7 research outputs found
Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04
A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V–Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 h the alloy transformed into three different distinct phases - BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au+ ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence X-ray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent “High Entropy Alloy” types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs
Development of Novel Low Activation Refractory High Entropy Alloys for Nuclear Fusion Applications
Nuclear fusion power has the potential to meet the growing global energy demand. However, for commercial fusion energy to be realised, serious reactor design concerns need to be addressed. Suitable materials for the first wall of the reactor need to be able to withstand harsh conditions including: extreme heat loads (0.25-0.5 MWm-2), significant transient heat fluxes between 10-20 MWm-2, and high energy neutron bombardments (14 MeV). A novel class of materials, high entropy alloys, have recently been identified to have interesting properties including high temperature capabilities and the self-healing effect upon irradiation.
Preliminary findings, reported in this thesis, demonstrated the excellent radiation damage resistance of the refractory high entropy alloy V2.5Cr1.2MoWCo0.04 in the as-cast condition. This metastable BCC phase showed no microstructural difference upon 5 MeV Au 2+ ion implantation at room temperature. Despite this, after a prolonged heat treatment at intermediate temperatures the phase transformation to a mixture of phases including a BCC, tetragonal, and an orthorhombic phase was observed.
Computational alloy design tools were implemented for the development of novel low activation refractory high entropy alloys for consideration as plasma-facing materials. High entropy alloy empirical parameters for the prediction of a BCC solid solution, and Thermo-Calc simulations for the reduction of intermetallic phases were used for the design of five novel refractory low activation high entropy alloys: V35Cr33Mn2Fe12W18, V46Cr3Mn1Ta10W40, V21Cr18Mn9Fe17Ta20W15, V36Cr18Fe8Ta17W21, and V26Cr17Fe32Ta25. The thermal stability of these alloys was assessed, and the elemental segregation of the alloys could not be reduced due to insufficient homogenisation time. Additionally, only VCrMnTaW and VCrMnFeW displayed BCC phases which were retained upon thermal treatment. From these results, the deficiencies of the design tools were highlighted. Vickers microhardness studies also showed a strong correlation between atomic size difference and as-cast hardness values.
The most promising alloy, V35Cr33Mn2Fe12W18 displayed strong microstructural similarities to V2.5Cr1.2MoWCo0.04, so potential for radiation resistance in the as-cast condition may result. Further high temperature X-ray diffraction experiments on V35Cr33Mn2Fe12W18 showed the formation of a σ phase at 750 °C which was irreversible upon cooling to room temperature.
In this work, the investigation of novel low activation refractory alloys and their potential as plasma-facing materials is assessed and insight is given into the design process of high entropy alloy systems with BCC solid solutions
Complex Concentrated Alloys (CCAs)
This book is a collection of several unique articles on the current state of research on complex concentrated alloys, as well as their compelling future opportunities in wide ranging applications. Complex concentrated alloys consist of multiple principal elements and represent a new paradigm in structural alloy design. They show a range of exceptional properties that are unachievable in conventional alloys, including high strength–ductility combination, resistance to oxidation, corrosion/wear resistance, and excellent high-temperature properties. The research articles, reviews, and perspectives are intended to provide a wholistic view of this multidisciplinary subject of interest to scientists and engineers
In situ TEM investigation of deformation and fracture mechanisms of ceramics and alloys
The mechanical properties of any materials are highly dependent on defects and defect interactions. To improve the mechanical performance and design better materials, it is critical to understand the way defects influence the mechanical properties fundamentally. Transmission electron microscope (TEM) is a powerful tool for the characterization of defects, and thus there is a long history of studying defects using TEM. In situ TEM straining stages were first developed in 1950s, for example, to enable direct observations of dislocations and their interactions with other defects such as twins, grain boundaries (GBs), and materials interfaces. With the recent development of load sensors, in situ TEM mechanical testing combines the power of TEM imaging and diffraction with quantitative load and displacement measurements to provide quantitative understanding of the deformation and fracture mechanisms in various materials. Here, we used in situ TEM to study the deformation and fracture mechanisms of ceramics and alloys.
Firstly, we demonstrated a novel method to evaluate the conditional fracture toughness of thin films and to correlate with in-situ study of fracture mechanisms. Nanocrystalline TiN thin films were investigated using this method. In-situ TEM bright field imaging reveals three crack propagation pathways, namely bridging, intergranular fracture and a mixed mode of transgranular and intergranular fracture. Our methodology is universal and can be applied to other ceramic material systems to evaluate the fracture toughness and study the deformation and failure mechanisms. To further understand the deformation mechanisms of nanocrystalline ceramics, we conducted in situ TEM compression testing on nanocrystalline TiN nanopillars. Grain rotation is detected during the deformation of nanocrystalline ceramics, which effectively alleviates the lattice strain.
Next, we studied the deformation mechanisms of a new type of alloy, high-entropy alloys (HEAs), with the help of in situ TEM and focused ion beam (FIB) fabrication. The deformation mechanism of HEA nanopillar is revealed by simultaneous measurement of mechanical response and dislocation dynamics. By observing dislocation activities leading to dislocation slip on a single slip plane in HEA nanopillars using in-situ TEM, a series of yielding events are revealed, including activation/deactivation of dislocation sources, intermittent propagation of dislocation arrays, collective dislocation jumps, and finally slip avalanches with large stress drops. The experimentally-obtained stress-dependent slip-size distributions and the spatial properties of the slips in the HEA nanopillars agree with the MFT-model predictions. We obtained a scaling collapse of the slip-avalanche size distributions as function of applied stress and dislocation activities that confirm MFT-scaling predictions and indicate that the applied stress is a critical tuning parameter.
Lastly, we studied the soliton-like dislocation waves in HEA nanopillars. The waves propagate initially smoothly with rise and falls in the wave width, followed by intermittent jumps. We show that the waves were formed by the operation of multiple Frank-Read dislocation sources. The propagation of dislocation waves is accompanied by intermittent bursts of dislocation activities over a large area of the nanopillars. Thus, the correlation study of mesoscopic mechanic testing and nm-scale dislocation imaging here provides unprecedented insights into the less observable dislocation processes during the quiescent periods between large avalanches and collective dislocation dynamics
Evaluation of Radiation Response in CoCrFeCuNi High-Entropy Alloys
CoCrFeCuNi high-entropy alloys (HEAs) prepared by arc melting were irradiated with a 100 keV He+ ion beam. Volume swelling and hardening induced by irradiation were evaluated. When the dose reached 5.0 × 1017 ions/cm2, the Cu-rich phases exhibited more severe volume swelling compared with the matrix phases. This result indicated that the Cu-rich phases were favorable sites for the nucleation and gathering of He bubbles. X-ray diffraction indicated that all diffraction peak intensities decreased regularly. This reduction suggested loosening of the irradiated layer, thereby reducing crystallinity, under He+ ion irradiation. The Nix-Gao model was used to fit the measured hardness in order to obtain a hardness value H0 that excludes the indentation size effect. At ion doses of 2.5 × 1017 ions/cm2 and 5.0 × 1017 ions/cm2, the HEAs showed obvious hardening, which could be attributed to the formation of large amounts of irradiation defects. At the ion dose of 1.0 × 1018 ions/cm2, hardening was reduced, owing to the exfoliation of the original irradiation layer, combined with recovery induced by long-term thermal spike. This study is important to explore the potential uses of HEAs under extreme irradiation conditions
Evaluation of Radiation Response in CoCrFeCuNi High-Entropy Alloys
CoCrFeCuNi high-entropy alloys (HEAs) prepared by arc melting were irradiated with a 100 keV He+ ion beam. Volume swelling and hardening induced by irradiation were evaluated. When the dose reached 5.0 × 1017 ions/cm2, the Cu-rich phases exhibited more severe volume swelling compared with the matrix phases. This result indicated that the Cu-rich phases were favorable sites for the nucleation and gathering of He bubbles. X-ray diffraction indicated that all diffraction peak intensities decreased regularly. This reduction suggested loosening of the irradiated layer, thereby reducing crystallinity, under He+ ion irradiation. The Nix-Gao model was used to fit the measured hardness in order to obtain a hardness value H0 that excludes the indentation size effect. At ion doses of 2.5 × 1017 ions/cm2 and 5.0 × 1017 ions/cm2, the HEAs showed obvious hardening, which could be attributed to the formation of large amounts of irradiation defects. At the ion dose of 1.0 × 1018 ions/cm2, hardening was reduced, owing to the exfoliation of the original irradiation layer, combined with recovery induced by long-term thermal spike. This study is important to explore the potential uses of HEAs under extreme irradiation conditions
Evaluation of Radiation Response in CoCrFeCuNi High-Entropy Alloys
CoCrFeCuNi high-entropy alloys (HEAs) prepared by arc melting were irradiated with a 100 keV He+ ion beam. Volume swelling and hardening induced by irradiation were evaluated. When the dose reached 5.0 x 10(17) ions/cm(2), the Cu-rich phases exhibited more severe volume swelling compared with the matrix phases. This result indicated that the Cu-rich phases were favorable sites for the nucleation and gathering of He bubbles. X-ray diffraction indicated that all diffraction peak intensities decreased regularly. This reduction suggested loosening of the irradiated layer, thereby reducing crystallinity, under He+ ion irradiation. The Nix-Gao model was used to fit the measured hardness in order to obtain a hardness value H-0 that excludes the indentation size effect. At ion doses of 2.5 x 10(17) ions/cm(2) and 5.0 x 10(17) ions/cm(2), the HEAs showed obvious hardening, which could be attributed to the formation of large amounts of irradiation defects. At the ion dose of 1.0 x 10(18) ions/cm(2), hardening was reduced, owing to the exfoliation of the original irradiation layer, combined with recovery induced by long-term thermal spike. This study is important to explore the potential uses of HEAs under extreme irradiation conditions