Microstructural Evaluation and Fracture Behavior of AZ31/Nb₂O₅ Metal Matrix Composite

Abstract

There have been remarkable improvements in the research field of magnesium over the last few decades, especially in the magnesium metal matrix composite in which micro and nanoparticles are used as reinforcement. The dispersion phase of nanoparticles shows a better microstructural morphology than pure magnesium. The magnesium metal matrix nanocomposite shows improved strength with a balance of plasticity as compared to the traditional magnesium metal matrix composite. In this research, Nb₂O₅ (0 wt.%, 3 wt.%, and 6 wt.%) nanoparticles were used to reinforce AZ31 with the stir casting method, followed by heat treatment, and finally, an investigation was conducted using microstructural analysis. Factors such as the degree of crystallinity, crystallite size, and dislocation density are affected by the concentration of Nb₂O₅ and heat treatment. With the compositional increase in Nb₂O₅ weight percentage, the grain size decreases up to 3% Nb₂O₅ and then increases gradually. The SEM image analysis showed a grain size reduction of up to 3% Nb₂O₅ and fracture morphology changed from basal slip to a mixture of basal slip and adiabatic shear band.

1. Introduction

Attention towards carbon emissions has been taken very seriously over the last decade, especially the carbon emissions produced by automobiles and some major manufacturing industries. Research on weight reduction could be a solution to carbon emission reduction [1,2,3]. With two-thirds the density of aluminum, the use of magnesium instead of aluminum can reduce carbon dioxide and carbon monoxide, as well as other unburnt residuals from fossil fuel. The concerning factor about magnesium is its strength and fracture behavior. Due to less creep and corrosion resistance, the use of magnesium is still lesser than aluminum [4]. The required improvement in strength can be achieved by reinforcement of the pure alloys, followed by heat treatment, coldwork, etc., and further followed by severe plastic deformation [4,5,6,7,8].

The fabrication method used to fabricate the composite was stir casting because of its availability, cost-effectiveness, and large-scale casting capacity [8,9,10,11]. The addition of reinforcement helps to distribute the secondary phase evenly along the matrix material and the agglomeration of aluminum can be reduced [8]. As reported by J. Zhu et al., the addition of Nb₂O₅ in TiAl composite increases its flexural strength and fracture toughness tremendously [12]. As we focused on the mechanical strengthening of AZ31 with a certain percentage of Nb₂O₅, the choice of Nb₂O₅ (reinforcement) depends on its properties, as well as its application. The properties of Nb₂O₅ include excellent thermal, chemical, and thermodynamic stability, high reflective indices, excellent mechanical properties, and excellent fracture toughness. The superior catalytic property of Nb₂O₅ enabled hydrogen absorption and desorption, along with biomedical and sensor applications [13]. Nb₂O₅ has not yet been fully explored in terms of its potential benefits and applications. Due to their low wear resistance, low hardness, and low malleability, Mg–Zn alloys have some disadvantages that limit their use. Nb₂O₅ as a reinforcement helps to overcome the limitation in the magnesium matrix composite. In the selected matrix, Nb₂O₅ reinforcement is added by the stir casting method. To verify the phase composition of the fabricated composite, XRD analyses were performed. Microstructural characterization was done by SEM image analyses and EDS to verify the elemental distribution in the composite. Mechanical testing, such as compression and microhardness, was performed, and the broken specimen surface was analyzed by SEM to understand the fracture mechanics.

2. Materials and Methods

The magnesium alloy AZ31 was used as a base material for the metal matrix composite (MMC), which consists of Al-5.95, Zn-0.64, Mn-0.26, Fe-0.005, Si-0.009, Cu-0.0008, Ni-0.0007, and Mg balance. The dispersion reinforcement used in these MMCs was an oxide called Nb₂O₅ with a particle size of 100 nm diameter. Various weight percentage of Nb₂O₅ was used and mentioned in

Table 1. Composition of AZ31 and reinforcement.

Types of Cast Ingots	Nb2O5 wt.%	Nb2O5 Size (nm)
Pure AZ31	0	-
Nb2O5/AZ31	3	100
Nb2O5/AZ31	6	100

. Figure 1 shows the method of the experiment in which stir casting was used to fabricate the AZ31/ Nb₂O₅ metal matrix composite based on the casting feasibility. During casting, at every 100 °C increase in temperature, a stabilization time of 15 min was given up to 700 °C and when the temperature crossed 760 °C, the final stabilization time of 30 min was given and the molten alloy was stirred for 5 min at 300 rpm to distribute the nanoparticles equally in the MMCs. Carbon dioxide (CO₂) and sulfur hexafluoride (SF₆) gases were used at 400 °C to avoid magnesium burning, and at 700 °C, argon gas was used to avoid oxidation. The molten mixed MMCs were then poured into a crucible under the furnace. The casted ingots were taken for billet cutting. Some sets of billets with various compositions of Nb₂O₅ were taken to examine the microstructural evaluation. In order to examine the microstructure, mechanical grinding and polishing procedures were followed. The samples were polished and then etched with 100 mL of ethanol, 10 mL of DI water, 5 mL of acetic acid, and 6 g of the picric acid solution for 40 s before being taken for microstructure evaluation. SEM images were taken with the JSM-6500F machine and XRD (Bruker D2 phaser model) was used to confirm the phase intensity and planes. The grain size was calculated using the ImageJ (version 1.53v21, Bethesda, MD, USA) analysis software. The formula used to calculate the parameters like dislocation density and microstrain are given below:

(i)

Crystallite diameter (D) = $\mathrm{k}\mathsf{\lambda }$/$\mathsf{\beta }\cos \mathsf{\theta }$

(ii)

Dislocation Density (δ) = 1/D²

(iii)

Microstrain (ε) = β/(4 tanθ)

• where β—values corresponding to FWHM value (Full width at half maximum) of XRD profile, k—Shape factor (0.89), λ—Wavelength of XRD radiation, θ—peak position.

The specimen sets were cut for the compression test according to ASTM E9-19, and the UTM-100 machine was used to perform the test. The tested specimens were taken for scan-electron microscopy (SEM) image analysis to analyze the fracture surface.

3. Results & Discussion

3.1. Microstructural Evaluation

3.1.1. SEM Image Analysis

The SEM images shows the grain morphology and the EDS shows the elemental distribution in the casted AZ31 composite showed in Figure 2. The EDS images reveal the proper distribution of magnesium throughout the grain, but the agglomeration of aluminum around the grain boundary and also the presence of magnesium in the grain boundary indicate the formation of the Mg₁₇Al₁₂ secondary phase. Figure 2d shows the even distribution of Zn along the matrix, which contributes to a single-phase Al₅Mg₁₁Zn₄.

The EDS images in Figure 3b,d–f correspond to Mg, Zn, Nb, and O with a proper distribution; however (c) represents Al, which combines with Mg, resulting in the formation of the secondary phase. The secondary phase (Mg₁₇Al₁₂) of 3% Nb₂O₅ + AZ31 exhibited a 12.5% lower intensity as compared to the pure as-cast AZ31 composite, and the secondary phase Al₃Mg₂ of 3% Nb₂O₅ + AZ31 exhibited a 30.6% lower intensity compared to pure as-cast AZ31 composite. Hence, it is self-explanatory that the addition of 3% Nb₂O₅ restricted the formation of the Al₃Mg₂, Mg₁₇Al₁₂ phase. The phase quantification is shown in

Table 2. Shows phase quantification and crystal structure.

Phase Quantification
Composition (wt.%)	Major Phase	Crystal Structure	Percentage
AZ31	Mg	Hexagonal	25.6
	Al12Mg17	Cubic	0.8
	Al3Mg2	Cubic	6.2
	Al5Mg11Zn4	Orthorhombic	67.4
	Mg4Nb2O9	Hexagonal	0
3% Nb2O5/AZ31	Mg	Hexagonal	20
	Al12Mg17	Cubic	0.6
	Al3Mg2	Cubic	4.3
	Al5Mg11Zn4	Orthorhombic	52.9
	Mg4Nb2O9	Hexagonal	22.1
6% Nb2O5/AZ31	Mg	Hexagonal	19.4
	Al12Mg17	Cubic	0.5
	Al3Mg2	Cubic	5.6
	Al5Mg11Zn4	Orthorhombic	51.8
	Mg4Nb2O9	Hexagonal	23

. The flax can be seen in the figure, but there is a very minute trace of aluminum in it, so the denser oxygen presence and niobium formed Mg₄Nb₂O₉. The secondary phase is distributed unevenly and is more discontinuous and separated. The dispersion of aluminum in Figure 2c is greater compared to Figure 3c, which shows the broadness of the grain boundary of Figure 2a compared to Figure 3a and that the change in microstructure and broadness of grain boundary took place due to the added reinforcement i.e., 3% Nb₂O₅ + AZ31.

The increase in the reinforcement of about 6% Nb₂O₅ + AZ31 has a different morphology compared to 3% Nb₂O₅ + AZ31, as shown in the SEM image of Figure 4. Further addition of reinforcement led to oxygen agglomeration, specifically in the grain boundary. Zinc and niobium correspond to Figure 4d,e with a uniform distribution, despite the fact that aluminum with an agglomerative character formed Mg₁₇Al₁₂, as reported by B.R. Sunil et al. [14] and following the same as shown in pure AZ31 and 3% Nb₂O₅ + AZ31.

3.1.2. XRD-Analysis and Phase Quantification

In Figure 5, the phase composition of Nb₂O₅/AZ31 composite samples was examined by XRD analysis. The XRD data shows a high intensity of α-Mg at (1 0 1) and a closely related quasicrystal line Al₅Mg₁₁Zn₄, which has an orthorhombic crystal structure instead of hexagonal due to distortion [15]. At approximately 36 degrees (in 2θ degree) Mg₁₇Al₁₂ at (4 1 1), and at 37 degrees β-Al₃Mg₂ at (11 3 3) were traced. The reason for the formation of β-Al₃Mg₂ was continuous heating after 723 K and usually pure β-Al₃Mg₂ was left after this temperature [16]. Though the percentage of these two secondary phases was considerably less i.e., 0.8%, 0.6%, and 0.5% of Mg₁₇Al₁₂ in pure AZ31, 3% Nb₂O₅/AZ31, and 6% Nb₂O₅/AZ31. Al₃Mg₂ formed in pure AZ31, 3% Nb₂O₅/AZ31, and 6% Nb₂O₅/AZ31 was 6.2%, 4.3%, and 5.6%. Another phase with a hexagonal crystal structure was obtained, i.e., Mg₄Nb₂O₉, which, according to K. Sarkar, V. Kumar, Shashank Bhushan Das et al., has a high band gap with low dielectric loss and excellent photoluminescence [17].

The effective reduction in grain size is shown in Figure 6, and a little variation can be observed in the microstrain and dislocation density in Figure 7 and Figure 8. The trace of secondary β-Al₁₂Mg₁₇ was found in all the compositions, but the uniformness in the distribution of the discontinuous secondary phase increased with the addition of reinforcement. The proper distribution of the secondary phase and the less agglomeration of aluminum precipitate in 3% Nb₂O₅ reduced the local inhomogeneity along the grain boundary [4]. However, a reduction in microstrain can be due to a reduction in dislocation density, as stated by R.S. Lei et al. [18]. The orthorhombic phase of Al₅Mg₁₁Zn₄ belongs to the quasicrystalline [19]. The orthorhombic phase has the valuable quality of storing energy [20], which might be because of its non-periodicity. Hydrogen absorption and desorption might be tested in the future.

3.2. Compression and Microhardness

Figure 9 shows the compressive load-bearing capacity and the relative change in length. The peak compressive stress of 323.25 MPa and fracture strain of 24.09% show the progress in

Table 3. Shows peak stress, fracture strain, and microhardness.

Composite	Ultimate Compressive Strength (MPa)	Strain (%)	Microhardness (HV)
Pure AZ31	272.36	22.69	55.06 ± 7.56
3 wt.% Nb2O5/AZ31	323.25	24.09	58.68 ± 2.85
6 wt.% Nb2O5/AZ31	297.23	25.25	59.68 ± 5.13

, the composite has an increment of 18.68% for 3% Nb₂O₅/AZ31 and 9.15% for 6% Nb₂O₅/AZ31 when compared with pure AZ31. The compressive strain value had an improvement of 6.17% for 3% Nb₂O₅ and 11.28% for 6% Nb₂O₅ compared to pure AZ31. The curve shows a small enhanced compressibility property compared to the pre-compression analyzed by H. Zhang et al. [21]. The slope in the curve shows that the increase in stress and strain seems proportional to a much larger distance for 3% Nb₂O₅/AZ31 compared to pure AZ31 because of the reinforcement.

The microhardness shown in Table 3 explains that the addition of reinforcement increases the microhardness by 6.57% for 3% Nb₂O₅ compared to monolithic AZ31. However, the increment percent slows down for further addition of Nb₂O₅ because for 6% Nb₂O₅, the increment is just 1.7% compared to 3% Nb₂O₅. The increase in hardness shows a decrease in intermetallic partial spacing for reinforced AZ31 compared to pure AZ31 [22] and also because of the hard ceramic particles (Nb₂O₅) present in the matrix, the composite exhibits a greater microhardness due to the high constraint on matrix deformation during indentation.

3.3. Fractography

The SEM image of the fracture surface obtained after the compression test shows various morphologies for different compositions.

3.3.1. Fracture Study of AZ31

The fracture surface morphology of AZ31 is shown in Figure 10. The secondary phase Mg₁₇Al₁₂ agglomerated impurity can be seen on the surface which has a clean surface morphology. The surface was debonded easily, which could be the reason for crack initiation. Some observable microcracks show initiation from the secondary phase flakes. Though the twins can be observed in the enlarged view the percentage is not much.

3.3.2. Fracture Study of 3% Nb₂O₅ + AZ31

The fracture surface morphology of 3% Nb₂O₅ + AZ31 is shown in Figure 11. The reinforcement shows an improvement in the fracture surface with fewer microcracks with basal slip and compression twin. Adiabatic shear bands (ASB) can be seen because of the high-stress concentration due to compression [23]. The low grain boundary diffusion of magnesium, along with compression stress, forms the white bands called ASB. The addition of Nb₂O₅-induced inclusion can be seen in the basal slip surface.

3.3.3. Fracture Study of 6% Nb₂O₅ + AZ31

The fracture surface morphology of 6% Nb₂O₅ + AZ31 is shown in Figure 12. The agglomeration of reinforcement and secondary phase can be seen with the formation of micropores. The enlarged view has no trace of extension twining but many adiabatic shear bands can be seen. The microcrack and shear band initiated from local stress developed by the agglomerated aluminum precipitate in the grain boundary leads to the reduction in peak compression stress.

4. Conclusions

The Nb₂O₅/AZ31 composite shows an improvement in its properties, which can be concluded as follows:

• Although there is no shift in plane obtained, the addition of reinforcement distributes the secondary phase evenly throughout the matrix, which reduces the local inhomogeneity in the case of 3% Nb₂O₅. Furthermore, an improvement in grain size reduction was observed.
• The compression stress–strain curve reviled that the stress endurance increased by 18.68% for 3% Nb₂O₅ reinforcement with peak stress of 323.25 MPa. However, in the case of microhardness, the increment was 6.5% (3% Nb₂O₅) and 8.3% (6% Nb₂O₅) compared to the AZ31 composite.
• The fracture analyses show maximum basal slip in AZ31 and 6% Nb₂O₅ + AZ31, but 3% Nb₂O₅ shows much more ASB, which indicates the resistance towards the applied stress.