Question 5.1: Diffusion of Ar/He and Cu/Ni Consider a box containing an im......
Diffusion of Ar/He and Cu/Ni
Consider a box containing an impermeable partition that divides the box into equal volumes (Figure 5-3). On one side, we have pure argon gas; on the other side, we have pure helium gas. Explain what will happen when the partition is opened. What will happen if we replace the Ar side with a Cu single crystal and the He side with a Ni single crystal?
Before the partition is opened, one compartment has no argon and the other has no helium (i.e., there is a concentration gradient of Ar and He). When the partition is opened, Ar atoms will diffuse toward the He side, and vice versa. This diffusion will continue until the entire box has a uniform concentration of both gases. There may be some density gradient driven convective flows as well. If we took random samples of different regions in this box after a few hours, we would find a statistically uniform concentration of Ar and He. Owing to their thermal energy, the Ar and He atoms will continue to move around in the box; however, there will be no concentration gradients.
If we open the hypothetical partition between the Ni and Cu single crystals at room temperature, we would find that, similar to the Ar/He situation, the concentration gradient exists, but the temperature is too low to see any significant diffusion of Cu atoms into the Ni single crystal and vice versa. This is an example of a situation in which a concentration gradient exists; however, because of the lower temperature, the kinetics for diffusion are not favorable. Certainly, if we increase the temperature (say to 600 °C) and wait for a longer period of time (e.g., ∼24 hours), we would see diffusion of Cu atoms into the Ni single crystal and vice versa. After a very long time, the entire solid will have a uniform concentration of Ni and Cu atoms. The new solid that forms consists of Cu and Ni atoms completely dissolved in each other, and the resultant material is termed a “solid solution,” a concept we will study in greater detail in Chapter 10.
This example also illustrates something many of you may know by intuition. The diffusion of atoms and molecules occurs faster in gases and liquids than in solids. As we will see in Chapter 9 and other chapters, diffusion has a significant effect on the evolution of microstructure during the solidification of alloys, the heat treatment of metals and alloys, and the processing of ceramic materials.