STRUCTURE OF METALS AND ALLOYS I
In this lab, we will take a close look at the structures of the metallic elements and a class of alloys known as solid solutions.
A majority of the metallic elements exhibit one of three structure types:
| Structure Type | Strukturbericht Type | Space Group | Atom Positions |
|---|---|---|---|
"bcc" |
A2 |
I 4/m 3bar 2/m |
2a |
ccp |
A1 |
F 4/m 3bar 2/m |
4a |
cph |
A3 |
P 63/m 2/m 2/c |
2c |
Build ball and stick models of these three structure types from the structural information given. Make one or two unit cells. In some cases, more than two cells will be helpful. Study the models carefully. Consider that each structure actually consists of hard sphere atoms which are touching wherever possible. Present drawings of the positions of one set of the most densely packed planes in the unit cells of the three structure types in your notebook. Calculate the packing factor (the space actually occupied by the hard sphere atoms/total space available) for each structure type. Which of these types is (are) close-packed? What different types of close packing are possible? (The answer is greater than 3.) How many actually exist for the pure metals? (Some library research will be necessary here.)
(Put all of your calculations, comments, and answers in your notebook and clearly mark what they are so that they can be graded.)
After the first session of this lab, look up the structure data for Sm, and construct a drawing in your notebook which shows the salient features of this structure. (Remember: structure means show ATOMS.) Why does Sm have this structure?
STRUCTURE OF METALS AND ALLOYS II
Two types of solid solutions are substitutional and interstitial. Carbon dissolves interstitially into iron. However, iron is allotropic, and, as expected the solubility of carbon in the various forms differs. Use the structure models constructed previously to calculate the sizes of both the octahedral and the tetrahedral (not necessarily regular) interstices for alpha (A2) and gamma (A1) iron. What do you think the relative solubilities of carbon in both forms of iron are? Presnt your results in a table. Do the values for the packing fraction suggest anything about the solubilities?
Put all of your calculations, comments, and answers in your notebook
and clearly mark what they are so that they can be graded. Turn in your
notebook at the next lab meeting.