Abstract
To study the deformation behavior and recrystallization of alloy 718 in annealed and aged state, compression tests were performed using Split-Hopkinson pressure bar (SHPB) at high strain rates (1000 to 4000 s−1), for temperatures between 20 ^circ C and 1100 ^circ C (293 K to 1373 K). Optical microscope (OM) and electron back-scatter diffraction (EBSD) technique were employed to characterize the microstructural evolution of the alloy. The stress–strain curves show that the flow stress level decreases with increasing temperature and decreasing strain rate. In addition, up to 1000 ^circ C, the aged material presents higher strength and is more resistant to deformation than the annealed one, with a yield strength around 400 MPa higher. For both states, dynamic and meta-dynamic recrystallization occurred when the material is deformed at 1000 ^circ C and 1100 ^circ C, leading to a refinement of the microstructure. As necklace structures were identified, discontinuous recrystallization is considered to be the main recrystallization mechanism. The recrystallization kinetics is faster for higher temperatures, as the fraction of recrystallized grains is higher and the average recrystallized grain size is larger after deformation at 1100 ^circ C than after deformation at 1000 ^circ C.
Highlights
It is imperative to note that energy-dispersive X-ray spectroscopy (EDS) works as best as only ‘‘semi-quantitative’’ in capability,[32] and the chemical composition presented here need to be viewed in a semi-quantitative light without more standardization
Variations of less than ± 3 pct in nickel, chromium and iron contents indicate that the composition of the alloy used in the current study is consistent with the recommendations for a standard alloy 718.[28]
– The flow stress decreases with increasing temperature and decreasing strain rate, which is the common behavior of metals and alloys
Summary
It was developed in the 1960s and it is, nowadays, one of the most widely used superalloys due to its high strength, high corrosion resistance and good mechanical properties at high temperature. It presents good fatigue, creep and wear resistance. Alloy 718 is a precipitate hardening alloy with a face-centred cubic Ni-Cr matrix, the c phase. Two precipitates contribute to its strength: the c0 phase (Ni3(Al,Ti)), which is face-centred cubic and spherical or cuboid, and the c00 phase (Ni3Nb(Ti,Al)), which is body-centred tetragonal and disc-shaped. The c00 precipitates are usually considered as the main hardening precipitates in this alloy
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