Identification of Laves‐ and Z‐phase formed in 9%Cr ferritic alloy after long‐term thermal treatment
Authors: Klimenkov Michael , Jäntsch Ute , Hoffmann Jan , Rieth Michael 
Published: December 2016

High‐performance reduced‐activation ferritic–martensitic steels (EUROFER 97) with 9%Cr (WMnVTa) content have been envisaged for application as a structural material for operations at up to 650°C in future fusion reactors as well as in Generation IV fission reactors [1]. The thermal stability of steel's microstructure is a basis for its application in structural components exposed for long time at a very hostile environment, which includes high neutron fluxes at high temperatures, as the one expected during operation of future fusion reactors. The desired mechanical properties of the EUROFER 97 were achieved by the formation of M
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C
6
, TaC, and VN precipitates on the grain and lath boundaries during alloy fabrication and thermal treatment [1]. The long term annealing for up to 100.000h leads to the changes in this desired precipitates distribution and formation of coarse Z‐ and Laves phases which might influence the mechanical properties of the steel. Transmission Electron Microscopy (TEM) Energy Dispersive X‐ray analysis (EDX) and Electron Energy Loss Spectroscopy has been used for identification and to study the morphology, structure, and chemical composition of new phases formed during long‐term thermal treatment.

Fig. 1 shows TEM investigation of precipitate's distribution in the specimen annealed at 550°C for 76.000h. The different phases are imaged with different colours in 2 dimensional elemental map (b). The investigation reveals the presence of tungsten rich precipitates additionally to the typical for EUROFER 97 three phases: M
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, TaC, and VN phases. These new W‐rich precipitates have been identified as Laves phase with (FeCr)W
2
composition. The corresponding EDX spectrum shown in Fig. 3a demonstrates typical Laves phase composition.

Fig. 2 shows a distribution of main compositional elements in a specimen annealed at 600°C for 35000 h, which was obtained using EDX elemental mapping. The investigated area is marked by a square in the HAADF image (Fig. 2). The new precipitates identified by this temperature are V‐rich Z‐phase. The smaller precipitates of M
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C
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imaged with red colour have a size that varies from 50 nm to 120 nm. The faceted Z‐phase have been found to be up to 700 nm in size. Ta and V are present in small precipitates of MX type (VN and TaC [1]) as well as in the Z‐phase. Based on these analytical results, it was found that the Z‐phase can be differentiated well from other phases, which also may contain Cr, V, and Ta or a combined composition of these elements [2]. M
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C
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and VN precipitates in ferritic steels can have a contrast similar to that of the matrix in HAADF and bright field images, they can only be studied using 2‐dimensional analytical imaging.

The results demonstrate that precipitation mechanism of secondary phases changes in the narrow temperature range between 550°C and 600°C. Formation Z‐phase precipitates with Cr2TaVN2 composition was detected only at 600° after annealing for as longer as 30.000h.