School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China
Cite this article:
Ping LI, Quan LIN, Yufeng ZHOU, Kemin XUE, Yucheng WU. TEM Analysis of Microstructure Evolution Process of Pure Tungsten Under High Pressure Torsion. Acta Metall Sin, 2019, 55(4): 521-528.
Refractory metal tungsten has wide applications in many fields such as aerospace, national defense, military and nuclear industry due to its excellent comprehensive mechanical properties. As the demand for high-performance materials in the new era is increasing, existing materials cannot meet the performance requirements under extreme conditions. The high pressure torsion (HPT) process can produce severe shear deformation and densify the material effectively, leading to ultrafine-grain structure with non-equilibrium grain boundaries and having a significant effect on improving the overall performance of pure tungsten materials. HPT process is used to prepare an ultrafine-grain material with excellent comprehensive performance, which can broaden the application field of refractory metal tungsten and promote the engineering application of high-performance materials. The HPT experiment was carried out on commercial pure tungsten at a relatively low temperature, and the microstructure evolution during HPT processing at various turning numbers has been investigated by means of EBSD, TEM and HRTEM. It was found that with the strain increasing, the grains were refined significantly, dislocation density and the ratio of non-equilibrium grain boundary increased obviously. Moreover, it was transparent that the low angle grain boundary transform into high angle grain boundary during HPT processing. At the same time, the dislocation structure moved to grain boundary gradually so that there was no obvious defect in fined grains. When the equivalent strain increased to 5.5, the deformation mode of grains transformed from intracrystalline sliding to grain boundary sliding, because the size of some grains was close to the mean free path of dislocation.
Fund: National Natural Science Foundation of China(No.51675154);Program for New Century Excellent Talents in University(No.NCET-13-0765);International Thermonuclear Experimental Reactor Project(No.2014GB121000)
Fig.1 Schematic (a) and specimen (b) of high pressure torsion (HPT) processing for tungsten
Fig.2 TEM images showing the microstructures of grains (a, c, e) and grain boundaries (b, d, f) of pure tungsten processed by HPT under 1 turn (a, b), 2 turns (c, d) and 5 turns (e, f) (Insets in Figs.2a, c and e show the SAED patterns of grains)
Fig.3 EBSD images showing the evolution of grain boundary angles of pure tungsten before (a) and after HPT under 1 turn (b), 2 turns (c) and 5 turns (d)
Fig.4 Grain boundary morphologies of pure tungsten processed by HPT under 1 turn (a~c), 2 turns (d~f) and 5 turns (g~i)
Fig.5
Fig.6
Fig.7 HRTEM images of tungsten grain inside and boundary processed by HPT(a~c) grain A inside, grain B inside and grain boundary under 1 turn, respectively(d~f) grain C inside, grain D inside and grain boundary under 2 turns, respectively(g~i) grain E inside, grain F inside and grain boundary under 5 turns, respectively
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