Thermal stability of the HPT-processed tungsten at 1250 – 1350 °C
Introduction
As one of the most attractive materials, tungsten (W) is used widely for extreme condition in aerospace, lighting and weapon industries [[1], [2], [3]]. In particular, its high melting point, low sputtering yield and low tritium retention enable pure tungsten to withstand the high temperatures, high heat loads and high energy particle irradiation arising at the first wall, which makes it the prime candidate for plasma-facing materials of future fusion reactors [[4], [5], [6]]. However, the high stationary operation temperatures (up to 1200 °C at the divertor) can significantly alter the microstructure of pure tungsten by abnormal grain growth [7,8]. And plasma surface interaction on the microstructure with low-density grain boundaries and grains near (111) surface orientation may also cause surface morphology changes severely [4,[6], [7], [8], [9], [10]]. In order to further improve the mechanical property and irradiation resistance in pure tungsten, the control of microstructures at high temperatures becomes a hot topic. To this end, HPT, as one of the most powerful severe plastic deformation (SPD) process, has become an efficient method to obtain ultrafine-grained microstructures with outstanding properties [[11], [12], [13], [14]].
With the aid of advanced experimental technologies, the structural mechanism responsible for strain-induced grain formation under SPD conditions is considered as continuous dynamic recrystallization (cDRX). And microstructures in subsequent annealing process are affected by cSRX homogeneously and entirely. In order to clarify the effect rules of the cSRX processes, A. Takayama et al. [15] have summarized the cSRX behavior of pure copper processed by multi-directional forging, which could be categorized in three stages: after a kind of incubation period with little change, the grain size first coarsen rapidly and transiently, and then a gradual increase in grain size occurs due to further coarsening. Earlier investigations by Yu.R. Kolobov et al. [16] pointed out that there are a wider grain size distribution and a higher microhardness for HPT-processed molybdenum with the annealing temperature increasing from 1273 K to 1473 K. In the works of Peter Cengeri et al. [17], microstructure evolution and property change caused by combined action of annealing temperature and pre-strain take place on pure copper and pure nickel under the HPT condition. In the more recent work by Z.S. Levin et al. [18], the change of microstructure and Vickers hardness for tungsten under equal channel angular extrusion conditions was also attributed to the effects of the annealing temperature and pre-deformation on the cSRX behavior. The partial recrystallization in this work was observed at the temperature as low as 1100 °C, and complete recrystallization required ~1400 °C. Apart from the changes described above, significant shear modulus drop, misorientation distribution changes, stored energy variation and texture transformation were also found in some cSRX process, demonstrating the significant effect of the distinctive recrystallization [12,15,19]. Additionally, the researchers found that the microstructure and mechanical properties of the material after a long period of cSRX process will be preserved and not reach the level of full-annealed state in some materials [15,19,20]. Therefore, such phenomenon makes the HPT process an appropriate strategy for pure tungsten to improve microstructure and properties at high temperatures.
In spite of the experimental investigations on the cSRX for pure tungsten and molybdenum carried out [[12], [13], [14],16,18,21,22], the effects of the main process parameters, such as large pre-strain and higher annealing temperature, are still unclear. In some researches, due to different components calculated by different test methods, the quantitative characterization on the stored energy of recrystallization has an inevitable error, and the major factor of the cSRX in the microstructure of pure tungsten is not certain. Moreover, the effect of annealing on the crystallographic orientation has not yet been considered, which may result in an under-prediction of radiation resistance. Therefore, in order to clarify these issues and further investigate the effect of the HPT process, an overall study for cSRX in pure tungsten should be conducted and discussed.
In this study, the annealing process of HPT-processed tungsten was mainly studied at 1250–1350 °C. The changes of microstructure under different conditions were characterized and compared. Then, based on the EBSD measurements, a detailed analysis of energy release was quantified. Finally, the effect of cSRX behavior on the hardness loss within a wide temperature range was done with the aid of Vickers hardness measurement.
Section snippets
Experimental details
The experiments were conducted using commercially pure (99.95%) tungsten with the average grain size of 65.5 μm. The as-received samples in the form of dimension of 10 × 2 mm were encapsulated snugly inside 304 L stainless steel cans with the diameter of 16 mm. The quasi-constrained HPT experiments were carried out at 1.5 GPa and 350 °C on a RZU2000HF pressing and torsion machine. The rotation rate was 1 rpm and processing was performed through total numbers of rotations, N, of 1, 2 and 5 turns.
Deformation behavior during HPT
The studies of the HPT-processed materials behavior [[11], [12], [13], [14],21] indicate that the inconsistent accumulated strain distribution induced by the different linear velocity results in the difference in the microstructure and properties along the radius direction. Since the decrease in thickness of the disks and slipping phenomenon during the HPT experiments cannot be ignored, the true strain at each position on the specimen can be estimated by [11]:
where h and h0
Conclusions
The cSRX behavior of HPT-processed tungsten, as a consequence of influence of the HPT process, was determined by an examination of microstructure and Vickers hardness to quantitatively investigate microstructure stability, energy release and hardness loss in the annealed state. The results point to the following conclusions:
- 1)
The cSRX in annealing process brings about the homogeneous microstructure with restricted grain growth, crystallographic orientation change and GNBs reduction in
Author statement
The authors confirm that all authors acknowledge that the material presented in this manuscript has not been previously published, except in abstract form, nor is it simultaneously under consideration by any other journal.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationshipsthatcouldhaveappearedtoinfluencetheworkreportedinthispaper.
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51675154, 51875158 and 51975175).
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