ENVIRONMENTAL EMBRITTLEMENT AND GRAIN-BOUNDARY SEGREGATION OF BORON AND CARBON IN NI-3(SI,TI) ALLOYS

The effect of the addition of small amounts of boron and carbon on the environmental embrittlement of Ni-3(Si, Ti) alloys was investigated b y the room temperature tensile test. It is shown that the addition of boron to Ni-3(Si, Ti) alloys completely suppresses the embrittlement i n air and distilled water over the whole range of strain rates, and sl ightly suppresses the embrittlement in H-2 gas. The addition of carbon to Ni-3(Si, Ti) alloys completely suppresses the embrittlement in air and distilled water at high strain rates, moderately suppresses the e mbrittlement at low strain rates and slightly suppresses the embrittle ment in H-2 gas over the whole range of strain rates. The distinction between the embrittlement in air (or distilled water) and H-2 gas is d ue to the different decomposition kinetics into hydrogen atoms for H2O and H-2 molecules. The beneficial action of dopant atoms on the envir onmental embrittlement is attributed to boron (and carbon) segregation to grain boundaries, where they compete for site occupation and/or di ffusion with hydrogen atoms, resulting in the alteration of the grain boundary to transgranular fracture mode.