A Comparative Study on the Hot Working Behavior of Inconel 718 and ALLVAC 718 Plus
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THE Inconel alloys are a group of Ni-based superalloys with wide applications for high-temperature components such as aero-engines and gas turbine disks.[1,2] Some unique properties such as high strength, desirable cold and hot workability, and moderate cost are the major reasons for the application of Inconel alloys. Some of the most well-known alloys in this family, such as Inconel 718, have been at the center of industrial and academic attention in the past decades. The solid solution of alloying elements, such as Fe, Cr, Mo, Ti, and Nb, and the precipitation of c¢¢ phase (Ni3Nb), are the major sources of hot strength in Inconel 718. However, the major drawback of this alloy is still c¢¢ particles, when they are coarsened by prolonged high temperature exposure during service. This would limit the working temperature of Inconel 718 to about 923 K (650 C) to avoid c¢¢ coarsening.[3,4] In order to resolve this challenge, scientists introduced ALLVAC 718 plus (718+) as an alternative to 718 with higher temperature endurance. The changes in the chemical composition of 718+, including increase in Al, Co, Cr, and Nb and decrease in Fe, would improve the temperature resistance to about 328 K (55 C). The changes also replaced c¢¢ with c¢
AMIR MOMENI is the Materials Science and Engineering Department, Hamedan University of Technology, Hamedan, 6516913733, Iran. Contact e-mail: [email protected] SEYED MEHDI ABBASI, MARYAM MORAKABATI, and HASAN BADRI are with the Metallic Materials Research Center (MMRC), Maleke Ashtar University of Technology, Tehran, 177415875, Iran. Manuscript submitted July 21, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
(Ni3Al), which was more resistant to coarsening at the service temperatures.[5] It is worthy to mention that some other phases such as d (delta) and g (eta) with chemical compositions near to c¢¢ may form in the Ni-based superalloys depending on the chemical composition.[6,7] It has been approved that the formation of such phases affects the grain c grain boundary mobility and therefore restrict grain coarsening rate.[8] In general, alloying elements which are added to ameliorate hot strength and corrosion resistance degrade hot workability during forming. The major reason for the degradation of hot workability in Ni-based superalloys is that the alloying elements generally decrease the stacking fault energy (SFE) of Ni. Among the alloying elements in superalloys only Al increases the SFE value[9,10] and others tend to decrease the SFE value of the matrix. As SFE has a direct influence on the dislocations mobility, it controls the mechanism of microstructural changes during hot working. In high-SFE materials, dislocations easily move, interact, and annihilate during hot working.[11] In such materials, dynamic recovery (DRV) is the major microstructural evolution which leads to the formation of a uniform substructure of dislocations. When DRV occurs in large strain deformations, the subgrains can grow or coalesce to form fine recrystallized grains. This kind of recrystall
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