Xinbao Zhao

Effect of carbon and boron additions on segregation behavior of directionally solidified nickel-base superalloys with rhenium

The phase transformation temperature, segregation behavior of elements and as-cast microstructure were investigated in experimental nickel-base superalloys with different levels of carbon and boron. The results show that the liquidus temperature decreases gradually but the carbide solvus temperature increases obviously with increasing carbon addition. Minor boron addition to the alloy decreases the liquidus temperature, carbide solvus temperature and solidus temperature slightly. Apart from rhenium, the segregation coefficients of the elements alter insignificantly with the addition of carbon. The segregation behavior of rhenium, tungsten and tantalum become more severe with boron addition. The volume fraction and size of primary carbides increase with increasing carbon addition. The main morphology of the carbides is script-like in the alloys with carbon addition while the carbide sheets tend to be concentrated and coarse in the boron-containing alloys.

Effect of solidification rate on MC-type carbide morphology in single crystal Ni-base superalloy AM3

In order to study the effect of the withdrawing rate on carbide morphology, MC-type carbide in single crystal superalloy AM3 was systematically investigated with sample growth rates from 3.5 μm/s to 500 μm/s. The carbide morphologies were investigated by scanning electron microscopy (SEM), and the electron probe microanalysis (EPMA) was used to characterize the carbide composition. The results indicate that the solidification rate is the important factor governing MC carbide growth morphology, size and distribution, composition and growth mechanism. With the increase of withdrawing rate, nodular, rod-like, Chinese script types of carbide morphology are observed. For the low withdrawing rate, with the increase of withdrawing rate, the carbide size becomes larger. For the case of dendritic interface, the carbide size becomes smaller with refinement of dendrites as withdrawing rate increases. The volume fraction of carbides increases with the withdrawing rate increasing.