Carbon Diffusion and Kinetics During the Lath Martensite Formation

Calculations verify that carbon diffusion may occur during the lath martensite formation. Accordingly, the diffusion of interstitial atoms or ions must be taken into account when martensitic transformation is defined as a diffusionless transformation. In derivation of the kinetics equation of the athermal martensitic transfomation, regarding the carbon diffusion, i .e. the enrichment of the austenite during the lath martensite formation, and ΔG(γ→α) being function of the temperature and the carbon content in austenite, the kinetics equation is modified to a general form as : [MATH] where Co and Cs are carbon contents in the austenite before and after quenching respectively. Consequently, the alloying element not only influences Ms, but also the diffusibility of carbon and both factors govern the amount of retained austenite in quenched steel which dominates in determing the toughness of the steel

Theoretical models of martensitic transformations

By consideration of the accommodation strain energy in parent phase, a modified Landau-Ginzburg model is suggested. Transformation hysteresis and athermal transformation can be displayed by the modified model and the results of numerical study of the model agree well with experiments, including surface martensite formation, autocatalysis and burst transformation. One-dimensional soliton model interpreting the relation between driving force and motion velocity of martensite is established. The calculated growth velocities are consistent or comparable with those reported values for some exemplified Fe-based and Au-Cu-Zn alloys. Taking account of the interface energy in nanocrystals yields an equation describing the relationship between the grain size and nucleation barrier for martensitic transformation in nanocrystalline materials

Effect of stacking fault energy and austenite strengthening on martensitic transformation in Fe-Mn-Si alloys

In the Fe-Mn-Si alloys with very low stacking fault energy ($\gamma_0$), the $\gamma$(fcc)$\to$(hcp) martensitic transformation substantially relies on an overlapping-of-stacking-faults mechanism. $\gamma_0$ strongly influences its $M_s$ temperature. $M_s$ gets lowered when $\gamma_0$ increases. It has been shown that both Mn and Si strengthen the austenitic matrix. However, Mn in a certain concentration range raises $\gamma_0$ of the alloy and thus lowers its $M_s$ while Si acts in the other way around. Carefully summarizing and treating the available theoretical and experimental data, an analytical expression is established to describe the $M_s$ as a function of $\gamma_0$ and the strengthening effect that is related with the strain energy. The results show that stacking fault energy plays an overwhelming role on the transformation in such alloys, even though the austenite strengthening also lowers $M_s$ to some extent

Martensitic transformation in nanostructured Fe-Ni alloys

In Fe-Ni thin films prepared by magnetron sputtering at room temperature, the bcc structure can exist stably in a larger range of Ni-content than that in Fe-Ni equilibrium diagram. Furthermore, the experiment verified that the bcc structure forms directly from collision of atoms during the sputtering process, rather than the product of fcc$\to$bcc martensitic transformation. The starting temperature for bcc$\to$fcc transformation in thin films is near to that of bulk Fe-Ni alloys. Theoretical calculations show that the nucleation barrier of martensitic transformation and the critical size of the martensitic embryo increase with decreasing grain size within the nanometer scale, implying that the martensitic transformation in nano-sized grains would be suppressed. Moreover, the autocatalytic tendency of martensitic transformation is significantly weakened within a transformed grain or within its neighboring grain due to the reduction of probability of martensite nucleation or the drop of the stress field at grain boundary produced by martensitic transformation

Thermodynamic consideration of the effect of alloying elements on martensitic transformation in Fe-Mn-Si based alloys

The critical driving force for martensitic transformation, the stacking fault energy of parent phase at the start martensite start temperature Ms and the influence of anti-ferromagnetic transition on the Gibbs energy of Fe-Mn-Si alloys have been reviewed. The Gibbs energies of the fcc($\gamma$) and hcp($\varepsilon$) phases as a function of temperature have also been evaluated for Fe-Mn-Si-Cr quaternary alloys. The critical driving force for martensitic transformation in a representative quaternary alloy (Fe-26.4Mn-6.2Si-5.2Cr) was determined to be-145.25 J/mol, significantly lower than the value for Fe-30Mn-6Si, implying that the substitution of Cr for Mn decreases the stacking fault energy of the austenite