Brittle-ductile transition temperature of recrystallized tungsten following exposure to fusion relevant cyclic high heat load

V Shah,J.A.W Van Dommelen,E Altstadt,A Das,M.G.D Geers

doi:10.1016/j.jnucmat.2020.152416

Abstract

The lifetime of tungsten (W) monoblocks under fusion conditions is ambivalent. In this work, the microstructure dependent mechanical behaviour of pulsed high heat flux (HHF) exposed W monoblock is investigated. Two different microstructural states, i.e. initial (deformed) and recrystallized, both machined from HHF exposed monoblocks are tested using tensile and small punch tests. The initial microstructural state reveals a higher fraction of low angle boundaries along with a preferred orientation of crystals. Following HHF exposure, the recrystallized state exhibits weakening of initial texture along with a higher fraction of high angle boundaries. Irrespective of the testing methodology, both the microstructural states display brittle failure for temperatures lower than 400∘C. For higher temperatures (>400∘C), the recrystallized microstructure exhibits more ductile behaviour as compared to the initial state. The observed microstructural state-dependent mechanical behaviour is further discussed in terms of different microstructural features. The estimated brittle-to-ductile transition temperature (BDTT) range is noticed to be lower for the recrystallized state as compared to the initial state. The lower BDTT in the recrystallized state is attributed to the high purity of the W in combination with its low defect density, thereby preventing segregation of impurities at the recrystallized boundaries and the related premature failure. Based on this observation, it is concluded that the common opinion of the aggravation of BDTT in W due to recrystallization is not unerring, and as a matter of fact, recrystallization in W could be instrumental for preventing the self-castellation of the monoblocks.

Highlights

The refractory bcc metal Tungsten (W) exhibits exceptional high temperature properties such as a high melting temperature, and a high thermal conductivity in combination with a low coefficient of thermal expansion
In fig. 2a, it can be observed that the initial microstructure consists of relatively large grains elongated with respect to the width direction (WD) of the monoblock
The mechanical behaviour of W monoblocks exposed to a few thousand cycles of pulsed high heat flux (HHF) was examined to understand the extent of the mechanical property changes, the brittle-to-ductile transition temperature (BDTT), as a consequence of the dynamic microstructure evolution by recrystallization and grain growth

Summary

Introduction

The refractory bcc metal Tungsten (W) exhibits exceptional high temperature properties such as a high melting temperature, and a high thermal conductivity in combination with a low coefficient of thermal expansion. The magnitude of the temperature gradient in the monoblock depends on the geometry of the monoblock, the distance to the cooling tube (armour thickness ≈ 6-8 mm) as well as the surface heat flux [2,4,9] This will result in a plethora of surface effects (such as erosion, crack nucleation and growth, droplet formation and plastic deformation assisted roughening [10,11,12,13,14,15]) along with bulk effects such as barrelling due to residual stresses [16,17,18], microstructural changes by recrystalliza-

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Conclusion