Discontinuous yielding in Ni-base superalloys during high-speed deformation

Discontinuous yielding in Ni-base superalloy during high-speed compression has been investigated. Flow stress curves of GH4049 can be divided into two types: the first type curves display common flow behavior revealing work hardening, stable, softening and steady stages; the second type curves present abnormal flow behavior revealing discontinuous yielding feature, which were characterized by a sharp peak stress (σP), obvious upper yield point (σU) and a lower yield point. Apparent activation energies for peak values and upper values were calculated to be QP=1162kJmol-1and QU=1106kJmol-1, respectively. Constitutive equations represent peak stress and upper stress as functions of strain rate and deformation temperature are described. First type curves present common work hardening behavior however, second type curves present spiral hardening behavior since discontinuous softening during high-speed deformation. When GH4049 superalloys present first type flow behavior, volume fraction of dynamic recrystallization (XDRX) can be described in terms of normal S-curves revealing slow-rapid-slow property. However, when alloys present second type flow behavior, XDRXcan be described in terms of double S-curves exhibiting sudden-steady-rapid-slow property.