Effect of Ti/Al Ratio on Precipitation Behavior during Aging of Ni-Cr-Co-Based Superalloys

Precipitation behaviors of Ni-Cr-Co-based superalloys with different Ti/Al ratios aged at 750, 800, and 850 degrees C for up to 10,000 h were investigated using scanning and transmission electron microscopy. The Ti/Al ratio did not significantly affect the diameter of the gamma ' phase. However, the volume fraction of the gamma ' phase increased with increasing Ti/Al ratios. The eta phase was not observed in alloys with a small Ti/Al ratio, whereas it was precipitated after aging at 850 degrees C for 1000 h in alloys with a Ti/Al ratio greater than 0.80. Higher aging temperatures and higher Ti/Al ratios led to faster eta formation kinetics and accelerated the degradation of alloys. It is thought that the increase in hardness with an increase in the Ti/Al ratio is attributed to the effective inhibition of the gamma ' phase on dislocation movement due to the increase in the volume fraction of the gamma ' phase and an increase in the antiphase boundary (APB) energy.