Effects of boron on the deformation behavior of Ni3Al

Abstract

The effects of boron additions (0 to 4000 wppm B) on the room temperature deformation behavior of Ni-24Al 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-24Al 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 Ni3Al. 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-25Al, the so-called stoichiometric effect, can be attributed to a diminished “softening”, combined with the rapid solid-solution strengthening observed in this alloy.