# In-Situ Nanoparticles: A New Strengthening Method for Metallic Structural Material
## 3 Materials Fabrication
### In-Situ Nanoparticle Reinforced Cu Alloys  
#### 3.2.1 Thermodynamic Calculation  
Fe and Co elements are selected to be added into Tin bronze and pure Cu during the casting process in order to fabricate in-situ nanoparticle reinforced Cu alloys with NPFG structure, which has numerous iron-rich nanoparticles (NP) dispersed within fine grains (20-40 \(\upmu\)m, FG) [38; 40; 41; 62; 63; 64; 65; 66; 67; 68; 69; 70]. The NPFG structure is in-situ formed during the casting process, so it can be widely applied in mass production of metallic structural material with complex shapes.  
Thermo-Calc software was used to calculate how the equilibrium molar fraction of each phase changes as the melt cools down to room temperature in Cu-10 Sn-2 Zn-1.5 Fe-0.5 Co alloy (Figure 5). It is obvious that in the equilibrium state FCC_L12 phase first precipitates in the form of face-centered cubic (FCC) structure from the melt with the starting temperature of 1310 K. Cu matrix (FCC_L12#2 phase) begins to solidify when the molten alloy cooled down to 1277 K, accompanying with the precipitation and growth of FCC_L12 phase. When temperature decreases to 1192 K, phase transition happens  
Figure 4: Schematic diagram of the processing method for the formation of nanoparticles in melt through a combination of multipoint dispersion supply processing (unique addition method of Ti wires) and electromagnetic stirring (strong convection stirring) [33].  
from FCC_L12 phase to BCC_B2 phase. Subsequently, the iron-rich phase continues to precipitate and grows as BCC structure. Compositions of these phases at different temperatures were calculated by using \"Single Point Equilibrium\" module and the results in Table 1 show that both the FCC and BCC iron-rich phases are mainly composed of Fe and Co elements, and the ratio of Fe/Co is about 3/1 [71].  
Precipitation behavior of iron-rich phase in Cu-10 Sn-2 Zn-1.5 Fe-0.5 Co solid matrix was also calculated by using \"Precipitation Simulation\" module [71]. The iron-rich phase is always regarded as spherical shape without considering the morphological evolution. At ambient temperature, the iron-rich phase has double-peak size distribution, including small particles with radius less than 2 nm and large particles with radius of 5-80 nm. The number of the small particles is 10\({}^{3}\)-10\({}^{4}\) times bigger than that of the large ones, which is consistent with the 3 DAP (3 Dimentional Atom Probe) and TEM (Transmission Electron Microscope) characterization results [69]. TCAL4 database in Thermo-Calc package was used to calculate size distribution at room temperature and the average radius evolution of iron-rich phase (BCC_B2) in the solid matrix of Cu-10 Sn-2 Zn-1.5 Fe-0.5 Co alloy with cooling rate of 1.4 K/s from 1310 to 293 K [71]. Average size evolution of nanoparticles shows that above 800 K, the iron-rich phase precipitates and grows with average size slowly increasing, which may be caused by long-term diffusion. Under 800 K, myriads of new iron-rich phase precipitate with small size through short-term diffusion or spinodal decomposition due to the lack of conditions for long-term diffusion. In this case, it is difficult for the precipitates to grow up to large scale, resulting in a sharp decline in average size, which is also an important reason for the formation of double-peak size distribution.