Design of novel BCC compositionally complex alloys based on low-activation elements

Abstract

The structural materials used in Generation IV nuclear reactors necessitate robust mechanical properties in high-temperature environments to ensure the reactors operate safely. Compared to conventional alloys, compositionally complex alloys (CCAs) demonstrate suppressed defect cluster evolution and reduced energy dissipation under radiation. Due to the introduction of high-activation elements in face-centered cubic (FCC) CCAs, body-centered cubic (BCC) counterparts with only low-activation elements are regarded as promising candidates. However, the insufficiency of plasticity impedes their potential application. The main objective of this thesis is to design novel low-activation BCC CCAs and validate their tensile properties. In the first part, a crucial feature space was developed and validated to rapidly discover ductile BCC CCAs. A classification and regression tree (CART) algorithm was applied to the comprehensive dataset to distinguish the “Class 1” samples with a compressive fracture strain larger than 50%. The applicability of the CART classifier was authenticated by training and testing F1 scores and accuracies. Consequently, Pugh’s ratio (κ) and valence electron concentration (VEC) are confirmed as two crucial attributes for identifying the objective CCAs. According to the proposed crucial κ-VEC feature space, the “Class 1” alloys are mostly located in the area where κ is larger than 3.129 or VEC is larger than 6.296. Then, for the first time, a novel low-activation VCrFeMn0.33 BCC CCA was developed and investigated, showing satisfactory tensile ductility and typical ductile fracture behavior at temperatures above 650 °C but brittle fracture below 500 °C, indicating the ductile-to-brittle transition temperature (DBTT) of current CCA falls between 500 °C and 650 °C. Same as the conventional ductile alloys, the CCA demonstrated a dislocation-dominated plasticity while straining above DBTT, a behavior seldom observed in non-refractory BCC CCAs. Additionally, the influences of isothermal treatment durations and heat treatment conditions on the tensile behaviors of the VCrFeMn0.33 CCA were assessed respectively. As the 650 °C isothermal treatment duration is prolonged, the strength decreases marginally, whereas the ductility declines noticeably and transitions to brittle fracture. The deterioration in ductility might be ascribed to phase transformations. The tensile performance of the as-rolled samples outperforms the as-recrystallized counterparts subjected to deformation at intermediate temperatures, indicating their superior crack propagation resistance. Finally, we first discovered a new compositionally complex oxide (CCO), Mn(V,Ti,Al)2O4, with a spinel structure that is embedded in a new BCC CCA matrix. Leveraging neutron diffraction and HRTRM, the crystallography and orientation relationship of CCO and CCA are thoroughly elucidated. The results of nano-indentation experiments demonstrate that the localized strength of CCA can be efficiently elevated from ~9.20 GPa to ~11.45 GPa by the introduction of CCO. This work illustrates that combining new CCO with new CCA could be a new strategy for developing new oxide-metal composites. Additionally, the potential radiation resistance properties of the low-activation BCC CCA are expected to be promoted through the involvement of this nanometer-sized CCO.