Abstract
Molybdenum ingots, which are electron beam melted without any additions, are not ductile at room temperature and liable to intergranular fracture under a low stress. The purposes of this experiment are (1) to investigate the variations in ductility of some kinds of molybdenum ingots which are electron beam melted with C, B, Zr and Ti, respectively, and (2) to make a study of the mechanisms for the improvement of their ductility by melting with the additional elements. By means of bend tests, measurements of intergranular fracture stresses and observations of fracture surfaces, following results were obtained. (1) The ductility of molybdenum ingots at room temperature was improved by electron beam melting with the optimum amounts of C, B and Ti, respectively. However, the addition of Zr did not improve the ductility of ingots. (2) The improvement in ductility was dependent on the strengthening of grain boundaries. (3) Judging from fractographic observations, it was considered that in the cases of C and B additions, the intergranular fracture strength was increased by the suitable amounts of intergranular precipitates. In the case of Ti addition, it was assumed that grain boundaries were strengthened by eliminating the segregation of some harmful impurities at grain boundaries. (4) In conclusion, the degree of improvement in ductility of molybdenum ingots by additional elements depends on the two opposite effects of the added elements, i.e., the lowering of ductility of matrix and the strengthening of grain boundary.
