WEB Evolution of bimodal L12 (γ') phase distribution and its effect on mechanical properties in nickel-based alloy 718Plus
Alloy 718Plus, a derivative of alloy 718, exhibits unimodal distribution of L12 ordered gamma prime (γ') precipitates, owing to their high thermal stability, retains optimum properties up to 704°C . This alloy is expected to replace alloy 718 in turbine disk applications and shall be continually exposed to cyclic loading, making material fatigue the major mode of failure [2, 3]. Bimodal distribution often introduces particle sizes corresponding to dislocation precipitate mechanism of shearing as well as looping, depending upon volume fraction and distribution. The influence of this microstructure on mechanical properties, fatigue life and relaxation in crack growth rates are being studied.
In order to generate a bimodal distribution of γ׳ precipitates, heat treatment was carried out at 905°C and 710°C. Larger particle size was decided by isolating the heat treatment, which gave peak hardness at 905°C. The peak aging condition was achieved at 1.5hr. The average γ' particle size was found to be 73.5nm with volume fraction of 8.6%, grain size of 59 microns and hardness of 296 VHN0.2. Lower temperature aging treatment of 710°C for 21 hrs resulted in precipitation of fine γ׳ particles of 5.7nm size and hardness of 426 VHN0.2.
The mechanical properties of the alloy with bimodal precipitate distribution were studied by conducting tensile tests at room temperature and at 650°C. Transmission electron microscopy was employed to investigate the evolution of γ׳ precipitates and also the dislocation-precipitate interaction mechanism. The results are rationalized on the basis of the present understanding of the subject reported in the literature.