Crystal Growth Due to Recrystallization upon Annealing Rapid

Apr 9, 2014 - ABSTRACT: Crystal growth due to recrystallization in rapidly solidified microstructures was experimentally observed. Deeply undercooled ...
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Crystal Growth Due to Recrystallization upon Annealing Rapid Solidification Microstructures of Deeply Undercooled Single Phase Alloys Quenched before Recalescence X. L. Xu and F. Liu* State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi, 710072, People’s Republic of China

ABSTRACT: Crystal growth due to recrystallization in rapidly solidified microstructures was experimentally observed. Deeply undercooled Ni−Cu alloy melts were quenched before rapid solidification by Ga−In liquid alloy. After the quenching, partially recrystallized microstructures were obtained and further investigated by means of electron backscattering diffraction and transmission electron microscopy. Dense crystalline defects such as dislocation networks which are typical characteristics of deformation microstructures were observed. After the partially recrystallized microstructures were annealed at different temperatures, further recrystallization in the microstructures was observed. However, few crystalline defects were observed in the naturally cooled alloy with the same undercooling. After rapidly solidified microstructures of the naturally cooled alloy were annealed at identical annealing conditions, no recrystallization was observed. With these directly observed experimental results, a new way to prepare microstructures by crystal growth through recrystallization in rapid solidification microstructures is found.

In this work, completely miscible binary single phase Ni80Cu20 alloys are selected as model alloys. Before rapid solidification, the deeply undercooled alloy melt was quenched by Ga−In liquid alloy. After the as-solidified microstructures were annealed, crystal growth due to recrystallization was observed. A new processing technology to prepare microstructures by crystal growth through recrystallization in rapid solidification microstructures is found. Experimental Procedures. Ni80Cu20 alloy samples, each weighing about 3 g, were prepared by in situ melting of pure Ni pieces (99.99% purity purity) and pure Cu pieces (99.99 wt % purity) under the protection of an argon (Ar) atmosphere in a vacuum chamber. A high purity quartz crucible containing the alloy sample was placed in the center of an induction coil. The melting process was conducted in the vacuum chamber. The vacuum chamber was first evacuated to a pressure of 2 × 10−3 Pa, and then ultrapure Ar was backfilled into the chamber until pressure of the chamber reached 0.05 Pa. The alloy was melted and cooled in the chamber cyclically until a desired undercooling was achieved. Temperature of the sample was monitored by an infrared pyrometer with an accuracy of ±5

Recrystallization is a very complex process happening in annealing experiments on plastically deformed metals and alloys. Traditionally, recrystallization can only be observed in the annealed metals and alloys which were in a cold worked state before annealing. So the only way to make recrystallization happen is to anneal the cold worked microstructures at appropriate temperatures, i.e., the recrystallization temperatures. Recrystallization temperatures are determined by both the nature of the materials and the amount of deformation. However, investigations for decades on rapid solidification of deeply undercooled metallic melts1,2 show that the grain refinement at high undercoolings may be mainly due to the socalled “stress-induced recrystallization”.7−13 As the most probable underlying physical mechanism for the grain refinement occurring at high undercoolings, the stress-induced recrystallization mechanism has been numerously investigated.3−11 However, direct experimental observation of recrystallization in rapid solidification microstructures has been lacking until now. The purpose of this work is to show that crystal growth due to recrystallization upon annealing rapid solidification microstructures of highly undercooled single phase alloys quenched before recalescence is possible. A new processing technology can be realized to improve the recrystalllization processing technology if the processing conditions are well designed. © 2014 American Chemical Society

Received: February 1, 2014 Revised: April 3, 2014 Published: April 9, 2014 2110

dx.doi.org/10.1021/cg500176t | Cryst. Growth Des. 2014, 14, 2110−2114

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K and response time of 10 ms. The deeply undercooled Ni−Cu alloy melts were quenched by Ga−In liquid alloy before recalescence. The as-solidified microstructures were revealed by etching the polished specimens with a mixture of nitric acid water solution (100 mL of HNO3 + 100 mL of H2O). Then, the microstructures were observed by optical microscopy (OP, Olympus GX71) and further studied by transmission electron microscopy (TEM, Technai F30 G2 300 kV) to show the substructures in the microstructures, e.g., dense dislocation networks, etc. The TEM specimens were prepared by standard procedures of mechanical grinding and ion milling. The electron backscatter diffraction specimens were mechanically polished and then polished by vibratory polishing. Electronic backscattering diffraction (EBSD) mapping was obtained by using a field emission gun scanning electron microscope (SEM) equipped with a fully automated electron backscatter diffraction analysis system. In this work, the grain boundaries with misorientation angles lower than 10° are defined as low angle grain boundaries. Results and Discussion. In the present experiments, first, the Ga−In liquid alloy was put into an injection apparatus. Second, this injection apparatus was placed on the top of the inner part of the chamber where the experiments were conducted. The undercooling experiments were executed by applying the traditional cyclical heating−cooling method2 but without the fluxing method2,7 until the desired undercoolings were steadily obtained. Thereafter, the liquid Ga−In alloy in the injection apparatus was directly injected onto the highly undercooled melts before rapid solidification. Therefore, rapid solidification was automatically triggered, and the resultant rapidly solidified microstructures together with the residual liquids were quenched and rapidly cooled. In addition, applying the same method, rapid solidification of alloy melts with the same undercoolings was triggered by a nickel needle, but these alloys were naturally cooled after recalescence. As can be seen from the microstructures of these two kinds of alloys, the average grain size in the microstructures of the quenched alloy (see Figure 1a) is much smaller than that in the naturally cooled alloy (see Figure 2a). The EBSD mapping (see Figure 1a) of the quenched alloy shows that there are a number of twinned crystals. Therefore, it can be inferred that the microstructures of the quenched alloy are partial recrystallization microstructures. Moreover, it can be seen from the pole figure (PF) in Figure 1c that the textures are nearly random. Misorientation is defined as the crystallography orientation difference between two adjacent grains. The frequency of a certain misorientation angle is defined as the length of the grain boundaries with the same certain misorientation angle divided by the total length of grain boundaries in the investigated area in the EBSD mapping. EBSD enables an easy identification of the ∑ 3 twin boundary ( 60°) between two mirror symmetrical crystals. As can be seen in Figure 1d, for the quenched alloy, a fraction of about 4.1% of grain boundaries is identified as ∑ 3 twin boundaries. In addition, the fraction of the low angle grain boundaries is about 75.2%. Figure 2a shows the EBSD mapping of the microstructures of the naturally cooled alloy. It can be seen from the PF in Figure 2c that the textures are nearly random. Figure 2d shows the misorientation angle distribution figure of the grain boundaries in the naturally cooled alloy. It is shown that the fraction of the high angle grain boundaries is about 62.5%, and the fraction of the misorientation angle of 60° which is the identification of the

Figure 1. (a) EBSD orientation mapping of the microstructures of the highly undercooled Ni−Cu alloy quenched after recalescence with undercooling ΔT = 225 K; (b) grain boundaries of (a); (c) the pole figure of (a) showing a nearly random texture; (d) misorientation distribution figure of the grain boundaries in (a).

Figure 2. (a) EBSD orientation mapping of the microstructures of the highly undercooled Ni−Cu alloy naturally cooled after recalescence with ΔT = 225 K; (b) grain boundaries of (a); (c) the inverse pole figure of (a) showing a nearly random texture; (d) misorientation distribution figure of the grain boundaries in (a).

∑ 3 twin boundaries occupies about 19.68%. Obviously, compared to Figure 1d, Figure 2d shows increases of volume fractions of not only the high angle grain boundaries but also the ∑ 3 twin boundaries. Therefore, it can be inferred from the changes of the volume fractions of the high angle grain boundaries and the ∑ 3 twin boundaries that the microstructures of the naturally cooled alloy obviously had recrystallized into more fully recrystallized microstructures. Deformation substructures such as dense dislocation networks (see Figure 3a−c) were also observed in the grains in the quenched alloy. These dense dislocation networks are very similar to the dislocation cell boundaries commonly observed in the cold worked metals and alloys.14,15 Furthermore, these dislocation cell boundaries are of different average sizes and linked to each other, as can be seen in Figure 3b. Figure 3c 2111

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Figure 4. (a−c) TEM bright field image of the rapidly solidified Ni− Cu alloy naturally cooled with ΔT = 225 K; (d) selected area electron diffraction pattern (SAED) of a twin boundary in (c).

Figure 3. TEM bright field image of substructures in the rapidly solidified Ni80Cu20 alloy quenched with ΔT = 225 K: (a−c) dense dislocation networks in deformed grains; (d) the SAED pattern of a high angle grain boundary in (c).

are twins in this alloy; see the typical SAED of twins as shown in Figure 4d. Recrystallization is a very complex process that proceeds by thermally activated site saturation nucleation and subsequent growth of newly formed strain free crystal nucleus at the expense of the deformed grains in metals and alloys.14,15 In order to reveal the recrystallization in rapid solidification microstructures of deeply undercooled alloys, recrystallization annealing experiments were performed for the abovementioned alloys at different annealing conditions. After the quenched alloy were annealed with a undercooling of about 225 K (see Figure 5a) at 973 K for about 30 min, the microstructures further recrystallized into microstructures due to newly formed recrystallization grains and their subsequent recrystallization growth (Figure 5b). After quenched alloy was

shows a recrystallizing grain at the expense of the deformed grains. As can be seen in Figure 3c, the recrystallization fronts of this recrystallizing grain are consuming the deformed grains; see the dense dislocations in the lower right corner in Figure 3c. Thus, in the present case, the microstructures in the quenched alloy are partially recrystallized microstructures. It is the stored energy due to the dense dislocations that allows the quenched alloy to still have substantial driving force for recrystallization. Some substructures which are not deformation microstructures also are present. The white circled area in Figure 3b shows the secondary dendrites grown from the secondary mushy zone, which is called “the second mushy zone solidification”.7 This second mushy zone solidification is the solidification of the residual alloy melts during the natural cooling process after rapid recalescence. Because the primary dendrite arms in these secondary solidification dendrites are kept away by distance from each other, the secondary dendrite arms are developed and can be observed distinctly. Meanwhile, it can be found that the parallel aligned dendrites grown from the above-mentioned second mushy zone are surrounded by a dense dislocation network. It is well-known that sometimes a small amount of impurities can affect recrystallization. To test whether the Ga− In diffused into the Ni−Cu grains, we have made many EDS line scanning tests on the microstructures of the quenched alloy. The results from the EDS tests show that there are no Ga−In in the grains of the quenched alloy. So we can rule out the possibility that Ga−In would go into the Ni−Cu grains and affect its recrystallization behavior. Further, it can be inferred from the EDS results that the compositions of the residual alloy in the white circled areas in Figure 3b are still the initial composition of the alloy. The substructures in the naturally cooled alloy were also revealed by TEM. It is found that there are few visible dislocations in the grains of the naturally cooled alloys, as shown in Figure 4a−c. It should be pointed out that the extinction fringes (also called the extinction contours) in Figure 4a−c should not be confused with dislocations. However, there

Figure 5. Microstructures of the quenched Ni80Cu20 alloys with ΔT = 225 K (a) and 240 K (c); (b) and (d) the quenched alloys of (a) annealed at 973 K for about 30 min and (c) annealed at 1073 K for about 20 min, respectively. 2112

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deformation textures are responsible for the recrystallization textures.14,16 However, it is well-known that in rapid solidification of highly undercooled metal and alloy melts, the remelting process of the primarily formed dendrites will produce a large amount of randomly oriented dendrite piece grains in the as-solidified microstructures.4 In addition, rapid solidification would produce large stress which would make the primarily formed dendrites highly deformed and even break up into randomly dispersed grain pieces.7 Thus, after rapid solidification, the microstructures consist of randomly oriented dendrite piece grains, and many of them are in “cold worked state”. Due to the initial randomly textured microstructures of the as-solidified alloy, the recrystallized microstructures whose textures are highly related to that of the initial microstructures will have nearly random textures, as can be seen in Figure 6c. It can be seen from the misorientation distribution in Figure 6d that the volume fraction of the low angle grain boundaries in Figure 6a is about 74.03%. It should be noted that the microstructures also have about 5.52% ∑ 3 twin boundaries, which indicate the presence of twins in the microstructures. It is obvious that these microstructures have typical characteristics of the recrystallization microstructures. However, after the microstructures of the naturally cooled alloy were annealed at identical conditions, the microstructures of this alloy transformed into coarse grained microstructures with a typical grain size of about 200 μm. Meanwhile, the grains are irregularly shaped with curved grain boundaries. The EBSD mapping in Figure 7a shows that grain growth had substantially occurred

annealed with a undercooling of about 240 K (see Figure 5c) at 1073 K for about 20 min, the fine microstructures also further recrystallized into microstructures due to newly formed recrystallization grains and their subsequent recrystallization growth (Figure 5d). However, care should be taken so that the recrystallization observations are not falsified by grain growth occurring at the same time toward the end of the recrystallization process.15 To test whether or not grain growth had participated in these microstructural transformations, the microstructures of the naturally cooled alloy were annealed at identical conditions, but recrystallization did not happen in the annealed microstructures. Therefore, it can be concluded that compared to the naturally cooled alloy, much more driving force for recrystallization was preserved in the microstructures of the quenched alloy. Thus, the quenched alloy would undergo recrystallization at appropriate annealing conditions. However, for the naturally cooled alloy, the recrystallization driving force in the microstructures had been largely dissipated during the recrystallization process in the natural cooling process. Therefore, when the naturally cooled alloy was annealed, its microstructures underwent no recrystallization. After the quenched alloy were annealed at 1273 K for about 30 min, its microstructures recrystallized into very coarse equiaxed grained microstructures with a typical grain size of about 80 μm; the grain boundaries are sharp and straight, as can be seen in Figure 6a. The EBSD mapping in Figure 6a shows

Figure 7. (a) EBSD orientation mapping of the microstructures of the naturally cooded Ni−Cu alloy with ΔT = 225 K after annealing at 1273 K for about 30 min; (b) the misorientation angle distribution figure of the grain boundaries in (a). Figure 6. (a) EBSD orientation mapping of the microstructures of the quenched Ni−Cu alloy with ΔT = 225 K after annealing at 1273 K for about 30 min; (b) the grain boundaries of (a); (c) the IPF of (a); (d) the misorientation angle distribution figure of the grain boundaries in (a).

during the annealing process of the naturally cooled alloy. As can be seen from Figure 7b, it is noted that the microstructures also have 17.86% ∑ 3 twin boundaries. This result indicates that the naturally cooled alloy also has a small amount of stored energy for recrystallization. Obviously it is mainly the abnormal grain growth that had substantially occurred in this annealed naturally cooled alloy. Therefore, the transformation mechanisms of the annealed microstructures of these two kinds of alloys are very different. For the quenched alloy, the microstructural transformation is mainly due to recrystallization, while the transformation in the naturally cooled alloy is mainly a grain growth process. Conclusions. Deeply undercooled Ni80Cu20 alloy melts were quenched by Ga−In liquid alloy before rapid solidification. Deformation microstructures such as dense dislocation networks were observed in the as-quenched microstructures. After further annealing of these partially recrystallized as-quenched microstructures, crystal growth due to recrystallization was obtained. However, few crystalline defects were observed in the

that the grains are mainly annealing twinned crystals. The textures of the microstructures are nearly random, as can be seen from the PF in Figure 6c. Our explanation for the nearly random textures is that in tradition physical metallurgy, the microstructures of the undeformed metals or alloys are usually highly oriented. After deformation, the orientation of the grains in the microstructures of the cold worked metals or alloys is related to that of the undeformed matrix. On the other side, recrystallization of a cold worked grain is the development of a newly formed strain free grain with a new orientation. The orientations of the recrystallized grains are highly related to the highly textured cold worked grains. Thus, the recrystallized grains are usually highly “textured”. So, it is considered that the 2113

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(16) Hong, S. H.; Lee, D. N. Evolution of the Cube Recrystallization Texture in Cold Rolled Copper Sheets. Mater. Sci. Eng., A 2003, 351, 133−147.

naturally cooled alloy with the same undercooling. After rapidly solidified microstructures of the naturally cooled alloy were annealed at identical annealing conditions, no recrystallization was observed. With these directly observed experimental results, a new processing technology to prepare microstructures by crystal growth through recrystallization in rapid solidification microstructures is found.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published to the web on April 4, 2014, with an error to the first paragraph of the Experimental Procedures section, the seventh paragraph of the Results and Discussion Section, and the corresponding author's email. The corrected version was reposted April 18, 2014.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of China National Funds for Distinguished Young Scientists (51125002) and National Basic Research Program of China (973 Program, 2011CB610403), the Fundamental Research Fund of Northwestern Polytechnical University (JC20120223, JC2001134), the 111 Project B08040.

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dx.doi.org/10.1021/cg500176t | Cryst. Growth Des. 2014, 14, 2110−2114