Effect of Al content on the structural stability of Σ3 (111) twin boundary in Ti1-xAlxN hard coatings: A first-principles study

Twin boundary (TB) is a promoting factor to enhance the mechanical properties of metals, but less work has been performed in transition-metal nitrides. In this work, we have conducted a comprehensive first-principles investigation of the effect of Al on the structural stability of Σ3 (111) TB in Ti1-xAlxN hard coatings. We find that the Ti1-xAlxN (x ≤ 0.25) Σ3 (111) TB structures with Duffy-Tasker type are unstable, which is attributed to the joint effect of high energies of surface and TB. Moreover, high Al content (x ≥ 0.25) can stabilize the Σ3 (111) TB structures in Ti1-xAlxN due to the energy difference decreasing with increasing Al content. And there is a structural transformation of Σ3 (111) TB from Duffy-Tasker type to Kingery type in Ti1-xAlxN (x ≥ 0.25), which is related to the distribution of Al in the TB. The detailed structural transformation behavior including electronic structures is discussed using Ti0.5Al0.5N Σ3 (111) TB as an example. It is found that the atoms on both sides of the TB move synchronously in opposite directions along the TB in Ti0.5Al0.5N, significantly different from that reported in CrN, where the motion of Cr and N atoms is asynchronous.