Effect of Fe Intermetallics on Microstructure and Properties of Al-7Si Alloys

Abstract

The present work deals with the effect of iron intermetallics on the microstructure and mechanical properties of Al-7% Si alloys. Two different iron additions were made, 0.6% Fe and 2% Fe, to study the effect of iron intermetallics on Al-Si alloys. Microstructure property correlations were carried out using SEM-EDS and tensile testing of alloys. Microstructure results show that the rise in iron content significantly increased the average size, thickness and number of intermetallic particles in the alloys. Nano-indentation study shows that the iron intermetallics are too brittle compared with the primary aluminium. Moreover, the hardness and Young's modulus of iron intermetallics are higher than those of primary aluminium. Tensile test results show that there is no significant difference in strength levels between Al-7%Si and Al-7Si-0.6Fe alloys. However, an increase in iron from 0.6% to 2% resulted in a significant decrease in tensile strength and elongation of the alloys. Two-dimensional SEM studies suggest that the increased number of needle-shaped b-phase intermetallic particles formed because of increased amounts of Fe could be the reason for early failure of the alloy. To further understand the early failure of iron-containing alloys, the fractured tensile specimens were studied using the 3D x-ray tomography technique. XCT results show that the failure in tensile testing of 2% Fe alloy was not mainly due to breaking of brittle b-phase intermetallic particles, but due to the morphology and particle-matrix interface debonding. XCT shows that the needleshaped particles are long, sharp-edged platelets in 3D, which act as stress raisers for crack initiation and propagation along the interphase.

The Effect of the β-Al5FeSi Phases on Microstructure, Mechanical and Fatigue Properties in A356.0 Cast Alloys with Higher Fe Content without Additional Alloying of Mn

“…The higher content of Fe in alloy C led to the formation of a shorter ( Table 3 ) and thicker ( Figure 3 ) needle phase in comparison to alloy B ( Figure 3 ). These results correlate with the findings of Mathew [ 12 ], who determined that materials with 2 wt.% of Fe have thicker phases than materials with 0.6 wt.% of Fe.…”

“…On the metallography samples, β-Fe has the shape of needles ( Figure 3 b) and is therefore at times incorrectly called iron needles. Many works show that the compact and difficult shape of such phases has a detrimental effect on mechanical properties [ 8 , 12 , 25 , 35 , 36 ]. The β phases were observed in the form of long plates ( Figure 3 ).…”

“…Moreover, the higher Fe content led to an increase in the mechanical properties ( Figure 4 ), but the increase did not exceed the common measurement error of 7%, according to standard (in comparison to alloy A) ( Figure 5 ). The increase in the mechanical properties of materials with the higher Fe content also decelerated [ 12 ]. This increase could correlate with the presence of the finer eutectic Si particles, as shows in Figure 3 .…”

“…The samples that were not broken before reaching the specified number of cycles were defined as run-out and marked by red dots in Figure 7 . The run-out was observed only for alloys A and C. This observation was probably related to results of the lengths of the Fe-rich phases and their quantity, because these phases are brittle and could act as concentrators of stress and stress risers for crack initiation [ 12 , 25 , 35 , 36 , 40 , 41 ]. However, the results shown in in Figure 7 reflect that the experimental alloys with higher Fe content had the better fatigue properties (alloy B being the best) under higher stress amplitude.…”

“…There are two main α Fe-rich phases formed be Al–Si alloys: α-Al 15 (Mn, Fe) 3 Si 2 (Chinese script-type structure with a cubic (bcc) crystal lattice) and α-Al 8 Fe 2 Si (phase is hexagonally shaped). The other, β, phases, identified in Al-Si alloys, are Al 3 FeSi 2 , Al 4 FeSi, Al 9 Fe 2 Si 2 , Al 5 FeSi, Al 4.5 FeSi, and Al 5 Fe 2 Si [ 9 , 11 , 12 , 13 , 14 ]. Additions of Mn, Cr, Cu, V, Mo, and W promote a body-centered cubic structure in the Fe-rich phase.…”