Microstructure evolution and mechanical properties of nanocrystalline FeAl obtained by mechanical alloying and cold consolidation

In the present study high energy mechanical milling followed by cold temperature pressing consolidation has been used to obtain bulk nanocrystalline FeAl alloy. Fully dense disks with homogenous microstructure were obtained and bulk material show grain size of 40 nm. Thermal stability of the bulk material is studied by XRD and DSC techniques. Subsequent annealing at a temperature up to 480 °C for 2 h of the consolidated samples enabled supersaturated Fe(Al) solid solution to precipitate out fine metastable Al5Fe2, Al13Fe4 and Fe3Al intermetallic phases. Low temperature annealing is responsible for the relaxation of the disordered structure by removing defects initially introduced by severe plastic deformation. Microhardness shows an increase with grain size reduction, as expected from Hall–Petch relationship at least down to a grain size of 74 nm, then a decrease at smallest grain sizes. This could be an indication of some softening for finest nanocrystallites. The peak hardening for the bulk nanocrystalline FeAl is detected after isochronal ageing at 480 °C.

Research highlights▶ In this paper, the Fe–40 at% Al alloy was prepared by mechanical alloying (MA), cold consolidation and subsequent annealing. ▶ Almost full consolidation of the MA powder occurred after 7 GPa where the consolidated material had a nanocrystalline structure with grain size of 40 nm. ▶ Heating of the consolidated samples resulted in formation of nanocrystalline Fe2Al5 and Al13Fe4, and Fe3Al intermetallic phases and finer grain size may be retained. ▶ Mechanical property measured in microhardness testing shows strengthening down to a grain size of 74 nm and Hall–Petch behaviour is thus demonstrated over the range of grain sizes from 95 to 74 nm. ▶ The peak hardening is detected after isochronal ageing at 480 °C.