EFFECT OF TI IN SOLUTION ON DUCTILE-BRITTLE TRANSITION CHARACTERISTICS OF POWDER-METALLURGY MOLYBDENUM ALLOYS

By adding TiC (target content of Ti: 0.1-1.0 mass%), molybdenum-titanium alloys having titanium in solution up to 1 mass% and a certain content of oxygen were produced by powder-metallurgy technique. For these materials, tensile tests were performed at temperatures from 173 to 363 K. Yield and fracture (or tensile) strengths and total elongations were calculated from the load-displacement curves obtained at each temperature. Then the critical stress and temperature that were parameters representing the low-temperature fracture strength and the ductility of the material, respectively, were evaluated after correcting for the grain size effect. Finally the effect of titanium in solution on the ductile-brittle transition characteristics was discussed in terms of apparent intergranular and transgranular fracture strengths and yield strength. The results are summarized as follows. (1) TiC additions induced a relative reduction of oxygen content and a slight decrease of grain size compared to titanium additions. (2) The critical stress increased with increasing content of titanium in solution. Both the intergranular and transgranular fracture strengths increased with increasing content of titanium in solution up to 0.3 mass%. With a further increase of titanium in solution up to 1 mass%, the apparent transgranular fracture strength tended to decrease, whilst the apparent intergranular fracture strength was almost constant. (3) Corresponding to the increase of critical stress, the critical temperature was decreased with increasing the titanium in solution up to 0.3-0.5 mass%. However, the critical temperature began to increase at 1 mass% of titanium in solution, notwithstanding the critical stress was almost constant. The latter result is due to the relative increase of yield strength with the increase of the content of titanium in solution. (4) The fracture mode was mainly intergranular fracture, though the proportion of transgranular fracture tended to increase slightly for the alloy having 1 mass% of titanium in solution.