Comparison of impact and slow bend behavior of PM ferrous alloys

Instrumented impact testing and slow bend (Three-Point Bending, TPB) testing were used to study the influence of strain rate on the bending behaviour of four types of powder metallurgical materials (material A: pure iron; material B: Fe-0.4C; material C: Fe-0.4C in the heat treated state; material D: Fe-4Ni-1.5Cu-0.5Mo-0.5C) produced at two density levels. Whereas materials A and B are characterized by the presence of a distinct yield point and a pronounced plastic deformation before fracture, materials C show a reduced plastic deformation and materials D are characterized by a continuous yielding behaviour both under impact and TPB testing. The ratio between the impact and TPB yield loads was always larger than unity and increased as the materials hardness decreased. This ratio is mainly influenced by the matrix microstructure. On the other hand, both matrix microstructure and porosity influence the ratio between the impact and TPB maximum loads, which is in any case close to unity. Concerning the fracture energies, all the materials except materials A display a fracture energy which is larger under impact than under TPB. The highest ductility of materials A is responsible for the formation of an intense plastic deformation and a stable microcrack system on the tensile side of the specimen during TPB, which produces values for the TPB fracture energy similar to the impact fracture energy values. (C) 1998 Elsevier Science S.A. All rights reserved.