Microstructure of Materials

Most of the materials around us are polycrystalline materials. Properties of these materials which are relevant to humanity like mechanical strength, hardness, ductility, fracture toughness, creep resistance, fatigue resistance, corrosion resistance etc. to name a few are strongly dependent on microstructure of polycrystalline materials. Therefore, it is useful to understand microstructure of materials. In this article microstructure of a polycrystalline material will be elaborated in a manner that a reader with a common knowledge will be able to comprehend it.
All the materials are made of atoms. These atoms combine (bond) together either similar atoms or dissimilar atoms or both to form large aggregates. The name of these large aggregate of atoms – depends on how these atoms have bonded together. Some atoms combine to form molecules and some atoms combine to form crystals. Molecules are much smaller units and combine together to form either crystals or non-crystalline polymers. Let us move ahead with crystals. When atoms (or molecules) combine together to form a periodic structure; this structure is known as crystals. The basic unit that repeats to form the entire crystal is the lattice. A crystal displays different kinds of symmetries and symmetry of a crystal also profoundly affects its properties. To provide some examples iron crystals can be either face centered cubic (fcc) or body centered cubic (bcc) depending upon the temperature. What is interesting to note that fcc iron is non-magnetic while the bcc iron is magnetic.
Now these crystals are the grains of a polycrystalline material. When large number of these crystals comes together to large aggregate what we have is the polycrystalline material that we use in everyday life. When two crystals come together is highly probable that their orientation will not match and this results in formation of a boundary between two crystals (grains) in a polycrystalline material. This is what is known as grain boundary. In most of the cases the size of these crystals or grains is too small to be seen by naked eye and a microscope is need to see these grains and the boundary between them that’s why the structure is known as microstructure (something that can be seen by a microscope). Another reason why these are called as microstructure is that size of these crystals (grains) is of the order of a few tens of micrometer. Besides size, another important feature of a crystal (grain) is its morphology or shape it could be nearly spherical, or cylindrical or dendritic (tree like) or lenticular (lens like) etc.
Depending upon the chemistry and processing condition a polycrystalline material may be composed of chemically and morphologically different kind of crystals (grains) in varying proportions. Besides, there may be intentional or unintentional flaws like dislocations, porosity, inclusions etc. As these flaws also affect property of a material significantly, nowadays a new trend has emerged to include these also a constituents of microstructure.
Therefore, when one talks of microstructure of a material he is essentially talking about the size, shape and relative proportion of the different phases including the flaws. Lets us elaborate this with an example of steel, let us take mild steel.Microstructure of mild steel consists of two different phased – Ferrite (small amount of interstitial elements like C, N etc) dissolved in bcc iron and Cementite (a compound of iron and carbon – Fe3C). Now let us talk about the morphology of these phases. It depends on the processing condition. But the microstructure I am talking about is a near equilibrium microstructure. The two phases are distributed in the following manner. There are two kinds of regions – one consists of only ferrite grains (of nearly spherical size) and the other region consists of alternate layers of ferrite and cementite lamellae. This structure consisting of alternate lamellae of ferrite and cementite is known as pearlite. Thus one can say that microstructure of mild steel consists of ferrite and pearlite.
How this microstructure can be altered / modified to a different kind of microstructure will form part of the next article that will talk about heat treatment of materials.