Melting Range
This is the temperature interval over which the alloy becomes molten. The low temperature, or solidus, is the temperature below which the alloy is completely solid. The high temperature, or liquidus, is the temperature above which the alloy is fully liquid. Between these temperatures, both solid and liquid metal exist.

	The melting range tells us a number of important things about an alloy.
	Casting temperature is typically 100-200°F above the liquidus.
	Higher melting range alloys typically require higher burnout temperatures.
	Alloys with liquidus temperatures much higher than about 2100°F must be cast into high heat (phosphate) investments.
	Alloys with liquidus temperatures much higher than about 2100°F are best cast with oxygen fired torches.
	The liquidus temperature of a solder should be below the solidus temperature of the alloy to be soldered.
	High solidus alloys may be (but not always are) less prone to distortions during porcelain firing.

Grain size
Most solid materials are crystalline, and most metallic materials are polycrystalline. This means that the bulk of the solid is made up of numerous tiny interlocking crystals of alloy.

Grain boundaries (the surfaces at which grains meet each other) typically behave as defects in the solid. Grain boundaries may be weaker than the grains themselves, for example, or non-metallic inclusions in the alloy may accumulate at grain boundaries. As the grain size decreases, the mechanical properties improve as a result of the reduction in scale of this network of defects. In effect, the weaknesses inherent in the alloy are distributed or "averaged out." A fine grained alloy will have higher Yield Strength, Ultimate Tensile Strength, Hardness, and Elongation than an alloy of the same composition with a coarser grain size.

For the same reasons, finer grain alloys tend to be less prone to hot tear cracks during casting and solidification than are coarse grain alloys. Hot tears are grain boundary cracks; by making each single grain boundary less critical to the integrity of the solid, small cracks at grain boundaries have less catastrophic effects on the entire casting.

Hardness
The hardness of a material represents its ability to resist abrasion or indentation from a harder material. This concept is reflected well by one of the original hardness scales which most people heard about at some point in grade school, called "Moh's scale. Moh's scale is the one that tells us that Diamond is the hardest material on earth. This was determined by rubbing two rocks together, and seeing which one left scratches on the other. Diamond scratches ruby, therefore diamond is harder than ruby. Ruby scratches Topaz, therefore ruby is harder than Topaz, and so on.

While Moh's scale is easy for us to understand, it is not very practical for dental alloys because most alloys are somewhere between Moh's 4 and 5, and the simple scratch test can not distinguish one from another. Instead, we use the Vickers hardness test. In the Vickers test, a small pyramid shaped diamond is pressed into the metal surface with a fixed amount of force. The indentation left in the metal is then measured. Larger indentations are made in softer materials, and smaller indentations are left in harder materials.

While in general, harder materials have higher yield strengths than softer materials, technicians should be cautioned that this is not always so, especially when comparing alloys of different types. Additionally, the elongation usually drops as hardness increases, which means harder alloys may be more brittle than softer alloys.

The hardness value by itself does not indicate an alloy's ability to support a load, or its ability to resist fracture.

Yield Strength
This is the force required to initiate a permanent deformation in an alloy when pulled in tension. It represents the load carrying capacity of the alloy, and is measured in the tensile test.

To understand the yield strength, one must first understand a little of how materials behave when they are stressed. When a uniform bar of an alloy is pulled in tension, two important things happen. Obviously, the stress on the bar rises. (Stress is defined as the force per cross sectional area) Less obviously, the length of the bar increases. A very slight increase in length is measurable, which increases proportionally as the magnitude of the stress increases. If the stress were to be removed, the bar would return to its original length. The distortion at this point is reversible, or elastic.

If the stress rises to sufficient value, a sudden extreme elongation will occur. This extreme elongation is no longer proportional to the stress, and it is no longer reversible. If the load were now removed, the length of the bar would be longer than it originally was. The bar is said to have yielded, and the stress level at which this yielding occurs is the Yield Strength.

Ultimate Tensile Strength - This is the maximum stress level which an alloy can withstand prior to fracture.

Consider a uniform bar of alloy, pulled in tension. As stress rises, the bar stretches slightly until the yield strength is reached, at which point a sudden extreme elongation occurs as permanent deformation sets in. Typically, the stress will continue to rise (as the tensile elongation deformation takes place) until just prior to fracture. The Ultimate Tensile Strength is that highest stress measured.

Modulus of Elasticity
This is a measurement of the stiffness of an alloy. Just like a glass rod is more stiff than a paper clip, some alloys are more stiff than others.

The modulus is the proportional relationship between stress and distortion prior to yield. It is the amount of elastic or reversible elongation which occurs as the result of the stress applied.

The stiffness of an alloy is independent of hardness, yield strength, elongation, and ultimate tensile strength. High strength or very hard alloys are not necessarily very stiff. Modulus is actually most closely related to the Coefficient of Thermal Expansion - both are primarily dependent upon the composition of the alloy. Palladium is the main agent used for increasing modulus; higher palladium alloys tend to have higher moduli, while high gold alloys tend to have lower moduli.

The modulus of elasticity is considered a rather important property for PFM work, and many consider it important in regards to implant structures as well.

The modulus represents the amount of distortion that occurs under a given stress, when that stress is below the yield strength. A lower modulus alloy will deflect more under the same stress than will a higher modulus alloy. Although this distortion is reversible and disappears once the stress is removed, the distortion experienced during the maximum load must remain very small or else the brittle porcelain fired to the alloy will crack.

In regard to implants, the stiffness issue may be related to how occlusal forces are distributed to multiple implants. There are two theories on this topic.

	Stiffer alloys are more suited for implant work because flexing under load is minimized, thus putting less localized stresses on individual implants on a multi-implant framework. Loads are distributed better on all implants.
	Less stiff alloys are more suited for implants because some of the occlusal forces are absorbed by the elastic flexing of the alloy.

Fortunately, the dental technician is not forced to answer this debate, because the range of moduli on alloys suitable for implant work is really quite limited. The technician often needs to balance many design parameters, including porcelain compatibility, melting range, cost, solderability/weldability, and any minimum gold content requirements placed on him or her by the implant component supplier (many will recommend, for example, the use of alloys with minimum gold of 50%-75%). In the end, the technician usually has a choice of only one or two alloys, and both typically have similar moduli.

As with the CTE, small differences in composition do not make for large differences in modulus. Differences in moduli seen between manufacturers for alloys that appear quite similar in composition are most likely due to the strong dependence of this measurement upon the equipment used for the test.