J. E. Gruzleski

Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

The crystallization behavior of iron-containing intermetallic compounds in industrial grade 319 aluminum alloy has been investigated by means of thermal analysis and metallography. In the absence of manganese, the iron compound crystallizes in theβ phase, at all cooling rates ranging from 0.1 °C/s to 20 °C/s under normal casting temperatures (750 °C). However, when the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron compound crystallizes in the α phase at high cooling rates. This is due to the fact that γ alumina, which forms at low melt temperatures (≤750 °C), acts as a nucleus for crystallization ofβ phase. When the melt is superheated to high temperature (≥85O °C), the γ alumina transforms to a alumina. This is a poor nucleus for the β-phase crystallization, and as a result, a phase forms. The importance of nucleation and growth undercooling for the crystallization of iron compounds is highlighted. In the presence of manganese, the iron compound crystallizes in a phase at low cooling rates and in both the α andβ phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.

Structure and properties of hypoeutectic Al-Si-Mg alloys modified with pure strontium

Pure metallic strontium has been used as a modifier for A356.0 alloys, and the relationship between microstructure and mechanical properties has been established. It is shown that mechanical properties depend on both the cooling rate and the amount of strontium in the melt. The effect of other elements (Fe, Mg) on the microstructure of strontium modified Al-Si-Mg hypoeutectic alloys has been studied. The mechanical properties are adversely affected through the formation of intermetallic compounds caused by the presence of these elements.

Electron microscope study of Al-Fe-Si intermetallics in 6201 aluminum alloy

The Al-Fe-Si intermetallics present in a commercial cast 6201 electrical conductor alloy have been studied using high resolution electron microscopy. The β-Al5FeSi phase is highly faceted and contains multiple (001) growth twins parallel to the growth direction. The α-Al8Fe2Si phase which forms in a Chinese script morphology has a nonfaceted interface with the aluminum matrix and exhibits no growth twinning. Formation of the β phase is believed to occurvia a peritectic decomposition of α-Al8Fe2Si at 612 °C. Observations made by transmission electron microscopy (TEM) support this hypothesis. When 30 ppm strontium is added to this alloy, the α phase is stabilized and very little β-Al5FeSi appears in the microstructure. A silicon-rich layer is found around the α-phase particles. It is proposed that strontium adsorbs to the α-phase interface, and in so doing, the diffusion of silicon into the α phase, necessary for its transformation to β, is prevented.

Dissolution of iron intermetallics in Al-Si alloys through nonequilibrium heat treatment

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.

Gravity segregation of complex intermetallic compounds in liquid aluminum-silicon alloys

Primary crystals of intermetallics that are rich in iron, manganese, and chromium form at temperatures above the liquidus, and because their density is higher than that of liquid alumi-num, they cause gravity segregation in the melt. Segregation may occur either in the mold at slow cooling rates or in the bulk liquid in furnaces or ladles. The kinetics of settling of these intermetallic compounds in a melt of Al-12.5 pct Si having 1.2 pct Fe, 0.3 pct Mn, and 0.1 pct Cr has been studied. Sedimentation was investigated at 630 ‡C for settling times of 30, 90, and 180 minutes in an electric resistance furnace. The effect of settling time and height of melt on the volume percent, number, and size of intermetallic compounds was studied by image anal-ysis. The volume percent of intermetallics increases with distance from the melt surface. Both the number of particles and the average size increase during sedimentation. The rate of settling varies with position in the melt due to depletion of intermetallics near the surface and an increase near the bottom. The settling velocities obtained experimentally were compared with terminal velocities calculated by Stokes’ law. Good agreement was generally found. The settling speed of intermetallics reaches the terminal velocity at very short times and very close to the liquid surface. Stokes’ law is therefore applicable to virtually all locations within the melt.

The electrical conductivity of cast Al−Si alloys in the range 2 to 12.6 wt pct silicon

The changes in electrical conductivity of cast Al−Si alloys, in the range of 2 to 12.6 wt pct silicon due to strontium additions (0.03 wt pct) have been investigated and explained in terms of variations in microstructure. The strontium-containing alloys exhibited a higher conductivity than alloys with no strontium, and this conductivity difference increased as the silicon and magnesium contents were increased and the solidification rate was decreased. It has been demonstrated that this difference is due to changes in microstructural features of the eutectic silicon upon modification.

Fracture toughness and its development in high purity cast carbon and low alloy steels

Fracture studies conducted using linear elastic fracture mechanics, crack opening displacement, and J-integral methods have shown that high room temperature toughness can be achieved in plain and low alloyed medium carbon cast steels. The high quality, carefully controlled, and heat treated steels prepared often yielded Jlc values which exceeded 0.1 MPa-m, or Klc values exceeding 150 MPa-m 1/2 Correspondingly, crack tip opening displacement values were of the order of 0.24 to 0.26 mm. Of the several elements examined chromium is the most effective for developing optimum properties at medium carbon levels. However, in general, chemical composition, in terms of major alloying elements, was found to be only a minor variable in determining fracture toughness of the heat treated steel. Fractographic measurements revealed that a squared power relationship exists between the stretched zone width and relative toughness (Klc/Φys) at both low and high toughness levels.

The effect of strontium on the electrical resistivity and conductivity of aluminum-silicon alloys

Electrical conductivity and resistivity measurements have been conducted on Al-Si and Al-Si-Sr alloys containing up to 12.6 wt pct silicon and 0.035 wt pct strontium. While silicon decreases the electrical resistivity of aluminum, strontium has no effect as it neither dissolves appreciably in aluminum nor changes the silicon solubility. Strontium does, however, retard the rate of silicon precipitation from supersaturated solid solution. When eutectic silicon appears in the microstructure strontium increases the conductivity of the alloy by up to 10 pct at the eutectic composition. Determination of the resistivity of directionally frozen eutectic samples demonstrates that this difference is due to changes in the silicon morphology.