Физика/2.Физика твердого тела.

Galina Shlyakhova^1,3, Vladimir Danilov^1,4, Boris Semukhin¹ and Lev Zuev^1,2

¹Institute of Strength Physics & Materials Science, SB RAS, Tomsk, Russia

²National Research Tomsk State University, Tomsk, Russia

³Seversk Technological Institute affiliated to NIYaU MIFI, Seversk, Russia

⁴Yurga Institute of Technology, TPU affiliate, Yurga, Russia

Plastic deformation macrolocalization and fracture in ultrafine grain titanium

The tests were performed for commercial pure titanium, using flat samples having ‘dog-bone’ shape; their work part had dimensions 40×6×1.5 mm. The material had been subjected to severe plastic deformation (SPD), i.e. abc-press forging and cold rolling with subsequent prerecrystallization annealing [1]. As-treated material had a submicrocrystalline structure, which contained equiaxed grains having size 0.2…0.6 μm which made up 65% by volume, the remainder being structural components having sizes < 0.2 μm. The samples were tested in uniaxial tension at a constant rate 8.33×10-5 s-1. Simultaneously, strain fields were recorded for the tensile samples using the method of double-exposure speckle photography. The distributions of local elongations exx(x) and local rotations ωz(x) along the extension axis were obtained [2].

The X-ray beam was scanned over the work surface of the test sample and the diffraction patternof said beam was recorded. The extent of misorientation of crystalline blocks was determined from the angular positions of sub-reflexes and the size of blocks corresponding to each sub-reflex was calculated from formula. As a result, a set of data on misorientation and block size was obtained for each particular spot to enable plotting histograms of block sizes and misorientations. Using the Gauss function, approximation was performed for the histograms obtained to yield average block size and average extent of misorientaion for a respective spot. A linear correlation is found to exist between the above two parameters. It can be concluded that the larger the size of crystalline blocks, the greater the extent of misorientation and of lattice distortion at the block border. According to the author in [3], lattice distortions are the most probable cause of local stresses. The extent of misorientation and the size of crystalline blocks are found to vary for different points of the test sample. The extent of misorientation q and the size of blocks D observed for the high-amplitude localization zone (x = 20 mm) are considerably smaller by comparison with the same values observed for the areas on the border of the same zone (see Fig. 1a, b). It is needless to say that the blocks occurring within the high-amplitude zone have small sizes. A high extent of misorientation of crystalline blocks occurring on the border of the same zone can be assigned to high local stresses. Similar reasonable inferences as to the high stress levels on the borders of localized plasticity nuclei were previously made in [2].

a) b)

Fig. 1. The dependencies of block misorientation (2) and size (3) on the distribution of local rotations (1)

Using the method of transmission electron microscopy, structural investigation was performed for material sampled from the area in which fracture was liable to occur. By and large, the characteristic sizes and morphology of structural components are found to remain unchanged. However, a fraction of subgrains would become non-equiaxial ones. It is also found that most structural components are elongated along the extension axis. The extent of non-equiaxiality is as high as 2.5. That subgrains and grains having sizes 0.1…0.4 μm account for 80% and nonequiaxial subgrains and grains having sizes 0.6…0.9 μm, the remaining 20%. Of particular interest are the research results obtained by the technique of atomic force microscopy in conjunction with the thin-foil method. Thin-foils are generally employed for transmission electron microscopy investigations. The surface relief of thin foil examined was found to contain deep equiaxed cups (etch pits) having average size 0.14 μm as well as nonequiaxed ones having average size 0.19 μm (maximal diameter) and 0.09 μm (minimal diameter), i.e. the extent of non-equiaxiality is ~2. It is noteworthy that non-equiaxed cups had longitudinal axis aligned along the sample loading axis. These results were matched with the data obtained by transmission electron microscopy. There is good reason to think that the appearance of deep cups on the foil surface is due to chemical polishing, which caused etching out of structural components of ultrafine grain titanium. The granular structure of material is recognizable from the partially etched-out grain boundaries. This observational result can be used for certification of materials having nano- and submicrocrystalline structure.

The co-authors are thankful to the Russian Foundation of Fundamental Research for partial financial support of the investigations which were performed in the frame of Grant No. 14-08-00299.

References

[1] G.V. Shlyakhova, A.Yu Eroshenko, V.I. Danilov, Yu.P. Sharkeev, A.I. Tolmachyov,

Microstructure and fracture features of ultra-fine-grained titanium VT1-0 produced by abc-pressing method, Deformation and Fracture of Materials 9 (2012) 24-28.

[2] L.B. Zuev, V.I. Danilov, S.A. Barannikova, V.V. Gorbatenko, Autowave model of localized plastic flow of solids, Phys. Wave Phenom. 17 (2009) 66-75.

[3]. G. F. Kuznetsov, X-ray topographic identification and measurement of plastic strains and elastic stresses in single crystallites of polycrystalline diamond layers, Technical Physics 48 (2003) 1546-1553.