Determination of the order in the intermetallic phase Ni₃Al as a function of temperature

The degree of long range order in the γ′-phase of the nickel–aluminum system was determined as a function of temperature by means of X-rays. The measurements were carried out on {h00} single crystals, using a high-temperature vacuum X-ray diffractometer. Two compositions were investigated: stoichiometric Ni₃Al and an off-stoichiometric nickel-rich alloy. Complete long-range order was present to within 65°C of the melting point, the highest temperature measured. Line breadth measurements indicated a large, temperature independent anti-phase domain size. From the intensity variation of the 400 reflection with temperature, a Debye temperature of 360°K was deduced. The linear coefficient of thermal expansion between 25 and 800°C was found to be 15 x 10⁻⁶ per °C.