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Structural and morphological study of a 2024 Al–Al2O3 composite produced by mechanical alloying in high energy mill
Materials and Design 37 (2012) 96–101
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Structural and morphological study of a 2024 Al–Al2O3 composite produced by mechanical alloying in high energy mill J.L. Hernández Rivera a,⇑, J.J. Cruz Rivera b, V. Paz del Ángel c, V. Garibay Febles c, O. Coreño Alonso d, R. Martínez-Sánchez a a
Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, C.P. 31109, Chihuahua, Mexico Facultad de Ingeniería-Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí, Sierra Leona 550, Lomas 2ª sección, C.P. 78210, SLP, Mexico Instituto Mexicano del Petróleo, Laboratorio de Microscopia Electrónica de Ultra Alta Resolución, Eje Central Lázaro Cárdenas 132, San Bartolo Atepehuacan, C.P. 07730, México D.F., México d Universidad de Guanajuato, División de Ingenierías, Dpto. Ing. Civil, Juárez 77 Col Centro, Guanajuato, Mexico b c
a r t i c l e
i n f o
Article history: Received 12 October 2011 Accepted 23 December 2011 Available online 3 January 2012 Keywords: A. Metal matrix composite B. Powders C. Mechanical alloying
a b s t r a c t 2024 Al composite reinforced with Al2O3 particles was obtained by mechanical alloying (MA) using Al, Cu and Mg elemental powders as raw materials and Al2O3 nanoparticles as reinforcement. The results shown that as the MA time increased, the non-reinforced (WR) and Al2O3 reinforced powders (R1A and R2A) morphology changed from flake-flattened to equiaxed. Regarding the average particle size, WR group displayed a continuous decreasing value even for a processing time of 10 h while R2A and R1A groups shown a constant value for the same time. This led to the conclusion that steady state of the process was reached in shorter times in presence of Al2O3 nanoparticles. It was found that the reinforcement was present in the matrix like isolated particles and small agglomerates which affected the dislocation motion, and it was assumed that this fact caused the increase observed in the microhardness values. There was no evidence of new phase precipitation through MA process. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Al alloys with superior mechanical and physical properties are promising materials for several industrial components where lightweight is required. One of the emerging technologies for the production of these alloys is the development of Al matrix composites. It is known that Al matrix confers some unique properties to the composites such as lightweight, environmental resistance, high specific strength and stiffness, high thermal and electrical conductivity and good wear resistance. Mechanical properties of Al composites can be improved by dispersion of ceramic particles into the metal matrix. By another hand, an important consideration during fabrication of composites is the reinforcement size. For example, Jia [1] reported that the mechanical strength of Al composites can be increased up to 20% if the reinforcement size is reduced from the order of micro to nanoscale. Several methods such as ultrasonically cast [2], gas pressure infiltration [3], squezze casting [4,5], semi-solid mechanical stir-
⇑ Corresponding author. Address: Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109, Chihuahua, Mexico. Tel.: +52 614 4391146. E-mail address: [email protected] (J.L. Hernández Rivera). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.12.035
ring [6] and mechanical alloying [7] have been used for the production Al composites. MA has been proved to be a process that overcomes some drawbacks (for example segregation phenomena) that commonly occur in composites produced from liquid processes. When this phenomena occurs the alloy elements distribution is not homogeneous and it can decrease the resulting mechanical properties. In addition, when liquid processes are used there are some density differences between matrix and reinforcement particles which do not allow a uniform distribution of the reinforcement. MA process could be used to obtain homogeneous nanocomposites due to the high microstructure refinement achieved in the matrix crystallite size. Generally, MA is performed in three steps [8]. Firstly, powders suffer impacts among them and balls with little fracture and plastic deformation. In the second step, plastic deformation and cold welding process predominate. Finally, powders harden and then fracture due to the continuous increase of plastic deformation giving rise to new and smaller surfaces. In this step, there is a balance between cold welding and fracture frequency. Therefore, it can be said that the ‘‘steady state condition’’ has been reached. Further processing times will not cause a significant effect either on the particle size, shape and morphology [9]. On the other hand, the successful processing of Al composites depends not only on the appropriate performance of the technique
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J.L. Hernández Rivera et al. / Materials and Design 37 (2012) 96–101
as was mentioned above, but also on the adequate reinforcement selection. In this context, Al2O3 is a well-studied and universally used ceramic that has attractive properties such as excellent wear and oxidation resistance and good high temperature strength. Due to these properties, it has been commonly incorporated to Al composites in order to increase the mechanical properties or to optimize the MA process. However, little literature exists about the role of this nanoreinforcement on the microstructural evolution of Al composites during MA. In this work it was studied the effect of the Al2O3 nanoparticles on the Al composites during MA and its influence on the size and powders morphology. Also, the effectiveness of MA in terms of achieving the steady state condition was widely described.
2. Experimental procedure Raw powders used to produce the Al-2024 alloy into commercial composition range were Al (purity of 99.9%, 325 mesh), Cu (purity of 99.9%, 200 mesh) and Mg (purity of 99.8%, 325 mesh). Traces of Mn, Si, Ti, and Zn powder were also added. Table 1 shows the quantities of the elements used to produce the alloy. MA was carried out in a Simoloyer high energy mill (ZOZ CM01) under Ar atmosphere. Balls-to-powder weight ratio was 13.3 and the process control agent was methanol. Powder mixtures were blended for 5 min prior to MA to obtain an adequate and homogeneous mix. 1 and 2 wt.% of Al2O3 nanoparticles were added to the matrix and MA times were 1, 3, 5 and 10 h. For comparison, samples neither mechanically alloyed nor reinforced were also produced. Table 2 shows the nomenclature employed for the composite powders. Characterization of microstructural evolution was carried out by scanning electron microscopy (SEM) using a Philips XL-30 microscope which was coupled with an energy dispersive spectroscopy unit (EDS). Two transmission electron microscopes (TEMs) were used for general observations and high resolution work. A FEI-TECNAI T20 and a FEI-Titan operated at 200 kV and 300 kV, respectively. The average particle size was measured using the Image J analyzer software. Measurements were obtained from 10 fields for each sample. X-ray diffraction experiments were carried out in a Rigaku DMAX-2200 diffractometer using 40 kV, 36 mA, step size of 0.02 and monochromatic radiation of Cu Ka with wavelength of 1.5405 nm. Average crystallite size of specimens was obtained from the processed and analyzed XRD patterns using MAUD software. This software is based on Rietveld refinement method in which experimental and theoretical patterns are fitted through iterative application of least-squares algorithm. The iteration was finished when sigma and Rw statistical indicators converged to smaller values than 2 and 15, respectively. These values are recommended by Lutterotti [10] because they found that the residuals or differences between experimental and calculated pattern were minimum when these indicators reached those values. As a result, reliable structural information such as crystallite size can be obtained for this procedure.
Table 1 Quantities of elemental powders used to obtain the 2024 Al alloy.
Table 2 Nomenclature of the MA composite powders. Specimen
MA time (h) 0
1
3
5
10
WR3 R1A3 R2A3
WR5 R1A5 R2A5
WR10 R1A10 R2A10
Nomenclature Without reinforcement (WR group) 1% Al2O3 (R1A group) 2% Al2O3 (R2A group)
WR0 R1A0 R2A0
WR1 R1A1 R2A1
3. Results and discussion 3.1. Scanning electron microscopy Figs. 1 and 2 show the powders microstructure of the WR and R2A groups, respectively. It can be observed light-gray color particles of Al with some lamellas inside in both micrographs. The white-colored ones correspond to Cu while the darker gray to Mg. For 1 h of processing it is clear that alloying elements remained coarse. It can be taken as a signal that this time was not enough to disperse them homogeneously throughout the matrix (Figs. 1 and 2, incises A and B). Apparently, these lamellas had preferred orientation. In the case of powders resulting from 10 h of MA, Cu and Mg lamellas were finer and exhibit a random orientation (Figs. 1 and 2, incises C and D). However, it can be clearly noticed that Cu and Mg lamellas were smaller and a homogeneous dispersion was obtained in the presence of Al2O3 for 10 h of MA (compare Figs. 1D and 2D). On the other hand, the WR powders exhibited a flake-flattened morphology for earlier times (Fig. 1A and B) but it changed to equiaxed when the MA time was increased to 10 h (Fig. 1C and D). In the R2A powders the flake-flattened morphology was observed after 1 h of MA and then it changed to a more equiaxed when a time of 10 h was achieved. It is known that when the alloying process is accelerated, the main attributable cause is the powder surface energy modification by presence of a reinforcement which reduces the time to achieve steady state [9,11]. If it is taken into account that finer distribution of alloy lamellas were observed inside Al powders reinforced with Al2O3 nanoparticles, it can be established that MA was accelerated by this reinforcement addition. Furthermore, according to previous studies [12,13] when MA steady state is reached, the average particle size becomes constant and does not change anymore. Fig. 3 shows the effect of MA time on the average particle size of WR and RA specimens. For the WR group there was an increase of this parameter as MA time increased until 3 h. Longer MA times caused a notably decrease in the particle size. It means that powders were mainly welded until 3 h and then fracture process predominated over this time. In the case of R1A and R2A powders, there was only a small initial increase in the average particle size and then, after 5 h of MA this parameter remained in a considerable stable value which can be taken as equilibrium between welding and fracture processes had been reached. This fact could give support to the assumption that the Al2O3 reduces the time for reaching the MA steady state. 3.2. X-ray diffraction
Element
wt.%
Al Cu Mg Mn Si Ti Zn Cr
92.66 4.26 1.49 0.59 0.50 0.15 0.25 0.10
Fig. 4A and B shows the X-ray diffraction results of WR and R2A groups, respectively. In general, it was observed that when the MA time increased, the width of Al, Cu and Mg peaks was higher and the intensity decreased. In Fig. 4A it can be seen that Cu main diffraction peak was slightly visible after 10 h. However, the peak corresponding to Mg disappeared completely after 3 h. Also, a slightly shift of these peaks to different 2h values was observed. For the R2A group (Fig. 4B), similar results were observed but in this case,
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(A)
(B)
(C)
(D)
Fig. 1. SEM micrographs of the WR group for: (A and B) 1 h and (C and D) 10 h.
Fig. 2. SEM micrographs of the R2A group for: (A and B) 1 h and (C and D) 10 h.
the intensity of the main peak of Cu for 10 h seemed to be lower than the WR group for the same time (compare Fig. 4A and B). It is well known that shortening of diffraction peaks could be related to a particle size decrease. Therefore, this statement complemented the results obtained in Section 3.1 on which it was demonstrated that Al2O3 nanoparticles caused finer sizes of alloy lamellas. On the other hand, the shift of the Al peaks is thought to be caused by incorporation of alloying elements into the Al lattice forming in that way a solid solution. Therefore, it is possible that a change in the Al lattice parameter caused the shift that was observed. To evaluate a solid solution formation, the Al peak shift was measured and then it was related to the lattice parameter
value. As can be seen in Fig. 5, there was a small change in the position of the (1 1 1) Al peak for the R2A group when MA time was increased from 0 to 10 h. This difference in the 2h angle was 0.075. Converting this value to Al lattice parameter, it corresponded to a change of 0.0008 nm. This increment was higher than 0.0004 nm, a value reported previously by Hernández-Rivera et al. [14]. Although composites were processed under similar experimental conditions, the composite obtained in that study was reinforced with graphite and processed for 5 h. Then, it could be expected that due to the higher MA time used in the present work (10 h), both the lattice distortion and corresponding lattice parameter must be different compared to those obtained in Ref. [14].
99
Centroid 2 theta 38.445
Centroid 2 theta 38.370
WR R1A R2A
400
300
R2A0 R2A10
Intensity, a.u.
Average particle size (µm)
J.L. Hernández Rivera et al. / Materials and Design 37 (2012) 96–101
200
100
0 0
1
2
3
4
5
6
7
8
9
10 11 37.5
Milling time (h)
38.0
Fig. 3. Change in the average particle size as function of MA time.
3.3. Transmission electron microscopy Fig. 7 shows the TEM images that correspond to WR specimens processed for 1 and 10 h of MA. In the dark field images (see the bright areas of Fig. 7B and D) it can be seen that the matrix crystal-
(A)
Al
Cu
(B)
Mg
Al
Cu
Intensity, a.u.
Intensity, a.u.
WR3 WR1
R2A5 R2A3 R2A1 R2A0
WR0 45
50
55
2 theta
60
Mg
R2A10
WR5
40
39.5
lite size decreased significantly as MA time increased as was discussed previously in Section 3.2. Other results that supported the last argument regarding the reduction of crystallite size were the selected area diffraction patterns (SADPs). Fig. 7A and C shows Al rings for WR sample for 1 and 10 h of MA, respectively. As can be seen, the pattern obtained for short time (1 h) exhibited isolated spots due to the bigger crystallite size. However, for 10 h of MA, more and smaller crystallites were observed and it caused more well-defined rings in the SADP. Similar results were obtained in specimens with 2% of Al2O3 but in this case, the refinement was higher (Fig. 8). Comparing R2A10 SADP´s with those obtained in WR10 specimen, the rings were more defined in the former which confirmed the sentence stated above (compare Fig. 7A, C with 8A, C). It is important to mention that the results reported in Section 3.2 involved a considerable amount of particles. In contrast, analyses by TEM were carried out locally in a single particle. Consequently, it can be considered that this fact caused the difference between the values obtained in both techniques. On the other hand high quantity of lattice defects was generated due to high amount of plastic deformation imparted during MA. Evidence of some of these defects such dislocations is shown in Fig. 9A and B). The importance of these defects rely on two roles that they could have in the composite microstructure development: the first one is through the formation of substructure and small angle grain boundaries, by this manner crystallite size is decreased and plastic deformation is accommodated. The second is the interaction with the reinforcement. This kind of interaction can be seen in Fig. 9C and D. In the first one, there was evidence of small agglomerates formed by the reinforcement. It was found that even in this arrangement the dislocation motion can be altered. As can be observed on the same figure the middle part of
WR10
35
39.0
Fig. 5. Graph that shows the (1 1 1) Al peak shift in the 2h position for two different MA time.
Considering that the value obtained was very small, it can be taken as a signal that only a partial rather than complete solid solution was reached even in the R2A group. Also the average crystallite size and microhardness measurements are presented to give more evidence about the microstructural evolution of composite. Fig. 6A shows the effect of MA time on average powders crystallite size for WR, R1A and R2A groups. After 1 h of MA, a considerable decrease in the crystallite size up to about 125 nm is observed for the three groups. Then, it decreased gradually up to approximately 60 nm after 10 h. This change is attributed to the arrangement of dislocations into lowangle crystallite boundaries during MA [15]. By the contrary, Fig. 6B illustrates that microhardness increased as a function of the MA time. The highest value, 230 HV, was obtained after 10 h of MA in powders with Al2O3 (R1A and R2A groups), a fact that was expected due to the higher hardness of the reinforcement material. For the same MA time, the WR group exhibited the lowest hardness value (175 HV). It is noteworthy to point out that the reduction in the average crystallite size could also have a partial contribution in the resultant composite hardness due to grain refinement strengthening mechanism (Fig. 6B). It is also important to mention that plastic deformation caused by MA process generated higher amounts of lattice defects in the Al matrix. It is thought that these defects and their interaction with Al2O3 nanoparticles contributed to the hardness increment of the R2A and R1A groups. This is explained in detail in Section 3.3.
30
38.5
2 theta
65
70
30
35
40
45
50
55
60
65
2 theta
Fig. 4. XRD patterns of powders at different MA times: (A) WR and (B) R2A group.
70
100
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240 WR R1A R2A
300 250
Microdureza (Hv)
Average crystallite size (nm)
350
200 150 100
200 160 WR R1A R2A
120 80 40
50 0
2
4
6
8
10
0
0
2
Milling time (h)
4
6
8
10
12
Milling time (h)
Fig. 6. (A) Average crystallite size and (B) microhardness as function of MA time.
Fig. 7. TEM micrographs of the WR group: (A and B) 1 h and (C and D) 10 h. The inset shows the SADP obtained in each case.
the dislocation is bended when it is close to an agglomerate of nanoparticles. Fig. 9D shows how a single nanoparticle locked a dislocation. Considering the latter result about the interaction between nanoparticles and lattice defects it can be explained the higher hardness of R2A powders compared with WR sample. HRTEM images shown in Fig. 10A evidenced that Al-matrix microstructure consisted of small crystallites with different orientations as a result of the substructure formation and crystallite size reduction during MA. Fig. 10B presents a Gaussian filtered image of 200, 200, 020 and 0–20 Al reflections obtained in the region marked with a circle in Fig. 10A which was oriented in [1 0 0] zone axis. It was obtained a distortion of 7% in the lattice parameter through the interplanar distance measurements that were carried out in the experimental micrograph. Then it was compared with the Al equilibrium values. This result can support the previous assumption about a partial incorporation of alloying elements into the Al lattice. This distortion value was higher than the one reported in Section 3.2. The variations determined among lattice parameter values for XRD and HRTEM can be explained if it is considered that for 10 h of MA there were still high composition gradients along the microstructure. It had greater effects in the XRD results while in the HRTEM results the gradient effect was smaller because latter technique was carried out locally in a single particle.
Fig. 8. TEM micrographs of the R2A group after MA for: (A and B) 1 h and (C and D) 10 h. The inset shows the SADP obtained in each case.
4. Conclusions Experimental results demonstrated that using Al2O3 nanoparticles as reinforcement had a significant effect on the morphology and size of 2024 Al powders. The WR group exhibited equiaxed morphology and a continuous decrease on the average particle size as the MA time increased, while the R2A group showed also equiaxed morphology and the average particle size remained in a considerable stable value after 5 h of MA. These results indicate that MA steady state was reached in shorter times in presence of Al2O3 nanoparticles. XRD results demonstrated that there was no evidence of formation of new phases during MA. Moreover, after 3 h the Mg peaks disappeared completely while those of Cu remained even for 10 h. The absence of the Mg peaks and the decrease in intensity in the Cu ones was attributed to the high reduction in the particle size and the formation of a partial solid solution. Al2O3 nanoparticles were present in the Al-matrix as both: isolated particles and small agglomerates. It was detected that these particles interact with matrix dislocations impeding their motion. Consequently, there was a higher increase of microhardness when Al2O3 nanoparticles were present. The variations found in the crystallite size and lattice parameter values (determined by XRD and
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101
Acknowledgments This research constitutes part of a thesis to be submitted (by JLHR) to CIMAV in partial fulfillment of the requirements for the PhD degree. The first author appreciates the scholarship Granted by CONACYT no. 27519 and is grateful for the kindly support of all members of LAMEUAR (ultra-high resolution transmission electron microscopy laboratory) at the Instituto Mexicano del Petróleo, during his research stay. The facilities for the TEM characterization in this institution are also appreciated. This research was also supported by CONACYT (106658). Thanks to 198 Redes Temáticas de Nanotecnología y Nanociencias Reg. 0124623. Finally, a special acknowledgment is extended to Daniel Lardizabal, Alberto Torres, Fernando Rodriguez, Teresita Tristan and Claudia Elias from CIMAV and Instituto de Metalurgia – UASLP, respectively, for their valuable technical assistance. References
Fig. 9. TEM micrographs showing: (A and B) the presence of dislocations in the Almatrix and (C and D) different arrays of Al2O3 nanoparticles interacting with dislocations.
Fig. 10. (A) HRTEM micrographs of the R2A10 specimen showing several crystallites of Al and (B) Gaussian filtered image of the 200, 200, 020 and 0–20 reflections.
HRTEM) were attributed to the presence of composition gradients inside of the powders.
[1] Jia DC. Influence of SiC particulate size on the microstructural evolution and mechanical properties of Al–6Ti–6Nb matrix composites. Mater Sci Eng A 2000;289:83–90. [2] Mula S, Padhi P, Panigrani SC, Pabi SK, Gosh S. On structure and mechanical properties of ultrasonically cast Al–2%Al2O3 nanocomposite. Mater Res Bull 2009;44:1154–60. [3] Travitzky NA. Effect of metal volume fraction on the mechanical properties of alumina/aluminum composites. J Mater Sci 2001;36:4459–63. [4] Miserez A, Stucklin S, Rossoll A, San Marchi C, Mortensen A. Influence of heat treatment and particle shape on mechanical properties of infiltrated Al2O3 particle reinforced Al–2 wt% Cu. Mater Sci Technol 2002;18:1461–70. [5] Luan BF, Wu GH, Liu W, Hansen N, Lei T. High strength Al2O3 composites – effect of particles, subgrains and precipitates. Mater Sci Technol 2005;21:1440–3. [6] Qin XH, Jiang DL, Dong SM. Nanometer, submicron and micron sized aluminum powder prepared by semi-solid mechanical stirring method with addition of ceramic particles. Mater Sci Eng A 2004;385:31–7. [7] Ozdemir I, Ahrens S, Mucklich S. Nanocrystaline Al–Al2O3 and SiC composites produced by high energy ball milling. J Mater Proc Technol 2008;205:111–8. [8] Suryanarayana C. Mechanical alloying. Prog Mater Sci 2001;46:1–30. [9] Razavi TSS, Yazdani R, Salahi E, Mobasherpour I. Production of Al–20 wt% Al2O3 composite powder using high energy milling. Powder Technol 2009;320:591–602. [10] Lutterotti L. MAUD tutorial for quantitative analysis.. [11] Hesabi RS, Sajjadi SA. Structural evolution during mechanical milling of nanometric and micrometric Al2O3 reinforced Al matrix composites. Mater Sci Eng A 2006;428:159–68. [12] Aikin BJM, Courtney TH. The kinetics of composite particle formation during mechanical alloying. Metall Trans A 1993;24:647–57. [13] Zebaryad SM, Simchi A, Rehiani SM. Microstructure evaluation of Al–Al2O3 composite produced by mechanical alloying method. Mater Des 2006;27:684–8. [14] Hernández-Rivera JL, Cruz Rivera JJ, Gómez Carlos, Coreño O, MartínezSánchez R. Synthesis of graphite reinforced aluminum nanocomposite by mechanical alloying. Mater Trans 2010;51:1120–6. [15] El-Eskandarany MS. Mechanical alloying for fabrication of advanced engineering materials. 1st ed. EUA: William Andrew Publishing; 2001. p. 36–40.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Structural and morphological study of a 2024 Al–Al2O3 composite produced by mechanical alloying in high energy mill J.L. Hernández Rivera a,⇑, J.J. Cruz Rivera b, V. Paz del Ángel c, V. Garibay Febles c, O. Coreño Alonso d, R. Martínez-Sánchez a a
Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, C.P. 31109, Chihuahua, Mexico Facultad de Ingeniería-Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí, Sierra Leona 550, Lomas 2ª sección, C.P. 78210, SLP, Mexico Instituto Mexicano del Petróleo, Laboratorio de Microscopia Electrónica de Ultra Alta Resolución, Eje Central Lázaro Cárdenas 132, San Bartolo Atepehuacan, C.P. 07730, México D.F., México d Universidad de Guanajuato, División de Ingenierías, Dpto. Ing. Civil, Juárez 77 Col Centro, Guanajuato, Mexico b c
a r t i c l e
i n f o
Article history: Received 12 October 2011 Accepted 23 December 2011 Available online 3 January 2012 Keywords: A. Metal matrix composite B. Powders C. Mechanical alloying
a b s t r a c t 2024 Al composite reinforced with Al2O3 particles was obtained by mechanical alloying (MA) using Al, Cu and Mg elemental powders as raw materials and Al2O3 nanoparticles as reinforcement. The results shown that as the MA time increased, the non-reinforced (WR) and Al2O3 reinforced powders (R1A and R2A) morphology changed from flake-flattened to equiaxed. Regarding the average particle size, WR group displayed a continuous decreasing value even for a processing time of 10 h while R2A and R1A groups shown a constant value for the same time. This led to the conclusion that steady state of the process was reached in shorter times in presence of Al2O3 nanoparticles. It was found that the reinforcement was present in the matrix like isolated particles and small agglomerates which affected the dislocation motion, and it was assumed that this fact caused the increase observed in the microhardness values. There was no evidence of new phase precipitation through MA process. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Al alloys with superior mechanical and physical properties are promising materials for several industrial components where lightweight is required. One of the emerging technologies for the production of these alloys is the development of Al matrix composites. It is known that Al matrix confers some unique properties to the composites such as lightweight, environmental resistance, high specific strength and stiffness, high thermal and electrical conductivity and good wear resistance. Mechanical properties of Al composites can be improved by dispersion of ceramic particles into the metal matrix. By another hand, an important consideration during fabrication of composites is the reinforcement size. For example, Jia [1] reported that the mechanical strength of Al composites can be increased up to 20% if the reinforcement size is reduced from the order of micro to nanoscale. Several methods such as ultrasonically cast [2], gas pressure infiltration [3], squezze casting [4,5], semi-solid mechanical stir-
⇑ Corresponding author. Address: Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109, Chihuahua, Mexico. Tel.: +52 614 4391146. E-mail address: [email protected] (J.L. Hernández Rivera). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.12.035
ring [6] and mechanical alloying [7] have been used for the production Al composites. MA has been proved to be a process that overcomes some drawbacks (for example segregation phenomena) that commonly occur in composites produced from liquid processes. When this phenomena occurs the alloy elements distribution is not homogeneous and it can decrease the resulting mechanical properties. In addition, when liquid processes are used there are some density differences between matrix and reinforcement particles which do not allow a uniform distribution of the reinforcement. MA process could be used to obtain homogeneous nanocomposites due to the high microstructure refinement achieved in the matrix crystallite size. Generally, MA is performed in three steps [8]. Firstly, powders suffer impacts among them and balls with little fracture and plastic deformation. In the second step, plastic deformation and cold welding process predominate. Finally, powders harden and then fracture due to the continuous increase of plastic deformation giving rise to new and smaller surfaces. In this step, there is a balance between cold welding and fracture frequency. Therefore, it can be said that the ‘‘steady state condition’’ has been reached. Further processing times will not cause a significant effect either on the particle size, shape and morphology [9]. On the other hand, the successful processing of Al composites depends not only on the appropriate performance of the technique
97
J.L. Hernández Rivera et al. / Materials and Design 37 (2012) 96–101
as was mentioned above, but also on the adequate reinforcement selection. In this context, Al2O3 is a well-studied and universally used ceramic that has attractive properties such as excellent wear and oxidation resistance and good high temperature strength. Due to these properties, it has been commonly incorporated to Al composites in order to increase the mechanical properties or to optimize the MA process. However, little literature exists about the role of this nanoreinforcement on the microstructural evolution of Al composites during MA. In this work it was studied the effect of the Al2O3 nanoparticles on the Al composites during MA and its influence on the size and powders morphology. Also, the effectiveness of MA in terms of achieving the steady state condition was widely described.
2. Experimental procedure Raw powders used to produce the Al-2024 alloy into commercial composition range were Al (purity of 99.9%, 325 mesh), Cu (purity of 99.9%, 200 mesh) and Mg (purity of 99.8%, 325 mesh). Traces of Mn, Si, Ti, and Zn powder were also added. Table 1 shows the quantities of the elements used to produce the alloy. MA was carried out in a Simoloyer high energy mill (ZOZ CM01) under Ar atmosphere. Balls-to-powder weight ratio was 13.3 and the process control agent was methanol. Powder mixtures were blended for 5 min prior to MA to obtain an adequate and homogeneous mix. 1 and 2 wt.% of Al2O3 nanoparticles were added to the matrix and MA times were 1, 3, 5 and 10 h. For comparison, samples neither mechanically alloyed nor reinforced were also produced. Table 2 shows the nomenclature employed for the composite powders. Characterization of microstructural evolution was carried out by scanning electron microscopy (SEM) using a Philips XL-30 microscope which was coupled with an energy dispersive spectroscopy unit (EDS). Two transmission electron microscopes (TEMs) were used for general observations and high resolution work. A FEI-TECNAI T20 and a FEI-Titan operated at 200 kV and 300 kV, respectively. The average particle size was measured using the Image J analyzer software. Measurements were obtained from 10 fields for each sample. X-ray diffraction experiments were carried out in a Rigaku DMAX-2200 diffractometer using 40 kV, 36 mA, step size of 0.02 and monochromatic radiation of Cu Ka with wavelength of 1.5405 nm. Average crystallite size of specimens was obtained from the processed and analyzed XRD patterns using MAUD software. This software is based on Rietveld refinement method in which experimental and theoretical patterns are fitted through iterative application of least-squares algorithm. The iteration was finished when sigma and Rw statistical indicators converged to smaller values than 2 and 15, respectively. These values are recommended by Lutterotti [10] because they found that the residuals or differences between experimental and calculated pattern were minimum when these indicators reached those values. As a result, reliable structural information such as crystallite size can be obtained for this procedure.
Table 1 Quantities of elemental powders used to obtain the 2024 Al alloy.
Table 2 Nomenclature of the MA composite powders. Specimen
MA time (h) 0
1
3
5
10
WR3 R1A3 R2A3
WR5 R1A5 R2A5
WR10 R1A10 R2A10
Nomenclature Without reinforcement (WR group) 1% Al2O3 (R1A group) 2% Al2O3 (R2A group)
WR0 R1A0 R2A0
WR1 R1A1 R2A1
3. Results and discussion 3.1. Scanning electron microscopy Figs. 1 and 2 show the powders microstructure of the WR and R2A groups, respectively. It can be observed light-gray color particles of Al with some lamellas inside in both micrographs. The white-colored ones correspond to Cu while the darker gray to Mg. For 1 h of processing it is clear that alloying elements remained coarse. It can be taken as a signal that this time was not enough to disperse them homogeneously throughout the matrix (Figs. 1 and 2, incises A and B). Apparently, these lamellas had preferred orientation. In the case of powders resulting from 10 h of MA, Cu and Mg lamellas were finer and exhibit a random orientation (Figs. 1 and 2, incises C and D). However, it can be clearly noticed that Cu and Mg lamellas were smaller and a homogeneous dispersion was obtained in the presence of Al2O3 for 10 h of MA (compare Figs. 1D and 2D). On the other hand, the WR powders exhibited a flake-flattened morphology for earlier times (Fig. 1A and B) but it changed to equiaxed when the MA time was increased to 10 h (Fig. 1C and D). In the R2A powders the flake-flattened morphology was observed after 1 h of MA and then it changed to a more equiaxed when a time of 10 h was achieved. It is known that when the alloying process is accelerated, the main attributable cause is the powder surface energy modification by presence of a reinforcement which reduces the time to achieve steady state [9,11]. If it is taken into account that finer distribution of alloy lamellas were observed inside Al powders reinforced with Al2O3 nanoparticles, it can be established that MA was accelerated by this reinforcement addition. Furthermore, according to previous studies [12,13] when MA steady state is reached, the average particle size becomes constant and does not change anymore. Fig. 3 shows the effect of MA time on the average particle size of WR and RA specimens. For the WR group there was an increase of this parameter as MA time increased until 3 h. Longer MA times caused a notably decrease in the particle size. It means that powders were mainly welded until 3 h and then fracture process predominated over this time. In the case of R1A and R2A powders, there was only a small initial increase in the average particle size and then, after 5 h of MA this parameter remained in a considerable stable value which can be taken as equilibrium between welding and fracture processes had been reached. This fact could give support to the assumption that the Al2O3 reduces the time for reaching the MA steady state. 3.2. X-ray diffraction
Element
wt.%
Al Cu Mg Mn Si Ti Zn Cr
92.66 4.26 1.49 0.59 0.50 0.15 0.25 0.10
Fig. 4A and B shows the X-ray diffraction results of WR and R2A groups, respectively. In general, it was observed that when the MA time increased, the width of Al, Cu and Mg peaks was higher and the intensity decreased. In Fig. 4A it can be seen that Cu main diffraction peak was slightly visible after 10 h. However, the peak corresponding to Mg disappeared completely after 3 h. Also, a slightly shift of these peaks to different 2h values was observed. For the R2A group (Fig. 4B), similar results were observed but in this case,
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(A)
(B)
(C)
(D)
Fig. 1. SEM micrographs of the WR group for: (A and B) 1 h and (C and D) 10 h.
Fig. 2. SEM micrographs of the R2A group for: (A and B) 1 h and (C and D) 10 h.
the intensity of the main peak of Cu for 10 h seemed to be lower than the WR group for the same time (compare Fig. 4A and B). It is well known that shortening of diffraction peaks could be related to a particle size decrease. Therefore, this statement complemented the results obtained in Section 3.1 on which it was demonstrated that Al2O3 nanoparticles caused finer sizes of alloy lamellas. On the other hand, the shift of the Al peaks is thought to be caused by incorporation of alloying elements into the Al lattice forming in that way a solid solution. Therefore, it is possible that a change in the Al lattice parameter caused the shift that was observed. To evaluate a solid solution formation, the Al peak shift was measured and then it was related to the lattice parameter
value. As can be seen in Fig. 5, there was a small change in the position of the (1 1 1) Al peak for the R2A group when MA time was increased from 0 to 10 h. This difference in the 2h angle was 0.075. Converting this value to Al lattice parameter, it corresponded to a change of 0.0008 nm. This increment was higher than 0.0004 nm, a value reported previously by Hernández-Rivera et al. [14]. Although composites were processed under similar experimental conditions, the composite obtained in that study was reinforced with graphite and processed for 5 h. Then, it could be expected that due to the higher MA time used in the present work (10 h), both the lattice distortion and corresponding lattice parameter must be different compared to those obtained in Ref. [14].
99
Centroid 2 theta 38.445
Centroid 2 theta 38.370
WR R1A R2A
400
300
R2A0 R2A10
Intensity, a.u.
Average particle size (µm)
J.L. Hernández Rivera et al. / Materials and Design 37 (2012) 96–101
200
100
0 0
1
2
3
4
5
6
7
8
9
10 11 37.5
Milling time (h)
38.0
Fig. 3. Change in the average particle size as function of MA time.
3.3. Transmission electron microscopy Fig. 7 shows the TEM images that correspond to WR specimens processed for 1 and 10 h of MA. In the dark field images (see the bright areas of Fig. 7B and D) it can be seen that the matrix crystal-
(A)
Al
Cu
(B)
Mg
Al
Cu
Intensity, a.u.
Intensity, a.u.
WR3 WR1
R2A5 R2A3 R2A1 R2A0
WR0 45
50
55
2 theta
60
Mg
R2A10
WR5
40
39.5
lite size decreased significantly as MA time increased as was discussed previously in Section 3.2. Other results that supported the last argument regarding the reduction of crystallite size were the selected area diffraction patterns (SADPs). Fig. 7A and C shows Al rings for WR sample for 1 and 10 h of MA, respectively. As can be seen, the pattern obtained for short time (1 h) exhibited isolated spots due to the bigger crystallite size. However, for 10 h of MA, more and smaller crystallites were observed and it caused more well-defined rings in the SADP. Similar results were obtained in specimens with 2% of Al2O3 but in this case, the refinement was higher (Fig. 8). Comparing R2A10 SADP´s with those obtained in WR10 specimen, the rings were more defined in the former which confirmed the sentence stated above (compare Fig. 7A, C with 8A, C). It is important to mention that the results reported in Section 3.2 involved a considerable amount of particles. In contrast, analyses by TEM were carried out locally in a single particle. Consequently, it can be considered that this fact caused the difference between the values obtained in both techniques. On the other hand high quantity of lattice defects was generated due to high amount of plastic deformation imparted during MA. Evidence of some of these defects such dislocations is shown in Fig. 9A and B). The importance of these defects rely on two roles that they could have in the composite microstructure development: the first one is through the formation of substructure and small angle grain boundaries, by this manner crystallite size is decreased and plastic deformation is accommodated. The second is the interaction with the reinforcement. This kind of interaction can be seen in Fig. 9C and D. In the first one, there was evidence of small agglomerates formed by the reinforcement. It was found that even in this arrangement the dislocation motion can be altered. As can be observed on the same figure the middle part of
WR10
35
39.0
Fig. 5. Graph that shows the (1 1 1) Al peak shift in the 2h position for two different MA time.
Considering that the value obtained was very small, it can be taken as a signal that only a partial rather than complete solid solution was reached even in the R2A group. Also the average crystallite size and microhardness measurements are presented to give more evidence about the microstructural evolution of composite. Fig. 6A shows the effect of MA time on average powders crystallite size for WR, R1A and R2A groups. After 1 h of MA, a considerable decrease in the crystallite size up to about 125 nm is observed for the three groups. Then, it decreased gradually up to approximately 60 nm after 10 h. This change is attributed to the arrangement of dislocations into lowangle crystallite boundaries during MA [15]. By the contrary, Fig. 6B illustrates that microhardness increased as a function of the MA time. The highest value, 230 HV, was obtained after 10 h of MA in powders with Al2O3 (R1A and R2A groups), a fact that was expected due to the higher hardness of the reinforcement material. For the same MA time, the WR group exhibited the lowest hardness value (175 HV). It is noteworthy to point out that the reduction in the average crystallite size could also have a partial contribution in the resultant composite hardness due to grain refinement strengthening mechanism (Fig. 6B). It is also important to mention that plastic deformation caused by MA process generated higher amounts of lattice defects in the Al matrix. It is thought that these defects and their interaction with Al2O3 nanoparticles contributed to the hardness increment of the R2A and R1A groups. This is explained in detail in Section 3.3.
30
38.5
2 theta
65
70
30
35
40
45
50
55
60
65
2 theta
Fig. 4. XRD patterns of powders at different MA times: (A) WR and (B) R2A group.
70
100
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240 WR R1A R2A
300 250
Microdureza (Hv)
Average crystallite size (nm)
350
200 150 100
200 160 WR R1A R2A
120 80 40
50 0
2
4
6
8
10
0
0
2
Milling time (h)
4
6
8
10
12
Milling time (h)
Fig. 6. (A) Average crystallite size and (B) microhardness as function of MA time.
Fig. 7. TEM micrographs of the WR group: (A and B) 1 h and (C and D) 10 h. The inset shows the SADP obtained in each case.
the dislocation is bended when it is close to an agglomerate of nanoparticles. Fig. 9D shows how a single nanoparticle locked a dislocation. Considering the latter result about the interaction between nanoparticles and lattice defects it can be explained the higher hardness of R2A powders compared with WR sample. HRTEM images shown in Fig. 10A evidenced that Al-matrix microstructure consisted of small crystallites with different orientations as a result of the substructure formation and crystallite size reduction during MA. Fig. 10B presents a Gaussian filtered image of 200, 200, 020 and 0–20 Al reflections obtained in the region marked with a circle in Fig. 10A which was oriented in [1 0 0] zone axis. It was obtained a distortion of 7% in the lattice parameter through the interplanar distance measurements that were carried out in the experimental micrograph. Then it was compared with the Al equilibrium values. This result can support the previous assumption about a partial incorporation of alloying elements into the Al lattice. This distortion value was higher than the one reported in Section 3.2. The variations determined among lattice parameter values for XRD and HRTEM can be explained if it is considered that for 10 h of MA there were still high composition gradients along the microstructure. It had greater effects in the XRD results while in the HRTEM results the gradient effect was smaller because latter technique was carried out locally in a single particle.
Fig. 8. TEM micrographs of the R2A group after MA for: (A and B) 1 h and (C and D) 10 h. The inset shows the SADP obtained in each case.
4. Conclusions Experimental results demonstrated that using Al2O3 nanoparticles as reinforcement had a significant effect on the morphology and size of 2024 Al powders. The WR group exhibited equiaxed morphology and a continuous decrease on the average particle size as the MA time increased, while the R2A group showed also equiaxed morphology and the average particle size remained in a considerable stable value after 5 h of MA. These results indicate that MA steady state was reached in shorter times in presence of Al2O3 nanoparticles. XRD results demonstrated that there was no evidence of formation of new phases during MA. Moreover, after 3 h the Mg peaks disappeared completely while those of Cu remained even for 10 h. The absence of the Mg peaks and the decrease in intensity in the Cu ones was attributed to the high reduction in the particle size and the formation of a partial solid solution. Al2O3 nanoparticles were present in the Al-matrix as both: isolated particles and small agglomerates. It was detected that these particles interact with matrix dislocations impeding their motion. Consequently, there was a higher increase of microhardness when Al2O3 nanoparticles were present. The variations found in the crystallite size and lattice parameter values (determined by XRD and
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Acknowledgments This research constitutes part of a thesis to be submitted (by JLHR) to CIMAV in partial fulfillment of the requirements for the PhD degree. The first author appreciates the scholarship Granted by CONACYT no. 27519 and is grateful for the kindly support of all members of LAMEUAR (ultra-high resolution transmission electron microscopy laboratory) at the Instituto Mexicano del Petróleo, during his research stay. The facilities for the TEM characterization in this institution are also appreciated. This research was also supported by CONACYT (106658). Thanks to 198 Redes Temáticas de Nanotecnología y Nanociencias Reg. 0124623. Finally, a special acknowledgment is extended to Daniel Lardizabal, Alberto Torres, Fernando Rodriguez, Teresita Tristan and Claudia Elias from CIMAV and Instituto de Metalurgia – UASLP, respectively, for their valuable technical assistance. References
Fig. 9. TEM micrographs showing: (A and B) the presence of dislocations in the Almatrix and (C and D) different arrays of Al2O3 nanoparticles interacting with dislocations.
Fig. 10. (A) HRTEM micrographs of the R2A10 specimen showing several crystallites of Al and (B) Gaussian filtered image of the 200, 200, 020 and 0–20 reflections.
HRTEM) were attributed to the presence of composition gradients inside of the powders.
[1] Jia DC. Influence of SiC particulate size on the microstructural evolution and mechanical properties of Al–6Ti–6Nb matrix composites. Mater Sci Eng A 2000;289:83–90. [2] Mula S, Padhi P, Panigrani SC, Pabi SK, Gosh S. On structure and mechanical properties of ultrasonically cast Al–2%Al2O3 nanocomposite. Mater Res Bull 2009;44:1154–60. [3] Travitzky NA. Effect of metal volume fraction on the mechanical properties of alumina/aluminum composites. J Mater Sci 2001;36:4459–63. [4] Miserez A, Stucklin S, Rossoll A, San Marchi C, Mortensen A. Influence of heat treatment and particle shape on mechanical properties of infiltrated Al2O3 particle reinforced Al–2 wt% Cu. Mater Sci Technol 2002;18:1461–70. [5] Luan BF, Wu GH, Liu W, Hansen N, Lei T. High strength Al2O3 composites – effect of particles, subgrains and precipitates. Mater Sci Technol 2005;21:1440–3. [6] Qin XH, Jiang DL, Dong SM. Nanometer, submicron and micron sized aluminum powder prepared by semi-solid mechanical stirring method with addition of ceramic particles. Mater Sci Eng A 2004;385:31–7. [7] Ozdemir I, Ahrens S, Mucklich S. Nanocrystaline Al–Al2O3 and SiC composites produced by high energy ball milling. J Mater Proc Technol 2008;205:111–8. [8] Suryanarayana C. Mechanical alloying. Prog Mater Sci 2001;46:1–30. [9] Razavi TSS, Yazdani R, Salahi E, Mobasherpour I. Production of Al–20 wt% Al2O3 composite powder using high energy milling. Powder Technol 2009;320:591–602. [10] Lutterotti L. MAUD tutorial for quantitative analysis.