On Buckling, Kink Boundaries and Kinking Nonlinear Elastic Solids
Authors:   , Shamma Mohamed , Barsoum Michel W. , Kontsos Antonios 
Published: July 2021

The mechanical behavior of materials has been under investigation for decades. However, there is always unknown information to be researched and characterized. Extensive research has been performed on materials deforming by slip and twin mechanisms due to their presence in a lot of materials used in practical applications. Conversely, much less attention was directed to investigating the dislocation mechanism responsible for the fully reversible energy dissipating mechanical response of Kinking Nonlinear Elastic (KNE) solids. Herein, the buckling dislocation mechanism responsible for the KNE mechanical behavior of MAX phases is investigated. The main features of the buckling dislocation mechanism are identified that are globally applicable for layered structured materials as well. This was done by analyzing the deformation of a single crystal layered structure. The single crystal layered structure is a setup consisting of a pile of paper supported by sponge and ply wood in which paper buckles during compression. The displacement of the paper layers is studied in relation to dislocation nucleation across the layers called dislocation walls (DW). Schmid factor maps where developed that recognize the shape and progress of traction applied on the layers during deformation. Hence, the evolution of buckling dislocation mechanism that occurs in layered structures has been discovered distinguishing between two possible buckling modes (extrusion and indentation) that depend on the layered structure surrounding support and constraints. In situ neutron diffraction and ultrasonic bias stress techniques were used to perform in situ experiments on selected MAX phases to obtain information about the dislocation mechanism during its activity. Linear elastic Ti2SC was investigated as well for comparison with Ti₃SiC₂ and Ti₂AlC MAX phases which demonstrate KNE mechanical response in bulk. The in situ ND results of textured Ti₂AlC showed typical results except for the (0004) grains which demonstrated an almost linear elastic response however for the first time a reversible loop is observed on microstrain measurements at the crystal level. The grains inclined at 45° showed a reversible loop as well. The results indicate that dislocation pile-ups are undoubtedly not the active dislocation mechanism. All of the features observed prove that buckling is the dislocation mechanism taking place in KNE materials. The reversibility of mechanism is believed to originate from the bonds within and between the layers, however, this aspect needs further investigation. Counterintuitive features in the mechanical response of textured Ti₂AlC are explained. The higher strain accommodation and energy dissipation of the N- relative (loading direction with respect to basal plane) to the P-orientation is explained through schmid factor analysis. In addition, the inverse expansion that occurs during buckling of layered structures in the N-sample orientation explains the grains expansion that occurs along the compressive axis. In summary, this work demonstrates the evolution of the buckling dislocation mechanism taking place in layered structured materials, and the normal and shear traction applied on the layers that govern the behavior. The in situ ND results eliminate the possible activity of slip and/or twin and support the buckling dislocation activity.