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

Ni₃Al (Ni₇₅Al₂₅) and Ni₃Al-2Mo/Ta (Ni_73.5Al_24.5Mo/Ta₂) single crystals were prepared by the Bridgman method under high temperature gradient (~170°C/cm) at the withdrawal rates of 5 and 20μm/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 (5μm/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 20μm/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.