Effect of Impurities on the Electronic Structure of Grain Boundaries and Intergranular Cohesion in Tungsten
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EFFECT OF IMPURITIES ON THE ELECTRONRC STRUCTURE OF GRAIN BOUNDARIES AND INTERGRANULAR COHESION IN TUNGSTEN GENRICH L. KRASKO Army Research Laboratory, Metals Research Branch, Watertown, MA 02172-0001 ABSTRACT The cohesion of a grain boundary (GB) is believed to be the controlling factor limiting the ductility of high-strength metallic alloys, and particularly W. Intergranular embrittlement is usually associated with segregation of impurities at the GBs. Impurities present in ppm concentrations can result in a dramatic decrease in plasticity. This paper reviews recent results on both semi-empirical and first-principles modelling of the energetics and the electronic structures of impurities on a 13 (111) GB in W. Our calculations have shown that impurities, such as N, 0, P, S, and Si weaken the intergranular cohesion resulting in "loosening" the GB. The presence of B and C on the contrary, enhances the interatomic interaction across the GB. The so-called sitecompetition effect should play an important role affecting impurity distribution in W GBs. Among the impurities analyzed, B in the GB has the lowest energy, and thus would tend to displace other impurity atoms from the GB. Microalloying with 10-50 ppm B may be an effective way of improving tungsten's ductility. These results are important for understanding the fundamental physics of intergranular embrittlement. INTRODUCTION The reduced cohesion of grain boundaries (GBs) is often the controlling factor limiting ductility, and hence performance and reliability of high-strength metallic alloys [1]. Intergranular embrittlement in metals is usually caused by impurities segregating towards the GBs [2-6]. A ductile-brittle transition temperature (DBTT) as low as -196C [7] was observed in high purity W single crystals obtained by electron beam zone melting with special impurity gettering. Impurities present in bulk concentrations of only 10-3-10-4 atomic percent can result in a dramatic decrease of plasticity. This drastic degradation of mechanical properties of metallic alloys, in particular W, poses significant technological and application problems. The detrimental effect of minute impurity concentrations can be readily understood. A simple estimate shows that a ppm amount of impurity is sufficient for saturating all the grain boundaries in a typical polycrystalline material. Sensitivity of the DBTT' to the grain size confirms the above physical concept: the larger the grain size, the smaller amount of impurity is needed to saturate the GB[3]. Finer-grain polycrystals are known to be less brittle. It should be noted that BCC crystals, being not as close packed as FCC or HCP, are particularly prone to GB embrittlement by impurity segregation. If impurities are the main cause of embrittlement, gettering these impurities is an obvious way of ductilizing W. A well-known, though extremely costly option is to use the so-called "Rhenium Effect" (see, e.g.[8]). A more promising way of removing "the harmful" impurities, such as 0, N, P, from the GBs is gettering by forming th
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