Microstructural evolution of nanotwinned Al-Zr alloy with significant 9R phase

Abstract

Aluminum (Al) alloys have a multitude of applications, notably in the automotive, aerospace and coating industries, yet they exhibit significantly lower mechanical strength than conventional steels. Nanotwins drastically improve mechanical strength while retaining ductility. However, the high SFE of Al largely prevents twinning. Here, we synthesize Al-Zr alloy films containing an abundance of incoherent twin boundaries and 9R phases. These alloys exhibit an extended solid solubility of Zr, retaining a columnar nanotwinned structure across all compositions. These films reach a hardness up to 4.2 GPa with 10 at% Zr and demonstrate the capacity for producing strong Al alloys with nanotwins.

Effect of De-Twinning on Tensile Strength of Nano-Twinned Cu Films

“…Owing to its coherence feature, the twin boundary has the smallest grain-boundary energy among all types of grain boundaries. For FCC Cu, the low stacking fault energy (~22 mJ/m 2 ) easily results in the nano-twinned structure in the Cu phase [ 1 , 2 , 3 , 4 ]. It has been reported that, with the high density of twin boundary, the strength, ductility, and conductivity of the Cu can be enhanced [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ].…”

Achieving strong and stable nanocrystalline Al alloys through compositional design

“…Figure 2d exhibits different solute effects on grain refinement and in general, a higher solute concentration generates a smaller grain size. Co, Ni and Fe show strikingly higher effectiveness in grain refinement compared to Ti, Zr and Mg [35][36][37][38]40]. 5-6 at% addition of Ni, Co and Fe managed to bring the grain size down to about 14, 5 and 5 nm, respectively.…”

“…For liquidquenched amorphous alloys, larger T g /T m (T g is glass transition temperature), difference in atomic radius and mixing enthalpy in general lead to a higher glass-forming ability [65], and this suggests that the amorphization is highly composition-and rate dependent [66,67]. An amorphous phase started to form as Fe content (C Fe ) reached ~ 8% in Al-Fe, C Cr ~ 13% in Al-Cr and C Zr ~ 15% in Al-Zr system [29,34,36]. Ophus et al revealed that 12% Mo was the onset of composition for the amorphous formation [68].…”

“…Figure 2c shows the composition effects on hardness of various binary NC Al alloys and it clearly suggests that Co [37], Ni [40] and Fe [35] are more effective in strengthening Al than other solutes. With 5-6 at% solutes, Al-Co, Al-Ni and Al-Fe produce hardnesses as high as 5.5-6 GPa, whereas the hardnesses of Al-X (X = Ag [35], Cr [34], Mg [38], Mo [35,71], Ti [39], Zr [36] or W [35]) alloys could hardly exceed 3 GPa.…”

“…In contrast, electrodeposited Ni-W achieved 5-7 nm grain size, but was classified as a metastable/unstable NC system [26]. In the course of our recent developments of various binary NC Al alloys with supersaturated solid solution using sputtering technique, moderate addition of Co, Ni, or Fe has manifested exceedingly high effectiveness in shrinking the grain size of Al down to a few dozens of or a few nanometers, in striking contrast to Ag, Cr, Mg, Mo, Ti, W, and Zr [34][35][36][37][38][39][40]. Notwithstanding these NC Al-TM alloys endowed with strengths greater than 1.5 GPa, first principles calculations in combination with the NC stability criterion suggest that those Al-TM systems are subjected to very large weighted fraction of unstable segregated GBs, reflective of low propensity for good thermal stability [41].…”