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 gamma'-(Ni,Pt)(3)Al and liquid in the Ni-Al-Pt system. The activities of All 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 x 10(-4) to 2 X 10(-5), at 1560 K. This occurred for Delta H-m(Al) of -203 +/- 10 kJ mol(-1) and the a(Al) decrease was due to Delta n (S) over bar (Al) increasing from -60 to -40 J mol(-1) K-1 with Pt addition. The large negative Delta(m)(H) over bar (Al) and Delta(m)(S) over bar (Al) indicate Al-atoms are highly ordered in gamma'-(Ni,Pt)(3)Al. Nickel activity, a(Ni), remained essentially constant, similar to 0.7, indicating an increasing ternary interaction between Ni-atoms and (Al + Pt)-atoms with Pt addition, where gamma(Ni) increased from about 0.7 to 1.2. This is supported by A H(Ni) in the range 6.1-7.1 +/- 1.5 kJ mol(-1) at 1520 K, and a positive Delta n (S) over bar (xs)(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 gamma(Ni) increasing from about 0.7 to 0.95. A dramatic change in the mixing behavior was observed between the X-Al = 0.24 and 0.27 series of alloys, where AnH(Al) and Delta(m)(S) over bar (Al) are seen to increase about 50 kJ mol(-1) and 20 J mol(-1) K-1 at T = 1566 K, respectively. In contrast, Delta(m)(H) over bar (Ni) decreased about 16 kJ mol(-1) at T = 1520 K and Delta(m)(S) over bar xs(Ni) changed from a positive to a negative value.