Grain structure, intermetallics

IGC of aluminum alloys is controlled by material selection and proper selection of thermal (tempering) treatments that can affect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to IGC is achieved by the use of heat treatments that cause general uniform precipitation throughout the grain structure. General guidelines for selecting suitable heat treatments for these alloys are available (45). 

This corrosion may be intercystalline in 2000 and 5000 alloys exhibiting a long-grained structure parallel to the direction of transformation. It can also be found in 7000 alloys with or without copper (which have a low susceptibility to intercrystalline corrosion). It results from the precipitation of parallel stripes of intermetallic phases such as AlgMn, Ali2CrMn, AlFeMn, etc., which are cathodic with respect to the solid solution, and between which there is an anodic zone depleted in Fe and Mn. 

Aluminum alloy microstructures are developed as a result of alloy composition and thermomechanical treatment. From a corrosion perspective, the dominant features of alloy microstructure are grain structure and the distribution of second phase (intermetallic) particles as constituent particles, dispersoids, or precipitates. Such particles have electrochemical characteristics that differ from the behavior of the surrounding alloy matrix, making alloys susceptible to localized forms of corrosion attack that has been termed microgalvanic corrosion. 

FIG. 27 An FIB cross-sectional micrograph of the bright-tin surface nodule shown in Fig. 26. Note the recrystallized grain structure in the nodule and the numerous SnCu intermetallic particles at the base of the nodule. (Courtesy of G. Galyon and L. Palmer, IBM Corporation.)... 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

T. Takasugi, Structure of Grain Boundaries , in Intermetallic Compounds Vol.I,... 

Differences in metallurgical structure. Grain boundaries, more reactive phases (solid solutions, intermetallic compounds, etc.). 

In the ribbon with x=5 the amorphous structural component was absent, X-ray study registered the existence of a-Al together with Al4Ce and Al3Sc crystalline intermetallics (Fig. 4c). However, ribbon hardness remained at almost the same level (Fig.7). Evidently, it can be explained by the formation of a very fine-scale structure of a eutectic type with a relatively small amount of a-Al grains (Fig. 6b). Small plates of both intermetallics of 20-60 nm in thickness were found in the eutectic by dark-field TEM investigation. A further increase of x caused a drop of hardness (Fig. 7), and in the ribbon Al91Ce2Sc7 no areas with eutectic structure were found (Fig. 

These examples serve to show that in metal systems chemical analogy has no necessary counterpart in analogy of structure, and that in general if a formula is assigned to an intermetallic phase it must be interpreted as representing composition alone without any implication of chemical combination. Even when the composition is known, however, the properties of the alloy are by no means completely specified. It may exist in an ordered or in a disordered state, or in a condition intermediate between these extremes, and its properties may, moreover, be profoundly dependent not only on structure but also on physical characteristics such as grain size, texture and the presence of faults . 

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