Microstructural and mechanical characterization of iron samples sintered in DC plasma

Samples of unalloyed iron were sintered in an abnormal glow discharge using a confined anode–cathode configuration. In this geometry the samples were placed on the holder, which acted as the discharge anode, while the cathode surrounded the sample undergoing bombardment by ions and energetic neutral atoms/molecules, leading to heating and sputtering. For comparison, another set of samples was sintered in a resistive furnace. The presence of large grains was observed for plasma-sintered unalloyed iron, which could be attributed to a large amount of atomic hydrogen in the electric discharge. These grains influence the mechanical properties of the sintered product. Three-point bending tests revealed lower yield strength and, quite surprisingly, ductility, compared to samples sintered in a furnace under the same temperature and time conditions. The decrease in ductility is associated with intergranular crack propagation in plasma-sintered samples. Such embrittlement could be attributed to the absorption of hydrogen during plasma sintering or higher impurity content at grain boundaries.