Scientia Sinica

30 i SCIENTIA SINICA Vol, V.

martensite in the specimen begins to decompose to form low-carbon martensite (containing 0.25% C) and €-carbide. When the temperature continues to rise, more €-carbide is formed in the specimen, so the internal friction increases with the rise of temperature. However, as the temperature becomes too high, the coherency between €-carbide and its parent phase will be easily destroyed. The optimum temperature of the peak is seen to be about 130°C. This may be due to that around this temperature, the €-carbide formed is plentiful and at the same time its coherency with its parent phase has not been destroyed much. In fact, the position of this internal friction peak changes with the speed at which the internal friction measurements were taken,

The stress-induced movement of a surface of coherency is associated with a definite time of relaxation, and should give rise to a stable internal friction peak. The constant changing of the observed 130°C peak is evidently due to the two opposing tendencies described above.

Work is in progress in utilizing the 130°C peak in studying the kinetics of the formation of €-carbide and the destruction of its coherency with its parent phase.

Temperature (°C) 220 200 180 160 140 120 100 80 60 40

0.007

0.006

0.005

0.004

0.003

Internal Friction (Q-!)

0.002

0.001

230 Dull 2:2, 2:3) 2425) 26 218 9 310) WS Ba) 3s 1000/1 —>

Fig. 9. Internal friction peak in a 0.72%C€ specimen air-cooled from 910°C and its disappearance with lowering temperature.

2. Some additional internal friction peaks

A specimen containing 0.72% C was air-cooled from 910°C. The internal friction measurement on this specimen gave some inexplicable results. A pronounced internal friction peak was observed around 110°C (the frequency of vibration used remained to be 2 cycles per second as before) as shown in Fig. 9. This peak is characterized by the fact that it does not rise again at