A preliminary study on the evolution of creep deformation substructure in Ni-base superalloy Allvac 718Plus has been performed. Specimens crept at 620 MPa and at temperatures ranging from 690-732ºC were examined utilizing diffraction contrast TEM characterization techniques.Creep was interrupted at 1-2.5% strain in order to study the deformation substructure following a limited amount of deformation. The dominant deformation modes at each of the test temperatures were highly planar in nature and involved shearing of the matrix and ′ precipitates on {111} glide planes. In addition, paired a/2<110> dislocations were evident which suggest an antiphase boundary shearing mechanism. Creep induced microtwinning was also observed at the highest creep temperature which was created by identical a/6<112> Shockley partial dislocations that shear the matrix and ′ precipitates on consecutive close packed {111} glide planes. IntroductionNi-based superalloys are predominately utilized in aircraft and land-based industrial gas turbine engines where they excel in the retention of mechanical properties during service at elevated temperatures. In order to improve both engine performance and efficiency significant emphasis has been placed on developing alloys that can operate at higher temperatures without significant loss in structural integrity. In the conventional 718 alloy, the upper bound service temperature is limited to ∼650º, which is governed primarily by microstructural instability through coarsening of the prime strengthening DO 22 structured Ni 3 Nb-based γ′′ precipitate phase followed by its transformation to the equilibrium orthorhombic Ni 3 Nb δ-phase [1][2][3][4][5][6]. With this in mind, Allvac 718Plus was developed with the specific goal of increasing the temperature capability by ∼55ºC while maintaining the excellent workability and weldability characteristics of 718 [7][8]. The alloy chemistry was modified such that the Fe content was reduced while the W, Co and Al/Ti ratio was increased. This combination of changes enables the precipitation of ordered L1 2 structured (Ni 3 Al,Ti) based γ′ precipitates that are coherently embedded in a solid solution γ matrix [9][10]. It is expected that the alteration of the prime-strengthening constituent from γ′′ to γ′ will have a significant effect on the thermal stability of this alloy as well as the high temperature mechanical properties.In Ni-based superalloys where γ′ is the prime-strengthening phase, a rich variety of deformation modes have been reported [11][12][13][14][15][16][17][18]. The precipitates themselves are unique in that during deformation, the γ′ precipitates impart resistance to plastic deformation since they act as effective γ γ γ γ 7th International Symposium on Superalloy 718 and Derivatives Edited by: