Thermal History-Dependent Coalescence Mechanisms and Sintering Dynamics in Al-6.8%Cu Nanopowders

TL;DR

The paper addresses how to control the microstructure of Al-6.8%Cu nanoparticle consolidates by considering the full thermal cycle. Using large-scale molecular dynamics with an EAM potential, the authors reveal a temperature-driven mechanistic crossover: dislocation-mediated plasticity governs sintering at low-to-moderate temperatures, while a transient liquid-like amorphous interfacial layer enables diffusion-dominated coalescence near $0.8\,T_m$. Crucially, the cooling rate kinetically governs the final crystalline-to-amorphous balance and defect content, enabling tailoring of the final density and microstructure. This work provides atomic-scale guidelines for thermal processing to design defect-stabilized, high-performance Al-Cu components with controlled porosity, density, and interfacial structure.

Abstract

Aluminum-Copper (Al-Cu) alloys are essential materials for weight reduction critical structures in the aerospace and automotive industries, yet achieving their maximum ultrahigh-strength potential remains limited by nanoscale defect control during powder metallurgy processing. We employ large-scale molecular dynamics simulations on Al-6.8%Cu nanoparticles to explore atomic-scale mechanisms governing the full thermal sintering cycle. We demonstrate that while the sintering temperature primarily initiates neck formation, the subsequent cooling rate is the dominant kinetic parameter dictating the final microstructure. Fast cooling rates trap a significantly higher density of stacking faults and can unexpectedly lead to the formation of an amorphous phase at the interparticle interfaces, a feature critically dependent on the rate of thermal dissipation. We confirm a clear shift in the coalescence mechanism from plastic deformation (dislocation slip) at low temperatures (300 K and 450 K) to mass transport via atomic diffusion at high temperatures (600 K). These findings provide essential, atomic-scale guidelines for controlling thermal processing, particularly cooling rates, to design defect-stabilized, high-performance Al-Cu components.

Figures (10)

• Figure 1: All-atom model of the Al-6.8%Cu nanoparticles.a, Top view of the initial configuration consisting of eight 5-nm-diameter Al-6.8%Cu spheres separated by 4 Å. Aluminum atoms are shown in gray and copper atoms in pink. b, The complete thermal profile used for the sintering simulation. The cycle includes initial structural relaxation at 300 K, heating to the sintering temperature of 600 K, a critical isothermal hold period, and the final controlled cooling stage. Snapshots beneath the plot illustrate the system's evolving configuration at the end of each major thermal stage.
• Figure 2: Melting behavior of surface and core regions in a Al-6.8%Cu NP.a, Temperature dependence of the potential energy per atom. Surface atoms (red) are defined as those within 2.5 Å of the outer surface, while the core (blue) consists of the remaining atoms. b, Cross-sectional view of a Al-6.8%Cu NP with a radius of 2.5 nm, sliced along the (010) plane. c, Final atomic configurations at different temperatures (300 K, 600 K, 700 K, and 800 K), showing FCC atoms in green and amorphous atoms in gray. The side bars indicate the relative fraction of FCC and amorphous atoms at each temperature, highlighting the progressive loss of crystalline order during heating.
• Figure 3: Microstructural evolution of Al-6.8%Cu NPs during sintering.a–d, Atomic configurations of Al-6.8%Cu NPs during annealing process up to 600 K shown in three-dimensional (top row) and (010)-plane cross-sectional (bottom row) views at: (a) the beginning of the relaxation stage, (b) mid-heating stage, (c) mid-holding stage, and (d) the end of the cooling stage. The (010)-plane section is taken through the center of the NPs. Hereafter, atoms and plots are color-coded according to their local crystal structure: FCC (green), HCP (red), BCC (blue), and amorphous (gray). e–f, Corresponding phase composition represented as pie charts showing the fraction of each structural type for the snapshots in (a–d).
• Figure 4: Temperature-dependent microstructural evolution of sintered Al-6.8%Cu NPs. Relative fractions of FCC, HCP, BCC, and amorphous phases extracted at the end of the sintering process for simulations performed at 300, 450, and 600 K, corresponding to approximately 0.4, 0.6, and 0.8 of the average melting point of the core atoms.
• Figure 5: Final microstructures and void fraction of Al-6.8%Cu NPs sintered at various temperatures.a, Cut-away view of the NP microstructure at the center of the sphere at 300 K, b, 450 K, and c, 600 K. The inset shows a cross-sectional view of the void region between eight NPs, with the sintering neck highlighted by a white arrow. d, In-situ TEM images of the sintering neck at 22 °C (295 K), 250 °C (523 K), and 350 °C (623 K), reproduced with permission from liu2023coalescence (Elsevier). e, Void fraction of the NPs, calculated as the total NP volume divided by the simulation box volume.