Effect of Al2O3 Nanoparticle on Cavitation Strengthening of Magnesium Alloys

In order to study the effect of Al2O3 nanoparticles in the cavitation-based strengthening process of magnesium alloys, the impact of a micro-jet generated by bubble collapse has been considered. The strengthening mechanism is based on the transfer of the energy of cavitation due to bubble collapse to Al2O3 particles, which then undergo collision with the surface of the sample. The hardness, surface morphology, element content and chemical state of the strengthened samples have been analyzed by microhardness tests, SEM (scanning electron microscopy) and XPS (X-ray photoelectron spectroscopy) techniques. The results show that: after 5 min of strengthening, nanoparticles can be found on the surface of the sample through SEM. Combined with XPS tests, the content of Al2O3 in the sample can be significantly increased, indicating that Al2O3 particles penetrate into the surface and increase its hardness by 29.1 HV.

through high-speed jets for modifying the material surface. Obtained results showed that high-speed water jets increase the residual compressive stress and improve the mechanical properties of the material surface. From the shot peening point of view, the resulting materials present smoother surface morphologies.
Studies show that morphology, texture, and grain size of the material surface affect the material properties. Accordingly, the mechanical shot peening can be combined with pure cavitation strengthening. In this regard, experiments were conducted on magnesium-aluminum (Mg-Al) alloys, where Al 2 O 3 nanoparticles are added in water and then embedded within the samples by the generated energy because of the cavitation-bubble collapse.

Test Methods
In this section, experiments were conducted by an ultrasonic cavitation testing machine (SLQS1000, Company name). Fig. 1 shows the test machine in this regard.
In the experiment, test parameters were set as follows: ultrasonic power of 800 W; ultrasonic frequency of 20 kHz; the amplitude-measuring transformer with a diameter of 15.8 mm at a distance of 0.5 mm to the sample surface; the test temperature was 25°C. Moreover, specimens made of AZ31 Mg alloy and dimensions of 20 mm × 20 mm × 0.2 mm were considered as the research object. The diameter of Al 2 O 3 particles was less than 500 nm. Meanwhile, all samples were polished before testing to reach the surface polish of 5000 mesh. After the test, samples were cleaned and dried and then preserved in sealed containers.
In order to perform the experiment, samples were put in a water solution, and then Al 2 O 3 nanoparticles were added to the water. Then the mixture was stirred evenly through ultrasonic-induced cavitation until bubbles collapsed near the sample wall, which originated from micro-jet Al 2 O 3 particles at a certain energy impact samples and infiltration of the surface. This process realizes the increment of surface hardness.
The samples were characterized and tested by applying a scanning electron microscope (SEM) (F50, FEI Inspect), an X-ray photoelectron spectrometer (XPS) (250Xi, Thermo Fisher EscaLab), and a Vickers hardness tester. Variations of different parameters, including the surface morphology, chemical state, and microhardness of the samples were monitored.

The Influence of the Treatment Mode
In order to analyze the influence of Al 2 O 3 particles on the strengthening process, compared with the pure cavitation strengthening (i.e., strengthening without Al 2 O 3 particles). Fig. 2 shows the surface morphology of the sample after 5 min of action.  These mechanisms originate from the micro-jet impact generated by the cavitation bubble collapse. In the initial stage, slight deformation appears at the point subjected to the micro-jet impact on the sample surface and shear failure is gradually generated at the edge so that erosion pits appear. Moreover, considering micro-jets differing in the impact direction and intensity, certain positions on the sample surface are more deformed so that the pitting diameter differs greatly. Fig. 3 schematically presents the micro-jet impact. Fig. 2b indicates that after 5 min of coupled strengthening with Al 2 O 3 particle, the surface morphology of the sample changes greatly. Under this circumstance, more pits appeared and some pits are gradually connected. Therefore, a smooth edge is achieved. Meanwhile, it is found that particles smaller than 500 nm exist in the pit, indicating that Al 2 O 3 particles infiltrate into the sample surface. In this case, the strengthening mechanism can be described by impacts of Al 2 O 3 particles on the sample surface rather than cavitation bubble collapse energy being directly applied to the samples. Therefore, a lower impact  force is imposed on the particle, and the transition zone a continuous and smooth pit is achieved. This issue is of significant importance to prevent unwanted deformation through micro-cracks and sharp edges [14]. Furthermore, Al 2 O 3 nanoparticles have a reasonable hardness and a better strengthening effect after embedding within the sample surface.
In order to prove the infiltration of Al 2 O 3 particles and analyze the changes in the chemical state of elements, the XPS full-spectrum, high-resolution spectrum of Mg1s and Al2p of the sample under two strengthening methods are shown in Fig. 4. Fig. 4a shows that regardless of the presence of Al 2 O 3 particles, the position of the main peak of the two strengthening methods does not change significantly after 5 min of action. Therefore, the composition of the substance is the same.
Moreover, Fig. 4b shows that the binding energy of the element Mg at the peak value after the coupled strengthening with Al 2 O 3 particles is 1303.1 eV, which corresponds to metal Mg according to previous research [15]. Furthermore, the binding energy at the peak value after the pure cavitation strengthening without Al 2 O 3 particles is 1303.6 eV, which also corresponds to the metallic Mg. The peak of the original samples obtains binding energy of 1303.9 eV, which corresponds to MgO. This is because the metal Mg on the sample surface is exposed to air for a long time and oxidized to MgO before the strengthening tests. Therefore, both modes of strengthening change the surface morphologies and exfoliate the MgO from the original surface, while exposing metal Mg at greater depth.  Fig. 4c illustrates that the binding energies of Al elements in the samples under the two strengthening methods appear at the same position, indicating similar compositions. However, the peak intensity between them differs significantly. It is found that the peak value under the coupled strengthening with Al 2 O 3 particles is higher than that under pure cavitation strengthening without Al 2 O 3 particles. The specific parameters are as follows: The Al2p peak of the original samples corresponds to the binding energy of 72.7 eV, mainly denoted as metallic Al. After the pure cavitation strengthening without Al 2 O 3 particles, the peak shifts to the position of high binding energy at 74.5 eV. This implies that the metallic Al is transformed into Al 3+ in the form of Al 2 O 3 due to the loss of electrons, which is attributed to the released heat during the cavitation bubble collapse. Therefore, considering the original samples, the binding energy varies, while the strength of the samples remains practically unchanged after pure cavitation strengthening without Al 2 O 3 particles treatment. This indicates that the valence state of Al elements changes, while their content remains unchanged. After coupled strengthening with Al 2 O 3 particles, the peak also appears at 74.5 eV, while there is larger binding energy. This indicates that Al 2 O 3 particles in the solution are embedded within the samples adjunct to the metal Al being transformed into Al 2 O 3 under the effect of heat release during cavitation bubble collapse. Therefore, Tab. 1 shows that the elemental Al content increases.  It can be found that the surface hardness of Mg alloy, after cavitation strengthening without Al 2 O 3 particles for 5 min, increased by 11.4 HV and by 29.1 HV after coupled strengthening with Al 2 O 3 particles. This indicates that the addition of Al 2 O 3 particles greatly enhances the strengthening effect.

The Influence of the Treatment Duration
The sample surface undergoes damage after 10 min. In this case, the binding capacity between Al 2 O 3 particles and the surface layer of the samples decreases. Moreover, Fig. 6 shows that most of the particles are gradually exfoliated from the sample surface.
As the action time increases, the sample undergoes cavitation erosion. As a result, the embedded Al 2 O 3 particles are released from the surface. Therefore, the content of Al 2 O 3 in the sample significantly decreases and becomes equivalent to the content of Al 2 O 3 under pure cavitation strengthening without Al 2 O 3 particles. This indicates that Al 2 O 3 particles are released from the surface of the sample and the chemical state of the Al element gradually becomes similar to that of pure cavitation, as shown in Fig. 7. Fig. 7 illustrates that the binding energy of the Al element in the samples under the two functional models appears in a similar position as the treatment duration changes. This shows that the same chemical state exists in different treatment durations. After being treated for 10 to 15 min, the peak amplitude decreases and the Al 3+ content decreases to a significant extent. This is mainly due to the morphological damage of the Al 2 O 3 particles, which are separated from the sample. After treatment for 15 to 20 min, the peak value under the pure cavitation strengthening effect is higher than that under the coupled strengthening effect. However, Tab. 3 shows that the difference between them is not significant.  It is observed that the strengthening effect of the two modes of treatment on the specimens conforms to the Gaussian distribution, which is expressed as follows: the optimal strengthening effect appears within 5 to 10 min. Then, the rate of the reduction of the microhardness increases under the effects of the coupled strengthening with Al 2 O 3 particles. This indicates that the microhardness is related to the morphology of the specimen and the amount of Al 2 O 3 particles.

Conclusion
Obtained results show that cavitation bubbles collapse near the sample wall, Al 2 O 3 particles under the action of micro-jet infiltration of the magnesium alloy surface, realize the reinforcement, and as the growth of the time to produce cavitation damage, Al 2 O 3 particles leave from the surface. The experiments show that compared with the pure cavitation strengthening method, in the proposed method, Al 2 O 3 particles penetrate the surface to improve the performance of the main factors. Therefore, the oxidation state of the Al content is significantly higher, and the surface hardness increases significantly.

Conflicts of Interest:
The authors declare that they have no conflicts of interest to report regarding the present study. 14. Soyama (7), 2296-2301.