On the role of intercrystalline defects in polycrystal plasticity

Abstract

Intercrystalline defects comprised of grain boundaries and triple junctions are expected to have an effect on the mechanical properties of polycrystalline materials. Grain boundary structure is often described by the Coincident Site Lattice (CSL) model [1], wherein interfaces having a highly ordered structure are characterised by S-values of less than 29. Reduced susceptibility of these grain boundaries to creep (grain boundary sliding) [2], intergranular cavitation [3], and fracture/cracking [4] have been attributed to their reduced defect content compared with interfaces characterised by higher-order S relationships (ie. S . 29) [5–8]. Several complementary models have been advanced for the structure of triple junctions in polycrystalline materials [9–13]; each of these models being directly or indirectly dependent on the CSL character of the three adjoining interfaces and completing the representation of polycrystalline materials as a balanced network of dislocations (ie. lattice, grain boundary, and triple line) [13]. The defect content of triple lines, which all previous models predict to diminish at junctions formed by low-S CSL interfaces [14], is expected to have a significant influence on the mechanical behaviour of materials. Rabukhin [15, 16] investigated the effect of triple junctions on the room temperature tensile properties of conventional polycrystalline wires of Al, Cu, and W, at various grain sizes. Increases in tensile strength, and reduced ductility were consistently observed on the transition from an “equiaxed-tobamboo” grain structure wherein triple junctions are eliminated from the microstructure. The grain size dependence of the proof stress was found to obey the Hall-Petch relationship [17, 18]; however, at constant grain size, lower values were always obtained with the equiaxed geometry. Moreover, triple line softening has been proposed [19] as a possible mechanism for the frequently observed “inverse” Hall-Petch phenomenon in nanocrystalline materials as a result of large predicted increases in triple junction volume fraction with decreasing grain size (below approximately 15nm) [20]. Recently, the present authors and others [21–24] have demonstrated that the relative proportion of low-S CSL and “random” (ie. S . 29) interfaces can be controlled in a wide range of materials; offering the possibility for the first time, of assessing the impact of intercrystalline defect structure on bulk mechanical properties. The objective of this study was to evaluate the effect of grain boundary and triple line structure on the mechanical behaviour of polycrystalline Ni of commercial purity. Pergamon Scripta Materialia, Vol. 39, No. 3, pp. 341–346, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00 PII S1359-6462(98)00173-0