718 Alloy

Alloy name: 718
Diagram No.: 1070
Type of diagram: CCT
Chemical composition in weight %: 51.5% Ni, 19.9% Fe, 18.2% Cr, 5.1% Nb, 2.9% Mo, 1.15% Ti, 0.57% Al, 0.035% C
Alloy group: Nickel-based alloys
Note: The CCT diagrams developed for IN-718 as function of homogenization treatment are shown in Figures 1, 2 and 3. A comparison of the results from these diagrams indicate a shift to longer times in the transformation reactions with increasing homogenization time. In addition, the results from the CCT diagram after the 90 hrs homogenization treatment appears to indicate two major differences when compared to the CCT diagrams after the 24 and 72 hrs homogenization treatments. For example, the first difference is the shift in the Delta-phase transformation start temperature to lower temperatures. The Delta-phase transformation temperature is depressed by about 12O'C. The second difference is that the nucleation of new and small MC-type of carbides precedes the formation of the Delta-phase. A possible explanation of this behavior could be based on the fact that at the homogenization temperature used in this study, 118O C, and the long holding times, the combined dissolution of Laves phase, small primary MC-type of carbides and other Nb-rich phase increases the supersaturation on Nb in the matrix and also reduces the chemical segregation of Nb at Gamma grain boundaries. By reducing the local concentration of Nb at grain boundaries the formation of Delta-phase is suppressed for relatively fast cooling rates. Hence, on cooling from the homogenization temperature the Nb eupersaturation of the matrix increases and the precipitation of MC-type carbide is favored over that of 6. Similar type of fine precipitation of MC-type carbides has been reported elsewhere. These MC-type carbides are NbC which later decompose to generate more Gamma” in the alloy upon further cooling.
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