Crystallographic and dimensional characteristics of structure elements in welds of high-temperature nickel alloy single crystals

Abstract

Crystallographic-orientational and dimensional-morphological features of microstructure in welded joints of high-temperature nickel alloy single crystals are considered, depending on the initial crystallographic orientation and its change over the weld solidification macrofront. EBSD analysis and optical metallography methods were used to study the structural features of individual zones of welded joints of high-temperature nickel alloy single crystals. their dependence on the initial crystallographic orientation of the welded joint and its change over the weld pool solidification macrofront is established. It is shown that alongside the predominant inheritance of base metal crystallographic orientation by the weld metal, formation of a grain structure of different crystallographic orientation, morphology, dimensions and nature, namely solidification and deformation-induced, can be in place. The mechanisms of formation of grains of the specified nature are considered. Boundaries of solidification type grains are the junction of dendrite blocks with different crystallographic orientation. Deformation type grains are the result of relaxation of microstresses at the stage of welded joint cooling, and they form in areas where the direction of acting stresses is the closest to the crystallographic orientation of easy slip. It was also found that deformation type grains can form both in the weld metal, and in the haZ. at the same temperature conditions of making the welded joints, the quantitative and dimensional-orientational parameters of deformation structure of the haZ metal correlate with similar parameters of weld structure elements. The possibility to produce welds without cracks with perfect single crystal structure is indicated.