Interface structure and strain development during compression tests of Al₂O₃/Nb/Al₂O₃ sandwiches

The interface structure of an Al₂O₃/Nb/Al₂O₃ sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a \( {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \), \( {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ \,||\, [2{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \), and on the other interface a \( {\hbox{(1{{${1}$}}0)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \) and \( {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ || [0{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \) orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)_Nb and \( (1{{\bar{{{1}}}}}0)_{{\rm Nb}} \) crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)_Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the \( (1{{\bar{{{1}}}}}0) \) face. The strain localization along a specific maximum shear direction results in the cleavage of Al₂O₃, always initiating from the interface possessing the \( {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \) and \( {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ \,||\, [0{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} \) orientation relationship.