We know various material types (e.g., metals, ceramics, polymers) can be used as raw materials to develop an implant or device. We also know composites comprise of more materials. The manufacturing processes shape and assemble the materials into the final product. Heating and cooling techniques are routinely employed during the manufacturing process to shape and deposit the materials. These techniques generate microstructures (typically visible only under a microscope) that play an important role in defining the final product properties. During material’s thermal processing, understanding the nature of phase transformations that give rise to various microstructures is crucial for control of final product properties. Central to this understanding of phase transformation is to have a good grasp of phase diagrams or “maps” for the design of processing methods. So let us delve a little bit more into phase transformation and phase diagrams.
We know from our basic science class that the solid, liquid and gas are the three most common states of matter. Plasma is another state; not so common on earth, but is perhaps most common in the universe. Moreover, recently scientists have identified Bose-Einstein condensates as a new state. Coming back to solid, liquid and gaseous states, we notice transformation or changes between these three states in our everyday life. Water melting is an example of solid to liquid transformation. Solid air fresheners are good examples of solid to gas transformation. If you mix different gases, the final mixture is homogenous. Thus, gas is always considered a single phase. Mix sugar with water, the solution is homogenous, and thus, it is also considered single phase. However, if you mix oil with water, they separate into two distinct layers or two phases. Hence, a liquid sample can exist in two or more phases. Solid can exist in even more phases. Consider the metal titanium with a body-centered cubic (BCC) or hexagonal close-packed (HCP) symmetry. Each of this symmetry is considered a different phase even though it is the same metal. An alloy of two metals with the same crystal structure, but different composition (amounts of each metal) are considered two different phases. Other phases could exist. For instance, based on material property (e.g., magnetic, superconducting) or stability (e.g., stable, metastable).
The figure shows a binary phase diagram as a temperature – composition map. The contours of the map are experimentally determined by noting cooling rates over a range of compositions. From the diagram, the phases of microstructures present at a given temperature and composition can be identified, understood or predicted. See reference in text on how this binary phase diagram is built.
Ask a chemist, materials engineer, or mineralogist, what is a phase diagram? They may most probably respond that it is a type of graph that looks like a map that contours the regions of various phases. Of course, if you pose the same question to a mathematician or even a physicist, you may get a totally different response – related to phase space. When it comes to employing composite materials to manufacture implants, we concentrate mainly on the solid and liquid state. The gaseous state is not of practical importance and is often ignored. Temperature and composition are considered key variables that influence material processing. Thus, the most dominant maps you will find will have these two variables as the axis of the graph – composition on the x-axis and temperature on the y-axis. These diagrams are called binary phase diagrams (see figure). In a phase diagram, you will notice different Greek letters, these are different solid phases with same atomic composition, but differing in atomic arrangement or crystal structure .
Phase transformation is the change between the phases identified in the phase diagram. Phase transformations can lead to desirable or undesirable properties which dictate acceptable or unacceptable material performance. A good example of phase transformation leading to unacceptable material performance is the recall of hip replacement product called Prozyr zirconia heads. A word of caution. Binary phase diagrams can be over-simplistic. More materials can be found in the compositions of many products. For example, in simplest form, steels are alloys of iron and carbon. However, medical grade stainless steel has additional materials such as Chromium and Nickel.