Effects of boron on the deformation behavior of Ni 3 Al
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The effects of boron additions (0 to 4000 wppm B) on the room temperature deformation behavior of Ni-24A1 have been examined with both quasi-static deformation and short pulse duration shock loading. Changes in compressive yield strength and hardness with the amount of boron suggest that strengthening effects are more complicated than predicted by usual interstitial solid-solution strengthening considerations. The nature of the dislocations changes from SISF-dissociated superdislocations in the Ni-24A1 base alloy to APB-dissociated dislocations with the additions of small amounts of boron. Therefore, the observed variation in strength with boron additions reflects a solid-solution strengthening contribution from the interstitial boron coupled with a "softening" effect arising from the greater mobility of the APB-bounding partials relative to the SISF-bounding partials. It is suggested that this "softening" is a necessary prerequisite for enhanced ductility in Ni 3 Al. In addition, the grain boundary fracture strength must be increased by boron additions. While this may occur through an increased grain boundary cohesive strength due to boron segregation, it is also expected that the interaction of the dislocations with the grain boundaries will be significantly altered as the nature of the dislocations changes. The lack of a "boron effect" in Ni-25A1, the so-called stoichiometric effect, can be attributed to a diminished "softening", combined with the rapid solid-solution strengthening observed in this alloy.
I. INTRODUCTION A major shortcoming of unalloyed ordered N13AI as a commercial material is its low temperature brittleness in polycrystalline form. Although single crystals of Ni3Al can exhibit tensile elongations >50%, polycrystalline Ni3Al is extremely brittle and fractures in an intergranular (IG) manner.1"5 Although polycrystalline Ni3Al was initially thought to be intrinsically brittle, George et al.6 suggested recently that the IG fracture is associated with moisture-induced environmental embrittlement. In 1979, Aoki and Izumi7 discovered that small additions of boron (hundreds of wppm) can cause significant increases in the ductility of Ni3Al and change the fracture mode from IG to predominantly transgranular (TG). The "boron effect" was later confirmed for cast and recrystallized8 and rapidly solidified9 Ni 3 Al. In addition, Liu et al.8 found that boron segregates to the grain boundaries and that alloy stoichiometry influences the degree of the ductilizing effect, with hypostoichiometric Ni-24A1 showing the greatest benefit, and stoichiometric and hyperstoichiometric alloys showing little or no ductility improvement. Despite extensive research over the past decade,10"14 the mechanism(s) responsible for the improvement of ductility with boron additions remains unclear, and many "'Present address: The Timken Company, Canton, Ohio 44706-0930.
1742 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 7, Jul 1994
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of the results are controversial and often contradictory. The
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