## 4 Discussion
The energy barrier is composed of an additional mechanical energy component due to the displacement of the atomic structure and an interface energy component. For comparison, references were considered in which the influence of the quenching rate was systematically investigated, \(T_{\text{MS}}\) was specified and a size effect was verifiable via corresponding images. Investigations whose temperature profiles simulated induction hardening processes and thus did not have a corresponding holding time at austenitization temperature were not considered. The limited number of references with the corresponding values are documented in Table 3.  
In [23] (Fe-22 wt.% Ni), a subsequent block formation (simultaneous in all packets and PAGs) is stated. The size dependency is explained by a thermally activated relaxation of austenite in the direct vicinity of the new blocks. A faster cooling rate leads to less relaxation, which is accompanied with a higher amount of elastic energy being introduced by the transformation. A reduced size of the block at a single block formation event is the consequence of this. The subsequent block formation mechanism is consistent with the small energy barrier indicated by the relatively low values of \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Material (Name)** & \(T_{\text{MS}}\) **in \({}^{\circ}\)C** & **Obvious Size Effect** & **Reference** & \(\frac{T_{0}-T_{\text{MS}}}{T_{0}}\) & \(\Delta G_{\text{chem}}^{\text{MS}}\) **in Jmol\({}^{-1}\)** \\  & & & & (Thermo-Calc TCFE 8) \\ \hline Fe-22 wt.\% Ni & 240 & Yes & [23] & 0.22 & \(-416\)\({}^{\star}\) \\ Fe-24 wt.\% Ni & 187 & Yes & [21] & 0.29 & \(-461\)\({}^{\star}\) \\ Fe-18 wt.\% Ni & 297 & Yes & [21] & 0.25 & \(-615\)\({}^{\star}\) \\
18\% Ni maraging steel & 217 & Yes & [21] & 0.44 & \(-1120\) \\ WHT1500 HF & 382 & Yes & [27] & 0.45 & \(-1718\) \\
50 CrMo4 & 280 & No & & 0.52 & \(-1875\) \\ AlSI 630 & 140 & No & [25] & 0.76 & \(-2400\) \\ \hline \hline \end{tabular} \({}^{\star}\) a conservative value of 0.005 wt.\% C is assumed.  
\end{table} 
Table 3: Comparison of relative differences between the equilibrium temperature and martensite start temperature \(\frac{T_{0}-T_{\text{MS}}}{T_{0}}\), differences of Gibbs free energies at the martensite start temperature \(\Delta G_{\text{chem}}^{\text{MS}}\) and size effects of several materials. in Table 3. With a small energy barrier, the induced stresses must be relatively small. Furthermore, the chemical driving force \(\Delta G_{\text{chem}}^{\text{MS}}\) is relatively small. If the energy barrier is exceeded, only a small amount of material has to be converted due to the low chemical driving force. The simultaneously induced stresses are too low to cause a mechanically driven nucleation of another block. Another martensite formation mechanism is stated in [25]. An energy barrier crossing event is accompanied by the formation of martensite clusters consisting of several blocks and packets. These martensite clusters extend over several PAGs, which are significantly smaller than in [23]. The plausibility of this fundamentally different formation mechanism can be explained by comparing \((T_{0}-T_{\text{MS}})/T_{0}\) and \(\Delta G_{\text{chem}}^{\text{MS}}\) values with those of the material in [23]. The significantly larger ratio of \((T_{0}-T_{\text{MS}})/T_{0}\) indicates a higher energy barrier. A significantly higher chemical driving force is therefore necessary to overcome this barrier. Higher induced stresses lead to a mechanically driven auto-nucleation mechanism (self-accommodated by the formation of corresponding martensite variants) which is stopped when the total of the additional mechanical energy and interfacial energy equals the driving force for martensite formation [25]. Higher mechanical stresses during the transformation are caused by elements such as C and Al, which influence the ratio of lattice constants [49; 50]. Furthermore, smaller austenite grain sizes contribute to higher mechanical stresses due to grain boundary strengthening.