Phase evolution during the reactive sintering of ternary Al-Ni-Ti powder compacts

Abstract The formation of various aluminides and intermetallic compounds in the Al–Ni–Ti ternary system offers great potential for a wide variety of applications. In the present work, reactions occurring during the heating of compacted ternary Al–Ni–Ti elemental powder mixtures were studied using differential scanning calorimetry (DSC). The effect of composition on the reaction behavior was studied by varying the aluminum addition (0–40 at.%) to nickel and titanium powders in equal proportions. The powder mixtures were compacted and heated in a DSC at 20 °C min −1 up to 1200 °C, using a continuous flow of pure and dry argon gas. The reaction behavior in binary Ni–Ti (1:1) powder compacts and Al–Ni–Ti samples containing 25 and 35 at.% aluminum, was studied in detail by interrupting the heating of samples at different stages of reaction. Phase evolution was followed by examining the sintered samples using X-ray diffraction technique, scanning electron microscopy and energy dispersive spectroscopy. The DSC plots showed two main exothermic peaks corresponding to reactions taking place below and above the melting point of aluminum. For all the Al–Ni–Ti samples studied, the first exothermic peak was observed in the interval 595°–625 °C, and the heat release increased with increase in the aluminum content of the samples. At 700 °C, microstructural studies showed that Al 3 Ni and Al 3 Ni 2 were the major constituents and only a thin layer of Al 3 Ti was observed around the titanium powder particles, indicating a diffusional barrier. The second exothermic peak was observed in the interval 938°–946 °C which corresponds to the reaction between nickel and titanium. Titanium-rich and nickel-rich ternary compounds, in addition to some binary compounds have been observed after this reaction in all the ternary compositions studied. The formation of AlNi 2 Ti (τ 4 ) phase together with some new ternary compounds was observed in most of the Al–Ni–Ti samples heated to 1200 °C. Among these, two phases correspond to compositions close to Al 2 Ni 3 Ti 5 and Al 36 Ni 28 Ti 36 , whose structural characteristics and stabilities are yet to be confirmed in literature.