12
The intermetallic N4W forms by the peritectoid reaction
Ni + NiW ^ = ^>Ni4W at 970 C. The crystal structure of N4W
is isostructural with N4M0 (Epremian and Harker, 1949).
Note the absence of any phase comparable to N3M0.
2.1.1.4 Ni-Ta
The Ni-Ta diagram as given by Shunk (1969) has been
modified to include the low temperature NigTa
intermetallic by Larson et al. (1970). It is shown in
Figure 2.4 (Moffatt, 1977). The NigTa intermetallic forms
sluggishly by peritectoid reaction with NigTa and Ni. It
is reported to be F.C.T. The NigTa phase has been found
to be monoclinic with a possible orthorhombic variant
(TiCog type), or as a tetragonal DO24 superlattice (TiAlg
type). The phase is thus not isostructural with either
NigMo DO22 or with orthorhombic equilibrium NigMo.
2.1.2 Ternary Diagrams
2.1.2.1 Ni-Al-Mo
High temperature Ni-Mo-Al isotherms show a
quasibinary eutectic between the NiAl and Mo phases
(Bagaryatski and Ivanovskaya, 1960). This allows the
diagram to be conveniently split in two at a line
connecting the NiAl and Mo phase fields. At the time this
investigation was originally begun, the Ni-rich low
temperature phase equilibria of this ternary system were