Inconel 718 was developed by International Nickel Company in 1962, has become the most widely used superalloy in the world. The alloy's popularity is due to its excellent combination of mechanical properties, moderate price and good processability. However, its maximum use temperature is restricted to 650ºC because the main strengthening γ″ phase responsible for the alloy's outstanding properties rapidly overages. There has been a substantial amount of work by numerous investigations over the years to increase the temperature capability of Inconel 718. Xie and Dong et al. has systematically studied the effect of variation of alloying elements including Al, Ti, Nb, P, B, S and Mg etc. content on structural stability and mechanical properties by experiments and thermodynamic calculations.This attempt was made to improve the temperature capability of Inconel 718 via controlling the morphology of γ″ and γ′ phase and increase their solvus temperature by variation of Al, Ti and Nb content, and W addition for solid solution strengthening. The phases precipitation behaviors, long time microstructure stability and mechanical properties of modified 718 type alloys have been systematically studied. The results indicate that compact morphology of γ″ and γ′ has been observed in grains. In addition, a new stable globular phase with higher solvus temperature mainly precipitated at grain boundaries, different to the δ phase in conventional 718 alloy, has been observed in modified 718 type alloys (this phase is temporarily named δ′′ phase for description convenience). After 680ºC aging for 1,000h, the combination of δ′′ granular phase at grain boundaries and compact morphology precipitation of γ″ and γ′ phase in grains characterize with superior thermal stability. Preliminary mechanical properties test results show that the alloy with this kind of microstructure can provide better stress rupture properties and fatigue resistance than that of conventional Inconel 718. It appears to provide a new approach for improvement of temperature capability of Inconel 718 and the new designed 718 type alloy can be used at 680ºC or higher temperatures.

In this paper, a series of hot compression tests were carried out under different hot deformation parameters, covering the temperature range of 1000ºC to 1170ºC, strain rate range of 0.01s -1 to 1s -1 and compression reduction of 30%, 50%, and 70%. After that, all specimens were heat treated under the standard heat treatment schedule of GH720Li to investigate its hot deformation characteristics, especially the grain growth behaviors. The further OM and SEM observations were adopted to investigate the interaction mechanism of the primary γ′ distribution and grain growth for GH720Li alloy. The results show that for GH720Li alloy, when hot deformed below γ′ solvus, the banded or bi-model structure always occurs; while hot deformed near or over γ′ solvus, the uniform grain size distribution can be obtained. Further microstructure analysis shows that the bi-model or banded grain size distribution in GH720Li alloy is mainly due to the nonuniform re-dissolving of primary γ′ during heating process; while the uniformity of primary γ′ phase is determined by the uniformity of elements in this alloy. In other word, the homogenizing process for ingot is the key reason for later grain size control.

The large-scale flue gas turbine disk 01450mm of GH4738 (Waspaloy) nickel based superalloy has been successfully forged by hot deformation using 800MN die forging hydraulic press, which contributed to microstructure evolution precision control models combined with integrated simulation method at different deformation conditions. Constitutive equations and grain structure evolution models of GH4738 alloy were constructed and used in Deform3D for achieving integrated simulation of the complete forging process for the turbine disk (from preheating billet for free forging to die forging). The integrated numerical simulation was used for an experimental disk with the diameter of 0300 mm before forging the large-scale disk. Comparison between integrated simulations and actual forging results show that the simulated microstructure are in accordance with the actual results and the integrated simulation method excelled traditional simple single stage simulation. Finally, the integrated numerical simulation of the 01450 mm turbine disk die forging was also carried out through orthogonal experiment methods according to initial average grain size, temperatures, strain and strain rate and so on, and the optimal deformation parameters were chosen for forging using a 800MN hydraulic press. The results indicate that this method has shown a high correlation with actual microstructure distribution of GH4738 superalloy disk. Besides, the mechanical properties, grain sizes and microstructural homogeneity of the large-scale flue gas turbine disk have reached up to the specification level of HG/T3650-2012 (equivalent to AMS 5704G). In conclusion, the microstructure evolution models and integrated simulation methods of GH4738 alloy have provided a new method for making large-scale disk forgings.