Alloying (continued)

Composition, Microstructure, and the Phase Diagram
Let’s finish this discussion on phase diagrams by briefly looking at three different compositions of elements A and B, and how their microstructures will differ because of their positions on the phase diagram. First a eutectic alloy, which is an alloy with composition right at the eutectic point, will be considered. Then compositions on both sides of the eutectic point will be discussed. An alloy with a composition that lies to the left of the eutectic point on the phase diagram is called a hypoeutectic alloy, and an alloy with a composition that lies to the right of the eutectic point is called hypereutectic alloy. At this point, only the condition of slow cooling, which will allow the alloy to solidify into it equilibrium condition, will be considered. The microstructure can be controlled by manipulating the speed of cooling the alloy, but this will be covered in the section on heat treatments.

Eutectic Alloys
First, consider the eutectic alloy of elements A and B as it is cooled from a temperature at location 1 to location 4 on the phase diagram. At location 1, the alloy is at a high enough temperature to make the mixture fully liquid. The circles below show a representation of the alloy's microstructure at each of the locations numbered on the phase diagram.

At location 1, there is nothing of interest as the alloy is completely liquid. As the alloy is slow cooled, it remains liquid until it reaches the eutectic temperature (location 2) where it starts to solidify at any favorable nucleation sites. From the microstructure image 2, it can be see that as the alloy solidifies it forms into alternate layers of alpha and beta phase. This layered microstructure is known as lamellar microstructure and the layers are often only of the order of 1 micron across. The reason that a eutectic alloy forms in this way has to do with the diffusion times required to form the solid.

The grains grow by adding alpha to alpha and beta to beta until they encounter another grain (location 3). Further nucleation sites will also continue to form within the liquid parts of the mixture. This solidification happens very rapidly as any given volume of liquid in the melt reaches the eutectic temperature. Remember that a eutectic composition solidifies at a single temperature like a pure element and not over a temperature range.

As the now sold alloy cools to location 4, the composition of the layers of alpha and beta continue to change as it cools. Atoms of A and B will diffuse between the two phases to produce the equilibrium compositions of alpha and beta phase at a given temperature. By drawing tie lines at various temperatures the eutectic point on the phase diagram, it can be seen that the solubility of A in the beta phase and B in the alpha phase decreases as the temperature decreases. Since this phase composition change is due to diffusion, which is a relatively a slow process), it is important that eutectic alloys be allowed to cool slowly to produce the correct microstructure.

Hypoeutectic Alloys
Next, consider an alloy of A and B that has an overall composition that places it to the left of the eutectic point. When an alloy falls to the left of the eutectic point it is called a hypoeutectic alloy. At location 1, the alloy is at a temperature that is high enough to put it in a fully liquid phase.

When the alloy is cooled, it remains in the liquid state until it reaches the temperature where it crosses the liquidus line (location 2). At this temperature, the alpha phase starts to solidify at any favorable nucleation sites. The alpha solidifies as dendrites which grow to become grains of alpha. The first solid phase to form is called the primary phase so, in this case, primary alpha is formed.

As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and further nucleation sites will form within the liquid part of the mixture. The melt will have that mushy consistency of chunks in liquid while it is in the “alpha + liquid” region of the phase diagram. Since the alpha phase is mostly element A (with a small amount of B atoms in solid solution), the remaining liquid becomes slightly richer in B as the liquid cools, which is indicated by the liquidus line. The composition of the solid alpha phase also becomes slightly richer in B atoms as the solid solution line shows.

This primary alpha phase growth and the accompanying phase composition shifts continue until enough A atoms have been removed so that the remaining liquid is of eutectic composition. This composition is achieved at the point where the temperature crosses the eutectic line (location 4). At this point the primary alpha phase stops forming. The remaining liquid starts to solidify into the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in the eutectic regions, and new solidification sites will continue to form. Remember that solidification occurs rapidly and without the need for a further decrease in temperature once the liquid reaches the eutectic line. At this point, the entire alloy has solidified into a mixture comprised of grains of alpha and grains of eutectic mixture (alpha and beta). The microstructure from this point at the eutectic line down to ambient temperature will look something like that shown in micro 5.

Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur in the grains of pure alpha, as the composition of alpha phase also changes with temperature.

Hypereutectic
Finally, consider an alloy of A and B that has an overall composition that places it to the right of the eutectic point. When an alloy falls to the right of the eutectic point it is called a hypereutectic alloy. This alloy will solidify like the hypoeutectic alloy did except it will pass through the “beta + liquid” region of the phase diagram rather than the “alpha + liquid” region. This will result in a microstructure comprised of grains of beta and grains of eutectic mixture (alpha and beta) rather than grains of alpha and grains of eutectic mixture (alpha and beta) as the hypoeutectic alloy had.

At location 1, the alloy is at a temperature that is high enough to put it in a fully liquid phase. When the alloy is cooled, it remains in the liquid state until it reaches the temperature where it crosses the liquidus line (location 2). At this temperature, the beta phase starts to solidify at any favorable nucleation sites. The beta solidifies as dendrites which grow to become grains of beta. The first solid phase to form is called the primary phase so, in this case, primary beta is formed.

As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and further nucleation sites will form within the liquid part of the mixture. Since the beta phase is mostly element B (with a small amount of A atoms in solid solution), the remaining liquid becomes richer in A as the liquid cools, which is indicated by the liquidus line. The composition of the solid beta phase also becomes slightly richer in A atoms as the solid solution line shows.

This primary beta phase growth and the accompanying phase composition shifts continue until enough B atoms have been removed so that the remaining liquid is of eutectic composition. This composition is achieved at the point where the temperature crosses the eutectic line (location 4). At this point the primary beta phase stops forming. The remaining liquid starts to solidify into the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in the eutectic regions, and new solidification sites will continue to form. At this point, the entire alloy quickly solidifies into a mixture of beta grains and eutectic mixture (alpha and beta) grains. The microstructure from this point at the eutectic line down to ambient temperature will look something like that shown in micro 5.

Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur in the grains of pure alpha, as the composition of alpha phase also changes with temperature.