Growth twins in high stacking fault energy metals: Microstructure, texture and twinning

Abstract

This work explores the formation of twin boundaries in high stacking fault energy metals (SFE > 125 mJ/m 2 ) by synthesizing thick films (> 10 μm) of Al, Al-5.3wt.%Mg, and Ni using magnetron sputtering. We report the observation of twin boundaries that are inclined with respect to the film growth direction across the entire thickness of the films. The formation of these inclined twin boundaries results in localized changes in the texture of the columnar grains.Microstructural analysis revealed that the fraction of twinned grains in the Al film (46%) is four times higher compared to other similar studies in Al, while the Al-5.3wt.%Mg and Ni films can be considered highly twinned since the fraction of twinned grains is 70% and 90%, respectively.The experimental observations provide an explanation on the formation of twin boundaries during the synthesis of the films, and emphasize that, in addition to stacking fault energy restrictions, high grain boundary mobility is a limiting factor on the nucleation of twin boundaries.

Characterization of Grain Boundary‐Engineered Aluminum–Magnesium Alloys

“…To understand the changes in special GBs among the three samples, one can look at a simplified model for Σ3 boundaries only. In Al alloys, a previous study demonstrated that sputtering rates ≥7 nm s −1 led to high fractions of twinned grains (46–70%), which agrees with the observed high fraction of special boundaries for sample C. This increase in twin boundaries was attributed to both the high sputtering rate and the decrease in GB mobility due to stress relaxation during film growth, [ 15 ] present during interrupted sputtering, making twins and special GBs more energetically favorable. [ 23,34 ] In samples A and B, the changes in sputtering rate did not yield a significant change in the GB fraction, which is likely due to the probability ( ρ) of twin boundary nucleation during film growth and its relationship to the deposition flux, where flux is proportional to the sputtering rate.…”

“…[ 14 ] For sputtering in particular, a previous study demonstrated that the fraction of twin boundaries in Ni and Al could be modified through the use of different sputtering rates. [ 15 ] Furthermore, a sputtered 5xxx series Al–Mg alloy demonstrated both higher fractions of low Σ (≤29) GBs and improved resistance to sensitization compared with conventional materials. [ 9 ] In this study, we go several steps further by focusing on tailoring the fraction of special GBs in a complex alloy as well as directly correlating the GB fraction to corrosion morphology.…”

“…[ 22 ] In sample C, the prevalence of Σ7 boundaries is tied to increased boundary migration as the high sputtering rate introduced additional thermal energy into the system, leading to higher temperatures [ 23 ] and greater strain in the sputtered material. [ 15,24 ] Specifically, it is expected that sample C had an average deposition temperature over 125 °C higher than samples A and B. [ 23 ] For Σ3 boundaries, higher sputtering rates simultaneously promote both nucleation and uncoupling from the GBs, [ 23 ] and in this instance, the resultant boundary migration induced increased boundary decomposition and grain growth, leading to higher energy Σ7 boundaries in the microstructure.…”

“…[ 24 ] Similarly, this higher sputtering rate can also account for the significant difference in the observed CSL fractions between the top surface and cross section for sample C: due to mobility as stated earlier, [ 24 ] or through increased stress relaxation from the interrupted sputtering process, which is more energetically favorable for the formation of special boundaries. [ 15 ] As the film grew, the high deposition rate increased the temperature at the free surface where new growth was occurring, [ 23 ] promoting even greater boundary mobility and diffusion [ 24,25 ] and activating more boundary annihilation toward the top of the sample, yielding fewer CSL boundaries. [ 26 ] Furthermore, it is also possible that the Σ3 and Σ7 boundaries possessed similar energies when they appeared together, as has been previously observed by Tschopp et al, [ 27,28 ] as the GB plane orientation can produce large variations in GB energy even for boundaries with the same Σ value.…”

“…Beyond the particular types of boundaries observed, it is also important to address the overall fraction of special boundaries in samples A–C while accounting for both composition and sputtering rate. [ 15,23,31,32 ] For example, the formation of nanotwins and special GBs has been shown to be inversely proportional to the stacking fault energy (SFE), [ 32,33 ] and is expected to be limited in Al‐ and Al–Mg alloys due to their high SFE values (>160 mJ m −2 ). [ 15 ] The samples in this study have minimal compositional variation (between 4.1 and 4.6 wt% Mg) and thus SFE is not a likely contributor to the observed microstructural variations.…”