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Recrystallization is one of the most important physical phenomena in condensed matter that has been utilized for materials processing for thousands of years in human history. It is generally believed that recrystallization is thermally activated and a minimum temperature must be achieved for the necessary atomic mechanisms to occur. Here, using atomistic simulations, we report a new mechanism of dynamic recrystallization that can operate at temperature as low as T = 10 K in metals during deformation. In contrast to previously proposed dislocation-based models, this mechanism relies on the generation of disclination quadrupoles, which are special defects that form during deformation when the grain boundary migration is restricted by structural defects such as triple junctions, cracks or obstacles. This mechanism offers an alternative explanation for the grain refinement in metals during severe plastic deformation at cryogenic temperature and may suggest a new method to tailor the microstructure in general crystalline materials.
To clarify the detailed atomistic mechanism of bulging, sub-grain rotation and twinning, a series of snapshots at intermediate steps between Fig. 1(b,c) are shown in Fig. 3. As shown in Fig. 3(a), at the onset of the sub-grain nucleation, Shockley partial dislocations nucleated in the area adjacent to the disclination quadrupole. It is important to emphasize that the dislocation nucleation did not start from the crack surface or GBs, which are the normal dislocation nucleation sites in crystalline metals during plastic deformation. On the contrary, the dislocations were nucleated from within the region surrounded by the disclination quadrupole, where the most significant rotation would be expected. Therefore, in contrast to previous models of SPD22 or DRX15,37,38 in which dislocation activities preceded the new sub-grain formation, the dislocations observed in Fig. 3 were the product rather than the cause of the atom rotation and the subsequent new grain formation. Although sometimes the dislocation-based model was also referred as LTDRX39, it is important to note that the characteristics of DRX consisting of bulging, sub-grain rotation and twinning was missing in these studies.
Furthermore, the corresponding shear stress-shear strain curve shown in Fig. 5 can be well understood by the different stages of SDGBM. As having been reported previously, the shear stress first monotonically increased with the shear strain until the stress became so high that the GB started to move, e.g., at 1.4% strain33. Once the GB started moving, a stick-slip behavior occurred, which is also consistent with previous studies33. When the GB reached the crack and was pinned, the SDGBM was hindered and the GB became bulged. This section can be seen from the monotonic increase in stress from 7% strain until the new grain was formed at 13.3% strain. It is also worth noting that the stress increase during GB bulging was mostly accumulated near the crack region which facilitated the sub-grain formation.
In summary, a new mechanism of LTDRX was found based on molecular dynamics simulations in crystalline Cu during deformation. The mechanism relied on the generation of disclination quadrupoles by SDGBM with restrictions by TJs, cracks or foreign obstacles. The disclination quadrupoles can induce dramatic rotation of the atoms within them and ultimately the nucleation of new sub-grains. This mechanism was found to be general, i.e. insensitive to temperature and independent of the loading mode, which may contribute to the grain refinement in metals during SPD at cryogenic temperatures. The novel mechanism of disclination-induced LTDRX may be used to design new methods of tailoring the microstructure in general crystalline materials.
In particular, Kononen et al. [6] studied if the pure titanium is susceptible to stress corrosion cracking in a topical fluoride solution by using a U-shape specimen exposed in different time periods. Throughout the paper it was concluded that topical fluoride solutions can cause SCC of commercially pure titanium. Nakamura et al. [7] investigated titanium SCC by using two different methods, the Slow Strain Rate Tensile method and the Constant load test. The sample was immersed in is 20% NaCl solution at 90C. The primary cracks was found to be approximately 10 μm. Sanderson et al. [8] in their paper dealt with U-bent tests and dynamic tensile tests which showed that SCC are formed in titanium alloys in NaCl environment at room temperature. It had been found that the cracks are developed because the chemical polishing creates a layer of very small hydride precipitates. Roy et al. [9] studied SCC of titanium by using double cantilever-beam technique. The sample was immersed in acidic brine pH 2.7 containing 5wt% NaCl at 90C continuosly. Simbi et al. [10] investigated the intergranular stress corrosion cracking of pure titanium in methanol hydrochloric acid at room temperature. The surface of the specimen was chemically polished in a solution containing 40% HF and 70% HNO3 concentrated acids. Hsiung et al. [11] studied the corrosion resistance of pure titanium in a mixture of 1% NaCl and 0% - 1% NaF at a constant pH of 6 under different elastic tensile strains. The results of polarization resistance reveal that the Rp decreased on increasing the tensile strain and increase in NaF concentration.
In the present work, for the first time, the design and experimental apparatus testing was evaluated towards formulating an appropriate set of conditions for creating a tree shape morphology on the titanium surface. In order to achieve the above goal ultrasonic loading was used for promoting crack propagation. In ultrasonic loading, cracks propagate during the tensile part of each cycle, while during the compression part crushing, attrition and dissolution of surface oxide films widen the conduits of the corrosive medium to the crack. All cracking network pictures taken from SEM are processed by the Matlab software to show the percentile amount of cracks.
The morphology of titanium foils was characterized using Scanning Electron Microscope (SEM). The largest stress is found at the point where the sample is bent. Further, an image processing code was created in the Matlab software, example is provided in Figure 3, which processes the image from SEM, and shows the average percentage of the cracked surface that has been created in each image. Then we compare all the results of the experiments in order to come to conclusions about the effect of the different conditions applied on each experiment.
examined in SEM and it was observed that on the sample surface there were some primary cracks. These were created due to the sample machining. All primary cracks are similarly oriented. Several experiments were in which the primary cracks were in a horizontal or vertical position in relation to the electrolyte surfaces conducted.
In the absence of ultrasound stress corrosion it was found that the average length of the cracks (in microns) of the reference sample (Figure 5) was about 4.19%. The average percentage of the reference sample represents the primary cracks found in the sample.
Next objective of this study was to explore the contribution of each one of the important experimental conditions reported above (e.g. temperature, electrolyte concentration, etc.), separately in order to decouple their effect and find out which conditions affect the increase of the size of the primary cracks. For clarification, it should be stated that in all samples below the primary cracks are horizontal in relation with the electrolyte surface.
Air presence: As a next step, another Ti-sample examined under experimental conditions 0/1/0/0/0/0/1. In this case, oxygen was provided in the electrolyte solution. The average percentage of cracks was 4.97%. The chemical corrosion due to air bubbling led to a small increase of cracks.
Electrochemical anodization: Another Ti-sample was tested under experimental conditions 0/0/1/0/0/0/1. A platinum electrode was put at 10mm distance from the sample in order for the electrochemical anodization to take place. In this case the average percentage was found to be 2.9%. One observes decreasing of the cracks compared to the average percentage of the reference sample which was 4.2%. Under conditions of anodization which can be considered a dynamic condition, the rate of oxide development and intrinsic characteristics of the material (e.g. coherency) and at the same time the adherence of the formed oxide with the underlying metal seem to change with time and this could lead to a decrease of cracks [16].
Based on the previously-presented results, it can be stated that the experimental conditions which result in an increase of the size of primary cracks are ultrasonic loading and provision of air to the electrolyte for the reasons explained above, where mechanically-forced or chemical-assisted corrosion was performed.
In the next group of experiments the ultrasonic loading is applied in all the cases explored. Apart from this, in each experiment there is a different experimental condition applied. In all of the samples below the primary cracks are horizontal in relation to the ultrasonic loading. The first experiment (1/0/0/0/0/0/1) has already been discussed previously in section 3.1, i.e. in which the ultrasonic loading is applied and the percentage of the primary cracks increases (compared to the reference sample).
The next experiment (1/1/0/0/0/0/1) is a combination of ultrasonic loading and provision air to th