The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings A, versus cooling rate is λ1=0.013（GV）-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.
The relationship between primary dendrite arm spacing and sample diameter was studied during directional solidification for Al-4%Cu (mass fraction) alloy. It is shown that primary dendrite spacing is decreased with the decrease of the sample diameter at given growth rate. By regressing the relationship between primary dendrite arm spacing and the growth rate, the primary dendrite arm spacing complies with 461.76v-0.53, 417.92v-0.28 and 415.83v-0.25 for the sample diameter of 1.8, 3.5 and 7.2 mm, respectively. The primary dendrite spacing, growth rate and thermal gradient for different sample diameters comply with 28.77v-0.35G-0.70, 23.17v-0.35G-0.70 and 23.84v-0.35G-0.70, respectively. They are all consistent with the theoretical model λ1 =k b v-a1G-b1, and b1/a1=2. By analyzing the experimental results with classical models, it is shown that KURZ-FISHER model fits for the primary dendrite spacing in smaller sample diameters with weaker thermosolute convection. Whereas TRIVEDI model is suitable for describing primary dendrite arm spacing with a larger diameter (d>2 mm) where convection should be considered.
The microstructure of experimental nickel-base single crystal superalloys with different levels of carbon has been studied. The results indicated that with increasing carbon addition, the liquidus temperature decreased obviously and the as-cast microstructures exhibited a decrease in the amount of γ/γ′ eutectic structure and an increase in the volume fraction of carbides. The carbides formed in these alloys were most script-type MC carbides which appeared continuous dendritic networks in the interdendritic region. The segregation behavior of element W was influenced by the carbon addition.
A single crystal Ni-based superalloy AM3 was processed at withdraw rates of 3.5, 10, 50, 100, 200, and 500 μm·s-1, respectively.The as-cast microstructures and solidification segregation ratio were characterized with various withdraw rates.The shape and size of carbide microstructures were determined.As expected, the primary and secondary dendrite arm spacings (PDAS and SDAS) decrease with the increase of withdraw rate.The highest volume fraction of eutectic γ/γ' is observed at the 100 μm·s-1 withdraw rate.The volume fraction of eutectic γ/γ' does not appear to be a strong function of the withdraw rate.With increasing withdraw rate, interface morphologies change in the sequence of planar, cellular, and dendrite.There is a general refinement of the microstructure as the withdraw rate increases.EPMA analysis showed that withdraw rate does not have obvious influence on the segregation of elements.
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.
