Both plots then decrease with increasing T (due to the -TS term), with the decrease being at a higher rate for the (more disordered) phase with higher entropy. However, when comparing two phases (with different entropies, but similar specific heats) in this way, the changes in enthalpy are often neglected. Martensite is a needle-shaped microstructure formed by a rapid, diffusionless transformation and significantly affects the mechanical properties of materials. It actually increases, despite the -TS term, because the enthalpy increases approximately linearly with temperature (with the proportionality constant being the specific heat of the material).
#Martensite start using jmatpro free
Moreover, for reasons that need not be detailed here, superelasticity and shape memory behaviour are NOT normally exhibited by steels.įootnote: Although commonly presented in this form, it's not strictly correct for the free energy to be shown as decreasing with increasing temperature. As it happens, while the austenite-to-martensite transformation that occurs in the Fe-C system obviously is a martensitic transformation, it is crystallographically complex and exhibits certain rather special characteristics.
In particular, it is important to avoid any confusion with the austenite and martensite phases that form in steels. It is important to be clear that this terminology is generic – i.e., these phases are not any specific ones, but refer to a type of phase. Sometimes, the austenitic phase is termed the “ parent” phase.
For superelastic and shape memory alloys, the two phases are normally termed “ austenite” (stable at higher temperatures) and “ martensitic” phase (stable at lower temperatures). The animation below shows the stability of two phases as a function of temperature. The quantity of primary concern here is the Gibbs free energy, G. Some simple features of the thermodynamics relevant to phase transformations are given here - see Ellingham Diagram TLP. Previous Next Martensitic Phase Transformations - Basic Thermodynamics