Microsegregation and Rayleigh number variation during the solidification of superalloy Inconel 718

The microstructure and composition of the residual liquid at different temperatures were investigated by scanning electron microscopy （SEM） and energy dispersive X-ray spectrometer （EDX） associated with the Thermo-calc software calculation of the equilibrium phase diagrams of Inconel 718 and segregated liquid. The liquid density difference and Rayleigh number variation during solidification were estimated as well. It is found that the heavy segregation of Nb in liquid prompts the precipitation of δ and Laves phase directly from liquid and the resultant quenched liquid microstructure consists of pro-eutectic γ＋eutectic,or complete eutectic according to the content of Nb from low to high. The liquid density increases with decreasing temperature during the solidification of Inconel 718 and the liquid density difference is positive. The largest relative Rayleigh number occurs at 1320°C when the liquid fraction is about 40vol%.

Segregation of niobium in laser cladding Inconel 718 superalloy

Inconel 718 superalloy is widely used in the aerospace and turbine industry. Segregation of niobium appears in the laser cladding Inconel 718 superalloy and consequently influences the phase transformation during the rapid solidification. In order to control the microstructure and improve the mechanical properties of the deposited coating, the the influence of solidification conditions on the segregation of niobium and the resultant formation of Nb-rich Laves phase was studied using the microstructure observation and EDS analysis. The results show that the cooling rate has considerable influence on the microstructure of the deposited coating. High cooling rate is beneficial for suppressing the segregation of Nb and reducing the formation of Laves phase, which is believed to be detrimental to the performance of the Inconel 718 alloy.

Microstructure and micro-hardness of the Inconel 718 superalloy coating prepared by cold gas dynamic spraying

Inconel 718 superalloy coating was prepared on a martensitic steel substrate by cold gas dynamic spraying （ CGDS ）. Microstructure and micro-hardness of the coating were characterized by means of scanning electronic microscope （SEM） ,energy dispersive spectrometer （EDS）, and micro-hardness and nano-indentation tests. The results have indicated that the coating＇ s thickness can reach more than 300 um, and there exists a good interfacial cohesion between the coating and the substrate. The quantity and size of the defects at the bottom, middle and top areas increase gradually. There is no significant change to the original microstructure of the Inconel 718 superalloy particle after the process of CGDS ;and an obvious torsional deformation on the particle＇ s profile is produced while little deformation at the center part occurs. The micro-hardness of the coating at the bottom, middle and top areas decreases in turn. Compared with the bottom area, the micro-hardness of the middle and top areas decreases by about 10% and 21%, respectively. The nano-hardness of the coating is much lower than the original particle,which decreases by about 13.5% at the bottom area and 28% at the top area,respectively. The distribution of micro-defects is an important factor to the micro-hardness of the coating.

Composition optimization of inconel 718 via cluster formula and experimental verification

Superalloys feature multi-elements and complex elemental ranges,which makes the proper composition selection difficult.In fact,more strict composition standards generally apply in practical productions.The objective of this paper is to understand and eventually to renew the composition standard via example of the most common grade Inconel 718.We have recently shown that t he alloy chemistry originates from a nearest-neighbor cluster[center-shell]plus a few next-neighbor glue atoms,or expressed in cluster for-mula[center-shell](glue atoms).By grouping the elements into Ni=(Ni,Co,Cu,Fe),Cr=(Cr,Mn,Si,Mo),and Nb=(Nb,Al,Ti),it is found that the reported alloys fall within a narrow composition zone Ni_(11.0-13.0)-Cr_(3.5-4.5)-Nb_(1) confined by cluster formulas of 16 and 18 atoms.This composition zone is also expressed in terms of 288-atom supercluster formulas,Ni_(198-208)-Cr_(63-72)-Nb_(16-18),which leads to coordi-nated elemental variations in wt.%:69.0≤Ni+Co+Cu+Fe≤72.7;19.8≤Cr+Mn+1.7 Si+0.6 Mo≤22.8;8.7≤Nb+3.2 Al+1.9 Ti≤9.8.Within this composition zone,Ni_(206)-Cr_(65.5)-Nb_(16.5) is further pinpointed and validated by our own experiments to possess the optimal match of strength and plasticity both at room and at 923 K.