Microstructure on Thermal Expansion Coefficient of In-Situ Al2O3/Al Composites by High Pressure Solidification

Al2O3 particle reinforced aluminum matrix composites exhibit excellent properties,such as high strength,high hardness, wear resistance and low coefficient of thermal expansion,making them the preferred material in aerospace,automotive light weight and electronic packaging industries. In the production,with the increase of Al2O3 volume in the matrix,the strength and hardness of the composites were increased,while the plastic toughness was decreased. The main reason was that the wettability of Al2O3 strengthening phase is poor,which leads to agglomeration when entering the matrix. In this paper,the nanometer Al2O3 particle reinforced Al matrix composites were prepared by in-situ high-pressure solidification(0.02,2.5 and 3 GPa)to refine Al2O3 particles and make them distribute. In these composites,Al2O3 particles were produced in-situ by the substitution reactions of Fe2O3 and Al. Compared with Al matrix composites which were directly added Al2O3 particles,the composites of in-situ Al2O3 particle possessed the characteristics of excellent interface bonding,good compatibility and nano-sized particles. Then the microstructure of the composites was analyzed by scanning electron microscope(SEM),X-ray diffraction(XRD)and energy dispersive spectrometer(EDS),and the effects of microstructure on density and thermal expansion of the composites were discussed. The results showed that Fe2O3/Al-12Si composites were composed of α-Al,β-Si,Fe2O3 and new product Al2O3. The microstructure showed that the composites were mainly composed of "nano network grain boundary+bulk silicon+matrix". Through EDS analysis of the "network grain boundary",it was found that the network grain boundary was composed of nano-sized Al2O3 particles,some Fe2O3 particles were not involved in the reaction and Fe phase produced by the reaction. The non-Fe diffraction peak in XRD was due to the low content of Fe phase produced and a certain solid solubility in Al. Additionally,with the increase of the pressure,the network boundaries became denser,and the higher Fe2O3 content was, the more obvious the network grain boundaries were. In summary,the solidification process could be briefly described as follows: when the sample was heated under high pressure,and the melting point of Al was low beginning to melt and the reaction of Fe2O3 in contact with the surrounding was accelerated. However,the displacement of atoms was limited under extremely high pressure,and Fe2O3 could not be supplemented in time. On the other hand,the product of Al2O3 at the melt front increased the resistance at the melt front. The final Al2O3 ceramic phase and some unreacted Fe2O3 were distributed at grain boundaries. Al2O3 phase and part of Fe2O3 that did not participate in the reaction were distributed at the grain boundary. The coefficient of thermal expansion(CTE)of Fe2O3/Al-12Si composites in the first heating process was different from that in the second heating process. During the first heating,the CTE of the Fe2O3/Al-12Si composites could be divided into three regions according to the temperature and the peak appears. The peak value was caused by the precipitation of solid solution Si in the matrix during heating. In addition,because CTE value of Fe2O3 was also smaller than that of Al alloy,CTE value of composites with 1%Fe2O3 content was 33.84×10-6 K-1 at 740 K,which was higher than that of the composites with 3%Fe2O3 content of 32.83×10-6 K-1. During the second heating,no peak of the coefficient of thermal expansion occurred due to the slow cooling after the first heating,and the maximum thermal expansion coefficient of the second heating was 75% lower than that of the first heating. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.