Ni-Al
Raney-type nickel–aluminium alloys have been used for catalytic applications in the chemical industry since the 1920s. These catalytic materials differ from other types of catalysts since their high surface area is achieved by selectively leaching away the aluminium. Gas atomized particles can show anomalous high contents of the Ni2Al3 phase. The rapid solidification processes of Ni-Al droplets can lead to microstructure modifications and possibly involve metastable phases. The kinetics of rapid solidification processes in undercooled Raney-type Ni-50wt.%Al (Ni-68.5at.%Al) alloys is studied in EML experiments.
Phase diagram and growth kinetics
Microstructure evolution
BSEI of microstructure of Ni-68.5at.%Al alloy for as-cast sample (a,b) and sample quenched from undercooling DT = 270 K (c,d).The microstructure of deeply undercooled and subsequently quenched samples has some common features with as-cast one. Few areas of regular dendrites represent branches directed in an angle close to 60° (c) . This morphology rather points to primary solidification of the Ni2Al3 phase with hexagonal crystal structure than to the cubic b-NiAl equilibrium phase.
The equilibrium primary solidifying b-NiAl phase is absent in the microstructures of both quenched undercooled samples and in the as-cast master alloys.