Effect of Aluminum Level on Boron Clustering in Ni 3 Al
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EFFECT OF ALUMINUM LEVEL ON BORON CLUSTERING IN NiA3 J. A. Horton, M. K. Miller, C. T. Liu, E. P. George, and J. Bentley Metals and Ceramics Division, Oak Ridge National Laboratory P.O. Box 2008, Oak Ridge, TN 37831-6376 ABSTRACT In alloys containing 0.24% boron, atom-probe field-ion microscopy (APFIM) revealed the presence of boron clusters in Ni-25 at. % A] and Ni-26 A] but not in Ni-24 Al. The observed boron clusters generally consisted of two to three boron atoms with a maximum size of 10 atoms. Quench rates that ranged from rapid solidification to furnace cooling had little effect on the clustering. The occurrence of the clustering coincides with a higher rate of boron strengthening as measured by an increase in the yield stress per atomic percent boron, and it also coincides with a reduced amount of boron segregation to grain boundaries. The levels of nickel and boron were highly variable on grain boundaries in rapidly solidified material and therefore no consistent indication of nickel enrichment at the grain boundaries associated with boron segregation was found. This result suggests that cosegregation of nickel with boron may not be necessary for the ductilization of Ni 3AI by boron, since the rapidly solidified material is also ductilized by boron and exhibits segregation of only boron to the grain boundaries. INTRODUCTION Previous atom-probe studies of boron-doped Ni3AI have shown that boron segregates inhomogeneously to most linear and planar defects including both low and high angle grain boundaries, twin boundaries, antiphase boundaries, superlattice intrinsic stacking faults, and dislocations [1-3]. Based on APFIM observations, it was suggested that a disordered fcc layer at the grain boundary explained the boron ductilization of Ni3AI [1]. Recent high resolution transmission electron microscopy results show direct evidence of a disordered layer in boron-doped Ni-24 at. % Al [4]. Whereas these results produce a consistent explanation of the boron ductilization of hypostoichiometric Ni3AI (
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