Environmental embrittlement and grain boundary segregation of boron in Ni3(Si,Ti) and Co3Ti alloys

Abstract

Recent observation for environmental embrittlement of Ni[sub 3](Si,Ti) and their alloys showed that boron doping has the effect of suppressing the environmental embrittlement. The room temperature tensile elongation of these alloys doped with boron was insensitive to the testing atmosphere and also strain rate, and was then higher than those of alloys without boron, while the room temperature tensile elongation of alloys without boron was lower in air than in vacuum, and also at a slower strain rate than at a higher strain rate. Boron is strongly enriched in grain boundaries in the case of Ni[sub 3]Al alloy. On the other hand, boron doping into Co[sub 3]Ti alloys does not have the effect of suppressing the environmental embrittlement. In this case, it is postulated that boron does not segregate, or little segregates, to grain boundaries in Co[sub 3]Ti alloys. However, direct observation for grain boundary chemistry has not yet been performed on alloys of both Ni[sub 3](Si,Ti) and Co[sub 3]Ti. The tensile results are summarized for undoped and boron-doped Ni[sub 3](Si,Ti) and Co[sub 3]Ti alloys, respectively. Both alloys of undoped Ni[sub 3](Si,Ti) and Co[sub 3]Ti tested in air showed reduced tensile elongation. However, boron-doped Ni[sub 3](Si,Ti) alloy tested in airmore » did not show reduced tensile elongation while boron-doped Co[sub 3]Ti alloy tested in air did. To verify the argument for environmental embrittlement and grain boundary segregation of boron, a comparison of the grain boundary chemistry between two alloys of boron-doped Ni[sub 3](Si,Ti) and Co[sub 3]Ti is necessary. In this work, the results of Atom Probe Field Ion Microscopy (APFIM) and Auger Electron Spectroscopy (AES) are shown for the grain boundary composition of Ni[sub 3](Si,Ti) and Co[sub 3]Ti alloys doped with boron, respectively. Based on these observations, some implication for the correlation between environmental embrittlement and the boron doping effect is discussed.« less