Influence of Mo and Ta additions on solidification behavior of Ni3Al single crystal alloys

Ni3Al (Ni75Al25) and Ni3Al–2Mo/Ta (Ni73.5Al24.5Mo/Ta2) single crystals were prepared by the Bridgman method under high temperature gradient (~170 °C/cm) at the withdrawal rates of 5 and 20 μm/s. The experimental results showed that the addition of Mo decreased the liquidus and solidus temperatures, and the addition of Ta increased the liquidus and solidus temperatures. Meanwhile, both Mo and Ta additions were found to increase the solidification range of experimental alloy. It has been found that for solidification of stoichiometric Ni3Al at low withdrawal rate (5 μm/s), β/γ′ eutectic first solidified from liquid (L→β+γ′) and then β phase completely transformed into pure γ′ (β→γ′) after the completion of solidification. However, at high withdrawal rate of 20 μm/s, the remaining β phase was found in the as-cast microstructure of Alloy Ni3Al. Different from stoichiometric Ni3Al, the solidification sequence of Alloy Ni3Al–2Mo was identified as L→β+γ′+Mo-rich phase (ternary eutectic reaction). In addition, the addition of Ta led to the formation of primary γ′ phase and then subsequent intercellular/interdendritic β/γ′ eutectic microstructure. Based on above study, a new Ta and Mo containing Ni3Al based single crystal alloy with superior tensile strength at ultra-high temperature (>1100 °C) was designed.