Effect of thermomechanical treatment on the properties of Fe-11Al and Fe-14Al alloys

Fe-Al alloys have the potential to be relatively inexpensive soft magnetic materials if their formability could be improved. An investigation has been made on the effect of thermomechanical treatment on the properties of Fe-11 wt%Al and Fe-14 wt%Al alloys (designated Fe-11Al and Fe-14Al respectively). For the former the room temperature mechanical properties were found to be determined principally by the recrystallised grain size. A good combination of properties for Fe-11Al, i.e. high strength and ductility, was obtained when the grain size was less than about 100 μm. The small grain size was produced by warm rolling at 600°C followed by 1 hour annealing at 600–700°C. On the other hand hot rolling followed by annealing resulted in large grain size, hence rendered the alloy brittle. The cold formability also exhibited a grain size dependence, with the Fe-11Al alloy with a fine recrystallised grain size having good cold rollability. In contrast Fe-14Al was brittle irrespective of the treatment given; ductility of less than 1% was observed in all cases and the cold rollability was limited. Ordering was not seen to be a factor affecting the observed mechanical properties and rollability of either alloy as all the thermomechanical treatments, other than an ordering treatment of 500 hours at 400°C, resulted in a disordered structure. The stress required to work these alloys at elevated temperatures were estimated from compression tests and it is apparent that for Fe-11Al the stress is greatly reduced (50%) from the room temperature value at 600°C and that at 750°C both alloys required a similar stress which was about 15% of the room temperature value. The magnetic properties of Fe-11Al compared favourably with Fe-14Al; the former has a higher saturation induction, a similar coercive force but a lower permeability than Fe-14Al.