高純度鉄およびその希薄合金の機械的性質

Abstract

In order to reveal some of the inherent mechanical properties of iron, we prepared specimens which were purer than any iron ever used for this purpose. The purity was represented with the resistivity ratio (the explanation of the resistivity ratio being given in the text) and was more than 3600 and in most cases over 5000. From these values, the purity was estimated to be better than 99.999%. The resistivity ratio of commercial high purity iron with a purity of slightly less than 99.999% is 2000 or less. Our iron will be referred to as ultra-high purity in the text. (1) The yield stress (0.1% off-set) is almost independent of the purity if the purity is 99.995% or better. The temperature-yield stress relation is convex upward between 280 and 170 K. This convixity is larger in purer specimens, e.g., the yield stress is larger for purer specimens in this temperature range. (2) At room temperature or above, three-stage hardening is observed. At 333 K work-softening, rather than work-hardening is observed. It is shown that work-hardening and work-softening of ultra-high purity iron are reasonably explained in terms of the fundamental properties of dislocations. (3) Dilute alloys of ultra-high purity iron with carbon, hydrogen and with both elements were investigated. By alloying the yield stress and flow stress are decreased in certain ranges of concentration and temperature. This is the solution softening. Hydrogen has been thought to cause slight hardening in previous research. We proved the hardening to be due to combind effect of hydrogen and impurity atoms. (4) Low temperature fracture, especially intergranular fracture, is investigated in terms of the grain boundary segregation of solute atoms.