6.1 Introduction

Metals and alloys have many uses in dentistry. Steel alloys are commonly used for the construction of instruments and of wires for orthodontics. Gold alloys and alloys containing chromium are used for making crowns, inlays and denture bases whilst dental amalgam, an alloy containing mercury, is the most widely used dental filling material.

With the exception of mercury, metals are generally hard and lustrous at ambient temperatures, and have crystalline structures in which the atoms are closely packed together. Metals are opaque and are good conductors of both heat and electricity.

The shaping of metals and alloys for dental use can be accomplished by one of three methods, namely, casting, cold working or amalgamation. Casting involves heating the material until it becomes molten, when it can be forced into an investment mould which has been prepared from a wax pattern. Cold working involves mechanical shaping of the metal at relatively low temperatures, taking advantage of the high values of ductility and malleability possessed by many metals. Some alloys can be mixed with mercury to form a plastic mass which gradually hardens by a chemical reaction followed by crystallization. The material is shaped by packing it into a tooth cavity whilst still in the plastic state. This specific technique of shaping by amalgamation is dealt with in detail in the chapter devoted to dental amalgam (Chapter 21).

6.2 Structure and properties of metals

Crystal structure

Metals usually have crystalline structures in the solid state. When a molten metal or alloy is cooled, the solidification process is one of crystallization

and is initiated at specific sites called nuclei. The nuclei are generally formed from impurities within the molten mass of metal (Fig. 6.1a). Crystals grow as dendrites or spherulites, which can be described as three-dimensional, structures emanating from the central nucleus (Fig. 6.1b). Crystal growth continues until all the material has solidified and all the dendritic or spherical crystals are in contact (Fig. 6.1c). Each crystal is known as a grain and the area between two grains in contact is the grain boundary.

After crystallization, the grains have approximately the same dimensions in each direction, measured from the central nucleus. They are not perfectly spherical or cubic however, nor do they conform to any other geometric shape. They are said to have an equiaxed grain structure. A change from an equiaxed structure to one in which the grains have a more elongated, fibrous structure can cause important changes in mechanical properties.

The atoms within each grain are arranged in a regular three-dimensional lattice. There are several possible arrangements such as cubic, body-centred cubic and face-centred cubic as shown in Fig. 6.2. The arrangement adopted by any one crystal depends on specific factors such as atomic radius and charge distributions on the atoms. Although there is a tendency towards a perfect crystal structure, occasional defects occur, as illustrated, twodimensionally, in Fig. 6.3. Such defects are normally referred to as dislocations and their occurrence has an effect on the ductility of the metal or alloy. When the material is placed under a sufficiently high stress the dislocation is able to move through the lattice until it reaches a grain boundary. The plane along which the dislocation moves is called a slip plane and the stress required to initiate movement is the yield stress.

In practical terms, the application of a stress greater than the yield stress causes the material to

Fig. 6.1 Diagram illustrating crystallization of a metal (a) from nuclei, (b) through dendritic growth, (c) to form grains.

Fig. 6.2 Some possible arrangements of atoms in metals and alloys: (a) cubic structure; (b) face-centred cubic; (c) body-centred cubic.

be permanently deformed as a result of movement of dislocations. Depending upon the circumstances, this can be a disadvantage or, alternatively, may be used to advantage, as in the formation of wires.

Grain boundaries form a natural barrier to the movement of dislocations. The concentration of grain boundaries increases as the grain size decreases. Metals with finer grain structure are

Fig. 6.3 (a) Simplified, diagrammatic indication of an imperfection in a crystal structure. (b) Under the influence of sufficient force atoms may move to establish a more perfect arrangement.

generally harder and have higher values of yield stress than those with coarser grain structure. Hence it can be seen that material properties can be controlled to some extent by controlling the grain size.

As stated in Chapter 2, the yield stress is not easily determined experimentally. The proportional limit is therefore often used as an indication of yield stress. Alternatively the proof stress is used. The proof stress is the stress required to produce a certain level of permanent strain. For example the 0.2% proof stress indicates the stress required to produce a strain of 0.002.

A fine grain structure can be achieved by rapid cooling of the molten metal or alloy following casting. This process, often referred to as quenching, ensures that many nuclei of crystallization are formed, resulting in a large number of relatively small grains as shown in Fig. 6.4a. Slow cooling causes relatively few nuclei to be formed which results in a larger grain size as shown in Fig. 6.4b. Some metals and alloys are said to have a refined grain structure. This is normally a fine grain structure which is achieved by seeding the molten material with an additive metal which forms nuclei for crystallization.

Cold working

In the previous section it was mentioned that permanent deformation takes place on the application of a sufficiently high force, due to the movement of dislocations along slip planes. For an applied tensile force the maximum degree of extension is a measure of the ductility of the metal or alloy. For an applied compressive force the maximum degree of compression is a measure of malleability. These changes occur when the stress is greater than the yield stress and at relatively low temperatures. Such cold working not only produces a change in microstructure, with dislocations becoming concentrated at grain boundaries,

but also a change in grain shape. The grains are no longer equiaxed but take up a more fibrous structure (Fig. 6.5a). The properties of the material are altered, becoming harder and stronger with a higher value of yield stress. The ductility or malleability is decreased because the potential for further cold working is reduced. Cold working is sometimes referred to as work hardening due to the effect on mechanical properties. When mechanical work is carried out on a metal or alloy at a more elevated temperature it is possible for the metal object to change shape without any alteration in grain shape or mechanical properties (Fig. 6.5b). The temperature below which work hardening is possible is termed the recrystallization temperature. Examples of cold working in dentistry include the following.

(1)The formation of wires, in which an alloy is forced through a series of circular dies of gradually decreasing diameter. The resulting fibrous grain structure is responsible for the special springy properties possessed by most wires.

(2)The bending of wires or clasps during the construction and alteration of appliances.

(3)The swaging of stainless steel denture bases.

Since metals and alloys have finite values of ductility or malleability there is a limit to the amount of cold working which can be carried out. Attempts to carry out further cold working beyond this limit may result in fracture. This limitation

Fig. 6.4 Control of metallic grain size by controlling the rate of cooling from the melt. (a) Rapid cooling – more nuclei, smaller grains. (b) Slow cooling – fewer nuclei, larger grains.

Fig. 6.5 Mechanical work carried out on a sample of metal or alloy. (a) Below the recrystallization temperature

– produces a fibrous grain structure. (b) Above the recrystallization temperature – retains an equiaxed grain structure.

should be remembered when carrying out alterations to clasps constructed from low-ductility alloys.

If a cold-worked metal or alloy with a fibrous grain structure is heated to above its recrystallization temperature it gradually reverts to an equiaxed form and becomes softer with a lower value of yield stress but a higher ductility. Hence, recrystallization can be used as a softening heat treatment. In many applications of wrought alloys however, it is something which must be avoided because of the adverse effect on mechanical properties. If the material is maintained above the recrystallization temperature for sufficient time, diffusion of atoms across grain boundaries may occur, leading to grain growth. The effect of grain size on mechanical properties has already been discussed and it is clear that grain growth should be avoided if the properties are not to be adversely affected.

Cold working may cause the formation of internal stresses within a metal object. If these stresses are gradually relieved they may cause distortion which could lead to loss of fit of, for example, an orthodontic appliance. For certain metals and alloys the internal stresses can be wholly or partly eliminated by using a low temperature heat treatment referred to as stress relief annealing. This heat treatment is carried out well below the recrystallization temperature and has no deleterious effect on mechanical properties since the fibrous grain structure is maintained.

6.3 Structure and properties of alloys

An alloy is a mixture of two or more metals. Mixtures of two metals are termed binary alloys, mixtures of three metals ternary alloys etc. The term alloy system refers to all possible compositions of an alloy. For example the silver–copper system refers to all alloys with compositions ranging between 100% silver and 100% copper.

In the molten state metals usually show mutual solubility, one within another. When the molten mixture is cooled to below the melting point one of four things can occur.

(1) The component metals may remain soluble in each other forming a solid solution. The solid solution may take one of three forms. It may be a random solid solution in which the component metal atoms occupy random sites in a common

crystal lattice. Another possibility is the formation of an ordered solid solution in which component metal atoms occupy specific sites within a common crystal lattice. The third type of solid solution is the interstitial solid solution in which, for binary alloys, the primary lattice sites are occupied by one metal atom and the atoms of the second component do not occupy lattice sites but lie within the interstices of the lattice. This is normally found where the atomic radius of one component is much smaller than that of the other.

Solid solutions are generally harder, stronger and have higher values of elastic limit than the pure metals from which they are derived. This explains why pure metals are rarely used. The hardening effect, known as solution hardening, is thought to be due to the fact that atoms of different atomic radii within the same lattice form a mechanical resistance to the movement of dislocations along slip planes.

(2)The component metals may be completely insoluble in the solid state. Examination of a binary alloy of two metals, A and B, showing this behaviour reveals the presence of some areas containing pure metal A and others containing pure metal B. This type of alloy is susceptible to electrolytic corrosion, as described in Section 2.7, particularly if the component metals have widely differing electrochemical potentials. Complete insolubility of two metals is rarely encountered in practice.

(3)The two metals may be partially soluble in the solid state. For metals A and B two distinct phases exist within the solid state. One phase consists of a solid solution of metal B in metal A, whilst the other phase consists of a solid solution of A in B. There is a limit to the solubility of the two metals one within the other in each of the two types of grain. The solubility is temperature dependent and normally decreases markedly as the temperature is reduced from the melting point down to room temperature. These partially soluble solid solutions are far more commonly encountered than the completely insoluble material covered in the previous paragraph.

(4)If the two metals show a particular affinity for one another they may form intermetallic compounds with precise chemical formulation (e.g.

Ag3Sn). Since intermetallic compounds have specific valence requirements there are fewer crystal