In this article we will discuss about:- 1. Process of Crystallization of Pure Metals 2. Dendritic Solidification 3. Allotropy of Metals 4. Effect of Grain Size on Properties 5. Solid Solution 6. Intermediate Phases.
Process of Crystallization of Pure Metals:
A crystal may be visualised as forming from a centre of freezing, or nucleus, which is composed of a small group of atoms oriented into one of the common crystal patterns. During the process of solidification many of these nuclei spring up, each nucleus being a potential crystal and able to grow to form a crystal large enough to be seen with the unaided eye.
As each nucleus is a growing crystal, and the atoms within it are all similarly oriented, no nucleus within the freezing melt may form with its planes or groups of atoms the same as those of any other nucleus. Thus, when the individual crystals have grown to the point where they have absorbed all of the liquid atoms and therefore, come in contact with each other along their boundaries, they do no line up, i.e., their planes of atoms change direction in going from one crystal to another.
This results in the solid states being composed of a number of crystals of different orientation, and we have a crystal aggregate or mixed crystals. Each crystal, therefore, is composed of a group of similarly oriented atoms, but on going from one crystal to the neighbouring crystals, the orientation changes.
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So far the nature of the crystal border is still not known completely but we may assume that it is an interlocking border line where the atoms of one crystal change orientation from the atoms of another crystal.
It may be that these are some left-over atoms along this border line separating the differently oriented crystals, such atoms not knowing to which crystal to attach themselves, and these act as non-crystalline cement between the various crystals. This condition may account for the greater strength of the crystal boundaries as compared to the strength of the individual crystals, and for many of actions that take place at the crystal boundaries.
Dendritic Solidification:
The crystals which form in the process of solidification of a metal may have many different structures (dendritic, lamellar, needle-type or acicular etc.) depending on the rate of cooling, and the type and amount of mixtures or impurities in the melt.
Perfect crystals of proper external shape can be obtained only if crystallisation develops under conditions when the degree of supercooling is very slight and the metal has a very high purity.
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In the great majority of cases, branched or tree-like crystals are obtained, which are called dendrities (Fig. 2.4).
Fig. 2.3 shows steps in the formation of a crystal. A crystal nucleus forms as shown in (a) and then proceeds to send out shoots or axes of solidification as shown in (b), (c), (d), forming the skeleton of a crystal in much the same way as frost patterns form. Atoms then attach themselves to the axes of the growing crystal from the melt in progressive layers, finally filling up these axes, and thus forming a completed solid or crystal, as shown in (e). Three dimensional view of dendritic growth is shown in Fig. 2.4.
Allotropy of Metals:
Existence of a given metal in two or more stable but different crystal structures is known as allotropy. The essence of allotropic transformations is that the atoms of a crystallic solid are converted from one crystalline form to another, i.e., they form a new crystal lattice. Modifications, stable at lower temperatures, are designated by the Greek letter α (alpha); β (beta) designate a second form of the same material that is stable at some higher temperatures; y (gamma) at still higher temperatures; etc.
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The allotropy of iron is of special importance. Iron exists in two allotropic forms: α iron, with a Body-Centered Cubic (B.C.C.) lattice, stable at temperatures upto 910°C and γ iron, with a face-centered cubic lattice, stable in the range from 910°C to 1400°C.
There are also three allotropic modifications of manganese (α-Mn, β-Mn and γ-Mn) with complex crystal lattices, two modifications of cobalt (α-Co and β-Co), two-of tin (α-Sn and β-Sn), two-of titanium (α-Ti and β-Ti). two-of zirconium (α-Zr and β-Zr), and two-of tellurium (α-Te and β-Te).
Changes in packing density of the crystal lattice, in conversion from one allotropic form to another, lead to changes in the volume of the material as well.
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Some common examples of allotropic transformations in metals are given in Table 2.1.
Properties of Alloys:
The properties of alloys are discussed below:
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1. Thermal and Electrical Conductivities:
I. These properties of a solid solution are less than those of the pure metals. According to Mathiessen’s rule when small quantities of an alloying element are added in solid solution to a metal the increase in resistance does not depend upon the temperature.
II. For mixture of insoluble phases, the thermal and electric resistances follow the law of mixtures.
I. Density is increased by heavier metal in solid solution and is decreased by a lighter metal.
II. In case of interstitial solid solution, there is little effect on density by the added element.
3. Specific Heat and Coefficient of Thermal Expansion:
These properties of alloys are governed by the law of mixtures.
4. Melting Point:
The greater the difference in valencies between the metals of alloy the wider is the melting range.
5. Boiling Point:
The boiling point (like melting point) is also converted into a range by the addition of alloying elements.
Effect of Grain Size on Properties of Metals:
A grain is a crystal with almost any external shape, but with an internal atomic structure based upon the space lattice with which it was born. The crystals found in all commercial metals and alloys are commonly called grains because of variation in their external shape.
The mechanical properties of metals that obtain maximum strength depend upon the arrangement of the grains, their shape and especially their size. Grain size directly controls the extent of slip interference by adjacent grains and strength, toughness, ductility and fatigue of metals are thus affected. The fine grain structure has a small grain size whereas coarse grain structure has a large grain size. The grain size is controlled by several factors out of which temperature and time of heating are important.
The important effects of grain size are given below:
1. Fine grained structure has higher strength than coarse grained structure.
2. Besides higher strength, fine grains provide better resistance to cracking and better machine finish.
3. Coarse grains make the surface rough.
4. Coarse grains make the metal less tough.
5. Coarse grained metal is difficult to polish.
6. Coarse grained structure has a better workability than fine grained structure.
7. Coarse grained materials at high temperature exhibit better creep resistance than fine grained ones.
Solid Solution:
In certain cases, the solidification of an alloy results in the formation of one kind of crystal in which both metals are present, but they cannot be detected by the microscope, although properties of the crystals are profoundly changed. In such a case we have a solid metal in which the interatomic state which existed in the liquid solution has been preserved after solidification, and it is known as a solid solution.
In simple, a ‘solid solution’ may be defined as a solution in the solid state which consists of two kinds of atoms combined in one type of space lattice.
In a solid solution the atoms occur in a definite geometrical pattern, which is usually a slightly distorted form of one of the constituent metals.
Solid solutions are conductors, but not as good as the pure metals on which they are based.
Some examples of solid solutions are:
(i) Cu-Zn alloys (Brasses)
(ii) Ni-Cu alloys (Monel metal)
(iii) Au-Ag alloys
(iv) Ag-Cu alloys (Sterling silver)
(v) Fe-Cr-Ni alloys (Certain stainless steels)
(vi) Fe-C alloys (Steels),
Types of Solid Solutions:
Solid solutions occur in either of two distinct types, namely-
1. Substitutional Solid Solution:
In substitutional solid solution, there is a direct substitution of one type of atom for another, so that solute atoms (Cu) enter the crystal to take positions normally occupied by solvent atoms (e.g., nickel atoms); refer to Fig. 2.5.
The alloy is said to be in a disordered condition if in the formation of a substitutional solid solution, the solute atoms do not occupy any specific position but are distributed at random in the lattice structure of the solvent as shown in Fig. 2.5. An ordered substitutional solid solution is shown in Fig. 2.6. Cu- Zn, Al-Cu, are some examples of ordered structures.
Hume-Rothery’s Rules Governing the Formation of Substitutional Solid Solutions:
Two metals can be completely soluble in each other only if they have:
I. Same lattice patterns.
II. Nearly equal atom diameters.
III. Equal number of valency electrons.
A. Where the difference in atomic diameters of the two metals exceeds 14-15% the ‘size factor’ is unfavourable and the range of solution is restricted.
B. The more electronegative the solute element and the more electropositive the solvent, or vice versa, the greater is the tendency to restrict the solution ranges and to form intermetallic compounds known as chemical affinity effect.
C. A metal of lower valency tends to dissolve a metal of higher valency more readily than vice-versa known as relative valency effect.
2. Interstitial Solid Solution:
The four elements hydrogen, carbon, nitrogen and boron have such small diameters that they can occupy the empty spaces (interstices) in the crystal lattices of many metals (Fig. 2.7). Such interstitial solid solutions usually have a limited composition range and are generally considered of secondary importance, but there are a few instances worthy of special attention.
The interstitial solution of carbon in iron constitutes the basis of steel hardening. Very small amounts of hydrogen introduced into steels during acid pickling (cleaning), plating, or welding operations cause a sharp decrease in ductility, known as hydrogen embrittlement.
Interstitial nitrogen is useful not only in nitriding process but also as an important factor in maintaining 18 Cr-8 Ni stainless steel in the austenitic condition.
In some alloys both interstitial and substitutional solid solutions are formed to an appreciable extent. For example, a chromium-nickel steel contains interstitially dissolved carbon and substitutionally dissolved chromium, nickel, and minor elements.
Intermediate Phases:
In several binary alloy systems, when the chemical affinity of metal is great, the mutual solubility becomes limited and intermediate phases are formed (rather solid solutions). These phases may range between the ideal solid solution and the ideal chemical compound.
They may have either narrow or wide ranges of homogeneity, and may or may not include a composition having a simple chemical formula. For example – the phase Cu AI exists in a homogeneity range that does not include the actual composition Cu AI.
Intermediate phases are of the two following types:
1. Intermetallic Compounds of Fixed Composition.
These compounds obey the usual valency laws, like ordinary chemical compounds, e.g., NaCI. A typical example of such Intermetallic compounds is Mg2Sn which contains 29.08% magnesium. This compound has a definite melting point which is lowered by addition of excess Mg or of Sn, which can be inferred from the equilibrium diagram (not shown).
2. Intermetallic Compounds of Variable Composition:
These compounds do not obey the valency laws and are known as Electron compounds.
Many of these compounds fall into three classes according to the ratio of valency electron to the number of atoms:
(i) Ratio 3/2 – beta (β), e.g., Cu Zn, Cu3Al;
(ii) Ratio 21/13 – gamma γ, e.g., Cu5Zn8, Cu3 Al4;
(iii) Ratio 7/4 – epsilon (ε), e.g., Cu Zn3, CuSn.