Wettability of NiAl by a liquid Ni–Si–B alloy

An investigation of the wettability of the intermetallic compound NiAl by a liquid Ni–4.5 wt% Si–3.2 wt% B filler metal is presented in this paper. Dynamic observations of spreading of Ni–Si–B droplets, conducted using hot-stage light microscopy, are correlated with post-cooling microscopy and analysis. The paper examines the influence of the oxide layer on the NiAl substrates, on the progression of spreading of the Ni–Si–B liquid. Termination of spreading of the Ni–Si–B droplets by the onset of isothermal solidification at the spreading front is considered. Spreading of the Ni–Si–B droplets was found to be rapid until the onset of isothermal solidification at the spreading front. However, once isothermal solidification commenced, negligible further spreading was observed. The Ni–Si–B filler metal was observed to spread by undermining of the substrate oxide. However, a marked reaction occurred between the substrate oxide and the Ni–Si–B filler metal. This reaction served to remove the substrate oxide layer. The paper contrasts the mechanisms of substrate oxide undermining and isothermal solidification of liquid Ni–Si–B droplets on NiAl with those occurring during the spreading of the same liquid on pure nickel and Ni–Cr alloys. This revised version was published online in November 2006 with corrections to the Cover Date.