Analysis of Thermally Generated Microstresses in Polycrystalline be due to the Presence of BeO Inclusions

Abstract

In polycrystalline hexagonal beryllium (Be), the microstresses due to the relatively small thermal expansion anisotropy were previously estimated, on an ideal elastic basis, to be comparable to those stresses measured for the general yield and fracture strengths of bulk Be material. Commercial beryllium materials normally contain inclusions of (hexagonal) beryllium oxide (BeO), and the mismatch of expansivity between these materials is now shown to be capable of producing even larger elastic microstresses which should produce additional localized yielded and fracture zones within the material. Despite the fact that the BeO inclusions are in compression, both compressive and tensile stresses are generated in the surrounding shell of Be material. The plastic zone size is estimated to be as much as ten times larger than an inclusion diameter so that an important parameter affecting the nature of plastic flow and cracking around an inclusion should be the polycrystal grain size. These several considerations are described on the basis of plasticity and dislocation models which are proposed for the material behavior.