Nanotwins readily form in numerous fcc metals with low stacking fault energy (SFE). However, growth twins rarely form in Al due to its high SFE. Here, by using thin inter- or buffer layers of a low SFE fcc metal (Ag), we overcome the SFE barrier and successfully grow high-density coherent and incoherent twin boundaries into Al [D. Bufford et al, Materials Research Letters, 1 (2013) 51-60; http://dx.doi.org/10.1080/21663831.2012.761654.]. We identify three mechanisms that enable growth twin formation in Al, and demonstrate enhanced mechanical strength in twinned Al.
Furthermore we will show that epitaxial Ag/Al multilayer films have high hardness (up to 5.5 GPa) in comparison to monolithic Ag and Al films (2 and 1 GPa). High-density nanotwins and stacking faults appear in both Ag and Al layers, and stacking fault density in Al increases sharply with decreasing individual layer thickness, h. Hardness increases monotonically with decreasing h, and no softening occurs. In comparison, epitaxial Cu/Ni multilayers reach similar peak hardness when h ≈ 5 nm, but soften at smaller h. High strength in Ag/Al films is primarily a result of layer interfaces, nanotwins, and stacking faults, which are strong barriers to transmission of dislocations. This research is funded by DOE-Office of Basic Energy Sciences.