Processing and superplastic properties of fine-grained iron carbide

Abstract

Fine-grained iron carbide material (80 vol pct iron carbide and 20 vol pct of an iron-base second phase) was prepared using two different powder metallurgy procedures: (1) hot isostatic pressing followed by uniaxial pressing and (2) hot extrusion followed by uniaxial pressing. Both procedures yield materials that are superplastic at elevated temperature with low values of the stress exponent (n = 2 to 1) and tensile elongations as high as 600 pct. The strain rate in then = 2 region is inversely proportional to approximately the cube of the grain size with an activation energy for superplastic flow between 200 and 240 kJ/mol. It is postulated that superplastic flow in the iron carbide material, in then = 2 region, is grain-boundary sliding accommodated by slip controlled by iron diffusion along iron carbide grain boundaries. The flow stress in compression is about 2 times higher than in tension in the region where grain-boundary sliding is the rate-controlling process. It is believed that the difference in flow stress is a result of the greater ease of grain-boundary sliding in tension than in compression. Tensile elongations were observed to increase with a decrease in stress and a decrease in grain size. These effects are quantitatively explained by a fracture mechanics model that has been developed to predict the tensile ductility of superplastic ceramics.