Microstructure and compressive flow stress of directionally solidified ternary Ni3(Al, Nb) and quaternary Ni3(Al, Nb, Ti) alloys with duplexphase

Abstract

The microstructure and compressive flow stress of directionally solidified ternary Ni3 (Al, Nb) and quaternary Ni3 (Al, Nb, Ti) alloys were examined. Three compositions of Ni–16.0 at.%Nb–9.0 at.%Al (Alloy 1), Ni–13.3 at.%Nb–7.5 at.%Al–4.2 at.%Ti (Alloy 2) and Ni– 10.7 at.%Nb–6.0 at.%Al–8.3 at.%Ti (Alloy 3) were selected for investigation. Alloy 1 was composed of the L12 and the D0a phases while the constituent phases varied for the D024 and the D0a phases for Alloys 2 and 3 with Ti content. The definite crystallographic relationship was observed between the D024 and the D0a phases to be (0001)D024// (010)D0a and ∹〉 D024//〈100〉D0a in Alloy 3. Compression tests were conducted along the growth direction in the temperature ranging from room temperature to 1000 °C. Alloy 1 exhibited high yield stress at low temperatures, but it rapidly decreased above 700 °C. Similar temperature dependence of yield stress was observed in Alloy 2, although the onset temperature of a rapid decrease in yield stress was somewhat lower. Alloy 3 with the highest Ti content showed the lowest compressive strength among the three alloys, while relatively good low-temperature ductility was obtained in Alloy 3. Yield stress of Alloy 3 exhibited anomalous strengthening behaviour accompanied by the basal slip in both D024 and D0a phases. Transition in operative slip systems from the basal slip to the prism slip occurred at the peak temperature of yield stress anomaly (600 °C), resulting in a gradual decrease in yield stress. Slip transfer behaviour between the D024 and the D0a phases was briefly discussed.