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Al-Mg composites
Journal of Alloys and Compounds 779 (2019) 404e411
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effects of Mg contents on microstructure and tensile properties of Al18B4O33w/Al-Mg composites Zhenling Wang a, b, Shawei Tang a, **, Yucheng Yu a, b, Jin Hu c, * a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Institute of Vanadium and Titanium, Panzhihua University, PanZhiHua 617000, PR China c National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2018 Received in revised form 22 November 2018 Accepted 24 November 2018 Available online 26 November 2018
Al18B4O33 whiskers reinforced Al-Mg matrix composites with different Mg contents are fabricated by squeeze casting method. The effects of Mg contents on the microstructure and the tensile properties of the composites are investigated. The results indicate that the Mg contents have strong influences on the type of interphase, the thickness of interfacial layer and the solid solubility of Mg along with the dislocation density in the matrices of the composites. The interface reaction products between the whiskers and Mg evolve from MgAl2O4 to MgAl2O4þMgO to MgO with the increase in Mg contents. The ultimate tensile strength (UTS) of the composites increases dramatically with the Mg contents, resulting from the combination of the interface strengthening and the solid solution strengthening of Mg. It is worth mentioning that the UTS of the composite containing 10 wt% Mg in the matrix attains a highest value (540 MPa) in as-cast Al18B4O33w/Al composites reported. © 2018 Elsevier B.V. All rights reserved.
Keywords: Discontinuous reinforcement Metal-matrix composites (MMCs) Interface/interphase Strength
1. Introduction Aluminum borate whisker (Al18B4O33 whisker, denoted by ABOw) reinforced aluminum matrix composite has attracted much interest owing to their excellent mechanical properties and low cost [1,2]. In general, high strength aluminum alloys, such as 2024Al, 6061Al, 7075Al and so on, are often used as a matrix material in ABOw/Al composites [3e8]. However, it is well known that ABOw can react with the magnesium included in matrix alloys during the casting and subsequent heat-treatment in ABOw reinforced aluminum (or magnesium) matrix composites. The severely interfacial reaction not only degrades the properties of ABOw/Al composites owing to the damage of whiskers, but also weakens the strength of aluminum alloy matrix due to the depletion of magnesium element in the alloy matrix [9e13]. Therefore, up to now, in order to improve the performance of ABOw/Al composites, lots of researchers put their eyes on the question how to inhibit the interface reaction [12e17]. Gao et al. [14] prepared Cu coating on
* Corresponding author. BOX 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: þ86 0451 86415894. ** Corresponding author. BOX 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. E-mail addresses: [email protected] (S. Tang), [email protected] (J. Hu). https://doi.org/10.1016/j.jallcom.2018.11.328 0925-8388/© 2018 Elsevier B.V. All rights reserved.
the surface of ABOw by an electroless plating method and fabricated Cu-coated ABOw reinforced 6061Al composite. They found that the UTS of the composite was enhanced by the Cu coating, however, the highest value of UTS was only about 350 MPa. Yue et al. [15] fabricated Cr2O3-coated ABOw reinforced 2024Al composites and the coating of whisker was prepared by means of a solgel method. Cr2O3-coated ABOw could hinder the interfacial reaction between aluminum borate whiskers and magnesium in the matrix, however, the coating could not improve effectively the UTS (360 MPa) of the composite due to the weak interfacial bonding. In order to restrain the interfacial reaction and improve the wettability between whisker and aluminum matrix, Ding et al. [16] deposited Al2O3 coatings on the surface of ABOw by means of the sol-gel method. However, the UTS of the composite could reach no more than 350 MPa. Obviously, the coatings effectively inhibit the reaction between the whisker and the matrix, but do not achieve the desired mechanical properties. Hu et al. [18] studied aluminum borate whisker reinforced pure aluminum composite, and found that no interface reaction between whisker and aluminum occurred. Li et al. [19] reported the mechanical properties of aluminum borate whisker reinforced pure aluminum matrix composites, and UTS of the composite was only 290 MPa.
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From the above, a conclusion can be drawn that, so far, there is no best way to achieve a high strength for aluminum borate whisker reinforced Al composites. Severely interfacial reaction can damage whiskers to reduce the enhancement effect of whisker, and the interface without interface reaction and the interface with coating have not achieved an expected performance. So is there a slightly reactive interface that enables composite materials to get excellent mechanical properties? To find out, we must find out the interface reaction mechanism between Mg and ABOw. As we all know, the interface plays an important role for high performance composites, and the study of interface reaction mechanism is the most basic and important. However, the reaction mechanism of Mg and ABOw is still controversial and lacking. Early findings showed that, the interface reaction product was only MgAl2O4 in ABOw reinforced aluminum matrix composites that contained Mg in the matrices [2e5,9e11], however, the interface reaction product was only MgO in ABOw reinforced magnesium matrix composites [20,21]. It was worth mentioning that the interface reaction products were MgAl2O4þMgO when the magnesium content in the matrix was 6.8 wt% in a ABOw/Al-Mg composite, which had been found in our previous study [22]. However, it is unfortunately that few researches are carried out about the effects of Mg contents on interfacial reaction between Al alloy containing Mg and ABOw during the fabrication of the composites by squeeze casting. It is of great importance to understand that the effects of Mg contents on the interfacial reaction between Al alloy and ABOw and the mechanical properties of ABOw/Al-Mg composites. This is because the interfacial reaction has been regarded as one of the main factors that significantly affect the properties of the composite. In the present work, ABOw/Al-Mg composites containing different Mg contents are fabricated by squeeze casting method. The microstructure, interfacial reaction and tensile behavior of the composites are investigated and discussed. 2. Materials and experimental method Al alloy with 2.5 wt% Mg, 6.8 wt% Mg and 10 wt% Mg utilized for matrix materials in these experiments are prepared in situ from commercially pure Al (99.5 wt%), magnesium (99.7 wt%). The incorporated material is ABOw with a diameter of 0.5～1 mm and length of 10～30 mm made in Shikoku Chemical Company of Japan. The preparation process of Al-Mg matrix composites is as follows: firstly, the raw ABOw is dispersed by mechanical stirring and ultrasonic vibration generator in a deionized water solution, and then the water solution contained the dispersed ABOw is poured into a filter mould for the preparation of a whisker preform; The whisker preforms in a diameter of 90 mm and a height of 20 mm are dried at room temperature for 48 h, then dried at 80～100 C for 24 h, finally the performs are sintered at 900 C for 1 h to obtain a high strength. The whisker performs and the mould are preheated at 550 C, and then the Al-Mg alloy at 750 C is cast into the mould with a pressure of 100 MPa. The Al-Mg matrix composites reinforced by Al18B4O33w (20% volume fraction) with different Mg contents are obtained. For simplification, the Al matrix composites contained 2.5 wt% Mg, 6.8 wt% Mg and 10 wt% Mg are named as C1, C2 and C3, respectively. The phase compositions of the composites are identified using a Philips X'Pert X-ray diffractometer with a Cu Ka radiation source at 40 kV and 40 mA. Interfacial microstructures of the composites are observed using a FEI G2F30 transmission electron microscopy (TEM) with an accelerating voltage of 120 kV, and the specimens used for TEM observation are mechanically abraded to a thickness of 40 mm, and then thinned by ion milling. Tensile tests are performed on an Insrton 5590 test machine with a speed of 0.5 mm/
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min at room temperature. Dimensions of the tensile specimens are shown in Fig. 1. The morphologies of tensile fractographs of the composites are examined by a Hitch S-4700N scanning electron microscope (SEM) with an accelerating voltage of 20 kV.
3. Results 3.1. Typical interfacial features of the ABOw/Al-Mg composites 3.1.1. Interfacial features of the ABOw/Al-2.5%Mg composite Fig. 2 shows TEM micrographs of C1 composite. It can be clearly seen that there are many larger grains on the whisker surface from the bright field image (Fig. 2 a)) and the dark field image (Fig. 2 b)). These grains should belong to interfacial reaction products according to our previous study [3,22,23]. The selected area electron diffraction pattern (SADP) given in Fig. 2d) shows that the interfacial phase is MgAl2O4, and the average size of the interfacial phase is about 100 nm. This shows that an interface reaction between ABOw and Al-Mg alloy has occurred, the reaction might occur according to Eq. (1). The micrograph of the transverse section of the whisker shows that the interfacial layer completely covers the whisker surface and its thickness is about 100e200 nm, as shown in Fig. 2c). This suggests that a serious interface reaction exists on the interface between whisker/matrix, and the interfacial reaction has badly damaged the integrity of the whisker by consuming the whiskers. 4Al18B4O33þ33 Mg/33MgAl2O4þ6Alþ16B
(1)
3.1.2. Interfacial features of the ABOw/Al-6.8%Mg composite Fig. 3 shows the interfacial microstructure of the C2 composite. It can be seen that many interfacial products present at the interface between the whisker and matrix, as shown in Fig. 3a) [22]. Fig. 3b) shows a clear interface between the whisker and the matrix. The interface is covered by the interfacial products. An interfacial layer with 60e80 nm thickness exists at the interface, which suggests that the degree of the interface reaction of the C2 composite is weaker than that of the C1 composite. The interfacial products contain a small quantity of larger grains and lots of fine grains. The interfacial products with fine grains are detected as MgO (Fig. 3c)), and the interfacial products with larger grains are confirmed as MgAl2O4, as shown in Fig. 3d) [20]. The presence of MgO at the interface in the composite may be expressed as Eq. (2): Al18B4O33þ33 Mg/33MgOþ18Alþ4B
Fig. 1. Dimension of the tensile specimen (mm).
(2)
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Fig. 2. TEM images of the ABOw/Al-2.5%Mg composite, a) bright field image; b) dark field image; c) transverse section of the whisker; d) The SADP patterns of interphases.
The interfacial reaction was found in many of Al18B4O33 whiskers reinforced aluminum alloys containing Mg and magnesium alloy [2e6,9e11,13e16]. MgAl2O4 and MgO are together found at the interface in the C2 composite, which deals with the higher Mg contents in the Al matrix. 3.1.3. Interfacial features of the ABOw/Al-10%Mg composite Fig. 4 is the TEM morphology and SADP of the interphases in the C3 composite. Many fine grains were observed on the surface of whisker. The SADP analysis indicates that the interphase in this composite is only MgO (as shown in Fig. 4a). The reaction between ABOw and Al-Mg alloy might occur according to Eq. (2). A thin interfacial layer (about 30 nm in thickness) exists at the interface in the C3 composite, as shown in Fig. 4b). In addition, the interfacial products with larger grains observed in C1 and C2 composites can be hardly observed at the interface in the C3 composite. It is suggests that the formation of MgAl2O4 at the interface is restrained, well then a correspondingly higher Mg contents may be retained in the matrix. 3.2. Matrix character of the ABOw/Al-Mg composites 3.2.1. Dislocation states in the matrices The dislocation states in the matrices of the ABOw/Al-Mg composites containing different Mg contents are presented in Fig. 5. A correspondingly low dislocation density can be observed in the matrix of the C1 composite (Fig. 5a), which suggests that the residual stress in the composite is relaxed during the fabrication
process of the composite. A slight increase of the dislocation densities in the matrix of the C2 composite (Fig. 5 b)) can be found, which means that the relaxation of residual stress in the composite is similar to the C1 composite. However the dislocation densities in the matrix of the C3 composite are high, and dislocation tangles exist in the matrix (see Fig. 5c). The high density of dislocations in the matrix suggests that heavier plastic deformation is induced in the matrix during the fabrication process of the composite due to residual stress relaxation. The fabrication method of the composites is same, and that the volume fraction of the whiskers in the three composites is also same. The change of the dislocation states in the matrices should be attributed to the difference in Mg contents. As the Mg content increases, the yield strength of Al-Mg alloy should increases. As a result, the residual stress relaxation in the composites should be more difficult, well then, the dislocation densities in the matrices of the composites should decrease with the increase in the Mg contents. These phenomena in Fig. 5 may be directly related to the interface reaction between the whisker and the matrix in the Al-Mg matrix composites. It is well known that dislocation is introduced in composites due to the difference of coefficients of thermal expansion (CTEs) between matrix and reinforcement. A high dislocation density near the interface should exist when the composite is cooled from the elevated temperature of squeeze casting process. And the greater the difference of CTE (this difference is abbreviated asDa) is, the higher the dislocation density is. The difference of Mg contents in the composites leads to the different interfacial products at the
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Fig. 3. The interfacial microstructure of the ABOw/Al-6.8%Mg composite, a) TEM image of the interphase [20]; b) TEM image of the interfacial layer; c) and d) The SADP patterns of interphases [20].
Fig. 4. TEM images of the interface for ABOw/Al-10%Mg composite, a) interphase and its SADP pattern (insert); b) TEM image of the interfacial layer.
interfaces, it should be responsibility for the difference of dislocation density close to the interface among C1, C2 and C3. From Reactions (1) and (2), one can calculate the volume expansion according to the principle of mass invariance and the material densities before and after the interfacial reaction. This is about 8.5 pct for MgAl2O4 and þ2.2 pct for MgO when an interface reaction occurs. During the cooling process, the volume of the composite will shrink, and the shrinkage of the Al matrix is much
greater than that of the ABOw (CTE: 23 106/K for Al, 4.2 106/K for ABOw). In this case, the Da value will be greatly reduced due to Reaction (1) with a 8.5 pct volume expansion, well then a low dislocation density will exist in the C1 composite (Fig. 5a). On the contrary, for the C3 composite with MgO interfacial reaction phase, the Da value will increase due to Reaction (2) with a þ2.2 pct volume expansion, a high dislocation density appears in Fig. 5c. And for the C2 composite with MgAl2O4þMgO interfacial reaction
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Fig. 5. The dislocation states of the ABOw/Al-Mg composites containing different magnesium contents in the matrices, a) 2.5% Mg; b) 6.8%Mg; c) 10%Mg.
phase, the Da value is between the former two, the dislocation density is also between the former two. With the Mg contents increases, the interface reaction products of the ABOw/Al-Mg composites evolve from MgAl2O4 (reaction (1)) to MgAl2O4þMgO to MgO (reaction (2)). So, the dislocation density of the Al-Mg composites increases with Mg content.
3.2.2. Solution states of Mg in the matrices Fig. 6 displays the XRD patterns of the ABOw/Al-Mg composites. The diffraction peaks assigned to Al and Al18B4O33 whisker are clearly identified. It is surprised to find that the positions of Al diffraction peaks appeared at 2q values of 64 , 78 and 82 are different in the three composites. The 2q angles of Al peaks in the C1 composite are appreciably lower than that of in the C2 composite, and yet the Al peaks in the C3 composite move towards a lower angle. According to Bragg's formula, one can conclude that the decrease in 2q values necessarily implies the increase in d values (2dsinq ¼ l). To our knowledge, stress and solid solubility in the matrix are the main factors to affect the change of the peak position. The change of stress in the matrices should be small because of the same fabrication method and the same volume fraction of ABOw in the composites. Therefore, the solid solubility of Mg in Al should be the primary reason to affect the peak position of Al. As we known, atomic radius of Mg is greater than that of Al, once magnesium atoms enter into the crystal lattice of Al, the d value must increase, resulting in Al peaks moving towards the lower angles. The Mg contents are different in the matrices of the ABOw/ Al-Mg composites, and the Mg cannot be consumed entirely by the interfacial reaction. The higher the Mg contents in the matrix are,
Fig. 6. The XRD patterns of the ABOw/Al-Mg composites containing different Mg contents in the matrices.
the larger the solid solubility of Mg in the matrix is. Therefore, the 2q angles of Al peaks decrease with the increase in Mg contents. A distinct decrease in 2q values of Al peaks appear in the C3 composite, indicating that more Mg atoms enter into the crystal lattice of Al due to high Mg content in the matrix and weakly interfacial
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reaction at the interface. The changes of the microstructures necessarily affect the tensile properties of the ABOw/Al-Mg composites. 4. Tensile behavior of the ABOw/Al-Mg composites Fig. 7 is the stress-strain curves of the ABOw/Al-Mg composites containing the different Mg contents in the matrices. It can be seen that the Mg contents has a highly significant effect on the tensile property of the composites. As shown in Fig. 6, the ultimate tensile strength (UTS) of the ABOw/Al-Mg composites increases evidently, from 306 MPa, 360 MPae540 MPa with the Mg contents change from 2.5 wt%, 6.8 wt% to 10 wt%. The elongation of ABOw/Al-Mg composites slightly decreases with the increase in Mg contents, in which the elongations of C2 and C3 composites are almost the same. Fig. 8 shows the tensile fractures of the ABOw/Al-Mg composites. From Fig. 8a), the debonding of whiskers (denoted by the arrow) can be seen, which results from a weak interfacial bonding. The serious interface reaction not only damages the integrity of the whisker but also introduces lots of brittleness phase at the interface, which makes the composite easy to fracture. In addition, a small quantity of dimples can also be seen in Fig. 8a), the presence of low dislocation densities in the matrix is beneficial for the matrix to deform. So the lowest UTS and the highest d appear in the C1 composite. Fig. 8b) shows the tensile fractures of the C2 composite. The fracture morphology of the C2 composite is similar to the C1 composite. The deboning and the pull-out of the whisker can be observed in Fig. 8b), which implies the composite also has a weak interfacial bonding, it is unfavourable on the improvement of the tensile properties of the composite. Fig. 8c) is the tensile fractograph of the C3 composite. Compared with the C1 and C2 composites, the tensile fractograph of C3 composite has a distinct change. There are many of fracted whiskers on the fractograph (denoted by the arrow) and no the debonding and pull-out of whiskers are observed. These imply a good interfacial bonding exists at the interface. Moreover, there are almost no dimples in the matrix. The highest UTS and lower d are obtained in the C3 composite. According to the above analysis, it can be found that the UTS of
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the composites increases with the Mg contents, the pull-out and debonding of whiskers decrease with the increase in the Mg contents. This suggests that the formation of MgO greatly improve the bonding strength of the interface due to the existing of the thin interface layer and the low degree of damage to the whiskers. 5. Discussions 5.1. Interfacial reaction of the composites Many researches indicated that ABOw was prone to react with Mg element in both aluminum alloy (contained Mg) and magnesium alloy. In this study, we find that Mg contents in the matrices of ABOw/Al-Mg composites obviously affect the types of the interfacial reaction products. The ABOw/Al-Mg composite system can be thought as an Al-Mg-B-O system. The ability for magnesium to form its oxide is reported to be about 8 times higher than that of aluminum at any temperature [20,24]. In addition, the study on thermodynamic stability of AleMg oxides in AleMg alloys [25e27] shows that the formation of MgAl2O4 and MgO is competitive processes. For the alloys with lower magnesium contents, the formation of MgAl2O4 is more favorable, but when the magnesium content in the alloy exceeds 4 wt%, it is easy to form MgO thermodynamically [25,27]. The interfacial reaction product in the C1 composite is only MgAl2O4 (Fig. 2) due to the low Mg content in the matrix. Moreover, a thick interface layer exists at the interface owing to the serious reaction between the ABOw and the matrix. In the C2 composite, the Mg content in the matrix alloy just exceeds 4 wt%, therefore, MgO is prone to form at the interface according to the thermodynamic analysis. Once MgO forms at the interface, the Mg content in the matrix near the interface necessarily decreases due to the consumption of Mg element, resulting in the formation of MgAl2O4 at the interface accordingly. Therefore, MgAl2O4 and MgO can be found in the C2 composite together (Fig. 3). Moreover, most surfaces of the whiskers are covered by the firstly formed MgO during the squeeze casting, there is little chance for the whisker to react with Mg adequately. Therefore, a small quantity of MgAl2O4 presents at the interface. Once more, a thin interfacial layer exists at the interface. In the C3 composite, MgO is the only interfacial reaction product due to the higher Mg contents in the matrix. The thinnest interfacial layer appears at the interface. The higher Mg contents decrease the degree of the interfacial reaction and result in the thinnest interfacial layer at the interface. Lloyd et al. [28] studied the interface reaction in alumina reinforced aluminium (contained Mg）composites and found that the size of the reaction products between the alumina particles and magnesium became smaller as the Mg contents increased. This implies that the nucleation process of the reaction product is quick when a high Mg content in the matrix. The fine MgO is observed at the interfaces in the C2 and C3 composites should relate to the quick nucleation of MgO. The quick nucleation of MgO at the interface leads to the active points on the whisker surface being covered rapidly, as a result, the degree of the interfacial reaction decreases. In this case, it is easy to understand that the thickness of the interfacial layer decreases with the increase in the Mg contents. 5.2. The tensile properties of the composites
Fig. 7. The tensile stress-strain curves of the ABOw/Al-Mg composites containing different Mg contents in the matrices.
From the above analysis, the Mg contents affect the morphology, type and thickness of the interphases at the interface and the solution of Mg as well as the dislocation density in the matrix of the composite. The UTS of the composite is suggested to be due to the combination of several factors. The solid solution strengthening of Mg and the interfacial bonding states are the main factors to
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Fig. 8. Tensile fractographs of the ABOw/Al-Mg composites containing different Mg contents in the matrices, a) 2.5% Mg; b) 6.8% Mg; c) 10% Mg.
influence the UTS of the composites due to a little change in dislocation states. The reasons that the UTS of the composites increases with the Mg contents in the matrices is as follows: As the Mg contents increase, the interfacial layers get more and more thin (Figss. 2e4) and the interfacial reaction degree reduces continuously. Hereby the integrality of whiskers in the composites enhances evidently, which make the composites hard to fracture. Moreover, more Mg comes into the matrix (Fig. 6) and the solution strengthening of Mg in the composites is more and more evident. As a result, the UTS of the composite increases with the Mg contents in the matrices. The highest UTS is obtained by the C3 composite, resulting from the minimum damage of whiskers (caused high interfacial bonding) and the maximum solid solution of Mg in the matrix. The effects of the interfacial bonding states and solution strengthening of Mg are all strong. 6. Conclusions 1. The Mg contents in the matrices greatly affect the interfacial phases in the ABOw/Al-Mg composites. As the Mg contents increases, the interface reaction products evolve from MgAl2O4 to MgAl2O4 þMgO to MgO. 2. The interfacial reaction degree between the whisker and the AlMg matrix along with the thickness of interfacial layer gradually reduces with the increasing of the Mg contents. The solid solubility degree of Mg in the matrices of the ABOw/Al-Mg composites increases with the Mg contents. 3. The Mg contents in the matrices affect the UTS of the ABOw/AlMg composites. The UTS of the composites increases dramatically, from 306 MPa, 360 MPae540 MPa with the Mg contents change from 2.5 wt%, 6.8 wt% to 10 wt%. The solid solution strengthening of Mg and the interfacial bonding states are the main factors to influence the UTS of the composites.
Foundation item Projects supported by Doctor Research Start-Up Foundation of
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doi.org/10.1016/j.msea.2003.09.085. [21] M.Y. Zheng, K. Wu, C.K. Yao, S. Kamado, Y. Kojima, Squeeze cast Al18B4O33 whisker-reinforced magnesium matrix composite, J. Mater. Sci. 21 (2002) 533e535. https://doi.org/10.1023/A:1015496619283. [22] Y.C. Yu, S.W. Tang, Z.L. Wang, J. Hu, Effects of coating contents on the interfacial reaction and tensile properties of Al2O3 coated-Al18B4O33w/Al-Mg matrix composites, Mater. Char. 107 (2015) 327e333. https://doi.org/10. 1016/j.matchar.2015.07.029. [23] Y.C. Yu, S.W. Tang, J. Hu, Effects of heat-treatment on the interfacial reaction and tensile properties of Al2O3 coated-Al18B4O33w/Al-Mg matrix composites, Mater. Des. 90 (2016) 416e423. https://doi.org/10.1016/j.matdes.2015. 11.011. [24] D.R. Gaskell, Introduction to Metallurgical Thermodynamics, Hemisphere, New York, 1981, p. 287. [25] B.C. Pai, S. Ray, Fabrication of aluminium-alumina (magnesia) particulate composites in foundries using magnesium additions to the melts, Mater. Sci. Eng. 24 (1976) 31e44. https://doi.org/10.1016/0025-5416(76)90092-6. [26] W.M. Zhong, G. L'Esperance, M. Suery, Interfacial Reactions in Al-Mg (5083)/ Al2O3p composites during fabrication and remitting, Metall. Mater. Trans. A 26A (1995) 2625e2635. https://doi.org/10.1007/BF02669420. [27] D.R. Kongshaug, J.B. Ferguson, B.F. Schultz, P.K. Rohatgi, Reactive stir mixing of Al-Mg/Al2O3np metal matrix nanocomposites: effects of Mg and reinforcement concentration and method of reinforcement incorporation, J. Mater. Sci. 49 (2014) 2106e2116. https://doi.org/10.1007/s10853-013-7903-7. [28] D.J. Lloyd, I. Jin, G.C. Weatherly, Controlling the interface reaction in alumina reinforced aluminium composites, Scr. Metall. Mater. 31 (4) (1994) 393e396. https://doi.org/10.1016/0956-716X(94)90006-X.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effects of Mg contents on microstructure and tensile properties of Al18B4O33w/Al-Mg composites Zhenling Wang a, b, Shawei Tang a, **, Yucheng Yu a, b, Jin Hu c, * a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Institute of Vanadium and Titanium, Panzhihua University, PanZhiHua 617000, PR China c National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2018 Received in revised form 22 November 2018 Accepted 24 November 2018 Available online 26 November 2018
Al18B4O33 whiskers reinforced Al-Mg matrix composites with different Mg contents are fabricated by squeeze casting method. The effects of Mg contents on the microstructure and the tensile properties of the composites are investigated. The results indicate that the Mg contents have strong influences on the type of interphase, the thickness of interfacial layer and the solid solubility of Mg along with the dislocation density in the matrices of the composites. The interface reaction products between the whiskers and Mg evolve from MgAl2O4 to MgAl2O4þMgO to MgO with the increase in Mg contents. The ultimate tensile strength (UTS) of the composites increases dramatically with the Mg contents, resulting from the combination of the interface strengthening and the solid solution strengthening of Mg. It is worth mentioning that the UTS of the composite containing 10 wt% Mg in the matrix attains a highest value (540 MPa) in as-cast Al18B4O33w/Al composites reported. © 2018 Elsevier B.V. All rights reserved.
Keywords: Discontinuous reinforcement Metal-matrix composites (MMCs) Interface/interphase Strength
1. Introduction Aluminum borate whisker (Al18B4O33 whisker, denoted by ABOw) reinforced aluminum matrix composite has attracted much interest owing to their excellent mechanical properties and low cost [1,2]. In general, high strength aluminum alloys, such as 2024Al, 6061Al, 7075Al and so on, are often used as a matrix material in ABOw/Al composites [3e8]. However, it is well known that ABOw can react with the magnesium included in matrix alloys during the casting and subsequent heat-treatment in ABOw reinforced aluminum (or magnesium) matrix composites. The severely interfacial reaction not only degrades the properties of ABOw/Al composites owing to the damage of whiskers, but also weakens the strength of aluminum alloy matrix due to the depletion of magnesium element in the alloy matrix [9e13]. Therefore, up to now, in order to improve the performance of ABOw/Al composites, lots of researchers put their eyes on the question how to inhibit the interface reaction [12e17]. Gao et al. [14] prepared Cu coating on
* Corresponding author. BOX 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: þ86 0451 86415894. ** Corresponding author. BOX 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. E-mail addresses: [email protected] (S. Tang), [email protected] (J. Hu). https://doi.org/10.1016/j.jallcom.2018.11.328 0925-8388/© 2018 Elsevier B.V. All rights reserved.
the surface of ABOw by an electroless plating method and fabricated Cu-coated ABOw reinforced 6061Al composite. They found that the UTS of the composite was enhanced by the Cu coating, however, the highest value of UTS was only about 350 MPa. Yue et al. [15] fabricated Cr2O3-coated ABOw reinforced 2024Al composites and the coating of whisker was prepared by means of a solgel method. Cr2O3-coated ABOw could hinder the interfacial reaction between aluminum borate whiskers and magnesium in the matrix, however, the coating could not improve effectively the UTS (360 MPa) of the composite due to the weak interfacial bonding. In order to restrain the interfacial reaction and improve the wettability between whisker and aluminum matrix, Ding et al. [16] deposited Al2O3 coatings on the surface of ABOw by means of the sol-gel method. However, the UTS of the composite could reach no more than 350 MPa. Obviously, the coatings effectively inhibit the reaction between the whisker and the matrix, but do not achieve the desired mechanical properties. Hu et al. [18] studied aluminum borate whisker reinforced pure aluminum composite, and found that no interface reaction between whisker and aluminum occurred. Li et al. [19] reported the mechanical properties of aluminum borate whisker reinforced pure aluminum matrix composites, and UTS of the composite was only 290 MPa.
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From the above, a conclusion can be drawn that, so far, there is no best way to achieve a high strength for aluminum borate whisker reinforced Al composites. Severely interfacial reaction can damage whiskers to reduce the enhancement effect of whisker, and the interface without interface reaction and the interface with coating have not achieved an expected performance. So is there a slightly reactive interface that enables composite materials to get excellent mechanical properties? To find out, we must find out the interface reaction mechanism between Mg and ABOw. As we all know, the interface plays an important role for high performance composites, and the study of interface reaction mechanism is the most basic and important. However, the reaction mechanism of Mg and ABOw is still controversial and lacking. Early findings showed that, the interface reaction product was only MgAl2O4 in ABOw reinforced aluminum matrix composites that contained Mg in the matrices [2e5,9e11], however, the interface reaction product was only MgO in ABOw reinforced magnesium matrix composites [20,21]. It was worth mentioning that the interface reaction products were MgAl2O4þMgO when the magnesium content in the matrix was 6.8 wt% in a ABOw/Al-Mg composite, which had been found in our previous study [22]. However, it is unfortunately that few researches are carried out about the effects of Mg contents on interfacial reaction between Al alloy containing Mg and ABOw during the fabrication of the composites by squeeze casting. It is of great importance to understand that the effects of Mg contents on the interfacial reaction between Al alloy and ABOw and the mechanical properties of ABOw/Al-Mg composites. This is because the interfacial reaction has been regarded as one of the main factors that significantly affect the properties of the composite. In the present work, ABOw/Al-Mg composites containing different Mg contents are fabricated by squeeze casting method. The microstructure, interfacial reaction and tensile behavior of the composites are investigated and discussed. 2. Materials and experimental method Al alloy with 2.5 wt% Mg, 6.8 wt% Mg and 10 wt% Mg utilized for matrix materials in these experiments are prepared in situ from commercially pure Al (99.5 wt%), magnesium (99.7 wt%). The incorporated material is ABOw with a diameter of 0.5～1 mm and length of 10～30 mm made in Shikoku Chemical Company of Japan. The preparation process of Al-Mg matrix composites is as follows: firstly, the raw ABOw is dispersed by mechanical stirring and ultrasonic vibration generator in a deionized water solution, and then the water solution contained the dispersed ABOw is poured into a filter mould for the preparation of a whisker preform; The whisker preforms in a diameter of 90 mm and a height of 20 mm are dried at room temperature for 48 h, then dried at 80～100 C for 24 h, finally the performs are sintered at 900 C for 1 h to obtain a high strength. The whisker performs and the mould are preheated at 550 C, and then the Al-Mg alloy at 750 C is cast into the mould with a pressure of 100 MPa. The Al-Mg matrix composites reinforced by Al18B4O33w (20% volume fraction) with different Mg contents are obtained. For simplification, the Al matrix composites contained 2.5 wt% Mg, 6.8 wt% Mg and 10 wt% Mg are named as C1, C2 and C3, respectively. The phase compositions of the composites are identified using a Philips X'Pert X-ray diffractometer with a Cu Ka radiation source at 40 kV and 40 mA. Interfacial microstructures of the composites are observed using a FEI G2F30 transmission electron microscopy (TEM) with an accelerating voltage of 120 kV, and the specimens used for TEM observation are mechanically abraded to a thickness of 40 mm, and then thinned by ion milling. Tensile tests are performed on an Insrton 5590 test machine with a speed of 0.5 mm/
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min at room temperature. Dimensions of the tensile specimens are shown in Fig. 1. The morphologies of tensile fractographs of the composites are examined by a Hitch S-4700N scanning electron microscope (SEM) with an accelerating voltage of 20 kV.
3. Results 3.1. Typical interfacial features of the ABOw/Al-Mg composites 3.1.1. Interfacial features of the ABOw/Al-2.5%Mg composite Fig. 2 shows TEM micrographs of C1 composite. It can be clearly seen that there are many larger grains on the whisker surface from the bright field image (Fig. 2 a)) and the dark field image (Fig. 2 b)). These grains should belong to interfacial reaction products according to our previous study [3,22,23]. The selected area electron diffraction pattern (SADP) given in Fig. 2d) shows that the interfacial phase is MgAl2O4, and the average size of the interfacial phase is about 100 nm. This shows that an interface reaction between ABOw and Al-Mg alloy has occurred, the reaction might occur according to Eq. (1). The micrograph of the transverse section of the whisker shows that the interfacial layer completely covers the whisker surface and its thickness is about 100e200 nm, as shown in Fig. 2c). This suggests that a serious interface reaction exists on the interface between whisker/matrix, and the interfacial reaction has badly damaged the integrity of the whisker by consuming the whiskers. 4Al18B4O33þ33 Mg/33MgAl2O4þ6Alþ16B
(1)
3.1.2. Interfacial features of the ABOw/Al-6.8%Mg composite Fig. 3 shows the interfacial microstructure of the C2 composite. It can be seen that many interfacial products present at the interface between the whisker and matrix, as shown in Fig. 3a) [22]. Fig. 3b) shows a clear interface between the whisker and the matrix. The interface is covered by the interfacial products. An interfacial layer with 60e80 nm thickness exists at the interface, which suggests that the degree of the interface reaction of the C2 composite is weaker than that of the C1 composite. The interfacial products contain a small quantity of larger grains and lots of fine grains. The interfacial products with fine grains are detected as MgO (Fig. 3c)), and the interfacial products with larger grains are confirmed as MgAl2O4, as shown in Fig. 3d) [20]. The presence of MgO at the interface in the composite may be expressed as Eq. (2): Al18B4O33þ33 Mg/33MgOþ18Alþ4B
Fig. 1. Dimension of the tensile specimen (mm).
(2)
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Fig. 2. TEM images of the ABOw/Al-2.5%Mg composite, a) bright field image; b) dark field image; c) transverse section of the whisker; d) The SADP patterns of interphases.
The interfacial reaction was found in many of Al18B4O33 whiskers reinforced aluminum alloys containing Mg and magnesium alloy [2e6,9e11,13e16]. MgAl2O4 and MgO are together found at the interface in the C2 composite, which deals with the higher Mg contents in the Al matrix. 3.1.3. Interfacial features of the ABOw/Al-10%Mg composite Fig. 4 is the TEM morphology and SADP of the interphases in the C3 composite. Many fine grains were observed on the surface of whisker. The SADP analysis indicates that the interphase in this composite is only MgO (as shown in Fig. 4a). The reaction between ABOw and Al-Mg alloy might occur according to Eq. (2). A thin interfacial layer (about 30 nm in thickness) exists at the interface in the C3 composite, as shown in Fig. 4b). In addition, the interfacial products with larger grains observed in C1 and C2 composites can be hardly observed at the interface in the C3 composite. It is suggests that the formation of MgAl2O4 at the interface is restrained, well then a correspondingly higher Mg contents may be retained in the matrix. 3.2. Matrix character of the ABOw/Al-Mg composites 3.2.1. Dislocation states in the matrices The dislocation states in the matrices of the ABOw/Al-Mg composites containing different Mg contents are presented in Fig. 5. A correspondingly low dislocation density can be observed in the matrix of the C1 composite (Fig. 5a), which suggests that the residual stress in the composite is relaxed during the fabrication
process of the composite. A slight increase of the dislocation densities in the matrix of the C2 composite (Fig. 5 b)) can be found, which means that the relaxation of residual stress in the composite is similar to the C1 composite. However the dislocation densities in the matrix of the C3 composite are high, and dislocation tangles exist in the matrix (see Fig. 5c). The high density of dislocations in the matrix suggests that heavier plastic deformation is induced in the matrix during the fabrication process of the composite due to residual stress relaxation. The fabrication method of the composites is same, and that the volume fraction of the whiskers in the three composites is also same. The change of the dislocation states in the matrices should be attributed to the difference in Mg contents. As the Mg content increases, the yield strength of Al-Mg alloy should increases. As a result, the residual stress relaxation in the composites should be more difficult, well then, the dislocation densities in the matrices of the composites should decrease with the increase in the Mg contents. These phenomena in Fig. 5 may be directly related to the interface reaction between the whisker and the matrix in the Al-Mg matrix composites. It is well known that dislocation is introduced in composites due to the difference of coefficients of thermal expansion (CTEs) between matrix and reinforcement. A high dislocation density near the interface should exist when the composite is cooled from the elevated temperature of squeeze casting process. And the greater the difference of CTE (this difference is abbreviated asDa) is, the higher the dislocation density is. The difference of Mg contents in the composites leads to the different interfacial products at the
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Fig. 3. The interfacial microstructure of the ABOw/Al-6.8%Mg composite, a) TEM image of the interphase [20]; b) TEM image of the interfacial layer; c) and d) The SADP patterns of interphases [20].
Fig. 4. TEM images of the interface for ABOw/Al-10%Mg composite, a) interphase and its SADP pattern (insert); b) TEM image of the interfacial layer.
interfaces, it should be responsibility for the difference of dislocation density close to the interface among C1, C2 and C3. From Reactions (1) and (2), one can calculate the volume expansion according to the principle of mass invariance and the material densities before and after the interfacial reaction. This is about 8.5 pct for MgAl2O4 and þ2.2 pct for MgO when an interface reaction occurs. During the cooling process, the volume of the composite will shrink, and the shrinkage of the Al matrix is much
greater than that of the ABOw (CTE: 23 106/K for Al, 4.2 106/K for ABOw). In this case, the Da value will be greatly reduced due to Reaction (1) with a 8.5 pct volume expansion, well then a low dislocation density will exist in the C1 composite (Fig. 5a). On the contrary, for the C3 composite with MgO interfacial reaction phase, the Da value will increase due to Reaction (2) with a þ2.2 pct volume expansion, a high dislocation density appears in Fig. 5c. And for the C2 composite with MgAl2O4þMgO interfacial reaction
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Fig. 5. The dislocation states of the ABOw/Al-Mg composites containing different magnesium contents in the matrices, a) 2.5% Mg; b) 6.8%Mg; c) 10%Mg.
phase, the Da value is between the former two, the dislocation density is also between the former two. With the Mg contents increases, the interface reaction products of the ABOw/Al-Mg composites evolve from MgAl2O4 (reaction (1)) to MgAl2O4þMgO to MgO (reaction (2)). So, the dislocation density of the Al-Mg composites increases with Mg content.
3.2.2. Solution states of Mg in the matrices Fig. 6 displays the XRD patterns of the ABOw/Al-Mg composites. The diffraction peaks assigned to Al and Al18B4O33 whisker are clearly identified. It is surprised to find that the positions of Al diffraction peaks appeared at 2q values of 64 , 78 and 82 are different in the three composites. The 2q angles of Al peaks in the C1 composite are appreciably lower than that of in the C2 composite, and yet the Al peaks in the C3 composite move towards a lower angle. According to Bragg's formula, one can conclude that the decrease in 2q values necessarily implies the increase in d values (2dsinq ¼ l). To our knowledge, stress and solid solubility in the matrix are the main factors to affect the change of the peak position. The change of stress in the matrices should be small because of the same fabrication method and the same volume fraction of ABOw in the composites. Therefore, the solid solubility of Mg in Al should be the primary reason to affect the peak position of Al. As we known, atomic radius of Mg is greater than that of Al, once magnesium atoms enter into the crystal lattice of Al, the d value must increase, resulting in Al peaks moving towards the lower angles. The Mg contents are different in the matrices of the ABOw/ Al-Mg composites, and the Mg cannot be consumed entirely by the interfacial reaction. The higher the Mg contents in the matrix are,
Fig. 6. The XRD patterns of the ABOw/Al-Mg composites containing different Mg contents in the matrices.
the larger the solid solubility of Mg in the matrix is. Therefore, the 2q angles of Al peaks decrease with the increase in Mg contents. A distinct decrease in 2q values of Al peaks appear in the C3 composite, indicating that more Mg atoms enter into the crystal lattice of Al due to high Mg content in the matrix and weakly interfacial
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reaction at the interface. The changes of the microstructures necessarily affect the tensile properties of the ABOw/Al-Mg composites. 4. Tensile behavior of the ABOw/Al-Mg composites Fig. 7 is the stress-strain curves of the ABOw/Al-Mg composites containing the different Mg contents in the matrices. It can be seen that the Mg contents has a highly significant effect on the tensile property of the composites. As shown in Fig. 6, the ultimate tensile strength (UTS) of the ABOw/Al-Mg composites increases evidently, from 306 MPa, 360 MPae540 MPa with the Mg contents change from 2.5 wt%, 6.8 wt% to 10 wt%. The elongation of ABOw/Al-Mg composites slightly decreases with the increase in Mg contents, in which the elongations of C2 and C3 composites are almost the same. Fig. 8 shows the tensile fractures of the ABOw/Al-Mg composites. From Fig. 8a), the debonding of whiskers (denoted by the arrow) can be seen, which results from a weak interfacial bonding. The serious interface reaction not only damages the integrity of the whisker but also introduces lots of brittleness phase at the interface, which makes the composite easy to fracture. In addition, a small quantity of dimples can also be seen in Fig. 8a), the presence of low dislocation densities in the matrix is beneficial for the matrix to deform. So the lowest UTS and the highest d appear in the C1 composite. Fig. 8b) shows the tensile fractures of the C2 composite. The fracture morphology of the C2 composite is similar to the C1 composite. The deboning and the pull-out of the whisker can be observed in Fig. 8b), which implies the composite also has a weak interfacial bonding, it is unfavourable on the improvement of the tensile properties of the composite. Fig. 8c) is the tensile fractograph of the C3 composite. Compared with the C1 and C2 composites, the tensile fractograph of C3 composite has a distinct change. There are many of fracted whiskers on the fractograph (denoted by the arrow) and no the debonding and pull-out of whiskers are observed. These imply a good interfacial bonding exists at the interface. Moreover, there are almost no dimples in the matrix. The highest UTS and lower d are obtained in the C3 composite. According to the above analysis, it can be found that the UTS of
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the composites increases with the Mg contents, the pull-out and debonding of whiskers decrease with the increase in the Mg contents. This suggests that the formation of MgO greatly improve the bonding strength of the interface due to the existing of the thin interface layer and the low degree of damage to the whiskers. 5. Discussions 5.1. Interfacial reaction of the composites Many researches indicated that ABOw was prone to react with Mg element in both aluminum alloy (contained Mg) and magnesium alloy. In this study, we find that Mg contents in the matrices of ABOw/Al-Mg composites obviously affect the types of the interfacial reaction products. The ABOw/Al-Mg composite system can be thought as an Al-Mg-B-O system. The ability for magnesium to form its oxide is reported to be about 8 times higher than that of aluminum at any temperature [20,24]. In addition, the study on thermodynamic stability of AleMg oxides in AleMg alloys [25e27] shows that the formation of MgAl2O4 and MgO is competitive processes. For the alloys with lower magnesium contents, the formation of MgAl2O4 is more favorable, but when the magnesium content in the alloy exceeds 4 wt%, it is easy to form MgO thermodynamically [25,27]. The interfacial reaction product in the C1 composite is only MgAl2O4 (Fig. 2) due to the low Mg content in the matrix. Moreover, a thick interface layer exists at the interface owing to the serious reaction between the ABOw and the matrix. In the C2 composite, the Mg content in the matrix alloy just exceeds 4 wt%, therefore, MgO is prone to form at the interface according to the thermodynamic analysis. Once MgO forms at the interface, the Mg content in the matrix near the interface necessarily decreases due to the consumption of Mg element, resulting in the formation of MgAl2O4 at the interface accordingly. Therefore, MgAl2O4 and MgO can be found in the C2 composite together (Fig. 3). Moreover, most surfaces of the whiskers are covered by the firstly formed MgO during the squeeze casting, there is little chance for the whisker to react with Mg adequately. Therefore, a small quantity of MgAl2O4 presents at the interface. Once more, a thin interfacial layer exists at the interface. In the C3 composite, MgO is the only interfacial reaction product due to the higher Mg contents in the matrix. The thinnest interfacial layer appears at the interface. The higher Mg contents decrease the degree of the interfacial reaction and result in the thinnest interfacial layer at the interface. Lloyd et al. [28] studied the interface reaction in alumina reinforced aluminium (contained Mg）composites and found that the size of the reaction products between the alumina particles and magnesium became smaller as the Mg contents increased. This implies that the nucleation process of the reaction product is quick when a high Mg content in the matrix. The fine MgO is observed at the interfaces in the C2 and C3 composites should relate to the quick nucleation of MgO. The quick nucleation of MgO at the interface leads to the active points on the whisker surface being covered rapidly, as a result, the degree of the interfacial reaction decreases. In this case, it is easy to understand that the thickness of the interfacial layer decreases with the increase in the Mg contents. 5.2. The tensile properties of the composites
Fig. 7. The tensile stress-strain curves of the ABOw/Al-Mg composites containing different Mg contents in the matrices.
From the above analysis, the Mg contents affect the morphology, type and thickness of the interphases at the interface and the solution of Mg as well as the dislocation density in the matrix of the composite. The UTS of the composite is suggested to be due to the combination of several factors. The solid solution strengthening of Mg and the interfacial bonding states are the main factors to
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Fig. 8. Tensile fractographs of the ABOw/Al-Mg composites containing different Mg contents in the matrices, a) 2.5% Mg; b) 6.8% Mg; c) 10% Mg.
influence the UTS of the composites due to a little change in dislocation states. The reasons that the UTS of the composites increases with the Mg contents in the matrices is as follows: As the Mg contents increase, the interfacial layers get more and more thin (Figss. 2e4) and the interfacial reaction degree reduces continuously. Hereby the integrality of whiskers in the composites enhances evidently, which make the composites hard to fracture. Moreover, more Mg comes into the matrix (Fig. 6) and the solution strengthening of Mg in the composites is more and more evident. As a result, the UTS of the composite increases with the Mg contents in the matrices. The highest UTS is obtained by the C3 composite, resulting from the minimum damage of whiskers (caused high interfacial bonding) and the maximum solid solution of Mg in the matrix. The effects of the interfacial bonding states and solution strengthening of Mg are all strong. 6. Conclusions 1. The Mg contents in the matrices greatly affect the interfacial phases in the ABOw/Al-Mg composites. As the Mg contents increases, the interface reaction products evolve from MgAl2O4 to MgAl2O4 þMgO to MgO. 2. The interfacial reaction degree between the whisker and the AlMg matrix along with the thickness of interfacial layer gradually reduces with the increasing of the Mg contents. The solid solubility degree of Mg in the matrices of the ABOw/Al-Mg composites increases with the Mg contents. 3. The Mg contents in the matrices affect the UTS of the ABOw/AlMg composites. The UTS of the composites increases dramatically, from 306 MPa, 360 MPae540 MPa with the Mg contents change from 2.5 wt%, 6.8 wt% to 10 wt%. The solid solution strengthening of Mg and the interfacial bonding states are the main factors to influence the UTS of the composites.
Foundation item Projects supported by Doctor Research Start-Up Foundation of
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