Precipitation processes in Al-Cu-Mg-Sn and Al-Cu-Mg-Sn-Ag

Highlights

• Sn and (Sn + Ag) are microalloyed in Al-2.5Cu-1.5Mg (wt%).

• Hardness and microstructure are examined after solution treatment and ageing.

• Adding Sn results in superior hardness and homogenises S phase distribution.

• Adding (Sn + Ag) reduces time to peak hardness and results in Ω phase precipitation.

Abstract

Microalloying trace elements into aluminium alloys have been shown to improve mechanical properties by altering the precipitation process. Here, trace amounts of Sn and (Sn + Ag) have been added to Al-1.1Cu-1.7Mg (at.%) and the effects have been investigated by a combination of hardness testing and transmission electron microscopy (TEM). Hardness testing shows that the addition of Sn increases the hardness throughout the ageing process, and in combination with Ag, further increases the hardness and shortens the time to reach the peak hardness. The increase in hardness via Sn microalloying is attributed to the homogeneous distribution of S phase (Al₂CuMg) precipitates. In the alloy microalloyed with both Sn and Ag, the microstructure is dominated by homogeneously distributed Ω phase (Al₂Cu) precipitates in the peak strengthened condition. Given that neither spherical β-Sn precipitates, nor any other obvious nucleation sites for the Ω phase precipitates were observed using TEM, the mechanism for development of such homogeneous precipitation remains to be determined.

Introduction

Microalloying elements in aluminium alloys have long been known to improve mechanical properties [1], [2], [3], [4], [5], [6], [7], [8], [9]. For example, adding trace amounts of Ag into Al-Cu-Mg alloys has been shown to stimulate an enhanced age hardening response [1], [5], [7], [10] and improve creep properties [2]. Trace addition of Sn into Al-Cu was shown to enhance the hardening response [3]. Therefore, Ag and Sn are both elements with good potential for improving Al-Cu-Mg alloy systems. Understanding the effects of microalloying Al with just Sn, or both Sn and Ag together, on the microstructural development and mechanical properties will greatly aid in the design of better aluminium alloys.

Microalloying of Sn in Al-Cu alloys increases the hardness, which is attributed to the refinement and homogeneous dispersion of the strength-providing θ′ precipitate phase (Al₂Cu) [11]. This is in contrast to heterogeneous θ′ phase nucleation at dislocations in binary Al-Cu without Sn [12]. Many hypotheses were proposed [3], [11], [13] until Ringer et al. [14], [15] showed conclusively from both TEM and atom probe field ion microscopy (APFIM) that such homogeneous distribution is due to nucleation from uniformly distributed, spherical β-Sn particles. This result has also been demonstrated by high resolution transmission electron microscopy (HRTEM) [16] and that such β-Sn precipitates have a stabilising effect on the thickness of the nucleated θ′ phase [17]. Atom probe tomography (APT) studies [18], [19], [20] have shown that these β-Sn precipitates form in the earliest stages of ageing from clusters of Sn atoms.

Fewer studies been have been carried out to measure the properties arising from microalloying additions of Sn in Al-Cu-Mg. Recently however, there has been some work on Al-Cu-Mg alloys in the α + θ + S field of the ternary phase diagram (i.e., high atomic Cu:Mg ratio at around 4.5:1) [21], [22], [23], [24], where it is shown that Sn stimulates an enhanced age hardening response. Using TEM, Shu et al. [21] observed the formation of insoluble Mg₂Sn particles in the as-quenched (AQ) condition. These particles stimulate the development of fine and uniform θ´ phase precipitates at the expense of the σ and Ω phases. It was proposed that this is due to the formation of Mg₂Sn, which removes Mg atoms from the matrix. Until now though, no study has been carried out to determine the effect of microalloying of Sn in an Al-Cu-Mg alloy having a composition in the α + S field (i.e., atomic Cu:Mg ratio close to 1:1).

The effect of microalloying addition of Ag to Al-Cu-Mg alloys are better understood, where it is known that the Cu:Mg ratio has a very important effect on how Ag changes the microstructural evolution via promotion of new precipitates [10], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38] that effect enhanced properties. For example, in Al-Cu-Mg base alloys with high atomic Cu:Mg ratios (~ 5:1) (i.e., in the α + θ + S phase field), trace amounts of Ag (~ 0.1 at.%) stimulate the formation of a fine and uniform dispersion of the Ω phase [1], [25], [27], [29], [32], [36], [37], [38], [39], [40], [41]. This phase is known to be orthorhombic in structure; in fact, it is a distortion of the θ phase (Al₂Cu) found in Al-Cu [25], [40], [42], [43], [44], and has the following orientation relationship [25]:

$${\left(001\right)}_{\Omega }//{\left(111\right)}_{\alpha }$$

$${\left[010\right]}_{\mathrm{\Omega }}//{\left[10\bar{1}\right]}_{\alpha }$$

$${\left[100\right]}_{\mathrm{\Omega }}//{\left[1\bar{2}1\right]}_{\alpha }.$$

In contrast, Ag stimulates the evolution of a different phase, X´ (Al₂CuMg [10]), in base Al-Cu-Mg alloys from the α + S field (atomic Cu:Mg ratio ~ 1:1). The X′ phase also lies on the {111}_α planes [10], [31]. However, unlike the Ω phase, it has a different structure and orientation relationship [31]:

$${\left(0001\right)}_{X^{\prime }}//{\left(111\right)}_{\alpha }$$

$${\left[1010\right]}_{X^{\prime }}//{\left[110\right]}_{\alpha }.$$

In the present study, detailed TEM examination has been performed to characterise the microstructural evolution of Al-1.1Cu-1.7Mg (at.%) microalloyed with both Sn and also (Sn + Ag). The aim of this study is to understand the role of such microalloying on the microstructural development during age hardening. Given that Sn has never been added to Al-Cu-Mg in the α + S field or to Al-Cu-Mg-Ag, the focus has been on the change in the type of precipitation observed and the homogeneity of its distribution.