Plane Script Fe
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Plane Script Fe
Iron (Fe) is the most common and the most detrimental impurity element in Al alloys due to the formation of Fe-containing intermetallic compounds (IMCs), which are harmful to mechanical performance of the Al-alloy components. In this paper we investigate the formation of Fe-containing IMCs during solidification of an Al-5Mg-2Si-0.7Mn-1.1Fe alloy under varied solidification conditions. We found that the primary Fe-containing intermetallic compound (P-IMC) in the alloy is the BCC α-Al15(Fe,Mn)3Si2 phase and has a polyhedral morphology with 1 1 0 surface termination. The formation of the P-IMCs can be easily suppressed by increasing the melt superheat and/or cooling rate, suggesting that the nucleation of the α-Al15(Fe,Mn)3Si2 phase is difficult. In addition, we found that the IMCs with a Chinese script morphology is initiated on the 1 0 0 surfaces of the P-IMCs during the binary eutectic reaction with the α-Al phase. Both the binary and ternary eutectic IMCs are also identified as the BCC α-Al15(Fe,Mn)3Si2 phase. Furthermore, we found that the Fe content increases and the Mn content decreases in the Fe-containing intermetallic compounds with the decrease of the formation temperature, although the sum of the Fe and Mn contents in all of the IMCs is constant.
During the past few decades, more than 20 different Fe-containing IMCs have been reported to exist in various Al alloys, including β-Al5FeSi (β-AlFeSi for short), α-Al15(Fe,Mn)3Si2 (α-AlFeSi for short), Al13Fe4 and Al6Fe, and so on.[4,5,6,7,8,9] Among these IMCs, β-AlFeSi and α-AlFeSi are the most common ones present in Al-Si based alloys widely used for automotive components.[10,11,12] Research effort has been focused on these two types of IMCs to control their formation during solidification. Technologically, physical processes, such as plastic deformation, ultrasonic vibration and electromagnetic stirring, were employed to break up and/or refine such Fe-containing IMCs with some degree of success. Plastic deformation during thermomechanical processing resulted in aligned IMCs with a reduced particle size, from a few tens of microns to a few microns. Ultrasonic vibration and electromagnetic stirring provide intensive forced convection during solidification processing and have been reported to be beneficial to the phase transition from the platelet-like β-AlFeSi to the more compact α-AlFeSi or Chinese script α-AlFeSi. However, the majority of the prior research has been concentrated on elemental additions (e.g., Mn, Cr, Co, Sr, Li, and K) to modify the morphology of the Fe-containing IMCs, from platelet to polyhedral, in order to reduce their detrimental effect on mechanical properties.[16,17,18,19,20,21]. For instance, Mn addition with the Fe/Mn ratio lower than 2:1 can promote the transition from platelet β-AlFeSi to polyhedral α-AlFeSi.
To further minimize the detrimental effect of the IMCs, the size of the α-AlFeSi phase must be refined and its morphology modified to be more compact. Few studies have been carried out to significantly refine the α-AlFeSi intermetallic phase. In particular, the α-AlFeSi phase with Chinese script morphology could be as large as a few millimeters in size, significantly deteriorating the mechanical properties of the cast components if it is not refined. The effect of solidification conditions, such as superheat and cooling rate, on the formation of the IMCs were investigated previously.[15,22] Several approaches including intensive shearing, rapid solidification, and twin roll casting were carried out.[3,22,23] It was indicated that the platelet β-AlFeSi can be suppressed and α-AlFeSi becomes smaller at high cooling rates.
Figure 1 shows the general microstructure of the alloy solidified in the TP1 mold with a pouring temperature of 680 C. The solidified microstructure consists of the primary IMC (P-IMC) with a compact and faceted morphology, the binary eutectic IMC (BE-IMC) with the typical Chinese script morphology, the α-Al dendrites, and the fine ternary eutectic IMC (TE-IMC).
Figure 2 is a SEM micrograph showing the 3D morphology of the P-IMC particles extracted from the TP1 sample cast at 680 C. The P-IMC particles are facetted and exhibit a polyhedral morphology. The typical angles between the facets were worked out statistically to be either 60 or 120 deg. The chemical composition of the P-IMC was quantified using EDS analysis in the TEM facility and the results are presented in Table II. The P-IMC particles contain 80.2 at. pct Al, 7.8 at. pct Fe, 6.8 at. pct Mn and 5.3 at. pct Si. Figure 3 presents a TEM bright field image of a P-IMC particle (Figure 3(a)) and the correspondent selected area electron diffraction (SAED) patterns taken from the particle with electron beam being parallel to [1 1 1] (Figure 3(b)) and [1 0 0] (Figure 3(c)) zone axis of the P-IMC particle. The results in Figure 3 confirm that the P-IMCs is the α-Al15(Fe,Mn)3Si2 phase with a body centered cubic (BCC) crystal structure. The lattice parameter of the α-Al15(Fe,Mn)3Si2 phase was determined to be 1.270 0.001 nm, which is slightly larger than the reported value of 1.256 nm in the literature. In addition, from the SAED pattern taken along the [1 1 1] zone axis of the P-IMC, the terminating surfaces of the faceted P-IMC particles are determined to be 1 1 0 planes, as marked in Figure 3(a). This is consistent with the angles between facets (60 or 120 deg) worked out statistically from Figure 2.
(a) TEM bright field image showing the faceted morphology of the P-IMC, and (b, c) its selected area electron diffraction (SAED) patterns taken from (b) [1 1 1] and (c) [1 0 0] zone directions of the P-IMC in (a). The terminating surfaces of the P-IMCs are identified as 1 1 0 planes
Figure 4(a) is an optical micrograph showing the typical Chinese script morphology of the BE-IMC phase which is associated with a P-IMC particle in the center. Figure 4(b) is an SEM image showing the 3D morphology of the BE-IMC phase. It is apparent that the BE-IMCs were initiated on and connected to the corners or the edges of the P-IMCs, and grew further to develop Chinese script morphology. The TEM-EDS results given in Table II suggest that the binary eutectic IMCs have an average composition of 78.8 at. pct Al, 9.0 at. pct Fe, 5.6 at. pct Mn and 6.6 at. pct Si, being very similar to that of the P-IMCs. It was noted from the composition measurement that, although Mn content in the BE-IMC phase was slightly lower than that in the P-IMCs, the sum of Fe and Mn contents in the BE-IMC phase is close to that in the P-IMCs. The SAED patterns taken from the BE-IMCs further confirmed that they are the α-Al15(Fe,Mn)3Si2 phase. The experimentally determined lattice parameter is 1.258 0.002 nm for the binary eutectic Fe-containing phase.
The connection of the BE-IMC phase with the P-IMCs was further examined by TEM. Figure 5(a) is a TEM bright field image showing that the BE-IMC phase grows from the corner of a P-IMC particle. According to the correspondent SAED pattern in Figure 5(b), the P-IMC particle is viewed along its [1 0 0] zone axis direction. Again, the P-IMC particles exhibit 1 1 0 facets. As indicated in the TEM image, the four corners of the P-IMC particle correspond to the 1 0 0 planes where the Chinese script BE-IMCs were initiated. The SAED patterns taken from both the P-IMC and the BE-IMC phases demonstrated that both the primary and its attached BE-IMCs have exactly the same crystallographic orientations, confirming that the BE-IMCs nucleated and grew naturally on the P-IMCs during the binary eutectic solidification.
(a) TEM bright field image showing a faceted P-IMC particle viewed along its [1 0 0] zone direction; (b) the corresponding selected area electron diffraction (SAED) pattern from the P-IMC. This result suggests that the BE-IMCs are initiated on the 1 0 0 planes of the P-IMC
Figure 6(a) is a TEM bright field image showing the morphology of the BE-IMC phase, and the HRTEM image in Figure 6(b) shows the interface between the BE-IMC and its adjacent Al, where the incident electron beam is parallel to the [1 0 0] zone direction of the BE- IMCs. Due to the same crystal structure with the P-IMC, the BE-IMC was initiated on the 1 0 0 planes of the P-IMC as the leading phase during the binary eutectic reaction. The HRTEM image in Figure 6(b) suggests that the BE-IMC phase is also 0 1 1 faceted, just like the P-IMCs. Although with a curved interface with its adjacent Al sometimes, the BE-IMC still has 0 1 1 facets with atomic scale 0 1 1 steps, as shown in Figure 7. The curved surface of BE-IMC is possibly caused by the fast growth of the BE-IMCs during the binary eutectic reaction.
(a) SEM micrograph showing the detailed microstructure of the ternary eutectic of ternary eutectic; (b) a HRTEM image of the interface between the TE-IMC viewed along its [1 0 0] zone direction and Mg2Si showing that the TE-IMC has a (0 1 1) faceted plane
However, this is not the case for the formation BE-IMCs when the P-IMCs are present. Heterogeneous nucleation of the BE-IMCs on the 1 0 0 planes of the existing P-IMCs becomes a merely epitaxial growth process due to their same crystal structure and similar chemical composition, requiring almost zero nucleation undercooling. This suggests that the BE-IMC will be the leading phase for the formation of the binary eutectic with the α-Al through a coupled growth mechanism, resulting in the Chinese script morphology for the BE-IMCs, as shown in Figures 3 and 4).
The Fe-containing intermetallic compound with a Chinese script morphology is formed through a binary eutectic reaction with the α-Al phase. The binary eutectic Fe-containing intermetallic compound (BE-IMC) is identified as the BCC α-Al15(Fe,Mn)3Si2 phase with a 1 1 0 surface termination. 041b061a72