Influence of the Substrate on the Creep of SN Solder Joints

The creep rate of Sn solder joints is noticeably affected by joint metallization. Cu|Sn|Cu joints have significantly higher creep rates than Ni|Sn|Cu joints, which, in turn, have higher creep rates than Ni|Sn|Ni joints. Replacing Ni by Cu on both substrates increases the creep rate at 333 K (60 °C) by roughly an order of magnitude. The increased creep rate appears with no apparent change in the dominant creep mechanism; the change in the constitutive equation for creep (the Dorn equation) is in the pre-exponential factor. The decreased creep rate on substituting Ni is accompanied by an increase in the hardness of the polygranular solder but a decrease in the nanohardness of the grain interiors. The source of the strong influence of the Ni substrate appears to be the introduction of an array of Ni3Sn4 intermetallic precipitates along the grain boundaries. These precipitates inhibit grain boundary sliding, boundary reconfiguration, and grain growth during creep. The intermediate creep rate of the asymmetric Ni|Sn|Cu joint has two causes: a decrease in grain boundary mobility due to precipitate decoration and a restriction in the free volume of the joint due to rapid intermetallic growth from the substrate on the Ni side. The sources of this anomalous intermetallic growth are discussed

On cells and microbands formed in an interstitial-free steel during cold rolling at low to medium reductions

The cold-rolled microstructure formed in ah interstitial-free (IF) steel from low to medium reductions has been investigated by using transmission electron microscopy. Dislocation cells formed early in the process and reduced in size until a critical strain was reached, beyond which they remained at a constant size of ∼1-μm diameter. The critical strain coincided with the formation of microbands, and it is proposed here that microbands carry all the deformation from their creation, rendering cell refinement unnecessary. Furthermore, since the dislocation densities of meshes and cells are almost identical and the stored elastic energy Ecell is always less than E mesh, it is proposed that cells are derived from the dislocation mesh structure. Microbands grow quickly to a length that is limited by grain boundaries. The thermodynamic condition favoring microband formation is (∂EMB/∂ρ) < (∂E Cell/∂ρ), and the microbands are not derived from cell structures but from dislocation sheets or walls on which deformation is concentrated.link_to_subscribed_fulltex

Advances in Microstructural Understanding of Wrought Aluminum Alloys

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