The role of pyramidal ⟨c plus a⟩ dislocations in the grain refinement mechanism in Ti-6Al-4V alloy processed by severe plastic deformation

This study focuses on the role of pyramidal < c + a > dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial {11 (2) over bar0} < 10<(1)over bar>0 > rolling texture. A large number of pyramidal < c + a > dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal < c + a > dislocations were discovered. First, pyramidal < c + a > slip coordinates the large c-axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic < c > and prismatic < c + a > dislocations) were produced for the first time by pyramidal < c + a > interacting with basal < a > dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (similar to 6.6 x 10(15) m(-2) in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal < c + a > dislocations in coarse grains; (II) basal < a > intersecting with prismatic < a > dislocations in coarse grains; and (III) basal < a > intersecting with pyramidal < c + a > dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal < c + a > slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.