The effect of grain size and Fe:Co ratio on the room temperature yielding of FeCo

Compression tests have been used to determine the Hall-Petch parameter and the ductility has been measured under tension in three FeCo alloys, Fe30Co, Fe50Co and Fe70Co (compositions given in atomic percent throughout) in both their ordered and disordered states. It was that disorder, either constitutional or thermal, decreased the Hall-Petch parameter and increased the ductility, results explained by the increased ease of slip transmission on disordering. Surface slip trace observations showed that all the alloys exhibited wavy slip in their disordered states whilst planar slip occurred in the ordered states. The compression tests also showed that Fe50Co had a higher yield strength in the disordered state than in the ordered state, irrespective of grain size, as noted by others. In contrast, irrespective of grain size, the yield strength of Fe70Co was much lower in the disordered state than in the ordered state. On the other hand, Fe30Co was stronger in the ordered state at fine grain sizes but weaker at coarse grain sizes. This behaviour was interpreted in terms of the short range order upon quenching from 800°C.