Microstructural examination of shear localisation during high strain rate deformation of Alloy 718

Abstract

Chip formation during metal cutting involves high strain rates and large deformations. Under many conditions, the deformation is concentrated in narrow bands due to shear localisation from adiabatic heating. In order to understand the localisation process, it is necessary to increase the knowledge regarding the microstructural evolution during deformation. However, the deformation that occurs during chip formation is hard to measure. Therefore, this study utilises top-hat specimens deformed at high strain rates in order to generate localised shear bands in the Ni-based superalloy Alloy 718, with defined measurable deformation. The resulting shear bands in the top-hat specimens are compared with those generated in metal cutting chips and studied in order to characterise the deformation occurring at a microstructural level. The resulting microstructures in the top-hat specimens and machining chips are found to be similar, with heavily localised deformation into narrow shear bands and homogeneously sheared microstructure adjacent to the bands. The centres of the shear bands are heavily deformed with ultra-fine grains, indicating dynamic recrystallisation during the deformation. The results indicate that the shear deformation produced by high strain rate testing of top-hat specimens can provide an excellent means of replicating the conditions for shear localisation during metal cutting. However, care should be taken to design the tests so that the local conditions are representative in terms of strains and strain rates.

Effects of different heat treatments on the dynamic shear response and shear localization in Inconel 718 alloy

“…9(c). Similar observations are also found by Johansson et al [20]. The band contrast is sensitive to the presence of the high and low angle grain boundaries and thus can be used to characterize the microstructure of the Inconel 718 alloy.…”

“…In the top hat shear tests conducted by Demange et al [19], it is noteworthy that the precipitation hardened material readily exhibited shear localization, while the solution treated material demonstrated no evidence of shear localization. More recently, in order to understand the localization process in precipitation hardened Inconel 718, Johansson et al [20] performed particular investigations on the microstructure of ASB formed in the precipitation hardened Inconel 718 during machining and the top hat shear tests, respectively. They compared the formed shear bands in the top-hat specimens with those generated in metal cutting chips.…”

Dynamic Shear Deformation of a Precipitation Hardened Al_0.7CoCrFeNi Eutectic High-Entropy Alloy Using Hat-Shaped Specimen Geometry

“…Therefore, a detailed investigation of the microstructural evolution of ASB in metallic materials before and after impact loading tests is desirable [12,13]. Hat-shaped specimens, commonly referred to as top hat specimens, have been widely used to study the dynamic deformation behavior under shear loading [14][15][16][17][18][19][20][21][22][23][24] using split-Hopkinson pressure bar (SHPB) apparatus. This sample geometry promotes shear localization resulting in deformation focused in a narrow shear band (~ 2-50 µ m wide).…”

“…This sample geometry promotes shear localization resulting in deformation focused in a narrow shear band (~ 2-50 µ m wide). Inside the shear bands, dynamically recrystallized grains of the order of 20-300 nm in diameter are observed [20], which can exhibit a shear texture. Observation of a rather wide zone with increasing rotation and elongation of the grains shows a rather abrupt transition from relatively homogeneous shear.…”

“…(2) Dynamic recrystallization (DRX) is observed in most the tested materials including copper [16], steel [17,18], pure titanium [15], titanium alloys [14,18], and aluminum alloy [19]. The shear texture is also observed in low-nickel-containing steel [17], pure titanium [15], and nickel alloy [20]. The DRX is confirmed by the emergence of ultra-fined grains with low dislocation density within the shear band [17].…”