Evolution Of Annealing Twin Boundary And Mechanical Behavior In A Nickel-iron Based Wrought Alloy

As a special boundary, annealing twin boundary is generally observed in face-centered cubic metals and alloys with low stacking fault energy after deformation and subsequent annealing processes. Compared with general high angle boundary, annealing twin boundary exhibits excellent properties related to degradation of boundaries attributing to the regular arrangement of atoms and low interfacial energy. Moreover, annealing twin boundary shows a great potential as a novel strategy for designing advanced superalloys with low stacking fault energies due to impede movement of dislocations during creep deformation. Consequently, it is helpful for the application of annealing twin boundary to study the evolution of annealing twin boundary during thermo-mechanical processing and the mechanical behaviors during tensile testing.In the present work, the method of thermo-mechanical processing was employed to prepare the samples, which possessed various grain sizes and different densities of annealing twin boundary. The evolutions of morphologies and densities of annealing twin boundaries were systemically studied via the pro-deformation, annealing temperature and time, and the factors controlling annealing twin boundary were revealed. The mechanical behaviors were observed in different samples under various conditions, with special emphasis on the serrated flows at low temperature, and the mechanisms of serrations were revealed via activation energies of serrations. Moreover, the relationships among serrations and grain size and annealing twin boundary were discussed at low tensile temperature. The samples with various densities of twin boundaries were prepared via thermo-mechanical processing on the basis of the evolving rules of annealing twin boundary with parameters. The relationships between annealing twin boundary and mechanical behaviors were studied systematically.During deformation process, the interactions between dislocations and annealing twin boundaries lead the continuous morphologies of twin boundaries to show the discontinuous morphologies due to the formation of series of stacking faults on twin boundaries. As a result, the boundaries gradually evolve to low-angle boundaries with discontinuous morphologies. The morphologies of twin boundaries in annealed samples clearly indicate that the degradations of boundaries with special orientations induce the formation of twin boundaries, and their development as a result of fast migrations of low-angle boundaries during recovery annealing process, and thus the cluster morphologies of twin boundaries connecting with low-angle boundaries have developed. However, the twin boundaries in shapes of straight single line/ parallel line pairs form due to the growth accidents during recrystallization annealing process. Moreover, the twin boundaries form in the form of discontinuous lines in the alloys after lowlevel strain and recrystallization annealing.During thermo-mechanical processing, severer deformation and higher annealing temperature can increase the twin boundary density per grain via reducing the critical size, and can modify the grain size distribution, and thus increase the annealing twin boundary density in the processed alloy. Moreover, during high annealing temperature, the dissolution of secondphase particles increases the velocity of boundary migrating, and therefore increases the annealing twin boundary density. The association of critical size for annealing twinning into the Pande’ model can reduce the error between experimental results and Pande’ model, and improve the accuracy of Pande’ model for prediction of annealing twin boundary density in the alloys after thermo-mechanical processing.The appearance of serrated flows during high temperature tensile testing is attributed to the interactions between moving dislocations and substitutional solutes atoms on the basis of their activation energies gained via various methods. However, the interactions between annealing twin boundaries and slip bands induce the serrations in large-grained alloys during tensile testing at 200 oC. The average serration amplitude shows an increasing tendency with the TBs density, because the annealing twin boundaries can exacerbate pile-up of dislocations during deformation due to the small number of neighboring grains. In contrast, in fine-grained alloy, the average number of neighboring grains is larger than that in large-grained alloys, and thus it is difficult for moving dislocation to pile around both grain boundaries and twin boundaries, and therefore reduces the probability of serrated flows during low-temperature deformation.During high temperature deformation processing, the interactions between annealing twin boundaries and dislocations is not in favor with the free energy of system, with the formation of series of stacking faults on twin boundaries, leading the increases in the strengths and the average strain hardening rate. However, the dislocations can slip on the conjugate glide plane due to the misorientation between matrix and twin, once the slip system is activated in neighboring twin grains under applied stress conditions. Consequently, the annealing twin boundary has less negative effect on the elongation of alloy. In addition, the contributions of annealing twin boundary to properties of alloy gradually decrease with the increasing of tensile temperature and the decreasing of strain rate, but the contribution of annealing twin boundary plays an important role in mechanical properties of alloy under high temperature and low strain rate conditions.