The magnesium alloy used in this research study had a nominal chemical composition, which is summarized in Table 1. This alloy was provided to the National University of Singapore (NUS, Singapore) by Tokyo Magnesium Company Limited (Yokohama, Japan). Blocks of the as-provided AZ31 alloy were carefully sectioned into smaller pieces and then precision machined to remove any and all oxide layers and other visible scales on the surface. All of the surfaces of the magnesium alloy were washed thoroughly with ethanol subsequent to machining. The alumina particles (Al₂O_3p) having a size of 50 nanometers (nm) along with micron-size nickel particles having a size of 44 microns (µm) were used as reinforcements for the AZ31 magnesium alloy to engineer the magnesium alloy-based metal matrix composite. The reinforcing (Al₂O₃)_p particles were procured from Baikowski (Japan) and the reinforcing nickel powder was obtained from Johnson Matthey (USA).

The extruded rods of the monolithic AZ31 alloy and dual particle-reinforced magnesium alloy composite (diameter = 8 mm and length = 300 mm) were precision machined to obtain cylindrical specimens that had threaded ends for use in both uniaxial tension tests and stress-controlled high cycle fatigue tests. The machining was carried out in conformance with specifications outlined in Standard E-8 of American Society for Testing Materials (ASTM) [21]. The major stress axis was taken to be along the extrusion direction of the rod stock. The overall length of the specimen measured 2.3 inches (58 mm) with a diameter of 0.25 inch (6.3 mm) at a thread section and diameter of 0.125 inch (3 mm) at the gage section. The gage length measured 0.5 inch (12.5 mm).

To understand the specific influence of processing on (a) grain size and shape, and (b) size, shape and distribution of the second-phase particles in the monolithic alloy (AZ31), and (c) the effect of dual particles, i.e., nano-size alumina particulates (Al₂O₃) _p and micron-size nickel powder, reinforcements on intrinsic microstructural features of the magnesium alloy composite, initial microstructural characterization of the as-provided composite material was carried out. No attempt was made in this study to determine the size of the reinforcing Al₂O₃ particulates dispersed in the magnesium alloy metal matrix. The as-provided extruded rods of the alloy (AZ31) and the composite [AZ31/1.5 vol.%(Al2O3) _P/1.5 vol.%Ni] were prepared for examination in a light optical microscope (Model: Mitutoyo FS70) and photographed using standard bright field illumination technique. Preparation involved first cutting samples that were perpendicular to the direction of extrusion. The cut samples were then mounted in bakelite. The mounted samples were prepared for optical microscopy observation using standard metallography preparation techniques. Coarse polishing was performed on progressively finer grades (320 grit, 400 grit, and 600 grit) of silicon carbide (SiC) impregnated emery paper. Finish polishing to obtain a mirror-like surface finish was accomplished using 1 micron alumina powder suspended in distilled water as the lubricant. The finish polished samples were then etched using an etchant. The etchant was acetic-picral, i.e., a solution mixture of 5 mL acetic acid, 6 g picric acid, 10 mL water and 100 mL ethanol. The polished surface of the test specimen was subsequently immersed in the etchant till a brown film formed, and then immediately rinsed in running water followed by drying using a hot air gun.

Uniaxial tension tests and stress-controlled high-cycle fatigue tests were performed on a fully automated, close-loop servohydraulic mechanical test machine (Model: INSTRON: 8500 Plus) in a laboratory environment (Relative Humidity of 55 pct) and at room temperature (25 °C). The servohydraulic machine was equipped with a 22 kip (100 KN) load cell and the tensile tests were performed at a constant strain rate of 0.0001 per second. A PC-based Data Acquisition Software (DAS) was used to record the stress and strain measurements during the monotonic tension test.

The stress amplitude-controlled high cycle fatigue tests were performed on both the monolithic alloy (AZ31) and the composite (AZ31/1.5 vol.% Al₂O₃/1.5 vol.% Ni) using a sinusoidal waveform and at stress ratios (R = minimum stress/maximum stress) of 0.1 and 1.0. The tests were conducted at a constant frequency of 5 Hz with the objective of establishing the variation of maximum stress (σ_maximum) with cyclic fatigue life (N_f). Under stress amplitude controlled fatigue testing the maximum stress was taken to be a fraction of the yield stress of the candidate material as obtained from the uniaxial tension test. The number of cycles to cause complete failure or separation is taken as the fatigue life (N_f).

A high magnification Scanning Electron Microscope (Model: FEI Quanta 200) was used to examine the fracture surfaces of the deformed and failed fatigue specimens. Observations were made over a range of magnifications to identify the macroscopic fracture mode and to concurrently establish the microscopic features and intrinsic mechanisms causing failure. Samples for observation were obtained from the fully deformed and failed fatigue test specimens by sectioning parallel to the fracture surface.

The optical microstructures of the monolithic alloy AZ31 and the dual particle, i.e., nanosize alumina particle and micron-sized nickel powder, reinforced composite were taken using bright a field illumination technique and are shown in Figure 1. Both the unreinforced alloy and the reinforced magnesium alloy or composite revealed a poly-grained structure but with an observable difference in their microstructural features or microstructural intricacies. The dual particle reinforced magnesium alloy composite (Figure 1b) revealed an observable difference in intrinsic microstructural features when compared one-on-one to the monolithic counterpart (Figure 1a). Most noticeable was the difference in both size and volume fraction of the two phases, i.e., the primary phase (the matrix) and the second phase (the eutectic). No attempt was made in this study to determine both the size and volume fraction of the phases present in the monolithic alloy and the dual particle reinforced magnesium alloy composite. The observed difference in intrinsic microstructural features of the dual particle-reinforced magnesium alloy composite is ascribed to the mutually interactive influences of the following competing mechanisms:

The monolithic alloy (AZ31) revealed a duplex microstructure consisting of light colored alpha (α) phase and the dark colored eutectic phase (β), which was found to be distributed both at and along the grain boundaries (Figure 1a). The alumina nanoparticulate and nickel particle reinforced magnesium alloy revealed a heterogeneous distribution of the reinforcing nickel particles through the microstructure with pockets of particle agglomeration resulting in nickel-particle-rich and nickel-particle-depleted regions (Figure 1b). Earlier studies have used the technique of X-ray diffraction and identified the eutectic phase (β) to be the intermetallic particle (Mg₁₇Al₁₂) [22,23]. An observable higher volume fraction of the eutectic phase (β) was observed in the reinforced magnesium alloy when compared one-on-one with the unreinforced counterpart (Figure 1a). However, no attempt was made in this study to determine the volume fraction of the phases present in the two materials, i.e., monolithic alloy and the composite. The influence of an increased volume fraction of the eutectic phase on the tensile properties and high cycle fatigue resistance is discussed in the following section.

The uniaxial tensile properties of this dual particle reinforced magnesium alloy composite along with the monolithic or unreinforced matrix alloy (AZ31) are summarized in Table 2. The values recorded are the mean values based on duplicate tests. An attempt is made to understand the synergistic and mutually interactive influences of the two reinforcing particles in governing the fine microscopic mechanisms and the resultant macroscopic mechanical behavior of the magnesium alloy composite under uniaxial loading. This enables us to make the following observations from the test results.

The engineering stress versus engineering strain curve of the AZ31/1.5 vol.% Al₂O₃/1.5 vol.% Ni composite and the monolithic alloy are compared in Figure 2. This figure reveals the dual particle reinforced magnesium alloy composite to have noticeably lower tensile strength but improved tensile ductility quantified by a reduction in the cross section area when compared to the monolithic alloy (AZ31).

The results of the stress-controlled high cycle fatigue tests are shown in Figure 3–Figure 6. The maximum strains generated during fatigue testing are predominantly elastic in nature while the maximum applied stresses are well below the tensile yield strength of both the composite material and the monolithic counterpart. The plastic deformation is confined to occur at the microscopic heterogeneities in the microstructure and is conducive to the initiation of a crack, which is essentially microscopic in nature. Continued growth of the fine microscopic cracks through the microstructure of this magnesium alloy-based composite during repeated cyclic stressing results in their eventual coalescence to form a visible macroscopic crack. Concurrent growth of both the fine microscopic and macroscopic cracks through the microstructure of the composite results in failure of the test specimen. During stress amplitude-controlled cyclic loading the property changes, if any, are expected to occur both at the crack tip and the region immediately in front of an advancing crack tip, while the material in the far field remains essentially unaffected. Further, under conditions of stress-controlled fatigue, the most part of the fatigue life is spent in the initiation of an observable crack that is macroscopic in size. The stress-amplitude-controlled fatigue test provides a useful measure of the endurance limit of the candidate composite material at the chosen load ratio (R = σ_minimum/σ_maximum).

The fracture surfaces of the cyclically deformed and failed test specimens were carefully and comprehensively examined in a scanning electron microscope (Model: FEI Quanta 200) to establish the macroscopic fracture mode and to concurrently characterize the intrinsic microscopic features decorating the fracture surface and thereby help establish the mechanisms governing fracture. Only representative fracture features of the nickel-particle reinforced magnesium alloy composite (AZ31/1.5 vol.% Al₂O₃/Ni) and the unreinforced magnesium alloy (AZ31) at the two load ratios of 0.1 and −1.0 taken at high value of maximum stress and resultant short fatigue life and low value of maximum stress and resultant enhanced fatigue life are shown in Figure 7–Figure 12.

The constraints to mechanical deformation induced by the physical presence of hard and essentially elastically deforming nano-sized aluminum oxide (Al₂O₃) particle and micron-size nickel particle reinforcements on the soft, ductile and essentially plastically deforming magnesium alloy (AZ31) metal matrix coupled with the resultant development of matrix triaxiality does exert an influence of flow stress on deformation capability of the magnesium alloy [23,24]. Under the influence of cyclic loading the fine microscopic voids appeared to have undergone only limited growth confirming a possible contribution from the nanosize Al₂O₃ and micron-size nickel particulate reinforcements on the failure of the composite. The locally existing triaxial stresses that exist as a direct consequence of far-field loading favor only a limited growth of the fine microscopic voids in the magnesium alloy metal matrix. The limited growth of the fine microscopic voids, during far-field loading, coupled with a lack of their coalescence with each other and companion macroscopic voids to be the dominant fracture mode for this magnesium alloy composite, clearly indicate that the deformation properties of the magnesium alloy (AZ31) are noticeably altered by the presence of both nano-size aluminum oxide and micron-size nickel particle reinforcements.

In fact, ductile crack initiation in this metal matrix composite results from void growth and eventual coalescence. This is made possible under the combined influence of “local” plastic strain and hydrostatic tensile stress. When the plastic strains exist in the presence of hydrostatic tension stress, the fine microscopic voids that nucleate around the reinforcing particles begin to grow. A gradual increase in size of the growing void intensifies the strain between neighboring voids until void-void interactions result in their coalescence culminating in the formation and presence of a macroscopic crack. Overall, the process of void growth and eventual coalescence leading to ductile crack initiation at the microscopic level are affected by void distribution, anisotropy spacing of the reinforcing particles, nature of void initiation, evolution of void shapes, void-to-void interactions and the nucleation and growth of secondary voids that facilitate coalescence. In this magnesium alloy composite the halves of the limited grown microscopic voids are the shallow dimples visible on the fatigue fracture surface [25,26,27,28]. Failure of the reinforcing aluminum oxide (Al₂O₃) particulates either through fracture or by decohesion at their interfaces with the magnesium alloy metal matrix was difficult to observe at the allowable magnifications of the SEM due to the small size (nanometer) of the reinforcing particulates. However, the constraints to mechanical deformation, at the fine microscopic level, during cyclic loading coupled with considering the influence of particle reinforcement on strain hardening of this magnesium alloy metal matrix are factors responsible for the improved ductility and marginally improved cyclic fatigue resistance (quantified by fatigue life) of the composite over the unreinforced counterpart. In regions and/or locations of clustering or agglomeration of the Al₂O₃ nanoparticle reinforcements, the short inter-particle distance is conducive to the linkage between the neighboring voids and resultant microscopic crack.

For test specimens of both the magnesium alloy composite and the monolithic alloy that were cyclically deformed under fully-reversed loading, the fracture surface did reveal some evidence of brittle fracture at the fine microscopic level. The compression part of the load cycle caused the voids to collapse into oblate or penny-shaped voids, which tended to create sharper stress concentrations than the spherical voids that form in tension-tension cyclic loading and under monotonic tension. On subsequent loading under tension, the highly localized stresses at the circumference of the collapsed voids will tend to favor earlier ruptures of the ligament between voids by promoting alternate failure mechanisms, including microscopic cleavage and microscopic cracking. Thus, the net result of the collapse of the void under fully-reversed loading helps to reduce the critical size of the void. This explains the presence of smaller voids and fine microscopic cracks on the fracture surface of the test specimen that was cyclically deformed under fully reversed loading.