Evaluation of microstructure evolution and mechanical properties of Al-10Zn-1.63Si/ Irvingia gabonensis particulates alloy composites

Abstract

This study demonstrates the feasibility of using Irvingia gabonensis shell particulates (IGSp) as alternative reinforcing materials in the development of aluminium-based composites. In this experimental study, the microstructure, phase composition, and mechanical behaviour of Al-10Zn-1.63Si/xIGSp (wt%, x = 1, 3, 5 and 7) composites were investigated. The Al-10Zn-1.63Si based composites were fabricated using the stir-casting technique. Different weight percentages (1, 3, 5 and 7) of IGSp were added to the Al-10Zn-1.63Si matrix. The chemical constituents of the IGSp were determined using X-ray fluorescence (XRF). The grain characteristics and phase(s) compositions were determined using Scanning Electron Microscopy (SEM) and X-ray diffractometer (XRD). The ultimate tensile strength, hardness, and impact strength of the developed composites were also determined. The SEM and XRD results revealed the presence of different phases: aluminium phosphate (Al₁₆P₁₆O₆₄), gahnite (ZnAl₂O₄), andalusite (Al₂SiO₅), Quartz (SiO₂) and aluminium silicate (Al₂O_3.5.SiO₂). Results show that addition of IGSp led to an increase in ultimate tensile strength, with the highest value (128 MPa) obtained at 3 wt% IGSp. The hardness of the composites increased with increasing concentrations of IGSp, reaching a maximum value of 285 HV after adding 7 wt% IGSp. The impact strength improved with the addition of IGSp, with the highest value (30 J) obtained at 1 wt% IGSp. The improvements in mechanical properties were attributed to the dispersion of three major phases: aluminium silicate (Al₂O_3.54.SiO₂), Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂. These phases contributed to the enhanced strength and hardness of the composites. The study noted a sudden decrease in ultimate tensile strength with higher concentrations of IGSp due to the increase in the intensities of Al₁₆P₁₆O₆₄ and precipitation of hard but brittle new phase; Al₂Si_60.6O126.33. The study concludes that IGSp has the potential to serve as an alternative reinforcing material for aluminium-based composites.

Keywords

Irvingia gabonensis surface morphology microstructure evolution hardness strength

Introduction

Aluminium alloys have great technological value and wide ranges of industrial applications such as aerospace, automobile, construction, etc., owing to their exceptional properties: lightweight, high strength, good formability and excellent corrosion resistance. However, pure aluminium has limitations in terms of mechanical properties, hence often alloyed or reinforced with other materials to enhance its properties.^1–8 Metal Matrix Composites (MMCs) are composite materials consisting of a metal matrix (like aluminium) and reinforcing materials (like ceramics or green plant waste). These composites offer improved properties compared to the base material alone. They find applications in industries like aerospace, automobile, and defence due to their enhanced creep resistance, heat conductivity, dimensional stability, stiffness, hardness, and strength.^2–15 The demand for eco-friendly and cost-effective composite materials has led to the exploration of using green plant waste as reinforcing materials. Compared to traditional ceramic reinforcements like silicon carbide and alumina, green plant waste has the advantage of lower density, making it an attractive choice for creating cost-effective composites.^16–20 Previous studies^21–31 reported the effectiveness of various eco-friendly green plant waste materials such as rice husk shells, groundnut shells, corn cob, eggshell, coconut shell, bamboo leaf, coconut fibre, periwinkle shells, horse eye bean seed shells, and African walnut kernel as reinforcing materials for improving the mechanical and wear resistance properties of Al alloy-based biocomposites, but little or no work has been done on the reinforcing characteristics of Irvingia gabonensis shell in Al alloy-based biocomposites.

Irvingia gabonensis, also known as African mango or bush mango, is a tree found in the southern part of Nigeria. It bears edible fruit and has therapeutic properties, hence used to treat various conditions like diarrhoea, skin issues, and yellow fever.^32–35 Additionally, researchers have identified Irvingia species as potential corrosion inhibitors for mild steel.^36,37 This research is aimed at investigating the reinforcing potential of Irvingia gabonensis shell particulates (IGSp) in Al-10wt%Zn-1.63wt%Si based composite. In this study, the influence of different concentrations of Irvingia gabonensis shell particulates on the microstructure evolution and mechanical properties of the Al-10wt%Zn-1.63wt%Si based composite was investigated.

Experimental procedure

In this experimental study, the aluminium wire (5 mm diameter), zinc powder (25 μm particle size), and silicon powder (25 μm particle size) of percentage purities 99.8%, 99.9% and 99.5% respectively were supplied by Sigma-Aldrich while the Irvingia gabonensis shells were obtained from Uzo-uwani, Enugu State, Nigeria. The Irvingia gabonensis shells were extracted, washed thoroughly with distilled water, and sun-dried for 5 days. The Irvingia gabonensis shells, after being dried, underwent milling and sieving using a series of sieves arranged in decreasing order of particle fineness. Particle size analysis was conducted in compliance with the BS 1377:1990 standard, with a particle size of 50 μm chosen for the study. The milling process employed a planetary ball mill (planetary mono mill Pulverisette 6, Fritsch, Germany) for duration of 60 min. Tungsten balls, 10 mm in diameter, were utilised in a ball-to-powder ratio of 10:1, operating at a rotational speed of 200 rpm during the ball-milling process. The chemical constituents of the Irvingia gabonensis shell particulates were determined using X-ray fluorescence (XRF, Phillips PW2400).

The Al-10Zn-1.63Si/IGSp biocomposites were prepared by double-layer feeding stir casting technique.³⁸ The IGSp were added in concentrations of 1, 3, 5, and 7 wt%. The Al-10Zn-1.63Si was produced as the matrix, while the Al-10Zn-1.63Si/xIGSp composites were produced keeping zinc and silicon contents constant. Al-10Zn-1.63Si matrix was produced by charging 1 kg of aluminium into a crucible pot preheated at 200°C. The temperature of the crucible pot was increased gradually until the aluminium melted at 660°C. The aluminium melt was superheated to 750°C to ensure adequate fluidity of the melt. The required amounts of zinc and silicon powders were introduced into the aluminium melt with the aid of an aluminium foil. The mixture was stirred vigorously using a mechanical stirrer coated with graphite at a speed of 500–700 rpm. The mixture was further super-heated to 780°C and the melt held at that temperature for about 10 min. Then, skimming was done to remove oxides and impurities from the melt. The based alloy system was cast in a preheated permanent steel mould (at 200°C) with internal dimensions of 250 mm by 16 mm and allowed to cool inside the steel mould to room temperature. For each composite formulation, the temperature of the molten Al-10Zn-1.63Si alloy was reduced to 580°C just below the liquidus point to keep the slurry in a semi-solid state. The required weight per cent of Irvingia gabonensis shell particulates was introduced into the slurry and stirred. The slurry was reheated to 720°C and then stirred for 2 min. After achieving a homogenous mixture, hexachloroethane was added into the mixture as a degassing tablet. The molten mixture was then poured into the preheated mould. The furnace temperature was maintained at 740°C–760°C while pouring temperature was maintained at 720°C.

Dome-shaped samples of gauge length 50 mm, gauge diameter 8 mm, and total length 120 mm were prepared from the cast samples for tensile strength test according to ASTME8/E8M-21 (2018) standard. The hardness and impact energy test samples of dimensions 20 × 16 mm² and 55 × 10 × 10 mm³ (with a 2 mm deep notch), respectively were also prepared following ASTM E18 and ASTM D638 standards, respectively. Tensile and impact energy tests were performed using a JPL tensile strength tester (Model: 130812) with a 10 kN capacity and pendulum impact energy testing equipment (Model: U1820). The surfaces of the samples for hardness test were ground and polished. Three indentations were made on each of the sample surfaces using a Vickers hardness tester (Model: VM-50) at a load of 183.9 kgf and 5 s dwell time. The diagonals of indentations were measured using a 20X Olympus BH optical microscope and the average diameter was determined. The values of the Vickers hardness were calculated using equation (1).³⁹ The microstructure and composition of the dispersed phases/intermetallic compounds were analysed using SEM/EDS [Carl Zeiss SEM (EVO/NA10)]. The phases were identified using X-ray diffractometer (XRD).

HV = 1.8544 . \frac{P}{d^{2}}

(1)

Where, HV = Vickers hardness (HV), P = applied load (kgf), d = average diagonals of indentations (µm).

Results and discussion

Chemical composition of the Irvingia gabonensis shell particles and the developed composites

Table 1 shows the chemical composition of the Irvingia gabonensis shell particulates and the developed composites. The results of the analysis show that the Irvingia gabonensis shell particulates contain majorly Ca, K, Fe, K, P, Ni, Mg, Zn, Nb and traces of other elements such as Na, Si, S and Mn. The Al-Zn-Si-xIGSp biocomposites contain Al, Zn and Si as the major elements with traces of other constituents from the shell particulates.

Table 1.

Chemical composition of the Irvingia gabonensis shells particulates (IGSp) and the developed composites.

Elements	Nominal composition (wt%)
	IGSp	AlZnSi	AlZnSi-1IGSp	AlZnSi-3IGSp	AlZnSi-5IGSp	AlZnSi-7IGSp
Al	-	88.486	86.053	83.362	81.662	81.1059
Si	1.000	1.628	1.684	2.038	2.160	2.160
Fe	13.2662	-	0.536	0.536	0.579	0.685
Ni	0.8430	-	0.00126	0.00126	0.10751	0.10751
Zn	0.4127	9.885	10.004	11.318	11.318	11.556
Mg	1.896	-	0.00034	0.15214	0.27714	0.27714
Na	2.06	-	0.0093	0.0093	0.11555	0.11555
S	6.696	-	0.0087	0.0087	0.11495	0.11495
P	14.386	-	0.0245	0.3135	1.0505	1.2625
Ca	34.759	-	1.0556	1.3836	1.6316	1.6316
K	22.442	-	0.5142	0.6142	0.6142	0.6142
Mn	0.7191	-	0.10682	0.25982	0.25982	0.25982
Nb	1.52	-	0.00128	0.00228	0.10853	0.10853
O	-	0.0010	0.0010	0.0012	0.0012	0.0013

Mechanical properties of Al-10Zn-1.63Si/Irvingia gabonensis shell particulates biocomposites

Figures 1 to 4 show the percentage elongation (%E), ultimate tensile strength (UTS), hardness, and impact energy of Al-10Zn-1.63Si matrix and Al-10Zn-1.63Si/xIGSp composites. Figure 1 shows that the matrix alloy (Al-10Zn-1.63Si) without IGSp exhibited a percentage elongation of 31.1%. Adding 1 wt% of Irvingia gabonensis shell particulate reduced the percentage elongation to 26.8%, which corresponds to a decrease of about 13.83. This reduction in percentage elongation could be attributed to the dispersion of the IGSp within the alloy structure, as indicated by SEM images (Figures 5 –7). The presence of hard phases: Al₂O_3.54.SiO₂, Al₁₆P₁₆O₆₄, as evidenced in the XRD patterns (Figures 8 –10) could also contribute to the decrease in percentage elongation. Incorporation of 7 wt% IGSp into the Al-10Zn-1.63Si/1IGSp composite resulted in a further reduction of the percentage elongation from 26.8% to 10.9%. This could be attributed to the dispersion of more particulates and the presence of hard phases in the alloy structure.

Figure 1.

Effect of IGSp concentrations on the %E of Al-10Zn-1.63Si composite.

Figure 2.

Effect of IGSp concentrations on the UTS of Al-10Zn-1.63Si composite.

Figure 3.

Effect of IGSp concentrations on the hardness of Al-10Zn-1.63Si composite.

Figure 4.

Effect of IGSp concentrations on the impact energy of Al-10Zn-1.63Si composite.

Figure 5.

SEM micrograph/EDS of Al-10Zn-1.63Si alloy system (Mag: 1.5 kx).

Figure 6.

SEM micrograph/EDS of Al-10Zn-1.63Si-3IGSp composite (Mag: 1.5 kx).

Figure 7.

SEM micrograph/EDS of Al-10Zn-1.63Si-7IGSp composite (Mag: 1.5 kx).

Figure 8.

X-ray diffraction patterns of Al-10Zn-1.63Si alloy system.

Figure 9.

X-ray diffraction patterns of Al-10Zn-1.63Si-3IGSp composite.

Figure 10.

X-ray diffraction patterns of Al-10Zn-1.63Si-7IGSp composite.

The variations of ultimate tensile strength and hardness of Al-10Zn-1.63Si alloy with different concentrations of Irvingia gabonensis shell particulates are presented in Figures 2 and 3. Analysis of the results indicates that the alloy system recorded ultimate tensile strength and hardness of 73 MPa and 252 $\pm 0.8$ HV respectively. Addition of 1 wt% Irvingia gabonensis shell particulates resulted in a significant increase in both ultimate tensile strength and hardness. The ultimate tensile strength increased by approximately 34.23% (from 73 to 98 MPa), and the hardness increased by approximately 1.58% (from 252 $\pm 0.8$ HV to 256 $\pm 1.4$ HV). The subsequent increase in concentrations of Irvingia gabonensis shell particulates led to further improvements in both ultimate tensile strength and hardness. The maximum ultimate tensile strength and hardness values were achieved at concentrations of 3 and 7 wt%, respectively. The improvement in mechanical properties can be attributed to the even dispersion of the particulates and the recrystallisation of specific intermetallic compounds, such as aluminium phosphate (Al₁₆P₁₆O₆₄) and aluminium silicate (Al₂O_3.54.SiO₂) phases (Figures 8 and 10). These results conformed to the findings of Bagheri and Abbasi.^4–7

The scanning electron microscope (SEM) images revealed that the hard IGSp particles are evenly and completely dispersed within the aluminium matrix. This uniform dispersion is essential for achieving desired material properties. As the concentration of IGSp increased, the phase assemblages changed and the intensity peaks of the Al₁₆P₁₆O₆₄ phase increased, indicating a greater presence of this phase in the material. Additionally, a new phase Al₂O_3.54SiO₂ is also identified in the XRD patterns. This suggests that the presence of IGSp influences the material’s crystalline structure and phase composition. The introduction of IGSp induced the process of recrystallisation, which in turn, resulted in improvements in both ultimate tensile strength and hardness of the composite material. Interestingly, a sudden drop in the ultimate tensile strength of the composite is observed when the concentration of IGSp increased from 3 to 5 wt%. This drop can be attributed to the increased intensity peaks of the Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂ phases.

Figure 4 shows the effect of Irvingia gabonensis shell particulate concentrations on the impact energy of Al-10Zn-1.63Si alloy system. Analysis of Figure 4 shows clearly that the addition of Irvingia gabonensis shell particulates significantly increased the impact energy of the developed composite with maximum values of 30 J obtained at 1 wt%IGSp addition. The impact energy of the studied composite decreased correspondingly with increasing concentration of IGSp. This trend of the impact energy values of the fabricated alloy composites can be attributed to the increasing intensity peaks of the hard phases (Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂) in the composite structure as evidenced in the X-ray diffraction patterns (Figures 8 –10).

Microstructure evolution and X-ray diffraction patterns of Al-10Zn-1.63Si and Al-10Zn-1.63Si-xIGSp alloy composites

The SEM images, EDS analysis, and XRD patterns of Al-10Zn-1.63Si and Al-10Zn-1.63Si-xIGSp alloy composites are presented in Figures 5 to 7 and Figures 8 to 10 respectively. Figure 5 shows the SEM images of Al-10Zn-1.63Si alloy system. The images reveal nodular grains scarcely dispersed in the aluminium matrix. The EDS analysis revealed Al, Zn, Na, Fe, P, Si, Ag, Mg, Cl, Ca, K, Y, C and Nb major elements. The diffraction peaks were matched with PDF cards and the major phases are identified as aluminium phosphate (Al₁₆P₁₆O₆₄, PDF# 76-0234), gahnite (ZnAl₂O₄, PDF# 96-901-3643), andalusite (Al₂SiO₅, PDF# 00-039-0376) and Quartz (SiO₂, PDF# 01-082-1407) (Figures 5 and 8). The SEM images of Al-10Zn-1.63Si-3IGSp and Al-10Zn-1.63Si-7IGSp alloy system composites are presented in Figures 6 and 7. The microstructures reveal even dispersion of IGSp in the Al matrix and fine grains identified as aluminium phosphate (Al₁₆P₁₆O₆₄), gahnite (ZnAl₂O₄), andalusite (Al₂SiO₅), Quartz (SiO₂) and Al₂Si_60.6O126.33. Comparing the SEM images of Al-10Zn-1.63Si and Al-10Zn-1.63Si-3IGSp, it is evidenced that the intensity peaks of aluminium phosphate (Al₁₆P₁₆O₆₄) increased. In addition, a new phase (Al₂O_3.5.SiO₂) emerged (Figures 8 and 9). The intensity peaks of the Al₁₆P₁₆O₆₄ and Al₂O_3.5.SiO₂ increased, leading to precipitation of aluminium silicate with a new composition (Al₂Si_60.6O_126.33). The increase in intensity peaks and precipitation of new phases directly affects the mechanical properties of the fabricated alloy system composites. The dispersion of these phases in the aluminium matrix led to obstruction of dislocation motion, thereby increasing the strength and hardness of the alloy composites.²⁵

Conclusion

The present research explored the microstructure evolution and mechanical properties of Al-10Zn-1.63Si-xIGSp biocomposites. The alloy composites demonstrated excellent ultimate tensile strength, hardness, and impact strength. These improvements in mechanical properties are associated with the dispersion of hard Irvingia gabonensis shell particulates in the matrix, precipitation and recrystallisation of hard phases: Al₂O_3.54.SiO₂ and Al₁₆P₁₆O₆₄ in the composite structure. Based on the results of the study, the following conclusions can be made:

Addition of Irvingia gabonensis shell particulates decreased the percentage elongation of Al-10Zn-1.63Si alloy owing to the dispersion of hard particles of Irvingia gabonensis shell and recrystallisation of hard phases (aluminium silicate (Al₂O_3.54.SiO₂) in the composite structure.

The ultimate tensile strength, hardness, and impact energy of Al-10Zn-1.63Si alloy increased significantly after adding Irvingia gabonensis shell particulates. These improvements are associated with the precipitation of Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂ phases.

The Al-10Zn-1.63Si-x wIGSp recorded peak ultimate tensile strength at 3 wt% IGSp additions. Further, an increase in IGSp induced precipitation of high contents of very hard, but brittle phases (Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂), leading to a decrease in ultimate tensile strength of the alloy composite.

This hardness of Al-10Zn-1.63Si-xIGSp alloy composite increased with increasing concentrations of IGSp with a maximum hardness value of 285 HV obtained with 7IGSp additions.

The addition of IGSp significantly increased the impact strength of the alloy composite with maximum values of 30J obtained at 1IGSp addition. Further increase in concentrations of IGSp led to a decrease in impact energy. This trend of the impact energy values is attributed to the increasing intensity peaks of the hard phases (Al₁₆P₁₆O₆₄ and Al₂O_3.54.SiO₂) in the composite structure.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Chukwuneke Jeremiah Lekwuwa

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