Electron-Microscopic Observations of Tensile-Fractured Surfaces of Prestrained Pure Chromium

Abstract

This work has been undertaken to examine the submicroscopic structure of tensile fractured surfaces of pure chromium specimens recrystallized or prestrained under various conditions by electron-microscopic examination of the fractured surface using the replica technique.Tensile testing has been done at room temperature. Brittle or ductile fracture has been observed depending upon the treatment of specimens. Methyl-methacrylate-carbon replicas shadowed with chromium have been used for the electron microscopic examination of fractured surfaces.The results obtained are as follows:(1) Specimens as recrystallized:The fractured surface consists of flat cleavage and cleavage steps. The cleavage steps are rather small in their density and some of them are presumed to be due to secondary cleavage.(2) Specimens prestrained by a little amount:The density of cleavage steps increases and the “river pattern” formed.(3) Properly prestrained and ductile specimens:Three kinds of fractured surfaces are mixed together.Type-A: Comparatively flat smooth surface.Type-B: Surface covered with shallow dimples.Type-C: Rough surface consisting of numerous, submicroscopic unit cleavage facets.(4) Overprestrained specimens:The fractured surface is composed of cleavage surfaces entirely covered with well-developed river patterns.(5) Specimens properly prestrained and annealed:The fracture surface is composed of ductile regions which are similar to the Type-A surface in (3) and brittle are just like the brittle cleavage in (4).Based upon the above observations, discussions have been given about the mechanism of ductile and brittle fracture in pure chromium.