Microstructural Characterization of Beryllium Treated Al-Si Alloys

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 673025, 10 pages http://dx.doi.org/10.1155/2015/673025 Research Article Microstructural Characterization of Beryllium Treated Al-Si Alloys M. F. Ibrahim,1 S. A. Alkahtani,2 Kh. A. Abuhasel,2 and F. H. Samuel1 1 Département des Sciences Appliquées, Université du Québec à Chicoutimi, Chicoutimi, QC, Canada G7H 2B1 Mechanical Engineering Department, College of Engineering, Prince Sattam bin Abdulaziz University, Al Kharj 11942, Saudi Arabia 2 Correspondence should be addressed to F. H. Samuel; Received 7 August 2015; Accepted 13 September 2015 Academic Editor: Francesco Delogu Copyright © 2015 M. F. Ibrahim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The present study was carried out on B356 and B357 alloys using the thermal analysis technique. Metallographic samples prepared from these castings were examined using optical microscopy and FESEM. Results revealed that beryllium causes partial modification of the eutectic Si, similar to that reported for magnesium additions. Addition of 0.8 wt.% Mg reduces the eutectic temperature by ∼10∘ C. During solidification of alloys containing high levels of Fe and Mg, but no Sr, formation of a Be-Fe phase was detected at 611∘ C, close to that of 𝛼-Al. The Be-Fe phase precipitates in script-like form at or close to the 𝛽-Al5 SiFe platelets. A new reaction, composed of fine particles of Si and 𝜋-Fe phase, was observed to occur near the end of solidification in high Mg-, high Fe-, and Be-containing alloys. The amount of this reaction decreased with the addition of Sr. Occasionally, Be-containing phase particles were observed as part of the reaction. Addition of Be has a noticeable effect on decreasing the 𝛽-Al5 FeSi platelet length; this effect may be enhanced by addition of Sr. Beryllium addition also results in precipitation of the 𝛽-Al5 FeSi phase in nodular form, which lowers its harmful effects on the alloy mechanical properties. 1. Introduction Bäckerud et al. [1] reported that the main reactions to be observed in the Al-7%Si-0.56%Mg alloy containing 0.14 wt.% Fe are (i) the formation of primary 𝛼-Al dendrites, (ii) the formation of the Al-Si eutectic phase along with the 𝛽-Al5 FeSi phase, and (iii) the formation of secondary eutectic phases. It will be observed that there are two possible reactions required for the formation of the 𝜋-phase. The first is a result of the transformation of the 𝛽-Al5 FeSi phase into the 𝜋-phase through a peritectic reaction. The general microstructure of Al-7%Si-Mg alloys consists of (i) primary 𝛼-Al, (ii) Mg2 Si displaying Chinese script morphology, (iii) 𝛽-phase (Al5 FeSi) with its plate-like morphology, and (iv) the script-like 𝜋phase (Al8 Mg3 FeSi6 ) [1]. Phragmén [2] reported that the composition of the 𝛽Al5 FeSi phase is 27 wt.% Fe and 13.5 wt.% Si with a density of 3.30–3.35 g/cm3 , appearing in the form of thin platelets or needles in the microstructure. The 𝛽-Al5 FeSi phase grows in a lateral or faceted mode and contains multiple (001) growth twins parallel to the growth direction [3]. The first suggestion of the stoichiometry of the 𝜋-phase, from the work of Foss et al. [4], was Al9 FeMg3 Si5 , which deviates from the Al8 Mg3 FeSi6 suggested by others [1, 5]. The 𝜋phase is a quaternary phase having a script-like morphology, often linked with 𝛽-Al5 FeSi [5]. The chemical composition of the 𝜋-phase, observed only in two possible morphologies, is 10.9 wt.% Fe, 32.9 wt.% Si, and 14.1 wt.% Mg, with a density of 2.82 g/cm3 [5–7]. It has been observed that the addition of strontium acts as an obstacle for the nucleation of the 𝛽-Al5 FeSi platelets, by reducing the number of sites ultimately available for nucleation. As a result, the 𝛽-Al5 FeSi phase precipitates at a smaller number of sites, leading to the precipitation of needles which are larger compared to those in the nonmodified alloy [8]. It has been reported that for a 319 alloy containing 0.46 wt.% Fe and solidified at a slow cooling rate, the optimum Sr levels lie closer to the limit of 400 ppm: as the Fe level increases, the optimum Sr level will be observed to shift towards the higher limit [9]. 2 Advances in Materials Science and Engineering 𝜋-phase volume fraction (%) 1.00 Table 1: Average chemical composition (wt%) of the 356 and 357 alloys studied. 0.80 Alloy code 0.60 0.40 0.20 0.00 0.3 0.4 0.5 0.6 0.7 0.8 Mg content (wt%) As-cast As-cast + Be 0.9 1 1.1 SHT SHT + Be Figure 1: An example of the effect of Be addition on the volume fraction of the 𝜋-phase in nonmodified Al-7Si-𝑥Mg-0.1Fe Becontaining alloys [5]. For Al-Si-Fe-Mg cast alloys, prior investigations have shown that the amount of 𝜋-phase and 𝛽-phase may remain constant regardless of the Mg content, whereas the amount of Mg2 Si increases [10]. Cáceres et al. [11] studied the microstructures of two Al-Si-Mg casting alloys, respectively, containing 0.4 wt.% and 0.7 wt.% Mg. It was observed that the iron intermetallic phases in high Mg-content alloys are larger than those are in alloys containing low levels of Mg and that the Fe-rich intermetallic phases in low-Mg alloys were exclusively small 𝛽-Al5 FeSi phase plates, while large 𝜋-phase (Al8 Mg3 FeSi6 ) particles were dominant in high-Mg alloys together with a small proportion of the 𝛽-Al5 FeSi phase. It has been observed that the addition of Mg to Al-Si-Mg casting alloys, at three different cooling rates, changes the solidification sequence and the type of iron intermetallic phase formed [1]. At Mg additions of more than 1.0 wt.% to molten 319 type alloys, the amount of the 𝛽-Al5 FeSi phase is reduced as a result of the transformation of the 𝛽-phase into the 𝜋-Al8 Mg3 FeSi6 phase [12]. It has been reported that increasing the Mg content from 0.4 to 0.7 wt.% in 357 alloys significantly increases the potential for the formation of the Mg-containing Al8 FeMg3 Si6 iron intermetallic phase [13]. The present study is an extension of the work carried by Elsharkawi, Figure 1 [5], on the effects of metallurgical parameters on the decomposition of the 𝜋-Al8 Mg3 FeSi6 phase in Al-Si-Mg alloys, with an emphasis on the role of Be, Sr in the microstructural features of B356 and B357 alloys, by first considering the role of increasing both the Mg and Fe content. A1 A1B A1S A1BS C3 C3B C3S C3BS Si 7.146 7.146 7.146 7.146 7.146 7.146 7.146 7.146 Element concentration (wt%) Fe Mg Ti Sr Be 0.09 0.40 0.168 0.00 0.00 0.09 0.40 0.168 0.00 0.05 0.09 0.40 0.168 0.02 0.00 0.09 0.40 0.168 0.02 0.05 0.60 0.80 0.168 0.00 0.00 0.60 0.80 0.168 0.00 0.05 0.60 0.80 0.168 0.02 0.00 0.60 0.80 0.168 0.02 0.05 Al Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. In the above table, codes A and C correspond to the Fe levels 0.09 and 0.6, respectively, while co (...truncated)