Defect structures in cold worked and small grain pure and boron-doped Ni 3 Al alloys
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DUCTION Improved low temperature ductility of boron-doped Ni3Al alloys occurs only in the hypostoichiometric (25 at. % > Al > 23 at.%) composition range. 1 ' 2 In stoichiometric and hyperstoichiometric alloys no such improvement has been observed. This effect has been found2"4 to be related to boron segregation to grain boundaries; i.e., considerable boron enrichment (=10%) occurs at grain boundaries in hypostoichiometric alloys, while in stoichiometric and hyperstoichiometric materials enrichment levels no higher than = 3 % have been observed.2'3 Even when boron segregation to grain boundaries is high, the degree of enrichment has been found to vary 4 ' 5 from boundary to boundary and within the same boundary. The mechanism by which boron improves ductility has been the subject of much research and debate. Two possibilities have been considered: (i) Boron increases the cohesive strength of grain boundaries.6'7 (ii) The presence of boron at grain boundaries makes them more susceptible to the propagation of slip.8 No clear evidence has been presented which eliminates either mechanism. Theoretical calculations9'10 indicate the cohesive energy of grain boundaries in brittle Ni3Al to be similar to that of ductile Ni. Experimental evidence for both mechanisms is largely phenomenological. " Department of Applied Science and Physics
Takasugi et al.6J found a lack of correlation between metallurgical and crystallographic properties and brittle failure of a large number of alloys while chemical factors such as a valence difference were shown to correlate well with improved ductility. Schulson et al.8 have argued, albeit somewhat indirectly, using the relationship between yield strength and grain size, that boron increases the susceptibility of grain boundaries in Ni3Al to slip. These authors have shown, using Transmission Electron Microscopy (TEM), both dislocation pileups at a grain boundary and slip band propagation across a grain boundary in borondoped Ni75Al25. Bond et al." have observed, again using TEM, dislocation pileups at grain boundaries in Ni76Al24 and Ni76Al24 + 100 wt ppm B. However, these authors also observed the generation of dislocations in a second grain after pileup had occurred at the grain boundary of an adjacent grain. Here we present positron annihilation (lifetime) data which show that dislocations, produced during cold working, do not interact with grain boundaries in boron-doped Ni 76 Al 24 , while in pure and boron-doped Ni74Al26 and pure Ni76Al24 some part of the population of dislocations does interact with grain boundaries. Positron annihilation spectroscopy12 utilizes the fact that a positron (anti-particle of the electron), sometime after entering matter, will annihilate with an electron producing a number, in metals most probably 2, of y-photons. These y-photons carry information pertaining to the initial state of the annihilating pair. In particular, the time of their
J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
55
S. G. Usmar and K. G. Lynn: Defect structures in Ni3AI alloys
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