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TEM investigations of recrystallization in rapidly solidified Ni-Fe-Pb ternary alloy
Materials Characterization 127 (2017) 73–76
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
TEM investigations of recrystallization in rapidly solidified Ni-Fe-Pb ternary alloy Z. Chen a,⁎, T. Liang a, Y. Zhang a, L.C. Feng a, X.Q. Yang b,⁎, Y. Fan a a b
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China School of Chemical and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China
a r t i c l e
i n f o
Article history: Received 17 October 2016 Received in revised form 1 February 2017 Accepted 2 February 2017 Available online 01 March 2017 Keywords: Undercooling Stress accumulation Solidification Defects Recrystallization
a b s t r a c t Using the molten glass purification combined with the cycle superheating methods, a refined grain was achieved in undercooled Ni-Fe-Pb ternary alloy. The characteristics of microstructures and substructures of the highundercooled refined grain were investigated by the transmission electron microscopy and the stress accumulation model. The grain refinement occurring at high undercooling was induced by the plastic deformation of dendrites and subsequent recrystallization, which was evidenced by high densities of substructures, e.g. dislocations, subgrains and migration of high angle grain boundaries. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Rapid solidification of bulk undercooled melts has been studied extensively since it is a good method for preparation of metals in metastable states, such as grain-refined materials [1,2]. Two grain refinement events taking place at low and high undercoolings of undercooled metallic melts have been detected and evidenced in a variety of alloys, such as Cu-Ni [3], Fe-Ni [4], Ni-C [5] and Ni-Pd [6], etc. It has been widely accepted that the grain refinement at low undercooling is induced by remelting of dendrites in postrecalescence [6]. However, the mechanism of grain refinement at high undercooling is still under debate, especially for the multicomponent alloys (such as Ni-Fe-Pb alloy, an important alloy used in engineering) [7–9]. Several mechanisms have been proposed for explaining the grain refinement at high undercooling, such as dendrites breakup owing to remelting upon recalescence [10–12], stress induced recrystallization [13], etc. As is known, if hypercooling is achieved in solidification, the whole alloy melt will completely solidify during rapid recalescence and the remelting effect will not work, since no remaining liquid exists [1,2]. So it is inclined to be accepted that the grain refinement at high undercooling results from recrystallization, and the recrystallization originates from the stress induced by shrinkage [1]. Upon rapid solidification, the rapid liquid/ solid (L/S) transformation leads to sharp volume shrinkage, thus resulting ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Chen), [email protected] (X.Q. Yang).
http://dx.doi.org/10.1016/j.matchar.2017.02.002 1044-5803/© 2017 Elsevier Inc. All rights reserved.
in solidification stress. The substantial stress originating from rapid solidification can surmount the yielding stress of the solidified skeleton and leads to plastic deformation in the as-solidified structure. Subsequently, the stored energy due to plastic deformation provides driving force for the recrystallization occurring after rapid recalescence [1]. Several alloys have been used to evidence the recrystallized mechanism, such as the undercooled Ni-Cu [14], Co-Pd [13], Ni-Sn [15] alloys. However, details of the recrystallization are still unclear for lack of enough convincing experimental evidences and not extended to multicomponent alloys. In the present paper, Ni-1.5 at.% Fe-1 at.% Pb was selected as an experimental alloy. The recrystallization process after recalescence can be detected after rapid solidification, so the stored energy accumulated in the as-solidified structure can be preserved, e.g. in the form of lattice defects. The direct experimental evidences and the recrystallization mechanism at high Δ T were investigated. 2. Experimental details Applying glass fluxing combined with cyclic superheating, high undercooling for Ni-Fe-Pb alloy samples was achieved. High purity elements of nickel (Ni), iron (Fe) and plumbum (Pb), better than 99.95 wt% were alloyed in situ to form 5 g samples which with the stoichiometric proportion of Ni-1.5 at.% Fe-1 at.% Pb. The undercooling experiments were carried out in quartz tubes in a high-frequency induction melting apparatus under the protection of B2O3 glass slag so as to denucleate the alloy by reaction, adsorption, and passivation of the foreign catalytic site. Subsequently, the alloy was induction melted, cyclically superheated and cooled
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under the protection of the molten flux until the desired undercooling was achieved. The thermal behavior was measured and recorded by infrared thermometer with a respond time of 1 ms and a measurement accuracy of ± 2 K. The specimens were etched by using a solution composed of 5 g FeCl3, 10 mL HCl and 100 mL alcohol for microstructural analysis on a PMG3-OLYMPUS optical microscope (OM). The grain size was determined by a Leco image analysis software. The microstructures were observed by transmission electron microscopy (TEM; JEM 2100). The detailed information of the TEM specimen preparation and characterization parameters was shown as follows: thin disks about 0.3 mm thickness were cut from the specimen's center by wire-electrode cutting for TEM observation. These disks were made electron transparent using a MTP-1A jet electropolisher. The electrolyte used for the polishing was a solution of 97% ethanol and 3% perchloric acid. The electropolishing condition for Ni-Fe-Pb alloy was −20 °С. The high-resolution transmission electron microscopy (HRTEM) images and the corresponding selected-area electron diffraction (SAED) patterns were performed using a TEM at an operating voltage of 200 kV.
3.2. The formation mechanism of solidification microstructures beyond ΔT⁎ The obvious grain refinement event occurs when Δ T ≥ Δ T⁎. The equixed grains with rather straight GBs (see Fig. 2g) strongly suggest that the formation of the solidification microstructure is due to recrystallization. Recrystallization is defined as the formation of a relatively defect free structure at the expense of the deformed matrix, whose driving force is stored energy in the plastically deformed matrix, in the form of lattice defects, i.e. dislocations [1]. Recrystallization occurs in the solid and leads to refined grains. The argument for recrystallization is also in agreement with the existence of the annealing twins in the microstructure, as indicated by circles in Figs. 1f and g and 3b. When rapid solidification is taking place at a rather high undercooling, the advance of the S/L interface proceeds extremely fast (see the dendrite growth velocity in Fig. 4 [16]). Consequently, a severe volume contraction due to rapid solidification will cause a high shrinkage stress developing in the system. A model was developed to calculate the shrinkage
3. Results and discussion 3.1. Solidification structures and sub-microstructures of the undercooled alloy The as-solidified morphologies subject to various Δ Ts and undercooled experimental curves of Ni-1.5 at.% Fe-1 at.% Pb alloy at 260 K are shown in Figs. 1 and 2, respectively. With increasing ΔT, the as-solidified morphologies subject to an obvious transition process. When the melt is suffered to a small undercooling (ΔT b 140 K), typical dendrites with cross-branching are formed (Fig. 1a–d). For ΔT ≥ 140 K, the granular crystals under high ΔT are solidified, as shown in Fig. 1e–g. Therefore, Δ T⁎ = 140 K is defined as the critical undercooling of the grain refinement under large ΔT. Moreover, the microstructure corresponding to the second grain refinement happening in the range of ΔT ≥ ΔT⁎ consists of equiaxed grains with rather straight grain boundaries (GBs), and partial grains contain twins, as indicated by circles (see Fig. 1f and g). By the way, the grain sizes reduce with the increase of ΔT when ΔT ≥ ΔT⁎ as shown in Fig. 2. The growth process occurs in the recalescence stage. The dendrite grow speed also increases with Δ T, especially under high Δ T, grows up at extremely high speed. Because the melt solidification occurs in a short moment accompanying recalescence and release of latent heat, the solid phase will be heated from a lower temperature to a higher temperature (sometimes melting point) in this process. When Δ T ≤ Δ T ⁎ , the melts are heated to melting point and the as-solidified dendrite skeleton will inevitably be remelted, so the first refinement is resulted, as shown in Figs. 1 and 2. When Δ T ≥ Δ T⁎, especially in large undecooling, the temperature after recalescence is far below the melting point, so the heat released by recalescence cannot destroy the initial dendrite morphology, thus cannot cause the grain refinement. Generally, the grain refinement at a large Δ T attributes to the stress that originates from the extremely rapid solidification process, which causes the dendrite fragmentation finally [1]. The above deduction can be verified in Figs. 1e–g, where the GBs in the grain-refined microstructures appear very narrow and straight. Since the characters of the sub-GBs are similar to those in the grains of plastic deformation [1], more experimental analysis (such as TEM analysis) are needed to verify the above judgment. Fig. 3 reveals the sub-microstructure in the grains of Ni-Fe-Pb alloy solidified at Δ T = 260 K. As shown in Fig. 3, high densities of dislocations (Fig. 3a), twin boundaries (Fig. 3b) can be observed clearly. These microstructural characteristics imply a recrystallization microstructure in the undercooled Ni-Fe-Pb alloy when Δ T ≥ Δ T⁎.
Fig. 1. The as-solidified morphologies subjected to various ΔTs: (a) ΔTs = 0 K; (b) ΔTs = 45 K; (c) ΔTs = 70 K; (d) ΔTs = 120 K; (e) ΔTs = 140 K; (f) ΔTs = 180 K; (g) ΔTs = 260 K.
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Fig. 2. Grain sizes detected at different melt undercoolings and recalescence curves of the Ni-Fe-Pb alloy undercooled by 70 and 260 K.
stress during rapid solidification and can be expressed by the following equation, σ s ðg s Þ ¼
160μa2 g l 2 g R f s t f λ22 s
g s −g coh þ S
! 1 1 1−g s βs − þ 2ln 1−g s 1−g coh 1−g coh S S ð1Þ
where a is the length of solid-liquid mixing zone in recalescence, μ is the viscosity coefficient, fRs is the solid faction after recalescence, λ 2 is the secondary dendrite arm spacing, t f is the solidification time of recalescence process, g l is the volume fraction of liquid, gs is the volume of solid, where g coh b g S b fRs . g coh is the minimum of S S solid volume fraction for the formation of dendrite skeleton, f Rs (= C P / ΔH f (T R − T n )) is the solid fraction after recalescence, where T R is the maximum recalescence temperature and T n is the nucleation temperature (which can obtained from recalescence curves), in which the relationship between fRs and Δ T can be calculated by the solute conversation using the physical parameters in Table 1, as shown in Fig. 4. The detailed derivation of the present model has been shown in Ref. [1]. Once σs(gs) exceeds the yielding strength of Ni-Fe-Pb alloy, the dendrites formed will be plastically
Fig. 3. The sub-microstructure in the grains of Ni-Fe-Pb alloy solidified at ΔT = 260 K.
deformed, and a storage of microstrain in the microstructure will be the resulted [15]. Further increasing ΔT causes a continuous increase of the stored microstrain, resulting in a continuous increase of the driving force for recrystallization. Once the driving force is high enough to initiate the nucleation, the recrystallization will take place. Consequently, a refined recrystallized microstructure can be obtained. Since the annealing twins are a typical characteristic of the recrystallized metal with fcc structure, its appearance (Fig. 3b) further evidences the occurrence of the recrystallization [17]. Extra free energy of the cold-worked state, which is difficult to be measured, is the driving force for recovery and recrystallization. A major part of the stored energy will be attributed to the introduction of extra dislocations in the system [18]. In order to provide more reliable evidence for the recrystallization mechanism of the grain refinement at Δ T ≥ Δ T ⁎ , the sub-GBs and many large-angle boundaries can be also detected in Fig. 5. There are many high-angle boundaries in the organization of each specimen. Fig. 5a shows the bright-field TEM image of two adjoining grains which are separated by the high-angle boundary in the as-solidified Ni-Fe-Pb alloy (Δ T = 260 K). Fig. 5b shows that SAED patterns of two adjoining grains in Fig. 5a. The present two
Fig. 4. Calculated dendrite growth velocity and stress accumulated in the dendrite skeleton upon rapid solidification as a function of initial undercooling.
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Table 1 Physical parameters for calculated stress development in dendrite network. Parameters
Dimension
Value
α μ tf βs gcoh s λ2
m Pa s s – – m
0.01 5 × 10−3 0.1 0.06 0.15 3 × e−5
adjoining grains have different diffraction spots on the specific zone axis orientation, such as the index of lattice planes on the left side of the GB is (2 2 0), (3 1 1) and (1 − 1 1), while on the right side is (3 − 1 − 2), (4 0 2) and (1 1 1). The plane angles on the left side is 51°, while on the right side is 59°. It is obvious that there are different structures and crystallographic orientations on both sides of the GB. To further determine the structural difference in both sides and to verify the accuracy of the above calibration, the HRTEM is used to know the arrangement of lattice atoms. The HRTEM result also confirms that the planes with the same index (1 1 1) on both sides of GB has a high misorientation and the plane spacing is also different. The formation of annealing twins in fcc metals and alloys are due to growth accidents on {1 1 1} propagating steps present on migrating GBs. As a result of the accidents, Shockley partial dislocations are generated contiguously to GBs. These partial dislocations repel each other and glide away from the boundaries to produce twin boundaries [18]. That is to say, the density of annealing twins increases with the increase of the deformation degree (Fig. 3). The differences of adjoining grains detected by SAET and HRTEM demonstrate that the recrystallization already happened. Recrystallization is a process of new distortionless grains instead of deformed grains. The new formed grains grow up with highangle GBs in this process, since the straight high-angle GBs are of the lowest energy. Because the accumulated stress of dendrite leads to more serious deformation of the dendritic skeleton with increasing undercooling, more re-crystal nuclei will be generated by recrystallization, so refinement phenomenon occurs [18]. Most actual applied materials are multicomponent alloys whose solidification processes are very complex. Thus it is of great importance to research the rapid solidification of multicomponent alloys. In the past few years, more and more research activities were
dedicated to multiphase solidification of ternary and multicomponent alloys [19]. The present experiments showed that the second component Pb induced the obvious microstructural refinement of Ni-Fe alloy. As for the undercooled Ni-Fe alloys, the grain sizes of natural cooling samples were above 100 μm (even reached 900 μm at ΔT = 230 K [20]) in all of the undercooled range [20]. The grain growth after recrystallization can be inhibited by the strongly segregated atoms Pb. Due to the inhibited effect of Pb atoms in the grain boundaries, the grain size of as-solidified NiFe-Pb reduced to about 20 μm. The in-depth study of stabilized effect induced by multicomponent alloys has been carried out in other paper [21]. 4. Conclusions Using the molten glass purification combined with the cycle superheating methods, TEM observation of recrystallization in rapidly solidified Ni-Fe-Pb ternary alloy was presented. The main conclusions are as follows. 1. A refined grain was achieved in undercooled Ni-Fe-Pb ternary alloy. The characteristics of microstructures and substructures of the high-undercooled refined grain were investigated by the transmission electron microscopy and the stress accumulation model. 2. The grain refinement occurring at high undercooling was induced by the plastic deformation of dendrites and subsequent recrystallization, which was evidenced by high densities of substructures, e.g. dislocations, subgrains and migration of high angle grain boundaries. Acknowledgements The work was supported by the Fundamental Research Funds for the Central Universities (2015QNA27) and China Postdoctoral Science Foundation (2016M591953). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Fig. 5. (a) The bright-field TEM images of two adjoining grains in the as-solidified Ni-Fe-Pb alloy (ΔT = 260 K); (b) the SAED patterns of the circled regions in (a) and (c) at both side of the boundary and the zone axis is [−1 1 2], with insert of (a) the magnified HRTEM morphologies.
F. Liu, G.C. Yang, Int. Mater. Rev. 51 (2006) 145–170. D.M. Herlach, Mater. Sci. Eng. R12 (1994) 177–272. M. Schwarz, A. Karma, K. Eckler, D.M. Herlach, Phys. Rev. Lett. 73 (1994) 1380–1383. T.Z. Kattamis, J. Cryst. Growth 34 (1976) 215–220. K. Eckler, A.F. Norman, F. Gärtner, A.L. Greer, D.M. Herlach, J. Cryst. Growth 173 (1997) 528–540. S.Y. Lu, J.F. Zhou, J. Cryst. Growth 309 (2007) 103–111. N. Wang, B. Wei, J. Cryst. Growth 247 (2003) 576–586. H.P. Wang, J. Chang, B. Wei, Phys. Lett. A 374 (2010) 2489–2493. K. Wang, H.F. Wang, F. Liu, H.M. Zhai, Acta Mater. 61 (2013) 1359–1372. H.F. Wang, F. Liu, G.C. Yang, Y.H. Zhou, Acta Mater. 58 (2010) 5402–5410. H.F. Wang, F. Liu, G.C. Yang, Y.H. Zhou, Acta Mater. 58 (2010) 5411–5419. Elinor G. Castle, Andrew M. Mullis, Robert F. Cochrane, Acta Mater. 66 (2014) 378–387. X.L. Xu, Y.Z. Chen, F. Liu, J. Cryst. Growth 377 (2013) 153–159. T. Zhang, F. Liu, H.F. Wang, G.C. Yang, Scr. Mater. 63 (2010) 43–46. Q. Chen, Z. Chen, F. Liu, R.X. Cui, T. Liang, J. Alloys Compd. 638 (2015) 109–114. W.J. Boettinger, S.R. Coriell, R. Trivedi, Rapid Solidification Processing: Principles and Technologies IV, Claitor's, Baton Rouge, LA, 1988 13–24. F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Second ed., 2004. J.W. Christian, Recovery, recrystallization and grain growth, The Theory of Transformation in Metals and Alloys. Pergamon Press, Oxford, 832–858. W.L. Wang, Z.Q. Li, B. Wei, Acta Mater. 59 (2011) 5482–5493. Z. Chen, Q. Chen, C.J. Shen, F. Liu, J. Alloys Compd. 646 (2015) 983–989. T. Liang, Z. Chen, X.Q. Yang, J.Y. Zhang, P. Zhang, Int. J. Mater. Res. (submitted).
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
TEM investigations of recrystallization in rapidly solidified Ni-Fe-Pb ternary alloy Z. Chen a,⁎, T. Liang a, Y. Zhang a, L.C. Feng a, X.Q. Yang b,⁎, Y. Fan a a b
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China School of Chemical and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China
a r t i c l e
i n f o
Article history: Received 17 October 2016 Received in revised form 1 February 2017 Accepted 2 February 2017 Available online 01 March 2017 Keywords: Undercooling Stress accumulation Solidification Defects Recrystallization
a b s t r a c t Using the molten glass purification combined with the cycle superheating methods, a refined grain was achieved in undercooled Ni-Fe-Pb ternary alloy. The characteristics of microstructures and substructures of the highundercooled refined grain were investigated by the transmission electron microscopy and the stress accumulation model. The grain refinement occurring at high undercooling was induced by the plastic deformation of dendrites and subsequent recrystallization, which was evidenced by high densities of substructures, e.g. dislocations, subgrains and migration of high angle grain boundaries. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Rapid solidification of bulk undercooled melts has been studied extensively since it is a good method for preparation of metals in metastable states, such as grain-refined materials [1,2]. Two grain refinement events taking place at low and high undercoolings of undercooled metallic melts have been detected and evidenced in a variety of alloys, such as Cu-Ni [3], Fe-Ni [4], Ni-C [5] and Ni-Pd [6], etc. It has been widely accepted that the grain refinement at low undercooling is induced by remelting of dendrites in postrecalescence [6]. However, the mechanism of grain refinement at high undercooling is still under debate, especially for the multicomponent alloys (such as Ni-Fe-Pb alloy, an important alloy used in engineering) [7–9]. Several mechanisms have been proposed for explaining the grain refinement at high undercooling, such as dendrites breakup owing to remelting upon recalescence [10–12], stress induced recrystallization [13], etc. As is known, if hypercooling is achieved in solidification, the whole alloy melt will completely solidify during rapid recalescence and the remelting effect will not work, since no remaining liquid exists [1,2]. So it is inclined to be accepted that the grain refinement at high undercooling results from recrystallization, and the recrystallization originates from the stress induced by shrinkage [1]. Upon rapid solidification, the rapid liquid/ solid (L/S) transformation leads to sharp volume shrinkage, thus resulting ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Chen), [email protected] (X.Q. Yang).
http://dx.doi.org/10.1016/j.matchar.2017.02.002 1044-5803/© 2017 Elsevier Inc. All rights reserved.
in solidification stress. The substantial stress originating from rapid solidification can surmount the yielding stress of the solidified skeleton and leads to plastic deformation in the as-solidified structure. Subsequently, the stored energy due to plastic deformation provides driving force for the recrystallization occurring after rapid recalescence [1]. Several alloys have been used to evidence the recrystallized mechanism, such as the undercooled Ni-Cu [14], Co-Pd [13], Ni-Sn [15] alloys. However, details of the recrystallization are still unclear for lack of enough convincing experimental evidences and not extended to multicomponent alloys. In the present paper, Ni-1.5 at.% Fe-1 at.% Pb was selected as an experimental alloy. The recrystallization process after recalescence can be detected after rapid solidification, so the stored energy accumulated in the as-solidified structure can be preserved, e.g. in the form of lattice defects. The direct experimental evidences and the recrystallization mechanism at high Δ T were investigated. 2. Experimental details Applying glass fluxing combined with cyclic superheating, high undercooling for Ni-Fe-Pb alloy samples was achieved. High purity elements of nickel (Ni), iron (Fe) and plumbum (Pb), better than 99.95 wt% were alloyed in situ to form 5 g samples which with the stoichiometric proportion of Ni-1.5 at.% Fe-1 at.% Pb. The undercooling experiments were carried out in quartz tubes in a high-frequency induction melting apparatus under the protection of B2O3 glass slag so as to denucleate the alloy by reaction, adsorption, and passivation of the foreign catalytic site. Subsequently, the alloy was induction melted, cyclically superheated and cooled
74
Z. Chen et al. / Materials Characterization 127 (2017) 73–76
under the protection of the molten flux until the desired undercooling was achieved. The thermal behavior was measured and recorded by infrared thermometer with a respond time of 1 ms and a measurement accuracy of ± 2 K. The specimens were etched by using a solution composed of 5 g FeCl3, 10 mL HCl and 100 mL alcohol for microstructural analysis on a PMG3-OLYMPUS optical microscope (OM). The grain size was determined by a Leco image analysis software. The microstructures were observed by transmission electron microscopy (TEM; JEM 2100). The detailed information of the TEM specimen preparation and characterization parameters was shown as follows: thin disks about 0.3 mm thickness were cut from the specimen's center by wire-electrode cutting for TEM observation. These disks were made electron transparent using a MTP-1A jet electropolisher. The electrolyte used for the polishing was a solution of 97% ethanol and 3% perchloric acid. The electropolishing condition for Ni-Fe-Pb alloy was −20 °С. The high-resolution transmission electron microscopy (HRTEM) images and the corresponding selected-area electron diffraction (SAED) patterns were performed using a TEM at an operating voltage of 200 kV.
3.2. The formation mechanism of solidification microstructures beyond ΔT⁎ The obvious grain refinement event occurs when Δ T ≥ Δ T⁎. The equixed grains with rather straight GBs (see Fig. 2g) strongly suggest that the formation of the solidification microstructure is due to recrystallization. Recrystallization is defined as the formation of a relatively defect free structure at the expense of the deformed matrix, whose driving force is stored energy in the plastically deformed matrix, in the form of lattice defects, i.e. dislocations [1]. Recrystallization occurs in the solid and leads to refined grains. The argument for recrystallization is also in agreement with the existence of the annealing twins in the microstructure, as indicated by circles in Figs. 1f and g and 3b. When rapid solidification is taking place at a rather high undercooling, the advance of the S/L interface proceeds extremely fast (see the dendrite growth velocity in Fig. 4 [16]). Consequently, a severe volume contraction due to rapid solidification will cause a high shrinkage stress developing in the system. A model was developed to calculate the shrinkage
3. Results and discussion 3.1. Solidification structures and sub-microstructures of the undercooled alloy The as-solidified morphologies subject to various Δ Ts and undercooled experimental curves of Ni-1.5 at.% Fe-1 at.% Pb alloy at 260 K are shown in Figs. 1 and 2, respectively. With increasing ΔT, the as-solidified morphologies subject to an obvious transition process. When the melt is suffered to a small undercooling (ΔT b 140 K), typical dendrites with cross-branching are formed (Fig. 1a–d). For ΔT ≥ 140 K, the granular crystals under high ΔT are solidified, as shown in Fig. 1e–g. Therefore, Δ T⁎ = 140 K is defined as the critical undercooling of the grain refinement under large ΔT. Moreover, the microstructure corresponding to the second grain refinement happening in the range of ΔT ≥ ΔT⁎ consists of equiaxed grains with rather straight grain boundaries (GBs), and partial grains contain twins, as indicated by circles (see Fig. 1f and g). By the way, the grain sizes reduce with the increase of ΔT when ΔT ≥ ΔT⁎ as shown in Fig. 2. The growth process occurs in the recalescence stage. The dendrite grow speed also increases with Δ T, especially under high Δ T, grows up at extremely high speed. Because the melt solidification occurs in a short moment accompanying recalescence and release of latent heat, the solid phase will be heated from a lower temperature to a higher temperature (sometimes melting point) in this process. When Δ T ≤ Δ T ⁎ , the melts are heated to melting point and the as-solidified dendrite skeleton will inevitably be remelted, so the first refinement is resulted, as shown in Figs. 1 and 2. When Δ T ≥ Δ T⁎, especially in large undecooling, the temperature after recalescence is far below the melting point, so the heat released by recalescence cannot destroy the initial dendrite morphology, thus cannot cause the grain refinement. Generally, the grain refinement at a large Δ T attributes to the stress that originates from the extremely rapid solidification process, which causes the dendrite fragmentation finally [1]. The above deduction can be verified in Figs. 1e–g, where the GBs in the grain-refined microstructures appear very narrow and straight. Since the characters of the sub-GBs are similar to those in the grains of plastic deformation [1], more experimental analysis (such as TEM analysis) are needed to verify the above judgment. Fig. 3 reveals the sub-microstructure in the grains of Ni-Fe-Pb alloy solidified at Δ T = 260 K. As shown in Fig. 3, high densities of dislocations (Fig. 3a), twin boundaries (Fig. 3b) can be observed clearly. These microstructural characteristics imply a recrystallization microstructure in the undercooled Ni-Fe-Pb alloy when Δ T ≥ Δ T⁎.
Fig. 1. The as-solidified morphologies subjected to various ΔTs: (a) ΔTs = 0 K; (b) ΔTs = 45 K; (c) ΔTs = 70 K; (d) ΔTs = 120 K; (e) ΔTs = 140 K; (f) ΔTs = 180 K; (g) ΔTs = 260 K.
Z. Chen et al. / Materials Characterization 127 (2017) 73–76
75
Fig. 2. Grain sizes detected at different melt undercoolings and recalescence curves of the Ni-Fe-Pb alloy undercooled by 70 and 260 K.
stress during rapid solidification and can be expressed by the following equation, σ s ðg s Þ ¼
160μa2 g l 2 g R f s t f λ22 s
g s −g coh þ S
! 1 1 1−g s βs − þ 2ln 1−g s 1−g coh 1−g coh S S ð1Þ
where a is the length of solid-liquid mixing zone in recalescence, μ is the viscosity coefficient, fRs is the solid faction after recalescence, λ 2 is the secondary dendrite arm spacing, t f is the solidification time of recalescence process, g l is the volume fraction of liquid, gs is the volume of solid, where g coh b g S b fRs . g coh is the minimum of S S solid volume fraction for the formation of dendrite skeleton, f Rs (= C P / ΔH f (T R − T n )) is the solid fraction after recalescence, where T R is the maximum recalescence temperature and T n is the nucleation temperature (which can obtained from recalescence curves), in which the relationship between fRs and Δ T can be calculated by the solute conversation using the physical parameters in Table 1, as shown in Fig. 4. The detailed derivation of the present model has been shown in Ref. [1]. Once σs(gs) exceeds the yielding strength of Ni-Fe-Pb alloy, the dendrites formed will be plastically
Fig. 3. The sub-microstructure in the grains of Ni-Fe-Pb alloy solidified at ΔT = 260 K.
deformed, and a storage of microstrain in the microstructure will be the resulted [15]. Further increasing ΔT causes a continuous increase of the stored microstrain, resulting in a continuous increase of the driving force for recrystallization. Once the driving force is high enough to initiate the nucleation, the recrystallization will take place. Consequently, a refined recrystallized microstructure can be obtained. Since the annealing twins are a typical characteristic of the recrystallized metal with fcc structure, its appearance (Fig. 3b) further evidences the occurrence of the recrystallization [17]. Extra free energy of the cold-worked state, which is difficult to be measured, is the driving force for recovery and recrystallization. A major part of the stored energy will be attributed to the introduction of extra dislocations in the system [18]. In order to provide more reliable evidence for the recrystallization mechanism of the grain refinement at Δ T ≥ Δ T ⁎ , the sub-GBs and many large-angle boundaries can be also detected in Fig. 5. There are many high-angle boundaries in the organization of each specimen. Fig. 5a shows the bright-field TEM image of two adjoining grains which are separated by the high-angle boundary in the as-solidified Ni-Fe-Pb alloy (Δ T = 260 K). Fig. 5b shows that SAED patterns of two adjoining grains in Fig. 5a. The present two
Fig. 4. Calculated dendrite growth velocity and stress accumulated in the dendrite skeleton upon rapid solidification as a function of initial undercooling.
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Table 1 Physical parameters for calculated stress development in dendrite network. Parameters
Dimension
Value
α μ tf βs gcoh s λ2
m Pa s s – – m
0.01 5 × 10−3 0.1 0.06 0.15 3 × e−5
adjoining grains have different diffraction spots on the specific zone axis orientation, such as the index of lattice planes on the left side of the GB is (2 2 0), (3 1 1) and (1 − 1 1), while on the right side is (3 − 1 − 2), (4 0 2) and (1 1 1). The plane angles on the left side is 51°, while on the right side is 59°. It is obvious that there are different structures and crystallographic orientations on both sides of the GB. To further determine the structural difference in both sides and to verify the accuracy of the above calibration, the HRTEM is used to know the arrangement of lattice atoms. The HRTEM result also confirms that the planes with the same index (1 1 1) on both sides of GB has a high misorientation and the plane spacing is also different. The formation of annealing twins in fcc metals and alloys are due to growth accidents on {1 1 1} propagating steps present on migrating GBs. As a result of the accidents, Shockley partial dislocations are generated contiguously to GBs. These partial dislocations repel each other and glide away from the boundaries to produce twin boundaries [18]. That is to say, the density of annealing twins increases with the increase of the deformation degree (Fig. 3). The differences of adjoining grains detected by SAET and HRTEM demonstrate that the recrystallization already happened. Recrystallization is a process of new distortionless grains instead of deformed grains. The new formed grains grow up with highangle GBs in this process, since the straight high-angle GBs are of the lowest energy. Because the accumulated stress of dendrite leads to more serious deformation of the dendritic skeleton with increasing undercooling, more re-crystal nuclei will be generated by recrystallization, so refinement phenomenon occurs [18]. Most actual applied materials are multicomponent alloys whose solidification processes are very complex. Thus it is of great importance to research the rapid solidification of multicomponent alloys. In the past few years, more and more research activities were
dedicated to multiphase solidification of ternary and multicomponent alloys [19]. The present experiments showed that the second component Pb induced the obvious microstructural refinement of Ni-Fe alloy. As for the undercooled Ni-Fe alloys, the grain sizes of natural cooling samples were above 100 μm (even reached 900 μm at ΔT = 230 K [20]) in all of the undercooled range [20]. The grain growth after recrystallization can be inhibited by the strongly segregated atoms Pb. Due to the inhibited effect of Pb atoms in the grain boundaries, the grain size of as-solidified NiFe-Pb reduced to about 20 μm. The in-depth study of stabilized effect induced by multicomponent alloys has been carried out in other paper [21]. 4. Conclusions Using the molten glass purification combined with the cycle superheating methods, TEM observation of recrystallization in rapidly solidified Ni-Fe-Pb ternary alloy was presented. The main conclusions are as follows. 1. A refined grain was achieved in undercooled Ni-Fe-Pb ternary alloy. The characteristics of microstructures and substructures of the high-undercooled refined grain were investigated by the transmission electron microscopy and the stress accumulation model. 2. The grain refinement occurring at high undercooling was induced by the plastic deformation of dendrites and subsequent recrystallization, which was evidenced by high densities of substructures, e.g. dislocations, subgrains and migration of high angle grain boundaries. Acknowledgements The work was supported by the Fundamental Research Funds for the Central Universities (2015QNA27) and China Postdoctoral Science Foundation (2016M591953). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Fig. 5. (a) The bright-field TEM images of two adjoining grains in the as-solidified Ni-Fe-Pb alloy (ΔT = 260 K); (b) the SAED patterns of the circled regions in (a) and (c) at both side of the boundary and the zone axis is [−1 1 2], with insert of (a) the magnified HRTEM morphologies.
F. Liu, G.C. Yang, Int. Mater. Rev. 51 (2006) 145–170. D.M. Herlach, Mater. Sci. Eng. R12 (1994) 177–272. M. Schwarz, A. Karma, K. Eckler, D.M. Herlach, Phys. Rev. Lett. 73 (1994) 1380–1383. T.Z. Kattamis, J. Cryst. Growth 34 (1976) 215–220. K. Eckler, A.F. Norman, F. Gärtner, A.L. Greer, D.M. Herlach, J. Cryst. Growth 173 (1997) 528–540. S.Y. Lu, J.F. Zhou, J. Cryst. Growth 309 (2007) 103–111. N. Wang, B. Wei, J. Cryst. Growth 247 (2003) 576–586. H.P. Wang, J. Chang, B. Wei, Phys. Lett. A 374 (2010) 2489–2493. K. Wang, H.F. Wang, F. Liu, H.M. Zhai, Acta Mater. 61 (2013) 1359–1372. H.F. Wang, F. Liu, G.C. Yang, Y.H. Zhou, Acta Mater. 58 (2010) 5402–5410. H.F. Wang, F. Liu, G.C. Yang, Y.H. Zhou, Acta Mater. 58 (2010) 5411–5419. Elinor G. Castle, Andrew M. Mullis, Robert F. Cochrane, Acta Mater. 66 (2014) 378–387. X.L. Xu, Y.Z. Chen, F. Liu, J. Cryst. Growth 377 (2013) 153–159. T. Zhang, F. Liu, H.F. Wang, G.C. Yang, Scr. Mater. 63 (2010) 43–46. Q. Chen, Z. Chen, F. Liu, R.X. Cui, T. Liang, J. Alloys Compd. 638 (2015) 109–114. W.J. Boettinger, S.R. Coriell, R. Trivedi, Rapid Solidification Processing: Principles and Technologies IV, Claitor's, Baton Rouge, LA, 1988 13–24. F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Second ed., 2004. J.W. Christian, Recovery, recrystallization and grain growth, The Theory of Transformation in Metals and Alloys. Pergamon Press, Oxford, 832–858. W.L. Wang, Z.Q. Li, B. Wei, Acta Mater. 59 (2011) 5482–5493. Z. Chen, Q. Chen, C.J. Shen, F. Liu, J. Alloys Compd. 646 (2015) 983–989. T. Liang, Z. Chen, X.Q. Yang, J.Y. Zhang, P. Zhang, Int. J. Mater. Res. (submitted).