Microstructural characterisation of deformation behaviour of nickel base superalloy IN625

Simple compression and microscopy techniques were employed to characterise the microstructural origin of the deformation behaviour of nickel base superalloy IN625 during large strain testing. The alloy exhibited a four-stage strain hardening response similar to that previously reported for low stacking fault energy face centred cubic alloys. At strains lower than about -0.06 (stage A), a falling regime of the hardening rate was observed. This stage was followed by a second stage (stage B) of slow increasing hardening rate, which was found to be coincident with the formation of Lomer-Cottrell locks. The second falling regime of strain hardening (stage C) was seen in the strain range of -0.25 to -0.65. The occurrence of this stage was attributed to the increasing ease of dislocation cross-slip with increasing strain and consequently to the decreasing Lomer-Cottrell lock efficiency in hindering dislocation movement. Beyond a strain of -0.65, a final slightly constant hardening regime (stage D) was developed. The initiation of this stage was concurrent with the onset of deformation twinning in the microstructure.