Mechanism transition of cross slip with stress and temperature in face-centered cubic metals

doi:10.1016/j.jmst.2020.04.035

Abstract

Abstract:

A<110>/2 screw dislocation is commonly dissociated into two <112>/6 Shockley partial dislocations on {111} planes in face-centered cubic metals. As the two partials are not purely screw, different mechanisms of cross-slip could take place, depending on the stacking fault energy, applied stress and temperature. It is crucial to classify the mechanisms of cross-slip because each mechanism possesses its own reaction path with a special activation process. In this work, molecular dynamics simulations have been performed systematically to explore the cross-slip mechanism under different stresses and temperatures in three different metals Ag, Cu and Ni that have different stacking fault energies of 17.8, 44.4 and 126.8 mJ/m², respectively. In Ag and Cu with low stacking fault energy, it is observed that the cross-slip mechanism of screw dislocations changes from the Fleischer obtuse angle (FLOA), to the Friedel-Escaig (FE), and then to the FL acute angle (FLAA) at low temperatures, with increasing the applied stress. However, when the temperature increases, the FE mechanism gradually becomes dominant, while the FLAA only occurs at the high stress region. In particular, the FLOA has not been observed in Ni because of its high stacking fault energy.

Key words: Cross-slip, Molecular dynamics simulation, Face-centered cubic metals, Stacking fault energy

K.Q. Li, Z.J. Zhang, J.X. Yan, J.B. Yang, Z.F. Zhang. Mechanism transition of cross slip with stress and temperature in face-centered cubic metals[J]. J. Mater. Sci. Technol., 2020, 57: 159-171.

Figures/Tables 15

Fig. 1. Classical mechanisms of cross-slip in FCC metals. (a) The FE mode, (b) the FLAA mode, and (c) the FLOA mode.

Fig. 2. (a) The schematic diagram of the simulation box for cross-slip, in which a right-handed screw dislocation splits into two Shockley partial dislocations γA and Bγ on the original slip plane. Under the applied shear strain γxz shown by the arrow, two partials are moving upward. (b) The two-dimensional expansion of Thompson tetrahedron of the Burgers vector on the original slip plane and the cross-slip plane.

Table 1 The values of lattice parameter a in ?, single-crystal elastic constants Cij in GPa, and SFE γ in mJ/m2. τo and τc are scaled by γxz.

Metal	a	C₁₁	C₁₂	C₄₄	γ	τ_o	τ_c
Ag	4.0905	124.2	93.9	46.4	17.8, 16^a, 17.8^b, 14.5^c	-5.4	25.6
Cu	3.6155	170.1	122.8	76.2	44.4, 41^a, 38.5^b, 41.8^c	-9.6	41.2
Ni	3.5200	242.2	151.3	127.8	125.8, 125^d, 144.5^b, 152.1^c	-12.3	72.9

Fig. 3. (a) The γ-surface on (111) in Ag. (b) The similar generalized SFE curves for Ag, Cu and Ni. The local minima corresponds to the SFE γ.

Fig. 4. Formation of two ESFs under the initial shear stress of 1.15 GPa at 30 K in Ag. The left are the snapshots of dislocation from simulation results. (The black arrows denote the glide direction of partial dislocations on two slip planes.) The right is copied from the left simulation results.

Fig. 5. Cross-slip accomplished by a non-classical FLOA mode while τc,0 = 1.28 GPa and T = 30 K in Ag.

Fig. 6. Cross-slip via the combination of the FLOA and the FE mechanisms (termed by FLOA_FE) when τc,0 = 1.34 GPa and T = 30 K in Ag.

Fig. 7. Cross-slip via the FE mechanism when τc,0 = 1.41 GPa and T = 30 K in Ag.

Fig. 8. Two nucleation sites for cross-slip via the FE mechanisms while τc,0 = 1.54 GPa and T = 30 K in Ag.

Fig. 9. Cross-slip via the combination of the FLAA and the FE mechanisms (termed by FE_FLAA) when τc,0 = 1.75 GPa and T = 30 K in Ag.

Fig. 10. The classical FLAA mechanism takes place when τc,0 = 1.82 GPa and T = 30 K in Ag.

Fig. 11. The time dependence of (a) the system temperature T and (b) the shear stress τc with different initial stresses τc,0. The symbols denote the occurrence of cross slip.

Fig. 12. Cross-slip via the classical FLOA mechanism while τc,0 = 1.28 GPa and T = 40 K in Ag.

Fig. 13. Cross-slip via the FE mechanism for τc,0 = 1.28 GPa at 50 K in Ag.

Fig. 14. The variation of cross-slip mechanism with the applied shear strain (γxz) and the temperature (T) in (a) Ag, (b) Cu, and (c) Ni. The dotted lines are used to distinguish the regions of different cross-slip modes.

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