A metallographic study of certain sintered zirconium base alloys

摘要

Following a literature survey and a number of exploratory experiments, mixtures of zirconium and chromium powders containing up to 18% by weight of chromium were cold pressed and sintered in vacuo at temperatures in the range 900°C -1100°C. for times up to 16 hours. The densities and hardnesses of the sintered alloys were determined and the resultant micro-structures were examined under the optical microscope. The density and hardness results were subjected to an analysis of variance to establish the significances and validities of the observed trends. It was found that both density and hardness increased with increasing temperature and time of sintering and with increasing chromium content, the rates of increase were less in the upper temperature ranges, and sintering temperature was a more significant variable than either of the other two. In no case were theoretical maximum densities obtained, and anomalous density decrements were observed associated with particular combinations of sintering temperature and time. Microscopic examinations of the alloys showed that in no case were equilibrium conditions reached, although the alpha-zirconium/ZrCr[2] eutectoid and some massive ZrCr[2] were formed, complete solution of the free chromium particles in the zirconium matrix was not obtained. Porosity, of two types, was seen. The normal sintering porosity showed a decrease in pore number and an increase in pore size with increased sintering. At the higher temperatures and longer times of sintering, other pores were seen associated with the free/free chromium/zirconium matrix interfaces. These pores were attributed to differential diffusion effects across the interfaces and account for the anomalous density effects. Certain phenomena associated with abnormal sintering conditions were also detected and examined. A number of conclusions were drawn on all aspects of the work and possible further work was outlined.