Copper Nickel

8/2/2019 Copper Nickel

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Copper - Nickel Example

Since my example problem is supposed to be an introduction to thephase diagram, its

calculations, and how it is perceived, most of my text is taken from Callister.

figure 1

This figure contains the copper-nickel phase diagram. Its system is termed as beingisomorphus.

A good interpretation of a binary phase diagram that is easy to understand and interpret is theCu-Ni system. This diagram has three differentphaseregions, the alpha region, the liquid region,

and the alpha + liquid region, which are defined by specificcompositionsand temperatures as

illustrated in figure 1. Both points A and B are located in the alpha and the alpha + liquid regionsrespectively. The phase boundaries are separated by two lines. The line separating the liquid and

the alpha + liquid regions is theliquidous line. The line separating the alpha and the alpha +

liquid regions is thesolidous line. The intersection of these two lines signify the meltingtemperatures of the twoconstituentsindividually. The Cu-Ni system is especially noted for its

complete liquid and solidsolubilityof its constituents, and is thusly identified as anisomorphous

system.
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figure 2

A section of the copper-nickel phase diagram is contained here. Both the compositionsand the phase amounts of each constituent will be determined at point B.

The determination of phase compositions in a single phase, is just how much of each phase ispresent at a given temperature. In figure 1 at point A, the alpha region is the only phase present;

therefore, the composition at 1100 degrees Centigrade is a weight percent of sixty for Ni and

forty for Cu. For a double phase region, the procedure intensifies. One has to first draw a

horizontal line at the given temperature from the first phase boundary to the second phaseboundary, which is depicted at point B in figure 2. This line is defined as thetie line. Next, at the

intersection of each phase boundary with the tie line, vertical lines are drawn straight down until

they intersect the x-axis where the composition of each constituent is identified. The compositionof the liquid phase, CL, is 32 wt% Ni - 68 wt% Cu. Thusly, the alpha phase, CALPHA, has a

composition of 43 wt% Ni - 57 wt% Cu.

Through the use of thelever rule, one can also determine the amount of each phase present. The

lever rule is an expression which allows one to compute the phase amounts in a two phase alloy

equilibrium situation. In a one phase region, thealloyis composed of 100% of that phase, suchas the alpha region in figure 2 at point A. For a two phase region, one has to rely on the

combination of the tie line and the lever rule expression.

As stated previously with the determination of phase compositions, the tie line is drawn at the

specified temperature between the two-phase region and the composition of the alloy is

identified (figure 2). The percentage of one phase is calculated by dividing the length of the tieline from the overall composition of the alloy to the phase boundary for the second phase by the

length of the total tie line, which is denoted by subtracting the compositions of the constituents,
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and multiplying by one hundred. The percentage of phase two is calculated by using the same

procedure.

Let's take a look at computing both the alpha and the liquid phases (figure 2). The tie line is

drawn at point B, which is specified at twelve hundred fifty degrees Centigrade. The overall

composition of the alloy is 35 wt% Ni, the composition of the alpha phase is 43 wt% Ni, and thecomposition of the liquid phase is 32 wt% Ni. (It is important to note that the composition for a

binary alloy needs to be specified in terms of one of the constituents.) The mass fraction for theliquid may be computed by:

You may prefer the use of one equation over another; however, both render the same solution.The mass fraction for the alpha phase maybe computed similarly.

Again, both equations render the same solution. Conclusively, at equilibrium when both thetemperature and the composition are known in a two-phase region for a binary alloy, then the

lever rule may be utilized to assist in the calculations of the relative amounts or fractions of

phases.

Now that an interpretation of the Cu-Ni System has been given, we need to take a deeper glance

into the development of microstructure that occurs for isomorphus alloys during solidification.

figure 3


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This figure represents the development of microstructure during the equilibriumsolidification of a 35 wt% Ni - 65 wt% Cu alloy.Movie* Note: You will need and

MPEG viewer to play this movie!

At thirteen hundred degrees Centigrade with an alloy composition of 35 wt% Ni - 65 wt% Cu,

we will observe the cooling process slowly (figure 3). Equilibrium will be continuous in thegiven phase as long as cooling occurs very slowly. At point a, the alloy is completely liquid. This

microstructure, which is represented by the circle inset in the figure, can be viewed by clickingon point a in figure 3. As the cooling process takes place, we reach the liquidous line. Here we

reach a change in both the microstructure and the composition of the alloy. Point b is located

roughly around twelve hundred seventy degrees Centigrade with its composition defined by thetie line. By clicking on point b in figure 3, one is able to see that the first solid (alpha) begins to

form. As cooling continues from this point further, both compositions and relative amounts of

each of the phases will change. With continued cooling, the fraction alpha phase will increase, as

the fraction of the liquid will decrease. Its microstructure can be viewed by clicking on points c,d, and e in figure 3. It is obvious that the compositions and relative amounts of each phase will

change; however, the overall alloy composition does maintain consistent at 35 wt% Ni - 65 wt%Cu.

Point c signifies that the cooling process is half complete. The microstructure displays an

approximate equal amount of alpha and liquid. At point d, there is a definite increase in alpha.Very little liquid can be viewed. Finally, point e is located after crossing the solidous line. Here

the remaining liquid solidifies. The outcome has a uniform 35 wt% Ni - 65 wt% Cu composition

which is then a polycrystalline alpha-solid solution (point e, figure 3). If the alloy is cooled

beyond point e, there will be no microstructural or compositional changes.

The solidification process is very extensive for equilibrium conditions must be maintained at all

temperatures. Cooling has to take place slowly in order for the readjustments to occur in thecompositions of the two phases, which will yield in the one phase diagram previously viewed.

Thediffusionprocess is the driving mechanism for the composition readjustments. Just as

diffusion depends on time in order to maintain equilibrium during cooling, the compositionreadjustments need sufficient time.

Example:

Problem

For a 35 wt% Ni - 65 wt% Cu alloy at twelve hundred fifty degrees Centigrade, what phases(s) is

(are) present? What is (are) the composition(s) of the phase(s)? Calculate the relative amount ofeach phase present in terms of mass fraction.

Solution

(a.) Locate this temperature-composition point on the phase diagram (point c in figure 3). It is

located within the alpha plus liquid region; therefore, both the alpha and the liquid phases will

coexist.
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(b) Since two phases are present, it becomes necessary to construct a tie line across the alpha

plus liquid region at twelve hundred fifty degrees Centigrade, as indicated in figure 3. Thecomposition of the alpha phase corresponds to the tie line intersection with the alpha per alpha

plus liquid solvus boundary about 43 wt% Ni - 57 wt% Cu. Similarly for the liquid phase, which

will have a composition of approximately 3 wt% Ni - 70 wt% Cu.

(c) Since the alloy consists of two phases, it is necessary to employ the lever rule.

There are variousapplicationsof the isomorphus Cu-Ni phase diagram that may be applied to

every day experiences contained within this section.

Additional Information:

Various mechanical properties, which are explored in depth in MSE 3305 - Structure Property

Relationships, of solid isomorphus alloys are affected by composition as other structural

variables (e.g., grain size) are held constant. Below the melting temperature of the lowest-

melting component, for all temperatures and compositions, only a single phase will exist. As a

result, an increase in strength and hardness, or solid solution hardening will be experienced by

each component through additions of the other component.
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Copper Nickel