Composition dependence of the precipitation behavior in NiAl–Cr(Mo)–(Ti,Hf) near eutectic alloys

Influence of Ti and Hf combined additions on the Ni2AlX (X = Ti,Hf) Heusler phase precipitation in as-cast NiAl–Cr(Mo) eutectics was investigated by XRD, SEM and TEM observation. The results show that with the increasing of Ti content, the Ni2Al(Ti,Hf) precipitation with blocky morphology at NiAl/Cr(Mo) interface and cellular boundary is nearly suppressed; however, this precipitation with cuboidal or granular morphology within the NiAl matrix promoted. The mechanisms of this transition of precipitation behavior are attributed to the decrease of interfacial energies due to the coherency between NiAl and Heusler phase Ni2Al(Ti,Hf) with the increasing of Ti addition. The size of Ni2Al(Ti,Hf) coherent precipitates inside the NiAl matrix decreased progressively with the increasing of Ti content in NiAl phase, showing a Gibbs–Thomson relation-like trend. In addition, another (Ti,Hf) solid solution phase along with the Ni2Al(Ti,Hf) precipitates was observed for lower Ti addition alloys, which is the precipitation from NiAl matrix prior to Ni2Al(Ti,Hf) precipitation. This may be ascribed to the fast formation kinetics of (Ti,Hf) solid solution phase relative to the Ni2Al(Ti,Hf) phase for lower level Ti-containing alloys.