Effect of Tension-Torsion Coupled Loading on the Mechanical Properties and Deformation Mechanism of GH4169 Superalloys

GH4169 superalloys are used in gas turbine engines and power plants owing to their excellent mechanical properties and corrosion resistance at temperatures exceeding 600 degrees C. Because of their service condition, involving high temperature and complex stress, much attention has been attracted to the effect of temperature and loading mode on the mechanical properties and deformation mechanism. The effect of the loading mode, especially the multiaxial or coupled loading, on the mechanism of plastic deformation is still an outstanding open question despite numerous investigations on the effect of temperature on mechanical properties. In this study, the effect of tension-torsion coupled loading on deformation behavior was investigated, where the microstructures and underlying mechanism were revealed using SEM, TEM, EBSD, and neutron diffraction. It is found that the mechanical properties are dependent on the tension-torsion loading. For the tension specimens, the yield and ultimate strengths increase with the pretorsion angle; for instance, at the pretorsion angle of 720 degrees, the increase rate is approximately 150% and 13%, respectively. At the pretension strain of 20%, the yield strength and elongation increase by approximately 31% and 16%, respectively. The density of dislocations increases in those samples after tensile and torsional deformations compared to the undeformed samples. Moreover, the density of dislocations for specimens deformed under the coupled loading is lower than those deformed under axial loading, indicating the dislocation annihilation effect. The yield strength is enhanced due to the strengthening effect of the initial dislocations produced during the preloading. The ultimate strength for the torsional specimens after pretension decreases because of the dislocation annihilation effect during the subsequent coupled loading. However, for the tensile specimens after pretorsion, such dislocation annihilation effect can be counteracted to some extent by the strengthening effect of the formed gradient structure by pretorsion on mechanical strength. These findings provide some insight into the regulation of the microdeformation mechanism process of materials through designing the coupled or multiaxial loading modes and the coordinated improvement of strength and toughness based on the achievement of gradient or hierarchical microstructure.