The Role of Local Disorder near Grain Boundary in Ductilization of Ni 3 Al
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THE ROLE OF LOCAL DISORDER NEAR GRAIN BOUNDARY IN DUCTILIZATION OF Ni 3AA JIAN SUN AND DONGLIANG LIN(T.L. Lin) Department of Materials Science and Engineering, Shanghai JiaoTong University, Shanghai 200030, P.R.China
ABSTRACT The local chemical order near grain boundaries (GBs) in NisAl was verified by measuring the local degree of order and chemistry with microdiffraction and EDS microanalysis. The composition of alloys are (a) Ni-24at%AI with and without 700ppm wt boron;(b) Ni-25 at 9Al with 700ppm wt boron;(c) Ni-26 at Yo Al with 700ppm wt boron. GBs studied in the present work are high angle boundaries. The results show that there is a local disordered region about 5-10nm wide on the side of the GB in non-stoichiometric Ni3 Al with or without boron, and no disordered region in stoichiometric NiAl. EDS microanalysis results show that the composition of GB almost keeps the matrix concentration of Ni in stoichiometric NisAl with boron, and is Ni-rich in hypostoichiometric and Al-rich in hyperstoichiometric NisAI with and without boron. The local disordered region existing in the vicinity of GBs is related to Ni enrichment or depletion near GBs in hypostoichiometric or hyperstoichiometric Ni3AI alloys. Boron seems not to be the main factor to control the local disorder and composition at GBs in Ni 3AI alloys. It can be concluded that the local disorder at GBs is not a decisive factor for ductilizing GBs in Ni3AI alloys.
INTRODUCTION The Ni3AI compound has received considerable attention during the past decade owing to its potential high temperature applications. However, grain boundaries (GBs) in Ni3Al are intrinsically brittle. Izumi et al.[1] first discovered that microalloying with a trace of boron increases the ductility of polycrystalline Ni 3Al. Liu et al.L2] have systematically investigated the beneficial effect of boron on ductility and fracture behaviour of NiAl alloys and have found that boron tends to segregate to GBs. The concentration of boron at GBs decreases with increasing bulk aluminium concentration from 24 at% to 25 ato, with a corresponding reduction of ductility from 50% to 6%. Two hypotheses have been suggested to explain the ductilizing effect of boron: (1) Boron enhanced cohesive strength of GB, and (2) boron facilitated slip transfer across GB. It was suggested that disordering in the GB regions might lead to enhance ductility by an increased number of permissible dislocation reactions with these brittle interfaces. Several experimental studies have attempted to check these hypotheses. Early results from high resolution energy dispersive x-ray spectroscopy and atom probe analysis[3,4] showed that the boundary regions were Ni-rich in boron doped Ni 3A1, whereas in boron free Ni5 Al the boundaries had the bulk composition, suggesting that the excess Ni at the boundary made it effectively partially disordered. However George et al. [5] and Krzanoski[6], using AES and STEM respectively, showed that GB in both boron doped and boron free NisAl had a similar Ni enrichment. Their results indicate
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