Conference Agenda

This study presents Strain Gradient Crystal Plasticity (SGCP) model within a thermodynamically consistent framework, formulated in terms of the MicroSlip, which is based on the gradient of cumulative shear strain for the irradiated crystals. The governing balance equations and the length scale are derived using the principle of virtual power and the Clausius-Duhem inequality. To solve both the classical and generalized balance laws, the Fast Fourier Transform (FFT) homogenization method is employed as an efficient computational approach. These balance laws are explicitly coupled within the FFT-based algorithm, ensuring an accurate numerical implementation.

Polycrystalline simulations are performed using the MicroSlip SGCP model, considering different length scales and higher order interface conditions at grain boundaries. The results indicate that the SGCP model exhibit additional strain hardening compared to the conventional Crystal Plasticity (CCP) framework, which is attributed to the dislocation pile-up mechanism within the microstructure. Moreover, increasing the length scale enhances the hardening response and leads to a broader distribution of shear bands. The influence of different grain boundary conditions, MicroFree, MicroContinuity, and MicroHard, is also investigated. It is observed that under the MicroFree and MicroContinuity conditions, shear bands propagate across grain boundaries, whereas in the MicroHard condition, they terminate at the boundaries. This phenomenon may be associated with grain boundary embrittlement, potentially caused by oxidation or hydrogen dissolution in the internals of nuclear power plant

Laser powder bed fusion (L-PBF) is an additive manufacturing technique that allows the creation of three-dimensional objects by selectively melting layers of metallic powder. By using a high energy density laser, it’s possible to achieve complex geometries, even with refractory metals like tungsten. However, L-PBF of tungsten presents significant challenges due to its inherent properties, such as a high melting point, high melt viscosity and surface tension, excellent thermal conductivity and a high ductile-to-brittle transition temperature. As a result, the L-PBF process for tungsten often leads to parts that are cracked or porous (lack-of-fusion porosity).

A combined approach of with an active oxygen getter alloying strategy was suggested as a pathway to completely eliminate cracks in L-PBF W parts [1]. Baseplate preheating reduces the thermal gradient during processing, thus minimizing the residual stresses accumulated in the material. Addition of carbon and in-situ formation of W-W₂C composite material has shown potential as an armour material of the divertor [2]. Additionally, it was successfully applied to supress crack formation during L-PBF of Mo [3].

In this work we investigate the effect of baseplate preheating to temperatures above the DBTT (up to 1000 °C) during L-PBF of W with the addition of 4 at.% of C. During solidification carbon reacts with W to form small carbide precipitates located at the grain boundaries and within the grains. As a result of constitutional undercooling significant grain refinement of the microstructure is observed with the transition from planar (typical for pure W) to cellular solidification. Baseplate preheating to 600 °C and 1000 °C resulted in increase in densification with nearly full density obtained at 1000 °C. Although microcracks were not completely eliminated, the crack length and crack density was significantly reduced.

Acknowledgements: This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Financial support from the FWO-ARIS Weave project ‘Functionally Graded Materials with Interpenetrating Phases made of Immiscible Alloys’ under Grant No G093822N (FWO) and N2–0324 (ARIS) is acknowledged.

[1] B. Vranken et al. Additive Manufacturing 46 102158 (2021)

[2] S. Novak, et al. Mater. Sci. and Engin.: A, 772, 138666 (2020),

[3] L. Kaserer, et al. Int. J. Refract. Met. Hard Mater. 84 105000 (2019)

Reduced activation ferritic/martensitic (RAFM) steels are the main candidates for the construction of structural components in future fusion and Gen IV nuclear reactors. To ensure safe and stable reactor employment, RAFM-based materials require efficient methods for their characterization under harsh operational conditions. Constantly enduring neutron irradiation, their mechanical properties degrade and may cause a failure of the component. However, neutron irradiation for research purposes is an expensive and long process, so it becomes a major limiting factor to steadily investigate its effect and deliver new research data. Hence, a safer, more paced, and cheaper solution of ion irradiation as a tool for surrogating the neutron damage is becoming more and more popular. As ions are characterized by their penetrating ability, the introduced damage is non-uniformly distributed and densely accumulated on the sub-surface. To correctly estimate their impact, nanoindentation technique is widely applied. The presented study demonstrates a semiempirical approach to effectively interconnect the ion and neutron radiation-induced hardening in RAFM steels, introduced in a range of irradiation conditions. The applied set of tools, based on nanoindentation and tensile tests, as well as their simulations using crystal plasticity finite element method, allows us to extract the irradiation effect on the material law, and accurately reproduce the experimental data. Ultimately, the analysis performed on an ion-irradiated specimen can provide the macroscale (neutron irradiated) yield stress values in a range of dpa doses, which accurately correlate with the literature. The complementary investigations of the microstructural evolution done by focused ion milling and transmission electron microscopy are compared with their computational analogue to confirm the predictive capability of the method. Globally, the presented research is aimed at the establishment and validation of the computationally experimental procedure to precisely deduce the temperature-depended irradiation hardening in metallic materials for nuclear applications.