Periodic Reporting for period 4 - microKIc (Microscopic Origins of Fracture Toughness)
Reporting period: 2021-09-01 to 2023-04-30
The resistance to crack propagation is undoubtedly one of the most important properties of structural materials. However, our current mechanistic understanding of the fracture processes in typical semi-brittle materials like steels, refractory metals, or semiconductors is not sufficiently advanced to predict the fracture toughness KIc and its dependence on the microstructure, temperature, or strain rate. Therefore, KIc is commonly regarded as a phenomenological material parameter for fracture mechanics models that require experimental calibration.
The aim of microKIc is to study fracture in model materials in order to gain a detailed understanding of the microscopic crack-tip processes during fracture initiation, propagation, and arrest, and to systematically study the interactions of cracks with the microstructure.
Using large-scale, massively parallel 3D atomistic simulations we could unequivocally show that the most important crack-tip plasticity mechanisms are all intrinsically 3D and can therefore not be captured in most currently used, quasi-2D models. We could furthermore identify several so far unknown, fracture-toughness-increasing mechanisms.
These mechanisms were included in a new mesoscale simulation tool that combines discrete dislocation dynamics with XFEM for modeling cracks. This open-source tool CraDis allows for the first time both realistic as well as parameter studies of crack propagation in the presence of plastic flow, including direct dislocation-crack tip interactions.
In conjunction with our theoretical approaches, we performed micromechanical fracture tests at different temperatures and strain rates on W, Cr and NiAl with well-defined microstructures, including precipitates, irradiation-induced voids, grain boundaries (GBs), and different dislocation populations. The outcomes of these experiments can be understood in the framework of the above mechanisms and collective effects that can be modeled with our new tool.
Information about atomic-scale dislocation and crack mechanisms, mesoscale plasticity, and experimental results are now being included in a continuum model as well as disseminated to the scientific community, e.g. through the well-attended, international “Fundamentals of Fracture Symposium” that we organized in April 2023.
The fundamental insights gained in this project and its wealth of data will contribute to better predictions of material failure, e.g. under irradiation in nuclear and fusion power plants, as well as advance the development of new fracture-resistant materials and design guidelines for safety-relevant structures and components.
The aim of microKIc is to study fracture in model materials in order to gain a detailed understanding of the microscopic crack-tip processes during fracture initiation, propagation, and arrest, and to systematically study the interactions of cracks with the microstructure.
Using large-scale, massively parallel 3D atomistic simulations we could unequivocally show that the most important crack-tip plasticity mechanisms are all intrinsically 3D and can therefore not be captured in most currently used, quasi-2D models. We could furthermore identify several so far unknown, fracture-toughness-increasing mechanisms.
These mechanisms were included in a new mesoscale simulation tool that combines discrete dislocation dynamics with XFEM for modeling cracks. This open-source tool CraDis allows for the first time both realistic as well as parameter studies of crack propagation in the presence of plastic flow, including direct dislocation-crack tip interactions.
In conjunction with our theoretical approaches, we performed micromechanical fracture tests at different temperatures and strain rates on W, Cr and NiAl with well-defined microstructures, including precipitates, irradiation-induced voids, grain boundaries (GBs), and different dislocation populations. The outcomes of these experiments can be understood in the framework of the above mechanisms and collective effects that can be modeled with our new tool.
Information about atomic-scale dislocation and crack mechanisms, mesoscale plasticity, and experimental results are now being included in a continuum model as well as disseminated to the scientific community, e.g. through the well-attended, international “Fundamentals of Fracture Symposium” that we organized in April 2023.
The fundamental insights gained in this project and its wealth of data will contribute to better predictions of material failure, e.g. under irradiation in nuclear and fusion power plants, as well as advance the development of new fracture-resistant materials and design guidelines for safety-relevant structures and components.
Different types of interatomic potentials for bcc metals were compared to DFT calculations (see Fig.1) which showed that none of the initially available potentials was able to correctly reproduce crack tip processes. This led to the development of a new MEAM potential and the optimization of a machine-learned ACE potential. Furthermore, the potentials’ ability to simulate the dislocation motion was assessed.
Based on these studies, The crack initiation toughness was determined for W and in stoichiometric and off-stoichiometric B2 NiAl (Fig.2) for different fracture systems, crack tip configurations and radii.
A large part of the project was devoted to large-scale, fully 3D atomistic studies of crack-microstructure interactions. In particular, the interaction of pre-existing dislocations (Fig. 4) with cracks was studied in detail, which proved common, textbook-level conceptions wrong. Further simulations were performed to study crack-GB interactions, and GB fracture (Fig.5) as well as crack-void interactions (Fig.3). Also here, new mechanisms of crack-defect interactions were identified.
The atomistic simulations were complemented with tailored micromechanical fracture tests. These included randomly oriented crack systems, cracks with different crack tip qualities, cracks at or near GBs, cracks interacting with precipitates (Fig.8) and fracture of irradiated, W as well as single crystalline, pre-deformed W to study the effect of populating dedicated slip systems. To elucidate the role of thermal activation, fracture experiments were performed at different temperatures and with different strain rates. These experiments were supplemented with finite element simulations (Fig.7).
The CNRS group in Grenoble managed to combine their 3D discrete dislocation dynamics code with an XFEM implementation of fracture and included many of the atomistically observed crack-dislocation interaction mechanisms. This allows now for the first time to perform detailed parameter studies on a realistic model of crack tip plasticity at the micron scale, including crack tip shielding and blunting effects, crack propagation, and changes in crack configuration and dislocation nucleation at the crack tip (Fig.6). The CraDis code is published as open-source.
Taken together, the experimental and simulation results show clearly, that the usual tenet, that increasing the yield stress through the introduction of microstructural obstacles to dislocation motion leads to an increased brittleness does not generally hold, even on the crystallite level. In particular, at low temperatures, dislocation source terms can become important, and fracture toughness is determined by a complex, 3D competition between dislocation nucleation, glide and multiplication, and crack advance, reorientation, deflection and blunting, which can all be influenced by microstructure design.
The results of the project were disseminated in journal articles and conference presentations. We organized a well-attended international symposium on the fundamentals of fracture at Europe’s largest Physics conference the DPG Spring Meeting at the beginning and end of the project (2018 and 2023). A media campaign furthermore resulted in a TV report and a whole-page feature in the regional newspaper.
Our insights in 3D fracture processes will be exploited in a continuum model for fracture in semi-brittle materials.
Based on these studies, The crack initiation toughness was determined for W and in stoichiometric and off-stoichiometric B2 NiAl (Fig.2) for different fracture systems, crack tip configurations and radii.
A large part of the project was devoted to large-scale, fully 3D atomistic studies of crack-microstructure interactions. In particular, the interaction of pre-existing dislocations (Fig. 4) with cracks was studied in detail, which proved common, textbook-level conceptions wrong. Further simulations were performed to study crack-GB interactions, and GB fracture (Fig.5) as well as crack-void interactions (Fig.3). Also here, new mechanisms of crack-defect interactions were identified.
The atomistic simulations were complemented with tailored micromechanical fracture tests. These included randomly oriented crack systems, cracks with different crack tip qualities, cracks at or near GBs, cracks interacting with precipitates (Fig.8) and fracture of irradiated, W as well as single crystalline, pre-deformed W to study the effect of populating dedicated slip systems. To elucidate the role of thermal activation, fracture experiments were performed at different temperatures and with different strain rates. These experiments were supplemented with finite element simulations (Fig.7).
The CNRS group in Grenoble managed to combine their 3D discrete dislocation dynamics code with an XFEM implementation of fracture and included many of the atomistically observed crack-dislocation interaction mechanisms. This allows now for the first time to perform detailed parameter studies on a realistic model of crack tip plasticity at the micron scale, including crack tip shielding and blunting effects, crack propagation, and changes in crack configuration and dislocation nucleation at the crack tip (Fig.6). The CraDis code is published as open-source.
Taken together, the experimental and simulation results show clearly, that the usual tenet, that increasing the yield stress through the introduction of microstructural obstacles to dislocation motion leads to an increased brittleness does not generally hold, even on the crystallite level. In particular, at low temperatures, dislocation source terms can become important, and fracture toughness is determined by a complex, 3D competition between dislocation nucleation, glide and multiplication, and crack advance, reorientation, deflection and blunting, which can all be influenced by microstructure design.
The results of the project were disseminated in journal articles and conference presentations. We organized a well-attended international symposium on the fundamentals of fracture at Europe’s largest Physics conference the DPG Spring Meeting at the beginning and end of the project (2018 and 2023). A media campaign furthermore resulted in a TV report and a whole-page feature in the regional newspaper.
Our insights in 3D fracture processes will be exploited in a continuum model for fracture in semi-brittle materials.
Several new methods were developed within the project, most notably the combination of discrete dislocation dynamics simulations with XFEM-based crack dynamics simulations (CraDis). But also on the experimental side, a new method for microscale cyclic crack growth characterization was developed to determine the effect of individual microstructural features on crack propagation. Several experiments were carried out for the first time, like fracture tests on irradiated tungsten or on single crystals with defined pre-deformations. On the atomistic simulation side, new, DFT-based potentials were developed that realistically describe fracture processes. Most importantly, several new mechanisms of energy dissipation caused by crack-microstructure interactions were identified.
Combining the method development and the simulation and experimental results has drastically improved our fundamental understanding of fracture in bcc-based semi-brittle materials. Our new, atomistically-informed open-source tool for mesoscale fracture simulations allows to model the influence of microstructure, temperature, and strain rate on fracture toughness. This enables now the targeted design of microstructures to locally tailor the fracture behavior. All results will be also included in the development of continuum-based methods for better prediction of fracture in safety-relevant structures.
Combining the method development and the simulation and experimental results has drastically improved our fundamental understanding of fracture in bcc-based semi-brittle materials. Our new, atomistically-informed open-source tool for mesoscale fracture simulations allows to model the influence of microstructure, temperature, and strain rate on fracture toughness. This enables now the targeted design of microstructures to locally tailor the fracture behavior. All results will be also included in the development of continuum-based methods for better prediction of fracture in safety-relevant structures.