Microstructure and mechanical properties of SiCp/AZ91 composites prepared by freeze casting and hot extrusion

doi:10.1016/j.metadv.2026.04.003

Abstract

Abstract:

In this work, the SiC_p/AZ91 composites prepared by freeze casting were subjected to hot extrusion, and then the microstructure, mechanical properties and fractural behavior were given and analyzed. The results showed that, the particle distribution of SiC_p/AZ91 composites changed from lamellar to uniform distribution after extrusion, which significantly improved the mechanical properties of the composites. With increasing volume fraction of SiC_p, the thermal expansion coefficient of the as-extruded SiC_p/AZ91 composites decreased significantly, accompanied by the improvement in elastic modulus. When the content of SiC_p was 25 vol.%, the ultimate tensile strength (UTS) and elastic modulus of the composite reached ∼513 MPa and ∼91 GPa, respectively, which exhibited the best combination of tensile strength and modulus. During the loading process, microcracks were more likely to initiate at the interface between the SiC_p and magnesium matrix, and excessive SiC_p could aggravate the stress concentration at the interface significantly, which led to the interconnection of microcracks and failure of the composite.

Key words: Freeze casting, Hot extrusion, Mechanical properties, Fracture mechanism

Cui-Ju Wang, Ao-Hua Su, Kun-Kun Deng, Xue-Jian Li, Zi-Long Zheng, Kai-Bo Nie. Microstructure and mechanical properties of SiC_p/AZ91 composites prepared by freeze casting and hot extrusion[J]. Metals Advances, 2026, 46: 75-85.

Figures/Tables 17

Fig. 1. SEM image of SiCp after pickling.

Table 1. Main chemical composition of AZ91 alloy.

Al	Zn	Mn	Fe	Si	Cu	Ni	Mg
8.500	0.800	0.300	0.004	0.050	0.025	0.010	Bal.

Fig. 2. (a-c) SEM microstructures and (d-f) ceramic layer thickness statistics of freeze casting SiCp/AZ91 composites: (a, d) 20 vol.% SiCp, (b, e) 25 vol.% SiCp, (c, f) 30 vol.% SiCp.

Fig. 3. (a-c) SEM microstructure and (d-f) EDS microstructure of the as-extruded SiCp/AZ91 composites by freeze casting: with the SiCp of (a, d) 20 vol.%, (b, e) 25 vol.%, (c, f) 30 vol.%.

Fig. 4. As-extruded of SiCp/AZ91 composites by freeze casting: (a) HAADF image, (b) EDS diagram.

Fig. 5. XRD patterns of as-cast and as-extruded 25 vol.% SiCp/AZ91 composites.

Fig. 6. TEM images of SiCp/AZ91 composites by freeze casting: (a) microstructure in the particle-dense region; (b) DRXed grains and dislocations around SiCp; (c) DRXed grains at the sharp corners of SiCp.

Fig. 7. HRTEM images of as-extruded SiCp/AZ91 composite: (a) HREM image at the interface of SiCp and Mg; (b) and (c) IFFT images of Mg and SiCp, respectively.

Fig. 8. Thermal expansion coefficient of as-extruded AZ91 alloy and SiCp/AZ91 composites.

Table 2. CTE of SiCp/AZ91 composites.

Composite	Temperature (°C)	ROM (×10⁻⁶/℃)	Turne (×10⁻⁶/℃)	Kerner (×10⁻⁶/℃)	Actual value (×10⁻⁶/°C)
20 vol.% SiC_p/AZ91	100	20.728	13.523	20.822	16.601
	200	21.207	13.875	21.301	18.016
	300	22.381	14.233	21.976	19.574
25 vol.% SiC_p/AZ91	100	19.639	12.277	19.749	14.353
	200	20.086	12.586	20.196	15.671
	300	20.716	12.892	20.826	17.199
30 vol.% SiC_p/AZ91	100	18.650	11.233	18.738	13.501
	200	19.229	11.498	19.317	14.596
	300	19.804	11.759	19.892	16.094

Fig. 9. Compressive stress-strain curves and properties of SiCp/AZ91 composites before and after extrusion: (a, b) before extrusion, (c, d) after extrusion.

Fig. 10. As-extruded tensile properties of freeze-cast SiCp/AZ91 composites: (a) tensile stress-strain curve, (b) elastic modulus, (c) comparison of mechanical properties between this study and other SiCp reinforced Mg matrix composites [7], [22], [23], [24], [25], [26], [27].

Fig. 11. Tensile fracture morphology of SiCp/AZ91 composites at room temperature after extrusion: with the volume fraction of SiCp (a, d, g) 20 vol.%, (b, e, h) 25 vol.%, (c, f, i) 30 vol.%.

Fig. 12. Flexural properties of freeze casting SiCp/AZ91 composites before and after deformation: (a) flexural stress-strain curve before deformation, (b) flexural stress-strain curve after deformation.

Fig. 13. Fracture toughness of freeze-cast SiCp/AZ91 composites before and after deformation was tested: (a) unilateral notch stress-displacement curve before deformation, (b) unilateral notch stress-displacement curve after deformation.

Table 3. Flexural properties of SiCp/AZ91 composites before and after extrusion with different particle contents.

Composites	Particle content (vol.%)	Flexural strength (MPa)	K_Ic (MPa m^1/2)	K_Jc (MPa m^1/2)	W (J m⁻²)
As-cast	20%	464.1	12.737	24.95	1590
	25%	526.8	11.738	20.94	786
	30%	460.4	9.864	17.72	513
As-extruded	20%	709.8	16.328
	25%	750.4	14.636
	30%	735.6	10.66

Fig. 14. Crack propagation path, local microcracks and fracture morphology with the volume fraction of SiCp (a-d) 20 vol.%, (e-h) 25 vol.% and (i-l) 30 vol.% composites.

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