Effect of forming conditions on the softening behavior in coarse grained structures

Polycrystalline nickel with a purity of 99.99 wt% and different starting grain sizes of 240 μm and 770 μm has been investigated. The effects of the initial grain size on the hot deformation behavior were studied by compression tests at various forming conditions. The microstructure was captured after deformation, using electron back scatter diffraction technique (EBSD). A variation of the initial grain size has unexpected effects on the acting softening mechanisms. A coarsening in the starting microstructure leads to a higher flow stress and a remarkable higher hardening at elevated temperatures. Furthermore, the nucleation of dynamic recrystallized grains is enormously retarded due to the less pronounced grain boundary bulging in a coarser grained microstructure compared to a finer one. As a consequence, discontinuous dynamic recrystallization generates a metastable grain size, that is finer than the steady state grain size.