Depth-Sensing Hardness Measurements to Probe Hardening Behaviour and Dynamic Strain Ageing Effects of Iron during Tensile Pre-Deformation

This work reports results from quasi-static nanoindentation measurements of iron, in the un-strained state and subjected to 15% tensile pre-straining at room temperature, 125 °C and 300 °C, in order to extract room temperature hardness and elastic modulus as a function of indentation depth. The material is found to exhibit increased disposition for pile-up formation due to the pre-straining, affecting the evaluation of the mechanical properties of the material. Nanoindentation data obtained with and without pre-straining are compared with bulk tensile properties derived from the tensile pre-straining tests at various temperatures. A significant mismatch between the hardness of the material and the tensile test results is observed and attributed to increased pile-up behaviour of the material after pre-straining, as evidenced by atomic force microscopy. The observations can be quantitatively reconciled by an elastic modulus correction applied to the nanoindentation data, and the remaining discrepancies explained by taking into account that strain hardening behaviour and nano-hardness results are closely affected by dynamic strain ageing caused by carbon interstitial impurities, which is clearly manifested at the intermediate temperature of 125 °C

Effect of statistically stored dislocations in tungsten on the irradiation induced nano-hardening analyzed by different methods

Tungsten self-ion irradiation was performed at 800 °C up to 0.01-1 dpa on two different W grades with essentially different dislocation density. Nanoindentation was applied to characterize the radiation hardening in two W grades with different microstructure. Different methods to analyze the indentation curves were applied to extract the bulk equivalent radiation hardening. It was shown that depending on the applied method, different outcomes may occur. The most satisfactory procedure was established and a consistent set of parameters was found. The bulk equivalent radiation hardening was found to saturate above 0.1 dpa. The characteristic distance between irradiation induced defects acting as dislocation pinning points was found to decrease up to 0.1 dpa, and then saturate/increase with irradiation dose. No essential difference in radiation hardening was observed between the studied W grades with essentially different initial dislocation density

Round Robin into Best Practices for the Determination of Indentation Size Effects

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Development of chromium for mid-flux region PFCs for DEMO divertor

Continuous efforts in the design of plasma-facing components (PFC) for DEMO divertor unravel new challenges to be met by the in-vessel materials. Embrittlement induced by 14 MeV neutrons in tungsten, main candidate for the first wall material, is one of the hottest issues to be addressed in the design of PFCs with a reliable structural integrity. For designing mid heat flux PFCs, chromium (Cr) is currently considered as material for the main body connected to tungsten armour tile and cooling pipe. Cr has the superior toughness property in the low temperature range where the commercial tungsten products are brittle. Cr is an excellent material to reduce the oxidation in case of loss-of-vacuum accident [1], as well as it does not experience such harsh transmutation as W does producing Re and Os yielding to disadvantageous precipitation and embrittlement. The advantages of Cr may help substantiation of water coolant design and usage of ITER experience for licensing of PFC components. However, the mechanical properties of Cr are extremely sensitive to purity. Because of that commercially produced Cr grade (Plansee) in non-irradiated state exhibits rather high ductile to brittle transition temperature (DBTT) ~150-250°C [2]. Here, we employ vacuum arc melting (VAC) equipment for fabrication of Cr and Cr-W alloy suitable for PFCs, which represents new promising alternative route with high upscale potential. VAM fabrication improves Cr quality by avoiding the introduction of interstitial impurities, while the produced grades can be further mechanically treated to design dedicated microstructure and enhance the yield strength. The produced heats of pure Cr and Cr-10W are investigated by means of chemical and microstructural analysis as well as mechanical testing and compared with the Plansee Cr. The VAM-produced pure Cr shows DBTT below room temperature proving the principal advantage of this fabrication route