Modeling and Simulation of Solute Effects on Deformation Mechanisms in FCC Alloys

Abstract

Face-centered cubic (FCC) equiatomic multi-principal element alloys (MPEAs), such as CrCoNiFeMn, CrCoNi and VCoNi, have generated significant interest in recent decades. These alloys are highly regarded for their remarkable mechanical properties, such as high tensile strength, excellent fracture toughness, and robust resistance to hydrogen embrittlement (HE). To explain the exceptional mechanical performance of FCC MPEAs, researchers have put forward three main mechanisms, namely twinning, phase transformation (fcc→hcp), and lattice distortion. These mechanisms provide insights into the underlying factors contributing to the remarkable characteristics exhibited by FCC MPEAs. The stacking fault energy (SFE) has been recognized as a significant indicator for the deformation mechanism. When SFE exceeds 40 mJ/m², dislocation glide is typically expected as the dominant deformation mechanism. In the range of 20 mJ/m² < SFE < 40 mJ/m² , twinning becomes more likely to occur. When the SFE is below 20 mJ/m², phase transformation is anticipated to play a significant role in the deformation process. CrCoNi, VCoNi and CrCoNiFeMn have been reported to exhibit similar SFE. However, there is a distinct difference in terms of their deformation mechanisms. While the twinning mechanism is observed in CrCoNi and CrCoNiFeMn, it does not operate in VCoNi. In addition, CrCoNi, VCoNi and CrCoNiFeMn exhibit notable resistance to HE, which has been attributed to the phenomenon known as hydrogen-enhanced deformation twinning (HEDT). It is important to note that the HEDT does not occur in Ni and stainless steels. The effect of H on deformation mechanism in different materials remains unclear and requires further investigation. In this thesis, a comprehensive study is conducted using first-principles calculations based on density functional theory (DFT) to elucidate the distinctions among different MPEAs including CrCoNi, VCoNi and CrCoNiFeMn. DFT-based Monte Carlo simulation is employed to investigate the SRO in different MPEAs. Results show that the degree of SRO significantly influences both the SFE and twinnability of different MPEAs. SRO always raises the mean intrinsic SFE relative to that in base special quasi-random structure (SQS) configurations. In SRO-saturated states, CrCoNi and CrCoNiFeMn retain high twinnability comparable to Ag, while VCoNi has narrow dislocation core separations similar to Al and twinnability lower than all elemental FCC metals. Additionally, this work explores the effects of SRO on the mean SFE of binary alloy Fe-22Mn steel and traditional stainless steels, such as 316L and 304. The results demonstrate that the SRO in Fe-22Mn is primarily magnetic driven, while the SROs observed in 316L and CrCoNi are chemical driven. Furthermore, the investigation delves into the influence of H on the deformation mechanism in three MPEAs. This is accomplished by studying the generalized-stacking fault energy (GSFE) and the twinnability. The results reveal that the presence of H promotes twinning in the three MPEAs when H occupies tetrahedral sites (TETs). However, twinning is inhibited when H occupies octahedral sites (OCTs). The diffusion coefficients of H (D_H) in CrCoNi and CrCoNiFeMn at 300 K are 10^-2 and 10^-4 times of D_H in Ni, respectively. The sluggish hydrogen diffusion in MPEAs can result in an accumulation of H at the surface only and increase the probability of H occupying TETs. In contrast, the rapid diffusion of H in Ni and SS316L facilitates the attainment of an equilibrium distribution of hydrogen within the experimental time period. The slow kinetics of H in MPEAs at 300 K results in the presence of H at TETs and promotes the HEDT. On the other hand, the fast kinetics of H in Ni and SS316L results in the residence of H at OCTs and inhibits the HEDT. Therefore, HEDT is likely governed by the slow diffusion and local accumulation of H while the apparent resistance of HE is not directly related to HEDT in some MPEAs.

Date of Award	30 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhaoxuan WU (Supervisor) & David Joseph SROLOVITZ (Supervisor)