The Role of Ni-P Coating Structure on Fly Ash Cenospheres in the Formation of Magnesium Matrix Composites

The commercial AZ91 magnesium alloy was used as the matrix alloy with the chemical composition specified by ASTM B93-94: 9 wt pct Al, 0.9 wt pct Zn, and 0.4 wt pct Mn. The as-received hollow aluminosilicate cenospheres with a particle size distribution varying from 63 to 125 μm were supplied by the Opole (Poland) electric power station. The chemical composition was as follows: 53 to 63 wt pct SiO₂, 21 to 31 wt pct Al₂O₃, 4.4 to 5.6 wt pct Fe₂O₃, 1.5 to 2.5 wt pct CaO, 0.3 to 0.7 wt pct MgO, 0.8 to 1.6 wt pct K₂O + Na₂O, and 0.1 to 0.7 wt pct TiO₂. The Ni-P coating on the cenospheres was performed by the electroless plating method described in work.[26] The thickness of the obtained Ni-P layer on the cenospheres was equal to about 0.4 μm. The cenospheres were used just after the coating process (composite named AZ91/FAs-C in this work) and after heat treatment of the Ni-P coated material at 773 K (500 °C) (composite named AZ91/FAs-CH). Figure 1 shows the scheme of the component preparation. The composite was fabricated by the negative pressure infiltration method.[27] In both cases, the steel mold was initially heated up to 773 K (500 °C). The volume fraction of the cenospheres in the fabricated composites was verified by the linear method.

The phase compositions of the investigated materials (coated cenospheres and composites) were analyzed by the X-ray diffraction (XRD) method using a Bruker D8 ADVANCE diffractometer. Cu_Kα X-ray radiation was used with an angular step equal to 0.02 deg. Reflexes from particular phases were identified according to ICDD PDF-4+ cards.[28] The Ni-P coating on the cenospheres was also examined by an Veeco atomic force microscope (AFM) with the tapping mode procedure. The NanoScope 7.20 program was used to determine the surface roughness parameters of the Ni-P coating like R_a (Center Line Average High) and SAD (Surface Area Difference). Analyses were performed on areas equal to 0.04 μm.

The composite specimens for the microstructure investigations were prepared by standard metallographic procedures including wet prepolishing and polishing with different diamond pastes without contact with water. To reveal the matrix alloy microstructure, the samples were etched in a 1 pct solution of HNO₃ in C₂H₅OH. The microstructure examinations were performed by an Olympus GX51 light microscope (LM) with differential-interfaces contrast (DIC) and a JOEL JSM-6610LV scanning electron microscope with an energy dispersive X-ray spectrometer (SEM + EDX). The microstructure of the interfaces between the components was also examined by transmission electron microscopy (TEM). A TECAI G² FEG transmission electron microscope with EDAX equipment was used. The samples for the TEM investigations were cut in the form of 3 mm discs. They were next dimpled and mechanically thinned using a Gatan 656 dimple and finally thinned using Leica-EM RES 101 equipment. Brightfield and darkfield techniques were used for the microstructure observations and selected area electron diffraction (SAED) patterns were employed to define the structural constituents.

Figure 2 shows the images of the used cenospheres with the Ni-P coating. It can be seen that the morphology of the used cenospheres reveals a spherical shape with a generally smooth exterior surface. The obtained Ni-P coating had about an 0.4 μm thickness, which is clearly visible especially on the plane perpendicular to the cenosphere walls (on the fracture surface) shown in Figure 2(c). The coating had a permanent structure and it was closely connected with the cenospheres. The Ni-P-coated cenospheres were heat treated at 773 K (500 °C). Heating at this temperature did not cause defects in the Ni-P coating like cracking or chipping. Figure 3 presents a comparison of the surface topography of the Ni-P coating after electroless plating (Figures 3(a) and (c)) and heat treatment at 773 K (500 °C) (Figures 3(b) and (d)), obtained by the AFM technique. Heat treatment caused a slight rise in the R _a parameter from 5.05 nm for the surface of the Ni-P coating to 5.15 nm for the heated material. The SAD parameter also increased from 16.40 to 16.93 after heat treatment of the Ni-P coating.

Figure 4 shows the X-ray diffraction micrographs of both types of used cenospheres. They confirmed that the used cenospheres are mainly composed of mullite and quartz. The broad diffraction reflexes also indicate the presence of an amorphous (glass) phase in the cenosphere walls. It should be noted that the applied heat treatment of the Ni-P-coated cenospheres caused differences in the phase composition of the Ni-P coating. The Ni-P coating on the cenospheres performed by the electroless plating method had an amorphous structure, which is revealed by the broad diffraction reflex from nickel (Figure 4(a)). The heat treatment of the cenospheres at 773 K (500 °C) caused the Ni and Ni₃P phases inside the coating to crystallize. The sharp deflection reflexes visible in Figure 4(b) clearly disclosed the presence of those crystalline phases in the Ni-P coating. Additionally, the X-ray analyses also revealed a significant presence of reflexes from the NiO phase. Because the heat treatment process was conducted in air, an NiO layer was formed on the Ni-P coating due to the oxidation reaction. The reason for the Formation of this oxide was the increase in the above-described surface roughness parameters, determined by the AFM technique.

Figures 5 and 6 present microstructure images of the composites fabricated with as-received Ni-P-coated cenospheres (composite AZ91/FAs-C) and after heat treatment (AZ91/FAs-CH), respectively. In both cases, hollow cenospheres were successfully incorporated into the matrix alloy. It should be noted that both the investigated composites were characterized by uniform distribution of intact cenospheres unfilled by the matrix alloy. In addition, in both cases the volume fraction of the cenospheres was about 60 vol pct. The presented composites' microstructure with cenospheres unfilled by the matrix alloy was a consequence of both the used Ni-P-coated cenospheres and the negative pressure infiltration method for composite fabrication. The relatively low pressure used during this method of composite fabrication did not cause cenosphere damage. Nevertheless, the composite with the as-received Ni-P-coated cenospheres (AZ91/FAs-C) was characterized by visible porosity between the cenospheres. Figure 5 shows the microstructure of this composite where entrapped air porosity in the matrix alloy was observed in the region between the hollow cenospheres. Although the steel mold was heated up to 773 K (500 °C), using cold cenospheres hampered the molten matrix stream by crystallizing phases. Analogical porosity was also observed by Rohatgi et al.[6] in the microstructure of A356/fly ash cenosphere syntactic foam. In the case of the AZ91/FAs-CH composite, both hot cenospheres and mold enabled filling of all the spaces between the cenospheres by the molten alloy matrix (Figure 6). This material was free from porosity or other casting defects.

For both composites, the microstructure of the matrix alloy had a dendritic morphology with very strong segregation of the alloying element, typical for cast AZ91 alloy. The microstructure consisted of a hexagonal closely packed α-Mg solid solution dendrite, depleted in alloying elements in comparison to the equilibrium conditions and α + γ divorced eutectic. In Mg-Al-type alloys the γ-phase (called also β-phase) is an intermetallic compound with a stoichiometric composition of Mg₁₇Al₁₂ (at 43.95 wt pct Al) and an α-Mn-type cubic unit cell. Due to the presence of zinc in the chemical composition of the AZ91 alloy, a ternary intermetallic compound Mg₁₇Al_11.5Zn_0.5 or Mg₁₇(Al,Zn)₁₂ type was created because zinc substitutes aluminum in the γ phase.[29,30] Additionally, the γ phase in the form of secondary discontinuous precipitates was revealed in the microstructure of both fabricated composites. A characteristic lamellar structure with a marked anisotropy of growth of a plate-like γ phase, located especially near the α + γ eutectic was visible in the microstructure of both composites (Figures 5 and 6). Discontinuous precipitates of the γ phase were formed from the solid state below the solvus temperature. This precipitation process is the cellular growth of alternating plates of the secondary phase and near-equilibrium matrix phase. This structural constituent is typical for AZ91 alloy solidified especially in a sand mold where the cooling rate of the cast is relatively slow.[31] In the presented case, due to the high temperature of the steel mold [773 K (500 °C)] the solidified composites cooled down sufficient slowly so that the precipitation process could take place.

Additionally, it should be noted that differences in the cenosphere/matrix interface morphology were revealed during microstructure observation. In the case of the AZ91/FAs-C composite, fine phases with blocky morphology were observed at the cenosphere/matrix alloy interfaces. Figures 5(c) and (d) illustrate very distinct phases created at the external walls of the cenospheres. The observed fine phases at the component interfaces testify to the reaction between the Ni-P coating and the AZ91 magnesium alloy matrix. According to Thermo-Calc software (Stockholm, Sweden) data, the possible reaction products that can form at the cenospheres/matrix alloy interface in the AZ91/FAs-C composite are Al₃Ni₂, Al₃Ni, AlNi₃, AlNi, MgNi₂, and Mg₂Ni. It should be noted that on the contrary in the case of the AZ91/FAs-CH composite, the interfaces between components were smooth. Differences in the interface structure morphology for both the investigated composites are clearly visible from comparison Figures 5 and 6.

The presence of the observed structural constituents was confirmed by X-ray analyses. Figure 7 presents X-ray diffraction micrographs of the investigated materials. It was revealed that for both composites the matrix alloy was composed of an α-Mg and γ phase. Additionally, reflexes coming from phases disclosed for the cenospheres (presented in Figure 4), like mullite and quartz, were also distinct in the X-ray diffraction patterns obtained from both the composites. The high volume fraction of the cenospheres influenced the intensity of those reflexes. It should be noted that the X-ray analyses also revealed the differences in the phase composition of the fabricated materials. For the AZ91/FAs-C composite, reflexes from the Al₃Ni₂ and Mg₂Ni intermetallic phases were identified (Figure 7(a)), whereas these structural constituents were not present on the X-ray diffraction pattern obtained from the AZ91/FAs-CH composite. On the other hand, the X-ray analyses of the AZ91/FAs-CH composite revealed a significant presence of Ni, Ni₃P, and NiO phases (Figure 7(b)). It should be noted that reflexes from the NiO were not disclosed during analyses of the AZ91/FAs-C composite. The microstructure of the AZ91/FAs-C composite was also observed by the scanning electron microscope. EDX line measurements were performed on polished specimens perpendicular to the interface layer between the cenospheres and matrix alloy for a total length of about 40 μm. The SEM + EDX results presented in Figure 8 allowed the authors to confirm the presence of Al₃Ni₂ and Mg₂Ni intermetallic phases at the cenosphere/matrix alloy interfaces in this composite. Linear distribution of the elements along the line cutting across the interface between the components displayed a significantly increased content of Al and Ni in one area and a raised Mg and Ni content in the other side. It should also be noted that a raised Ni content was continuous outside the cenosphere wall. On the other hand, the content of oxygen was significantly increased only in the cenosphere wall. The results obtained from SEM+EDX analyses (Figure 8) in combination with the results acquired from the X-ray analyses (Figure 7(a)) testified that at the cenosphere/matrix alloy interfaces Al₃Ni₂ and Mg₂Ni intermetallic phases were formed. The above phases were formed due to the reaction between the Ni-P coating and liquid alloy matrix during the AZ91/FAs-C composite fabrication process. Although the reaction time between the Ni-P coating and the molten alloy matrix during fabrication was relatively short, especially in the case of the used cenospheres at ambient temperature, the presence of Al₃Ni₂ and Mg₂Ni phases in the investigated AZ91/FAs-C composite microstructure was demonstrated. Leon-Patiño et al.[32,33] also revealed an Al₃Ni₂ phase in aluminum matrix composites reinforced with Ni-P-coated SiC particles fabricated by the infiltration method. On the other hand, an Mg₂Ni phase was also observed by Hassan et al.[34] in an Mg-Ni composite and by Tsai et al.[35] in the Mg-Ni system.

The cenosphere/matrix alloy interfaces in the AZ91/FAs-CH composite were investigated by the TEM technique. TEM observations clearly confirmed the presence of Ni-P coating on the cenospheres. Figure 9 shows a TEM image of the Ni-P coating on a fly ash cenosphere in contact with the matrix alloy. Additionally, the thin permanent external layer of the Ni-P coating located on the matrix side is visible. The linear distribution of elements along the line cutting across the interface between the components displayed a significant increase in Ni and P content in the coating and raised oxygen content in the external layer of the Ni-P coating (on the matrix side). The increased content of oxygen testified to the presence of NiO, which was confirmed by SAED analyses.

Figure 10 presents the TEM image of the interface between the components with SAED patterns obtained from different places of the observed Ni-P coating and EDAX results of surface distribution of elements obtained from the marked area. The associated SAED pattern obtained from the external layer of the Ni-P coating confirmed the identification of the NiO phase. The presence of NiO at the Ni-P coating external layer was also confirmed by the surface distribution of elements revealed during TEM + EDAX analyses presented in Figure 10. On the other hand, the associated SAED patterns obtained from different places of the Ni-P coating revealed the presence of both Ni₃P and Ni phases inside the coating. It should also be noted that that rings visible on the diffractions received from both NiO and Ni testified to the fine-crystalline structure of these phases. The above phases identified during TEM analyses of the fabricated AZ91/FAs-CH composite were also revealed during X-ray analyses of the Ni-P-coated cenospheres heated to 773 K (500 °C) (Figure 4(b)). It should be noted that MgO which potentially can be created was not revealed during TEM analyses. Although the NiO external layer on Ni-P coating is potentially prone to reduction by the liquid magnesium to form MgO, the short time of the infiltration method nevertheless precluded reaction between components.

The presented investigation results indicated that structure of the Ni-P coating had an influence on the interface boundary between the components in magnesium matrix composites with cenospheres. Ni-P coating with amorphous structure as-received from the electroless plating method was prone to reaction with the liquid AZ91 magnesium alloy matrix. The Al₃Ni₂ and Mg₂Ni reaction products created during AZ91/FAs-C composite fabrication were located at the external layer of the Ni-P coating in the form of blocky phases visible in Figures 5 and 8. Although the reaction between the Ni-P coating and matrix alloy took place, the layers of the Ni-P coat and Al₃Ni₂ and Mg₂Ni intermetallic phases prevented the reactions between the cenospheres and the AZ91 alloy. A different interface structure was obtained during AZ91/FAs-CH composite fabrication. The heat treatment of the Ni-P-coated cenospheres caused crystallization of the Ni₃P and Ni phases inside the coating and creation of an NiO layer at its external layer. Those phases were stable during the composite fabrication process and formed perpetual layers on the cenosphere walls. The presented results unequivocally proved that this Ni-P coating prevents component reactions. Both types of component interfaces obtained during fabrication of the investigated composites are presented schematically in Figure 11. It should be noted that despite obtaining different interface structures between the cenospheres and AZ91 magnesium alloy, both types protected the cenospheres from reactions with the liquid alloy matrix.