Understanding Metals: General Metallurgy, Pt. II

This installment of Paul Finelt’s Understanding Metals deals with understanding alloy phase diagrams. It is probably the most technical and complicated article of the series, but once the reader is over this ‘hump,’ practical application for the jeweler and manufacturer will follow…

Introduction
Last month we discussed 1) crystal structures; 2) alloying elements and their effect on metals in the solid state; and 3) the liquid state and solidification.

In the current article we will continue our discussion of the liquid state and solidification. We will pursue a deeper understanding of the chemical and structural changes that can and do occur in metals as they are processed by casting, rolling and forming.

Phase diagrams:
Metallurgical road maps

Phase diagrams are to the metallurgist what anatomy is to the medical profession. It would be difficult to envision developing or understanding metals without understanding the phase diagram.

Phase diagrams have their analogy in every area dealing with chemistry (e.g. ceramics, plastics) and are vital everywhere materials are developed.

Hang on…
What we are about to discuss can be extremely boring! However, if you stick by and read this article carefully, you will be rewarded with a basic understanding of the most important and interesting tool in our trade!

So… loosen up those muscles (especially the ones around the neck) and plunge into this most interesting of subjects.

Salt and water:
A common example

We previously alluded to the fact that both solid and liquid metals react in a manner similar to that of other more common materials.

Let’s review the example of salt in water. A certain amount of salt (the solute) dissolves in water (the solvent) at a given temperature. If the temperature of water is raised, the amount of salt dissolvable in the water is increased; and if the temperature is reduced, the solid salt will come out of the water solution (visible at the bottom of the container).

Also, if the temperature of the water remains the same and salt is added (stirring continuously), a maximum quantity of salt is reached that can be dissolved in the water. The solution is called saturated at that exact quantity of salt in the water. If any more salt is added at that temperature, the water/salt solution becomes supersaturated with salt. The additional salt does not dissolve, but remains as a solid at the bottom of the container.

If the water were then heated to the point where the solution would be able to hold just the quantity of salt not in solution, the supersaturated solution would become a saturated solution again.

What we have described above is the varying of temperature and composition (the amount of one element or compound).

Secrets of gold alloys
Part I: The silver and gold phase diagram (Ag-Au)

As many of us are aware, most gold alloys have silver (up to about 20%) as well as varying amounts of copper, zinc and nickel as primary alloying elements.

Keep in mind that to produce 14k gold (14/24 = 58.333% gold), 58.33% of the alloyed metal must be gold, but the balance can have any composition that will yield the desired properties of color, workability, castability, etc.

Figure I is the phase diagram for silver and gold. These two elements form what is known as a solid solution. Think about that phrase for now. We’ll fill in detail later.

Along the bottom scale of the diagram is the amount (in weight percent) of gold. Of course, the difference is silver. At the left side of the chart is temperature (larger figures are centigrade, smaller figures are farenheit).

The far left point of the diagram (100% silver) indicates the melting point of pure silver is 961.93°C, or 763.47°F.

At the far right side is the melting point of pure gold- 1064.43°C, or 1947.97°F.

The two melting points are connected by an upper solid line and a lower dotted line. These lines are technically known as the liquidus (solid line) and solidus (dotted line).

It is interesting to note that the reason the solidus is shown here as a dotted line is that researchers are not exactly sure where the solidus lies.

The liquidus is the line above which the compositions are liquid. As you might imagine, the solidus is the line below which the compositions are solid. The compositions are indicated at the bottom scale.

You will notice an “L” in the area above the liquidus line indicating that anything above the line is liquid. Below the line you will see “(Ag, Au),” which indicates that the two elements form a solid solution (we’ll discuss this in a moment).

Let’s take a look at the alloy made of 75% gold (18k). The balance is silver, since the diagram at hand is only for these two elements.

We draw a line vertically from the 75% point at the bottom scale up to the liquidus line (Fig. I, line a). Then draw a horizontal line from the point on the liquidus line over to the temperature scale at the left side of the diagram (Figure I, line b).

This temperature is approximately 1910° F.

If you take another line from the solidus to the temperature scale, you find that it is only a few degrees (F) less than the liquidus. A very small range. Hurrah!

Now you know that silver and gold melt rather quickly as they solidify (a few degrees is very quickly as alloys go).

You may have also noticed that the pure elements solidify at a point. That means they solidify instantly (no range) upon reaching their melting (or solidifying) temperature.

Take a few minutes to go back over the past few paragraphs. Make sure you understand the physical meaning of what was discussed. Notice that you now can predict the phase (solid or liquid) that the gold and silver alloys (of your choice) will have at any temperature! That is quite an accomplishment.

Solid solutions
A solid solution is very simple. It means that the elements, when solid, are in solution (dissolved) with one another. They form a homogenous “phase” or crystalline structure throughout all compositions.

As we will see, some elements are not completely soluble (do not completely dissolve) in others.

The concept of solid solutions may be a difficult one to understand (and to explain adequately). Just keep in mind that the solid stuff we work with every day is not much different from water and salt (or water and sugar for those of you on sodium-free diets).

It is also important to note that there are not many metals like this, although we will discuss at least one other alloy with similar characteristics.

Take” a rest now…relax and enjoy…then start reading about….

Copper-nickel phase diagram (Cu-Ni)

These two elements form the basis for many commercial alloys. At various compositions, they form the alloys known as Monel and cupro-nickel. Copper and nickel along with zinc form the alloy system called “nickel silver.” Take away the zinc, add a small amount of beryllium and you have the alloy beryllium copper.

With some rather substantial modification, nickel silver-type alloys form the basis of the white gold alloys you purchase from an alloy supplier. Low amounts of nickel are also found in some yellow gold alloys.

Since I have touched on the subject of commercial, purchased alloy, let me make an important statement:

The alloys you purchase from reputable suppliers to the jewelry industry have been developed over many years and as such have been proven in the field. An attempt to manufacture alloy by someone not totally familiar with the metallurgy of the alloy at hand should not be undertaken.

Your risk is ending up with scrap and gold-which may very well not be of the gold content (karat) you believe it to be. I urge you, do not take that risk. Now, on to the copper- nickel phase diagram.

This phase diagram is shown in Figure 2. Note that it looks very much like the silver-gold diagram we just reviewed. And yes, copper and nickel do form a homogenous solid solution. See, you already think you ‘re an expert. Read on.

We know that the area marked “L” above the liquidus (upper-most solid line) is in the range where the compositions will be all liquid metal. And we know that below the lower solid line (solidus), the metal will be in solid phase. (Incidentally, the solidus line is not dotted here because in this case researchers are sure where the solidus lies).

So what’s in the middle? The answer is…MUSH. In that region, both solid and liquid exist as the metal cools or is heated. Anyone who has melted knows that alloys do not solidify instantly. The time it takes for a given size casting or ingot to solidify is dependent upon more factors than are presented in any phase diagram.

However, it should be apparent from this diagram that there is definitely a “mushy” temperature range where both the solid and liquid phases coexist. It is important to understand this so that when we go on to the subjects of deoxidation and solidification, you can better visualize the physical events.

Summary
We’ve discussed the very basic aspects of phase diagrams, and some points have been made about the importance of understanding these “road maps.”

In future articles we will explore other types of alloys (a bit more complex) and discuss how different alloys solidify. We will discuss how we can minimize shrinkage defects and improve our deoxidation practice; there is a practical application to all this!

We would certainly appreciate feedback on the nature, content and usefulness of these articles. We hope that at the conclusion of this series you will have established a basic understanding of metals and how they work.

AJM September 1985

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