## 3 Results and Discussion
### CTOD Examination and Failure Mechanism  
According to the results of the CTOD test, the Kq of the samples increased from 37.53 MPa m\({}^{1/2}\) (For 1.6511) to 54.9 MPa m\({}^{1/2}\) (for 1.6511 Si2V). Figure 15 shows the force-crack tip opening displacement relationships for the 1.6511, 1.6511 Si2, and 1.6511 Si2V samples. As can be seen in the figure, 1.6511 Si2V experienced more force and aperture before failure than the other samples. A very significant difference between the CFB samples graph is the existence of a pop-up in the stages before the failure. The leap in the graph indicates an increase in the fracture toughness of the sample due to the TRIP phenomenon during loading. The pop-up amount increased by increasing the amount of retained austenite.  
Figure 14: Graph of the energy absorbed during the impact test of the samples.  
Figure 13: SEM images of the fracture surfaces of 1.6511 Si2 after impact testing (**a**) two different fracture surfaces, a ductile fracture (dimples in A area) and brittle crack face (B area); and (**b**) crack blunting by the second phase (C area).  
Figure 16 and Table 3 show the average length of the crack. In the vanadium-containing specimen, the average length of the crack growth zone is 9.56 mm, while in other specimens it is more than 10.2 mm. The variation in crack length is lower in samples containing more silicon. It shows that more retained austenite causes a lower crack growth length.  
The crack length in the silicon sample is not much different from that in the sample without Si, and since the sample matrix contains CFB silicon, it seems illogical to consider crack blindness as the main mechanism for increasing the resistance in samples with a CFB matrix. On the other hand, the existence of significant pops-up and an increase in force for the same displacement in steels with a CFB matrix compared to non-CFB matrix steels can be attributed to two phenomena. One is the increase in strength due to the solid solution as  
Figure 16: Cross section of the CTOD sample fracture surfaces and length of the crack (a to i) for (**a**) 1.6511, (**b**) 16511 Si2V, and (**c**) 1.6511 Si2. Note: In CFB samples, the crack lengths are more uniform compared with the no CFB sample.  
Figure 15: Force against the opening of the crack tip opening displacement of sample 1.6511 (blue), 1.6511 Si2 (green), and 1.6511 Si2V (red).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Crack** & **Crack** & **Crack** & **Crack** & **Crack** & **Crack** & **Crack** & **Crack** \\  & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** & **Length in** \\  & **Zone a** & **Zone b** & **Zone c** & **Zone d** & **Zone e** & **Zone f** & **Zone g** & **Zone h** & **in Zone I** \\  & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** \\ \hline
1.6511 & 8.923 & 10.232 & 10.949 & 11.184 & 11.124 & 10.351 & 10.232 & 10.232 & 9.131 \\
1.6511 Si2V & 9.276 & 9.486 & 9.605 & 9.605 & 9.755 & 9.575 & 9.755 & 9.545 & 9.426 \\
1.6511 Si2 & 9.253 & 10.062 & 10.421 & 10.151 & 9.882 & 10.122 & 10.541 & 10.541 & 10.271 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Crack length of the sample after failure in the CTOD test.  
a result of the increase in silicon, and the other is the TRIP phenomenon. On the other hand, due to the slight increase in vanadium content in the 1.6511 Si2V sample, the increase in the probability of the TRIP phenomenon occurring with the increase in retained austenite can be considered as the main factor of resistance to crack growth.