BCC to FCC

crystalline phase change - body and face center cubic - iron crystal structure

What it shows:
Iron atoms are arranged in a body-centered cubic pattern (BCC) up to 1180 K. Above this temperature it makes a phase transition to a face-centered cubic lattice (FCC). The transition from BCC to FCC results in an 8 to 9% increase in density, causing the iron sample to shrink in size as it is heated above the transition temperature.

How it works:
A three meter length of iron wire is horizontally stretched above the lecture bench. A Variac supplies the adjustable AC heating current. As the current is increased, the wire will heat up, expand, and sag. The hotter, the more the sag.

If the wire is heated to below the transition temperature and allowed to cool (heating current turned off), the wire shrinks back to its original length as is evident by a reduction of the sag to its original. However, if the wire is heated to a temperature above 1180 K and then allowed to cool, it behaves in a remarkable way. Initially there is a reduction in the sag as it begins to cool (no surprise). But when it reaches the transition temperature and goes from FCC to BCC, its density decreases, resulting in an increase in overall length (about 2%) and a visible increase in the sag. As it continues to cool back to room temperature the wire shrinks back to approximately its original length. Note that the increase in sag (at the transition temperature) happens very quickly and it is helpful to repeat the demonstration for the class.

Setting it up:
Use music or "piano" wire. 1   Gauge no. 12 (0.029" dia) works well. The wire is secured to lab clamps/supports on top of the lecture bench. Run hefty leads from the Variac to the wire as quite a bit of current will be required (˜ 9 amps). Set the Variac voltage to 0, switch on the Variac, and then increase the voltage slowly to about 95 volts. 2   The wire will glow red hot. Note the amount of sag and then switch off the Variac. The sagging wire will rise a few centimeters, halt momentarily and drop two to three centimeters, and then continue rising back to its original height. A vertical scale (meter stick) placed behind the wire may help to make its motion more evident. A small weight suspended from the middle of the wire by a paper clip also aids visibility.

Comments:
This demonstration has been around for some time (see Sutton, for example). The BCC phase is known as the alpha or ferrite phase of iron; the FCC phase is known as the gamma or austenite phase. Rating ***

References:
A.A. Bartlett, "Demonstration of a crystalline phase change in a solid," The Physics Teacher 13, 545-547 (1975).

R.M. Sutton, Demonstration Experiments in Physics, (McGraw-Hill, NY, 1938) p 197.

1 Precision Brand Products, Downers Grove, IL 60515. Gauges run from 4/0 (0.006" dia) up to no. 45 (0.180" dia).
2 Switching the Variac on otherwise will blow the 10Amp fuse; when the wire is relatively cold, its resistance is quite low and it will draw upwards of 15Amps if the voltage is turned up. Make sure the Variac has a slow-blow fuse.