Constrained yielding in niobium single crystals bonded to sapphire

Effects of processing conditions and ambient environment on the microstructure and fracture strength of copper/niobium/copper interlayer joints for alumina

“…These stress distributions can potentially cause joined structures to fail in the ceramic at applied loads lower than required to fracture the bulk ceramic. The mechanical constraint on a thin metal joining layer reduces the extent of plastic deformation the metal/ceramic bimaterial structure can undergo [8,50] . Other factors such as surface roughness can shield cracks, thus contributing to a high fracture energy [7] .…”

“…The solubility of copper in niobium is quite limited (≈1.2 at% maximum at the eutectic temperature) [23] , and substantial amounts of residual copper in the joint can potentially limit the high-temperature application of such joints. The more extensive plastic deformation in the joined assembly; however, niobium/sapphire joints with thicker metal layers produced by diffusion bonding have failed at lower stresses in compression tests [8] . Additionally, impractical processing times may be required for complete dissolution and diffusion of the copper to occur.…”

“…Generally, this required relatively high temperatures (920-1950°C) and pressures (2-16 MPa) [8,14,19,20,[73][74][75][76][77][78][79][80][81][82][83][84][85] in comparison to those used in the current study.…”

“…Additionally extensive surface preparation and cleaning techniques were often required, and some joints were fabricated in ultra high vacuum (UHV) conditions [8,75,79,81,86,86] .…”

“…Such joints often exhibit increased fracture toughness in comparison to the bulk ceramic, since the metal layer will exhibit some plasticity [5][6][7][8] . Structures which undergo some plastic deformation instead of catastrophic failure would be preferred, and joining smaller ceramic parts is beneficial in comparison to fabricating larger parts for this reason also.…”