抄録
In order to clarify the effect of nitrogen on the intergranular brittleness in molybdenum, bicrystals with the weakest ⟨110⟩ symmetric tilt boundary of a misorientation angle of about 33 degrees were specially prepared and were purified. Various amounts of nitrogen, 15 to 88 mass ppm, were doped into the purified specimens. The mechanical properties of purified and nitrogen-doped specimens were measured as a function of nitrogen content by a four-point bending test at the temperature of liquid nitrogen. The fracture surface was observed by SEM.
It is shown that the specimen containing 55 mass ppm nitrogen is fractured transgranularly and the other specimens completely intergranularly. The fracture behaviour depends strongly on nitrogen content. Small amounts of nitrogen, 15 to 30 mass ppm, enhance the intergranular brittleness, while considerable amounts of nitrogen, 55 to 88 mass ppm, suppress the brittleness. In particular, the 55 mass ppm nitrogen is most effective to suppress the brittleness; the ductility is as high as that of pure single crystals and the fracture stress is 1335 MPa. The fracture pass through a component crystal suggests that the cohesion of grain boundary should be higher than the measured fracture stress. The observed fracture behaviour is discussed in connection with the state (precipitation or segregation) of nitrogen at the grain boundary.
