The role of nanotwins and grain boundary plane in the thermal, corrosion, and sensitization behavior of nanometals

Grain boundaries play a significant role in the properties of polycrystalline metals, and the objective of grain boundary engineering is to incorporate special grain boundaries into materials to achieve better properties. While a special grain boundary is conventionally defined as a boundary with coincidence site lattice Σ ≤ 29, it should be noted that the grain boundary plane can also largely affect grain boundary structure and properties. Nanotwins in particular, which are bundles of coherent Σ3 boundaries, have extremely low energy and improved properties compared with ordinary grain boundaries. While it is well‐studied that nanotwins could enhance the mechanical properties of a metal, the effect of nanotwins on the thermal stability and corrosion resistance still needs to be explored and is the subject of this research. In addition to nanotwins which have {111} oriented boundary planes, this study also explored how differently oriented grain boundary planes could affect the sensitization behavior of an Al‐Mg alloy. ❧ For the thermal stability study, sputtered Cu foils with two types of microstructures, highly nanotwinned and non‐nanotwinned columnar grains, were sequentially heat‐treated at 200, 300, and 400℃. The highly nanotwinned Cu (grain size, ∼ 700 nm) remained thermally stable up to 300℃, whereas the non‐nanotwinned Cu (grain size, ∼ 400 nm) had a rapid grain growth even at 200℃. Additionally, the effect of nanotwins on corrosion behavior was evaluated through testing Cu samples containing various fractions of nanotwins: a) highly nanotwinned columnar grains, b) partially nanotwinned columnar grains, c) non‐nanotwinned columnar grains, and d) microcrystalline grains. These samples were tested in 3.5% NaCl solution (pH ~ 8.0) by linear polarization, potentiodynamic polarization, and immersion corrosion methods. It was found that highly nanotwinned Cu had a lower corrosion current density and a more protective passive layer compared with non‐nanotwinned, partially nanotwinned, and microcrystalline Cu samples. The improved thermal stability and corrosion resistance of highly nanotwinned Cu were attributed to the ordered structure of nanotwins, the special grain boundary network, and the {111} texture due to the presence of nanotwins. ❧ While a {111} oriented coherent boundary plane of nanotwins can enhance the thermal and corrosion behavior, this study further identified grain boundary plane orientations and explored their roles in the sensitization behavior of conventional and sputtered Al 5456 samples. Both types of samples were heated at 175℃ and subsequently etched in 10% H₃PO₄ in order to reveal the Mg-rich β phase. The orientations of selected grain boundary planes were measured and related to their corresponding β phase thicknesses. It was found that grain boundaries with plane orientations close to {110} appear particularly vulnerable to β precipitation. Moreover, the sputtered Al 5456 had much higher resistance to β precipitation than the conventional Al 5456, which could be due to its special columnar grain boundary plane orientations. This study highlights the potential of incorporating nanotwins and altering grain boundary plane orientations in order to improve thermal stability, corrosion resistance, and sensitization behavior.