*3.2. Effect of Zr Content on Mechanical Properties of Composites*

Figure 7 shows the hardness of the pore wall of the composites with different Zr content. The hardness of the pore wall increases continuously with the increase of Zr content, Through the analysis of the hardness value, the highest hardness value (145.4 HV0.1) is

obtained in the case of Zr content 30 wt.%, which is 71.6% higher than that of 84.73HV0.1 in the case of Zr content 5 wt.%. Based on the above analysis, the low pore wall hardness of composites is attributed to the weak bonding of powder particles with low content of Al3Zr. With the increase of Zr content, the main composition of the pore wall changes to Al3Zr, leading to higher hardness.

**Figure 7.** The hardness value of pore wall with different Zr content.

Figure 8 shows the compressive stress–strain and energy absorption capacity curves of the composites with different Zr content (space holder content is 60 vol.%). The curve in Figure 8a was divided into three regions: (1) Elastic region: the elastic deformation occurs, and the compression process is similar to dense materials, and the compression curve shows that the stress increases linearly with the increase of strain. The yield stress of the material is the value when the stress reaches the highest point for the first time. Moreover, the slope of the material with different Zr content in the elastic region is similar, indicating that the composition of the material has little effect on the elastic stage, and the main difference in the elastic region is the different yield stress of the material; (2) plateau region: most of the curves in this region is wavy or meandering. This phenomenon can be explained by the fact that the pore wall collapsed and deformed under the compressive load, and some pores ruptured rapidly with the increase of compressive strain, which eventually led to the stress weakening [27]. When the compression load is distributed to the uncompressed cavity, the support of the pore wall will make the stress increase again. So, the degree of curve bending is determined by the combined action of macroscopic and microstructural instability of porous materials [28,29]; (3) densified region: the pores in this area have been compacted, the material is in a dense state, and the compressive strain makes the stress rise rapidly.

**Figure 8.** (**a**) Compressive stress–strain and (**b**) energy absorption capacity curves of composite materials with different Zr contents.

Compared with the upper limit of 50% strain, the curves in Figure 8b show that the energy absorption capacity of the material increases at first and then decreases with the increase of Zr content. In the case of Zr content 20 wt.%, the energy absorption capacity reaches the highest of 11.68 MJ/m3, which is 315.6% higher than that of 5Zr (2.81 MJ/m3). This phenomenon can be explained by the fact that the energy absorption capacity is mainly related to the area under the stress–strain curve in the case of samples with similar porosity. Due to the good stiffness and elastic modulus of Al3Zr, the performance of the material in the plateau stress region is improved as well as the energy absorption performance.

Figure 9 presents the yield stress and plateau stress of the composites with different Zr content. It can be seen that the numerical gap between yield stress and plateau stress decreases gradually. This phenomenon can be explained by the fact that the density of porous materials increased with the increase of Zr content, and the microstructure of the material gradually stabilised. Under the influence of these comprehensive factors, the plateau stress value gradually approached yield stress. The figure also shows that yield stress and plateau stress increase at first and then decrease with the increase of Zr content, and reached their highest point at a Zr content of 20 wt.%. The yield stress was 28.11 MPa, which was 198.1% higher than 5Zr (9.43 MPa), and the plateau stress was 24.87 MPa, 358.8% higher than 5Zr (5.42 MPa). Among them, the stress rise of the plateau is more than the yield stress, which is due to the low density and many defects in the case of Zr content 5 wt.%, the pores will collapse quickly after entering the plateau region, resulting in a rapid decrease in the platform stress.

**Figure 9.** The yield stress and plateau stress of composites with different Zr contents.

In the case of Zr content 25 wt.% and 30 wt.%, there is mainly brittle phase Al3Zr in the pore wall, cracks generated at defects will propagate easily, which could decline the material properties.

Figure 10 presents the SEM images of compression fracture of composites with different Zr contents. There are no obvious fracture characteristics in the area around the white box in Figure 10a, indicating that the white box area is one of the starting points for the fracture failure of the pore wall under the compression load. Moreover, some pore walls in the material are not in a strictly closed state, which causes the regional stress concentration, collapse and deformation in the compression process. At the fracture of Figure 10c, there are continuous dimples of different sizes, indicating that the fractured part is coherent. This may be due to the crack propagating in all directions because the thickness and structure of the pore wall are uniform, the fracture occurs in the whole area. However, it is difficult to directly observe the initial position of the fracture. Unlike the morphology of the sample in Figure 10c, there is a smooth area surrounded by cracks at the fracture of the pore wall in Figure 10e. Because the thickness distribution of the pore wall around this area is different and this irregular porous structure will cause stress concentration. The stress concentration in the thinner part of the pore wall will be greater than that in the

thicker pore wall, Zettl [30] also concluded that the initial defect is the preferred medial part of the pore wall and the damage begins in the thinner part of the pore wall.

**Figure 10.** The SEM image of compression fracture of composites with different Zr content (**a**,**b**) 10Zr; (**c**,**d**) 20Zr; (**e**,**f**) 30Zr.

From the SEM of the pore walls, it can be seen that the fracture surfaces of 10Zr and 20Zr have a large number of dents and torn edges, indicating that the pore walls are ductile fractures. The dents of the fractures of 30Zr are larger and shallower than those of 20Zr, which indicates that brittle fractures occurred in the pore walls. Based on the explanation of the morphology and properties of porous materials with different Zr content, it can be considered that there are two fracture modes in the pore wall of Al3Zr/2024Al porous composites during compression. One is the fracture between ductile phase 2024Al, which is mainly displayed in Figure 10b. There are a large number of tear edges formed by the fracture between 2024Al. When the crack develops near the Al3Zr particles, the fine Al3Zr will hinder the crack extension and form dimples. As a result, the compression properties of the composites are enhanced. With the increase of the Al3Zr number, the resistance of crack extension also increases, and the number of dimples in the fracture surface of the pore wall also increases. Another one is the brittle fracture of Al3Zr, the interior of the material is mainly brittle phase Al3Zr when too much Al3Zr is formed. Due to the lack of ductile phase as support, the crack will develop rapidly after compression fracture, which leads to the fracture of some agglomerated Al3Zr particles earlier than the ductile phase and makes the dimples on the fracture surface larger and shallower. Finally, this brittle structure causes the decline of material properties.

According to the previous explanation of pore wall morphology, the reaction of Zr and 2024Al from 5Zr to 20Zr forms finely dispersed Al3Zr particles that hinder crack growth by reducing the stress concentration. The reaction also improved the density and reduced the number of defects on the pore wall. Under the combined effect of these factors, the composite performance is significantly improved. However, when the Zr content continues to increase, the composite is mainly brittle phase Al3Zr, the crack will quickly spread to the surrounding parts under a compression load, thereby degrading the properties of the sample. Therefore, Al3Zr should be composited with toughness to compensate for its brittleness, to obtain porous composites with excellent compressive and energy-absorbing properties.
