The effect of direct ageing treatment on microstructure and stress-rupture properties of GH761 alloy was studied. Phosphorus was added to the tested alloy for its beneficial effect on stress-rupture life. It was found that the grain size of the tested alloy became much finer after direct ageing treatment, in comparison with the commercial GH761 alloy under standard heat treatment condition. The optimum direct ageing treatment has been determined as follows: 830 ℃×4 h, air cooling+720 ℃×24 h, air cooling. After such treatment, the higher stress-rupture properties of the test alloy at 650 ℃, 690 MPa were obtained. The role of phosphorus is still effective under the condition of direct ageing treatment.
YANG Shu-linSUN Wen-ruLIU FangGUO Shou-renYANG Hong-caiHU Zhuang-qi
Continuing the effort to redesign IN718 alloy in order to provide microstructural and mechanical stability beyond 650℃, IN718 alloy was modified by increasing the Al, P and B contents, and the microstructure and mechanical properties of the modified alloy were compared with those of the conventional alloy by SEM and TEM. The precipitation of the grain boundaries of the two alloys is different. The Cr-rich phase, Laves phase andα-Cr phase are easily observed in the modified alloy. Theγ' andγ' phases in the modified alloy are precipitated in a 'compact form'. The tensile strengths of the modified alloy at room temperature and 680℃are obviously higher than those of the conventional one. The impact energy of the modified alloy is only about half of that of the conventional alloy. Ageing at 680℃up to 1 000 h lowers the tensile properties and impact energy of both the conventional and modified 718 alloys, except increasing the ductility at 680℃. It is concluded that the modified alloy is more stable than the conventional one.
