# In-Situ Nanoparticles: A New Strengthening Method for Metallic Structural Material
## 3 Materials Fabrication
### In-Situ Nanoparticle Reinforced Steel  
#### 3.1.1 Thermodynamic Calculation  
We have applied the in-situ nanoparticles reinforcement technology in low carbon steel [52; 53; 54; 55], ship building steel [56; 57; 33] and naval steel [58]. Titanium (Ti) was selected as the main forming element of microalloying in-situ nanophase in our research. On one hand, Ti can act as the deoxidizer effectively in molten steel in that the affinity of Ti with O is higher than that with N, S and C which generally exist in steels. On the other hand, with optimized amount, Ti was conducive to forming large amounts of nano-oxide second phase which evenly distribute in the melt at the same time [59]. Therefore, mechanical properties of steels are expected to be distinctly improved.  
Thermo-Calc software was used to calculate the thermodynamic equilibrium phase in Ti microalloying steel and investigate the precipitated phases with specific element composition of each phase existed in steel. As an example, the calculation results of the Ni-Cr-Mo HSLA steel with Ti addition are listed in Figure 2 and calculated temperature interval is 400 \({}^{\circ}\)C\(\sim\)1600 \({}^{\circ}\)C. Corresponding details of composition and calculating parameter could be found in ref [59]. Figure 2a,b show the precipitated phase composition diagram of Ti microalloying steel. The precipitation temperatures of main phases arrange as LIQUID\(>\)FCC_A1#3\(>\)FCC_A1#2\(>\)HCP_A3#1\(>\)M6C\(>\)M23C6. The Ti-containing precipitated phase should be mainly considered in the analysis. The FCC_A1#2 phase contains carbide and nitride of Ti/Nb and the precipitation temperature is about 1150 \({}^{\circ}\)C. The FCC_A1#3 phase consists of titanium carbide and titanium nitride, which has the higher precipitation temperature around 1580 \({}^{\circ}\)C. Specifically, composition diagram of liquid phase is displayed in Figure 2c and it can be observed that Ti and O elements always exist in the liquid phase. This phenomenon indicates that Ti reacted with O element in the molten steel to form titanium oxide at exactly the time when Ti wires were added into the melt. Once titanium oxide forms in the melt, it remains in the matrix after solidification. Hence, calculated results show that it is possible to obtain in-situ titanium oxide particles in the melt during casting process.  
To investigate the size of titanium oxide particles, thermodynamic analysis of Ti\({}_{3}\)O\({}_{5}\) precipitates was performed in Ti0.05%-O0.002% steel system. The calculation result shows that entire solid solution temperature (TAN) is 1555.21 \({}^{\circ}\)C and liquid temperature is 1512.75 \({}^{\circ}\)C [59]. Reaction of Ti and O starts immediately when Ti is added into the melt, resulting in Ti\({}_{3}\)O\({}_{5}\) precipitates. The solubility of Ti, O ([Ti], [O]) and molar fraction of precipitates in high-strength steel changes as a function of temperature is shown in Figure 3a,b, respectively. The solubility of Ti and O elements reduces with the decrease of temperature, while the total molar fraction t of Ti\({}_{3}\)O\({}_{5}\) precipitate increases with the decrease of temperature. At 1510 \({}^{\circ}\)C, [Ti] is 0.04876519%, [O] is 0.00131259% and molar fraction t of Ti\({}_{3}\)O\({}_{5}\) precipitate is 8.592 \(\times\) 10\({}^{-6}\) mol, which indicates that the molar fraction of Ti\({}_{3}\)O\({}_{5}\) precipitate is small during the solidification process [59]. If we choose suitable casting technique to render such a small molar fraction of Ti\({}_{3}\)O\({}_{5}\) to homogeneously distribute in so wide a range of molten steel, these in-situ Ti\({}_{3}\)O\({}_{5}\) particles are expected to be in nanosize.  
Figure 3: (**a**) The solid solubility of Ti, O and (**b**) molar fraction of precipitates in Ti0.05%-O0.002% series high-strength steel change as a function of temperature [59].  
Figure 2: (**a**) The precipitated phase composition diagram and (**b**) its local magnification of Ti microalloying steel; (**c**) the elemental composition of liquid phase [59].