Koval O. The influence of structure and temperature on fracture toughness and fracture mechanisms of molybdenum

The thesis is devoted to a study of influence of structural features of molybdenum alloys on their fracture behavior, fracture toughness and fracture mechanisms, in a wide temperature range. For the study molybdenum alloys of different purity (ЦМ-10, MT, MЧВП) were chosen. For these alloys the influence of grain size on mechanical properties and fracture mechanism was studied in a wide temperature range. It was found that ЦМ-10 alloy fails by the cleavage and the cleavage with relaxation and demonstrate non-monotone dependence of fracture toughness on temperature in 77-573 K temperature range. The value of activation energy of the process determining the temperature dependence of fracture toughness corresponds to the energy of activation of dislocation motion. It was found that in a case of ЦМ-10 alloy dependence of the fracture toughness on grain size does not follow the Hall-Petch equation at elevated temperatures. In contrast to ЦМ-10 alloy, the MT alloy fails by brittle intergranular mechanismwithin the whole temperature range studied. In case of MT alloy the dependence of the fracture toughness from temperature and grain size is monotonous. For MЧВП alloy, all possible fracture mechanisms relevant to ВСС-metals with dependence on their grain size are observed at room temperature. To enhance mechanical properties, Mo+4%Re and low alloyed molybdenum, for instance, so-called relaxation processing was proposed resulting in two times increase of fracture toughness. Dependence of product of fracture toughness on yield strength on temperature was analyzed in a wide range of temperatures. It was found out that in case of transcrystalline and intercrystalline fracture mechanisms product is constant