Simultaneously optimizing pore morphology and enhancing mechanical properties of Al-Si alloy composite foams by graphene nanosheets

doi:10.1016/j.jmst.2021.04.050

Abstract

Abstract:

The integrity and regularity of pore morphology play an important role in determining the mechanical properties of the metallic foam materials. The conventional methods on refining pore morphology are mainly focused on the optimization of fabrication techniques, however, they are usually inconvenient and complicated. Recently, incorporating nano reinforcement is considered to be a suitable way to fabricate metallic composite foams accompanied by optimized pore morphology and enhanced mechanical properties. In this work, through a facile and rapid powder metallurgy foaming method, the aluminum-silicon (Al-Si) alloy composite foams reinforced by graphene nanosheets (GNSs) are successfully fabricated. The microstructure analyses reveal that, for the Al-Si alloy foams incorporating the GNSs (GNSs/Al-Si composite foams), the pore size is transformed to be smaller, the pore size distributions become more homogeneous and the pore shape is also refined to a regular and roundish state. Meanwhile, the shape of Si precipitates is found transforming from an irregular long strip (length of ~20 μm, width of ~5 μm) to a fine particle state (diameter of ~5 μm). Moreover, the compressive testing results show that, the 0.4 wt% GNSs/Al-Si composite foams own the optimal compression stress of 11.7 ± 0.5 MPa, plateau stress of 10.0 ± 1.0 MPa and energy absorption capacity of 6.8 ± 0.7 MJ/m³, which have improvement of 58.1%, 53.8% and 51.1% in comparison with the Al-Si alloy foams counterpart, respectively. The present findings may pave a new way for developing new generation of metallic composite foams that with stable microstructure and excellent mechanical performance.

Key words: Metal matrix composites, Aluminum foam, Graphene, Energy absorption

Weiting Li, Xudong Yang, Kunming Yang, Chunnian He, Junwei Sha, Chunsheng Shi, Yunhui Mei, Jiajun Li, Naiqin Zhao. Simultaneously optimizing pore morphology and enhancing mechanical properties of Al-Si alloy composite foams by graphene nanosheets[J]. J. Mater. Sci. Technol., 2022, 101: 60-70.

Figures/Tables 15

Fig. 1. Schematic diagram about the fabrication process of the GNSs@Cu/Al-Si composite foams. The GNSs@Cu was firstly prepared through in-situ template-assisted and the high-temperature calcination approaches. Then, the GNSs@Cu/Al-Si composite foams were synthesized by powder metallurgy foaming method.

Fig. 2. SEM and TEM images of the GNSs@Cu powders. SEM images of (a) 3D cellular network of the GNSs and (b) the Cu nanoparticles with the diameter of 20-100 nm. TEM images of (c) the GNSs after etching Cu nanoparticles and (d) the GNSs with the thickness of 3 nm.

Fig. 3. 2D visualization images of X-ray CT of the foams: (a) the Al-Si foams, (b) the 0.2 wt% GNSs@Cu/Al-Si composite foams, (c) the 0.4 wt% GNSs@Cu/Al-Si composite foams and (d) the 0.8 wt% GNSs@Cu/Al-Si composite foams. The pore morphologies inside the composite foams are homogenous for all foams.

Fig. 4. Digital photographic images of cross sections of the foams: (a, e) the Al-Si foams, (b, f) the 0.2 wt% GNSs@Cu/Al-Si composite foams, (c, g) the 0.4 wt% GNSs@Cu/Al-Si composite foams and (d, h) the 0.8 wt% GNSs@Cu/Al-Si composite foams. Compared to the Al-Si foams, the pore morphologies of the composite foams are notable refined.

Fig. 5. The statistical analysis of the pore parameters of the composite foams with different GNSs@Cu content. (a) Diameter, (b) standard deviation of pore size, (c) circle shape factor and (d) aspect ratio factor. The longitudinal coordinates are the frequency histograms of these four pore parameters located in specific ranges.

Table 1 The statistical analysis of the pore average parameters of the composite foams with different GNSs@Cu content.

GNSs contents (wt%)	Total pore numbers for calculation	Average pore size $\bar{D}$ (mm)	Standard deviation of average pore size \|D²\|	Circle shape factor $\bar{F_{cs}}$	Aspect ratio factor $\bar{F_{ar}}$
0	280	3.59	0.65	0.85	1.59
0.2	287	2.96	0.56	0.88	1.49
0.4	295	2.45	0.52	0.91	1.38
0.8	254	2.92	0.69	0.84	1.62

Fig. 6. SEM and EDS element mapping images around the pores: (a) the Al-Si foams and (b) the 0.4 wt% GNSs@Cu/Al-Si composite foams.

Fig. 7. TEM characterizations of micropores. (a) TEM image of micropores of the 0.2 wt% GNSs@Cu/Al-Si composite foams. (b, c) High-resolution TEM images of selected area of (a). (d) TEM image of micropores of the 0.4 wt% GNSs@Cu/Al-Si composite foams. (e, f) High-resolution TEM image of selected area of (d).

Fig. 8. Microstructure characterizations of the nanoparticles in the Al matrix. (a) XRD results of the precipitates of the 0.4 wt% GNSs@Cu/Al-Si composite foams. TEM images of the (b) large amount precipitates in the 0.4 wt% GNSs@Cu/Al-Si composite foams and (c) peanut-like shape Si and Al2Cu precipitates (inset is the FFT pattern of Si and Al2Cu). (d) TEM and (e) high-resolution TEM images of rod shape Al4C3 precipitates. (insets are the FFT pattern of yellow box).

Fig. 9. Microstructure characterizations of the eutectic Si phase of the Al-Si foams. (a, b) OM and (c-e) SEM images. (f-h) EDS element mapping of (e). The Si precipitates exhibit 10-20 μm in length and 3-5 μm in width.

Fig. 10. Microstructure characterizations of the eutectic Si phase of the 0.4 wt% GNSs@Cu/Al-Si composite foams. (a, b) OM and (c-e) SEM images. (f-h) EDS element mapping of (e). The Si precipitates exhibit 3-5 μm in average diameter.

Fig. 11. TEM images of the eutectic Si precipitates of the 0.4 wt% GNSs@Cu/Al-Si composite foams: (a) single 141° reentrant and (b) double 141° reentrants are obviously observed at the edges of Si precipitates.

Fig. 12. Mechanical properties of the foams. (a) Compressive stress-strain curves, (b) energy absorption capacity curves and (c) the trend and nonlinear fitting of compression stress, plateau stress and energy absorption capacity of the GNSs@Cu/Al-Si composite foams with different GNSs@Cu content.

Table 2 The parameters of Gaussian distribution curves for the compression stress, plateau stress and energy absorption capacity, respectively.

Parameters	The curve of compression stress	The curve of plateau stress	The curve of energy absorption capacity
X0	0.39	0.35	0.34
y0	-2.48	2.04	1,54
σ	0.46	0.31	0.30
A	15.82	6.03	4.54
R-square	0.9766	0.9681	0.9868

Fig. 13. Digital photographic images of the macro deformation process: (a-d) the Al-Si foams and (e-h) the 0.4 wt% GNSs@Cu/Al-Si composite foams.

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