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Recrystallization behavior of tungsten processed by equal channel angular extrusion at low homologues temperature: Microstructure, hardness, and texture
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Recrystallization behavior of tungsten processed by equal channel angular extrusion at low homologues temperature: Microstructure, hardness, and texture
T
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Z.S. Levina, , B.G. Bradyb, D.C. Foleyc, K.T. Hartwiga a
Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3003, USA ARL South at Texas A&M University, FCDD-RLW-MF, College Station, TX 77843-3003, USA c Shear Form, Inc., 207 Dellwood St., Bryan, TX 77801, USA b
A B S T R A C T
This work examines the recrystallization behavior of bulk pure tungsten subjected to severe plastic deformation by Equal Channel Angular Extrusion at low temperature. Grain size, morphology, and orientation are examined as a function of subsequent heat treatment temperature. Four temperature ranges are identified wherein the material undergoes recovery, boundary migration, recrystallization, and grain growth. A comparison between this material and warm-worked material is also made, illustrating the effects of stored energy on recovery and recrystallization. Experimentally determined Hall-Petch values agree with previous work. Finally, the plastic deformation behavior of worked and recovered materials are compared with bend test results showing that heat treatment can be used to lower strength while maintaining ductility in heavily deformed tungsten.
1. Introduction Tungsten is a metal with outstanding properties that make it well suited for high temperature and demanding applications, including high temperature radiation shielding and high temperature structural components. Tungsten's property attributes include high strength (> 3GPa), high melting point (3240 °C), low sputtering yield, and low deuterium tritium retention. Tungsten is one of the few materials suited for plasma facing components in the ITER fusion reactor [1]. The major issue facing the use of tungsten in such applications is its susceptibility to brittle fracture below the ductile-to-brittle transition temperature (DBDT) of 200–300 °C [2], which drastically limits is usability. W.D. Coolidge developed a solution to the poor ductility in tungsten in the early 20th century at the by General Electric Company during the development of a ductile tungsten filament for incandescent light bulbs. It was noticed that through a process of pressing and sintering tungsten powders followed by working through a temperature step down approach, that tungsten becomes more ductile with increased working below the recrystallization temperature [3]. The cause of improved ductility was determined to be the elongation of the original tungsten grain boundaries and the formation of a 〈110〉 texture along the wire drawing or extrusion direction [4]. This temperature step down approach is now widely used in fabrication of tungsten wire and sheet. However, due to the area reduction required to render tungsten ductile, this approach has not been suitable for the manufacture of larger
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dimension tungsten products. More recent investigations of tungsten have used severe plastic deformation (SPD) methods like equal channel angular extrusion (ECAE), which does not reduce the cross sectional area, permitting the accumulation of large strains in bulk material [5–8]. Most of the deformations for these investigations were done near or above the recrystallization temperature to reduce strength and impart ductility. However, this elevated temperature processing can have a significant impact on the resulting mechanical behavior, as recrystallization can render even the most ductile tungsten filament or sheet into a brittle material with little to no ductility [9,10]. This change in mechanical behavior is caused by elimination of the changes made during the deformation process, specifically the reduction of grain size, reorientation in texture, the generation of numerous dislocations, and a conversion from equiaxed to elongated grains [11]. These equiaxed grains are susceptible to the low energy intergranular fracture that is the primary mode of failure in polycrystalline tungsten [12–14]. Due to the difficulty in processing tungsten at temperatures below the DBTT, it is often necessary to perform an intermediate heat treatment in order to reduce the loads necessary to further work the material. However, due the embrittling effects of recrystallization, any heat treatment must be conducted below this temperature to minimalize recrystallization. The effects of various heat-treatments needs to be investigated as cold working can alter the recrystallization temperature, through the storage of energy in lattice strain and reduced grain size.
Corresponding author. E-mail address: [email protected] (Z.S. Levin).
https://doi.org/10.1016/j.ijrmhm.2019.05.012 Received 22 February 2019; Received in revised form 20 April 2019; Accepted 15 May 2019 Available online 18 May 2019 0263-4368/ © 2019 Published by Elsevier Ltd.
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
Z.S. Levin, et al.
The objective of the current work is to better understand the influence of grain size, hardness, texture and grain morphology on annealing and recrystallization behavior of heavily deformed tungsten. The goal is to determine a heat treatment temperature that will effectively reduce the strength of heavily worked ductile tungsten without a significant reduction in deformability. Identification of the onset and termination of recrystallization will be examined, as well as the effect of ECAE processing temperature on recrystallization evaluated through Vickers hardness. The impact of post deformation heat-treatment on mechanical behavior measured by 3-point bend tests at ambient temperature is also examined. 2. Materials and methods In order to process tungsten via ECAE, it was necessary to a) protect the material from contamination, and b) render the material compatible with the tooling available. To do this, tungsten rod provided by Plansee measuring 12 mm in diameter with a purity exceeding 99.9% was encapsulated within a 316 stainless steel can with cross section of 25 mm × 25 mm and sealed under inert atmosphere. This assembly, referred to as a billet, was then placed into the ECAE tool, which was heated to the extrusion temperature ~300 °C. The die/tool geometry consisted of 25 mm square cross section entrance and exit channels with sliding walls to reduce friction, and a 90o die angle. The billet was allowed to reach equilibrium for one hour prior to extrusion. This extrusion procedure was repeated four times with no billet rotation between extrusions giving a total accumulated plastic strain of ~4.6. The processing technique is referred to as ECAE Route 4A. Tungsten test samples were extracted from the billet using electrical discharge machining (EDM). In order to prepare specimens for heat-treatment, the EDM surface was removed by surface grinding. Specimens were then sealed inside quartz tubes under a vacuum of better than 1 × 10−5 Torr. Below 1225 °C samples in quartz tubes were heat treated in a standard muffle furnace. At 1225 °C and above, heat treatments were done in an inert Argon gas furnace. The duration of all heat treatments was 1 h, followed by furnace cooling. Samples were prepared for microscopy by mechanical grinding with silicon carbide polishing pads followed by electrolytic polishing in a 1 wt% sodium hydroxide (NaOH) and water solution. Grain size determinations were made on the extrusion plane, sometimes referred to as the flow plane (billet side plane), with a Quanta 600 scanning electron microscope (SEM) using secondary and backscatter electron detectors. The length (l) and width (w) of individual grains with the number of grains examined n~500, were measured in order to ensure a statistically significant estimate for average grain size. These length and width data were also used to characterize morphology based on the grain aspect ratio (l/w). Vickers hardness measurements were taken on the flow plane with a 300 g load. Evaluation of the effectiveness of thermal processing was done using a custom built 3-point bend apparatus with a 7 mm bottom span. More detail on this apparatus, as well as tungsten ductility and fracture behavior, can be found elsewhere [15,16].
Fig. 1. Grain size and grain aspect ratio measurements for 4A ECAE processed tungsten.
growth first appears to occur between 700 and 800 °C, and continues to grow gradually approaching 0.4 μm at 1100 °C. A rapid increase in grain size occurs above 1100 °C reaching ~7 μm at 1300 °C. This dramatic increase in grain size is consistent with the known recrystallization temperature of tungsten. Above 1300 °C, grain growth is gradual increasing to ~10 μm at 1600 °C. In order to quantitatively examine the morphology of tungsten grains and the corresponding relationship with recrystallization behavior, the aspect ratio of each grain was also calculated and plotted versus annealing temperature as summarized in Fig. 1. Unexpectidly, changes in aspect ratio do not coincide with the regions of extensive grain growth. Instead, it appears that the avergae aspect ratio decreases continuously from ~5 in the as-worked state to 1.5 at near 1300 °C. While the data point standard deviations indicated by error bars in Fig. 1. are relatively large, the large number of data points provide a statically significant trend between the heat treatment temperature and the aspect ratio. The results of a linear regression fit of the data between 300 °C and 1300 °C indicate, a y-intercept of 5.4 with a standard error of 0.21, and a slope of −0.0028, with a standard error of 1.945E-4. The t-values for the y-intercept and slope are 24.7 and −14.44, with a Probability > |t| of 2.6–16 and 2.2E-9 respectively. This leads us to accept the rejection of the null hypothesis and determine that a relationship between the heat-treatment temperatures and the aspect ratio is highly likely. Above 1300 °C, the aspect ratio decreases only slightly in the measured range. The knee of this curve indicates that recrystallization is completed near 1300 °C. Recrystallization microstructures of the As-received, as-worked, and worked plus heat treated tungsten are shown in Fig. 2 micrographs. All scale bars in Fig. 2 denote 5 μm. The impact of 4-pass ECAE processing is clear by comparing the as-received and as-worked micrographs. ECAE processing not only refines the microstructure but also resulted in the elongation of grains and sub grains, and reorients their long axis to the extrusion direction. It should also be noted that the end of the asworked grains typically have a highly reduced cross section, which results in the large aspect ratio of (l/w)~5 for this material. The asworked material also contains grains in the nanosized range along with the occasional micron sized grains. Heat-treating at 900 °C produces a coarsening of the tungsten microstructure. The elongated grains observed in the as-worked material are smaller and sharp protrusions have been largely eliminated. It also appears that grains fall within bands that are oriented along the shear plane and toward the extrusion direction. The grain shape is also more uniform than the as-worked material, and the grain aspect ratio has decreased to ~3. The onset of recrystallization can be seen in specimens
3. Results 3.1. Microstructure A summary of the grain size (GS) and grain aspect ratio measurements for 4A ECAE as-worked and heat-treated tungsten in the temperature range of 300-1600 °C, is shown in Fig. 1. Grain size is plotted on a log scale in order to elucidate changes below recrystallization. Grain sizes appear to fall into four regions in this figure: as-worked700 °C, 700–1100 °C, 1100–1300 °C, and > 1300 °C. In the first region, 300–700 °C, the grain size appears to be relatively stable ~0.25 μm, as there is no significant GS difference over this temperature range. Grain 2
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
Z.S. Levin, et al.
Fig. 2. SEM images of as-received, as-worked and worked and annealed commercially pure tungsten. The annealing temperatures are indicated. Extrusion direction oriented horizontally left to right.
characteristic {110}||ED texture. The influence of the initial microstructure can still be seen at this temperature, with the alignment of grains into rows, running diagonally across the sample cross section. Heat-treatment at 1450 °C completely recrystallizes the microstructure; however the texture is not random and retains some of the orientation dependence observed in the deformed microstructure (e.g. the green orientations corresponding to {110}||ED), albeit at a lower intensity. The differences between the large and small grains in partially recrystallized tungsten can be seen in the high magnification EBSD and grain average misorentation micrographs of the 1175 °C heat-treated tungsten in Fig. 4. In the inverse pole figure map in Fig. 4(a), a region of un-recrystallized tungsten can be seen between the much larger and equiaxed grains. There is a clear difference between the regions of larger recrystallized grains and the smaller grains trapped between, which show distributions of orientation for each grain. Besides being much larger, these recrystallized grain boundaries are uniform and smooth and are nearly equiaxed, while the grains in between have rough interlocking edges, and are still elongated with respect to the extrusion direction. The difference in geometrically necessary dislocation (GND) density between the regions can be calculated according to the methods of Field et al. [18]; the GND map, calculated based on {110}〈111〉 type dislocations with acquisition step size of 40 nm and a maximum misorientation of 2° shows a significantly higher density of GNDs within these unrecrystallized regions according to Fig. 4(b). Furthermore, this region contains a number of low angle grain boundaries, which exceed the 2° maximum misorientation for GND calculations, as shown in Fig. 4(c).
heat-treated to 1100 °C. Here the grain size is larger, and these grains are no longer contained within bands as seen in the as-worked and 900 °C heat-treated materials. At heat-treatment temperatures of 1225 °C, very large grains appear, yet some of the much smaller grains remain, producing a bimodal distribution of grain size, which is characteristic of incomplete recrystallization. A fully recrystallized tungsten microstructure can be seen in the 1600 °C micrograph. These grains are much larger and more equiaxed than those in the as-received material, possibly indicating that the as-received material was worked at < 1600 °C prior to ECAE processing. 3.2. Texture The texture evolution of this four pass route ECAE processed tungsten through recrystallization is shown in Fig. 3. The inverse pole figure legend is also shown with respect to the extrusion direction for ease of orientation identification. Heat-treatment at 600 °C in Fig. 3(a) shows little impact on the worked texture: there is a substantial banding along the extrusion direction (ED), with a {101}||ED texture seen as green with minimal {100}||ED (red) and {111}||ED (blue) orientation relations. This texture is typical of heavily sheared BCC metals [17]; however the precise orientation of the {110} with respect to the shearing axis may change based on the magnitude of shear strain as a result of the rigid body rotations associated with simple shear. Heattreating at 800 °C Fig. 3(b), shows a slight decrease in the maximum intensity, but retains the same texture as Fig. 3(b). There is also the first appearance of stress relaxation, which can be seen by the presence of some equiaxed grains with uniform color within the larger green (110)||ED bands. At 1100 °C the presence of a few large recrystallized grains appear, however the measured texture does not deviate significantly from the traditional shearing texture. Shown in Fig. 3(d), by 1175 °C the tungsten microstructure is almost completely recrystallized, with large, nearly equiaxed grains interspersed with some heavily textured regions with very small grains, which appear to retain the
3.3. Hardness Vickers hardness testing was used to characterize the effects of heattreatment on mechanical behavior. Post-heat treatment micro-hardness measurements from this work are compared to a previous study [7], where ECAE processed pure tungsten was worked to a similar strain but 3
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
Z.S. Levin, et al.
Fig. 3. EBSD texture map of ECAE processed tungsten. The extrusion direction is oriented horizontally left to right.
400 °C produces a substantial decreases in hardness, while the material processed at 300 °C shows a more gradual decrease in hardness at this annealing temperature. Another notable feature of the recrystallization of 300 °C-processed tungsten is the dramatic decrease in hardness between 1100 °C and 1225 °C. In the 1000 °C and 1200 °C processed tungsten the onset of recrystallization appears to be similar to the 300 °C-processed material, around 1100 °C, yet the hardness decreases more gradually. While the hardness of the 300 °C material flattens out above 1225 °C it is not until 1400 °C that the 1000 °C and 1200 °C materials reach a similar
at higher temperatures (1000 °C and 1200 °C) and following a different ECAE route (4E or route 2C/ rotation 90o/route 2C). All results are presented in Fig. 5. As expected, the 300 °C processed material is substantially harder than the tungsten processed at higher temperatures where recovery and recrystallization can occur. This difference in hardness is likely due to a Hall-Petch type dependence on grain size. It is noted that the 300 °C material retains greater hardness than the materials processed at 1000 °C or 1200 °C even after intermediate annealing between 300 and 1100 °C. In the 1000 °C and 1200 °C processed tungsten, annealing at
Fig. 4. (a) EBSD texture map of ECAE processed tungsten heat-treated at 1175 °C for 1 h. (b) Corresponding GND Density map, and (c) image quality/grain boundary map highlighting the high density of dislocations and low angle grain boundaries within regions of the sample that have not been recrystallized. 4
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
Z.S. Levin, et al.
Fig. 1. The strengthening coefficient of 3.6 and the dislocation pinning constant 365 are similar to those reported previously, which are 10 and 350 respectively [19]. This data is of primary importance due to the common use of hardness measurements to provide an estimation of grain size. This Hall-Petch curve is unique as the grain sizes are smaller than previously reported due to the low temperature of processing and the high level of plastic strain. 3.5. Heat-treatment effect on mechanical behavior One of the primary goals of this research was to determine the thermal processing window for this heavily cold worked tungsten in order to enable additional cold work. This is an issue with tungsten due to its ability to work harden, which results in very high press loads when conducting ECAE. For many pure metals annealing would be an effective way to reduce the strength; however, with tungsten, recrystallization must be avoided because of its embrittling influence. This study proves useful for establishing the thermal window for subsequent heat-treatment without causing embrittlement. Based on the grain size, hardness and EBSD data, a temperature of 800 °C was chosen, as this temperature was the highest observed without significant recrystallization occurring. The effect of an 800 °C heat-treatment can be seen in Fig. 7, in the flexural stress-strain curves of the asworked and two samples of 800 °C heat-treated material. Heat-treating at 800 °C has no significant impact on the strain to failure while reducing strength by > 500 MPa. It also appears that this heat-treatment decreases the work hardening as compared to the asworked material. This may be due to the dislocation reorientation and locking that occurs with this thermal processing step, however due to the limited data available a definitive cause cannot be determined.
Fig. 5. Vickers hardness data at various annealing temperatures for four pass extrusions on ECAE tungsten processed at 300 °C from this investigation and, 1000 °C and 1200 °C reported elsewhere [7].
hardness, despite their initial hardness being substantially lower than the 300 °C material. This difference is likely due to the greater stored energy in the 300 °C material, which recrystallizes more completely at lower temperatures due to the greater amount of stored lattice strain energy. The similar hardness levels in the fully recrystallized condition indicate that the impurity levels are comparable between the two studies. 3.4. Hall-Petch
4. Discussion
A Hall-Petch plot containing grain size and Vickers hardness data is shown in Fig. 6. The range of grain sizes from sub-micron to nearly 10 μm provides a robust inverse square relationship between grain size and the Vickers hardness for this commercial purity tungsten. The error bars in this figure only indicate standard deviations for Vickers hardness as displaying the small standard deviation in grain size as an inverse square distorts the graph, and has been previously displayed in
An evaluation of the as-worked and annealed tungsten microstructure reveals several important features of the recrystallization and grain growth behaviors of this heavily worked material. Annealing of heavily worked tungsten at well below the recrystallization temperature alters the grain size and grain morphology. While changes in the grain size are not detectable below 700 °C, the continuous decrease in grain aspect ratio indicates that microstructural changes are occurring. The decrease in aspect ratio is likely due to surface energy driven rearrangement of the jagged tungsten grains as
Fig. 7. Room temperature flexural stress-strain curves of a typical as-worked tungsten sample and two separate 800 °C annealed tungsten samples all processed by ECAE route 4A at 300 °C.
Fig. 6. Hall-Petch plot of as-worked and annealed 4A at 300 °C ECAE processed tungsten. 5
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
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study hot-rolled plate. They showed that recrystallization does not occur at 1100 °C in one-hour anneals but will begin at this temperature given a long enough holding time [21]. They observed recrystallized grains after a 1200 °C one hour anneal. While this aligns with the findings of this work, the 4A300 ECAE-processed material appears to recrystallize a much higher fraction of the microstructure by 1175 °C/ 1 h than the 1200 °C/1 h sheet. Impurity differences could play a role, but we suspect that the higher stored energy of the SPD material is the cause. This can be seen in much earlier work as well. Klopp and Raffo found that rod warm swaged 40% area reduction was only 5% recrystallized after one hour at 1500 °C while an 83% area reduced bar was fully recrystallized after the same treatment [22]. Farrell et al. saw recrystallization in sheet at 1250 °C/1 h but none at 1200 °C [23]. The small difference in one hour recrystallization onset temperature between that work and this one may be due to their application of a five minute 1150 °C heat treatment to the sheet prior to additional annealing. It seems clear that both the initiation and completion of recrystallization are impacted by prior plastic deformation and the resulting stored energy.
seen in Fig. 2. The high density of grain boundaries within these regions is likely to be the driving force behind this change in morphology. As the heat-treatment temperature increases, the increased lattice mobility enables a small increase in grain size between 700 °C and 1100 °C. Based on microstructural observations, this modest grain growth is driven primarily by strain energy stored within the lattice and does not involve the large-scale lattice reorientation that is associated with recrystallization. True recrystallization for the 4A material initiates near 1100 °C, as was reported for tungsten processed at higher temperatures [7]. However, the onset temperature is not as apparent in the Vickers hardness data for the 300 °C-processed tungsten, as the decrease in hardness with an increase in annealing temperature is more gradual. The lack of a distinct transition in the hardness curve is due to the small changes in microstructure that occur with annealing. While the onset of recrystallization occurs at similar annealing temperatures for all three processing temperatures (Fig. 5), its completion is highly dependent on the processing temperature. This dependence is likely due to the smaller grain size and greater stored lattice strain in the 300 °C processed material. The aspect ratio data provides additional information about microstructure changes that are difficult to observe though measurement of either grain size or hardness. The constant decrease in aspect ratio from the as-worked state to 1225 °C suggests that even at temperatures below recrystallization some rearrangement of the microstructure occurs. The aspect ratio data also indicates that recrystallization is complete by the abrupt change in the curve slope above 1225 °C. This change, coupled with grain size trends, indicates complete recrystallization between 1225 °C and 1400 °C. This is consistent with the micrograph of tungsten annealed at 1225 °C, which shows signs of a bimodal grain size distribution, indicating partial recrystallization. The four distinct regions in the grain size vs. annealing temperature curve in Fig. 1 suggest that in each of these regions; as-worked-700 °C, 700 °C–1100 °C, 1100 °C–1300 °C, and > 1300 °C, a different mechanism controls grain growth. In the first region only the highly elongated ends of grains are being absorbed into the surrounding material. These regions have a high fraction of grain boundary volume that requires little energy to reorient. In the second region from 700 °C–1100 °C, the slight increase in grain size suggests that a different mechanism is active and perhaps is driven by stress relief of stored lattice energy through annihilation of dislocations. The third region corresponds to the recrystallization of tungsten and no doubt the grains that form as seen in Fig. 2, are mostly defect free and have almost completely consumed the initial grains, which is confirmed by the EBSD micrographs in Fig. 3. Recrystallization is complete between 1300 °C and 1400 °C, and higher temperatures result in grain growth. Below 700 °C the difference in grain size is not immediately evident, however, some changes are apparent from the decrease in grain aspect ratio. The changes in grain size are obscured by the way in which grain size is measured along the length and width, which are at a maximum and minimum respectively for the heavily elongated material. The similarity between Hall-Petch data presented here and previous investigations is notable in part because of the differences in the grain boundaries. The sintered material used in prior works would likely have relatively higher grain boundary impurity concentration compared to the heavily worked 4A ECAE processed material. The ECAE/SPD processed material comparison in Fig. 5 shows that the material in this study begins to recrystallize at nearly the same point as tungsten deformed at higher temperatures. However, 4A300 recrystallization completes at a lower temperature and the fraction of recrystallized grains is much higher at intermediate temperatures. Conventionally produced tungsten provides a comparison beyond SPD processing. Al, K, and Si doped (AKS) filament is the typical wire form of tungsten, to the extent that a recent study published in this journal failed to specify that the wire in their study was doped [20]. Plate and sheet forms of pure tungsten are commercially available and have been extensively studied. Tsuchida et al. used microhardness and OIM to
5. Findings and conclusions To summarize the key results from this work: 1. Heat treatment at temperatures as low as 300 °C following ECAE processing at 300 °C may cause some slight microstructural changes in pure tungsten - namely smoothing of the coarse interlocking grains and possible annealing out of dislocations. 2. Notable grain growth starts at a temperature around 700 °C, and this type of grain growth occurs up to 1100 °C. 3. Regardless of processing temperature, the onset of recrystallization occurs around 1100 °C for tungsten of this commercial purity. 4. Increasing the processing temperature elevates the temperature at which recrystallization is complete. 5. Grain size and hardness have a strong Hall-Petch type relationship. 6. Grain aspect ratio can be used to determine changes in microstructure that may not be possible through grain size and hardness measurements alone. 7. Heat-treating heavily cold worked tungsten at 800 °C will reduce the strength by around 500 MPa, without a significant reduction in the ductility or strain to failure. 8. ECAE processing produces {111} and {101} texturing along the extrusion directions. The following conclusions are drawn from the results: Through ECAE processing and heat-treating it is possible to exercise a large degree of control over the microstructure and hardness of pure tungsten. It appears that while microstructure changes occur at annealing temperatures as low as 300 °C, different grain remodeling occurs in four separate temperature regions. We hypothesize that in the lowest temperature region, 300-700 °C, changes are caused by the blunting of elongated grains with pointed ends as these features seem to be the only ones affected by heat-treatment in this region. Between 700 and 1100 °C, grain growth is driven by the energy stored within the lattice caused by cold working. Partial recrystallization was observed in heat-treatments as low as 1100 °C, but complete recrystallization required ~1400 °C. Fewer unrecrystallized grains remain in the 300 °C material than in material processed at higher temperatures, in this 1100-1400 °C range. Lattice relaxation and minor grain growth are driven by some activation of slip or increase in diffusion that is not significantly impacted by grain size or dislocation density. However, the temperature of complete recrystallization appears to be independent of processing temperature, grain size, and dislocation density, and thus is dependent only on total stored energy. Our observations of changes in grain size, grain shape, texture and hardness of heavily cold worked tungsten provide increased 6
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opportunity for the application of tungsten in demanding engineering environments for energy and defense.
[9] Z. Jeffries, Metallography of tungsten, Transactions of the American Institute of Mining and Metallurgical Engineers, vol. 60, 1919, pp. 588–643. [10] P.P. Bourque, D.F. Bahr, M.G. Norton, Effect of thermal treatment on failure modes in tungsten wire, Mater. Sci. Eng. A 298 (2001) 73–78. [11] C. Ren, Z.Z. Fang, M. Koopman, B. Butler, J. Paramore, S. Middlemas, Methods for improving ductility of tungsten - a review, Int. J. Refract. Met. Hard Mater. 75 (2018) 170–183 2018/09/01/. [12] J.C. Bilello, Nucleation of brittle fracture in sintered tungsten at low temperatures, Trans. Met. Soc. AIME 242 (703–8) (Apr. 1968) 1968. [13] P. Gumbsch, Brittle fracture and the brittle-to-ductile transition of tungsten, J. Nucl. Mater. 323 (2003) 304–312 Dec. [14] R.W. Margevicius, J. Riedle, P. Gumbsch, Fracture toughness of polycrystalline tungsten under mode I and mixed mode I/II loading, Mater. Sci. Eng. A 270 (1999) 197–209 9/30/. [15] Z.S. Levin, K.T. Hartwig, Strong ductile bulk tungsten, Mater. Sci. Eng. A 707 (2017) 602–611 2017/11/07/. [16] Z.S. Levin, A. Srivastava, D.C. Foley, K.T. Hartwig, Fracture in annealed and severely deformed tungsten, Mater. Sci. Eng. A, 734 734 (2018) 244–254 2018/05/ 03/. [17] J. Baczynski, J.J. Jonas, Texture development during the torsion testing of α-iron and two IF steels, Acta Mater. 44 (1996) 4273–4288 1996/11/01/. [18] D.P. Field, P.B. Trivedi, S.I. Wright, M. Kumar, Analysis of local orientation gradients in deformed single crystals, Ultramicroscopy 103 (2005) 33–39 2005/04/ 01/. [19] U.K. Vashi, R.W. Armstrong, G.E. Zima, The hardness and grain size of consolidated fine tungsten powder, Metal. Transac. 1 (1970) 1769–1771 June 01. [20] C.J.M. Denissen, J. Liebe, M. van Rijswick, Recrystallisation temperature of tungsten as a function of the heating ramp, Int. J. Refract. Met. Hard Mater. 24 (2006) 321–324 7//. [21] K. Tsuchida, T. Miyazawa, A. Hasegawa, S. Nogami, M. Fukuda, Recrystallization behavior of hot-rolled pure tungsten and its alloy plates during high-temperature annealing, Nuclear Mater. Energy 15 (2018) 158–163 2018/05/01/. [22] W.D. Klopp, P.L. Raffo, Effects of Purity and Structure on Recrystallization, Grain Growth, Ductility, Tensile, and Creep Properties of Arc-melted Tungsten, Country unknown/Code not available1964-11-01 (1964). [23] K. Farrell, A.C. Schaffhauser, J.O. Stiegler, Recrystallization, grain growth and the ductile-brittle transition in tungsten sheet, J. Less Common Metals 13 (1967) 141–155 1967/08/01/.
Acknowledgments The authors thank Robert Barber for his assistance in processing, Dr. Anup Bandyopadhyay for his assistance with heat-treatments, and Dr. Shreyas Balachandran, for thoughtful discussions. This work was funded in part by the SMART Scholarship Program as well as Army SBIR Grant W15QKN15C0031. References [1] T. Hirai, F. Escourbiac, S. Carpentier-Chouchana, A. Fedosov, L. Ferrand, T. Jokinen, et al., ITER tungsten divertor design development and qualification program, Fusion Eng. Des. 88 (2013) 1798–1801. [2] E. Lassner, W.-D. Schubert, Tungsten: Properties, Chemistry, Technology of the Elements, Alloys, and Chemical Compounds, Springer Science & Business Media, New York, 1999. [3] W. D. Coolidge, "Tungsten and method of making the same for use as filaments of incandescent electric lamps and for other purposes," ed: Google Patents, 1913. [4] M.R. Ripoll, E. Reisacher, H. Riedel, Texture induced tension–compression asymmetry of drawn tungsten wires, Comput. Mater. Sci. 45 (2009) 788–792. [5] L.J. Kecskes, K.C. Cho, R.J. Dowding, B.E. Schuster, R.Z. Valiev, Q. Wei, Grain size engineering of bcc refractory metals: top-down and bottom-up—application to tungsten, Mater. Sci. Eng. A 467 (2007) 33–43. [6] Z. Pan, Y.Z. Guo, S.N. Mathaudhu, L.J. Kecskes, K.T. Hartwig, Q. Wei, Quasi-static and dynamic mechanical properties of commercial-purity tungsten processed by ECAE at low temperatures, J. Mater. Sci. 43 (2008) 7379–7384. [7] S.N. Mathaudhu, A.J. deRosset, K.T. Hartwig, L.J. Kecskes, Microstructures and recrystallization behavior of severely hot-deformed tungsten, Mater. Sci. Eng. A 503 (2009) 28–31. [8] H. Yuan, Y. Zhang, A.V. Ganeev, J.T. Wang, I.V. Alexandrov, Strengthening and toughening effect on tungsten subjected to multiple ECAP, Mater. Sci. Forum 667669 (2010) 701–706.
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Recrystallization behavior of tungsten processed by equal channel angular extrusion at low homologues temperature: Microstructure, hardness, and texture
T
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Z.S. Levina, , B.G. Bradyb, D.C. Foleyc, K.T. Hartwiga a
Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3003, USA ARL South at Texas A&M University, FCDD-RLW-MF, College Station, TX 77843-3003, USA c Shear Form, Inc., 207 Dellwood St., Bryan, TX 77801, USA b
A B S T R A C T
This work examines the recrystallization behavior of bulk pure tungsten subjected to severe plastic deformation by Equal Channel Angular Extrusion at low temperature. Grain size, morphology, and orientation are examined as a function of subsequent heat treatment temperature. Four temperature ranges are identified wherein the material undergoes recovery, boundary migration, recrystallization, and grain growth. A comparison between this material and warm-worked material is also made, illustrating the effects of stored energy on recovery and recrystallization. Experimentally determined Hall-Petch values agree with previous work. Finally, the plastic deformation behavior of worked and recovered materials are compared with bend test results showing that heat treatment can be used to lower strength while maintaining ductility in heavily deformed tungsten.
1. Introduction Tungsten is a metal with outstanding properties that make it well suited for high temperature and demanding applications, including high temperature radiation shielding and high temperature structural components. Tungsten's property attributes include high strength (> 3GPa), high melting point (3240 °C), low sputtering yield, and low deuterium tritium retention. Tungsten is one of the few materials suited for plasma facing components in the ITER fusion reactor [1]. The major issue facing the use of tungsten in such applications is its susceptibility to brittle fracture below the ductile-to-brittle transition temperature (DBDT) of 200–300 °C [2], which drastically limits is usability. W.D. Coolidge developed a solution to the poor ductility in tungsten in the early 20th century at the by General Electric Company during the development of a ductile tungsten filament for incandescent light bulbs. It was noticed that through a process of pressing and sintering tungsten powders followed by working through a temperature step down approach, that tungsten becomes more ductile with increased working below the recrystallization temperature [3]. The cause of improved ductility was determined to be the elongation of the original tungsten grain boundaries and the formation of a 〈110〉 texture along the wire drawing or extrusion direction [4]. This temperature step down approach is now widely used in fabrication of tungsten wire and sheet. However, due to the area reduction required to render tungsten ductile, this approach has not been suitable for the manufacture of larger
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dimension tungsten products. More recent investigations of tungsten have used severe plastic deformation (SPD) methods like equal channel angular extrusion (ECAE), which does not reduce the cross sectional area, permitting the accumulation of large strains in bulk material [5–8]. Most of the deformations for these investigations were done near or above the recrystallization temperature to reduce strength and impart ductility. However, this elevated temperature processing can have a significant impact on the resulting mechanical behavior, as recrystallization can render even the most ductile tungsten filament or sheet into a brittle material with little to no ductility [9,10]. This change in mechanical behavior is caused by elimination of the changes made during the deformation process, specifically the reduction of grain size, reorientation in texture, the generation of numerous dislocations, and a conversion from equiaxed to elongated grains [11]. These equiaxed grains are susceptible to the low energy intergranular fracture that is the primary mode of failure in polycrystalline tungsten [12–14]. Due to the difficulty in processing tungsten at temperatures below the DBTT, it is often necessary to perform an intermediate heat treatment in order to reduce the loads necessary to further work the material. However, due the embrittling effects of recrystallization, any heat treatment must be conducted below this temperature to minimalize recrystallization. The effects of various heat-treatments needs to be investigated as cold working can alter the recrystallization temperature, through the storage of energy in lattice strain and reduced grain size.
Corresponding author. E-mail address: [email protected] (Z.S. Levin).
https://doi.org/10.1016/j.ijrmhm.2019.05.012 Received 22 February 2019; Received in revised form 20 April 2019; Accepted 15 May 2019 Available online 18 May 2019 0263-4368/ © 2019 Published by Elsevier Ltd.
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
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The objective of the current work is to better understand the influence of grain size, hardness, texture and grain morphology on annealing and recrystallization behavior of heavily deformed tungsten. The goal is to determine a heat treatment temperature that will effectively reduce the strength of heavily worked ductile tungsten without a significant reduction in deformability. Identification of the onset and termination of recrystallization will be examined, as well as the effect of ECAE processing temperature on recrystallization evaluated through Vickers hardness. The impact of post deformation heat-treatment on mechanical behavior measured by 3-point bend tests at ambient temperature is also examined. 2. Materials and methods In order to process tungsten via ECAE, it was necessary to a) protect the material from contamination, and b) render the material compatible with the tooling available. To do this, tungsten rod provided by Plansee measuring 12 mm in diameter with a purity exceeding 99.9% was encapsulated within a 316 stainless steel can with cross section of 25 mm × 25 mm and sealed under inert atmosphere. This assembly, referred to as a billet, was then placed into the ECAE tool, which was heated to the extrusion temperature ~300 °C. The die/tool geometry consisted of 25 mm square cross section entrance and exit channels with sliding walls to reduce friction, and a 90o die angle. The billet was allowed to reach equilibrium for one hour prior to extrusion. This extrusion procedure was repeated four times with no billet rotation between extrusions giving a total accumulated plastic strain of ~4.6. The processing technique is referred to as ECAE Route 4A. Tungsten test samples were extracted from the billet using electrical discharge machining (EDM). In order to prepare specimens for heat-treatment, the EDM surface was removed by surface grinding. Specimens were then sealed inside quartz tubes under a vacuum of better than 1 × 10−5 Torr. Below 1225 °C samples in quartz tubes were heat treated in a standard muffle furnace. At 1225 °C and above, heat treatments were done in an inert Argon gas furnace. The duration of all heat treatments was 1 h, followed by furnace cooling. Samples were prepared for microscopy by mechanical grinding with silicon carbide polishing pads followed by electrolytic polishing in a 1 wt% sodium hydroxide (NaOH) and water solution. Grain size determinations were made on the extrusion plane, sometimes referred to as the flow plane (billet side plane), with a Quanta 600 scanning electron microscope (SEM) using secondary and backscatter electron detectors. The length (l) and width (w) of individual grains with the number of grains examined n~500, were measured in order to ensure a statistically significant estimate for average grain size. These length and width data were also used to characterize morphology based on the grain aspect ratio (l/w). Vickers hardness measurements were taken on the flow plane with a 300 g load. Evaluation of the effectiveness of thermal processing was done using a custom built 3-point bend apparatus with a 7 mm bottom span. More detail on this apparatus, as well as tungsten ductility and fracture behavior, can be found elsewhere [15,16].
Fig. 1. Grain size and grain aspect ratio measurements for 4A ECAE processed tungsten.
growth first appears to occur between 700 and 800 °C, and continues to grow gradually approaching 0.4 μm at 1100 °C. A rapid increase in grain size occurs above 1100 °C reaching ~7 μm at 1300 °C. This dramatic increase in grain size is consistent with the known recrystallization temperature of tungsten. Above 1300 °C, grain growth is gradual increasing to ~10 μm at 1600 °C. In order to quantitatively examine the morphology of tungsten grains and the corresponding relationship with recrystallization behavior, the aspect ratio of each grain was also calculated and plotted versus annealing temperature as summarized in Fig. 1. Unexpectidly, changes in aspect ratio do not coincide with the regions of extensive grain growth. Instead, it appears that the avergae aspect ratio decreases continuously from ~5 in the as-worked state to 1.5 at near 1300 °C. While the data point standard deviations indicated by error bars in Fig. 1. are relatively large, the large number of data points provide a statically significant trend between the heat treatment temperature and the aspect ratio. The results of a linear regression fit of the data between 300 °C and 1300 °C indicate, a y-intercept of 5.4 with a standard error of 0.21, and a slope of −0.0028, with a standard error of 1.945E-4. The t-values for the y-intercept and slope are 24.7 and −14.44, with a Probability > |t| of 2.6–16 and 2.2E-9 respectively. This leads us to accept the rejection of the null hypothesis and determine that a relationship between the heat-treatment temperatures and the aspect ratio is highly likely. Above 1300 °C, the aspect ratio decreases only slightly in the measured range. The knee of this curve indicates that recrystallization is completed near 1300 °C. Recrystallization microstructures of the As-received, as-worked, and worked plus heat treated tungsten are shown in Fig. 2 micrographs. All scale bars in Fig. 2 denote 5 μm. The impact of 4-pass ECAE processing is clear by comparing the as-received and as-worked micrographs. ECAE processing not only refines the microstructure but also resulted in the elongation of grains and sub grains, and reorients their long axis to the extrusion direction. It should also be noted that the end of the asworked grains typically have a highly reduced cross section, which results in the large aspect ratio of (l/w)~5 for this material. The asworked material also contains grains in the nanosized range along with the occasional micron sized grains. Heat-treating at 900 °C produces a coarsening of the tungsten microstructure. The elongated grains observed in the as-worked material are smaller and sharp protrusions have been largely eliminated. It also appears that grains fall within bands that are oriented along the shear plane and toward the extrusion direction. The grain shape is also more uniform than the as-worked material, and the grain aspect ratio has decreased to ~3. The onset of recrystallization can be seen in specimens
3. Results 3.1. Microstructure A summary of the grain size (GS) and grain aspect ratio measurements for 4A ECAE as-worked and heat-treated tungsten in the temperature range of 300-1600 °C, is shown in Fig. 1. Grain size is plotted on a log scale in order to elucidate changes below recrystallization. Grain sizes appear to fall into four regions in this figure: as-worked700 °C, 700–1100 °C, 1100–1300 °C, and > 1300 °C. In the first region, 300–700 °C, the grain size appears to be relatively stable ~0.25 μm, as there is no significant GS difference over this temperature range. Grain 2
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
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Fig. 2. SEM images of as-received, as-worked and worked and annealed commercially pure tungsten. The annealing temperatures are indicated. Extrusion direction oriented horizontally left to right.
characteristic {110}||ED texture. The influence of the initial microstructure can still be seen at this temperature, with the alignment of grains into rows, running diagonally across the sample cross section. Heat-treatment at 1450 °C completely recrystallizes the microstructure; however the texture is not random and retains some of the orientation dependence observed in the deformed microstructure (e.g. the green orientations corresponding to {110}||ED), albeit at a lower intensity. The differences between the large and small grains in partially recrystallized tungsten can be seen in the high magnification EBSD and grain average misorentation micrographs of the 1175 °C heat-treated tungsten in Fig. 4. In the inverse pole figure map in Fig. 4(a), a region of un-recrystallized tungsten can be seen between the much larger and equiaxed grains. There is a clear difference between the regions of larger recrystallized grains and the smaller grains trapped between, which show distributions of orientation for each grain. Besides being much larger, these recrystallized grain boundaries are uniform and smooth and are nearly equiaxed, while the grains in between have rough interlocking edges, and are still elongated with respect to the extrusion direction. The difference in geometrically necessary dislocation (GND) density between the regions can be calculated according to the methods of Field et al. [18]; the GND map, calculated based on {110}〈111〉 type dislocations with acquisition step size of 40 nm and a maximum misorientation of 2° shows a significantly higher density of GNDs within these unrecrystallized regions according to Fig. 4(b). Furthermore, this region contains a number of low angle grain boundaries, which exceed the 2° maximum misorientation for GND calculations, as shown in Fig. 4(c).
heat-treated to 1100 °C. Here the grain size is larger, and these grains are no longer contained within bands as seen in the as-worked and 900 °C heat-treated materials. At heat-treatment temperatures of 1225 °C, very large grains appear, yet some of the much smaller grains remain, producing a bimodal distribution of grain size, which is characteristic of incomplete recrystallization. A fully recrystallized tungsten microstructure can be seen in the 1600 °C micrograph. These grains are much larger and more equiaxed than those in the as-received material, possibly indicating that the as-received material was worked at < 1600 °C prior to ECAE processing. 3.2. Texture The texture evolution of this four pass route ECAE processed tungsten through recrystallization is shown in Fig. 3. The inverse pole figure legend is also shown with respect to the extrusion direction for ease of orientation identification. Heat-treatment at 600 °C in Fig. 3(a) shows little impact on the worked texture: there is a substantial banding along the extrusion direction (ED), with a {101}||ED texture seen as green with minimal {100}||ED (red) and {111}||ED (blue) orientation relations. This texture is typical of heavily sheared BCC metals [17]; however the precise orientation of the {110} with respect to the shearing axis may change based on the magnitude of shear strain as a result of the rigid body rotations associated with simple shear. Heattreating at 800 °C Fig. 3(b), shows a slight decrease in the maximum intensity, but retains the same texture as Fig. 3(b). There is also the first appearance of stress relaxation, which can be seen by the presence of some equiaxed grains with uniform color within the larger green (110)||ED bands. At 1100 °C the presence of a few large recrystallized grains appear, however the measured texture does not deviate significantly from the traditional shearing texture. Shown in Fig. 3(d), by 1175 °C the tungsten microstructure is almost completely recrystallized, with large, nearly equiaxed grains interspersed with some heavily textured regions with very small grains, which appear to retain the
3.3. Hardness Vickers hardness testing was used to characterize the effects of heattreatment on mechanical behavior. Post-heat treatment micro-hardness measurements from this work are compared to a previous study [7], where ECAE processed pure tungsten was worked to a similar strain but 3
International Journal of Refractory Metals & Hard Materials 83 (2019) 104966
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Fig. 3. EBSD texture map of ECAE processed tungsten. The extrusion direction is oriented horizontally left to right.
400 °C produces a substantial decreases in hardness, while the material processed at 300 °C shows a more gradual decrease in hardness at this annealing temperature. Another notable feature of the recrystallization of 300 °C-processed tungsten is the dramatic decrease in hardness between 1100 °C and 1225 °C. In the 1000 °C and 1200 °C processed tungsten the onset of recrystallization appears to be similar to the 300 °C-processed material, around 1100 °C, yet the hardness decreases more gradually. While the hardness of the 300 °C material flattens out above 1225 °C it is not until 1400 °C that the 1000 °C and 1200 °C materials reach a similar
at higher temperatures (1000 °C and 1200 °C) and following a different ECAE route (4E or route 2C/ rotation 90o/route 2C). All results are presented in Fig. 5. As expected, the 300 °C processed material is substantially harder than the tungsten processed at higher temperatures where recovery and recrystallization can occur. This difference in hardness is likely due to a Hall-Petch type dependence on grain size. It is noted that the 300 °C material retains greater hardness than the materials processed at 1000 °C or 1200 °C even after intermediate annealing between 300 and 1100 °C. In the 1000 °C and 1200 °C processed tungsten, annealing at
Fig. 4. (a) EBSD texture map of ECAE processed tungsten heat-treated at 1175 °C for 1 h. (b) Corresponding GND Density map, and (c) image quality/grain boundary map highlighting the high density of dislocations and low angle grain boundaries within regions of the sample that have not been recrystallized. 4
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Fig. 1. The strengthening coefficient of 3.6 and the dislocation pinning constant 365 are similar to those reported previously, which are 10 and 350 respectively [19]. This data is of primary importance due to the common use of hardness measurements to provide an estimation of grain size. This Hall-Petch curve is unique as the grain sizes are smaller than previously reported due to the low temperature of processing and the high level of plastic strain. 3.5. Heat-treatment effect on mechanical behavior One of the primary goals of this research was to determine the thermal processing window for this heavily cold worked tungsten in order to enable additional cold work. This is an issue with tungsten due to its ability to work harden, which results in very high press loads when conducting ECAE. For many pure metals annealing would be an effective way to reduce the strength; however, with tungsten, recrystallization must be avoided because of its embrittling influence. This study proves useful for establishing the thermal window for subsequent heat-treatment without causing embrittlement. Based on the grain size, hardness and EBSD data, a temperature of 800 °C was chosen, as this temperature was the highest observed without significant recrystallization occurring. The effect of an 800 °C heat-treatment can be seen in Fig. 7, in the flexural stress-strain curves of the asworked and two samples of 800 °C heat-treated material. Heat-treating at 800 °C has no significant impact on the strain to failure while reducing strength by > 500 MPa. It also appears that this heat-treatment decreases the work hardening as compared to the asworked material. This may be due to the dislocation reorientation and locking that occurs with this thermal processing step, however due to the limited data available a definitive cause cannot be determined.
Fig. 5. Vickers hardness data at various annealing temperatures for four pass extrusions on ECAE tungsten processed at 300 °C from this investigation and, 1000 °C and 1200 °C reported elsewhere [7].
hardness, despite their initial hardness being substantially lower than the 300 °C material. This difference is likely due to the greater stored energy in the 300 °C material, which recrystallizes more completely at lower temperatures due to the greater amount of stored lattice strain energy. The similar hardness levels in the fully recrystallized condition indicate that the impurity levels are comparable between the two studies. 3.4. Hall-Petch
4. Discussion
A Hall-Petch plot containing grain size and Vickers hardness data is shown in Fig. 6. The range of grain sizes from sub-micron to nearly 10 μm provides a robust inverse square relationship between grain size and the Vickers hardness for this commercial purity tungsten. The error bars in this figure only indicate standard deviations for Vickers hardness as displaying the small standard deviation in grain size as an inverse square distorts the graph, and has been previously displayed in
An evaluation of the as-worked and annealed tungsten microstructure reveals several important features of the recrystallization and grain growth behaviors of this heavily worked material. Annealing of heavily worked tungsten at well below the recrystallization temperature alters the grain size and grain morphology. While changes in the grain size are not detectable below 700 °C, the continuous decrease in grain aspect ratio indicates that microstructural changes are occurring. The decrease in aspect ratio is likely due to surface energy driven rearrangement of the jagged tungsten grains as
Fig. 7. Room temperature flexural stress-strain curves of a typical as-worked tungsten sample and two separate 800 °C annealed tungsten samples all processed by ECAE route 4A at 300 °C.
Fig. 6. Hall-Petch plot of as-worked and annealed 4A at 300 °C ECAE processed tungsten. 5
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study hot-rolled plate. They showed that recrystallization does not occur at 1100 °C in one-hour anneals but will begin at this temperature given a long enough holding time [21]. They observed recrystallized grains after a 1200 °C one hour anneal. While this aligns with the findings of this work, the 4A300 ECAE-processed material appears to recrystallize a much higher fraction of the microstructure by 1175 °C/ 1 h than the 1200 °C/1 h sheet. Impurity differences could play a role, but we suspect that the higher stored energy of the SPD material is the cause. This can be seen in much earlier work as well. Klopp and Raffo found that rod warm swaged 40% area reduction was only 5% recrystallized after one hour at 1500 °C while an 83% area reduced bar was fully recrystallized after the same treatment [22]. Farrell et al. saw recrystallization in sheet at 1250 °C/1 h but none at 1200 °C [23]. The small difference in one hour recrystallization onset temperature between that work and this one may be due to their application of a five minute 1150 °C heat treatment to the sheet prior to additional annealing. It seems clear that both the initiation and completion of recrystallization are impacted by prior plastic deformation and the resulting stored energy.
seen in Fig. 2. The high density of grain boundaries within these regions is likely to be the driving force behind this change in morphology. As the heat-treatment temperature increases, the increased lattice mobility enables a small increase in grain size between 700 °C and 1100 °C. Based on microstructural observations, this modest grain growth is driven primarily by strain energy stored within the lattice and does not involve the large-scale lattice reorientation that is associated with recrystallization. True recrystallization for the 4A material initiates near 1100 °C, as was reported for tungsten processed at higher temperatures [7]. However, the onset temperature is not as apparent in the Vickers hardness data for the 300 °C-processed tungsten, as the decrease in hardness with an increase in annealing temperature is more gradual. The lack of a distinct transition in the hardness curve is due to the small changes in microstructure that occur with annealing. While the onset of recrystallization occurs at similar annealing temperatures for all three processing temperatures (Fig. 5), its completion is highly dependent on the processing temperature. This dependence is likely due to the smaller grain size and greater stored lattice strain in the 300 °C processed material. The aspect ratio data provides additional information about microstructure changes that are difficult to observe though measurement of either grain size or hardness. The constant decrease in aspect ratio from the as-worked state to 1225 °C suggests that even at temperatures below recrystallization some rearrangement of the microstructure occurs. The aspect ratio data also indicates that recrystallization is complete by the abrupt change in the curve slope above 1225 °C. This change, coupled with grain size trends, indicates complete recrystallization between 1225 °C and 1400 °C. This is consistent with the micrograph of tungsten annealed at 1225 °C, which shows signs of a bimodal grain size distribution, indicating partial recrystallization. The four distinct regions in the grain size vs. annealing temperature curve in Fig. 1 suggest that in each of these regions; as-worked-700 °C, 700 °C–1100 °C, 1100 °C–1300 °C, and > 1300 °C, a different mechanism controls grain growth. In the first region only the highly elongated ends of grains are being absorbed into the surrounding material. These regions have a high fraction of grain boundary volume that requires little energy to reorient. In the second region from 700 °C–1100 °C, the slight increase in grain size suggests that a different mechanism is active and perhaps is driven by stress relief of stored lattice energy through annihilation of dislocations. The third region corresponds to the recrystallization of tungsten and no doubt the grains that form as seen in Fig. 2, are mostly defect free and have almost completely consumed the initial grains, which is confirmed by the EBSD micrographs in Fig. 3. Recrystallization is complete between 1300 °C and 1400 °C, and higher temperatures result in grain growth. Below 700 °C the difference in grain size is not immediately evident, however, some changes are apparent from the decrease in grain aspect ratio. The changes in grain size are obscured by the way in which grain size is measured along the length and width, which are at a maximum and minimum respectively for the heavily elongated material. The similarity between Hall-Petch data presented here and previous investigations is notable in part because of the differences in the grain boundaries. The sintered material used in prior works would likely have relatively higher grain boundary impurity concentration compared to the heavily worked 4A ECAE processed material. The ECAE/SPD processed material comparison in Fig. 5 shows that the material in this study begins to recrystallize at nearly the same point as tungsten deformed at higher temperatures. However, 4A300 recrystallization completes at a lower temperature and the fraction of recrystallized grains is much higher at intermediate temperatures. Conventionally produced tungsten provides a comparison beyond SPD processing. Al, K, and Si doped (AKS) filament is the typical wire form of tungsten, to the extent that a recent study published in this journal failed to specify that the wire in their study was doped [20]. Plate and sheet forms of pure tungsten are commercially available and have been extensively studied. Tsuchida et al. used microhardness and OIM to
5. Findings and conclusions To summarize the key results from this work: 1. Heat treatment at temperatures as low as 300 °C following ECAE processing at 300 °C may cause some slight microstructural changes in pure tungsten - namely smoothing of the coarse interlocking grains and possible annealing out of dislocations. 2. Notable grain growth starts at a temperature around 700 °C, and this type of grain growth occurs up to 1100 °C. 3. Regardless of processing temperature, the onset of recrystallization occurs around 1100 °C for tungsten of this commercial purity. 4. Increasing the processing temperature elevates the temperature at which recrystallization is complete. 5. Grain size and hardness have a strong Hall-Petch type relationship. 6. Grain aspect ratio can be used to determine changes in microstructure that may not be possible through grain size and hardness measurements alone. 7. Heat-treating heavily cold worked tungsten at 800 °C will reduce the strength by around 500 MPa, without a significant reduction in the ductility or strain to failure. 8. ECAE processing produces {111} and {101} texturing along the extrusion directions. The following conclusions are drawn from the results: Through ECAE processing and heat-treating it is possible to exercise a large degree of control over the microstructure and hardness of pure tungsten. It appears that while microstructure changes occur at annealing temperatures as low as 300 °C, different grain remodeling occurs in four separate temperature regions. We hypothesize that in the lowest temperature region, 300-700 °C, changes are caused by the blunting of elongated grains with pointed ends as these features seem to be the only ones affected by heat-treatment in this region. Between 700 and 1100 °C, grain growth is driven by the energy stored within the lattice caused by cold working. Partial recrystallization was observed in heat-treatments as low as 1100 °C, but complete recrystallization required ~1400 °C. Fewer unrecrystallized grains remain in the 300 °C material than in material processed at higher temperatures, in this 1100-1400 °C range. Lattice relaxation and minor grain growth are driven by some activation of slip or increase in diffusion that is not significantly impacted by grain size or dislocation density. However, the temperature of complete recrystallization appears to be independent of processing temperature, grain size, and dislocation density, and thus is dependent only on total stored energy. Our observations of changes in grain size, grain shape, texture and hardness of heavily cold worked tungsten provide increased 6
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opportunity for the application of tungsten in demanding engineering environments for energy and defense.
[9] Z. Jeffries, Metallography of tungsten, Transactions of the American Institute of Mining and Metallurgical Engineers, vol. 60, 1919, pp. 588–643. [10] P.P. Bourque, D.F. Bahr, M.G. Norton, Effect of thermal treatment on failure modes in tungsten wire, Mater. Sci. Eng. A 298 (2001) 73–78. [11] C. Ren, Z.Z. Fang, M. Koopman, B. Butler, J. Paramore, S. Middlemas, Methods for improving ductility of tungsten - a review, Int. J. Refract. Met. Hard Mater. 75 (2018) 170–183 2018/09/01/. [12] J.C. Bilello, Nucleation of brittle fracture in sintered tungsten at low temperatures, Trans. Met. Soc. AIME 242 (703–8) (Apr. 1968) 1968. [13] P. Gumbsch, Brittle fracture and the brittle-to-ductile transition of tungsten, J. Nucl. Mater. 323 (2003) 304–312 Dec. [14] R.W. Margevicius, J. Riedle, P. Gumbsch, Fracture toughness of polycrystalline tungsten under mode I and mixed mode I/II loading, Mater. Sci. Eng. A 270 (1999) 197–209 9/30/. [15] Z.S. Levin, K.T. Hartwig, Strong ductile bulk tungsten, Mater. Sci. Eng. A 707 (2017) 602–611 2017/11/07/. [16] Z.S. Levin, A. Srivastava, D.C. Foley, K.T. Hartwig, Fracture in annealed and severely deformed tungsten, Mater. Sci. Eng. A, 734 734 (2018) 244–254 2018/05/ 03/. [17] J. Baczynski, J.J. Jonas, Texture development during the torsion testing of α-iron and two IF steels, Acta Mater. 44 (1996) 4273–4288 1996/11/01/. [18] D.P. Field, P.B. Trivedi, S.I. Wright, M. Kumar, Analysis of local orientation gradients in deformed single crystals, Ultramicroscopy 103 (2005) 33–39 2005/04/ 01/. [19] U.K. Vashi, R.W. Armstrong, G.E. Zima, The hardness and grain size of consolidated fine tungsten powder, Metal. Transac. 1 (1970) 1769–1771 June 01. [20] C.J.M. Denissen, J. Liebe, M. van Rijswick, Recrystallisation temperature of tungsten as a function of the heating ramp, Int. J. Refract. Met. Hard Mater. 24 (2006) 321–324 7//. [21] K. Tsuchida, T. Miyazawa, A. Hasegawa, S. Nogami, M. Fukuda, Recrystallization behavior of hot-rolled pure tungsten and its alloy plates during high-temperature annealing, Nuclear Mater. Energy 15 (2018) 158–163 2018/05/01/. [22] W.D. Klopp, P.L. Raffo, Effects of Purity and Structure on Recrystallization, Grain Growth, Ductility, Tensile, and Creep Properties of Arc-melted Tungsten, Country unknown/Code not available1964-11-01 (1964). [23] K. Farrell, A.C. Schaffhauser, J.O. Stiegler, Recrystallization, grain growth and the ductile-brittle transition in tungsten sheet, J. Less Common Metals 13 (1967) 141–155 1967/08/01/.
Acknowledgments The authors thank Robert Barber for his assistance in processing, Dr. Anup Bandyopadhyay for his assistance with heat-treatments, and Dr. Shreyas Balachandran, for thoughtful discussions. This work was funded in part by the SMART Scholarship Program as well as Army SBIR Grant W15QKN15C0031. References [1] T. Hirai, F. Escourbiac, S. Carpentier-Chouchana, A. Fedosov, L. Ferrand, T. Jokinen, et al., ITER tungsten divertor design development and qualification program, Fusion Eng. Des. 88 (2013) 1798–1801. [2] E. Lassner, W.-D. Schubert, Tungsten: Properties, Chemistry, Technology of the Elements, Alloys, and Chemical Compounds, Springer Science & Business Media, New York, 1999. [3] W. D. Coolidge, "Tungsten and method of making the same for use as filaments of incandescent electric lamps and for other purposes," ed: Google Patents, 1913. [4] M.R. Ripoll, E. Reisacher, H. Riedel, Texture induced tension–compression asymmetry of drawn tungsten wires, Comput. Mater. Sci. 45 (2009) 788–792. [5] L.J. Kecskes, K.C. Cho, R.J. Dowding, B.E. Schuster, R.Z. Valiev, Q. Wei, Grain size engineering of bcc refractory metals: top-down and bottom-up—application to tungsten, Mater. Sci. Eng. A 467 (2007) 33–43. [6] Z. Pan, Y.Z. Guo, S.N. Mathaudhu, L.J. Kecskes, K.T. Hartwig, Q. Wei, Quasi-static and dynamic mechanical properties of commercial-purity tungsten processed by ECAE at low temperatures, J. Mater. Sci. 43 (2008) 7379–7384. [7] S.N. Mathaudhu, A.J. deRosset, K.T. Hartwig, L.J. Kecskes, Microstructures and recrystallization behavior of severely hot-deformed tungsten, Mater. Sci. Eng. A 503 (2009) 28–31. [8] H. Yuan, Y. Zhang, A.V. Ganeev, J.T. Wang, I.V. Alexandrov, Strengthening and toughening effect on tungsten subjected to multiple ECAP, Mater. Sci. Forum 667669 (2010) 701–706.
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