Dislocation glide as an aid to precipitation at low temperatures
✍ Scribed by J.C. Fisher; J.H. Hollomon
- Publisher
- Elsevier Science
- Year
- 1955
- Weight
- 133 KB
- Volume
- 3
- Category
- Article
- ISSN
- 0001-6160
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✦ Synopsis
3 10 20 30 40 60 100 Time in Minutes FIG. 2. Results of measurements.
the ordinate indicates the position of the wire to which it elastically sprang back after the applied torque was released.
It is seen that no abnormal after-effect is observed with the aluminum wires which untwist at room temperature (the first 20 minutes) and continue to untwist at high temperature. The copper and silver wires behave di~erently since they untwist at room temperature and then reverse when heated up. The effect is more pronounced in silver where the amount of further twist approaches 0.1 per cent surface shear strain. The most peculiar effect is observed with the Nichrome wires. As all the other wires, they exhibit a normal after-effect at room temperature.
However, when heated up, they first twist further by a relatively large amount for a short time and then gradually untwist.
It may be that these phenomena can be explained in a way similar to that with which C. S. Barrett has explained his effects. Dislocations pile up under the oxide layer near the surface as a result of the plastic deformation. When the metal is heated, the stress needed for the dislocations to break through the oxide layer be-,comes smaller than the stress at the spearhead of the dislocation pile. Dislocations will then break through to the surface, and the wire will twist further, as is the'case in copper and silver, In aluminum this does not occur probably because of the much stronger oxide layer. The behavior of the Nichrome is more complex.