Effects of Si addition on the microstructure evolution of Al–Cu–Mg alloys in the α + S + T phase field

Abstract

The precipitation behaviour and age-hardening response of Al-1.5Cu-4.0Mg (wt.%) alloys microalloyed with Si have been investigated by means of hardness measurement, TEM and HRTEM. Compared to the ternary alloy, the quaternary alloys exhibit a higher hardness. It is found that the underaged microstructure in the Al-1.5Cu-4.0 Mg alloy contains some fine precipitates, which are identified as the T phase by FFT spectra. The peak-aged microstructures of the ternary alloy is dominated by the T phase, while the peak-aged microstructures of the Si-containing alloys are dominated by the S phase. The volume fraction of the S phase is found to increase as more Si is added.

Influence of a microalloying addition of Ag on the precipitation kinetics of an Al–Cu–Mg alloy with high Mg:Cu ratio

“…7 of this work, that in turn coincide with those reported by Chopra et al [31] and Hirosawa et al [47]. However, these precipitates were incorrectly identified as corresponding to the T phase [51].…”

“…However, the formation of small equiaxed precipitates in between S phase laths was reported by Hirosawa et al [47] in an Al-1%Cu-3%Mg (wt.%) after ageing at 170°C for 96 h. The precipitates were identified as Z phase and exhibited a cube-cube OR with the Al matrix. More recently, equiaxed precipitates in an Al-Cu-Mg alloy of the same composition as this work were reported to form during ageing at 200°C [51]. The diffraction pattern of the precipitates show reflections identical to those reported in Fig.…”

Microstructure Evolution and Mechanical Properties of Al–Cu–Mg Alloys with Si Addition

“…It divides the diagram into two sections on the Al-rich corner: α-Al + Al 2 Cu (θ) + S and α-Al + Mg 32 (Al, Cu) 49 (T) + S. A quasi-binary eutectic reaction (L => α-Al + S) occurs at a Cu/Mg ratio of 2.40, resulting in various non-variant reactions in the Al-rich corner of Al–Cu–Mg ternary alloys [ 2 ]. Low-Mg-containing alloys in the α-Al + θ + S section of the diagram are primarily composed of a primary α-Al matrix and eutectic α-Al-θ phases, along with a small amount of eutectic α-Al-S phases [ 3 , 5 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. High-Mg-containing Al–Cu–Mg commercial alloys have compositions lying in the α-Al + S + θ and α-Al + S sections of the diagram [ 1 , 2 , 19 , 20 , 21 ].…”

“…Low-Mg-containing alloys in the α-Al + θ + S section of the diagram are primarily composed of a primary α-Al matrix and eutectic α-Al-θ phases, along with a small amount of eutectic α-Al-S phases [ 3 , 5 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. High-Mg-containing Al–Cu–Mg commercial alloys have compositions lying in the α-Al + S + θ and α-Al + S sections of the diagram [ 1 , 2 , 19 , 20 , 21 ]. However, due to their higher Mg content, these alloys have a significantly greater amount of eutectic α-Al-S phases than low-Mg-containing alloys.…”

“…However, the development of Al–Cu–Mg alloys with compositions in the α-Al + S + T section of the Al–Cu–Mg ternary phase diagram has received very little attention so far, according to the authors’ knowledge [ 19 , 20 , 21 ]. High-Mg-containing Al–Cu–Mg alloys exhibit significantly higher tensile strength compared to Al–Si-based commercial alloys in the as-cast condition.…”