Thermodynamics of titanium alloys: I. Development of a triple Knudsen cell and its use to study the activity of copper in the Ti−Cu system

A new experimental technique for the determination of thermodynamic activity of alloys has been developed utilizing a triple Knudsen cell with a pure enriched natural isotope as the standard reference state. The alloy for which activity is to be determined is placed in one of the two effusion chambers of the triple cell and the pure isotopic standard in the other. The molecular beams from each chamber effuse into a third upper chamber and through a collimating hole into the ion source of a Bendix time-of-flight mass spectrometer. Since the recorded intensities are proportional to the vapor pressures within the chambers, a simple calculation based upon their ratio gives a direct determination of the activity of the solute in the alloy. The accuracy of the triple cell technique for the experimental determination of activities was checked by the measurement of the copper activity in a Ni?Cu alloy containing 30.6 wt pct Cu. A value ofRT γ_Cu=1778±142 cal per g-atom at 1450°K was obtained, which is in excellent agreement with values in the literature. The activity of copper in the bcc β phase when then determined for four Ti?Cu alloys (X _Cu=0.033 to 0.085), using pure enriched Cu⁶⁵ as the standard reference state. The composition range investigated was limited to alloys belowX _Cu=0.10, the maximum solubility within the experimental temperature range between 1423° and 1573°K, the activity of copper in the bcc β phase can be expressed using pure liquid copper as the standard state by the following equation:RT Ina _Cu=RT InX _Cu+X _Ti ² (1.92±0.30) in kcal per g-atom. The activity of titanium is given byRT Ina _Ti=RT InX _Ti+X _Cu ² (1.92±0.30) in kcal per g-atom.