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Titanium particulate reinforced AZ31 magnesium matrix composites with improved ductility prepared using friction stir processing
Materials Science & Engineering A 772 (2020) 138793
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Titanium particulate reinforced AZ31 magnesium matrix composites with improved ductility prepared using friction stir processing Isaac Dinaharan a, Shuai Zhang b, Gaoqiang Chen b, Qingyu Shi b, &lowest; a b
IDM-Joint Lab, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China The State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium matrix composite Friction stir processing Titanium Microstructure Tensile strength
Conventional ceramic particulate reinforcements cause a major loss in ductility of magnesium matrix composites (MMCs). Metallic particles possessing higher melting point can offer a solution to this issue. Titanium (Ti) particles (0,7,14 and 21 vol%) were reinforced into magnesium alloy AZ31 using friction stir processing (FSP) performed by a conventional sturdy vertical milling machine. The microstructure and the tensile behavior of the fabricated composites were studied in detail. The micrographs revealed a uniform distribution of Ti particles all over the stir zone irrespective of the volume content of Ti. Ti particles did not decompose or react with the matrix and its alloying elements. Ti particles established a proper interface with the matrix AZ31. Ti particles survived the severe plastic strain without breakage. The grains in the matrix were refined extremely because of dynamic recrystallization and the pinning effect of Ti particles. A large number of dislocations are found in the composite. Ti particles improved the tensile strength of the composite and helped to retain appreciable ductility.
1. Introduction Magnesium matrix composites (MMCs) reinforced with various particles exhibit light weight which makes them an attractive alternative material to commonly used aluminum matrix composites in several applications in automobiles and allied industries [1–3]. The reinforce ment particles are usually selected from any one category of ceramics such as oxides, carbides, borides and nitrides. Ceramic particles improved the strength of MMCs but deprived the ductility owing to its non-deformable nature [4–7]. Metallic particles possessing higher melting point and hardness such as titanium, nickel, tungsten, molyb denum etc. can be used to reinforce MMCs to offset the issue of ductility [8,9]. Titanium (Ti) is an appropriate choice for MMCs due to the following reasons. Both magnesium and titanium are characterized by hexagonal crystal structure (HCP) which would alleviate compatibility problems. The mechanical deformation of Ti is several folds higher than typical ceramic particles which would help to enhance ductility. Further, Ti does not mix with magnesium either in solid or liquid state to form a resultant solid solution or any kind of intermetallic compounds [10,11]. Producing MMCs is a daunting task among all metallic based com posites due to the risk of fire hazard and high shrinkage. Few
investigators attempted to produce Ti reinforced MMCs using various methods which include disintegrated melt deposition [12], powder metallurgy [13,14], accumulative diffusion bonding [15], stir casting [16,17], vacuum hot pressing [18], mechanical alloying [19,20]. Ti particles were successfully reinforced into different magnesium alloys such as AZ31, AZ61, AZ91, AM50, AME505 and pure magnesium using those techniques. Although Ti particles were reinforced by both liquid and solid-state methods, the desirable design features were lacking. Improper distribution [12], clustering of particles [13], pores at micro and macro levels [13], weak interfaces [14], coarse grains [14,16], emergence of detrimental compounds [17,19], high energy and time consumption [15,19] and distortion of crystal structure [20] were re ported. These unwanted microstructural features limited the tensile behavior and other factors increased the production cost. Friction stir processing (FSP) is accepted as a novel solid-state method to produce high performance MMCs and has attained popu larity in the last decade [21]. A harder rotating third body known as the tool is forced into the metallic plates to generate frictional heat. The matrix material is deformed combinedly by frictional heat and plastic deformation. The mechanical action of the tool reinforces the particles into the plasticized matrix. FSP consumes lower energy to produce metallic composites. The nature of the distribution is unaffected by the
* Corresponding author. E-mail addresses: [email protected] (I. Dinaharan), [email protected] (Q. Shi). https://doi.org/10.1016/j.msea.2019.138793 Received 16 October 2019; Received in revised form 5 December 2019; Accepted 7 December 2019 Available online 9 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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Table 1 The chemical composition of magnesium alloy AZ31B. Element
Al
Zn
Mn
Si
Ca
Ni
Fe
Others
Mg
wt.%
3.45
0.95
0.47
0.16
0.05
0.06
0.02
0.14
Balance
Fig. 1. (a) SEM micrograph, (b) particle size distribution and (c) XRD pattern of as received titanium powder.
reinforced nano hydroxyapatite MMCs and studied in vitro bioactivity. Particles and grain refinement caused biomineralization. Balakrishnan et al. [29] produced TiC particle reinforced AZ31 MMCs and analyzed the microstructural features. They reported a reasonable distribution of TiC particles in the composite. Navazani and Dehghani [30] prepared ZrO2 reinforced AZ31 MMCs and reported an improvement in tensile strength due to fine dispersion and grain refinement. Ahmadkhaniha et al. [31] developed nano alumina particle (Al2O3np) reinforced AZ91 MMCs and showed an improved wear behavior. Simar et al. [32] pre pared short carbon fiber reinforced AZ91D MMCs and analyzed the role of heat treatment. Dinaharan et al. [33] synthesized fly ash reinforced AZ31 MMCs and obtained an improved wear behavior to a similar composite produced by stir casting method. A short literature survey revealed that several kind of reinforcement particles were effectively reinforced to prepare MMCs with improved properties. However, literature on metallic particle reinforcements are lacking. Therefore, the present study aims to prepare AZ31/Ti MMCs using FSP and investigate the microstructural features using conven tional and advanced microscopic techniques. The influence of micro structural evolution on tensile behavior is reported.
Table 2 FSP conditions. Parameter
Values
Rotational speed (rpm) Traverse speed (mm/min) Passes Shoulder diameter (mm) Shoulder concavity (mm) Pin diameter (mm) Pin length (mm) Pin shape Tool material Tool tilt angle (� ) Groove width (mm) Groove depth (mm) Ti content (vol%)
950 30 5 24 0.2 7–5 5.7 Frustrum of a cone H13 steel 1 0, 0.4, 0.8 and 1.2 5 0,7,14 and 21
physical properties of the reinforcement particle. The heat evolved does not cause melting of the substrate which restricts possible interfacial reactions [22–24]. Asadi et al. [25] produced SiC particle reinforced AZ91 MMCs and investigated the effect of process parameters. They observed a reduction in grain size as the traverse speed was advanced. Khayyamin et al. [26] developed SiO2 reinforced AZ91 MMCs and obtained an improvement in tensile properties. Lu et al. [27] synthesized carbon nano-tubes (CNTs) reinforced AZ31 MMCs and evaluated the wear resistance and observed the wear mechanism. Ratna Sunil et al. [28] fabricated pure magnesium
2. Experimental procedure Hot rolled magnesium alloy AZ31 plates of size 150 mm � 100 mm x 8 mm were procured to produce the composite. Table 1 details the composition of as received plates tested by XRF. Commercially pure Ti 2
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Fig. 2. XRD patterns of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
particles (~2.7 μm) of grade 2 were used as reinforcement phase. Fig. 1 presents the morphology of particles, particle size distribution (measured using MALVERN Mastersizer 2000) and XRD of as received Ti particles. Machined groove strategy was followed to stuff the particles on the plate surface and two step FSP (pinless tool followed by pin tool) procedure was applied [34]. A conventional vertical milling machine (M/s BYJC X5032) was used for FSP. The processing conditions are listed in Table 2 which were optimized from an earlier work on the study of process parameters. Specimens to observe the microstructure evolu tion were sectioned across the FSP seam. Polishing was done using a semi-automatic polishing machine. Fine polishing was accomplished using alumina suspension and 5 μm diamond spray. A reagent consisting of 1 ml acetic acid, 1 ml nitric acid and l g oxalic acid in 150 ml water was used to reveal the microstructure. Micrographs were recorded using an optical microscope (OLYMPUS BX51 M), a scanning electron mi croscope (FEI Quanta 200) and a transmission electron microscope. EBSD was carried out in a FEI Quanta FEG SEM equipped with TSL-OIM software. Sub-sized tensile specimens having 25 mm gauge length; 4 mm width and 4 mm thickness were machined from the processed zone. Tensile test at a strain rate of 0.5 mm/min was tested using a comput erized tensile tester (MTS Exceed E45). Fracture surfaces of selected tensile tested specimens were recorded.
the laws of physical metallurgy. There are no other elements or com pounds detected in significant quantity in any of the composites. Representative SEM micrographs of the stir zone as a function of Ti content is shown in Fig. 3. Fig. 3a is the stir zone of the AZ31 magnesium alloy which is unreinforced. Traces of secondary eutectic particles (Mg17Al12) are seen which appear in white color and nodular shape. The SEM micrographs in Fig. 3b–d displays the reinforcement of Ti particles and the nature of distribution in the composite. The packed Ti particles are scattered in all directions in the stir zone. The number and density of particles increase as the volume content is increased. The variation in the spacing of particles i.e. inter particle distance is minimum which points out a uniform distribution. Particle free regions and clustering of particles are not spotted in the micrographs. Fig. 4 displays the optical micrographs of AZ31/21 vol% Ti com posites which were captured at random locations within the stir zone. It is interesting to observe an unchanging distribution irrespective of the spot location of the micrograph. All parts of the stir zone are uniformly reinforced with Ti particulates. There is not a single location which does not have any distribution. The bottom portion of the stir zone (Fig. 4d) does not display onion ring pattern. Fig. 5 presents the elemental dis tribution in AZ31/21 vol% Ti MMC. The distribution of Ti across the micrograph confirms the uniform distribution of Ti particles in the composite. Fig. 6a–c presents enlarged views of the AZ31/Ti composites at higher magnification which enable to have a closer look at the interface existing between the particle and the matrix alloy. A continuous inter face is observed around the particles without interruption. There are no foreign materials spotted at the interface. The matrix alloy is fused together with the particle on all sides without any pores. Line EDAX
3. Results XRD diffraction patterns of AZ31/Ti MMCs at different volume fractions are depicted in Fig. 2. The peaks of the magnesium matrix and the reinforcement Ti are distinctly observed in the figure. The height of Ti peaks grows as the volume fraction is increased in accordance with 3
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Fig. 3. SEM micrographs of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
Fig. 4. Optical photomicrograph of AZ31/21 vol%. Ti MMCs at various locations within the stir zone: (a) top portion, (b) interface at the retreading side, (c) middle portion and (d) bottom portion.
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Fig. 5. (a) SEM micrograph of AZ31/21 vol% Ti MMC and distribution of elements; (b) Mg, (c) Al, and (d) Ti.
Fig. 6. SEM micrographs of AZ31/Ti MMCs containing Ti particles; (a) 14 vol%, (b) and (c) 21 vol%, and (d) line EDAX across a particle.
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Fig. 7. EBSD (IPF þ grain boundary) maps of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
across the particle and the matrix alloy was recorded (Fig. 6d) to verify any sort of diffusion or traces of other compounds. The elemental spectrums of Mg and Ti are crossing sharply at the interface. There is no depletion of Ti spectrum or increase in the spectrum of Al at the interface. The reinforcement particles are subjected to severe straining during FSP. A change in the shape and the size of the reinforcement particles were noticed in several investigations [23,24]. There is no significant change in the size of Ti particles comparing Fig. 1 with Figs. 3 and 6. The initial size and morphology are almost retained after FSP. Fig. 7 reveals the EBSD images of the base metal as well as the pre pared composites which help to visualize the grain structure inside the material. The base metal shows a coarse grain structure recording an average grain size of 93 μm. Conversely, AZ31/Ti composites consist of a fine grain structure. There is a drastic reduction in the size of grains from 93 μm to 10.2 μm in AZ31/7 vol% Ti MMC. The increase in the volume content of Ti particles further reduced the grain size to 4.3 μm in AZ31/ 21 vol% Ti MMC. The corresponding misorientation maps in Fig. 8 shows that more percentage (40–50%) of high angle boundaries is generated compared to base metal. Fig. 9 presents a montage of TEM micrographs of AZ31/21 vol% Ti composites revealing various microstructural features. Fine grains and particle distribution are observed in Fig. 9a. It ensures absence of par ticle clustering in the composite. The interface is crystal clear and no gap is existing between the matrix and the particle (Fig. 9b). The matrix alloy AZ31 of the composite is characterized by large amount of dislo cations as seen in Fig. 9c and d. The selected area electron diffraction (SAED) pattern (Fig. 9e) confirms the hexagonal closed packed structure
of Ti particles. The stress strain graphs recorded during the tensile test of AZ31/Ti MMCs and the bar chart of extracted values are shown in Fig. 10. The effect of Ti particle incorporation into the magnesium alloy AZ31 is obvious from these plots. Ti particles led to an improvement in UTS from 226 MPa (0 vol%) to 283 MPa (21 vol%) in the present investigation. The yield strength of the composite also improved due to the presence of Ti particles. Yield strength was recorded from 98 MPa (0 vol%) to 193 MPa (21 vol%). There is a tremendous increase in the yielding point. Tensile graphs prove that Ti is a beneficial reinforcement for AZ31. It is interesting to observe the percentage elongation of the composites which was noted to be 14.5% at 0 vol% and 9.4% at 21 vol%. The overall ductility of the composite is better compared to ceramic particle rein forced MMCs. SEM observations of the fracture surface of AZ31/Ti MMCs are recorded in Fig. 11. The unreinforced matrix alloy AZ31 shows slightly elongated dimples in Fig. 11a. A network of fine dimples is visible on the fracture surfaces of the composite (Fig. 11b–d). Ti particles are spotted across the fracture surfaces. Fig. 11e shows the presence of full and fractured Ti particles which gives evidence for superior interfacial bonding. 4. Discussion It is generally known that magnesium and Ti do not dissolve in each other irrespective of the temperature. They constitute an immiscible system. However, it is possible to develop metastable phases as a result of severe plastic deformation as reported by Edalati et al. [10]. This is 6
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Fig. 8. Misorientation distribution of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
commonly reflected in broadening of magnesium peaks in the XRD pattern. No such broadening of magnesium peaks is observed in Fig. 2 which confirms the absence of metastable phases. This may be due to the exposure to severe deformation for a shorter duration. The matrix used in this investigation is not pure magnesium but magnesium alloy AZ31. The alloying elements Al and Ti have good chemical affinity to each other and tend to form deleterious compounds such as Ti3Al at elevated temperature and prolonged straining [19,35]. No peaks of Ti3Al are present in the XRD pattern which rules out the existence of Ti3Al. The processing temperature and the exposure duration during FSP are insufficient to cause considerable diffusion of Al into Ti to form Ti3Al. Nevertheless, advanced microstructural characterization is necessary to find out any traces of such compounds at the interface. Mg17Al12 particles usually experience dissolution into the magne sium matrix at higher temperature [36]. Few survived the dissolution effect while most of them were dissolved during processing. FSP leads to a desirable uniform distribution which is essential to boost mechanical properties. A good distribution is a result of appropriate choice of pro cess parameters. Poor particle distribution was observed in several studies. The key factor is the application of multiple passes (5 in this study) which provided better distribution and eliminated clusters and unreinforced regions. The mechanism of composite formation via FSP involves plasticization of the magnesium matrix under the combined action of frictional heat and the tool movement. The plasticized material is transported from the advancing side to the retreading side. The rotating tool mixes the packed particles with the plasticized material and squeezes out in the space between the cold base metal and the tool pin. The tool advancement results in forging of the composite. Segregation of reinforcement particles along the grain boundary region is a common issue in casting routes. Such kind of segregation is not present in the micrographs due to the absence of solidification process. It can be said that many particles are located inside the grains i.e. intragranular dis tribution. The distribution is constant irrespective of the volume content
under the set of processing conditions. A composite should possess a constant distribution throughout its span. It is an uphill task to achieve a constant distribution in casting methods. Various factors such as density difference, force of the solidi fication front etc. cause undesirable motion of particles in the composite before cooling down to atmospheric temperature. The frictional heat does not cause melting of the matrix alloy which avoids all the issues originating from the physical properties of the reinforcement. The movement of the reinforcement particle is dictated by the tool rotation and arrested after the consolidation. Therefore, the distribution is pre served during the cooling stage. Nevertheless, variation in the distri bution of particles within the stir zone was reported in composites prepared by FSP method [25,31]. A constant distribution is the outcome and the choice of optimized process parameters. Onion ring pattern would look like stacking of particle rich and particle depleted layers alternately [37]. There remains a 2 mm AZ31 plate between the end of the stir zone and back up plate which promotes dissipating heat fast. The temperature along the vertical direction is reduced and causes a lack of mixing of vertical and rotational material flow necessary to form onion ring structure. A good interface in Fig. 6a and b suggests that the plasticized ma terial completely covered the surface of the particle. Ti particles did not have abrupt sharp corners or irregular polygonal surfaces (as seen in Fig. 1) which may interrupt the smooth material flow and create pores at the interface [23]. Further, the mechanical movement of the tool is able to break any oxide layer present on the surface of the particle and es tablishes an intimate contact with the matrix. A sharp change in elemental spectrum at the interface suggests that there was no possible reaction between the alloying element Al and Ti to form any compounds at the interface. The nature of the interface is a crucial factor in MMCs to transfer the tensile load. If the interface is characterized by pores and other compounds, the mechanical performance will not improve. A strong interface is a vital factor compared to uniform distribution. 7
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Fig. 9. TEM micrograph of AZ31/21 vol% Ti MMC showing; (a) fine grains and particle distribution, (b) interface of a single particle, (c) and (d) dislocation density and (e) diffraction pattern of Ti particle.
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Fig. 10. (a) Stress strain graphs of AZ31/Ti MMCs and (b) extracted values.
The following could be the possible reasons for the retention of initial size and morphology of Ti particles. The toughness of Ti particles is higher to commonly used ceramic particles. Because the nominal area under the stress strain diagram of Ti is several times higher to the area of a ceramic particle obtained in a tensile test. Ti particles absorb the imposed strain to a larger extend. Secondly, unlike a non-deformable ceramic particle, deformable Ti particle does not oppose the plasti cized material flow and moves along with it. Finally, the smaller size of Ti particles has less tendency to fracture compared to a larger particle of size greater than 10 μm. The rolled direction of the base metal was perpendicular to pro cessing direction causing to display larger grains (Fig. 7a). The large grain refinement in the composites (Fig. 7b–d) is obtained due to the phenomenon known as dynamic recrystallization which is well accepted in literature. The extreme deformation of the material plastically at elevated temperature results in dynamic recrystallization. The grain size reduction due to an increase in volume fraction can be attributed to the pinning effect of Ti particles and increased grain nucleation sites. Huang and Shen [38] observed high angle boundaries and attributed them to adequate dynamic recrystallization. Though the size of Ti particles is small, they are distributed effec tively in the composite (Fig. 9a). Most of the particles are located inside the grains while few particles are sitting across the grain boundaries. This observation confirms the intragranular distribution of particles. A strong interfacial bonding is realized (Fig. 9b). The possible causes of dislocation formation are; (a) heavy plastic deformation of the matrix alloy during processing and (b) strain misfit between Ti particle and AZ31 matrix. The frictional heat did not destroy the dislocations completely. Dislocation filled strain fields are reported to add strength to the composite. The factors which account for an enhancement of tensile behavior are explained below one by one. According to the rule of mixture, the reinforcement needs to be stronger compared to matrix material to provide tangible strengthening [39]. Ti is stronger to that of AZ31 which is the primary cause of strengthening. Ti particles are reinforced uni formly in the matrix of the composite by FSP process which is advan tageous to benefit from Orowan strengthening. The progression of dislocations is retarded due to bowing effect. The direction of motion of dislocation is continuously changed multiple times [40]. The uniform distribution supports the external load by evenly sharing it to each particle. The microstructures exhibited ample evidence for a strong metallurgical bonding between the Ti particle and the AZ31 matrix. This assists to accept the transfer of tensile load to the particle effectively. The composites showed the existence of dislocation filled strain fields. These strain fields do not allow free movement of dislocations and provide resistance. Lastly, the exceptional grain refinement achieved through FSP is able to provide additional strengthening as per
Hall–Petch relationship. It is much more complicated to assess the effect of each individual factors. They interact with each other while strengthening the composite. These factors multiply as the quantity of Ti particles increase in the composite to higher volume fraction to further boost up the tensile behavior. Ti particles did not cause significant loss in ductility but helped to retain to a reasonable percentage. There are two main causes for the improvement in ductility. Unlike a ceramic particle, Ti particles are deformable under tensile load. Secondly, Ti particles conduct heat at a higher rate to that of a ceramic particle. Therefore, unnecessary work hardening of the matrix near the particle does not occur during tensile test which helps to achieve adequate plastic flow of the matrix. The fine dimples on the fracture surface originate due to the extremely refined grain structure of the composite. The increase in Ti content promoted flatness of the fracture surface due to a reduction in material flow caused by high degree of strengthening. Nevertheless, the fracture surfaces indicate that the composites failed by ductile mode of fracture. 5. Conclusions � Magnesium composites using AZ31 alloy as matrix material and Ti particles as reinforcement material were effectively produced using friction stir processing (FSP). The volume content was varied from 0 to 21 in steps of 7 vol%. � Ti particles were preserved in elemental form without any kind of chemical decomposition or deleterious reaction with magnesium and its alloying elements. � Ti particles were distributed homogenously in the composites and the distribution was independent upon the location observed. A proper interfacial bonding between Ti particles and the matrix was established without any kind of reaction layer or pores. The high plastic strain during processing did not cause fracture of Ti particles owing to its smaller size, smooth contours and high toughness. � The composite exhibited remarkable grain refinement obtained through dynamic recrystallization and pinning effect of Ti particles. Large number of dislocations filled strain fields were detected due to deformation and strain misfit. � Ti particles led to an improvement in UTS from 226 MPa (0 vol%) to 283 MPa (21 vol%) in the present investigation. The percentage elongation of the composites which was noted to be 14.5% at 0 vol% and 9.4% at 21.%. Ti particles enhanced the yield point of the composites and were beneficial to achieve overall strengthening as well as considerable deformation of the composite before failure in contrast to ceramic particle reinforcements. The fracture surfaces revealed ductile mode of fracture.
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Fig. 11. SEM micrographs of fracture surfaces of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, (d) and (e) 21 vol%.
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Author contributions
[15] K. Kitazono, S. Komatsu, Y. Kataoka, Mechanical Properties of titanium particles dispersed magnesium matrix composite produced through accumulative diffusion bonding process, Mater. Trans. 52 (2011) 155–158. [16] K.N.B. Malik, E. Przełozynska, The influence of Ti particles on microstructure and mechanical properties of Mg-5Al-5RE matrix alloy composite, J. Alloy. Comp. 728 (2017) 600–606. [17] K.N.B. Malik, E. Przełozynska, Analyses of AM50-Tip metal-metal composite microstructure, J. Alloy. Comp. 731 (2018) 1181–1187. [18] H. Yu, H. Zhou, Y. Sun, L. Ren, Z. Wan, L. Hu, Microstructures and mechanical properties of ultrafine-grained Ti/AZ31 magnesium matrix composite prepared by powder metallurgy, Adv. Powder Technol. 29 (2018) 3241–3249. [19] H. Yu, Y. Sun, L. Hu, Z. Wan, H. Zhou, Microstructure and properties of mechanically milled AZ61 powders dispersed with submicron/nanometer Ti particulates, Mater. Char. 127 (2017) 272–278. [20] K. Maweja, M. Phasha, N.V.D. Berg, Microstructure and crystal structure of an equimolar Mg–Ti alloy processed by Simoloyer high-energy ball mill, Powder Technol. 199 (2010) 256–263. [21] B. Ratna Sunil, G. Pradeep Kumar Reddy, H. Patle, R. Dumpala, Magnesium based surface metal matrix composites by friction stir processing, Journal of Magnesium and Alloys 4 (2016) 52–61. [22] Z.Y. Ma, Friction stir processing Technology: a review, Metall. Mater. Trans. A 39 (2008) 642–658. [23] V. Sharma, U. Prakash, B.V. Manoj Kumar, Surface composites by friction stir processing: a review, J. Mater. Process. Technol. 224 (2015) 117–134. [24] S. Rathee, S. Maheshwari, A.N. Siddiquee, M. Srivastava, A review of recent progress in solid state fabrication of composites and functionally graded systems via friction stir processing, Crit. Rev. Solid State Mater. Sci. (43) (2018) 334–366. [25] P. Asadi, G. Faraji, M.K. Besharati, Producing of AZ91/SiC composite by friction stir processing (FSP), Int. J. Adv. Manuf. Technol. 51 (2010) 247–260. [26] D. Khayyamin, A. Mostafapour, R. Keshmiri, The effect of process parameters on microstructural characteristics of AZ91/SiO2 composite fabricated by FSP, Mater. Sci. Eng. A 559 (2013) 217–221. [27] D. Lu, Y. Jiang, R. Zhou, Wear performance of nano-Al2O3 particles and CNTs reinforced magnesium matrix composites by friction stir processing, Wear 305 (2013) 286–290. [28] B. Ratna Sunil, T.S. Sampath Kumar, U. Chakkingal, V. Nandakumar, M. Doble, Friction stir processing of magnesium–nanohydroxyapatite composites with controlled in vitro degradation behavior, Mater. Sci. Eng. C 39 (2014) 315–324. [29] M. Balakrishnan, I. Dinaharan, R. Palanivel, R. Sivaprakasam, Synthesize of AZ31/ TiC magnesium matrix composites using friction stir processing, Journal of Magnesium and Alloys 3 (2015) 76–78. [30] M. Navazani, K. Dehghani, Fabrication of Mg-ZrO2 surface layer composites by friction stirprocessing, J. Mater. Process. Technol. 229 (2016) 439–449. [31] D. Ahmadkhaniha, M. Heydarzadeh Sohi, A. Salehi, R. Tahavvori, Formations of AZ91/Al2O3 nano-composite layer by friction stir processing, Journal of Magnesium and Alloys 4 (2016) 314–318. [32] A. Simar, A. Mertens, S. Ryelandt, F. Delannay, L. Brassart, Mean-field model analysis of deformation and damage in friction stir processed Mg-C composites, Mater. Sci. Eng. A 723 (2018) 324–333. [33] I. Dinaharan, S.C. Vettivel, M. Balakrishnan, E.T. Akinlabi, Influence of processing route on microstructure and wear resistance of fly ash reinforced AZ31 magnesium matrix composites, Journal of Magnesium and Alloys 7 (2019) 155–165. [34] C.J. Lee, J.C. Huang, P.J. Hsieh, Mg based nano-composites fabricated by friction stir processing, Scr. Mater. 54 (2006) 1415–1420. [35] H. Yu, Y. Sun, Z. Wan, H. Zhou, L. Hu, Nanocrystalline Ti/AZ61 magnesium matrix composite: evolution of microstructure and mechanical property during annealing treatment, J. Alloy. Comp. 741 (2018) 231–239. [36] W. Xunhong, W. Kuaishe, Microstructure and properties of friction stir butt-welded AZ31 magnesium alloy, Mater. Sci. Eng. A 431 (2006) 114–117. [37] M. Azizieh, A.H. Kokabi, P. Abachi, Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al2O3 nanocomposites fabricated by friction stir processing, Mater. Des. 32 (2011) 2034–2041. [38] G. Huang, Y. Shen, The effects of processing environments on the microstructure and mechanical properties of the Ti/5083Al composites produced by friction stir processing, J. Manuf. Process. 30 (2017) 361–373. [39] H.S. Kim, On the rule of mixtures for the hardness of particle reinforced composites, Mater. Sci. Eng. A 289 (2000) 30–33. [40] S. Queyreau, G. Monnet, B. Devincre, Orowan strengthening and forest hardening superposition examined by dislocation dynamics simulations, Acta Mater. 58 (2010) 5586–5595.
Isaac Dinaharan and Shuai Zhang executed the experiments and obtained the results. Isaac Dinaharan and Gaoqiang Chen designed the experiments and analyzed the results. Isaac Dinaharan wrote the manuscript and revised it. Qingyu Shi supervised the research work, provided the fund and proof read the manuscript. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Declaration of competing interest There are no conflicts between the authors or with the institution. Acknowledgements We thank Dr. Zhang and Dr. Zheng from ZKKF (Beijing) Science and Technology Co., Ltd for TEM observation and Sprint Testing Solutions, Mumbai for EBSD observation. References [1] A. Dey, K.M. Pandey, Magnesium metal matrix composites-a review, Rev. Adv. Mater. Sci. 42 (2015) 58–67. [2] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. [3] M. Gupta, W.L.E. Wong, Magnesium-based nanocomposites: lightweight materials of the future, Mater. Char. 105 (2015) 30–46. [4] X.F. Sun, C.J. Wang, K.K. Deng, K.B. Nie, X.C. Zhang, X.Y. Xiao, High strength SiCp/AZ91 composite assisted by dynamic precipitated Mg17Al12 phase, J. Alloy. Comp. 732 (2018) 328–335. [5] A. Khandelwal, K. Mani, N. Srivastava, R. Gupta, G.P. Chaudhari, Mechanical behavior of AZ31/Al2O3 magnesium alloy nanocomposites prepared using ultrasound assisted stir casting, Composites Part B 123 (2017) 64–73. [6] M. Rezayat, M.H. Parsa, H. Mirzadeh, J.M. Cabrera, Microstructural investigation of Al-Mg B4C composite deformed at elevated temperature, J. Alloy. Comp. 763 (2018) 643–651. [7] B.N. Sahoo, S.K. Panigrahi, Effect of in-situ (TiC-TiB2) reinforcement on aging and mechanical behavior of AZ91 magnesium matrix composite, Mater. Char. 139 (2018) 221–232. [8] M. Ali, M.A. Hussein, N. Al-Aqeeli, Magnesium-based composites and alloys for medical applications A review of mechanical and corrosion properties, J. Alloy. Comp. 792 (2019) 1162–1190. [9] W.L.E. Wong, M. Gupta, Development of Mg/Cu nanocomposites using microwave assisted rapid sintering, Compos. Sci. Technol. 67 (2007) 1541–1552. [10] K. Edalati, H. Emami, A. Staykov, D.J. Smith, E. Akiba, Z. Horita, Formation of metastable phases in magnesium–titanium system by high-pressure torsion and their hydrogen storage performance, Acta Mater. 99 (2015) 150–156. [11] K. Maweja, M. Phasha, N.V.D. Berg, Microstructure and crystal structure of an equimolar Mg–Ti alloy processed by Simoloyer high-energy ball mill, Powder Technol. 199 (2010) 256–263. [12] S.F. Hassan, M. Gupta, Development of ductile magnesium composite materials using titanium as reinforcement, J. Alloy. Comp. 345 (2002) 246–251. [13] P. Perez, G. Garce, P. Adeva, Mechanical properties of a Mg–10 (vol.%)Ti composite, Compos. Sci. Technol. 64 (2004) 145–151. [14] J. Umeda, M. Kawakami, K. Kondoh, E. Ayman, H. Imai, Microstructural and mechanical properties of titanium particulate reinforced magnesium composite materials, Mater. Chem. Phys. 123 (2010) 649–657.
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Titanium particulate reinforced AZ31 magnesium matrix composites with improved ductility prepared using friction stir processing Isaac Dinaharan a, Shuai Zhang b, Gaoqiang Chen b, Qingyu Shi b, &lowest; a b
IDM-Joint Lab, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China The State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium matrix composite Friction stir processing Titanium Microstructure Tensile strength
Conventional ceramic particulate reinforcements cause a major loss in ductility of magnesium matrix composites (MMCs). Metallic particles possessing higher melting point can offer a solution to this issue. Titanium (Ti) particles (0,7,14 and 21 vol%) were reinforced into magnesium alloy AZ31 using friction stir processing (FSP) performed by a conventional sturdy vertical milling machine. The microstructure and the tensile behavior of the fabricated composites were studied in detail. The micrographs revealed a uniform distribution of Ti particles all over the stir zone irrespective of the volume content of Ti. Ti particles did not decompose or react with the matrix and its alloying elements. Ti particles established a proper interface with the matrix AZ31. Ti particles survived the severe plastic strain without breakage. The grains in the matrix were refined extremely because of dynamic recrystallization and the pinning effect of Ti particles. A large number of dislocations are found in the composite. Ti particles improved the tensile strength of the composite and helped to retain appreciable ductility.
1. Introduction Magnesium matrix composites (MMCs) reinforced with various particles exhibit light weight which makes them an attractive alternative material to commonly used aluminum matrix composites in several applications in automobiles and allied industries [1–3]. The reinforce ment particles are usually selected from any one category of ceramics such as oxides, carbides, borides and nitrides. Ceramic particles improved the strength of MMCs but deprived the ductility owing to its non-deformable nature [4–7]. Metallic particles possessing higher melting point and hardness such as titanium, nickel, tungsten, molyb denum etc. can be used to reinforce MMCs to offset the issue of ductility [8,9]. Titanium (Ti) is an appropriate choice for MMCs due to the following reasons. Both magnesium and titanium are characterized by hexagonal crystal structure (HCP) which would alleviate compatibility problems. The mechanical deformation of Ti is several folds higher than typical ceramic particles which would help to enhance ductility. Further, Ti does not mix with magnesium either in solid or liquid state to form a resultant solid solution or any kind of intermetallic compounds [10,11]. Producing MMCs is a daunting task among all metallic based com posites due to the risk of fire hazard and high shrinkage. Few
investigators attempted to produce Ti reinforced MMCs using various methods which include disintegrated melt deposition [12], powder metallurgy [13,14], accumulative diffusion bonding [15], stir casting [16,17], vacuum hot pressing [18], mechanical alloying [19,20]. Ti particles were successfully reinforced into different magnesium alloys such as AZ31, AZ61, AZ91, AM50, AME505 and pure magnesium using those techniques. Although Ti particles were reinforced by both liquid and solid-state methods, the desirable design features were lacking. Improper distribution [12], clustering of particles [13], pores at micro and macro levels [13], weak interfaces [14], coarse grains [14,16], emergence of detrimental compounds [17,19], high energy and time consumption [15,19] and distortion of crystal structure [20] were re ported. These unwanted microstructural features limited the tensile behavior and other factors increased the production cost. Friction stir processing (FSP) is accepted as a novel solid-state method to produce high performance MMCs and has attained popu larity in the last decade [21]. A harder rotating third body known as the tool is forced into the metallic plates to generate frictional heat. The matrix material is deformed combinedly by frictional heat and plastic deformation. The mechanical action of the tool reinforces the particles into the plasticized matrix. FSP consumes lower energy to produce metallic composites. The nature of the distribution is unaffected by the
* Corresponding author. E-mail addresses: [email protected] (I. Dinaharan), [email protected] (Q. Shi). https://doi.org/10.1016/j.msea.2019.138793 Received 16 October 2019; Received in revised form 5 December 2019; Accepted 7 December 2019 Available online 9 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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Table 1 The chemical composition of magnesium alloy AZ31B. Element
Al
Zn
Mn
Si
Ca
Ni
Fe
Others
Mg
wt.%
3.45
0.95
0.47
0.16
0.05
0.06
0.02
0.14
Balance
Fig. 1. (a) SEM micrograph, (b) particle size distribution and (c) XRD pattern of as received titanium powder.
reinforced nano hydroxyapatite MMCs and studied in vitro bioactivity. Particles and grain refinement caused biomineralization. Balakrishnan et al. [29] produced TiC particle reinforced AZ31 MMCs and analyzed the microstructural features. They reported a reasonable distribution of TiC particles in the composite. Navazani and Dehghani [30] prepared ZrO2 reinforced AZ31 MMCs and reported an improvement in tensile strength due to fine dispersion and grain refinement. Ahmadkhaniha et al. [31] developed nano alumina particle (Al2O3np) reinforced AZ91 MMCs and showed an improved wear behavior. Simar et al. [32] pre pared short carbon fiber reinforced AZ91D MMCs and analyzed the role of heat treatment. Dinaharan et al. [33] synthesized fly ash reinforced AZ31 MMCs and obtained an improved wear behavior to a similar composite produced by stir casting method. A short literature survey revealed that several kind of reinforcement particles were effectively reinforced to prepare MMCs with improved properties. However, literature on metallic particle reinforcements are lacking. Therefore, the present study aims to prepare AZ31/Ti MMCs using FSP and investigate the microstructural features using conven tional and advanced microscopic techniques. The influence of micro structural evolution on tensile behavior is reported.
Table 2 FSP conditions. Parameter
Values
Rotational speed (rpm) Traverse speed (mm/min) Passes Shoulder diameter (mm) Shoulder concavity (mm) Pin diameter (mm) Pin length (mm) Pin shape Tool material Tool tilt angle (� ) Groove width (mm) Groove depth (mm) Ti content (vol%)
950 30 5 24 0.2 7–5 5.7 Frustrum of a cone H13 steel 1 0, 0.4, 0.8 and 1.2 5 0,7,14 and 21
physical properties of the reinforcement particle. The heat evolved does not cause melting of the substrate which restricts possible interfacial reactions [22–24]. Asadi et al. [25] produced SiC particle reinforced AZ91 MMCs and investigated the effect of process parameters. They observed a reduction in grain size as the traverse speed was advanced. Khayyamin et al. [26] developed SiO2 reinforced AZ91 MMCs and obtained an improvement in tensile properties. Lu et al. [27] synthesized carbon nano-tubes (CNTs) reinforced AZ31 MMCs and evaluated the wear resistance and observed the wear mechanism. Ratna Sunil et al. [28] fabricated pure magnesium
2. Experimental procedure Hot rolled magnesium alloy AZ31 plates of size 150 mm � 100 mm x 8 mm were procured to produce the composite. Table 1 details the composition of as received plates tested by XRF. Commercially pure Ti 2
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Fig. 2. XRD patterns of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
particles (~2.7 μm) of grade 2 were used as reinforcement phase. Fig. 1 presents the morphology of particles, particle size distribution (measured using MALVERN Mastersizer 2000) and XRD of as received Ti particles. Machined groove strategy was followed to stuff the particles on the plate surface and two step FSP (pinless tool followed by pin tool) procedure was applied [34]. A conventional vertical milling machine (M/s BYJC X5032) was used for FSP. The processing conditions are listed in Table 2 which were optimized from an earlier work on the study of process parameters. Specimens to observe the microstructure evolu tion were sectioned across the FSP seam. Polishing was done using a semi-automatic polishing machine. Fine polishing was accomplished using alumina suspension and 5 μm diamond spray. A reagent consisting of 1 ml acetic acid, 1 ml nitric acid and l g oxalic acid in 150 ml water was used to reveal the microstructure. Micrographs were recorded using an optical microscope (OLYMPUS BX51 M), a scanning electron mi croscope (FEI Quanta 200) and a transmission electron microscope. EBSD was carried out in a FEI Quanta FEG SEM equipped with TSL-OIM software. Sub-sized tensile specimens having 25 mm gauge length; 4 mm width and 4 mm thickness were machined from the processed zone. Tensile test at a strain rate of 0.5 mm/min was tested using a comput erized tensile tester (MTS Exceed E45). Fracture surfaces of selected tensile tested specimens were recorded.
the laws of physical metallurgy. There are no other elements or com pounds detected in significant quantity in any of the composites. Representative SEM micrographs of the stir zone as a function of Ti content is shown in Fig. 3. Fig. 3a is the stir zone of the AZ31 magnesium alloy which is unreinforced. Traces of secondary eutectic particles (Mg17Al12) are seen which appear in white color and nodular shape. The SEM micrographs in Fig. 3b–d displays the reinforcement of Ti particles and the nature of distribution in the composite. The packed Ti particles are scattered in all directions in the stir zone. The number and density of particles increase as the volume content is increased. The variation in the spacing of particles i.e. inter particle distance is minimum which points out a uniform distribution. Particle free regions and clustering of particles are not spotted in the micrographs. Fig. 4 displays the optical micrographs of AZ31/21 vol% Ti com posites which were captured at random locations within the stir zone. It is interesting to observe an unchanging distribution irrespective of the spot location of the micrograph. All parts of the stir zone are uniformly reinforced with Ti particulates. There is not a single location which does not have any distribution. The bottom portion of the stir zone (Fig. 4d) does not display onion ring pattern. Fig. 5 presents the elemental dis tribution in AZ31/21 vol% Ti MMC. The distribution of Ti across the micrograph confirms the uniform distribution of Ti particles in the composite. Fig. 6a–c presents enlarged views of the AZ31/Ti composites at higher magnification which enable to have a closer look at the interface existing between the particle and the matrix alloy. A continuous inter face is observed around the particles without interruption. There are no foreign materials spotted at the interface. The matrix alloy is fused together with the particle on all sides without any pores. Line EDAX
3. Results XRD diffraction patterns of AZ31/Ti MMCs at different volume fractions are depicted in Fig. 2. The peaks of the magnesium matrix and the reinforcement Ti are distinctly observed in the figure. The height of Ti peaks grows as the volume fraction is increased in accordance with 3
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Fig. 3. SEM micrographs of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
Fig. 4. Optical photomicrograph of AZ31/21 vol%. Ti MMCs at various locations within the stir zone: (a) top portion, (b) interface at the retreading side, (c) middle portion and (d) bottom portion.
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Fig. 5. (a) SEM micrograph of AZ31/21 vol% Ti MMC and distribution of elements; (b) Mg, (c) Al, and (d) Ti.
Fig. 6. SEM micrographs of AZ31/Ti MMCs containing Ti particles; (a) 14 vol%, (b) and (c) 21 vol%, and (d) line EDAX across a particle.
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Fig. 7. EBSD (IPF þ grain boundary) maps of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
across the particle and the matrix alloy was recorded (Fig. 6d) to verify any sort of diffusion or traces of other compounds. The elemental spectrums of Mg and Ti are crossing sharply at the interface. There is no depletion of Ti spectrum or increase in the spectrum of Al at the interface. The reinforcement particles are subjected to severe straining during FSP. A change in the shape and the size of the reinforcement particles were noticed in several investigations [23,24]. There is no significant change in the size of Ti particles comparing Fig. 1 with Figs. 3 and 6. The initial size and morphology are almost retained after FSP. Fig. 7 reveals the EBSD images of the base metal as well as the pre pared composites which help to visualize the grain structure inside the material. The base metal shows a coarse grain structure recording an average grain size of 93 μm. Conversely, AZ31/Ti composites consist of a fine grain structure. There is a drastic reduction in the size of grains from 93 μm to 10.2 μm in AZ31/7 vol% Ti MMC. The increase in the volume content of Ti particles further reduced the grain size to 4.3 μm in AZ31/ 21 vol% Ti MMC. The corresponding misorientation maps in Fig. 8 shows that more percentage (40–50%) of high angle boundaries is generated compared to base metal. Fig. 9 presents a montage of TEM micrographs of AZ31/21 vol% Ti composites revealing various microstructural features. Fine grains and particle distribution are observed in Fig. 9a. It ensures absence of par ticle clustering in the composite. The interface is crystal clear and no gap is existing between the matrix and the particle (Fig. 9b). The matrix alloy AZ31 of the composite is characterized by large amount of dislo cations as seen in Fig. 9c and d. The selected area electron diffraction (SAED) pattern (Fig. 9e) confirms the hexagonal closed packed structure
of Ti particles. The stress strain graphs recorded during the tensile test of AZ31/Ti MMCs and the bar chart of extracted values are shown in Fig. 10. The effect of Ti particle incorporation into the magnesium alloy AZ31 is obvious from these plots. Ti particles led to an improvement in UTS from 226 MPa (0 vol%) to 283 MPa (21 vol%) in the present investigation. The yield strength of the composite also improved due to the presence of Ti particles. Yield strength was recorded from 98 MPa (0 vol%) to 193 MPa (21 vol%). There is a tremendous increase in the yielding point. Tensile graphs prove that Ti is a beneficial reinforcement for AZ31. It is interesting to observe the percentage elongation of the composites which was noted to be 14.5% at 0 vol% and 9.4% at 21 vol%. The overall ductility of the composite is better compared to ceramic particle rein forced MMCs. SEM observations of the fracture surface of AZ31/Ti MMCs are recorded in Fig. 11. The unreinforced matrix alloy AZ31 shows slightly elongated dimples in Fig. 11a. A network of fine dimples is visible on the fracture surfaces of the composite (Fig. 11b–d). Ti particles are spotted across the fracture surfaces. Fig. 11e shows the presence of full and fractured Ti particles which gives evidence for superior interfacial bonding. 4. Discussion It is generally known that magnesium and Ti do not dissolve in each other irrespective of the temperature. They constitute an immiscible system. However, it is possible to develop metastable phases as a result of severe plastic deformation as reported by Edalati et al. [10]. This is 6
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Fig. 8. Misorientation distribution of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, and (d) 21 vol%.
commonly reflected in broadening of magnesium peaks in the XRD pattern. No such broadening of magnesium peaks is observed in Fig. 2 which confirms the absence of metastable phases. This may be due to the exposure to severe deformation for a shorter duration. The matrix used in this investigation is not pure magnesium but magnesium alloy AZ31. The alloying elements Al and Ti have good chemical affinity to each other and tend to form deleterious compounds such as Ti3Al at elevated temperature and prolonged straining [19,35]. No peaks of Ti3Al are present in the XRD pattern which rules out the existence of Ti3Al. The processing temperature and the exposure duration during FSP are insufficient to cause considerable diffusion of Al into Ti to form Ti3Al. Nevertheless, advanced microstructural characterization is necessary to find out any traces of such compounds at the interface. Mg17Al12 particles usually experience dissolution into the magne sium matrix at higher temperature [36]. Few survived the dissolution effect while most of them were dissolved during processing. FSP leads to a desirable uniform distribution which is essential to boost mechanical properties. A good distribution is a result of appropriate choice of pro cess parameters. Poor particle distribution was observed in several studies. The key factor is the application of multiple passes (5 in this study) which provided better distribution and eliminated clusters and unreinforced regions. The mechanism of composite formation via FSP involves plasticization of the magnesium matrix under the combined action of frictional heat and the tool movement. The plasticized material is transported from the advancing side to the retreading side. The rotating tool mixes the packed particles with the plasticized material and squeezes out in the space between the cold base metal and the tool pin. The tool advancement results in forging of the composite. Segregation of reinforcement particles along the grain boundary region is a common issue in casting routes. Such kind of segregation is not present in the micrographs due to the absence of solidification process. It can be said that many particles are located inside the grains i.e. intragranular dis tribution. The distribution is constant irrespective of the volume content
under the set of processing conditions. A composite should possess a constant distribution throughout its span. It is an uphill task to achieve a constant distribution in casting methods. Various factors such as density difference, force of the solidi fication front etc. cause undesirable motion of particles in the composite before cooling down to atmospheric temperature. The frictional heat does not cause melting of the matrix alloy which avoids all the issues originating from the physical properties of the reinforcement. The movement of the reinforcement particle is dictated by the tool rotation and arrested after the consolidation. Therefore, the distribution is pre served during the cooling stage. Nevertheless, variation in the distri bution of particles within the stir zone was reported in composites prepared by FSP method [25,31]. A constant distribution is the outcome and the choice of optimized process parameters. Onion ring pattern would look like stacking of particle rich and particle depleted layers alternately [37]. There remains a 2 mm AZ31 plate between the end of the stir zone and back up plate which promotes dissipating heat fast. The temperature along the vertical direction is reduced and causes a lack of mixing of vertical and rotational material flow necessary to form onion ring structure. A good interface in Fig. 6a and b suggests that the plasticized ma terial completely covered the surface of the particle. Ti particles did not have abrupt sharp corners or irregular polygonal surfaces (as seen in Fig. 1) which may interrupt the smooth material flow and create pores at the interface [23]. Further, the mechanical movement of the tool is able to break any oxide layer present on the surface of the particle and es tablishes an intimate contact with the matrix. A sharp change in elemental spectrum at the interface suggests that there was no possible reaction between the alloying element Al and Ti to form any compounds at the interface. The nature of the interface is a crucial factor in MMCs to transfer the tensile load. If the interface is characterized by pores and other compounds, the mechanical performance will not improve. A strong interface is a vital factor compared to uniform distribution. 7
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Fig. 9. TEM micrograph of AZ31/21 vol% Ti MMC showing; (a) fine grains and particle distribution, (b) interface of a single particle, (c) and (d) dislocation density and (e) diffraction pattern of Ti particle.
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Fig. 10. (a) Stress strain graphs of AZ31/Ti MMCs and (b) extracted values.
The following could be the possible reasons for the retention of initial size and morphology of Ti particles. The toughness of Ti particles is higher to commonly used ceramic particles. Because the nominal area under the stress strain diagram of Ti is several times higher to the area of a ceramic particle obtained in a tensile test. Ti particles absorb the imposed strain to a larger extend. Secondly, unlike a non-deformable ceramic particle, deformable Ti particle does not oppose the plasti cized material flow and moves along with it. Finally, the smaller size of Ti particles has less tendency to fracture compared to a larger particle of size greater than 10 μm. The rolled direction of the base metal was perpendicular to pro cessing direction causing to display larger grains (Fig. 7a). The large grain refinement in the composites (Fig. 7b–d) is obtained due to the phenomenon known as dynamic recrystallization which is well accepted in literature. The extreme deformation of the material plastically at elevated temperature results in dynamic recrystallization. The grain size reduction due to an increase in volume fraction can be attributed to the pinning effect of Ti particles and increased grain nucleation sites. Huang and Shen [38] observed high angle boundaries and attributed them to adequate dynamic recrystallization. Though the size of Ti particles is small, they are distributed effec tively in the composite (Fig. 9a). Most of the particles are located inside the grains while few particles are sitting across the grain boundaries. This observation confirms the intragranular distribution of particles. A strong interfacial bonding is realized (Fig. 9b). The possible causes of dislocation formation are; (a) heavy plastic deformation of the matrix alloy during processing and (b) strain misfit between Ti particle and AZ31 matrix. The frictional heat did not destroy the dislocations completely. Dislocation filled strain fields are reported to add strength to the composite. The factors which account for an enhancement of tensile behavior are explained below one by one. According to the rule of mixture, the reinforcement needs to be stronger compared to matrix material to provide tangible strengthening [39]. Ti is stronger to that of AZ31 which is the primary cause of strengthening. Ti particles are reinforced uni formly in the matrix of the composite by FSP process which is advan tageous to benefit from Orowan strengthening. The progression of dislocations is retarded due to bowing effect. The direction of motion of dislocation is continuously changed multiple times [40]. The uniform distribution supports the external load by evenly sharing it to each particle. The microstructures exhibited ample evidence for a strong metallurgical bonding between the Ti particle and the AZ31 matrix. This assists to accept the transfer of tensile load to the particle effectively. The composites showed the existence of dislocation filled strain fields. These strain fields do not allow free movement of dislocations and provide resistance. Lastly, the exceptional grain refinement achieved through FSP is able to provide additional strengthening as per
Hall–Petch relationship. It is much more complicated to assess the effect of each individual factors. They interact with each other while strengthening the composite. These factors multiply as the quantity of Ti particles increase in the composite to higher volume fraction to further boost up the tensile behavior. Ti particles did not cause significant loss in ductility but helped to retain to a reasonable percentage. There are two main causes for the improvement in ductility. Unlike a ceramic particle, Ti particles are deformable under tensile load. Secondly, Ti particles conduct heat at a higher rate to that of a ceramic particle. Therefore, unnecessary work hardening of the matrix near the particle does not occur during tensile test which helps to achieve adequate plastic flow of the matrix. The fine dimples on the fracture surface originate due to the extremely refined grain structure of the composite. The increase in Ti content promoted flatness of the fracture surface due to a reduction in material flow caused by high degree of strengthening. Nevertheless, the fracture surfaces indicate that the composites failed by ductile mode of fracture. 5. Conclusions � Magnesium composites using AZ31 alloy as matrix material and Ti particles as reinforcement material were effectively produced using friction stir processing (FSP). The volume content was varied from 0 to 21 in steps of 7 vol%. � Ti particles were preserved in elemental form without any kind of chemical decomposition or deleterious reaction with magnesium and its alloying elements. � Ti particles were distributed homogenously in the composites and the distribution was independent upon the location observed. A proper interfacial bonding between Ti particles and the matrix was established without any kind of reaction layer or pores. The high plastic strain during processing did not cause fracture of Ti particles owing to its smaller size, smooth contours and high toughness. � The composite exhibited remarkable grain refinement obtained through dynamic recrystallization and pinning effect of Ti particles. Large number of dislocations filled strain fields were detected due to deformation and strain misfit. � Ti particles led to an improvement in UTS from 226 MPa (0 vol%) to 283 MPa (21 vol%) in the present investigation. The percentage elongation of the composites which was noted to be 14.5% at 0 vol% and 9.4% at 21.%. Ti particles enhanced the yield point of the composites and were beneficial to achieve overall strengthening as well as considerable deformation of the composite before failure in contrast to ceramic particle reinforcements. The fracture surfaces revealed ductile mode of fracture.
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Fig. 11. SEM micrographs of fracture surfaces of AZ31/Ti MMCs containing Ti particles; (a) 0 vol%, (b) 7 vol%, (c) 14 vol%, (d) and (e) 21 vol%.
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Author contributions
[15] K. Kitazono, S. Komatsu, Y. Kataoka, Mechanical Properties of titanium particles dispersed magnesium matrix composite produced through accumulative diffusion bonding process, Mater. Trans. 52 (2011) 155–158. [16] K.N.B. Malik, E. Przełozynska, The influence of Ti particles on microstructure and mechanical properties of Mg-5Al-5RE matrix alloy composite, J. Alloy. Comp. 728 (2017) 600–606. [17] K.N.B. Malik, E. Przełozynska, Analyses of AM50-Tip metal-metal composite microstructure, J. Alloy. Comp. 731 (2018) 1181–1187. [18] H. Yu, H. Zhou, Y. Sun, L. Ren, Z. Wan, L. Hu, Microstructures and mechanical properties of ultrafine-grained Ti/AZ31 magnesium matrix composite prepared by powder metallurgy, Adv. Powder Technol. 29 (2018) 3241–3249. [19] H. Yu, Y. Sun, L. Hu, Z. Wan, H. Zhou, Microstructure and properties of mechanically milled AZ61 powders dispersed with submicron/nanometer Ti particulates, Mater. Char. 127 (2017) 272–278. [20] K. Maweja, M. Phasha, N.V.D. Berg, Microstructure and crystal structure of an equimolar Mg–Ti alloy processed by Simoloyer high-energy ball mill, Powder Technol. 199 (2010) 256–263. [21] B. Ratna Sunil, G. Pradeep Kumar Reddy, H. Patle, R. Dumpala, Magnesium based surface metal matrix composites by friction stir processing, Journal of Magnesium and Alloys 4 (2016) 52–61. [22] Z.Y. Ma, Friction stir processing Technology: a review, Metall. Mater. Trans. A 39 (2008) 642–658. [23] V. Sharma, U. Prakash, B.V. Manoj Kumar, Surface composites by friction stir processing: a review, J. Mater. Process. Technol. 224 (2015) 117–134. [24] S. Rathee, S. Maheshwari, A.N. Siddiquee, M. Srivastava, A review of recent progress in solid state fabrication of composites and functionally graded systems via friction stir processing, Crit. Rev. Solid State Mater. Sci. (43) (2018) 334–366. [25] P. Asadi, G. Faraji, M.K. Besharati, Producing of AZ91/SiC composite by friction stir processing (FSP), Int. J. Adv. Manuf. Technol. 51 (2010) 247–260. [26] D. Khayyamin, A. Mostafapour, R. Keshmiri, The effect of process parameters on microstructural characteristics of AZ91/SiO2 composite fabricated by FSP, Mater. Sci. Eng. A 559 (2013) 217–221. [27] D. Lu, Y. Jiang, R. Zhou, Wear performance of nano-Al2O3 particles and CNTs reinforced magnesium matrix composites by friction stir processing, Wear 305 (2013) 286–290. [28] B. Ratna Sunil, T.S. Sampath Kumar, U. Chakkingal, V. Nandakumar, M. Doble, Friction stir processing of magnesium–nanohydroxyapatite composites with controlled in vitro degradation behavior, Mater. Sci. Eng. C 39 (2014) 315–324. [29] M. Balakrishnan, I. Dinaharan, R. Palanivel, R. Sivaprakasam, Synthesize of AZ31/ TiC magnesium matrix composites using friction stir processing, Journal of Magnesium and Alloys 3 (2015) 76–78. [30] M. Navazani, K. Dehghani, Fabrication of Mg-ZrO2 surface layer composites by friction stirprocessing, J. Mater. Process. Technol. 229 (2016) 439–449. [31] D. Ahmadkhaniha, M. Heydarzadeh Sohi, A. Salehi, R. Tahavvori, Formations of AZ91/Al2O3 nano-composite layer by friction stir processing, Journal of Magnesium and Alloys 4 (2016) 314–318. [32] A. Simar, A. Mertens, S. Ryelandt, F. Delannay, L. Brassart, Mean-field model analysis of deformation and damage in friction stir processed Mg-C composites, Mater. Sci. Eng. A 723 (2018) 324–333. [33] I. Dinaharan, S.C. Vettivel, M. Balakrishnan, E.T. Akinlabi, Influence of processing route on microstructure and wear resistance of fly ash reinforced AZ31 magnesium matrix composites, Journal of Magnesium and Alloys 7 (2019) 155–165. [34] C.J. Lee, J.C. Huang, P.J. Hsieh, Mg based nano-composites fabricated by friction stir processing, Scr. Mater. 54 (2006) 1415–1420. [35] H. Yu, Y. Sun, Z. Wan, H. Zhou, L. Hu, Nanocrystalline Ti/AZ61 magnesium matrix composite: evolution of microstructure and mechanical property during annealing treatment, J. Alloy. Comp. 741 (2018) 231–239. [36] W. Xunhong, W. Kuaishe, Microstructure and properties of friction stir butt-welded AZ31 magnesium alloy, Mater. Sci. Eng. A 431 (2006) 114–117. [37] M. Azizieh, A.H. Kokabi, P. Abachi, Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al2O3 nanocomposites fabricated by friction stir processing, Mater. Des. 32 (2011) 2034–2041. [38] G. Huang, Y. Shen, The effects of processing environments on the microstructure and mechanical properties of the Ti/5083Al composites produced by friction stir processing, J. Manuf. Process. 30 (2017) 361–373. [39] H.S. Kim, On the rule of mixtures for the hardness of particle reinforced composites, Mater. Sci. Eng. A 289 (2000) 30–33. [40] S. Queyreau, G. Monnet, B. Devincre, Orowan strengthening and forest hardening superposition examined by dislocation dynamics simulations, Acta Mater. 58 (2010) 5586–5595.
Isaac Dinaharan and Shuai Zhang executed the experiments and obtained the results. Isaac Dinaharan and Gaoqiang Chen designed the experiments and analyzed the results. Isaac Dinaharan wrote the manuscript and revised it. Qingyu Shi supervised the research work, provided the fund and proof read the manuscript. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Declaration of competing interest There are no conflicts between the authors or with the institution. Acknowledgements We thank Dr. Zhang and Dr. Zheng from ZKKF (Beijing) Science and Technology Co., Ltd for TEM observation and Sprint Testing Solutions, Mumbai for EBSD observation. References [1] A. Dey, K.M. Pandey, Magnesium metal matrix composites-a review, Rev. Adv. Mater. Sci. 42 (2015) 58–67. [2] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. [3] M. Gupta, W.L.E. Wong, Magnesium-based nanocomposites: lightweight materials of the future, Mater. Char. 105 (2015) 30–46. [4] X.F. Sun, C.J. Wang, K.K. Deng, K.B. Nie, X.C. Zhang, X.Y. Xiao, High strength SiCp/AZ91 composite assisted by dynamic precipitated Mg17Al12 phase, J. Alloy. Comp. 732 (2018) 328–335. [5] A. Khandelwal, K. Mani, N. Srivastava, R. Gupta, G.P. Chaudhari, Mechanical behavior of AZ31/Al2O3 magnesium alloy nanocomposites prepared using ultrasound assisted stir casting, Composites Part B 123 (2017) 64–73. [6] M. Rezayat, M.H. Parsa, H. Mirzadeh, J.M. Cabrera, Microstructural investigation of Al-Mg B4C composite deformed at elevated temperature, J. Alloy. Comp. 763 (2018) 643–651. [7] B.N. Sahoo, S.K. Panigrahi, Effect of in-situ (TiC-TiB2) reinforcement on aging and mechanical behavior of AZ91 magnesium matrix composite, Mater. Char. 139 (2018) 221–232. [8] M. Ali, M.A. Hussein, N. Al-Aqeeli, Magnesium-based composites and alloys for medical applications A review of mechanical and corrosion properties, J. Alloy. Comp. 792 (2019) 1162–1190. [9] W.L.E. Wong, M. Gupta, Development of Mg/Cu nanocomposites using microwave assisted rapid sintering, Compos. Sci. Technol. 67 (2007) 1541–1552. [10] K. Edalati, H. Emami, A. Staykov, D.J. Smith, E. Akiba, Z. Horita, Formation of metastable phases in magnesium–titanium system by high-pressure torsion and their hydrogen storage performance, Acta Mater. 99 (2015) 150–156. [11] K. Maweja, M. Phasha, N.V.D. Berg, Microstructure and crystal structure of an equimolar Mg–Ti alloy processed by Simoloyer high-energy ball mill, Powder Technol. 199 (2010) 256–263. [12] S.F. Hassan, M. Gupta, Development of ductile magnesium composite materials using titanium as reinforcement, J. Alloy. Comp. 345 (2002) 246–251. [13] P. Perez, G. Garce, P. Adeva, Mechanical properties of a Mg–10 (vol.%)Ti composite, Compos. Sci. Technol. 64 (2004) 145–151. [14] J. Umeda, M. Kawakami, K. Kondoh, E. Ayman, H. Imai, Microstructural and mechanical properties of titanium particulate reinforced magnesium composite materials, Mater. Chem. Phys. 123 (2010) 649–657.
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