Microstructure and properties of in-situ Al–Si/Al 2 O 3 composites prepared by displacement reaction

Abstract

In this work, we have studied the displacement reaction of Al and SiO₂ powder blends by mechanical alloying (MA) for 20 h and low-temperature sintering. The microstructural evolution of powder and compacted specimens were studied by X-ray diffraction and scanning electron microscopy. The extent of the milling on the displacement reaction was studied by differential thermal analysis (DTA). The results show that the reaction between Al–SiO₂ couple does not initiate after milling up to 20 h but occurs after thermal treatment of powders. DTA results indicate a lowering in the reaction temperature of Al–SiO₂ couple after milling. Following the DTA results, we densified the Al–SiO₂ powder compacts at 700°C to produce an in-situ Al–Si/Al₂O₃ composite. The physical, mechanical, and tribological performance of the composite at different milling times are also reported here.

Keywords

Mechanical alloying in-situ displacement reaction sintering microstructure

Introduction

The aerospace and automobile industries are continuously in search of high-performance material for commercial and military aircraft. The potential candidates include particulate reinforced aluminium matrix composite (AMC). These AMCs possess high strength to weight ratio, excellent mechanical and wear properties over conventional aluminium alloys [1]. However, these AMCs have a problem of poor wetting of reinforcement to aluminium matrix and may cause the failure of the engineering components in long term. From the past literature, it is obvious that the in-situ alumina particulates produced as reinforcement in AMCs impart excellent mechanical bonding to the matrix. These in-situ AMCs are regarded to be more advanced and novel as compared to their ex-situ counterparts [2]. Several processing methods are available for fabricating in-situ AMCs, e.g. exothermic dispersion, self-propagated high-temperature synthesis, reactive hot pressing, mechanosynthesis, mechanical thermal synthesis (MTS), etc. [3 –9]. Among these methods, MTS has shown potential growth during the last decades due to its simplicity and low cost [9]. The MTS process eases the displacement reaction of metal and oxide powders. These composite powder particles contain numerous reacting diffusion couples and can give rise to numerous types of AMCs embedded with oxide particulates after heat treatment. In various aluminium alloys, Si is a popular alloying element to impart fluidity to the most of alloys used in microjoining at high temperatures through the formation of various compounds such as oxides, nitrides, and carbides [10 –12]. Therefore, SiO₂ is one of the most promising candidates to produce AMCs with in-situ reinforcements. This is due to the displacement reaction of Al and SiO₂ at certain temperatures where Si will be alloyed to the Al matrix. Compared to other oxide systems, limited research activities are available on Al–SiO₂ system to fabricate AMCs by milling. Most of the works are devoted to the melting infiltration techniques which involve very high temperatures [13,14]. Therefore, in this work, Al–SiO₂ composite powders were milled for 20 h. The displacement reaction between Al and SiO₂ couple can be shown as [15]: (1) where ΔH = −208 kJ mol⁻¹ of SiO₂, ΔG = −1773 kJ.

After milling, the powder compacts were low temperature sintered at 700°C under Ar atmosphere. The physical, mechanical and wear properties of Al–Si/Al₂O₃ composite are also investigated.

Experimental

Materials and methods

Milled composite powder

The powder materials used in this work were Al (purity > 99%, average size 60 µm, Sigma-Aldrich, USA) and SiO₂ (purity 99.5%, >325 mesh (44 µm), Sigma-Aldrich, USA). Near eutectic Al–12%Si composition was chosen for the study. The powder mixture was milled with WC steel vials and balls using a planetary ball mill (RETSCH PM-400, Germany). The ball to powder weight ratio was 10:1 and toluene was used as a process control agent. The mill was rotated at a constant disc speed of 600 rev min⁻¹ and vial speed of 300 rev min⁻¹ for a duration of 20 h.

Microstructure and phase evolution

The microstructure of the powder particles and the sintered compacts was examined using scanning electron microscopy (SEM) (Hitachi 4800S, Japan) operating at 20 kV. The various phases formed after MA and sintering were studied by a Brucker's X-ray diffractometer (D8 Advance) with Co-Kα radiation (λ = 1.789 Å) operating at 30 mV and 35 mA. The milled powder was analysed for the crystallite size and lattice strain using modified Scherrer formula [16].

Thermal analysis and sintering

The differential thermal analysis (DTA) was done using a Perkin Elmer Diamond DSC/DTA analyser under Ar gas atmosphere from 50 to 1000°C at a heating rate of 10°C min⁻¹. The powders blends were cold compacted at 650 MPa to produce green compacts of dimension 10 × 4 mm followed by sintering at 700°C for 2 h in an Ar atmosphere.

Microhardness and porosity

The microhardness of the sintered compacts was determined using the Vickers hardness tester (Mitutoyo HM 200, Japan) at a load of 50 gf for 20 s. The average value of seven measurements was reported. The percentage porosity (P) was estimated by Archimedes principle using water as impregnating medium: where ρ_c and ρ_t are the calculated and theoretical densities of the sample and represents the relative density [17].

Wear and friction behaviour

The prepared Al–Si/Al₂O₃ composites were wear-tested by a pin-on-disc tribometer (TRB3, Anton Paar, GmbH) against low alloy steel disc (hardness 62 Rc). The sample size was 10 mm dia. × 4 mm length in a cylindrical shape. The normal load was kept at 10 N for a sliding speed of 2.5 m s⁻¹ for a distance of 1200 m. The coefficient of friction (COF) and volume loss of the samples after the test were recorded with sliding distance. The wear debris and surface morphology were examined in SEM.

Results and discussion

Phase analysis of Al–SiO₂ and in-situ bulk Al–Si/Al₂O₃ composites

No new phases were observed after 20 h milling as shown in Figure 1(a). The crystallite size of the Al calculated from the Scherrer was found to be 30 nm. The lattice strain was found to be 0.6. Initially (5 h), the powder particles were bigger and get refined in due course of milling. The ductile aluminium particles tend to weld and join (Figure 1(b)). On further milling up to 20 h, hardened Al–SiO₂ composite particles were fractured into fine particles as shown in Figure 1(b) (inset). [16].

Figure 1

XRD pattern of Al–SiO₂ milled powder mixture (a) after 20 h milling, (b) SEM morphology of the powder blends milled for 5 and 20 h (inset). (c) DTA curve of the 20 h milled Al–SiO₂ powder, (d) XRD pattern of 20 h milled Al–SiO₂ powder after sintering at 700°C. (e) and (f) The morphology and EDS composition analysis of powder compacts after sintering at 700°C.

The DTA analysis of the milled powder (5 and 20 h) is shown in Figure 1(c). The DTA curve shows endothermic (Endo) and exothermic peaks (Exo1 and Exo2). The various onset temperatures decrease after higher milling time (Table 1). The onset and melting points of Al were 625 and 655°C (after 5 h), which reduce to 615 and 624^oC, respectively, after 20 h milling [9,18]. This lowering in Al melting point is ascribed to the fine size of powder of Al–SiO₂ diffusion couples and surface instabilities [18]. Intense exothermic peaks were found after 20 h at 695°C after 20 h (as compared to 685°C at 5 h), which shows that most of the reaction is completed in highly milled composite powder at high temperature. The exothermic peak in Figure 1(c) signifies the displacement reaction that occurred in the Al–SiO₂ system is as follows:

Table 1

The various temperatures in DTA curve of the milled Al–SiO₂ powder mixture.

Milling time (h)	Endo		Exo 1		Exo 2
Milling time (h)	T_onset (°C)	T_peak (°C)	T_onset (°C)	T_peak (°C)	T_onset (°C)	T_peak (°C)
5	625	655	575	625	650	685
20	615	624	–	–	679	695

Compared to Exo1 peak, the Exo2 peak is wider which suggests that the reaction proceeds for a longer time. The Endo peak follows the Exo2 of 5 and 20 h milled powders which shows that the exothermic reaction is a liquid to solid reaction [18].

After densification of the 20 h milled compacts, the intensity of Al peaks becomes smaller and new phases are noticed in the X-ray diffraction (XRD) pattern (Figure 1(d)). The various phases are Al, Si, SiO₂, and Al₂O₃. A minute residual SiO₂ peak is also observed. The formation of the Al₂O₃ shows the displacement reaction between Al and SiO₂ powder particles after thermal treatment (Figure 1(e,f)). Using ImageJ Version 1.46, the size of the particulates was measured after 20 h. The mean particulate size after 20 h of milling was 3.2 µm with a size distribution varying from 2–5 µm with a volume fraction of Al₂O₃ around 29%.

Density and microhardness

The microhardness of the bulk composites is shown in Figure 2. The composite hardness increases with increasing milling time. This improvement in microhardness can be well correlated by the formation of in-situ reinforcement to a greater extent at higher milling times. The microhardness of the composites increases and sintered porosity decreases. The microhardness of sintered compacts was maximum at 20 h (Figure 2(a)). Besides, the density of sintered compacts was maximum at 15 h (Figure 2(b)). This can be ascribed to the dispersion strengthening caused by harder alumina particulates and low porosity at 15 h [19,20].

Figure 2

(a) Microhardness, (b) density, and (c) porosity of in-situ Al–Si/Al₂O₃ composites as a function of milling time.

The displacement reaction can proceed via two types of diffusion mechanisms: (i) solid-state diffusion of Al–Si particles and (ii) reactive sintering of Al and SiO₂ particles and formation of Al₂O₃. Solid-state diffusion of Al–SiO₂ does not induce any change in the fraction of Al₂O₃ reinforcement and density improves with the extent of densification [18,20,21]. As shown in Figure 2(c), the porosity was increased in 20 h milled samples which indicates an increased work hardening of powders severe plastic deformation at 20 h. The work-hardened powder particles got fractured into flake-like particles and hence poor compressibility is noticed which may be the reason for increased porosity in 20 h milled sample. However, for reactive sintering, the fraction of Al₂O₃ increases resulting in an increase in hardness considerably which increases with densification rate [18 –21].

Wear volume loss

Figure 3(a) shows the wear volume loss of the various composites at 10 N load with a progressive increase in sliding distance. The volume loss of the 5 h milled composite was highest. The volume loss decreased considerably with further milling due to the denser and refined microstructure of composites [21].

Figure 3

(a) Cumulative volume loss and (b) COF vs. sliding distance of different composites milled for various times 5, 10, 15 and 20 h at Load = 10 N.

Lower volume loss indicates a higher wear resistance of the composites which is correlated to the refined particles and high hardness of Al–Si matrix obtained due to the dispersed Al₂O₃ particulates. However, the volume loss of 20 h milled composite was slightly higher as compared to 15 h milled composite. This can be due to the presence of too much Al₂O₃ in the Al–Si matrix. The mismatch in the thermal expansion properties of Al and Al₂O₃ causes an increase in pore development at the Al–Si/Al₂O₃ interface. Second, the possibility of agglomeration of Al₂O₃ in the semisolid Al melt can also contribute to the reduction in wear rate [21,22].

Figure 3(b) shows the COF which increases and reaches a steady-state region where the COF becomes stable. The initial increase in COF is due to the increase in the apparent contact area for full contact of the pin. The average COF of Al–Si/Al₂O₃ composites at 10 N shows a minimum value of 0.05 which improves from 0.1 to 0.3 with milling time.

COF increases due to the Al₂O₃ particles which support the major fraction of load applied as compared to soft Al matrix. The in-situ Al₂O₃ particulates may scratch out the Al matrix producing debris which spread and roll out on the counterface and pin bottom, blocking the direct contact between the two surfaces [23]. This phenomenon reduces the COF and wear rate, appreciably. The increased COF of the samples might be due to the fracture of agglomerated particles during sliding and the development of wear cracks. The hard Al₂O₃ abrasive particles act as a lubricant against the Al matrix at higher sliding distances for 20 h milled composite [21 –23]. Therefore, a high Al₂O₃ fraction may be responsible for the lowering in COF in these in-situ composites.

Wear morphology

Figure 4 shows the wear surfaces of the Al–Si/Al₂O₃ composites. Severe grooving and dimples parallel to the sliding direction are found. The width of the continuous grooves increases with normal loads and sliding distance (Figure 4(a)). Extensive surface ploughing and delamination are observed which indicate the occurrence of plastic deformation during the wear. The grooves are mild at 15 h but again depth increases in 20 h milled composite (Figure 4(b,c)). The depth and width of the grooves imply the amount of material removed which depends primarily upon the normal load and the hardness of the composite [21,22].

Figure 4

Wear surface (a–c) and corresponding debris (d–f) generated in various composites milled for various hours and load 10 N. (a, d) 10 h, (b, e) 15 h, and (c, f) 20 h. The EDS spectrum shows the composition of the debris recorded across regions 1 and 2 in (g).

Numerous irregular debris were generated by plastic deformation as shown in Figure 4(d-f). The composition of the debris is shown in Figure 4g. All types of debris consist of Al-Si mixed Al₂O₃ flakes. The size of the debris is bigger in 10 h milled composite while it is uniform in 15 and 20 h milled composite. The ploughing over the wear surface by hard asperities cause asperities generation and their flattening by further contact forming extrusions [18,20 –23]. The debris morphology shows aggregates of Al₂O₃ embedded in Al–Si and Al–Si rich with a smaller amount of Al₂O₃ showing mixed adhesive wear and fracture of the matrix.

Conclusions

Al–Si matrix reinforced with in-situ Al₂O₃ was produced successfully by MTS. The in-situ displacement reaction was solid-liquid. The onset reaction temperature decreases with milling due to the formation of numerous fine and high surface energy Al–SiO₂ diffusing species in the powder mixture. The sintering treatment after 20 h milling triggers the reaction kinetics considerably. The wear surface morphology suggests a wear mechanism that is adhesive and dominates at 15 h due to a high degree of consolidation and lower porosity.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

Ashutosh Sharma is working as Assistant Professor in the Department of Materials Science and Engineering, Ajou University, Korea. He is an expert in lead free soldering, brazing, electrochemical plating and corrosion. His current research interests include powder metallurgy and laser additive manufacturing.

Hansung Lee is a PhD student in the Department of Energy Systems Research, Ajou University, Korea. He is working on high entropy alloys, additive manufacturing, surface engineering and powder metallurgy.

Byungmin Ahn is currently Professor in Department of Materials Science and Engineering and Department of Energy Systems Research, Ajou University, Korea. He also holds a position of Adjunct Research Professor in the Department of Chemical Engineering and Materials Science, University of Southern California, USA, where he received his Ph.D. degree from.

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