The influence of solid-state and liquid-phase bonding on fatigue at Al/Al 2 O 3 interfaces
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(1) Clean metal/oxide interfaces devoid of segregants are strong and tough. Adhesion energies exceed 200 J m22. (2) Segregants and contaminants embrittle and weaken the interface: C and S may reduce the adhesion energy to 1 to 5 J m22. Gettering of these contaminants by alloy additives such as Cr again results in high adhesion energies, exceeding 100 J m22.[5]
shown upon introducing Ag at interfaces with Nb.[9,10] The interface between Al alloys and Al2O3 is generally tough, virtually regardless of the presence of segregants. Accordingly, cracks extend by the growth and coalescence of voids in the alloy, near the interface, by means of plasticity mechanisms similar to those that cause ductile fracture.[24] Strong, tough metal/oxide interfaces are susceptible to fatigue crack growth, at rates somewhat higher than those found in the monolithic alloy.[3,25] This trend is also expected to influence the thermomechanical fatigue characteristics of interfaces in multicomponent devices and systems. The intent of the present study was to explore those features of Al/Al2O3 interfaces that affect fatigue crack growth, with an emphasis on the crack growth threshold. The objective was to devise a threshold-based design criterion. For this purpose, interfaces made by solid-state bonding (SSB) and liquid phase bonding (LPB) have been studied. In the latter, the importance of impurities that segregate to the interface and form intermetallic reaction products are examined. Previous studies of fatigue crack growth at metal/oxide interfaces, especially Al/Al2O3, have revealed three predominant features.[3,25]
When the interfaces are strong, cracks blunt and activate interface failure by initiating and coalescing debonds ahead of the crack front. This occurs at weak sites along the interface. Such behavior has been demonstrated on interfaces involving Al2O3 bonded to Al, Cu, Ni, g-Ni, or Nb.[1–14] At embrittled interfaces, cracks remain sharp and subject to atomic scale opening displacements. Brittle behavior has been demonstrated by injecting C and S into interfaces with Au[22] and Ni,[23] respectively. Similar effects have been
(1) Striations are evident on the Al side of the fatigued interface, indicative of a classical mechanism of crack growth based on the extent of crack tip blunting per cycle. (2) There is a fatigue threshold at an energy release rate range, DGth, comparable to that for the alloy itself. (3) Cyclic crack growth rates at energy release rates above the threshold exceed those for the alloy by about an order of magnitude.
I.
INTRODUCTION
FACTORS that influence the adhesion of metal/oxide interfaces have been established by means of measurements, first principles calculations, and interface crack growth simulations. Adhesion measurements have been made on well-characterized interfaces between Al2O3 (typically sapphire) and the metals: Al,[1,2,3] Ni,[4,5] Cu,[6] Au,[7,8] Nb,[9–13] and Pt.[14] First principles calculations of atomic decohesion demonstrate aspects of bonding attributed to the work of bond rupture
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