Excess Phases and Strengthening Phases in Aluminum Alloys
Aluminum alloys are metals composed of two or more metallic elements, such as aluminum and other elements like silicon, zinc, magnesium, copper and iron. Each alloy has different properties and by manipulating the type and amount of each element, the properties of the alloy can be tailored to suit the intended application. One of the main ways to manipulate these alloying elements is through the formation of excess phases and strengthening phases.
Excess phases are metal compounds that form as part of the microstructure when two or more elements are alloyed. These compounds are often referred to as intermetallic compounds (IMCs) and are formed during the solidification phase when the metal is cooling. The composition and structure of these compounds differs for the different alloying elements, however, they all have a defined crystal structure that is distinct from the parent metal. Since these IMCs often have different properties than the parent metal, they can have a significant effect on the properties of the alloy. For instance, some IMCs may be harder and stronger than the parent metal, while others may be softer and ductile.
Strengthening phases, on the other hand, are often the result of thermomechanical processing of the alloy. Examples of these processes include cold rolling, cold drawing and heat treatment. During these processes, the metal is deformed and/or subjected to a range of temperatures which results in the formation of different phases. For example, during cold working, the deformation of the metal causes slip lines on the grain which leads to the formation of a different phase known as strain hardening. This difficuly in turn increases the strength of the aluminum alloy.
In addition to the formation of excess phases and strengthening phases, the properties of an aluminum alloy can also be improved by the addition of alloying elements and the manipulation of their location in the structure. For example, when an alloy containing silicon is quenched, the silicon segregates towards the grain boundaries, forming an interconnected network of harder silicon particles. This hardening effect is known as dynamic precipitation hardening (DPH) and is the basis of many high strength aluminum alloys.
Overall, the introduction of excess phases and strengthening phases in aluminum alloys can have many effects on the properties of the product, allowing engineers to tailor the properties to suit the desired application. However, it is also important to consider the associated cost and other potentially adverse effects, such as reduced ductility or increased susceptibility to corrosion, in order to ensure the optimal balance between performance and cost.
