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Abstract. Nickel-iron Alloy 718 is widely used for fabricating parts by superplastic deformation. Refinement of grains down to a nanostructure (NS) size improves the alloy's processing properties. Thermomechanical treatment has been carried out to form a NS state in bulk alloy by multiple isothermal forging (MIF) at gradually decreasing temperatures. Investigation of superplastic properties and processing behavior of Alloy 718 has been performed. The alloy with a grain size of 80 nm displays superplasticity (SP) at a temperature which is lower than for a conventional fine grained alloy by about 350°C. The values of the relative elongation δ and the strain rate sensitivity coefficient m are 350% and 0.37, respectively. The experimental data on the influence of grain size on solid-state weldability in the range of SP have been obtained. The application of the effect of low temperature SP yields lower temperatures of superplastic forming (SPF) and pressure welding (PW) as compared with conventional SP of fine-grained material. The experiment of the combined process of SPF and PW by counter-forming of two polished sheets, demonstrates its low temperature processing feasibility using NS specimens. The SPF processing of NS sheets in a cylindrical die has been investigated. It has revealed that macro-deformation is uniform in cross and longitudinal sections. Mechanical properties of Alloy 718 in NS condition and after strengthening heat treatment have been discussed.

This paper will describe the refinement of a coarse-grained alloy 718 microstructure to microcrystalline (MC), submicrocrystalline (SMC), and nanocrystalline (NC) structures by high-strain plastic deformation. Dynamic recrystallization occurs during high-strain hot plastic deformation leading to the formation of an MC structure with a grain size of 1-4 pm. The 6 phase precipitated as plates, and during deformation, these plates were broken up into smaller particles. These particles take the form of ellipsoids located mainly at the triple junctions of the recrystallized grains. A SMC structure was formed in alloy 7 18 during severe plastic deformation in the temperature range 0.67-0.60 T,. During dynamic and static recrystallization, the MC structure transformed to the SMC structure, and 6-phase plates precipitated uniformly throughout the matrix. The size of these precipitates was similar to the matrix grain size (0.2-0.5 pm). Further reduction in the processing temperature to 0.5 T, provided additional refinement of the microstructure to a grain size of less than 0.1 pm. The formation of NC structures in alloy 718 by cold severe plastic deformation (0.18 T,) was also studied. The microstructure evolution during combined torsion and compression was examined. With increasing strain, a cellular structure was formed, which was then refined to a granular structure with a grain size of 30-40 nm, with high angle grain boundaries and a high level of residual stress within the grains. The influence of the initial grain size on deformation mechanisms during superplastic deformation of alloy 718 was also studied. For grain sizes of 0.1-0.08 pm, superplasticity (6=350%, m=0.37) was observed at a temperature of 600°C and a strain rate of 1. 5~ ~O-~s-l. Supcralloys 718, 625. 706 and Various Derivatives Edited b~ E.A. Loria TbIS (The Minerals. Metals & Materials Society). 2001

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This paper will describe the generation of micro- and sub-microcrystalline structures in two Ni-based alloys that are typically strengthened by phases, such as γ′ and γ″+δ. The relationship between the superplastic behavior and microstructure will be discussed. High strain deformation processing in the temperature range of 0.9 Tm to 0. 6Tm results in reduction of the initial coarse-grained structure (>100 µm) to a range of structures including microcrystalline (MC) (grain size <10 µm) and sub-microcrystalline (SMC) (grain size <1 µm) with increasing deformation. The influence of alloy chemistry and constituent phases on dynamic and static recrystallization is considered, and their effect on grain refinement is described. Low-temperature and high strain rate superplasticity can be observed in dispersionstrengthened alloys with SMC structures. It was established that in dispersion-hardened Ni alloys with SMC structures, superplasticity can be observed at temperatures 200-250°C lower than in alloys with MC structure.

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A newly developed heavily alloyed polycrystalline nickel-base superalloy containing rhenium and intended as a structural material for turbine discs in gas turbine engines has been studied. Homogenisation and heterogenisation heat treatments were developed for the as-cast superalloy, which led to improving the hot workability as compared to that of as-cast material. electron backscatter diffraction analysis of samples isothermally compressed at temperatures near the solvus temperature showed that continuous dynamic recrystallisation occurred in the superalloy leading to the formation of refined recrystallised structure. These results were used for the development of canned forging processing under quasi-isothermal conditions. The superalloy subjected to canned forging followed by aging exhibited superior high strength and high-temperature capability, while retaining reasonable ductility as compared to known disc superalloys.