Effect of Cr element on microstructure of Al-3.2Mg alloys was investigated using optical microscopy, scanning electron microscopy (SEM) and EDS. Moreover, the tensile and hardness test were carried out. The results show that a minor additive of Cr can refine effectively the secondary dendrite arm spacing and decrease the number of coarse interdendritic structures. In the whole series of the alloys, the secondary dendrite arm spacing of Al–3.2Mg-0.1Cr alloy was the smallest, and the spacing presented an increasing trend with the Cr content. With high levels of Cr, the intermetallic compounds are formed by Cr with Al, Mg and Fe, the shape of the second phase particles were transformed from bulky bone shape to fine bone shape and long striped. The strength and the hardness of Al–3.2Mg alloys were greatly improved when the additions of Cr were 0.05% and 0.1%, but the ductility of the alloys remained at a low level by adding 0.05%Cr. Therefore, combining with the comprehensive properties and the microstructure, the best addition of Cr in the alloy was 0.10% in the whole series.
A sort of Aluminum-metal coating, which is characterized by a bi-layered structure of Fe2O3+Cr2O3+Al plus NiO+Al layers, was successfully prepared on steel substrate by melting-casting technique. The microstructure, the bonding status of the interface between the coating and the substrate, and the element distribution of the coating were studied. The results indicate that, there is an excellent bonding between the coating and the substrate, and no hole and gap in the composite is found out. Moreover, there is obvious element diffusion in the interface. The coating is composed of block phase, columnar grain, and dendrites. In addition, the elements present a uniform distribution in the interface but an obvious segregation in the middle of the coating.
Formation mechanism of TiC in Al matrix at a rather low melting temperature using in situ technology was studied. And effect of the melting temperature on the microstructure of in-situ TiC/Al composite was investigated. The results indicate that, the resultants of TiC and Al3Ti were found out in the matrix; however, no evidence showed that Al4C3, a resultant phase existed probably in TiC/Al composite in accordance with the relative reference, was found out. Compared to the sample of T750, more TiC and less Al3Ti existed in the sample of T850, and morphography of Al3Ti changed from block to bar with the increasing melting temperature. The melting temperature affected extremely the reaction speed and the resultants.
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