Table of Contents
- 1 Metals, Alloys, Grains and Crystals
- 1.1 Solids, liquids and the chemistry of metals
- 1.1.1 What’s the difference between a crystal and a grain?
- 1.1.2 Why gold is soft–How grain structure affects hardness and strength
- 1.1.3 Strengthening soft metal structures
- 1.1 Solids, liquids and the chemistry of metals
Metals, Alloys, Grains and Crystals
This series represents a mini course in dental alloys for the beginner, and persons seriously interested in gaining a basic working knowledge of dental alloys are advised to take the time to start at the beginning.
If all five pages are read in order, the reader will gain a good understanding of just what dental alloys really are, their internal crystalline structures, how they differ from each other and how different alloys are utilized in various applications.
Solids, liquids and the chemistry of metals
Although it is not readily apparent from everyday experience, metals are very much like water in that they can exist in solid, liquid and gaseous forms. Water freezes at 32°F. Below that temperature, water exists as a crystalline solid, and above that temperature, it exists as an amorphous liquid. In a crystal, the molecules take on a uniform orientation and configuration relative to each other.
What’s the difference between atoms and molecules?
A molecule is smallest physical unit of an element or compound. Compounds are chemical combinations of different elements. A molecule of water is composed of two atoms of hydrogen combined with one atom of oxygen. Thus the smallest component in water that can still be called water is the molecule H2O which is composed of three atoms. On the other hand, even though gold forms cubic crystalline units containing 14 atoms, it still retains its identity as gold as a single atom. Thus a molecule of gold is composed of a single gold atom.
Water forms hexagonal structures and these are familiar to everyone in the form of snowflakes. On the other hand, when ice melts, it turns back into water in which the molecules lose their ordered configuration and become an amorphous jumble. (Amorphous means “lacking a definite form or shape”.) The transition between water and ice happens at a definite temperature because all the molecules and the potential bonds between them are identical, so the transition happens under uniform conditions. Of course the temperature does not change instantaneously throughout any ice mass, so near the melting temperature, some water will be found in the form of ice, and some in the form of liquid water. In a situation like this, the “slush” is in a multiphasic state, in this case with two phases; ice and water. The solid ice particles are called grains, and as the temperature drops, these grains grow larger as more and more water molecules adhere to the growing crystals of ice. Each grain is composed of a single fairly continuous crystalline structure.
The analogy between metals and water is fairly exact. All metals have definite melting temperatures, above which they exist as amorphous liquids and below which they exist as crystalline solids. Like ice, when cooled slowly from its liquid state, a metal will form crystals slowly throughout a melt which contains both a liquid metal phase and solid crystalline grains. The grains form and grow separately throughout the liquid phase until the entire system “freezes”. The grains freeze in random orientations and the size of the grains will depend on the length of time they were allowed to grow before they were frozen in place. Thus, the microscopic structure of a solid metal will display a random jumble of grains of different sizes randomly oriented throughout the metallic mass.
And like aqueous solutions, liquid metals may be mixed together. Some metals are soluble in each other, while others are partially soluble at lower temperatures and insoluble at higher temperatures. Some metals, while in the molten state, will chemically react with others to form new chemical compounds. When two or more molten metals are mixed together and allowed to cool to a solid crystalline state, the result is called an alloy.
Solid alloys form mixed crystalline structures with complex microscopic internal structures composed of grains from various phases. (The image above shows a polished, etched alloy made of nickel and iron from a meteor. This alloy is thought to compose most of the earth’s solid core.) The melting temperature of each phase differs from that of the others depending on its chemical composition. Thus each type of grain in the body may have a different shape, depending on its chemical composition, as well as a different size and orientation.
Say you have two metals, A and B. They are partially soluble in each other and react to form a third substance, C, which is a compound of A and B. All three substances will constitute three separate phases, and the solid alloy will contain separate grains of each, all jumbled together as in the image above.
What’s the difference between a crystal and a grain?
Metal atoms have large numbers of electrons in their valence shell. These become delocalized and form a “sea” of electrons surrounding a giant lattice of positive ions. Metallic bonds, therefore are something like covalent bonds except that large numbers of electrons are shared by massive numbers of atoms. This trading back and fourth of electrons is what holds metallic crystals together, sort of like a massive, communal, covalent group hug. As you will see below, each metal forms a specific type of crystalline structure based upon the internal atomic properties for that metal.
Given enough time and ideal conditions, the crystal lattice can grow to be very large with a perfect internal crystalline structure. A single crystal of any metal could theoretically grow to be infinite in size. Nature, however, seldom provides ideal conditions for any project, and the reality is that almost every metal exists in a polycrystalline state composed of a jumble of crystals at odd angles and of varying sizes. When this happens, each individual crystal in the body is called a grain.
Each grain, however, is usually not a perfect crystal. Growth of crystals in nature does not proceed in a regular fashion. Instead, growth is likely to be more random with some positions in the lattice left vacant and other positions in which atoms are located in irregular places within the lattice. Thus grains are fairly regular crystalline structures, but with lots of imperfections which distort the crystal lattice.
The physical properties of any given alloy depends to a large extent on the nature of its internal microscopic crystalline structure, and this can be seriously affected by factors such as the speed at which it is allowed to cool from its molten state as well as subsequent heating and cooling cycles.
The phases in a cooling metal solution separate out into tiny grains which are evenly distributed throughout the alloy. The size of the grains depends on the speed of cooling. In general, the alloys that have the least permanent deformation during service also have finer grain structures. Thus small grain size is an advantage in a dental alloy. The longer it takes for an alloy to cool down from its molten state to its solid state, the more time the grains have to grow, and the larger they will become. So smaller grain size is achieved by rapid cooling of the molten metal. Another trick is the inclusion of tiny amounts of a very high melting metal such as iridium, rhenium, or ruthenium. These metallic elements are used as “grain refiners” and they solidify very early in the cooling process. They then act as nuclei around which the other metal grains can form. Click here for more on grain refiners.
Why gold is soft–How grain structure affects hardness and strength
Pure gold, all by itself is fairly soft and malleable. It is not a suitable material for large restorations or denture frameworks because its softness (malleability) leads to serious wear and deformation while in service in the mouth. On the other hand, the addition of very small amounts of soluble metals into a solution of molten gold creates a much harder alloy. Pure cast gold is only one fifth as strong and one sixth as hard as a typical gold based casting alloy. In order to understand why this is so, it is necessary to delve into the structure of crystals and grains.
Gold forms a face centered cubic crystal. Of course not all metals form this shape in crystalline form. Some form hexagonal platelike shapes, some form long needles. But face centered structures are common in metallurgy, and it is a form shared by gold, palladium, platinum, nickel and silver. The diagram on the left shows what a face centered cubic form would look like if you could see all the atoms that make it up, but it is a bit confusing so the diagram on the right is provided to make it easier to conceptualize.
A single crystalline unit like the face centered is quite strong and difficult to break apart, however the bonds between the atoms are able to stretch to a certain extent. A force applied to top left layer of of atoms in single face centered cube might temporarily distort the cubic form, but it would bounce back once the force ceased to be applied. Non-permanent distortion of this sort is called elastic deformation.
During the cooling of any metal, the naturally occurring crystalline structures stack together into larger and larger crystals. The shape of any crystal depends on the natural shape of the native crystalline structure. A face centered cubic crystal will extend in all directions forming a larger and larger cube until it bumps up against another crystal growing in a different orientation. Silica (Silicone Oxide), has a tetrahedral (pyramid shaped) molecular structure and forms six sided crystals with six sided pyramids on top. In general, each grain of any given substance will maintain a fairly coherent crystalline structure, and the difference between one grain and its neighbor is mostly in the orientation and size of each crystal.
When enough shear (side to side) force is placed on a perfect crystal, the individual molecules that make it up begin to slip past each other causing a permanent deformation. This form of slippage involves offsetting the molecular units one or more places along the natural planes that make up the crystalline lattice. Owing to the strength of the atomic bonds that keep the crystalline structure in its pristine state, a lot of force must be applied to make a pure crystal of any material deform permanently.
Most crystals, however, do not form perfectly. Frequently, there are vacancies in the atomic lattice and these may configure themselves in a number of ways.
Sometimes they arrange themselves as point defects remaining as single point imperfections in an otherwise perfect crystalline lattice. More frequently, they may alter the arrangement of other parts of the lattice that radiate away from them as in the diagram above. This type of defect, known as an edge dislocation, weakens the crystalline structure greatly. Edge dislocations are especially frequent in face centered cubic crystalline elements such as gold, and this is a major reason that gold is such a plastic (soft) metal. As you can see from the diagram, the hour glass structure caused by the missing line of atoms in the lattice causes bending of the interatomic bonds between neighboring atoms in the lattice. This bending causes an elastic deformation which wants to relieve itself, allowing the bent interatomic bonds to become straight again. However, since the defect is firmly embedded inside the lattice, it cannot do so unless energy is added to help it along.
Edge dislocations can be relieved is by placing shear stresses on the crystalline lattice. Shear is graphically demonstrated in the image of the red faced cubic crystal above. Because of the presence of the edge dislocation, the amount of shear force necessary to cause a permanent plane slippage is much less than it would be in a perfect crystal lattice. Depending on the metal involved, it can take a little as 0.5% of the force to permanently distort an imperfect crystalline structure as it would to distort a perfect crystal of the same metal.
When shear force is applied as in the diagram above, (courtesy Phillips Science of Dental Materials, eleventh edition) the horizontal plane containing the vacancy becomes the slip plane. If a sufficiently large shear stress is applied across the top and bottom faces of the metal crystal as shown, the bonds in the row of atoms adjacent to the dislocation are broken and new bonds are forged with with the next row, resulting in movement of the dislocation by one interatomic distance. If the force is continued, this process happens again and again until the dislocation reaches the boundary of the crystal.
Strengthening soft metal structures
It is apparent from the above discussion that there is little to hinder the movement of edge dislocations in pure metal grains. Edge dislocations are the dominant carrier of plasticity in pure metals. Therefore, in order to strengthen a metal body, a mechanism must be found to impede the progression of edge dislocations. There are three major ways to do this:
1.Cold working, or strain hardening
These terms are defined as mechanical deformation below the recrystallization temperature. It consists of applying shear stresses to the metal body. The process of applying shear forces across imperfect crystalline lattice structures causes permanent deformation by working edge dislocations to the boundaries of a grain. It also relieves stresses in the lattice and allows the re-formation of a more perfect, and therefore harder crystalline structure as is apparent from the diagram above. A more perfect crystalline structure is harder than an imperfect one containing edge dislocations, but it is also more brittle as well.
Shear forces can be applied to a metal in several ways.
Bending is one simple way to apply shear. When a soft metal wire is bent repeatedly, the edge dislocations in the area of the bend are worked out of the crystalline lattice causing it to become harder and more brittle. As bending continues, numerous micro cracks develop within the grains causing them to break into many smaller grains. As the number of micro cracks multiply, the grains rub against each other causing friction and heat, and eventually the wire breaks.
The most common industrial process used to harden metals in this way is called “forging” . This is done with hydraulic presses, pounding the metal with hammers or running it through rollers to flatten or further shape it. This process causes the grains to deform in the shape of the finished object. Wrought wire is made this way and is composed of grains that look like bundles of spaghetti running along the length of the wire. The term “wrought” is an archaic past tense of the verb “to work”. Wrought wire is cold forged, but many industrial processes heat the metal object while it is being forged in order to soften it and to avoid brittleness during prolonged forging operations. This is the origin of the village blacksmith and his forge.
After forming a more perfect, harder metallic structure, forging leads to the addition of vast numbers of additional dislocations throughout the increasingly brittle metal body. These new dislocations appear in the form of micro cracks throughout the grain structure. This causes the large grains to break into smaller grains which further strengthens the metal. (See #2 below to see why.)
Grain boundaries block the movement of dislocations. Thus, one of the keys to strengthening metal objects is to force the formation of smaller grains throughout the body as it cools below its melting temperature. This is done in a number of ways. The most common method is to cool the metal very quickly after it is cast. This is done in dental labs by quenching the still hot, invested casting in cold water. This not only shatters the investment away from the casting, but it also “freezes” the casting before the crystals have had time to grow to substantial size. Another method of achieving small grain size is to add small quantities of grain refiners such as ruthenium, iridium and rhenium to the alloy. These have a high melting temperature and crystallize before the other phases of the metal. The vast numbers of these tiny crystals forces the formation of very small crystal grains in the other phases that form around them.
3. Formation of an Alloy
Mixing two or more molten metals together forms an alloy. An alloy of two soft metals creates a much harder structure than either of them alone. Silver and tin, two soft metals, when alloyed will form pewter. Pewter was, for centuries, the basis of high class, unbreakable tableware among wealthy people.
Whenever two or more molten metals are mixed together and allowed to cool, the resulting grain structure becomes very complex because each metal will solidify into grains with inherently different shapes and sizes. This is complicated by the fact that the metals may chemically combine to form a third phase, or sometimes even more. The close approximation of grains precipitated from different phases makes the slippage of edge dislocations within any grain difficult. This hardens the alloy.
Finally, each grain of any phase will include atoms from each of the other phases. Grains contaminated with foreign atoms are called solid solutions. Foreign molecules within an otherwise pure grain cause localized distortions of the crystal lattice. However the presence of the foreign molecule within the lattice acts as a sort of lock and key preventing the movement of slip planes and further hardening the alloy.