Periodic Reporting for period 1 - MeNaWir (Mechanics of Nanoporous W under irradiation)
Reporting period: 2022-09-01 to 2024-08-31
Fusion reactors demand materials capable of withstanding unprecedented amounts of irradiation, heat flux and temperature. Refractory-based nanoporous metals offer a combination of mechanical and radiation performance that makes them ideal potential candidates for nuclear applications. Introducing a new material for this application requires characterizing its response under the extreme environment of fusion reactors; however, experimental testing facilities allowing to perform tests under these conditions are extremely limited. Modelling and simulation emerge as a valuable tool, allowing replacing many of such tests by simulations, and will be drivers in fusion materials development. The MeNaWir project (Mechanics of Nanoporous W under irradiation) aims at shedding light into elusive aspects of the mechanical behaviour of nanoporous refractory metals, focusing on nanoporous tungsten (np-W) and the effect of radiation damage on its mechanical properties. This will be accomplished by building a multiscale computational framework for the exploration of the mechanical properties of nanoporous metals for nuclear applications. This framework will encompass state-of-the-art modelling techniques from different fields, including Crystal Plasticity - Fast Fourier Transform (CP-FFT) simulations, radiation damage models and plasticity at the nanoscale. To achieve this goal, MeNaWir brings together a researcher with expertise in nanoporous metals and materials modelling at the atomistic scale with a world-recognized supervisor with expertise in modelling the non-linear behaviour and fracture of complex microstructures at the continuum scale and a research institute with a record of excellence, technology transfer, and top-level training in Materials Science. The specific objectives are to develop a framework for the computational assessment of nanoporous materials for the nuclear industry and to provide a window of optimal np-microstructures to guide the fabrication of np-W parts. The successful completion of the project will build the foundations for computational assessment of nanoporous materials for the nuclear industry and could lastingly impact innovation on materials for fusion.
The materials used as protection in fusion reactors demand components with radiation tolerance under intense fluxes. Nanoporous metals (np-metals) are a class of functional materials that exhibit a bi-continuous network of nanoscale pores and solid ligaments, forming an open-cell porous material. Np-metals also offer improved mechanical properties, enabled by nanoscale effects. In particular, refractory np-metals have a remarkable potential for nuclear applications2a,b. These np-metals combine the excellent mechanical and radiation performance of the base material with an improved tolerance to radiation, thanks to the presence of a large amount of internal surfaces, which act as perfect sinks for radiation-induced defects. Tungsten (W), a refractory metal, is an ideal choice of base material for np-metals for components of fusion reactors, combining an exceptional radiation tolerant nanostructure with the dimensional stability, high melting point, good thermal conductivity, low sputtering rate and good mechanical properties at high temperatures.
Materials for the nuclear industry must ensure reliability and performance of the components. In the past, developments in this area often hinged on costly and ineffective trial-and-error experiments. Nowadays, materials exploration and assessment in a cost effective way must incorporate “computational experiments” with a correct integration of the appropriate physics and scales involved. This Integrated Computational Materials Engineering (ICME) paradigm has already been successfully applied for the nuclear industry6a, but considering only classical bulk metals (e.g. W and Fe).
The computational study of the mechanical behaviour of nanoporous W for nuclear applications requires models that capture not only the standard aspects of nanoporous material deformation but also the influence of radiation damage on the mechanical properties. CP simulations are the preferred option to determine a crystal’s non-linear response to external arbitrary loads. Moreover, CP models have a strong physical basis, which allows incorporating the effects of radiation-induced defects. The combination of CP models with spectral solvers, CP-FFT, a technique in which the hosting group has also made fundamental contributions, have proven as accurate as CP-FEM with a much lower computational cost, which allows the use of more complex polycrystals/voxels.
As summary, CP-FFT arises as an ideal framework to handle the simulation of np-W with radiation effects. A voxelization strategy and the use of large RVEs will allow generating, in a simple manner, realistic RVEs and the effects of bulk material polycrystalline microstructure and damage radiation effects can be addressed using CP. We propose a modelling and simulation framework for the general treatment of the mechanical response of nanoporous materials, which will be particularized for np-W. This project, combined with third party experiments, will allow for optimized np-W microstructures and a characterization of their response under extreme conditions that will eventually lead to np-W specifically tailored for nuclear applications in advanced fission and fusion reactors.
• The main scientific and technological achievements include the development of a physically-based method for the computational assessment of nanoporous microstructures. This includes the development of a CP-FFT software for modelling nanoporous metals. This implies the achievement of advances on modelling techniques that are transversal to a variety of disciplines, from nuclear engineering to aerospace engineering. These were foreseen in the signed GA, effectively advancing SoA with respect to specific objective 1.
• From an economic perspective, this project provided a new set of tools for the development and assessment of materials for the nuclear industry that could help on the definition of materials requirements as an output from the scientific community to the material supply chain. Technically, nanoporous metals offer a high strength to weight ratio, high temperature resistance and the potential to withstand extreme environments, potentially impacting in technological developments for the aerospace industry. This exemplifies a broader economic and societal impact.
• This project is expected to contribute to Europe’s long-term goal of low carbon electricity generation by the next generation of fusion reactors.
• The presentation of the project results at international conference raised the interest of three companies (potential users). Formal communications have started with one of them. This includes the sign of a non-disclosure agreement and a series of exploratory meetings.
The materials used as protection in fusion reactors demand components with radiation tolerance under intense fluxes. Nanoporous metals (np-metals) are a class of functional materials that exhibit a bi-continuous network of nanoscale pores and solid ligaments, forming an open-cell porous material. Np-metals also offer improved mechanical properties, enabled by nanoscale effects. In particular, refractory np-metals have a remarkable potential for nuclear applications2a,b. These np-metals combine the excellent mechanical and radiation performance of the base material with an improved tolerance to radiation, thanks to the presence of a large amount of internal surfaces, which act as perfect sinks for radiation-induced defects. Tungsten (W), a refractory metal, is an ideal choice of base material for np-metals for components of fusion reactors, combining an exceptional radiation tolerant nanostructure with the dimensional stability, high melting point, good thermal conductivity, low sputtering rate and good mechanical properties at high temperatures.
Materials for the nuclear industry must ensure reliability and performance of the components. In the past, developments in this area often hinged on costly and ineffective trial-and-error experiments. Nowadays, materials exploration and assessment in a cost effective way must incorporate “computational experiments” with a correct integration of the appropriate physics and scales involved. This Integrated Computational Materials Engineering (ICME) paradigm has already been successfully applied for the nuclear industry6a, but considering only classical bulk metals (e.g. W and Fe).
The computational study of the mechanical behaviour of nanoporous W for nuclear applications requires models that capture not only the standard aspects of nanoporous material deformation but also the influence of radiation damage on the mechanical properties. CP simulations are the preferred option to determine a crystal’s non-linear response to external arbitrary loads. Moreover, CP models have a strong physical basis, which allows incorporating the effects of radiation-induced defects. The combination of CP models with spectral solvers, CP-FFT, a technique in which the hosting group has also made fundamental contributions, have proven as accurate as CP-FEM with a much lower computational cost, which allows the use of more complex polycrystals/voxels.
As summary, CP-FFT arises as an ideal framework to handle the simulation of np-W with radiation effects. A voxelization strategy and the use of large RVEs will allow generating, in a simple manner, realistic RVEs and the effects of bulk material polycrystalline microstructure and damage radiation effects can be addressed using CP. We propose a modelling and simulation framework for the general treatment of the mechanical response of nanoporous materials, which will be particularized for np-W. This project, combined with third party experiments, will allow for optimized np-W microstructures and a characterization of their response under extreme conditions that will eventually lead to np-W specifically tailored for nuclear applications in advanced fission and fusion reactors.
• The main scientific and technological achievements include the development of a physically-based method for the computational assessment of nanoporous microstructures. This includes the development of a CP-FFT software for modelling nanoporous metals. This implies the achievement of advances on modelling techniques that are transversal to a variety of disciplines, from nuclear engineering to aerospace engineering. These were foreseen in the signed GA, effectively advancing SoA with respect to specific objective 1.
• From an economic perspective, this project provided a new set of tools for the development and assessment of materials for the nuclear industry that could help on the definition of materials requirements as an output from the scientific community to the material supply chain. Technically, nanoporous metals offer a high strength to weight ratio, high temperature resistance and the potential to withstand extreme environments, potentially impacting in technological developments for the aerospace industry. This exemplifies a broader economic and societal impact.
• This project is expected to contribute to Europe’s long-term goal of low carbon electricity generation by the next generation of fusion reactors.
• The presentation of the project results at international conference raised the interest of three companies (potential users). Formal communications have started with one of them. This includes the sign of a non-disclosure agreement and a series of exploratory meetings.
With a multiscale perspecpective, encompassing experimental and computational efforts, the main activities and achievements can be summarized as follows:
- We presented a novel approach to simulate the deformation of submicron specimens, accounting for the discrete events produced by the slip of internal dislocation and surface nucleated ones in a stochastic manner. The framework considers the slip events as eigenstrain fields that produce a displacement jump across a slip plane and whose activation is driven by a Monte Carlo (MC) method. Physically-based laws are incorporated to account for activation probabilities, dislocation mobility and surface nucleation. Two factors of stochasticity are taken into account: a random selection of the position of defects and a random selection of plastic events after a MC process on a sampling array of possible displacement rates, computed using a similar concept as in Orowan’s equation. Implementation on a fast FFT solver, results on an efficient, computationally-cheap algorithm which allows to simulate accurately the deformation of a specimen in a wide range of strain rates and sizes in a fraction of the time needed using techniques as MD or DDD.
- By performing an atomistic modeling campaign we managed to unravel a key interplay of mechanisms responsible for the hardening response on nanoporous metals under compression, with focus on nanoporous tantalum. A model material for other np metals, like nanoporous W.
- Experimental activities were also performed, exploring the elastic properties and yield stress of nanoporous W under compression.
- We presented a novel approach to simulate the deformation of submicron specimens, accounting for the discrete events produced by the slip of internal dislocation and surface nucleated ones in a stochastic manner. The framework considers the slip events as eigenstrain fields that produce a displacement jump across a slip plane and whose activation is driven by a Monte Carlo (MC) method. Physically-based laws are incorporated to account for activation probabilities, dislocation mobility and surface nucleation. Two factors of stochasticity are taken into account: a random selection of the position of defects and a random selection of plastic events after a MC process on a sampling array of possible displacement rates, computed using a similar concept as in Orowan’s equation. Implementation on a fast FFT solver, results on an efficient, computationally-cheap algorithm which allows to simulate accurately the deformation of a specimen in a wide range of strain rates and sizes in a fraction of the time needed using techniques as MD or DDD.
- By performing an atomistic modeling campaign we managed to unravel a key interplay of mechanisms responsible for the hardening response on nanoporous metals under compression, with focus on nanoporous tantalum. A model material for other np metals, like nanoporous W.
- Experimental activities were also performed, exploring the elastic properties and yield stress of nanoporous W under compression.
The computational framework presented here opens the possibility to provide valuable analysis and interpretations of other small-scale testing techniques, such as microtensile testing and microbending. Future developments of dislocation mobility laws for FeCrAl alloys and multi-principal element alloys would allow for a systematic investigation of these relevant metals, while the possibility to include other micro and nanoscale geometries further extends the potential applications map to include nanolattices and nanoporous materials, among other nanostructures of interest.
We demonstrated that both linear hardening and exponential hardening in np metals under compression takes place not only by dislocation plasticity but also due to topological changes.
We demonstrated that both linear hardening and exponential hardening in np metals under compression takes place not only by dislocation plasticity but also due to topological changes.