The Influence Of Deformation And Heat Treatment On The Formation Of Annealing Twins In Nickel Baseband

It has been widely reported that nickel and its alloys are known as substrates for second generation of high temperature superconducting tapes. During thermo-mechanical treatment, annealing twin(Σ3) can be easily formed in nickel alloys because of its low- to medium-stacking fault energy, and these twin boundaries have an adversely effect on the conductivity of high temperature superconducting coated conductors. In this work, the evolution of micro-hardness, microstructures and texture, especially the annealing twin, during thermo-mechanical treatment have been systematically investigated. Moreover, the formation of the annealing twin and its relationship with cubic texture are analyzed. The main conclusions achieved are as follows:The research about annealed substrate with different temperatures shows that:(1) During the process of recrystallization, the formation of annealing twins is related not only to the grain boundary migration, but also to the evolution of cubic texture, both of which occur at the same time and depend on each other.(2) During grain growth after fully recrystallization, the fraction of Σ3 and HAGBs decrease significantly with further increasing temperature due to the encountering with others of growing cube-oriented grains to form new LAGBs.The research about low temperature annealed substrate shows that:(1) Due to lower driving force at low temperature, the impingement of the recrystallized grains leads to the development of the cluster and the decrease of the grain growth rate, which suppress the sustainable growth of annealing twin.(2) Meanwhile, during this process, a certain number of triple junctions of Σ1-Σ3-Σ3 types with high stability can effectively inhibit continuous migration of grain boundaries and thus suppress the growth of cube texture and annealing twins. As a result, there has been a lot of isolated twins existing within the cube oriented grains, and they didn’t disappear during the subsequent prolonged annealing.The research about substrate with different total reductions and varied annealed temperature shows that:(1) Samples with lower reductions can easily produce high frequency of special grain boundary, and even formed clustering of special boundaries which break the connectivity of HAGBs. Moreover, incoherent twins were found during low temperature annealing, which lead to the proportion of high order special boundaries increases.(2) With the increasing of deformation and annealing temperature, the percentage of HAGBs as well as grain surrounded by HAGBs, decreased rapidly. Simultaneously, the increasing of cubic texture and LAGBs lead to connectivity of grain boundary network completed gradually.