Notch effects on tensile behavior of Ni 3 AI and Ni 3 AI + B
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P.S. K H A D K I K A R , J.J. L E W A N D O W S K I , and K. V E D U L A Tensile tests have been carried out at room temperature on double-notched specimens of Ni3AI and Ni3A1 containing boron to observe the effect of boron on the notched specimen tensile behavior of Ni3A1. The results indicate that a predominantly intergranular fracture can be induced in the presence of a high constraint (notch), even in boron-doped Ni3A1, although a finite elongation and transgranular tearing-type fracture near the notch root were also observed. Fracture initiation was observed to occur in a brittle intergranular fashion in regions ahead of the notch in notched specimens of both Ni3A1 and Ni3A1 microalloyed with boron. An increase in the intergranular fracture stress of Ni3A1 due to boron was noted from the semiquantitative analysis (based on existing finite element models) of the notched tensile specimen data.
I.
INTRODUCTION
THE effects of boron
addition to Ni3A1 have been studied by several groups of investigators I~ ~61 ever since the beneficial effect of boron in improving the room temperature tensile ductility of polycrystalline Ni3A1 was initially discovered by Aoki and Izumi in 1979. m In smooth tensile specimens, improvement in the room temperature ductility via boron additions was observed to accompany a change in fracture mode from brittle intergranular fracture to predominantly transgranular fracture. This change in fracture mode was attributed to the boron enrichment of grain boundaries as revealed by Auger spectroscopy. [2,11-~3~Based on observations of the segregation behavior of boron, it was concluded t2,~3] that boron ductilizes Ni3A1 by improving the grain boundary cohesive strength. The proposed mechanism of ductility improvement is based on the effects of increased grain boundary strength in enabling higher levels of stress to be applied to a polycrystalline specimen, thereby activating slip before reaching the grain boundary fracture strength. However, no direct evidence of the increased grain boundary strength is available. Another possible mechanism which may contribute to the ductilizing effect of boron in Ni3A1 was suggested in more recent work by Khadkikar, Vedula, and Shabel IS1 and Schulson et al.,t91 who studied the grain s i z e - y i e l d strength relationships for Ni3AI and Ni3A1 + B. Both works rSm revealed that boron additions to polycrystalline Ni3A1 tested in both tension and compression produced a decrease in the Hall-Petch slope. These results were interpreted t8'9} to conclude that additions of boron facilitate slip propagation across the grain boundaries, thereby reducing stress concentrations at the boundaries. Although in situ straining of both Ni3A1 and Ni3A1 with boron in a transmission electron microscope t~~ has revealed initiation of slip in adjacent grains at dislocation pileups at grain boundaries, no discernible effects due to boron were noted. Hack et al. [~4} attempted to model the smooth tensile specimen fracture behavior of Ni3Al by considering the P.S. KHADKIKAR,
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