Temperature dependence of the microhardness of beryllium and Be-Cu alloys monocrystals

The hardness of beryllium has been investigated many times [1-9], although there are no data on the temperature and concentration dependences of this characteristic. Useful data on the anisotropy of the strength and ductility characteristics can be obtained from the variation of the microhardness with temperature and impurity concenration. Of particular interest is the microhardness of crystals along the c axis. Data on the plastic deformation of beryllium crystals with compression along the c axis are contradictory [10], and therefore the variation of the microhardness with temperature and impurity concentration in combination with data on the mechanical properties should yield significant information on the embrittlement of beryllium. In Vickers hardness tests with a pyramidal indentor the microhardness of beryllium single crystals and Be-Cu alloys along the c axis reaches values of 1000-1200kg/mm2 at low temperatures, which are comparable with the values for some crystals with covalent bonds. With increasing temperatures and alloying with copper the anisotropy of the microhardness decreases, but the absolute values remain high up to 400degK. The high anisotropy of the hardness of beryllium is due to its lattice and not to the presence of imputities. The results obtained and previous results on the critical shear stresses in different slip systems indicate strong anisotropy of the bonding forces in beryllium crystals and beryllium alloys with copper