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=1162 kJ mol−1 and QU=1106 kJ mol−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, XDRX can be described in terms of double S-curves exhibiting sudden–steady–rapid–slow property.