The interplay between grain boundary stability and grain growth in the strengthening and weakening of annealed nanograined Al and Cu: A molecular dynamics study

S Zouaoui and H Haouas and A Hassani and A Hasnaoui, JOURNAL OF ALLOYS AND COMPOUNDS, 1040, 183660 (2025).

DOI: 10.1016/j.jallcom.2025.183660

Grain refinement into the nanoscale significantly enhances the mechanical properties of metals and alloys, yet it poses significant challenges in ensuring their thermal stability. Nanograined (NG) materials, which are made up of extremely fine grains, owe their mechanical properties to the dynamics of grain boundaries (GBs). These GBs undergo processes like diffusion, migration, grain rotation, and grain coarsening. However, the specific mechanisms driving these phenomena during thermal annealing remain unclear, particularly for grains in the sub -10 nm range. GB plasticity is critical for determining how these materials behave under stress. It allows NG structures to balance strength and ductility, which is crucial for practical applications in metals and alloys. This study uses molecular dynamics simulations to explore how annealing affects NG aluminum (Al) and copper (Cu). Specifically, it focuses on extremely fine grains between 5 and 8 nm in size and considers the stacking fault energy (SFE) unique to each material. Isothermal annealing under different conditions of time and temperature revealed distinct strengthening-weakening transitions. NG-Al, with its high SFE, demonstrated superior GB relaxation, achieving an increase in yield stress and enhanced resistance to grain coarsening. In contrast, NG-Cu, characterized by low SFE, exhibited significant GB migration and softening due to dislocation activity. Key mechanisms analyzed include GB diffusion, GB migration, grain rotation, and grain coarsening, which underpin the microstructural evolution and mechanical behavior of these materials. This work deepens the understanding of the mechanisms underlying the strengthening- weakening transitions arising from thermal annealing conditions, offering valuable insights for designing ultrahigh-strength and ductile nanograined materials.

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