Partial Thermodynamic Properties of γ′-(Ni,Pt)3Al in the Ni-Al-Pt system

Measurements were made to determine how Pt influences the partial thermodynamic properties of Al and Ni in γ′-(Ni,Pt)3Al and liquid in the Ni-Al-Pt system. The activities of Al and Ni were measured with a multiple effusion-cell mass spectrometer (multi-cell KEMS). For a constant X Al = 0.24, adding Pt, from X Pt = 0.02-0.25, reduces a(Al) almost an order of magnitude, from 2 × 10−4 to 2 × 10−5, at 1560 K. This occurred for \({\varvec{\Delta_{\rm m}} \bar{\hbox{H}}\hbox{(Al)}}\) of −203 ± 10 kJ mol−1 and the a(Al) decrease was due to \({\varvec{\Delta_{\rm m}} \bar{\hbox{S}}\hbox{(Al)}}\) increasing from −60 to −40 J mol−1 K−1 with Pt addition. The large negative \({\varvec{\Delta_{\rm m}} \bar{\hbox{H}}\hbox{(Al)}}\) and \({\varvec{\Delta_{\rm m}} \bar{\hbox{S}}\hbox{(Al)}}\) indicate Al-atoms are highly ordered in γ′-(Ni,Pt)3Al. Nickel activity, a(Ni), remained essentially constant, ∼0.7, indicating an increasing ternary interaction between Ni-atoms and (Al + Pt)-atoms with Pt addition, where γNi increased from about 0.7 to 1.2. This is supported by \({\varvec{\Delta_{\rm m}} \bar{\hbox{H}}\hbox{(Ni)}}\) in the range 6.1-7.1 ± 1.5 kJ mol−1 at 1520 K, and a positive \({\varvec{\Delta_{\rm m}} \bar{\hbox{S}}^{\rm xs} \hbox{(Ni)}}\), which suggest disorder on the Ni-lattice. For a consistent X Al = 0.27, adding Pt, from X Pt = 0.10–0.25, also reduces a(Al) but only by a factor of about 3, while a(Ni) remained essentially constant, with γNi increasing from about 0.7 to 0.95. A dramatic change in the mixing behavior was observed between the \({X_{\rm Al} = 0.24}\) and 0.27 series of alloys, where \({\varvec{\Delta_{\rm m}} \bar{\hbox{H}}\hbox{(Al)}}\) and \({\varvec{\Delta_{\rm m}} \bar{\hbox{S}}\hbox{(Al)}}\) are seen to increase about 50 kJ mol−1 and 20 J mol−1 K−1 at T = 1566 K, respectively. In contrast, \({\varvec{\Delta_{\rm m}} \bar{\hbox{H}}\hbox{(Ni)}}\) decreased about 16 kJ mol−1 at T = 1520 K and \({\varvec{\Delta_{\rm m}} \bar{\hbox{S}}^{\rm xs} \hbox{(Ni)}}\) changed from a positive to a negative value.