Investigation of Deformation Mechanisms in Strong and Ductile Alloys

The stacking fault energy is an important factor in determining the types of plastic deformation mechanisms that occur in face-centered cubic materials and consequently influences their mechanical properties. Experimental observations
have shown that as the stacking fault energy decreases, the deformation mechanism transitions from pure dislocation glide to a combination of twinning and dislocation glide, and eventually to a combination of martensite formation and

dislocation glide at even lower stacking fault energy values. For austenitic steels, a stacking fault energy of 45 mJ⋅m^-2 marks the lower limit for pure dislocation glide, while stacking fault energy values in the range of 20-45 mJ⋅m^-2 result in a mixture of dislocation glide and twinning. When the stacking fault energy is even lower, materials tend to form martensite alongside dislocation glide. The presence of deformation twinning and martensite formation in addition to dislocation glide leads to higher work hardening rates compared to dislocation glide alone. Therefore, tailoring the mechanical properties by adjusting the stacking fault energy, such as through controlling the chemical composition, is considered a viable approach to create materials with unprecedented combinations of strength and ductility. 

With the emergence of high-entropy alloys, computational materials engineering methods have become crucial for designing potentially promising compositions within the vast space of alloy compositions. Consequently, the computational determination of stacking fault energy, typically using density functional theory, has garnered significant interest. While experiments and density functional theorybased calculations generally agree on stacking fault energy values for stable face centered cubic materials, there are severe discrepancies for metastable facecentered cubic materials. Density functional theory calculations predict negative stacking fault energy values, whereas experimental values remain positive. 

In this Ph.D. thesis, it was demonstrated that this discrepancy arises because density functional theory treats the stacking fault energy as a variable of state, unaffected by kinetic effects like the mobility of Shockley partial dislocations. In contrast, experimental stacking fault energy values are influenced by the resistance to the movement of Shockley partial dislocations. To reconcile the experimental and theoretical stacking fault energy values, it is proposed to subtract the excess stacking fault energy stemming from the resistance to the movement of Shockley partial dislocations from the experimental values. This correction assumes that the critical resolved shear stress for Shockley partial dislocations approximately corresponds to the critical resolved shear stress for twinning. By considering the correlation with the critical resolved shear stress for twinning, the correction can also explain the grain size dependence observed in experimental stacking fault energy values. Consequently, the corrected experimental values for metastable face-centered cubic materials become negative and align well with the stacking fault energy values obtained from density functional theory calculations. Moreover, this suggested correction has been tested on stable face-centered cubic alloys and pure metals, yielding consistent and quantitatively comparable results between experimental and theoretical stacking fault energy values for the first time.