So far, we briefly consider the mechanism of displacive transformation. In this section, we will think about miscellaneous issues on the martensite transformation and then think about bainite transformation in a brief way. At first, let's see the critical temperature for initiating martensite transformation which is called as Ms temperature. When we cool down the ferrous alloy, the martensite transformation happens below a specific temperature that is M/s temperature. Ms temperature is not the material constant but there are many empirical equations which express the Ms temperature of the alloy as a function of chemistry. The theoretical implication of Ms temperature started with the critical temperature below which diffusionless transformation is thermodynamically possible. This graph schematically shows the relative change of free energy of austenite and martensite with respect to the change of the temperature for given carbon concentration C0. The free energy of austenite is lower than that of martensite at sufficiently high temperature, but as the temperature decreases there will be a critical temperature where the free energy of both phases are the same. That temperature is T0 temperature. BelowT0 temperature, the free energy of martensite is lower than that of austenite for given carbon concentration and the transformation without the compositional change is thermodynamically possible. However, in practice, Ms temperature is somewhat lower than the T0 temperature. It is because additional driving force by super-cooling is necessary to overcome the strain energy associated with the shape deformation for the occurrence of the displacive transformation. The degree of super-cooling is dependent on various factors such as size of the shape deformation and the strength of austenite which control the amount of strain energy associated with the transformation. Stress and deformation is known to affect the displacive transformation behavior. Representative effect is their influence on the martensite starting temperature. The stress in the elastic deformation region increases the Ms temperature by providing additional mechanical driving force for the occurrence of transformation. When we apply the stress, the Ms temperature increases up to Ms sigma where further increase of stress causes a plastic deformation in austenite. Further increase of stress above Ms sigma starts to generate various kind of deformation structure which create additional nucleation site. Then the deformation structure as well as the mechanical driving force assists the formation of martensite which further increase the martensite starting temperature up to Md temperature. The Md temperature is the upper limit of martensite starting temperature over which the transformation is no longer activated by any stress or deformation. For the industrial application, martensite is very important to microstructure because it is the strongest phase in steel. This graph compares the hardness of iron-carbon alloy depending on the carbon concentration and their microstructure. You can see the strength of martensite is far higher than that of ferrite or pearlite for given carbon concentration and the gap become more significant as the carbon concentration in the alloy increases. The strength of martensite is originating from several factors. The most important one is solid solution hardening effect from carbon. As I mentioned, the martensite has carbon supersaturated BCC structure. It means that the BCC structure accommodate more carbon than it can do under equilibrium condition. The size of carbon is larger than that of available interstitial site of BCC structure and it causes a significant lattice distortion, making the motion of dislocation difficult during the deformation and increasing the strength of the material. Secondary, the formation of martensite, as I mentioned, intrinsically generates dislocation by slip or twinning. Those dislocations contribute to the hardening effect by acting as obstacle to the motion of other dislocation. And the martensite has very hierarchical microstructure having very fine sub-structure like packet and block, which provide grain refinement effect for further strengthening. Finally the interaction of solute atom with dislocations is likely to decrease the mobility of dislocation which also make the martensite strong microstructure. As a final subject of this section, I'd like to briefly mention the bainite transformation. One important thing is that the underlying mechanism of lattice change accompanying the bainite transformation has a displacive nature same to the martensite transformation. The only difference from the martensite transformation is that carbon diffusion occurs after formation of carbon supersaturated bainite plate, which causes the precipitation of carbide in the final microstructure. The bainite transformation occurs at intermediate temperature range between ferrite, pearlite and martensite transformation. And, as you can see here, the microstructure of bainite is more similar to that of martensite except for the presence of fine carbide. We often divide the bainite into upper bainite and lower bainite depending on the location of the carbide. The word upper and lower are related with the formation temperature of bainite. When the bainite forms at relatively high temperature, the diffusion of carbon is more active. Then the carbon effectively diffuse into surrounding austenite after formation of bainite plate. In that case the carbide are likely to form between the bainite plate. We call this microstructure as upper bainite. However, when the formation temperature of bainite is relatively low then the diffusion of carbon cannot occur in that active way and some of carbon possibly remains inside of the bainite plate. Then the carbide in the final microstructure are likely to locate inside of the plate as well as the outside of the plate. We call this microstructure as lower bainite.
