Behavior Of Macrocoarse Structure Evolution In GH4698 Superalloy

Superalloy GH4698 is one kinds of Ni-based superalloy, which is suitable to used for large-sized turbine discs. The mixed-grain microstructure or the grain size uneven phenomenon often appears during the forging process of turbine disc, even leading to the failure of relative components. Some studies have been carried on this phenomenon, but the essence still remains to be further defined.In order to clear the abnormal grain growth behavior of GH4698 alloy, GH4698 was solution treated(ST) at different temperature and times. The research uses EBSD technology to analyze the neighbor grain orientation difference and low ∑ value CSL grain boundaries. TEM also be conducted to study the dislocation motions and behavior of grain boundaries migration. The results have important engineering significance.The results show that there is an obvious coarsening of GH4698 alloy deformed at 1025 ℃ and 1120 ℃ after ST for eight hours. Grains of samples deformed at 1025℃ are bulky but uniform with an average grain diameter of 0.5mm. While, mixed crystal phenomenon including supersize grains exist in the samples deformed at 1120 ℃, and the grain size can be more than 1.5mm. In order to prevent the emergence of the macro-coarse grain phenomenon, groups of different ST time have been compared to determine its formation mechanism. Grains of samples deformed at 1025℃ start to grow up when samples are solution treated at 1100℃ for 5mins. Grains are uniform isometric during ST with an average grain size of 200μm. Supersize grains and fine original grains coexist in the samples deformed at 1120℃ and after ST at 1100℃ for 20mins. And finally supersize grains of more than 200p.m exist. Grain orientations change a lot during abnormal grains growing up, especially the increase of small angel orientations. Grains coarsen by the way of forming of twins. The content of twin in the forging microstructure is 40% when superalloy GH4698 is deformed at low temperature. However, the content of twin in the forging microstructure is more than 50% at high temperature. Twins growing up and reaction lead to supersize grains. Therefore, the appropriate deformation temperature should be selected during the actual process to prevent the formation of mixed crystal.