Structure –property relation  

Properties  of materials depend strongly on structure of metals. We will be concerned with steel.

Pure iron is highly ductile. Addition of carbon increases strength of iron.  system can alloy with several elements to promote either the formation of carbides (such as Ti, Zr, V, Nb, W, Mo, Cr etc.) or nitrides (such as Al, Ti etc)  or to stabilize austenite (such as Ni, Mn etc.) or to stabilize ferrite(such as Cr, W, Mo, V, Si etc.)

Steel is a polycrystalline material and its microstructure consists of grains (also called phases or crystals) oriented in different directions, and grain boundaries. A polycrystalline cube 10mm on edge, with grains 0.001 mm in diameter, would contain crystals with a grain boundary area of several square meters. Grain boundaries are important.

Grain boundary is the region of misfit between the grains. Due to different atomic configurations, it acts as sinks for impurity atoms which tend to segregate at interfaces


The equilibrium diagram of Fe-C system shows the following phases:

• ferrite: interstitial solid solution of carbon in bcc iron. Maximum solid solubility of carbon is and decreases to  .
  
  
• Austenite: FCC crystal structure and solid solubility of carbon is and decreases to  .
  
  
• Cementitie  : An intermetallic compound, hard and brittle in nature. It is stable at room temperature. It contains 6.67 % carbon.
  
  

The above phases are obtained when steel from the austenitic region is cooled very slowly. However, several other phases can be obtained by varying cooling rates. In the table given on the next page the different phases are summarised which can be obtained during phases transformation of steel