The changes in properties due to each stage of the annealing process are demonstrated by the three-point bending graphs. Apparatus and method for this technique are covered in the The structure and mechanical behaviour of wood TLP.
These results show that at higher annealing temperatures, the work for plastic deformation decreases. However, it is difficult to see from the graph how much the microstructures have changed during the annealing.
Referring to the micrographs below, it can be seen that the microstructure of the work hardened copper specimen has small grains, and there is a directionality to the grains - they are relatively long and thin. In the specimen annealed at 350°C, the material has recovered and recrystallised, as seen by the slightly larger, equiaxed grains. The specimens which have been heated at 500°C and 800°C have a similar shape of grain, but grain growth has occurred, and as such, the grains are larger as the temperature of anneal increases.
Relating this to the graph, it can be seen that recrystallisation reduces the yield stress. This is because there has been a large reduction in dislocation density. As a result the remaining dislocations are much more mobile since there are not as many pinning points, and so slip occurs at a much lower stress. Grain growth also has a significant effect on the plastic flow, with the increase in size reducing the flow stress.
In this experiment it has not been possible to observe the effects of recovery to the properties of a material. It is likely that the work hardened specimen has already recovered at room temperature due to the high purity of the copper.
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Work hardened copper |
Copper annealed at 350°C |
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Copper annealed at 500°C |
Copper annealed at 800°C |
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(Click on an image to view a larger version.) |
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