Impurities grain boundary

In solution drag, the width of the grain bovmdaiy, w, is the zone over which impurities interact. The grain boimdary thickness is taken to be independent of grain size. The diffusion coefficients of the host, D , and the impvirity, D/, will depend on the structure in the grain boundary. Impurities can therefore have an effect on the grain boundary velocity, Vg, by either their effect on the Co term or an effect on the diffusion coefficients for host and impurity. 

Failure analysis Surface contamination Particle identification Fracture due to grain boundary impurities... 

This situation requires heterogeneous nu-cleation of the p phase on a grain boundaries. Preferred locations of heterogeneous precipitation include grain boundaries, impurity particles, and dislocations [25]. 

Crack Nucleation. New cracks form in ceramics when an external stress forces DISLOCATIONS towards barriers such as GRAIN BOUNDARIES, impurities or other POINT DEFECTS (see CRYSTAL structure). This build-up of dislocations forms a microcrack, which grows initially by this plastic DEFORMATION PROCESS. See CRACK PROPAGATION. 

Metals are made harder and stronger by structural defects that interfere with the movement of dislocations across a crystal. These defects may consist of grain boundaries, impurities, or precipitates that are insoluble in the base metal. The greater the number of interfering defects, the greater will be hardening or strengthening effects. 

When one is faced, however, with choosing the most appropriate test of the corrosion worthiness of a future Monel alloy specimen, there is no doubt that the contrived exposure in acid conditions would be the most reliable. All these studies have identified the grain boundary impurities as the root of the corrosion problem in the alloy. The experiments in acid are specific to the boundaries and illustrate the merits of an electrochemical approach. 

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