Structure and morphology of S-phase precipitates in aluminum

Abstract

This work presents a crystallographic and morphological analysis of S-phase precipitation in Al alloys. Using quantitative high resolution electron microscopy, four models for the crystal structure of the S-phase (Al₂CuMg) in Al-based alloys are critically evaluated, and a new model is proposed. This model is identical to that of Perlitz and Westgren, but with an exchange of Cu and Mg. Two distinct precipitate morphologies are observed. Both are laths elongated along 〈100〉 directions common to the matrix and the precipitate and lie on {021} planes of the matrix. Type I precipitates have interfaces of the type $\text{(021)}{}_{\mathrm{<mtext>Al</mtext>}}\text{‖(001)}{}_{\mathrm{<mtext>S</mtext>}}$ while type II precipitates have interfaces of the type $\text{(021)}{}_{\mathrm{<mtext>Al</mtext>}}\text{‖(043)}{}_{\mathrm{<mtext>S</mtext>}}$, i.e. the two types differ in the S-phase lattice plane that is conjugate to the {021}_Al habit plane. The interface plane of type I precipitates tends to be atomically flat containing only growth ledges while that of type II precipitates is stepped. The orientation relationship of the two types of precipitate differs by a rotation of about 5° around the lath axis. The difference between the two types of precipitate is discussed in terms of their lattice correspondence, and type II precipitates are shown to follow an invariant line strain. Moiré analysis of lattice distortions revealed that {020}_Al planes remain undistorted while {002}_Al planes suffer significant shear during S-phase nucleation.

Introduction

Al–Cu–Mg and Al–Li–Cu–Mg alloys are of significant interest for aerospace and other applications, because of their light weight, mechanical strength and corrosion resistance. Their mechanical properties are based on a dispersion of S-phase precipitates which have been shown to alter the deformation mode [1]. S-phase precipitates have the composition Al₂CuMg and form as laths along 〈100〉_Al, with {012}_Al habit. In early investigations Bagaryatskii 2, 3, 4 and Silcock [5] observed the following crystallographic orientation relationship:

$$\text{[100]}{}_{\mathrm{<mtext>S</mtext>}}\text{//[100]}{}_{\mathrm{<mtext>Al</mtext>}}\text{(}\text{lath}\text{axis}\text{)}\text{[001]}{}_{\mathrm{<mtext>S</mtext>}}\text{//[021]}{}_{\mathrm{<mtext>Al</mtext>}}\text{(}\text{habit}\text{plane}\text{)}\text{[010]}{}_{\mathrm{<mtext>S</mtext>}}\text{//[01}\text{2}\text{̄}\text{]}{}_{\mathrm{<mtext>Al</mtext>}}\text{.}$$

However, despite extensive study 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, their crystal structure, morphology and interface structure are still not well understood. In the ternary Al–Cu–Mg alloy, Cu/Mg-rich precipitates (GPB zones) have been observed in addition to well-developed S-phase, causing considerable hardening 25, 31, 32. According to Silcock [5] these zones are cylindrical, 1–2 nm in diameter and about 4 nm long. GPB zones are also formed during the early stage of aging at 190°C and are followed by nucleation of S′-phase precipitates (reported to be differing from S-phase precipitates mainly by their degree of coherency). Over-aging occurs by growth of S′-phase precipitates with a corresponding re-solution of GPB zones. According to Bagaryatskii 2, 3, GPB zones do not act as nuclei for the S′-phase. Flower and Gregson [15] also noted that the connection between GPB zones and S′ formation has not been clearly established. However, in a quaternary Al–2.7% Cu–1.5% Mg–0.2% Si alloy, Weatherly [32] found strong evidence for the transformation of the GPB zones to S′ precipitates at 190°C.

Apart from standard heterogeneous nucleation sites, a distribution of vacancy clusters may provide sites for the precipitation of S-phase laths. Alternatively, the clusters may become enriched with Cu and Mg and then develop into S-phase precipitates as proposed for classical GPB zones [15]. More recent results, based on DSC measurements 33, 34, support the idea that the S-phase forms via the reaction: GPB zones→S′→S. However, since the formation enthalpies for S and S′ precipitates are identical [33] and HREM observations show the same structure [24], S and S′ may be considered to be the same phase.

Several models that have been proposed for the crystal structure of Al₂CuMg (termed S, S′, and S″) are listed in Table 1. The first model of the S-phase was given by Perlitz and Westgren (PW) [6] based on X-ray diffraction. The unit cell of the PW model is orthorhombic with unit cell dimensions $\text{a=0.4 }\text{nm}$, $\text{b=0.923 }\text{nm}$, and $\text{c=0.714 }\text{nm}$, space group Cmcm, containing 16 atoms in the ratio $\text{Al}\text{:}\text{Cu}\text{:}\text{Mg}\text{=2:1:1}$. The atomic volume of this structure is within 0.43% of that of the Al matrix. Laves and Witte [35] suggested that this phase extends toward Al₅Cu₂Mg₂, implying that equal amounts of the Cu and Mg atoms in Al₂CuMg are replaced by Al. Alternatively, Nishimura [36] proposed a homogeneous S-phase of composition Al₁₃Cu₇Mg₈, implying some degree of Mg substitution for Al. Mondolfo [12] suggested a modified PW model with slightly different lattice parameters ($\text{a=0.4 }\text{nm}$, $\text{b=0.925 }\text{nm}$, and $\text{c=0.718 }\text{nm}$).

Cuisiat et al. [29] offered a model for an S″-phase with unit cell dimensions identical to Mondolfo's, but with space group Im2m containing only eight instead of 16 atoms. Based on high resolution electron microscopy and electron diffraction, Yan et al. [28] proposed an orthorhombic structure with space group Pmm2 (No. 25), lattice parameters $\text{a=0.4 }\text{nm}$, $\text{b=0.461 }\text{nm}$, $\text{c=0.718 }\text{nm}$ and four atoms per unit cell in the ratio $\text{Al}\text{:}\text{Cu}\text{:}\text{Mg}\text{=2:1:1}$. According to the models of Cuisiat and Yan the density of the S-phase is only half that of the aluminum matrix. However, X-ray diffraction experiments performed on Al–Li–Cu–Mg alloys by Perez-Landazabal et al. [37] to clarify this controversy supported the models of PW [6] and Mondolfo [12].

It is obvious that S-phase precipitates play a very important role in the microstructure of Al–Cu–Mg alloys, but their morphology and interface structure are not well understood. Since the mechanical behavior of Al alloys strengthened by the S-phase cannot be fully understood without a clear understanding of their crystallography and morphology, the goal of this investigation was to characterize the crystal structure, evolution and interface structure of the S-phase using quantitative electron beam characterization techniques.