Experiments have been done on bi and tetra-crystal competitive growth in a nickel base single crystal superalloy to imitate the competitive growth during directional solidification. The grain orientations were controlled by seed crystals. Results show that the competitive growth is dependent on the relative orientations of grains and their relation with the direction of maximum temperature gradient. Grains with their preferred growth direction (<001> in nickel base superalloy) nearer to maximum temperature gradient will grow faster than those with their preferred growth direction far away from maximum temperature gradient in the condition of bi-crystal as well as tetra-crystal competitive growth. A third grain may appear and grow as a single crystal if two seed crystals with their preferred growth directions are far away from that of maximum temperature gradient in a divergent way in bicrystal competitive growth.
Low cycle fatigue behavior of a DZX40M directionally solidified Co-based superalloy has been investigated at room temperature in air under different total strain amplitudes ranging from 2.5 × 10(-3) to 6.0 × 10(-3). The results show that the cyclic hardening of the alloy appears at the first few cycles in the initial hardening stage, and then the saturation stage which is the major part of the whole fatigue life begins. Examination by TEM indicates that the initial hardening of the alloy is caused by the pile-up of the stacking faults at the intersection of stacking fault, while the saturation of the alloy during low cycle fatigue is arising from the transition of the stacking fault on the (111) to the closely spaced stacking fault ribbon on the (111).