AT LOOKING MICROSTRUCTURES To look at the microstructure of a material we use a reflected light microscope. For this the surface of the material should be very smooth and to achieve this it is polished using successively finer grades of silicon carbide paper and finally very fine (1 micron) diamond. We are trying to see the different phases present how big they are, what shape they are and how they're distributed, what they are and how much of each one is there. Sometimes these phases have different reflectivities when the material is polished (e.g. an Al-Si casting alloy), but most often they don't, and so the sample has to be etched using an acid solution to contrast the phases. When looking at microstructures you will need to have some sort of base-line for your observations. One tool that can be used is the equilibrium phase diagram. If you have an idea of the alloy system , then a binary equilibrium phase diagram can be a starting point to what phases and structures you might expect to see. Unfortunately, commercial alloys are rarely binary, and processes are NEVER equilibrium, but it will give a starting point. Specific heat treatment designed to change the properties can result in numbers of non-equilibrium phases present, and these can also be identified. CASTINGS All castings will have some common features because of how they're formed. Generally the grain size is quite large, and they may have casting defects present, the most typical one being porosity (holes in the metal due to incomplete filling of the mould for various reasons). Because porosity is holes it will appear as black areas in the microstructure, and the shape of these holes can tell how they were formed. Gas porosity for instance will be round, as the gas bubble tries to minimise its surface area. The structure of castings will depend on their cooling rate, with faster cooling rates giving a finer structure. Cast structures will be dendritic in form, and may exhibit segregation. This occurs because of the rejection of solute at the advancing solid/liquid interface, so that the solid that forms has a continuously varying composition. Note that grains (areas of metal of the same crystallographic orientation) may contain lots of dendrites. Castings can be heat-treated to improve their properties, but the porosity will not disappear, and generally the segregation never completely disappears either, although heat treatment may disguise the cast structure. Castings are not subsequently worked by hot or cold working the idea is to get a near net shape product but mating surfaces, threads may etc be machined.
This is a cast cobalt-chrome medical implant which has been etched to show the segregation and grain boundaries. There are three large grains visible and some chill
zone grains around the edges. The pattern of dendritic segregation is obvious. There is one major phase present, with minor phases showing between the dendrites.
This shows an Al-9Si-1.8Cu alloy. The aluminium dendrites are white, and the eutectic phases which are predominantly silicon fill the interdendritic areas. As this has not been etched, the grain boundaries cannot be seen. WROUGHT STRUCTURES. Cold or hot working will break up the as- solidified structure, so dendrites will no longer be present. The grain structure will be more equiaxed if hot worked, or elongated in the direction of working if cold worked.
The two microstructures above are both of a .05%C steel, polished and etched in nital to show the phases present and grain boundaries; one is annealed, and the other is cold rolled. The main phase present is ferrite (refer to the Fe-C phase diagram). If the amount of carbon is increased, then large proportions of a structure containing both ferrite and Fe3C called pearlite, occurs. This is the dark etching area in the photo below. It is possible to use the phase diagram and the lever rule to get an approximate carbon composition, although things are complicated by the presence of other alloying elements and the non-equilibrium processing history. The other end of the lever will be .8%C, with a structure that is 100% pearlite. This ...