Twin boundaries: Strong or weak?

Fatigue cracking along twin boundaries (TBs) has significantly different features in Cu and Cu–Al alloys. In Cu and Cu–5 at.% Al alloy, the TBs are intrinsically strong and resist fatigue cracking. With the decrease in stacking fault energy (SFE), some fatigue cracks were observed along or near the TBs in Cu–16 at.% Al alloys. The current findings confirm that the TBs are not always strong enough to prevent fatigue cracking, which is also affected by SFE or slip mode.