Aluminum Cast Alloys Based on the Al–Ce–Si–Mg System: An Influence of Silicon on Crystallization, Phase Composition, and Tensile Properties

Abstract

An attempt at strengthening the aluminum-cerium-based alloys through additions of silicon was assessed using the experimental Al5Ce3Si0.5Mg (wt. pct) cast hypoeutectic composition, designed based on the commercial A356 (Al–7Si–0.3Mg, wt pct) grade by substituting a portion of Si with Ce. To determine a role of Si, the Al5Ce0.5Mg (wt. pct) reference was cast and tested under identical conditions. An addition of 3 wt. pct Si to the Al5Ce0.5Mg base increased the room temperature yield stress almost three times, from 47 to 135 MPa, but reduced its elongation by an order of magnitude from 8 pct to that below 1 pct. A presence of Si led also to essential changes in the alloy crystallization with the melting range widened substantially from 10 °C to 91 °C mainly due to a reduction in the solidus level. As-cast microstructure of the Al5Ce0.5Mg base consisting the primary αAl along with 44 pct of the αAl + Al11Ce3 eutectic was replaced in the Al5Ce3Si0.5Mg alloy with the αAl + AlCeSi2 coarse lamellae ternary eutectic, bulky compounds having the Ce-rich core of AlCe2Si with external shell of Al2CeSi2 along with the quaternary eutectic of fine αAl, Si, Al2MgSi/Al2Mg2Si, and Al–Si–Mg-Ce phases. The fractographic analysis revealed that additions of Si caused a transition from largely ductile fracture to the predominantly brittle mode with crack paths propagating mainly along the interface between the coarse AlCeSi2 lamellae and Al(Si, Mg) solid solution, which explains the measured drastic reduction in the alloy elongation. Graphical Abstract

Microstructure and mechanical properties of suction-cast aluminium–cerium alloys

“…As a result, the dislocation density increased, leading to greater dislocation pile-ups near the precipitates and, ultimately, brittle failure. The influence of the CeAlSi 2 phase on the cracking and brittle fracture of Al alloys was reported by Czerwinski and Shalchi Amirkhiz [30] as well. According to the findings, cast Al–Ce alloys lack sufficient mechanical strength and have limited strengthening, making them unsuitable for engineering applications at the moment.…”

“…Okamoto [12] examined the Al–Ce alloys thermodynamically and proposed the existence of the Al 11 Ce 3 phase in addition to the α-Al phase in Al–12Ce alloys. In contrast, Gröbner et al [39] and Czerwinski and Shalchi Amirkhiz [40] in their research on the solidification thermodynamics of the Al–Ce–Si system suggested the presence of CeAlSi 2 , Al 11 Ce 3 , and α-Al phases when Ce content was greater than 11%.

Figure 2 XRD peaks of (i) Al–12Ce, (ii) Al–12Ce–4Si, and (iii) Al–12Ce–4Si–0.4Mg alloys in the (a) GC and (b) SC conditions; variation of (c) crystallite size, (d) dislocation density, and (e) microstrain values for the GC and SC Al–12Ce–X alloys.

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“…Okamoto [12] examined the Al-Ce alloys thermodynamically and proposed the existence of the Al 11 Ce 3 phase in addition to the α-Al phase in Al-12Ce alloys. In contrast, Gröbner et al [39] and Czerwinski and Shalchi Amirkhiz [40] in their research on the solidification thermodynamics of the Al-Ce-Si system suggested the presence of CeAlSi 2 , Al 11 Ce 3 , and α-Al phases when Ce content was greater than 11%.…”

“…The phase composition following casting is determined not only by electronegativity but also by the thermodynamics of solidification. After adding more than 10% Ce, the thermodynamic analysis of the ternary Al-Ce-Si system using CALPHAD and FactSage shows the formation of α-Al, Al 11 Ce 3 , and CeAlSi 2 phases [39,40].…”

“…Sims et al [29] proposed that adding 0.4Mg to Al–12Ce increased its tensile strength while decreasing its ductility. Recently, Czerwinski and Shalchi Amirkhiz [30] recently reported that adding 3%Si to the Al–5Ce–0.5Mg alloy tripled the room temperature yield stress (YS) while decreasing elongation by an order of magnitude from 8% to less than 1%. The literature on the mechanical performance of Al–12Ce alloys with Si and/or Mg content, on the other hand, is limited.…”