# In-Situ Nanoparticles: A New Strengthening Method for Metallic Structural Material
## 4 In-Situ Nanoparticles  
### Nanoparticles in As-Cast Steel  
Figure 8 shows representative TEM micrograph of thin foil specimens of the investigated as-cast Q235 steel [59], Q195 steel [33], Ni-Cr-Mo HSLA steel [58] and Ni-Cr-Mo-V-Nb-Ti ship steel [59]. Obviously, the matrix of all steels contains large numbers of fine precipitates with about 5-15 nm diameter, and these fine precipitates distribute homogenously in the whole sample. For example, average diameter and volume fraction of the nanoparticles in Q195 steel and Ni-Cr-Mo-V-Nb-Ti ship steel is 6.7 nm, 0.345% and 7.84 nm, 0.654%, respectively. The results fit well with SAXS results delineating mean size and volume fraction of nanoparticles in Ref. [57; 58; 59]. Combined with the EDS results, diffraction calibration analysis proves that the nanoparticles in all steels are Ti\({}_{3}\)O\({}_{5}\) with monoclinic structure, which usually has a specific crystallographic orientation relationship with \(\alpha\)-Fe matrix (bcc structure). For instance, in Ni-Cr-Mo HSLA steel the precipitate and \(\alpha\)-Fe matrix exhibit \((112)_{\text{matrix}}//(212)_{\text{Ti3O6}}\) relationship [58]. Figure 8: TEM images and corresponding selected area diffraction pattern (SADP) of in-situ nanoparticles in as-cast Q235 steel [59], Q195 steel [33], Ni-Cr-Mo HSLA steel [58] and Ni-Cr-Mo-V-Nb-Ti ship steel [59]. Figure 7: NPFG structure in-situ formed in cast Cu-10 Sn-2 Zn-1.5 Fe-0.5 Co (wt. %) alloy [64]. Large amounts of iron-rich particles in-situ form in the melt at first, which can absorb Cu atoms in the melt. During the solidification process, a small part of nanoparticles can serve as nucleation cores and most particles are captured by the solid/liquid interface. Resultantly, typical NPFG structure is observed in as-cast ingots. In all investigated as-cast steels, in-situ nanoparticles directly formed in the melt hold coherent or semi-coherent relationship with the matrix. Interestingly, this relationship can be retained after subsequent hot rolling and even high-temperature tempering, which is conducive to improving the comprehensive mechanical properties. Take the Ni-Cr-Mo-V-Nb-Ti ship steel as an example [57]. Representative HRTEM (High Resolution Transmission Electron Microgragh) images of the nanoparticles in as-cast state and 700 \({}^{\circ}\)C tempered state are displayed in Figure 9 so as to illustrate the coherency. Morie Patterns can be obviously observed in as-cast and tempered states, which reveals a coherency between the matrix and particles. In addition, coherent relationship could be further confirmed through mismatch degree between particles and the matrix, which can be calculated by the formula [73]:  
\[\sigma=\frac{2(d_{1}-d_{2})}{d_{1}+d_{2}}, \tag{1}\]  
where \(\sigma\) is the mismatch degree, \(d_{1}\) is the plane spacing of Ti\({}_{3}\)O\({}_{5}\) and \(d_{2}\) is the plane spacing of \(\alpha\)-Fe matrix. Misfit parameter for coherent interface is 0-0.05, for semi-coherent interface is 0.05-0.25 and for non-coherent interface is >0.25. Based on the above formula, calculated results present that the mismatch degree of as-cast state and tempered state is 0.202 and 0.009, respectively. So, we can testify that all nanoparticles are coherent with the matrix. Excellent thermal stability is another advantage of these in-situ nanoparticles. Bright field TEM images and quantitative statistics results of nanoparticles at different states can be seen in Figure 10[57; 59]. TEM micrographs show that the size, shape and distribution of Ti\({}_{3}\)O\({}_{5}\) particles change little from those of the as-cast state to tempered state. From statistics results, we find that the maximum variation of mean radius is 6.33 nm, indicating very little fluctuation of size and less obvious Oswald ripening compared with nano-Cu precipitates in common HSLA steel [74; 75; 76]. Actually, this growing process is decided by the extremely slow diffusion coefficient of Ti [57; 77; 78]. Plus, the melting point of Ti\({}_{3}\)O\({}_{5}\) is very high (2180 \({}^{\circ}\)C).