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Non-axially oriented dendritic microstructures and their mechanical characteristics of single-crystal Al-4.5%Cu alloy
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Non-axially oriented dendritic microstructures and their mechanical characteristics of single-crystal Al-4.5%Cu alloy Yumin Wang a, Shuangming Li a, *, Bin Yang a, Zhenpeng Liu a, Hong Zhong a, Hui Xing b, Huamiao Wang c a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, Xi’an, 710072, China MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary, Shaanxi Key Laboratory of Condensed Matter Structure and Properties, Northwestern Polytechnical University, Xi’an, 710129, China c State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Degenerate pattern Surface tension anisotropy Directional solidification Mechanical properties
This study investigated microstructure formation as a function of orientation-dependent surface tension anisotropy during directional solidification of Al-4.5%Cu alloy. Axial dendrites (AD), tilted dendrite (TD), and degenerate pattern (DP) were identified in the solidified microstructures of the single crystals. A single-crystal specimen with a stable DP, which was characterized by successive splitting tips, was carefully prepared and specifically investigated. The corresponding mechanical properties were compared with those of single-crystal AD and TD specimens. The ductility and toughness of the single-crystal specimen with a DP were approxi mately 91% and 87% higher than those of the other specimens, respectively. Moreover, the DP specimen showed a stronger work hardening effect than the other specimens, and the tip splitting in this specimen gave rise to refined θ0 precipitates. Field emission scanning electron microscopy (FESEM) and transmission electron micro scopy (TEM) characterizations uncovered the underlying mechanisms associated with this enhancement. This study provides insightful knowledge regarding microstructure control and enriches the understanding of me chanical properties for different dendritic patterns.
1. Introduction Dendrites are the most frequently observed growth pattern during casting and welding of metallic materials. The dendritic morphology mainly results from a morphological instability in the solid-liquid interface and directly affects the mechanical properties of final prod ucts [1,2]. Therefore, a fundamental understanding of the dendritic structure formation and the evolution of the morphological transition is of particular importance. Recently, the significance of including the anisotropy of the solidliquid surface tension in the dendritic morphology selection has been well characterized [3,4]. Furthermore, the surface tension anisotropy relies on the crystallographic orientation with respect to the crystal lattice [3,5,6]. For a cubic material, dendrites with stable tips can grow along the principal crystallographic <100> axes within the {001} plane. However, an irregular seaweed pattern emerges within the {111} crys tallographic plane, where the surface tension is nearly isotropic [7–10]. Apart from these two extreme cases, a complex dendritic structure forms
as a result of intermediate anisotropy, e.g., a degenerate pattern [11,12]. A degenerate pattern (DP) is unsteady and strongly disordered because of the splitting of the tips and the unstable growth of the trunks. As a consequence, the fabrication of DP structures is challenging. Metallic systems with low surface tension, such as aluminum, magne sium et al. can solidify into DP [12]. Aluminum is a valuable lightweight alloy used in automotive and aerospace applications and dendrite is a common growth pattern during solidification. In this study, we suc cessfully fabricated the degenerate pattern by precisely controlling orientation-dependent surface tension anisotropy in Al-4.5%Cu alloy. It is the first time to report the experimental results about the mechanical properties of degenerate pattern. In addition, single crystals with axial dendrite (AD) and tilted dendrite (TD) were prepared for comparison, showing that single crystals with DP have better mechanical properties than those with AD or TD. Our results clearly deepen the understanding of the mechanical properties of complex dendritic structure, a topic crucial to the development of lightweight alloy.
* Corresponding author. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.msea.2019.138665 Received 28 August 2019; Received in revised form 28 October 2019; Accepted 9 November 2019 Available online 11 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yumin Wang, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138665
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Fig. 1. Longitudinal microstructures near the solid-liquid interface directionally solidified the Al-4.5%Cu alloy at various growth velocities (V ¼ 5, 15, 25 and 50 μm/ s) with different orientation-dependent surface tension anisotropies: (a)–(d) δ ¼ 0� ; (e)–(h) δ ¼ 15� ; and (i)–(l) δ ¼ 45� .
2. Experimental procedure
represent the diffusivity of solute in the liquid and the freezing range, respectively [12,13]. At the pulling velocity of 1 μm/s, the single crystal of Al-4.5%Cu alloy has grown with a planar interface.
2.1. Single-crystal preparation Al-4.5%Cu alloy was prepared with Al (99.99 wt%) and Cu (99.99 wt %) under argon atmosphere in a vacuum induction furnace. The ~900 � C alloy melt was poured into a copper mold with a diameter of 150 mm and length of 200 mm. The sample was machined from the ingot and put in a mold (16 mm in internal diameter and 200 mm in length). Eventually, by employing a block starter and a spiral selector of grains, the single-crystal Al-4.5%Cu alloy was prepared in an improved Bridgman vertical vacuum furnace. During the single-crystal alloy preparation, the melt was preheated to 850 � C above the melting point of Al-4.5%Cu alloy by a graphite heating element and held for 30 min to stabilize the melt, then the mold was withdrawn at a predetermined velocity of 1 μm/s. A temperature gradient (G � 22 K/mm) was applied vertically along the sample as re ported previously [12,13]. Based on the constitutional undercooling criteria, Vcs¼ (GDL)/ΔT0�1 μm/s was selected, where DL and ΔT0
2.2. Directional solidification experiments Single-crystal seeds with desirable orientations were used to control the initial surface tension anisotropy. Similar to the configuration in Ref. [14], the seed (1.5 � 5 � 40 mm) was placed in a specially-designed alumina crucible with 1.5 � 5 � 150 mm corresponding to x � y � z axis, respectively. We denote the crystal orientation as (100)[001]δ, where the misaligned angle δ represents the degree between z-axis and [001]. Seed was placed with plane (100) parallel to the vertical y-z plane and [001] was arranged at an angle δ with respect to z-axis. In this study, the seeds sectioned from a single crystal with (100)[001]0� , (100)[001]15� and (100)[001]45� orientations were used to adjust the surface tension [15]. During directional solidification experiment, the seed was partly melted. After being heated at ~850 � C for 1 h, the samples were 2
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Fig. 2. Transversal microstructures near the solid-liquid interface directionally solidified the Al-4.5%Cu alloy various growth velocities (V ¼ 5, 15, 25 and 50 μm/s) with different orientation-dependent surface tension anisotropies: (a)–(d) δ ¼ 0� ; (e)–(h) δ ¼ 15� ; and (i)–(l) δ ¼ 45� .
directionally solidified at various velocities (5, 15, 25 and 50 μm/s). Finally, when the solidification distance reached about 50 mm, the sample was quenched into liquid metal bath (Ga–25 wt% In–13 wt% Sn alloy) to “freeze” morphology of the solid/liquid interface.
after tensile test was carried out on a field emission scanning electron microscopy (FESEM Verios G4) equipped with an energy dispersive spectrometer (EDS). Thin foils for transmission electron microscope (TEM) measurement were prepared by ion milling in a LEICA-RES102 instrument, and an FEI Talos F200X TEM with the accelerating voltage of 200 kV.
2.3. Characterization
3. Results and discussion
Directionally solidified samples were polished and etched with a diluted Keller solution (1 mL HF, 3 mL HNO3 and 46 mL H2O). The microstructure was observed under an Olympus TG-3 optical micro scope. The texture analysis was conducted on the polished y-z surface by using Panalytical X-ray diffractometer, equipped with Cu Kα radiations. The (100) and (110) pole figures, reflected from the face-centered cubic α-Al phase, were measured. Tensile specimens were wire cut along the z-axis of directionally solidified samples at V ¼ 15 μm/s. These specimens were pulled with a constant strain rate of 1�10-3 s-1 in an INSTRON 5848 tensile testing machine to fracture at room temperature. A non-contact video exten someter was applied to measure the strain during the tensile tests. The tensile test with respect to a morphology (AD, TD or DP) was repeated three times to ascertain its reliability. Microstructure characterization
3.1. Dendritic pattern evolution 3.1.1. Different dendritic morphologies in directional solidification Fig. 1 presents the longitudinal microstructures (y-z plane) of the Al4.5%Cu alloy as a function of the misaligned angle (δ) under different growth velocities. A cellular interfacial growth pattern was observed at V ¼ 5 μm/s for the (100)[001]0� orientation (Fig. 1(a)). As the growth velocity increases to 15 μm/s (Fig. 1(b)), the cellular-to-axial dendrite transition occurs, and the secondary dendritic arms form on the primary trunks. As the growth velocity further increases to 25 and 50 μm/s, the axial dendrite features remain, except for refined dendritic size (Fig. 1(c) and (d)).
Fig. 3. The (100) and (110) pole figures of the Al-4.5%Cu alloy in (100)[001]δ orientation of the same sample at growth velocity 15 μm/s: (a) AD, (b) TD and (c) DP. The pulling direction z and lateral direction y are indicated. 3
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Fig. 4. FESEM produced back scattered electron micrograph showing the second phase particle distribution (a1-c1) and corresponding EDS maps depicting the elemental enrichment of Al and Cu for (a1-a3) AD, (b1-b3) TD and (c1-c3) DP.
Tilted dendrites are developed when the misaligned angle (δ) changes to 15� (Fig. 1(e)). Moreover, the tilted dendritic trunks aligned ~15� towards the z-axis. At V ¼ 15 μm/s, the asymmetry becomes more pronounced and larger side-branching forms in this inclined direction, as shown in Fig. 1(f). When δ increases to 45� (maximum value for a cubic crystal), as indicated in Fig. 1(i)–(l), a disordered morphology, named the degenerate pattern (sometimes referred to as “seaweed-like” pattern), occurs at velocities of 5, 15, 25 and 50 μm/s. The successive splitting tips are observed to alternately grow on the left and right sides over time. The surviving tips continue to split and interact with their neighbors. Fig. 2 shows the corresponding transverse microstructures of the Al4.5%Cu alloy at various growth velocities with misaligned angles of δ ¼ 0� , 15� and 45� . The cross-sectional position was taken at ~30 mm behind the quenching interface of the samples. As indicated in Fig. 2(b)– (d), the primary dendrites have a four-fold symmetric side-branch at δ ¼ 0� , and the dendrites become denser and more refined with increasing growth velocity. When the misaligned angle changed to δ ¼ 15� , the solidified microstructures (Fig. 2(e)–(h)) consisted of wellaligned asymmetric side branches that arrayed regularly in rows. When the misaligned angle (δ) changed to 45� , the transverse section in the sample was entirely composed of plate-like features (Fig. 2(i)). As the growth velocity increased, the steady-state cross-section micro structure still exhibited plate-like features with some refinement, as presented in Fig. 2(i)–(l). In summary, the misaligned angle caused the solid-liquid interfacial morphology to change dramatically such that the microstructure solid ified into axial dendrite, tilted dendrite and then finally into a degen erate pattern with a rotation of the crystal about the [100] axis from 0� , 15� –45� . Moreover, the (100) and (110) pole figures of the microstructures grown at V ¼ 15 μm/s are shown in Fig. 3, in which the pulling direction z and lateral direction y are indicated. The [100] poles are all mainly concentrated in the center of the circle in the (100) pole figures, which means that (100) is parallel to the y-z plane. The (110) pole figure in Fig. 3(a) exhibits two maxima tilted ~45� towards the z-axis (high lighted by the red arrows), revealing that the [001] direction is parallel to the z-axis. These crystallographic orientations are consistent with the
designed seed orientations. In addition, the (110) pole figure exhibits two maxima that are highlighted by the red arrows tilted ~30� and 60� towards the z-axis in Fig. 3(b), whereas the pole is parallel to the z-axis in Fig. 3(c). These orientations inherit the planar-front seed well and confirm the good quality of the single crystals. Interestingly, despite the tip splitting of the degenerate pattern, the crystal is still well oriented, which is related to the surface tension anisotropy of the alloy. There is no compositional phase that affects the growth orientation of the dendritic microstructure in the Al-4.5% Cu alloy because the Al4.5% Cu alloy is a typical solid solution. According to the phase diagram of the Al–Cu alloy, the dendritic microstructure of α-Al grows during directional solidification. The dendritic roots usually have much higher concentrations than C0/k due to the non-equilibrium solidification ef fect, where C0 is the initial alloy concentration and k is the equilibrium distribution coefficient. This phenomenon leads to interdendritic pre cipitation of the eutectic phase, including the compositional phase CuAl2. Hence, the compositional phase CuAl2 forms in the interdendritic spaces that contain supersaturated liquid pools due to solute rejection in the late stages of solidification. This compositional phase will not affect the dendritic orientation or morphology in our article. However, the dendritic morphology can affect the distribution of the second phase CuAl2. The backscattered electron micrographs of the ADs, TDs and DPs were characterized with the help of a field emission scanning electron microscope equipped with an energy-dispersive spectroscopy (EDS) detector used for elemental mapping (Fig. 4). The micrographs of the AD and TD in Fig. 4(a1) and (b1) show the directional dendrite shape α-Al with CuAl2 phase along the interdendritic region. In contrast, the DP has finer α-Al with a fine network of CuAl2 phase (Fig. 4 (c1)). The microstructure of the DP shows a more homogeneous distri bution of CuAl2 due to the second phase precipitated along the narrow grooves of the splitting branches, which is beneficial for enhancing mechanical properties. 3.1.2. Surface tension anisotropy The orientation-dependent surface tension anisotropy plays a vital role in the formation of AD, TD and DP. As previously demonstrated, the surface tension anisotropy of a liquid-solid depends on the orientation of the local normal vector of the interface [16]. A simple model of the 4
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Fig. 5. (a) Computed anisotropy graphic illustration of (100)[001]δ orientation for δ ¼ 0� , 15� and 45� using the value of the anisotropy parameter ε4 ¼ 1% with Eqs. (4)–(6). (b) Surface tension in (100) plane for δ ¼ 0� , 15� and 45� in polar coordinates.
surface tension anisotropy γ for a cubic crystal is given the dimensional expression, where a primed coordinate system is attached to the crystal axes, as shown in the literature [17]: n �� � h i4 � � �o 4 4 γ sl ¼ γ0 1 þ 4ε4 n’x þ n’y þ n’z (1)
where θ2 [0, π] and ϕ2 [0, 2π] are the polar and azimuthal angles, respectively. Hence, the surface tension in the (100)[001]0� , (100) [001]15� and (100)[001]45� cases is reduced as follows: � � γðϕ; θÞ γ 0 ¼ 1 þ ε4 4 cos4 θ þ sin4 θð3 þ cos 4 ϕÞ (4) ðð100Þ½001�0� Þ
where γ0 is the integral mean surface tension, ε4 is the anisotropy parameter, and n’i are the Cartesian components of the unit direction vector. Under this initial case, the plane (100)//y-z plane and [001]//zaxis. Appropriate preliminary rotations of the crystal axes are used in each case ((100)[001]0� , (100)[001]15� and (100)[001]45� ) to bring the [001] into different misalignment δ degree with the z-axis. This rotation is given as follows: 0 1 2 30 1 n’ 1 0 0 nx B x’ C B ny C ¼ 4 0 cos δ sin δ 5@ ny A (2) @ A 0 sin δ cos δ n ’ z n
� γðϕ; θÞ γ 0 ¼ 1 þ ε4 4 cos4 sin4 θ pffiffi 7 þ sin4 sin4 θ þ 2 3sin3 sin3 θ cos θ þ 3 sin2 sin2 cos2 θ 2 � pffiffi 7 2 3sin ϕ sin cos3 θ þ cos4 θ 2 ðð100Þ½001�15� Þ � � γðϕ; θÞ γ 0 ¼ 1 þ 2ε4 cos4 θ þ 6 cos2 sin2 sin2 ϕ þ 2 cos4 sin4 θ þ sin4 sin4 θ ðð100Þ½001�45� Þ (6)
z
with 0 1 0 1 nx cos ϕ sin θ @ ny A ¼ @ sin ϕ sin θ A cos θ nz
(5)
The equilibrium shapes of surface tension γ/γ0 (Wulff shape) asso ciated with the changing of δ ¼ 0� , 15� and 45� are shown in Fig. 5(a). The normalized shape (γ/γ 0) is plotted as a color scale. The surface tension γ/γ 0 in the {100} plane for δ ¼ 0� , 15� and 45� is presented in Fig. 5(b) using polar coordinates.
(3)
Fig. 6. (a) Tensile engineering stress-strain curves of Al-4.5%Cu alloy for specimens with AD, TD and DP respective. (b) Mechanical properties of specimens. For comparison, dendrite with 45� misorientation is also displayed. The inset in (a) shows the dimension of specimen for tensile tested. 5
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Fig. 7. (a) Calculated the toughness of Al-4.5%Cu alloy for specimens with AD, TD and DP, respectively. (b) Contribution of calculated uniform deformation (UD) and post uniform deformation (PUD) in the examined specimens.
In the (100)[001]0� orientation, the direction of maximum surface tension in Fig. 5(b) parallels the z-axis in the experiment. The AD grows in the maximum surface tension direction. In the (100)[001]15� orien tation, the direction of the maximum surface tension deviates from the zaxis by 15� , giving rise to the growth of TD. In the (100)[001]45� orientation, two directions of maximum surface tension are arranged symmetrically with respect to the z-axis. The competition between the directions of maximum surface tension results in the formation of the DP. Hence, the relative magnitude and anisotropy of the surface tension govern the crystal growth morphology.
materials. Furthermore, the DP specimen exhibits higher strength than the 45� dendrite specimen and the corresponding increments in σ0.2 and σb are 13 and 18 MPa, respectively. These increases in strength can be ascribed to the finer structure of DP because both specimens have the same orientation. Hence, the finer structure in the DP specimen can be considered responsible for this increased strength. However, the different orientation of the single crystal makes the DP specimen exhibit lower strength than the AD and TD specimens. Ductility directly affects material toughness, which is considered to be the ability to absorb energy during tensile examination [18]. We calculate the ductility by integrating the area underneath the stress-strain curves, and the results are reported as the mechanical en ergy absorbed by the unit volume of the material [18,19]. The ductility consists of two parts, uniform deformation energy (UD) and the post-uniform deformation energy (PUD), where UD is affected by the mechanisms associated with uniform deformation and PUD reflects the post-uniform ductility. The total absorbed energy combined with the UD and PUD contributions is illustrated in Fig. 7. As displayed in Fig. 7(a), the maximum toughness of 85.4 MJ m-3 is obtained in DP specimens, whereas the AD and TD specimens exhibit toughness values of 45.6 and 44.9 MJ m-3, respectively. The high toughness in the DP specimen in dicates better mechanical energy absorption. Additionally, different contributions of UD and PUD in total toughness are presented in Fig. 7 (b). The results show that the percentage contribution of UD in the DP specimen can achieve 81.7% or above, whereas in the AD and TD specimens, the corresponding contributions decrease to 66.0% and 76.4% or less. The degenerate pattern has significantly enhanced the toughness and uniform deformation energy, which is believed to post pone the instability point and provide high work hardening for the material [18].
3.2. Mechanical properties 3.2.1. Tensile tests Fig. 6(a) plots the typical tensile engineering stress-strain curves of the specimens for three different dendritic morphologies (AD, TD and DP) at room temperature, and the inset image shows the dimensions of the tested specimens. The effect of the dendritic morphology on the mechanical properties of Al-4.5%Cu alloy is evident. The corresponding yield strength (σ 0.2), ultimate tensile strength (σb), uniform elongation (δu) and elongation to failure (δf) are summarized in Fig. 6(b). These findings imply that the ultimate tensile strength (σ b) decreases slightly from 174 in the AD specimens to 157 in the TD specimens to 152 MPa in the DP specimens. The yield strength (σ 0.2) also exhibits the same trend. However, the uniform elongation appreciably increases from 15.5% in the AD specimens to 26.1% in the TD specimens to 52.2% in the DP specimens. Remarkable elongation to failure (δf ¼ 62.8%) is obtained in the DP specimen, wherein the elongation to failure is 111% and 91% higher than that in the AD and TD specimens. This remarkable increase in uniform elongation (δu) and elongation to failure (δf) indicates that the degenerate morphology has a favorable effect on ductility. As shown in Figs. 1 and 4, the DP specimen has a finer structure than the AD and TD specimens. In addition, the DP orientation (δ ¼ 45� ) is different from that of the AD (δ ¼ 0� ) and TD (δ ¼ 15� ). The strength of the material in this work is affected not only by the fine morphology, but also by the crystallographic orientation. Therefore, we produced another dendritic specimen with the same orientation of DP (δ ¼ 45� ). This dendritic specimen was sectioned from a single crystal with a dendritic morphology, which was directionally solidified at the same growth velocity. Then, the 45� dendrite specimen was stretched under identical conditions, and the results are shown in Fig. 6(a). As displayed in Fig. 6(a), the 45� dendrite specimen exhibits a yield strength (σ0.2) of 61 MPa and an ultimate tensile strength (σb) of 134 MPa, which are lower than those exhibited by the AD (79 and 174 MPa) and TD (74 and 157 MPa) specimens. Since the AD, TD and 45� dendrite specimens all have dendritic morphologies, it seems that the decrease in strength can be attributed to the anisotropy of the
3.2.2. Work hardening Work hardening is one of the most important factors in the evalua tion of plastic deformation. The deformability, ductility and toughness of materials are intimately linked to the work hardening capacity [20, 21]. The hardening capacity (HC), considered as a ratio of the true ul timate tensile strength to the true yield strength, is an important indi cator in the analysis of the work hardening effect. Here, HC is calculated by the following normalized parameter [22]: HC ¼
σtUTS σty σtUTS ¼ t σty σy
1
(7)
whereσty and σtUTS are the true yield strength and true ultimate tensile
strength, respectively. The true stress-strain curves are calculated by the following equation [23]:
εt ¼ lnð1 þ εÞ
6
(8)
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Fig. 8. (a) True stress-strain curves of Al-4.5%Cu alloy for specimens with AD, TD and DP, respectively. (b) Work hardening capacity HC and work hardening exponent n of the examined specimens.
Fig. 9. Fracture surface of directionally solidified Al-4.5%Cu alloy of AD (a1 and a2), TD (b1 and b2) and DP (c1 and c2). Enlarged views of the fracture surface (a2, b2 and c2) from rectangle region in (a1, b1 and c1), respectively.
σt ¼ σ � ð1 þ εÞ
where K is the strength coefficient. From Fig. 8, the average value of n ¼ 0.33 in the DP specimen is slightly higher than that in the AD and TD specimens (0.22 and 0.27, respectively), demonstrating that the DP specimen has stronger resistance to plastic deformation.
(9)
where εt and σ t represent the true strain and stress, respectively, and ε and σ denote the engineering strain and stress, respectively. The calcu lated true stress-strain curves of the alloys at room temperature are presented in Fig. 8(a), and the corresponding values of work hardening capacity HC are displayed in Fig. 8(b). This figure indicates that the maximum HC is exhibited by the DP specimen, suggesting that this specimen has a stronger work hardening effect than the other specimens. To evaluate the work hardening effect of metallic materials, the work hardening exponent n is considered. The exponent n is calculated by the Hollomon relationship [24]:
σt ¼ Kðεt Þn
3.2.3. Fracture surface As indicated in Fig. 6, the DP specimen exhibits better ductility than the AD and TD specimens, which can also be reflected from their different fracture surfaces. The field emission scanning electron micro scopy (FESEM) images of the tensile fracture surface in three different dendritic specimens are shown in Fig. 9. As shown in Fig. 9(a1)-(c1), the characteristic fracture of the spec imen emerges in the entire region. It is clear that all tensile fracture surfaces are featured by tear ridges (as indicated by the blue arrow) and
(10) 7
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Fig. 10. TEM images and corresponding selected area diffraction patterns after tensile testing of different morphologies: (a) AD, (b) TD and (c) DP. Note the same in the scale bar.
ductile fracture dimples (as indicated by the yellow arrow). For the AD and TD specimens (Fig. 9(a1), (a2), (b1), and (b2)), the predominant fracture are tear ridges and dimples, and these features indicate that the fracture mode is a mixed ductile and brittle fracture. However, the fracture features of a few tear ridges and a large dimple surrounded by several micro-size dimples (marked with a white circle) for the DP specimen (Fig. 2(c1 and c2)) demonstrate that the ductile fracture mode dominates. The number of tear ridges on the fracture surface of the DP specimens is obviously less than those of the AD and TD specimens. Furthermore, a layered surface is evident in Fig. 9(c1), which forms through slip during post-uniform deformation (marked with a red rectangle). The width of this surface has a relation with the elongation and ductility of the specimen due to the slip in the crystalline planes [25]. Consequently, the DP specimen exhibits better plasticity than the other specimens, which is in agreement with large values of δu and δf.
the precipitate phase may be closely related to the growth pattern. Regular dendrite, AD or TD, typically grows with stable and long pri mary dendritic trunks, however, DP is formed through successive tip splitting of primary branches of the solidification front. Thus, the θ0 phase grown in the DP has a limited time and space for coarsening, leading to form the precipitate phase with a small size. 4. Conclusions Specimens with three different microstructures of axial dendrite, tilted dendrite and degenerate pattern were prepared through direc tional solidification by precisely controlling the surface tension anisot ropy. The DP specimen exhibited improved ductility and toughness at room temperature. The DP specimen exhibited a uniform elongation of 52.2% and a toughness of 85.4 MJ m-3, which were much higher than those for the AD (15.5% and 45.6 MJ m-3) and TD (26.1% and 44.9 MJ m-3) specimens. Additionally, the DP specimen showed a stronger work hardening effect than the other specimens. The TEM ex periments indicated that the refinement of the θ0 precipitates in the DP specimen was partly ascribed to the short solidification time of the splitting tips. This finding clearly shows that by controlling the orientation-dependent surface tension anisotropy, a novel approach to tailor the dendritic microstructure can be developed, enabling a deeper understanding of the mechanical properties of dendritic patterns.
3.2.4. TEM observation As well known, the main strengthening precipitate in Al–Cu binary alloy is θ0 phase [26]. These precipitates act as obstacles to decrease the dislocation mobility. In order to gain the optimal mechanical property, the size of precipitates has to be carefully controlled. Fig. 10 presents TEM images observed along <100>Al zone axes and corresponding selected area electron diffraction pattern of the specimens after tensile testing. It has been believed that these orthogonal needle precipitates θ0 preferred to precipitate along a certain direction [27,28]. This phe nomenon is so-called stress orientation effect [29]. Compared to the size of θ0 precipitate in the AD and TD specimen (Fig. 10(a) and (b)), the length and width of precipitate phase are significantly decreased in the DP specimen, as observed in Fig. 10(c). This distinctly different size of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 8
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the work reported in this paper.
[13] L.Y. Yang, S.M. Li, X.Q. Chang, H. Zhong, H.Z. Fu, Twinned dendrite growth during Bridgman solidification, Acta Mater. 97 (2015) 269–281. [14] S. Akamatsu, G. Faivre, Anisotropy-driven dynamics of cellular fronts in directional solidification in thin samples, Phys. Rev. E 58 (3) (1998) 3302–3315. [15] F. Li, L. Jin, Z. Xu, Z.Q. Guo, Determination of three-dimensional orientations of ferroelectric single crystals by an improved rotating orientation x-ray diffraction method, Rev. Sci. Instrum. 80 (8) (2009), 085106. [16] F.Y. Yu, Y.Z. Ji, Y.H. Wei, L.Q. Chen, Effect of the misorientation angle and anisotropy strength on the initial planar instability dynamics during solidification in a molten pool, Int. J. Heat Mass Transf. 130 (2019) 204–214. [17] K.F. Gurski, G.B. McFadden, The effect of anisotropic surface energy on the Rayleigh instability, Proc. R. Soc. A Math. Phys. Eng. Sci. 459 (2038) (2003) 2575–2598. [18] H. Pouraliakbar, M.R. Jandaghi, G. Khalaj, Constrained groove pressing and subsequent annealing of Al-Mn-Si alloy: microstructure evolutions, crystallographic transformations, mechanical properties, electrical conductivity and corrosion resistance, Mater. Des. 124 (2017) 34–46. [19] Y. Li, S.M. Li, L.Y. Yang, H. Zhong, Microstructure and properties of twinned dendrites in directionally solidified A356 alloy, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 734 (2018) 7–19. [20] X.H. Chen, L. Lu, Work hardening of ultrafine-grained copper with nanoscale twins, Scr. Mater. 57 (2) (2007) 133–136. [21] D.D. Zhang, D.P. Zhang, F.Q. Bu, X.L. Li, B.S. Li, T.L. Yan, K. Guan, Q. Yang, X. J. Liu, J. Meng, Excellent ductility and strong work hardening effect of as-cast MgZnZr-Yb alloy at room temperature, J. Alloy. Comp. 728 (2017) 404–412. [22] N. Afrin, D.L. Chen, X. Cao, M. Jahazi, Strain hardening behavior of a friction stir welded magnesium alloy, Scr. Mater. 57 (11) (2007) 1004–1007. [23] Z. Xue, M.G. Pontin, F.W. Zok, J.W. Hutchinson, Calibration procedures for a computational model of ductile fracture, Eng. Fract. Mech. 77 (3) (2010) 492–509. [24] N.D. Alexopoulos, Z. Velonaki, C.I. Stergiou, S.K. Kourkoulis, Effect of ageing on precipitation kinetics, tensile and work hardening behavior of Al-Cu-Mg (2024) alloy, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 700 (2017) 457–467. [25] H. Pouraliakbar, M.R. Jandaghi, Mechanistic insight into the role of severe plastic deformation and post-deformation annealing in fracture behavior of Al-Mn-Si alloy, Mech. Mater. 122 (2018) 145–158. [26] T. Dorin, M. Ramajayam, J. Lamb, T. Langan, Effect of Sc and Zr additions on the microstructure/strength of Al-Cu binary alloys, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 707 (2017) 58–64. [27] Y.L. Lu, J. Wang, X.C. Li, W. Li, R.L. Li, D.S. Zhou, Effects of pre-deformation on the microstructures and corrosion behavior of 2219 aluminum alloys, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 723 (2018) 204–211. [28] J. Wang, Y.L. Lu, D.S. Zhou, L.Y. Sun, R.X. Li, W.T. Xu, Influence of homogenization on microstructural response and mechanical property of Al-Cu-Mn alloy, Materials 11 (6) (2018) 17. [29] J.Q. Chen, Y.L. Deng, X.B. Guo, Revisit the stress-orienting effect of theta ’ in Al-Cu single crystal during stress aging, Mater. Char. 135 (2018) 270–277.
Acknowledgements This research is financially supported by the National Natural Sci ence Foundation of China (No. 51974257, 51474174), Research Funds of the State Key Laboratory of Solidification Processing in NWPU (No. 2019-TS-01, SKLSP201810). Yumin Wang thanks Dr. Bin. Zhang for his suggestions on our manuscript. References [1] J.L. Du, A. Zhang, Z.P. Guo, M.H. Yang, M. Li, F. Liu, S.M. Xiong, Atomistic underpinnings for growth direction and pattern formation of hcp magnesium alloy dendrite, Acta Mater. 161 (2018) 35–46. [2] E.Y. Guo, A.B. Phillion, B. Cai, S.S. Shuai, D. Kazantsev, T. Jing, P.D. Lee, Dendritic evolution during coarsening of Mg-Zn alloys via 4D synchrotron tomography, Acta Mater. 123 (2017) 373–382. [3] T. Haxhimali, A. Karma, F. Gonzales, M. Rappaz, Orientation selection in dendritic evolution, Nat. Mater. 5 (8) (2006) 660–664. [4] F.Y. Yu, Y.H. Wei, Effect of surface tension anisotropy and welding parameters on initial instability dynamics during solidification: a phase-field study, Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 49A (8) (2018) 3293–3305. [5] B.Q. Wang, S.M. Li, Y.M. Wang, H. Zhong, R. Hu, Y. Liu, X.M. Luo, Single-crystal growth of iridium with 100 and 110 orientations by electron beam zone melting, Vacuum 154 (2018) 141–147. [6] B. Utter, E. Bodenschatz, Dynamics of low anisotropy morphologies in directional solidification, Phys. Rev. E 66 (5) (2002), 051604. [7] B. Utter, E. Bodenschatz, Double dendrite growth in solidification, Phys. Rev. E 72 (1) (2005), 011601. [8] N. Haque, R.F. Cochrane, A.M. Mullis, Disorder-order morphologies in drop-tube processed Ni3Ge: dendritic and seaweed growth, J. Alloy. Comp. 707 (2017) 327–331. [9] E.G. Castle, A.M. Mullis, R.F. Cochrane, Evidence for an extensive, undercoolingmediated transition in growth orientation, and novel dendritic seaweed microstructures in Cu-8.9 wt.% Ni, Acta Mater. 66 (2014) 378–387. [10] M.R. Hauwiller, X.W. Zhang, W.I. Liang, C.H. Chiu, Q. Zhang, W.J. Zheng, C. Ophus, E.M. Chan, C. Czarnik, M. Pan, F.M. Ross, W.W. Wu, Y.H. Chu, M. Asta, P.W. Voorhees, A.P. Alivisatos, H.M. Zheng, Dynamics of nanoscale dendrite formation in solution growth revealed through in situ liquid cell electron microscopy, Nano Lett. 18 (10) (2018) 6427–6433. [11] S. Akamatsu, G. Faivre, T. Ihle, Symmetry-broken double fingers and seaweed pattern in thin-film directional solidification of a nonfaceted cubic-crystal, Phys. Rev. E 51 (5) (1995) 4751–4773. [12] Y.M. Wang, S.M. Li, Z.P. Liu, H. Zhong, L. Xu, H. Xing, Probing the degenerate pattern growth of {100}<011> orientation in a directionally solidified Al-4.5 wt% Cu alloy, J. Mater. Sci. Technol. 35 (7) (2019) 1309–1314.
9
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Non-axially oriented dendritic microstructures and their mechanical characteristics of single-crystal Al-4.5%Cu alloy Yumin Wang a, Shuangming Li a, *, Bin Yang a, Zhenpeng Liu a, Hong Zhong a, Hui Xing b, Huamiao Wang c a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, Xi’an, 710072, China MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary, Shaanxi Key Laboratory of Condensed Matter Structure and Properties, Northwestern Polytechnical University, Xi’an, 710129, China c State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Degenerate pattern Surface tension anisotropy Directional solidification Mechanical properties
This study investigated microstructure formation as a function of orientation-dependent surface tension anisotropy during directional solidification of Al-4.5%Cu alloy. Axial dendrites (AD), tilted dendrite (TD), and degenerate pattern (DP) were identified in the solidified microstructures of the single crystals. A single-crystal specimen with a stable DP, which was characterized by successive splitting tips, was carefully prepared and specifically investigated. The corresponding mechanical properties were compared with those of single-crystal AD and TD specimens. The ductility and toughness of the single-crystal specimen with a DP were approxi mately 91% and 87% higher than those of the other specimens, respectively. Moreover, the DP specimen showed a stronger work hardening effect than the other specimens, and the tip splitting in this specimen gave rise to refined θ0 precipitates. Field emission scanning electron microscopy (FESEM) and transmission electron micro scopy (TEM) characterizations uncovered the underlying mechanisms associated with this enhancement. This study provides insightful knowledge regarding microstructure control and enriches the understanding of me chanical properties for different dendritic patterns.
1. Introduction Dendrites are the most frequently observed growth pattern during casting and welding of metallic materials. The dendritic morphology mainly results from a morphological instability in the solid-liquid interface and directly affects the mechanical properties of final prod ucts [1,2]. Therefore, a fundamental understanding of the dendritic structure formation and the evolution of the morphological transition is of particular importance. Recently, the significance of including the anisotropy of the solidliquid surface tension in the dendritic morphology selection has been well characterized [3,4]. Furthermore, the surface tension anisotropy relies on the crystallographic orientation with respect to the crystal lattice [3,5,6]. For a cubic material, dendrites with stable tips can grow along the principal crystallographic <100> axes within the {001} plane. However, an irregular seaweed pattern emerges within the {111} crys tallographic plane, where the surface tension is nearly isotropic [7–10]. Apart from these two extreme cases, a complex dendritic structure forms
as a result of intermediate anisotropy, e.g., a degenerate pattern [11,12]. A degenerate pattern (DP) is unsteady and strongly disordered because of the splitting of the tips and the unstable growth of the trunks. As a consequence, the fabrication of DP structures is challenging. Metallic systems with low surface tension, such as aluminum, magne sium et al. can solidify into DP [12]. Aluminum is a valuable lightweight alloy used in automotive and aerospace applications and dendrite is a common growth pattern during solidification. In this study, we suc cessfully fabricated the degenerate pattern by precisely controlling orientation-dependent surface tension anisotropy in Al-4.5%Cu alloy. It is the first time to report the experimental results about the mechanical properties of degenerate pattern. In addition, single crystals with axial dendrite (AD) and tilted dendrite (TD) were prepared for comparison, showing that single crystals with DP have better mechanical properties than those with AD or TD. Our results clearly deepen the understanding of the mechanical properties of complex dendritic structure, a topic crucial to the development of lightweight alloy.
* Corresponding author. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.msea.2019.138665 Received 28 August 2019; Received in revised form 28 October 2019; Accepted 9 November 2019 Available online 11 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yumin Wang, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138665
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Fig. 1. Longitudinal microstructures near the solid-liquid interface directionally solidified the Al-4.5%Cu alloy at various growth velocities (V ¼ 5, 15, 25 and 50 μm/ s) with different orientation-dependent surface tension anisotropies: (a)–(d) δ ¼ 0� ; (e)–(h) δ ¼ 15� ; and (i)–(l) δ ¼ 45� .
2. Experimental procedure
represent the diffusivity of solute in the liquid and the freezing range, respectively [12,13]. At the pulling velocity of 1 μm/s, the single crystal of Al-4.5%Cu alloy has grown with a planar interface.
2.1. Single-crystal preparation Al-4.5%Cu alloy was prepared with Al (99.99 wt%) and Cu (99.99 wt %) under argon atmosphere in a vacuum induction furnace. The ~900 � C alloy melt was poured into a copper mold with a diameter of 150 mm and length of 200 mm. The sample was machined from the ingot and put in a mold (16 mm in internal diameter and 200 mm in length). Eventually, by employing a block starter and a spiral selector of grains, the single-crystal Al-4.5%Cu alloy was prepared in an improved Bridgman vertical vacuum furnace. During the single-crystal alloy preparation, the melt was preheated to 850 � C above the melting point of Al-4.5%Cu alloy by a graphite heating element and held for 30 min to stabilize the melt, then the mold was withdrawn at a predetermined velocity of 1 μm/s. A temperature gradient (G � 22 K/mm) was applied vertically along the sample as re ported previously [12,13]. Based on the constitutional undercooling criteria, Vcs¼ (GDL)/ΔT0�1 μm/s was selected, where DL and ΔT0
2.2. Directional solidification experiments Single-crystal seeds with desirable orientations were used to control the initial surface tension anisotropy. Similar to the configuration in Ref. [14], the seed (1.5 � 5 � 40 mm) was placed in a specially-designed alumina crucible with 1.5 � 5 � 150 mm corresponding to x � y � z axis, respectively. We denote the crystal orientation as (100)[001]δ, where the misaligned angle δ represents the degree between z-axis and [001]. Seed was placed with plane (100) parallel to the vertical y-z plane and [001] was arranged at an angle δ with respect to z-axis. In this study, the seeds sectioned from a single crystal with (100)[001]0� , (100)[001]15� and (100)[001]45� orientations were used to adjust the surface tension [15]. During directional solidification experiment, the seed was partly melted. After being heated at ~850 � C for 1 h, the samples were 2
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Fig. 2. Transversal microstructures near the solid-liquid interface directionally solidified the Al-4.5%Cu alloy various growth velocities (V ¼ 5, 15, 25 and 50 μm/s) with different orientation-dependent surface tension anisotropies: (a)–(d) δ ¼ 0� ; (e)–(h) δ ¼ 15� ; and (i)–(l) δ ¼ 45� .
directionally solidified at various velocities (5, 15, 25 and 50 μm/s). Finally, when the solidification distance reached about 50 mm, the sample was quenched into liquid metal bath (Ga–25 wt% In–13 wt% Sn alloy) to “freeze” morphology of the solid/liquid interface.
after tensile test was carried out on a field emission scanning electron microscopy (FESEM Verios G4) equipped with an energy dispersive spectrometer (EDS). Thin foils for transmission electron microscope (TEM) measurement were prepared by ion milling in a LEICA-RES102 instrument, and an FEI Talos F200X TEM with the accelerating voltage of 200 kV.
2.3. Characterization
3. Results and discussion
Directionally solidified samples were polished and etched with a diluted Keller solution (1 mL HF, 3 mL HNO3 and 46 mL H2O). The microstructure was observed under an Olympus TG-3 optical micro scope. The texture analysis was conducted on the polished y-z surface by using Panalytical X-ray diffractometer, equipped with Cu Kα radiations. The (100) and (110) pole figures, reflected from the face-centered cubic α-Al phase, were measured. Tensile specimens were wire cut along the z-axis of directionally solidified samples at V ¼ 15 μm/s. These specimens were pulled with a constant strain rate of 1�10-3 s-1 in an INSTRON 5848 tensile testing machine to fracture at room temperature. A non-contact video exten someter was applied to measure the strain during the tensile tests. The tensile test with respect to a morphology (AD, TD or DP) was repeated three times to ascertain its reliability. Microstructure characterization
3.1. Dendritic pattern evolution 3.1.1. Different dendritic morphologies in directional solidification Fig. 1 presents the longitudinal microstructures (y-z plane) of the Al4.5%Cu alloy as a function of the misaligned angle (δ) under different growth velocities. A cellular interfacial growth pattern was observed at V ¼ 5 μm/s for the (100)[001]0� orientation (Fig. 1(a)). As the growth velocity increases to 15 μm/s (Fig. 1(b)), the cellular-to-axial dendrite transition occurs, and the secondary dendritic arms form on the primary trunks. As the growth velocity further increases to 25 and 50 μm/s, the axial dendrite features remain, except for refined dendritic size (Fig. 1(c) and (d)).
Fig. 3. The (100) and (110) pole figures of the Al-4.5%Cu alloy in (100)[001]δ orientation of the same sample at growth velocity 15 μm/s: (a) AD, (b) TD and (c) DP. The pulling direction z and lateral direction y are indicated. 3
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Fig. 4. FESEM produced back scattered electron micrograph showing the second phase particle distribution (a1-c1) and corresponding EDS maps depicting the elemental enrichment of Al and Cu for (a1-a3) AD, (b1-b3) TD and (c1-c3) DP.
Tilted dendrites are developed when the misaligned angle (δ) changes to 15� (Fig. 1(e)). Moreover, the tilted dendritic trunks aligned ~15� towards the z-axis. At V ¼ 15 μm/s, the asymmetry becomes more pronounced and larger side-branching forms in this inclined direction, as shown in Fig. 1(f). When δ increases to 45� (maximum value for a cubic crystal), as indicated in Fig. 1(i)–(l), a disordered morphology, named the degenerate pattern (sometimes referred to as “seaweed-like” pattern), occurs at velocities of 5, 15, 25 and 50 μm/s. The successive splitting tips are observed to alternately grow on the left and right sides over time. The surviving tips continue to split and interact with their neighbors. Fig. 2 shows the corresponding transverse microstructures of the Al4.5%Cu alloy at various growth velocities with misaligned angles of δ ¼ 0� , 15� and 45� . The cross-sectional position was taken at ~30 mm behind the quenching interface of the samples. As indicated in Fig. 2(b)– (d), the primary dendrites have a four-fold symmetric side-branch at δ ¼ 0� , and the dendrites become denser and more refined with increasing growth velocity. When the misaligned angle changed to δ ¼ 15� , the solidified microstructures (Fig. 2(e)–(h)) consisted of wellaligned asymmetric side branches that arrayed regularly in rows. When the misaligned angle (δ) changed to 45� , the transverse section in the sample was entirely composed of plate-like features (Fig. 2(i)). As the growth velocity increased, the steady-state cross-section micro structure still exhibited plate-like features with some refinement, as presented in Fig. 2(i)–(l). In summary, the misaligned angle caused the solid-liquid interfacial morphology to change dramatically such that the microstructure solid ified into axial dendrite, tilted dendrite and then finally into a degen erate pattern with a rotation of the crystal about the [100] axis from 0� , 15� –45� . Moreover, the (100) and (110) pole figures of the microstructures grown at V ¼ 15 μm/s are shown in Fig. 3, in which the pulling direction z and lateral direction y are indicated. The [100] poles are all mainly concentrated in the center of the circle in the (100) pole figures, which means that (100) is parallel to the y-z plane. The (110) pole figure in Fig. 3(a) exhibits two maxima tilted ~45� towards the z-axis (high lighted by the red arrows), revealing that the [001] direction is parallel to the z-axis. These crystallographic orientations are consistent with the
designed seed orientations. In addition, the (110) pole figure exhibits two maxima that are highlighted by the red arrows tilted ~30� and 60� towards the z-axis in Fig. 3(b), whereas the pole is parallel to the z-axis in Fig. 3(c). These orientations inherit the planar-front seed well and confirm the good quality of the single crystals. Interestingly, despite the tip splitting of the degenerate pattern, the crystal is still well oriented, which is related to the surface tension anisotropy of the alloy. There is no compositional phase that affects the growth orientation of the dendritic microstructure in the Al-4.5% Cu alloy because the Al4.5% Cu alloy is a typical solid solution. According to the phase diagram of the Al–Cu alloy, the dendritic microstructure of α-Al grows during directional solidification. The dendritic roots usually have much higher concentrations than C0/k due to the non-equilibrium solidification ef fect, where C0 is the initial alloy concentration and k is the equilibrium distribution coefficient. This phenomenon leads to interdendritic pre cipitation of the eutectic phase, including the compositional phase CuAl2. Hence, the compositional phase CuAl2 forms in the interdendritic spaces that contain supersaturated liquid pools due to solute rejection in the late stages of solidification. This compositional phase will not affect the dendritic orientation or morphology in our article. However, the dendritic morphology can affect the distribution of the second phase CuAl2. The backscattered electron micrographs of the ADs, TDs and DPs were characterized with the help of a field emission scanning electron microscope equipped with an energy-dispersive spectroscopy (EDS) detector used for elemental mapping (Fig. 4). The micrographs of the AD and TD in Fig. 4(a1) and (b1) show the directional dendrite shape α-Al with CuAl2 phase along the interdendritic region. In contrast, the DP has finer α-Al with a fine network of CuAl2 phase (Fig. 4 (c1)). The microstructure of the DP shows a more homogeneous distri bution of CuAl2 due to the second phase precipitated along the narrow grooves of the splitting branches, which is beneficial for enhancing mechanical properties. 3.1.2. Surface tension anisotropy The orientation-dependent surface tension anisotropy plays a vital role in the formation of AD, TD and DP. As previously demonstrated, the surface tension anisotropy of a liquid-solid depends on the orientation of the local normal vector of the interface [16]. A simple model of the 4
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Fig. 5. (a) Computed anisotropy graphic illustration of (100)[001]δ orientation for δ ¼ 0� , 15� and 45� using the value of the anisotropy parameter ε4 ¼ 1% with Eqs. (4)–(6). (b) Surface tension in (100) plane for δ ¼ 0� , 15� and 45� in polar coordinates.
surface tension anisotropy γ for a cubic crystal is given the dimensional expression, where a primed coordinate system is attached to the crystal axes, as shown in the literature [17]: n �� � h i4 � � �o 4 4 γ sl ¼ γ0 1 þ 4ε4 n’x þ n’y þ n’z (1)
where θ2 [0, π] and ϕ2 [0, 2π] are the polar and azimuthal angles, respectively. Hence, the surface tension in the (100)[001]0� , (100) [001]15� and (100)[001]45� cases is reduced as follows: � � γðϕ; θÞ γ 0 ¼ 1 þ ε4 4 cos4 θ þ sin4 θð3 þ cos 4 ϕÞ (4) ðð100Þ½001�0� Þ
where γ0 is the integral mean surface tension, ε4 is the anisotropy parameter, and n’i are the Cartesian components of the unit direction vector. Under this initial case, the plane (100)//y-z plane and [001]//zaxis. Appropriate preliminary rotations of the crystal axes are used in each case ((100)[001]0� , (100)[001]15� and (100)[001]45� ) to bring the [001] into different misalignment δ degree with the z-axis. This rotation is given as follows: 0 1 2 30 1 n’ 1 0 0 nx B x’ C B ny C ¼ 4 0 cos δ sin δ 5@ ny A (2) @ A 0 sin δ cos δ n ’ z n
� γðϕ; θÞ γ 0 ¼ 1 þ ε4 4 cos4 sin4 θ pffiffi 7 þ sin4 sin4 θ þ 2 3sin3 sin3 θ cos θ þ 3 sin2 sin2 cos2 θ 2 � pffiffi 7 2 3sin ϕ sin cos3 θ þ cos4 θ 2 ðð100Þ½001�15� Þ � � γðϕ; θÞ γ 0 ¼ 1 þ 2ε4 cos4 θ þ 6 cos2 sin2 sin2 ϕ þ 2 cos4 sin4 θ þ sin4 sin4 θ ðð100Þ½001�45� Þ (6)
z
with 0 1 0 1 nx cos ϕ sin θ @ ny A ¼ @ sin ϕ sin θ A cos θ nz
(5)
The equilibrium shapes of surface tension γ/γ0 (Wulff shape) asso ciated with the changing of δ ¼ 0� , 15� and 45� are shown in Fig. 5(a). The normalized shape (γ/γ 0) is plotted as a color scale. The surface tension γ/γ 0 in the {100} plane for δ ¼ 0� , 15� and 45� is presented in Fig. 5(b) using polar coordinates.
(3)
Fig. 6. (a) Tensile engineering stress-strain curves of Al-4.5%Cu alloy for specimens with AD, TD and DP respective. (b) Mechanical properties of specimens. For comparison, dendrite with 45� misorientation is also displayed. The inset in (a) shows the dimension of specimen for tensile tested. 5
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Fig. 7. (a) Calculated the toughness of Al-4.5%Cu alloy for specimens with AD, TD and DP, respectively. (b) Contribution of calculated uniform deformation (UD) and post uniform deformation (PUD) in the examined specimens.
In the (100)[001]0� orientation, the direction of maximum surface tension in Fig. 5(b) parallels the z-axis in the experiment. The AD grows in the maximum surface tension direction. In the (100)[001]15� orien tation, the direction of the maximum surface tension deviates from the zaxis by 15� , giving rise to the growth of TD. In the (100)[001]45� orientation, two directions of maximum surface tension are arranged symmetrically with respect to the z-axis. The competition between the directions of maximum surface tension results in the formation of the DP. Hence, the relative magnitude and anisotropy of the surface tension govern the crystal growth morphology.
materials. Furthermore, the DP specimen exhibits higher strength than the 45� dendrite specimen and the corresponding increments in σ0.2 and σb are 13 and 18 MPa, respectively. These increases in strength can be ascribed to the finer structure of DP because both specimens have the same orientation. Hence, the finer structure in the DP specimen can be considered responsible for this increased strength. However, the different orientation of the single crystal makes the DP specimen exhibit lower strength than the AD and TD specimens. Ductility directly affects material toughness, which is considered to be the ability to absorb energy during tensile examination [18]. We calculate the ductility by integrating the area underneath the stress-strain curves, and the results are reported as the mechanical en ergy absorbed by the unit volume of the material [18,19]. The ductility consists of two parts, uniform deformation energy (UD) and the post-uniform deformation energy (PUD), where UD is affected by the mechanisms associated with uniform deformation and PUD reflects the post-uniform ductility. The total absorbed energy combined with the UD and PUD contributions is illustrated in Fig. 7. As displayed in Fig. 7(a), the maximum toughness of 85.4 MJ m-3 is obtained in DP specimens, whereas the AD and TD specimens exhibit toughness values of 45.6 and 44.9 MJ m-3, respectively. The high toughness in the DP specimen in dicates better mechanical energy absorption. Additionally, different contributions of UD and PUD in total toughness are presented in Fig. 7 (b). The results show that the percentage contribution of UD in the DP specimen can achieve 81.7% or above, whereas in the AD and TD specimens, the corresponding contributions decrease to 66.0% and 76.4% or less. The degenerate pattern has significantly enhanced the toughness and uniform deformation energy, which is believed to post pone the instability point and provide high work hardening for the material [18].
3.2. Mechanical properties 3.2.1. Tensile tests Fig. 6(a) plots the typical tensile engineering stress-strain curves of the specimens for three different dendritic morphologies (AD, TD and DP) at room temperature, and the inset image shows the dimensions of the tested specimens. The effect of the dendritic morphology on the mechanical properties of Al-4.5%Cu alloy is evident. The corresponding yield strength (σ 0.2), ultimate tensile strength (σb), uniform elongation (δu) and elongation to failure (δf) are summarized in Fig. 6(b). These findings imply that the ultimate tensile strength (σ b) decreases slightly from 174 in the AD specimens to 157 in the TD specimens to 152 MPa in the DP specimens. The yield strength (σ 0.2) also exhibits the same trend. However, the uniform elongation appreciably increases from 15.5% in the AD specimens to 26.1% in the TD specimens to 52.2% in the DP specimens. Remarkable elongation to failure (δf ¼ 62.8%) is obtained in the DP specimen, wherein the elongation to failure is 111% and 91% higher than that in the AD and TD specimens. This remarkable increase in uniform elongation (δu) and elongation to failure (δf) indicates that the degenerate morphology has a favorable effect on ductility. As shown in Figs. 1 and 4, the DP specimen has a finer structure than the AD and TD specimens. In addition, the DP orientation (δ ¼ 45� ) is different from that of the AD (δ ¼ 0� ) and TD (δ ¼ 15� ). The strength of the material in this work is affected not only by the fine morphology, but also by the crystallographic orientation. Therefore, we produced another dendritic specimen with the same orientation of DP (δ ¼ 45� ). This dendritic specimen was sectioned from a single crystal with a dendritic morphology, which was directionally solidified at the same growth velocity. Then, the 45� dendrite specimen was stretched under identical conditions, and the results are shown in Fig. 6(a). As displayed in Fig. 6(a), the 45� dendrite specimen exhibits a yield strength (σ0.2) of 61 MPa and an ultimate tensile strength (σb) of 134 MPa, which are lower than those exhibited by the AD (79 and 174 MPa) and TD (74 and 157 MPa) specimens. Since the AD, TD and 45� dendrite specimens all have dendritic morphologies, it seems that the decrease in strength can be attributed to the anisotropy of the
3.2.2. Work hardening Work hardening is one of the most important factors in the evalua tion of plastic deformation. The deformability, ductility and toughness of materials are intimately linked to the work hardening capacity [20, 21]. The hardening capacity (HC), considered as a ratio of the true ul timate tensile strength to the true yield strength, is an important indi cator in the analysis of the work hardening effect. Here, HC is calculated by the following normalized parameter [22]: HC ¼
σtUTS σty σtUTS ¼ t σty σy
1
(7)
whereσty and σtUTS are the true yield strength and true ultimate tensile
strength, respectively. The true stress-strain curves are calculated by the following equation [23]:
εt ¼ lnð1 þ εÞ
6
(8)
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Fig. 8. (a) True stress-strain curves of Al-4.5%Cu alloy for specimens with AD, TD and DP, respectively. (b) Work hardening capacity HC and work hardening exponent n of the examined specimens.
Fig. 9. Fracture surface of directionally solidified Al-4.5%Cu alloy of AD (a1 and a2), TD (b1 and b2) and DP (c1 and c2). Enlarged views of the fracture surface (a2, b2 and c2) from rectangle region in (a1, b1 and c1), respectively.
σt ¼ σ � ð1 þ εÞ
where K is the strength coefficient. From Fig. 8, the average value of n ¼ 0.33 in the DP specimen is slightly higher than that in the AD and TD specimens (0.22 and 0.27, respectively), demonstrating that the DP specimen has stronger resistance to plastic deformation.
(9)
where εt and σ t represent the true strain and stress, respectively, and ε and σ denote the engineering strain and stress, respectively. The calcu lated true stress-strain curves of the alloys at room temperature are presented in Fig. 8(a), and the corresponding values of work hardening capacity HC are displayed in Fig. 8(b). This figure indicates that the maximum HC is exhibited by the DP specimen, suggesting that this specimen has a stronger work hardening effect than the other specimens. To evaluate the work hardening effect of metallic materials, the work hardening exponent n is considered. The exponent n is calculated by the Hollomon relationship [24]:
σt ¼ Kðεt Þn
3.2.3. Fracture surface As indicated in Fig. 6, the DP specimen exhibits better ductility than the AD and TD specimens, which can also be reflected from their different fracture surfaces. The field emission scanning electron micro scopy (FESEM) images of the tensile fracture surface in three different dendritic specimens are shown in Fig. 9. As shown in Fig. 9(a1)-(c1), the characteristic fracture of the spec imen emerges in the entire region. It is clear that all tensile fracture surfaces are featured by tear ridges (as indicated by the blue arrow) and
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Fig. 10. TEM images and corresponding selected area diffraction patterns after tensile testing of different morphologies: (a) AD, (b) TD and (c) DP. Note the same in the scale bar.
ductile fracture dimples (as indicated by the yellow arrow). For the AD and TD specimens (Fig. 9(a1), (a2), (b1), and (b2)), the predominant fracture are tear ridges and dimples, and these features indicate that the fracture mode is a mixed ductile and brittle fracture. However, the fracture features of a few tear ridges and a large dimple surrounded by several micro-size dimples (marked with a white circle) for the DP specimen (Fig. 2(c1 and c2)) demonstrate that the ductile fracture mode dominates. The number of tear ridges on the fracture surface of the DP specimens is obviously less than those of the AD and TD specimens. Furthermore, a layered surface is evident in Fig. 9(c1), which forms through slip during post-uniform deformation (marked with a red rectangle). The width of this surface has a relation with the elongation and ductility of the specimen due to the slip in the crystalline planes [25]. Consequently, the DP specimen exhibits better plasticity than the other specimens, which is in agreement with large values of δu and δf.
the precipitate phase may be closely related to the growth pattern. Regular dendrite, AD or TD, typically grows with stable and long pri mary dendritic trunks, however, DP is formed through successive tip splitting of primary branches of the solidification front. Thus, the θ0 phase grown in the DP has a limited time and space for coarsening, leading to form the precipitate phase with a small size. 4. Conclusions Specimens with three different microstructures of axial dendrite, tilted dendrite and degenerate pattern were prepared through direc tional solidification by precisely controlling the surface tension anisot ropy. The DP specimen exhibited improved ductility and toughness at room temperature. The DP specimen exhibited a uniform elongation of 52.2% and a toughness of 85.4 MJ m-3, which were much higher than those for the AD (15.5% and 45.6 MJ m-3) and TD (26.1% and 44.9 MJ m-3) specimens. Additionally, the DP specimen showed a stronger work hardening effect than the other specimens. The TEM ex periments indicated that the refinement of the θ0 precipitates in the DP specimen was partly ascribed to the short solidification time of the splitting tips. This finding clearly shows that by controlling the orientation-dependent surface tension anisotropy, a novel approach to tailor the dendritic microstructure can be developed, enabling a deeper understanding of the mechanical properties of dendritic patterns.
3.2.4. TEM observation As well known, the main strengthening precipitate in Al–Cu binary alloy is θ0 phase [26]. These precipitates act as obstacles to decrease the dislocation mobility. In order to gain the optimal mechanical property, the size of precipitates has to be carefully controlled. Fig. 10 presents TEM images observed along <100>Al zone axes and corresponding selected area electron diffraction pattern of the specimens after tensile testing. It has been believed that these orthogonal needle precipitates θ0 preferred to precipitate along a certain direction [27,28]. This phe nomenon is so-called stress orientation effect [29]. Compared to the size of θ0 precipitate in the AD and TD specimen (Fig. 10(a) and (b)), the length and width of precipitate phase are significantly decreased in the DP specimen, as observed in Fig. 10(c). This distinctly different size of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 8
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the work reported in this paper.
[13] L.Y. Yang, S.M. Li, X.Q. Chang, H. Zhong, H.Z. Fu, Twinned dendrite growth during Bridgman solidification, Acta Mater. 97 (2015) 269–281. [14] S. Akamatsu, G. Faivre, Anisotropy-driven dynamics of cellular fronts in directional solidification in thin samples, Phys. Rev. E 58 (3) (1998) 3302–3315. [15] F. Li, L. Jin, Z. Xu, Z.Q. Guo, Determination of three-dimensional orientations of ferroelectric single crystals by an improved rotating orientation x-ray diffraction method, Rev. Sci. Instrum. 80 (8) (2009), 085106. [16] F.Y. Yu, Y.Z. Ji, Y.H. Wei, L.Q. Chen, Effect of the misorientation angle and anisotropy strength on the initial planar instability dynamics during solidification in a molten pool, Int. J. Heat Mass Transf. 130 (2019) 204–214. [17] K.F. Gurski, G.B. McFadden, The effect of anisotropic surface energy on the Rayleigh instability, Proc. R. Soc. A Math. Phys. Eng. Sci. 459 (2038) (2003) 2575–2598. [18] H. Pouraliakbar, M.R. Jandaghi, G. Khalaj, Constrained groove pressing and subsequent annealing of Al-Mn-Si alloy: microstructure evolutions, crystallographic transformations, mechanical properties, electrical conductivity and corrosion resistance, Mater. Des. 124 (2017) 34–46. [19] Y. Li, S.M. Li, L.Y. Yang, H. Zhong, Microstructure and properties of twinned dendrites in directionally solidified A356 alloy, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 734 (2018) 7–19. [20] X.H. Chen, L. Lu, Work hardening of ultrafine-grained copper with nanoscale twins, Scr. Mater. 57 (2) (2007) 133–136. [21] D.D. Zhang, D.P. Zhang, F.Q. Bu, X.L. Li, B.S. Li, T.L. Yan, K. Guan, Q. Yang, X. J. Liu, J. Meng, Excellent ductility and strong work hardening effect of as-cast MgZnZr-Yb alloy at room temperature, J. Alloy. Comp. 728 (2017) 404–412. [22] N. Afrin, D.L. Chen, X. Cao, M. Jahazi, Strain hardening behavior of a friction stir welded magnesium alloy, Scr. Mater. 57 (11) (2007) 1004–1007. [23] Z. Xue, M.G. Pontin, F.W. Zok, J.W. Hutchinson, Calibration procedures for a computational model of ductile fracture, Eng. Fract. Mech. 77 (3) (2010) 492–509. [24] N.D. Alexopoulos, Z. Velonaki, C.I. Stergiou, S.K. Kourkoulis, Effect of ageing on precipitation kinetics, tensile and work hardening behavior of Al-Cu-Mg (2024) alloy, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 700 (2017) 457–467. [25] H. Pouraliakbar, M.R. Jandaghi, Mechanistic insight into the role of severe plastic deformation and post-deformation annealing in fracture behavior of Al-Mn-Si alloy, Mech. Mater. 122 (2018) 145–158. [26] T. Dorin, M. Ramajayam, J. Lamb, T. Langan, Effect of Sc and Zr additions on the microstructure/strength of Al-Cu binary alloys, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 707 (2017) 58–64. [27] Y.L. Lu, J. Wang, X.C. Li, W. Li, R.L. Li, D.S. Zhou, Effects of pre-deformation on the microstructures and corrosion behavior of 2219 aluminum alloys, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 723 (2018) 204–211. [28] J. Wang, Y.L. Lu, D.S. Zhou, L.Y. Sun, R.X. Li, W.T. Xu, Influence of homogenization on microstructural response and mechanical property of Al-Cu-Mn alloy, Materials 11 (6) (2018) 17. [29] J.Q. Chen, Y.L. Deng, X.B. Guo, Revisit the stress-orienting effect of theta ’ in Al-Cu single crystal during stress aging, Mater. Char. 135 (2018) 270–277.
Acknowledgements This research is financially supported by the National Natural Sci ence Foundation of China (No. 51974257, 51474174), Research Funds of the State Key Laboratory of Solidification Processing in NWPU (No. 2019-TS-01, SKLSP201810). Yumin Wang thanks Dr. Bin. Zhang for his suggestions on our manuscript. References [1] J.L. Du, A. Zhang, Z.P. Guo, M.H. Yang, M. Li, F. Liu, S.M. Xiong, Atomistic underpinnings for growth direction and pattern formation of hcp magnesium alloy dendrite, Acta Mater. 161 (2018) 35–46. [2] E.Y. Guo, A.B. Phillion, B. Cai, S.S. Shuai, D. Kazantsev, T. Jing, P.D. Lee, Dendritic evolution during coarsening of Mg-Zn alloys via 4D synchrotron tomography, Acta Mater. 123 (2017) 373–382. [3] T. Haxhimali, A. Karma, F. Gonzales, M. Rappaz, Orientation selection in dendritic evolution, Nat. Mater. 5 (8) (2006) 660–664. [4] F.Y. Yu, Y.H. Wei, Effect of surface tension anisotropy and welding parameters on initial instability dynamics during solidification: a phase-field study, Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 49A (8) (2018) 3293–3305. [5] B.Q. Wang, S.M. Li, Y.M. Wang, H. Zhong, R. Hu, Y. Liu, X.M. Luo, Single-crystal growth of iridium with 100 and 110 orientations by electron beam zone melting, Vacuum 154 (2018) 141–147. [6] B. Utter, E. Bodenschatz, Dynamics of low anisotropy morphologies in directional solidification, Phys. Rev. E 66 (5) (2002), 051604. [7] B. Utter, E. Bodenschatz, Double dendrite growth in solidification, Phys. Rev. E 72 (1) (2005), 011601. [8] N. Haque, R.F. Cochrane, A.M. Mullis, Disorder-order morphologies in drop-tube processed Ni3Ge: dendritic and seaweed growth, J. Alloy. Comp. 707 (2017) 327–331. [9] E.G. Castle, A.M. Mullis, R.F. Cochrane, Evidence for an extensive, undercoolingmediated transition in growth orientation, and novel dendritic seaweed microstructures in Cu-8.9 wt.% Ni, Acta Mater. 66 (2014) 378–387. [10] M.R. Hauwiller, X.W. Zhang, W.I. Liang, C.H. Chiu, Q. Zhang, W.J. Zheng, C. Ophus, E.M. Chan, C. Czarnik, M. Pan, F.M. Ross, W.W. Wu, Y.H. Chu, M. Asta, P.W. Voorhees, A.P. Alivisatos, H.M. Zheng, Dynamics of nanoscale dendrite formation in solution growth revealed through in situ liquid cell electron microscopy, Nano Lett. 18 (10) (2018) 6427–6433. [11] S. Akamatsu, G. Faivre, T. Ihle, Symmetry-broken double fingers and seaweed pattern in thin-film directional solidification of a nonfaceted cubic-crystal, Phys. Rev. E 51 (5) (1995) 4751–4773. [12] Y.M. Wang, S.M. Li, Z.P. Liu, H. Zhong, L. Xu, H. Xing, Probing the degenerate pattern growth of {100}<011> orientation in a directionally solidified Al-4.5 wt% Cu alloy, J. Mater. Sci. Technol. 35 (7) (2019) 1309–1314.
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