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HRTEM-HRSTEM characterization and PFC simulation of clustering phenomenon in the Al-Cu system

V. Fallah1, A. Korinek2, N. Ofori-Opoku2, N. Provatas3, S. Esmaeili1.

Principal Investigators

Shahrzad Esmaeili1, Dept of Mechanical & Mechatronics Engineering
Nikolas Provatas3, Dept of Physics Centre for the Physics of Materials

(1) University of Waterloo
(2) McMaster University
(3) McGill University

Introduction

Early-stage solute clustering in solution treated precipitation hardening alloys strongly influences their aged microstructure and thus their mechanical properties. In this study, the clustering phenomenon in a solution treated and naturally aged Al–Cu alloy is characterized using High Resolution Transmission Electron Microscopy (HRTEM) and High Resolution Scanning Transmission Electron Microscopy (HRSTEM). The structural evolution of early clusters revealed by HRTEM-HRSTEM is compared against the Phase Field Crystal (PFC) simulations of dislocation-induced clustering [1,2] in a naturally-aged Al–2.5 at.% Cu alloy.

Results

Below we present our HRTEM-HRSTEM observations revealing, for the first time, the spherical-to-ellipsoidal morphology evolution, the preferred orientation and the dislocation-induced structural development of early clusters in a naturally aged Al-Cu alloy. The above observations are further confirmed by the PFC simulation results.  
 
Fig.1a shows the HRTEM micrograph of an early cluster in a solution treated Al–2.5 at.% Cu sample naturally aged for 14 days. The corresponding FFT, given in Fig.1b, shows streaking in {111} orientation. Also, the Fourier masked micrographs of Fig.1a reveals the association of dislocations with the above cluster, as marked by the circles in Fig.1c-d.       
 
HRSTEM observations revealed the earliest small compositional fluctuations appearing mainly with a spherical morphology, such as the one labeled with an arrow on Fig.2b. We introduce such spherical compositional fluctuations as precursors to the well-developed clusters/GP zones with an ellipsoidal morphology elongated along the close-packed directions <011>FCC, as can be clearly seen in Fig.1a and Fig.2. Also, the white contrast in Fig.2 represents the comparative local content of Cu, as the element with the higher atomic number in the Al-Cu system. This contrast varies from one cluster to another indicating the coexistence of clusters of various Cu contents.

Fig.1 (a) The HRTEM micrograph and (b) the corresponding FFT of a cluster in the Al–2.5 at.% Cu sample naturally aged for 14 days; (c, d) {111} orientation Fourier-masked micrographs of (a).

Fig.2 HAADF/HRSTEM micrographs of Cu-rich clusters in {011} orientation in the Al-rich matrix of Al–2.5 at.% Cu sample naturally aged for 14 days.

We further analyzed the morphological and structural evolution of early clusters using PFC simulations of the clustering phenomenon in the presence of quenched-in dislocations. In the PFC methodology, we employed the following free energy functional of a binary alloy described by two contributions, ideal and excess energy [1]:

(1)

where ΔFid is the ideal energy driving the system into uniform fields. ΔFex is the excess energy which favors periodic structures by two-point correlations between the atoms introducing elasticity and crystalline symmetry. kB is the Boltzmann constant, T is the temperature, ρ0 is the average density and V is the volume of the unit cell. The free energy functional in Eq.1 can be reduced to the following form as a function of density, n, and concentration, c:

(2)

where η, χ and ω are fitting parameters and the coefficient α sets the scale and energy of the compositional boundaries. Ceff is an effective correlation function interpolated between the individual two-point correlation functions of pure materials A and B. ΔFmix denotes the ideal entropy of mixing:

(3)

where c0 is the reference composition. The equations of motion for the density and concentration fields, n and c, respectively, follow dissipative dynamics. After reproducing the equilibrium properties of the Al-Cu system (i.e. the construction of the equilibrium phase diagram) the simulation of clustering was carried out by introducing quenched-in dislocations into a supersaturated Al–2.5 at.% Cu alloy.

Fig.3 PFC simulation of morphological evolution of a growing cluster in a naturally aged Al–2.5 at.% Cu alloy; Simulation time, t, is a dimensionless variable.

Our PFC simulations showed that the early clusters (Fig.3) can from in association with the quenched-in dislocations