Chapter two - Structure and Microstructure of Aluminum Alloys in As-Cast State

Abstract

The concept of “structure” comprises all elements of crystalline and interdendritic ingot features. Intergranular structure is usually called “microstructure.” It is always analyzed with the help of microscopes—optical and electronic. The term “microstructure” is typically used only in those cases when an alloy intergranular structure is studied at optical microscope magnifications not exceeding X-1000–2000. However, if the structure is analyzed using electron microscopy with high magnifications that help image dislocations and the products of decomposition of supersaturated solid solution, then one should use the term “substructure.” Microstructure is one of the most important features of casting aluminum alloys. Their physical properties are strongly influenced by the chemical and phase composition. Dislocations and grain boundaries to a significant extent affect, and consequently control, the alloys'mechanical properties. One can describe the relevant metallurgical features and the processing steps leading to their formation as follows: constituent particles (solidification); dispersoids (ingot thermal treatment); grain structure (bulk deformation) and its evolution; dislocations (cold work after quenching); low-temperature clustering reactions; coherent, semicoherent, and noncoherent precipitates (including Guinier–Preston zones); and both metastable and thermodynamically stable (final heat treatment). Additionally there are such features as twins, stacking faults, and different kinds of phase interfaces (both diffuse and sharp).