The Temperature Dependence of Grain Boundary Segregation in B-DOPED Ni 3 AI Bicrystals
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THE TEMPERATURE DEPENDENCE OF GRAIN BOUNDARY SEGREGATION IN B-DOPED Ni3AI BICRYSTALS URSULA OTTERBEIN, SIEGFRIED HOFMANN AND MANFRED RUHLE Max-Planck-Institut fOr Metallforschung, Institut for Werkstoffwissenschaft, Seestr. 92, D-7000 Stuttgart 1, Germany. ABSTRACT Equilibrium grain boundary segregation at symmetrical tilt grain boundaries of Ni3AI bicrystals containing 0.17 at% B and traces of S has been studied by Auger electron spectroscopy and compared directly to surface segregation by applying a special specimen geometry. From the experimentally determined interfacial concentrations after annealing at different temperatures, segregation free energies for B and S at both types of interfaces have been determined using the Langmuir-McLean equation. The effect of B and S segregation on the grain boundary cohesive energy has been calculated. INTRODUCTION Intermetallics with Li2 structure such as Ni3AI possess scientifically interesting features at high temperatures like anomalous yield strength-temperature behaviour and large Youngs modulus. Ni3AI serves as model material for understanding Li2 properties. Polycrystalline Ni3AI possesses very poor ductility due to intrinsic brittleness of its grain boundaries. 1However, small additions of B enhance the ductility by up to 50% in Ni2rich samples . B segregates to grain boundaries inboth Ni-rich and Al-rich samhas been also experimentally observed and is ples . Ni grain boundary 3enrichement independent of3B5 doping ,4.The reason for the ductilizing effect of B is not yet clear. Some authors . think that B segregation enhances the cohesive strength of the grain boundaries, whereas others6,7 assume that disordering near the grain boundaries eases transmission of slip across the grain boundaries. Inthis study the segregation behavior of B and S in Ni3AI was examined. Because differences in segreprevious experiments 3 on polycrystalline samples showed large gation levels from boundary to boundary due to anisotropy 8, only bicrystals with well defined orientations were studied. The change in cohesive energy with B was determined from the differences in impurity concentrations at grain boundaries and free surfaces. This requires the use of a special structure of the bicrystalline interfaces, which enables analysis of adjacent free surface and grain boundary regions under identical conditions. CONNECTION BETWEEN SEGREGATION TO INTERFACES AND COHESIVE ENERGY
For adiabatic separation the ideal work of fracture per unit area of the grain bounof energies by the difference dary is equal to the cohesive energy 1coh which is given 2 2 of the grain boundary YGB and the two free surfaces of symmetrical boundaries) 9:
fs (Ycoh =
YFS - YGB in the
case
The interfacial energies yo (index Ddenotes GB or FS) are changed by the segregation of impurities by Aya= NaG4 10 , where Nd is the number of impurity atoms per GD should be independent unit area and Go their Gibbs free energy at the interface. of N4>. For NGB = 2NFS and following Rice et al.1 1 the change of Ycoh by impurity segre
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