, a homogeneous pure metal or alloy is a single-phase system. A state in which a liquid alloy or metal coexists with its crystal is a two-phase system. Alloy may have two and more phases in solid state, for instance, aluminum and silicon in Al-Si alloy (Fig. 1.26).

As the result of various physicochemical interactions of the components the following phases can be formed in alloys:

- liquid solutions;

- solid solutions;

- chemical compounds;

- phase, or mechanical mixtures.

Fig. 1.26. Double-phase structure of Al-Si alloy

 

As a rule, in liquid state components of alloys are unlimitedly dissolved each in other forming liquid solutions.

Solid solutions are phases in which one component of the alloy retains its crystal lattice, and atoms of the other component (or components) are located in the lattice of the first one, called the solvent, changing its size (lattice constant). Distinction is made between substitutional and interstitial solid solutions (Fig. 1.27). In forming a substitutional solid solution, the atoms of the dissolved component (called solute atoms) substitute for a part of the atoms of the solvent (called matrix atom) in its crystal lattice. In forming an interstitial solid solution, the solute atoms are accommodated in the interstices (interatomic spaces) of the crystal lattice of the solvent.

Fig. 1.27. Substitutional (a) and interstitial (b) solid solutions

 

All metals are mutually soluble to some degree in solid state. For instance, up to
5,5 % Cu can be dissolved in aluminium and up to 39 % Zn in copper without changing the type of their crystal lattice. A continuous series of solid solutions may be formed in cases when the components can substitute each other in the crystal lattice in any quantitative ratio.

Chemical compounds are formed according to the law of normal valence. They have typical features that distinguish them from solid solutions:

- they have a crystal lattice that differs from those of the components forming the compound;

- there is a simple multiple relation of the atoms of elements: this enables their composition to be specified by a simple formula A_nB_m, where A and B are elements, n and m are small whole numbers;

- the properties of the compound differ greatly from these of its components;

- the compound has a constant melting point;

- the formation of the chemical compound is accompanied by a substantial thermal affect.

Mechanical (phase) mixtures are formed during crystallization of double alloys: Fe-C,
Pb - Sn, Al-Cu and so on. They are combinations of two pure metals, or metal and solid solution or solid solution and chemical compound. Mechanical mixtures have constant chemical composition, for example, 0,8%C and 99,2% Fe, 13% Pb and 87% Sb and constant temperature of crystallization or recrystallization.

The industrial alloys consist of many components (two, five, ten and more), therefore they have numerous phases and complex structure composition. The structure refers to the shape, size or mutual arrangement of the corresponding phases in metals and alloys. The structure defines properties of metals and alloys.

There are some methods to control the structure of metals and alloys. They distinguish concept of macrostructure and microstructure.

Macrostructure is constitution of a metal or alloy investigated by the naked eye or by low-power magnification (not more than x50).

Microstructure is constitution of metal or alloy observed by means of optical metallurgical microscope with magnification ranges from x50 to x2000.

Macrostructure can be examined either on a fracture or on specially prepared macrosection (Fig. 1.28). A study of a fracture is the simplest way to reveal the crystalline structure of metals. A fracture reveals grain size and shape, special features in smelting and casting of the metal, heat treatment used and, consequently, certain properties of the metal. Fractures may be: brittle, plastic, combine (intermediate), fatigue.

Fig. 1.28. Macrostructure of steel: (a) - “as cast”; (b) - forged; (c) - rolled

 

If the macrostructure is studied on special macrosection, the specimens are cut out of large billets (ingots, forgings, etc.) or machine parts and are then ground, polished and etched with special reagents called etchants. An examination of a macrosection can disclose the shape and arrangement of the grains in cast metals; directions of grain flow lines (of the deformed crystallites) in smith and closed-die forgings; defects impairing the continuity of the metal (shrinkage porosity, gas holes, cracks, etc.); chemical non-homogeneity of an alloy, caused by the crystallization process or resulting from heat -treatment or chemical heat-treatment (carbonizing, nitriding, etc.).

The microstructural analysis reveals the mutual arrangement of the phases, their shapes and sizes. Microstructure is studied on a microsection made of the metal to be analyzed. This is a small specimen, prepared by careful surface grinding, polishing and etching by special reagent. The microstructure of metals is observed by means of an optical or an electron microscope. The useful magnification of an optical microscope does not exceed x2000. This enables details of the structure to be observed if they are larger than 2000  or 2.0·10^-7m (Fig. 1.29).

 

a          x1000                    b  x500

c    x1000

Fig. 1.29. Microstructure of hipoeutectoid steel with 0.4%C (a),

eutectoid steel with 0.8%C (b) and hypereutectoid steel with 1,1%C

 

Today the electron microscope is extensively used to study the structure of metals. Its effective magnification may reach x5000000. The use the electron beams of exceptionally short wave-length (0.04 to 0.12x10^-8m) enables details of the object to be distinguished that are near
1  (10^{-10 m) in size (subgrains, dislocations and separate atoms).}