Compression tests, using computerized deformation apparatus, were performed to clarify the anomalous positive temperature dependence of yield stress in hypo-and hyper-stoichiometric $Fe_3Al$ intermetallic compounds. The anomalous yield behavior tested at aging temperature showed great differences depending on the off-stoichiometric compositions or phase transformation modes. For the first order transformation alloy with 24.1% Al hypo-stoichiometry, a peak in yield stress was observed in the $α +DO_3$ two phase field, and the peak shifted to the lower temperature side with increasing aging time. Microstructural evidence showed that change in the degree of order played an important role in the behavior before phase separation, while precipitation of α phase had an additional influence on the behavior after phase separation.
In contrast, for the second order transformation alloy with 27.2% Al hyper-stoichiometry, a peak in yield stress was observed in the B2 single phase field independent of aging time, and only the change in the degree of order was a dominant structural factor responsible for the behavior. Transmission electron microscopy was carried out in order to identify the type of dislocations governing this behavior. The result indicated that the positive temperature dependence of yield stress was due to the generation of the NNN APB(next nearest neighbor antiphase boundary) and the NN (nearest neighbor) APB during transition from $DO_3$ superdislocations to B2 superdislocations and from the B2 to unit dislocations, respectively.
The result of room temperature tests of the aged specimens also showed different yield behavior depending on the compositions. That is, the yield behavior of the hypo-stoichiometric alloy showed maximum in the $α + DO_3$ field, confirming the α precipitation hardening. Whereas, the 27.2% Al alloy of the hyper-stoichiometry showed minimum stress in the B2 field near the $DO_3$-B2 transformation temperature. This peculiar behavior in the 27.2% Al alloy can be explained by the change in the type and energy of APB with the degree of order, which was supported by the observation of dislocations. In addition, this opposite behavior of the room temperature test to the elevated temperature test means that the generation of dislocations should be a thermally activated process.
The analysis of the activation energy for the generation of various types of dislocation at Frank-Read sources suggested that the types of generated dislocation depend on the thermal energy, the applied stress and the energy of NN and NNN APB's. This analysis, in good agreement with the above experimental results, reconfirmed the APB generation mechanism. More extensive experiments were performed for the purpose of verifying the yield and flow behaviors predicted from the theoretical analysis, producing satisfactory agreement.
