Published online by Cambridge University Press: 25 February 2011
The conditions established in disordered FCC systems for predicting the slip system that will be activated by a grain boundary to relieve a local stress concentration have been applied to the ordered FCC alloy Ni3Al. The slip transfer behavior in hypo-stoichiometric Ni3Al with (0.2 at. %B) and without boron was directly observed by performing the deformation experiments in situ in the transmission electron microscope. In the boron-free and boron-doped alloys, lattice dislocations were incorporated in the grain boundary, but did not show evidence of dissociation to grain boundary dislocations or of movement in the grain boundary plane. The stress concentration associated with the dislocation pileup at the grain boundary was relieved by the emission of dislocations from the grain boundary in the boron-doped alloy. The slip system initiated in the adjoining grain obeyed the conditions established for disordered FCC systems. In the boron-free alloy, the primary stress relief mechanism was grain-boundary cracking, although dislocation emission from the grain boundary also occurred and accompanied intergranular crack advance. Because of the importance of the grain boundary chemistry in the models for explaining the boron-induced ductility in hypo-stoichiometric Ni3Al, the chemistry of grain boundaries in well-annealed boron-doped and boron-free alloys was determined by using EDS. No Ni enrichment was found at the grain boundaries examined. These observations are discussed in terms of the different models proposed to explain the ductility improvement in the boron-doped, hypo-stoichiometric alloy.
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