Morphological changes,structural evolutions and grain growth kinetics of mechanically alloyed(MAed)Fe50Al50,Fe42.5Al42.5Ti5B10 and Fe35Al35Ti10B20(mole fraction,%)powders were investigated by XRD and SEM,when being isothermally annealed at 1 073-1 373 K.The effect of different Ti and B addition on the grain growth of FeAl phase was also discussed.The results show that the nanocrystalline FeAl and in-situ TiB2/FeAl nanocomposite powders can be synthesized by subsequent heat treatment.Besides the relaxation of crystal defects and lattice stress,the transformation from Fe-based solid solution into B2-FeAl and TiB2 occurs upon heating of the MA-processed alloys.Although the grain growth takes place,the grain sizes of both FeAl and TiB2 are still in nanometer scale.The activation energies for the nanocrystalline FeAl growth in the three alloys are calculated to be 534.9,525.6 and 1 069.6 kJ/mol respectively,according to kinetics theory of nanocrystalline growth.Alloys with different TiB2 contents exhibit unequal thermal stability.The presence of higher content TiB2 plays significant role in the impediment of grain growth.
The interface reaction between the SiC particles （ SiCp ） and Fe was stndicd during sintering the SiCp reinforced Fe matrix composites at 1423 K for 1 h. In the composite having 3wt% （weight ratio） SiCp （the 3SiCp/ Fe composite）, the interface reaction products of Fe3 Si, the carbon precipitates, and Fe3 C or pearlite were generated. Fe3 Si coustructs the bright matrix of the reaction zone in the original situation of the SiCp. The carbon precipitates are randondy embedded in the reaction zone. Fe3 C or pearlite exists at the grain boundaries of the Fe matrix. As increasing the SiCp concentration in the SiCp/ Fe composite, the inteusity of the interface reaction between SiCp and Fe iacreases. After the 10SiCp/ Fe composite （ having 10wt .% SiCp ） sintered at 1423 K for 1 h, all of SiCp are decomposed, and replaced by the reaction zone composed of Fe3 Si and the carbon precipitates. No Fe3 C or pearlite was genertaed during the reaction. The effects of the techniques of oxidizing of SiCp , coating SiCp by interaction with the Cr powder, and alloying the Fe matrix by adding the Cr element on the interface stability of the SiCp/ Fe composite system were also investigated, respectitely. The oxide membrane and the coating layer on SiCp can inhibit the interface reaction between SiCp and Fe by isolating SiCp from the Fe matrix during sintering. The interface reaction does not occur in the 3 SiCp/ Fe- 10 Cr composite but in the 3 SiCp/ Fe-5 Cr composite. In the SiCp/ Fe-Cr alloy composites, the interface reaction between SiCp and the Fe- Cr alloys is weaker than that between SiCp and Fe . The Cr element behaves as a diluent, it causes a redaction in the interface reaction, which is proportional to the amount of the element added.
The nanocrystalline β-SiC powder was successfully synthesized by ball milling the Si-50%C elemental powder. During ball milling, a solid solution of C in Si, Si(C), firstly forms, followed by SiC. The formation of SiC is controlled by the mixing mechanism of the gradual diffusion reaction(GDR) and the mechanically induced self-propagating reaction(MSR). The amount of β-SiC increases with milling time increasing. After 40 h milling, there exists only β-SiC in the milled powder. The grain size of β-SiC is about 6.4 nm after the powder is milled for 60 h. After the 60 h-milled Si-50%C elemental powder is heat treated at 1 100 ℃ for 1 h, the grain size of β-SiC does not change, but the lattice ordering degree of β-SiC increases.
