Effects of 0.5 weight percent boron on cobalt aluminide

A study was made on the effects of adding 0.5 weight percent boron to the ordered intermetallic Cobalt Aluninide. Thirty gram (30g) specimens were induction melted in an pure argon atmosphere and cast into a copper alloy bar mold. The addition of boron raised the modulus of rupture from 30.1 to 44.3 ksi; increased the Knoop microhardness from 543 to 693; reduced the average grain size from 190 to 70 microns; and gave a more equiaxed grain structure. X-ray diffraction detected the presence of a second phase, AlB , 12 in the material containing boron. Scanning electron micro-scopy revealed that the fracture mode in both types of material was transgranular cleavage and that the fracture surface of the material containing boron contained many particles in the size range of 3 microns that the fracture was forced to go around. It was concluded that the increase in the Modulus of Rupture was due to a reduction in flaw size from the smaller, more equiaxed, grains, as well as to the interference in crack propogation caused by the particles --Abstract, p. i