Microstructure And Mechanical Properties of Ultrasonically Processed Al-7Si Alloy / Y2O3 Nanocomposite

doi:10.21203/rs.3.rs-509564/v1

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Microstructure And Mechanical Properties of Ultrasonically Processed Al-7Si Alloy / Y₂O₃ Nanocomposite

https://doi.org/10.21203/rs.3.rs-509564/v1

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published 14 Jan, 2022

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The aim of the present study is to investigate the microstructure and mechanical properties of the A356 aluminum metal matrix composite reinforced with Y₂O₃ particles. The composite is synthesized by adding 1 and 2 vol.% of reinforcement via stir casting assisted by ultrasonic treatment (UT). Microstructural contemplates shows improvement in the dispersion of nano Y₂O₃ particles and decrease in the porosity level due to the ultrasound aided synthesis. The UT refines the size of the Y₂O₃ particles as well as helps to improve its dispersion. The secondary dendrite arm spacing of 2 vol.% Y₂O₃ reinforced samples with 5 min UT is found to be significantly reduced to 12 µm as compared to that of the as-cast A356 alloy. Addition of 2 vol.% of nano Y₂O₃ has significantly improved the hardness of the A356 alloy from 60 HV to 108 HV. A considerable increment in the YS and TS of the A356 alloy is observed with the of Y₂O₃ and found to further improve with UT. However, small reduction in ductility is observed with the addition of Y₂O₃ as well as ultrasonic treatment.

Optical Materials and Devices

A356 alloy

Y2O3

Ultrasonic treatment

Grain refinement

Mechanical properties

Aluminum based metal matrix composites are widely used in aerospace, automotive, mining structural and military applications due to their low weight to high strength ratio, wear resistance, high hardness, elevated temperature resistance and greater stiffness compared to as-cast aluminum alloy. Raghu, R et al. [1] detailed the applications of aluminium based composites in various engineering applications. The properties and performance of the composites depends on their matrix alloy, reinforcement material and processing technique. Jayakrishnan et al. [2] demonstrated the properties of TiB₂ in-situ composites to be dependent on the A356 alloy and the processing technique. Also Satish Kumar et al. [3] studied the effect of reinforcement and the processing technique on the Al-4Mg Alloy/MgA_l2O₄ in-situ composites. Generally, aluminium 356 alloys are widely used in a variety of engineering applications due to its castability, corrosion and wear resistance. Radhika et al.[4] detailed the wear resiatance of LM25/SiO₂ composites and optimized its wear process parameters and Arunagiri et al. [5] studied the castability and adhesive wear behaviour of LM25/AlB₂ composites for applications.

The mechanical properties of these A356 alloy increased with nano particles like Al₂O₃, TiC, SiC and B₄C. However, achieving uniform dispersion of nano particles in the matrix is difficult due to its high surface area as reported by Shian et al. [6]. Su et al. [7] fabricated Al2024/nano-Al₂O₃ composites via mechanical stirring and ultrasonic vibration. From the microstructural analysis, the added nano-particles were reported to be uniformly distributed and have good bonding with the matrix alloy. Du Yuan et al. [8] successfully produced A356 alloy/nano-SiC_p composite using stir casting assisted by ultrasonic vibration and achieved uniform dispersion of nano-SiC_p in A356 alloy matrix. Compared to A356 alloy, ultrasonic treated A356 alloy/2 wt.% nano-SiC_p composite exhibited significant improvement in mechanical properties like YS, TS and % El by 62 %, 22 % and 24 % respectively. Shian et al. [6] developed aluminium 6061 alloy/nano-sized Al₂O₃ and SiC reinforced composites using ultrasonic vibration. No much improvement was observed in YS and TS of the composite samples. Whereas, the percentage of elongation was reported to increase in the composite sample as compared to the matrix alloy. Arpan et al.[9] made-up Al-WC nano-composites using UT with stirring. Significant improvement in the hardness and wear resistance of the UT composite samples was reported to be observed.

Recent studies have reported the ultrasonic manufacturing technology as an efficient method in microstructure refinement, degassing and uniform dispersion of reinforcement particles. However, very limited research articles are available on Al-7Si/nano Y₂O₃ composites. Hence, effect of ultrasonic treatment on the microstructure and mechanical properties of A356/Y₂O₃ nanocomposites.

Aluminum A356 cast alloy and yittria (Y₂O₃) powder (99 % pure) were chosen as matrix alloy and reinforcement for the present study. The chemical composition of A356 cast alloy was determined using an optical emission spectroscope (Bruker, Q8 MAGELLAN) and the acquired Table 1. Average size of the yittria powder was determined using a SEM (JEOL 6360 model).

Figure 1a and b shows the micrograph of the Y₂O₃ particles confirming the presence of particles with average size to be ~ 5 µm. Approximately 750 g of A356 ingot was charged in to the ceramic crucible and melted using a pit type electrical resistance furnace at a temperature of 750°C. Required quantity of Y₂O₃ powder (1 and 2 vol. %) was preheated to 400°C to remove the volatile impurities, if any. The preheated powders were subsequently added to A356 alloy melt by conventional stir casting method using a zircon coated 3 bladed mild steel stirrer and spatula. In order to prevent the particle settlement due to the density differences between molten matrix (ρ_Al−melt = 2.375 g.cc) and reinforcement (ρ_Y2O3 = 5.01 g.cc), an uninterrupted continuous stirring at600 rpm for 10 min, subsequently transferred into a cast iron mold of 80 mm length and 20 mm diameter. Thus the prepared stir cast composites are henceforth designated as micro composites in this article.

The stir cast composites were re-melted to a temperature of 730°C and the melt was subjected to UT for 5 min. Schematic representation of the stir cast and ultrasonic treatment setup is shown in Figure. 2. A magnetostrictive transducer (RELTEC, Russia) is used to generate ultrasonic waves with a frequency of 20.4 kHz and a stainless steel (SS304) sonotrode of 40 mm diameter is used to transfer the generated ultrasonic waves at an intensity of 128 W/cm² to the melt. To prevent the temperature drop and wall-crystal formation effect, the sonotrode was coated with zircon and preheated to 730°C prior to introduction into the melt. Immediately after completion of ultrasonic treatment, the sonotrode was removed and the molten composite was cast into the mould.

Samples for microstructural studies were sliced from the casting and were ground through 240 to 2000 grit papers, polished with alumina paste and finally etched by means of keller’s reagent. Microstructural studies were carried out by using a polarized light microscopy (Carl Zeiss Axio Scope A1). TEM with a potential of 200 kV. The Secondary Dendrite Arm Spacing (SDAS) was measured by detailed analysis of optical microstructures of the samples using ‘Image J’ image analyzing software. The density of the as-cast alloy and its composites was measured using Archimede's principle. Vickers hardness testing machine loaded with a diamond indenter, per ASTM E384 standards. An average of five readings is accounted as the respective sample hardness. To study the room temperature tensile properties of the composites, the samples were machined as per ASTM E8M standard and the tensile specimen of 6 mm diameter and 30 mm gage length was tested on a computerized test.

Table 1

elements of A356 (Al-7Si ) Alloy
ELEMENT	Si	Fe	Cu	Mg	Mn	Zn	Ti	Al
Vol.%	6.5–7.5	0.20	0.20	0.25–0.45	0.10	0.10	0.20	Balance

3.1 MICROSTRUCTURE ANALYSIS:

The density and porosity of the UT A356 alloy and its composite is calculated as per the procedure followed by Jayakrishnan et al. [10] in their study on ultrasonic treatment of strontium added A356 alloy. Table 2 shows the density values of the A356 alloy and Y₂O₃ reinforced composites calculated by means of Archimede’s principle. The porosity content of the composites produced via stir casting method (2.2 %) is found to be more than that of the as-cast A356 alloy (1.05 %). The increment in porosity values can be related to increase in more contact area in the molten aluminum due to the presence of the reinforcement particles and porosity associated in the individual reinforcement particles.

The porosity of the stir cast composite was found to increase due to the agglomeration of the particles in the matrix as shown in Fig. 3. Porosity increased with increase in vol.% of Y₂O₃ particles in the case of stir cast samples. Composite samples with UT have low porosity content and this degasification. After UT the density of the 2 vol.% Y₂O₃ reinforced composite sample improved significantly from 2.69 g/cm³ to 2.72 g/cm³. The % of porosity is found to reduce from 2.2 % to 0.8 % due to UT and the measured density was very close to its theoretical density and the values are tabulated Table 2.

Table 2

Average size of SDAS and density of A356 alloy and its composites
S.No	Sample	Average SDAS (µm)	Experimental Density (g/cm³)
1	A356 alloy	20.2 ± 2.0	2.6700 ± 0.0026
2	A356/1 vol.% Y₂O₃ stir cast	18.9 ± 1.8	2.6904 ± 0.0026
3	A356/2 vol.% Y₂O₃ stir cast	17.3 ± 1.7	2.6972 ± 0.0026
4	A356/1 vol.% Y₂O₃ 5 min UT	15.8 ± 1.5	2.6982 ± 0.0026
5	A356/2 vol.% Y₂O₃ 5 min UT	12.7 ± 1.2	2.7242 ± 0.0027

Optical Micrographs of A356 cast alloy, A356/1 vol.% Y₂O₃ nanocomposite stir cast, A356/2 1 vol.% Y₂O₃ nanocomposite stir cast, A356/1 vol.% Y₂O₃ nanocomposite with 5 min UT, A356/2 vol.% Y₂O₃ nanocomposite with 5 min UT are respectively shown in Fig. 4 (a-e) respectively. Micrograph of the as-cast A356 alloy (Fig. 4a) shows coarse dentritic structure of α- aluminum and eutectic silicon phase.

From the micrographs of the composite samples it can be observed that the SDAS of samples with UT is significantly reduced compared to the samples without UT. Apart from helping in reduction of particle size, UT also helps to enhance the homogeneous distribution of Y₂O₃ nanoparticles in the A356 matrix. It also promotes more heterogeneous nucleation events in the melt. In addition, UT can activate the inactive nucleant particles by surface cleaning and thus the SDAS refinement in UT samples can be rationalized. Similar results have been explained by Jayakrishnan et al.[2] in their study on UT of A356/TiB₂ in-situ composites. Y₂O₃ nano particles reduces the Al and Si will increase and restricts the eutectic silicon growth. The average value of measured SDAS of the A356 alloy and its composites is presented in Table 2 for ready reference. As-cast A356 alloy has a SDAS of about 29 µm. However, it is observed that 1 vol. % and 2 vol. % of Y₂O₃ nano particles added samples with UT has a SDAS of about 17.8 µm and 12.7µm respectively.

Figure 5a shows the SEM image of A356 alloy and Fig. 5 (b & c) shows the SEM images of A356/ 1–2 vol.% Y₂O₃ stir cast composite without UT and Fig. 5 (d & e) shows SEM images of the A356/ 1–2 vol.% Y₂O₃ composite with 5 min UT. A small region is magnified and shown as an inset in Fig. 5d. Nano particles are found to be spherical in shape and the size of the Y₂O₃ particles significantly reduced from 5µm to 500 nm due to UT. Jayakrishnan et al.[11] achieved reduction in the size of TiB₂ particles from micron size to nano size after 5 minutes of UT and the results of the present study correlates the same. SEM micrograph show .Y₂O₃ particles are found to be present in both the eutectic Si phase and primary matrix phase, which confirms that the added nano Y₂O₃ particles were trapped into the solid solution and are not rejected by the solid-liquid interface throughout solidification. The particles present in the eutectic Si phase restrict the grain growth generation and forms an Y₂O₃ rich eutectic domain along with eutectic silicon. Similar effect was reported by Bouaeshi et al.[12] in their reports on Y₂O₃ addition of aluminum. Puga et al. [13] also supported the above mechanism in their study on the properties of AlSi₉Cu₃ alloy. Khalifa et al.[14] explained the formation of eutectic domain along with silicon and it effect .Yu Pana et al [15] and Kewei Xie at al. [16] observed significant refinement in grian structure with the addition of TiC to Ti and AlNp to Al respectively.

No other intermetallics were observed from the SEM micrographs and it may be due to the presence of thermodynamically stable nano Y₂O₃ particles. Alike outcomes have been accounted by Rahul Gupta et al.[17] in their study on creep properties of ultrasonically processed in-situ Al₃Zr-Al alloy composites. The UT acoustic streaming and cavitation non-linear effect gives efficient stirring and significantly modified the primary Al and eutectic Si morphology.

TEM bright field micrograph of the Y₂O₃ particle (Fig. 6a) shows the size of particle to be about 500 nm. Figure 6a reveals the excellent bonding of the particles with the matrix and is free from interfacial reactions. EDS analysis (Fig. 6b) further confirms the presence of Y₂O₃ particles in the composite. The simulated and the observed Selected Area Diffraction (SAD) pattern of nano Y₂O₃ particles are shown in Fig. 6 (c&d). The SAD pattern indexing also confirms the presence of Y₂O₃ particles, as the d-spacing is in accordance with that of the Y₂O₃ particles. Figure 6e shows the presence of large number of dislocations near the grain boundary. These dislocations will aid in significantly improving the mechanical properties of the matrix alloy.

3.3 HARDNESS MEASREMENT:

Figure 7 shows the average hardness values for the composites reinforced with 1 vol.% and 2 vol.% of nano Y₂O₃ particle without UT is about 75 HV and 95 HV repectively Y₂O₃. Similarly, the average hardness values for the composites reinforced with 1 vol.% and 2 vol.% of nano Y₂O₃ particle with 5 min UT is about 85 HV and 108 HV respectively. Addition of 2 vol.% of nano Y₂O₃ has significantly improved the hardness of the A356 alloy from 60 HV to 108 HV. It can be seen that the microhardness sample increases with UT and the synergistic effect of Y₂O₃ particles and UT significantly increase the hardness of A356 alloy about 80 %. This increase in hardness is due to the significant size reduction in SDAS and less porosity of the UT samples as proposed by Yuan et al.[18] and by Zhiwei et al. [19] in their study on TiB₂p/Al-12Si-4Cu in-situ composites.

3.4 TENSILE PROPERTIES:

1vol.% nano Y₂O₃ particles added composites exhibited an increase in YS and UTS of 86 MPa and 138 MPa respectively as shown in Fig. 8. Further, 2 vol.% addition of Y₂O₃ particles resulted in a significant improvement in YS of 101 MPa and UTS 150 MPa. The YS and UTS are found to be improved by 27 % and 13 % respectively when compared to the matrix alloy. The tensile properties of the samples reinforced with 2 vol.% nano Y₂O₃ particle added composite with 5 min UT exhibited an YS of 109 MPa and UTS 167 MPa with % of improvement by 27 % and 26 % respectively. The ultrasonic cavitation treatment significantly enhanced the YS and UTS of the composite. Yuan et al. [17] in their study on A356/nano-SiC_p reinforced composites. The UT also improves the % of elongation and the elongation of 1 vol.% nano Y₂O₃ particle added composite with UT is higher than that of 2 vol.% nano Y₂O₃ particle added composite with UT. Generally composites reinforced with micrometre-sized ceramic particles will reduce the ductility of the matrix, whereas the nano Y₂O₃ particle reinforced composites improved the tensile properties by retaining the ductility. Similar reported by Yuan et al.[8] A356 alloy /nano SiC particles reinforced composites prepared by UT.

Significant improvement in the mechanical properties is due to the existence of uniformly dispersed nano Y₂O₃ particles and is probably contributed to the various strengthening mechanisms. The dominant mechanisms observed in the present study. Large difference in CTE between the reinforcement Y₂O₃ (8.1×10^− 6/K) and matrix (25×10^− 6/K) may lead to development of stresses during cooling resulting in the formation of large number of dislocations around the particles as shown in Fig. 5e.

Orowan strengthening mechanism is one of the dominant strengthening effects and mainly depends on the reinforcement particle size, bond strength with the matrix and interparticle spacing. Under tensile load, the softer matrix alloy deforms more plastically compared to the harder nano Y₂O₃ particles. During plastic deformation of the composite samples, closely spaced nano Y₂O₃ particles will become a barrier to the dislocation motion thus hindering their motion. As a consequence, dislocations will bend and form a loop in the region of Y₂O₃ particles. Consecutive dislocation incessantly forms loop in the region of Y₂O₃ which bring on back stress and obstructs the dislocation movement further. In this manner, Y₂O₃ nano particle facilitates in strengthening of the A356 alloy matrix. Compared to the stir cast composites (without UT), inter particle spacing in the nanocomposites (with UT) will be very less and thus offers more resistance to the dislocation movement and thus further strengthens the matrix.

According to Hall-Petch theory, the fine grains with more grain boundaries will significantly improve the strength of the matrix. Microstructure of A356 alloy/Y₂O₃ nano composites with UT has finer grains as compared to the A356 alloy. Dislocations pile up makes the further deformation of the material very difficult. Therefore it can be confirmed that the finer grains can significantly contribute to strengthening of the composites.

Fractograph examinations of tensile tested samples (for both stir cast and UST) have been carried out to acquire a proof on the Y₂O₃ particles dispersion in the A356 alloy matrix material and the attachment among the A356 alloy and the particle. Figure 9a and Fig. 9 (b-e) shows the SEM micrographs of A356 alloy and Y₂O₃ particle added composites after tensile test. SEM image of the A356 alloy shows a ductile fracture, where as the composites shows a cleavage fracture. Good interfacial bonding between the Y₂O₃ particles and the A356 alloy prompts effective load transfer from the Y₂O₃ particles to A356 alloy. This in turn reduces the crack propagation leading to major improvement in the tensile properties of the composites with reduction in ductility [20–26].

Addition of Y₂O₃ particles and presence of UT was found to significantly refine the α-Al crystals from dendrite structure to fine equiaxed structure. Uniformly distributed Y₂O₃ particles preferred to be present inside α-Al crystals which lead to refinement of α-Al crystals. 2 vol.% Y₂O₃ composite samples with 5 min UT exhibited a finer structure with 12 µm sized α-Al crystals. Addition of Y₂O₃ nano particles into the A356 alloy was found to improve the tensile strength as well as the ductility of the composites. Among the synthesized samples, 2 vol.% Y₂O₃ reinforced with 5 min UT composite showed best mechanical properties as YS and UTS of UT sample were improved by 27 % and 26 % compared to that of A356 alloy. CTE mismatch strengthening was the main strengthening mechanism that contributed to the increment of the YS of the composites. Homogeneous distribution of Y₂O₃ particles and presence of refined α-Al crystals improved the ductility of the UT composite sample.

Acknowledgements I would like to thank especially the Nanotechnology center PSG-IAS for supporting the Production and Characterization of composites.

Authors’ Contributions Dr. T.Satish Kumar declared on behalf of all co-authors that all authors contribute equally in the paper. All the authors are agreeing on the present list of authors and no one has any objection. The paper has not been used or published paper/part of paper any one author.

Funding Not applicable.

Data Availability Not applicable.

Ethics Approval and Consent to Participate Not applicable.

Consent for Publication Not applicable.

Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Editorial decision: Major revisions
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You are reading this latest preprint version

Microstructure And Mechanical Properties of Ultrasonically Processed Al-7Si Alloy / Y₂O₃ Nanocomposite

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Abstract

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Introduction

Experimental Procedure

Results And Discussion

3.1 MICROSTRUCTURE ANALYSIS:

3.3 HARDNESS MEASREMENT:

3.4 TENSILE PROPERTIES:

Conclusions

Declarations

References

Status:

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