Microstructure and Texture of Hydrostatic Extrusion Deformed Ni Single Crystals and Polycrystal
The differences in the microstructure and texture of two Ni single crystals, with different initial orientations ( and ), and of polycrystalline nickel, before and after severe plastic deformation (SPD) produced by hydrostatic extrusion (HE), have been investigated. The crystals were deformed by a two-step HE process with a total deformation value of . The global texture, mechanical properties, and microstructure were examined after the deformation. In every investigated sample, the presence of fibre texture was noted, while the starting orientation of a Ni single crystal was preserved in 50% of the volume. The results obtained were compared with the relevant literature data.
The strength of materials can be increased by forming obstacles to the movement of dislocations, which may be other dislocations, grain boundaries, or the particles of a second phase. One of the ways to strengthen materials is grain refinement through the use of severe plastic deformation methods. In order to better the understanding of deformation textures in polycrystalline nickel, some research has already been carried out on Ni single crystals. Nickel single crystals and polycrystals were mostly deformed by equal channel angular pressing (ECAP) [1–4]. So far, limited investigations have been conducted on hydrostatic extrusion (HE) of Ni single crystals  and polycrystals . Moreover, the evolution of microstructure and fibre texture during the hydrostatic extrusion process has been studied on polycrystalline materials [7–11]. The effect of the initial orientation of the material on its subsequent texture and microstructure is not sufficiently clear. This work is intended to complement these studies.
An overview of the literature data indicates that texture formation during extrusion and drawing processes in metals and alloys with an FCC crystal system leads to the creation of and fibre texture. Their proportions in the overall texture depend on, among other factors, the type of deformation  and their location in the sample [1, 13] ( fibre is dominant in the centre and fibre near the surface of the wire). The proportion of each fibre after extrusion or drawing is decided mainly by the initial orientation; it depends on whether the polycrystal (random) or single crystals with different initial orientations are being deformed.
The goal of this study is to observe the changes in the texture after hydroextrusion of nickel single crystals with different initial orientations and to compare them with the texture changes of polycrystalline nickel undergoing exactly the same deformations in the process of hydroextrusion. During the discussion of the results, a comparison of similar experiments on other single crystals and polycrystals of the FCC crystal system has also been included. This study is focused on the evolution of the microstructure and texture of Ni single crystals and polycrystal during plastic deformation realized by hydrostatic extrusion. Two Ni single crystals with different starting orientations ( and ), as well as polycrystalline Ni material, have been examined during a two-step HE process that achieves a total true strain of . Only the results for Ni single crystals, after an HE process with a different total true strain value, have been presented before . In this study, the authors wanted to investigate whether the differences would be noticeable during the same deformation of a single crystal and polycrystal.
2. Material and Experiment
Polycrystalline nickel of 99.95% purity, with a grain size of ~10.6 μm, and two Ni single crystals were investigated. Figure 1 shows schematically the sample preparation for extrusion and the two single crystals’ orientation before the deformation. The samples were cylindrical in shape and were cut out along the main crystallographic axes ( and ) of the single crystals using a spark erosion saw. The thin foils for Transmission Electron Microscopy (TEM) analysis were cut out from cross sections of the samples and prepared by applying the standard technique of electrolytic polishing using Struers apparatus.
The HE process was carried out in two steps (from ϕ9 mm to ϕ7 mm and then to ϕ5 mm). The total strain of the extruded wires can be calculated by the following equation:where ε indicates the true strain of the extruded single crystal nickel wires; and denote the diameters of the undeformed (ϕ9 mm) and extruded wires, respectively. All investigations were conducted on cross sections parallel to the extrusion direction. Changes of the mechanical properties were observed by measuring the microhardness of the initial states and those after deformation using the Vickers method. The microstructure after HE was examined in thin films of the material (specially prepared for TEM), using a Hitachi S5500 SEM/STEM, and the local texture was determined by the EBSD measurement (Hitachi SU70 SEM). The global texture of the materials was analysed by the X-ray diffraction method, using a Bruker D8 DISCOVER powder diffractometer equipped with a Cr X-ray source.
3. Results and Discussion
Figure 2 compares the microhardness of the two and Ni single crystals and polycrystalline Ni in both the initial state and the state after HE. The microstructure of all the deformed samples is typical for materials with high stacking fault energy (SFE). It is evident from the micrographs shown in Figure 3 that the density of dislocations is very high inside and near the grains. The grain boundaries are not well defined due to the high dislocation density. The TEM observations allowed the average grain size () to be determined.
The average subgrain parameter of deformed single crystals with the initial orientation of and amounted to 0.21 μm and 0.18 μm, respectively, whereas for polycrystalline Ni 99.95% it was 0.2 μm. All of these examinations were carried out on a surface perpendicular to the extrusion direction (ED).
The global texture was measured by the X-ray diffraction method, for the initial and deformed states of the polycrystalline nickel. Those investigations showed increased tangling of the fibre texture after HE. The volume fraction of its components is shown in Table 1. Pole figures of the deformed materials show the differential volume fraction of the two components, and , for both single crystals and polycrystalline material; see Figure 4. These results indicate that the initial orientation breaks up entirely during the hydrostatic extrusion process, where large portions of the extruded sample have been located close to a fibre. For all deformed samples, both single crystals and polycrystalline nickel, the presence of fibre in the texture is observed. The fibre character of the texture can be proved by the Orientation Distribution Functions (ODFs) calculated for the deformed materials (Figure 5).
3.1. Single Crystal
The initial orientation of the single crystal appears to be unstable during the processes of extrusion and drawing, which has been confirmed in other papers [5, 9]. These results apply not only to nickel, but also to other single crystals with a FCC system . After a deformation of , a total disappearance of this component occurs, with a component appearing in roughly 40% of the volume of the material. The complete lack of a component and the presence of large amounts of random components should also be noted. However, after a deformation that was twice as large , the component was still dominant (roughly 60% of the volume), and the presence of the component became evident (roughly 30% of the volume). At this point, only small amounts of random components were found.
3.2. Single Crystal
The initial orientation of a single crystal appeared to be stable; after a deformation of , roughly 50% of the volume remained in this orientation. In addition, the component was found in about 20% of the volume. The above results correspond well with results obtained during extrusion or drawing of other single crystals, or metals with a FCC system, for example, Cu  and Al . These researchers noted variable amounts of and components, depending on the metal, which has been connected with the influence of SFE, and the size of the deformation. A similar tendency has been observed by Chen et al. , when they deformed monocrystalline Cu of the same initial orientation. These results indicated that the component forms only with higher degrees of deformation.
3.3. Comparison of and
Comparison of the two initial orientations of the single crystals deformed by hydrostatic extrusion shows that after a deformation of the component is unstable, since it disappears completely, while the component appears to be stable and was present in 50% of the volume of the sample after exactly the same deformation. This component is also present in a large (30%) volume in a deformed single crystal with initial orientation, after .
3.4. Comparison of Single Crystals with a Polycrystal
Comparing deformed nickel single crystals (having and initial orientation) with polycrystalline nickel shows that presence of the and components varies between them, depending on the initial texture/orientation and the degree of deformation. Their common attribute is the lack of the component after the deformation. Similar to the single crystals, an increased degree of deformation in polycrystals fosters the increased presence of the strong and stable component. The effect on microstructure and texture of and Ni single crystals after HE has been examined previously . The authors found that hydrostatic extrusion (HE ) applied on the investigated single crystals yielded a strongly refined microstructure with an average subgrain diameter of approximately 300 nm. The analysis of the texture and microstructure in the HE-treated 99.5% Ni with varying degrees of deformation has been taken up by Kulczyk et al. . For all the applied total strain values, they obtained by the HE process a heterogeneous, submicrometer microstructure. The authors concluded that for a total strain of up to the grains are elongated, which is in agreement with the results presented in this study. After HE , for single crystal and polycrystalline materials alike, elongated grains in the direction of extrusion were obtained. These results can also be compared with the studies of Reed and McHargue . They sum up the data on extrusion in FCC poly- and single crystals. The comparison of quantitative texture results obtained in the discussed scientific papers with those shown in the study is presented in Figure 6.
In all of the discussed studies, dominance of the texture component has been observed, regardless of the specified total strain value and initial texture. The exception is HE-treated 2 Ni single crystal, where the initial orientation has been maintained in 51% by volume. The stability of the starting orientation in FCC single crystals after deformation has also been observed in another type of deformation . It can also be noted that by applying higher total strain on single crystals the dominating fibre breaks down to two components: and . Mechanical properties, such as yield strength and tensile strength, are affected by texture components. This has been confirmed by the microhardness results obtained in this study. The HE-treated polycrystalline material, with the texture component being the major component present, has the largest HV0.2 value of all the investigated samples.
Deformation of Ni single crystals and polycrystal by hydrostatic extrusion leads to the formation of fibre texture, which is typical for this process. The initial texture of the polycrystalline material and the initial orientation of the single crystal influence the evolution of the material’s texture after its deformation, especially the percentage content of individual components. The degree of deformation also has an impact on the material composition, as the increased deformation fosters the creation of the component. The presence of this strong and stable component is observed regardless of initial texture, accompanied by the component, while the component is not present in any researched case, even when is the initial orientation.
Theoretical studies of the development of deformation texture predict that orientations which have three or more equally stressed slip systems symmetrically arranged about the major flow direction. The presence of a double fiber texture for and fibers has been predicted for FCC materials. Differences in texture might be caused by the temperature and manner of the deformation process, the starting texture of the material, the relative hardening of the active and latent slip systems, and the stacking fault energy (SFE) . The volume fraction of in the texture decreases with increasing SFE. This is related to mechanical twinning. The orientations near will twin first as a result of shear stress on twinning. In metals with high SFE (for pure Ni SFE = 128 mJ/m2), twinning is more difficult, and the component becomes weaker.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported in part by the National Centre of Science in Poland, Project no. 2013/09/B/ST8/03754.
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