Microstructure characteristics of a hypereutectic Al-Si alloy manufactured by rapid solidification/powder metallurgy process

Abstract

Hypereutectic Al-Si-Fe-Cu-Mg alloys manufactured by rapid solidification and powder metallurgy (RS/PM) process contain high volume fraction and finely dispersed Si particles and Al-Si-Fe intermetallic compounds (1–10). As a result, the unfavorable effects of coarse primary Si and Al-Si-Fe intermetallic compound caused by conventional ingot metallurgy (IM) process on mechanical properties of the alloy are remarkably eliminated (2, 7). In addition, the sizes of the matrix grain and the other second phases are refined and the solid solubility limit of alloying elements is increased by this process, thus the properties of the alloy are much more improved. In this paper, the microstructure and phase morphology of a RS/PM Al-Si-Fe-Cu-Mg alloy with high silicon content were reported. The nominal composition of the alloy was designed Al-17Si-6Fe-4.5Cu-0.5Mg(wt%). The powders were prepared by means of ultrasonic N2-gas atomization process at pressure of 2 MPa and temperature of 900 ◦C. Consolidation of the alloy powders was carried out by canning, degassing and hot extrusion with a reduction ratio 14:1 after holding at 450 ◦C for 1 h. The phases of the extruded alloy were identified by a D/max-rB X-ray diffractometer (XRD) using Cu-Kα1 radiation. The microstructure was analyzed by S-570 scanning electron microscope (SEM). The SEM specimens were cut directly from the extrudate, followed by grinding, polishing and etching with Killer’s reagent. The size and distribution of the second phase particles in the extruded alloy were measured by image analysis technique. The analysis of the phase morphology was performed using Philips-EM420 transmission electron microscope (TEM) attached with energy dispersive spectroscopy (EDS), operating at an accelerating voltage of 100 KV. Thinning of TEM samples after mechanical grinding of the extrudate was performed by ion beam milling. As shown in Fig. 1, the microstructure of the alloy was primarily consisted of Al, Si, Al5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7 phases. In contrast to the alloy powder [11], new phases Al5FeSi and Al7Cu2Fe were formed in the microstructure of the extruded alloy. By comparing the microstructures between the powder and extruded alloy, it can be concluded that in the course of vacuum degassing or heating before extrusion, Al5FeSi was formed by phase transformation, regarding Al9FeSi3 as its master phase and Al7Cu2Fe was precipitated from the matrix. Fig. 2 illustrated the microstructure of the extruded alloy under SEM. A large amount of fine particle phases were uniformly distributed in aluminum matrix. The results of image analysis of the particle phases were shown in Fig. 3. It was shown that the average size of these particles was only 0.5 μm and maximum size was less than 2 μm. Because of their small size, the chemical compositions of the particle phases could not be ascertained by EDS. It was presumed that these phases were mixture of Si, Ai5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7. According to the previous research results [11], there was a great amount of fine needle-shaped intermetallic compound Al9FeSi3 in the powder alloy, yet herein we found it disappeared after hot extrusion. It was