Effect of Ti on surface energy, unstable stacking energy, Peierls-Nabarro potential energy and fracture behavior of Nb

In this work, the cell structure models of Nb alloy with different Ti contents (0, 11.1 at.% Ti and 25 at.% Ti (referred as to Nb-0Ti, Nb-11.1Ti and Nb-25Ti alloys, hereafter)) were established by the first principle, and the surface energy ( γ s ), unstable stacking energy ( γ us ), and Peierls-Nabarro potential energy (U P-N ) of dislocation on different slip systems were calculated. The deformation and toughening behaviors of Nb dependent on the Ti contents were investigated and discussed. The results revealed that the U P-N values of the edge and screw dislocations on the low Miller indices {1 1 0} planes were lower than those on the high Miller indices {1 1 2} and {1 2 3} planes. With an increasing Ti content from 0 to 25 at.%, the U P-N values on the corresponding slip systems ({1 1 0}, {1 1 2} and {1 2 3}) significantly decreased, which were resulted from decreasing of the anisotropic shear modulus c and the bond strength of Nb-Ti, while the fracture toughness (K Q ) of the bulk Nb-Ti alloys increased from 21.1 ± 1.2 (MPa·m 1/2 ) to 45.7 ± 2.7 (MPa·m 1/2 ). It was confirmed that with the Ti addition the decreasing U P-N value leads to slipping of more <1 1 1> {1 1 2}dislocations in addition to the <1 1 1> {1 1 0} dislocations, subsequently, increases the dislocation density and the proportion of the screw dislocation, and finally improves the K Q values of the Nb-Ti alloys with a dimple dominated fracture mode. • For Nb, the edge and screw dislocations on low Miller indices {1 1 0} plane show lower P-N potential energy (U P-N ) than those on high Miller indices {1 1 2} and {1 2 3} planes. • For a fixed slip system, the U P-N of edge dislocation is lower than those of screw dislocation. • Ti addition can enhance ductility and toughness of Nb by significantly reducing the U P-N . • A decreasing U P-N values activates mobility of dislocations on <1 1 1 > {1 1 0} and < 1 1 1 > {1 1 2} systems and increases dislocation density.