Effects of SiC Nanoparticle Content on the Microstructure and Tensile Mechanical Properties of Ultrafine Grained AA6063-SiCnp Nanocomposites Fabricated by Powder Metallurgy

doi:10.1016/j.jmst.2016.09.022

Abstract

Abstract:

Ultrafine grained AA6063-SiC_np nanocomposites with 1, 5 and 10 vol.%SiC_np have been fabricated by a novel powder metallurgy process. This process combines high energy ball milling of a mixture of 6063 alloy granules made from machining chips and SiC nanoparticles and thermomechanical powder consolidation by spark plasma sintering and hot extrusion. The microstructure and tensile mechanical properties of the samples were investigated in detail. Increasing the SiC nanoparticle content from 1 to 10 vol.%, the yield strength and ultimate tensile strength increased from 296 and 343 MPa to 545 and 603 MPa respectively, and the elongation to fracture decreased from 10.0%, to 2.3%. As expected, a higher SiC nanoparticle content generates a stronger inhibiting effect to grain growth during the thermomechanical powder consolidation process. Analysis of the contributions of various strengthening mechanisms shows that a higher SiC nanoparticle content leads to a higher contribution from nanoparticle strengthening, but grain boundary strengthening still makes the largest contribution to the strength of the nanocomposite. When the SiC nanoparticle content increased to 10 vol.%, the failure of the nanocomposite was initiated at weakly-bonded interparticle boundaries (IPBs), indicating that with a high flow stress during tensile deformation, the failure of the material is more sensitive to the presence of weakly-bonded IPBs.

Key words: Metal matrix nanocomposite, Ultrafine grained material, Powder metallurgy, Tensile mechanical properties

Yao X., Zhang Z., Zheng Y.F., Kong C., Quadir M.Z., Liang J.M., Chen Y.H., Munroe P., Zhang D.L.. Effects of SiC Nanoparticle Content on the Microstructure and Tensile Mechanical Properties of Ultrafine Grained AA6063-SiC_np Nanocomposites Fabricated by Powder Metallurgy[J]. J. Mater. Sci. Technol., 2017, 33(9): 1023-1030.

Figures/Tables 17

Fig. 1. (a) Morphology of SiC nanoparticles, (b) morphology of AA6063 granules made by crushing recycled AA6063 machining chips and (c) particle size distributions of the three as-milled powders.

Table 1 Fractions of AA6063 granules and SiC nanoparticles in the mixture used for making the AA6063-(1, 5, 10) vol.%SiCnp nanocomposite powders by HEMM (wt%)

Composition	6063 granules	SiC nanoparticles
AA6063-1 vol.%SiC_np	98.8	1.2
AA6063-5 vol.%SiC_np	94.2	5.8
AA6063-10 vol.%SiC_np	88.4	11.6

Fig. 2. SEM images showing the morphology and sizes of as-milled (a) AA6063-1 vol.%SiCnp, (b) AA6063-5 vol.%SiCnp and (c) AA6063-10 vol.%SiCnp nanocomposite powder particles.

Fig. 3. XRD patterns of (a) as-milled nanostructured AA6063-(1, 5, 10)vol.%SiCnp nanocomposite powders and (b) as-extruded 61SNC, 65SNC and 610SNC rods.

Table 2 Characteristics of as-milled nanostructured AA6063-(1, 5, 10) vol.%SiCnp nanocomposite powders

Powder	Average size, d, (μm)	Size range (μm)	Crystallite size (μm)	Lattice strain (%)
AA6063-1 vol.%SiC_np	195.5	15.9-796.2	53 ± 5	0.1 ± 0.01
AA6063-5 vol.%SiC_np	37.8	1.6-316.9	83 ± 15	0.19 ± 0.02
AA6063-10 vol.%SiC_np	23.5	0.7-70.9	94 ± 12	0.24 ± 0.01

Fig. 4. SEM backscattered electron images showing the sizes and morphology of SiC nanoparticles in (a) 61SNC, (b) 65SNC and (c) 610SNC rods.

Fig. 5. Size distributions of SiC nanoparticles in the as-extruded samples.

Table 3 Average SiC nanoparticle sizes and distribution ranges in as-extruded 61SNC, 65SNC and 610SNC rods

Rod	SiC nanoparticle size (nm)
61SNC	88 ± 32
65SNC	91 ± 34
610SNC	90 ± 37

Fig. 6. TEM (a, c, e) BF images and (b, d, f) DF images of (a, b) 61SNC, (c, d) 65SNC and (e, f) 610SNC rods, respectively. The white and yellow arrows indicate the SiC nanoparticles at the grain boundaries and inside the grains, respectively.

Fig. 7. SAED patterns of (a) 61SNC, (b) 65SNC and (c) 610SNC rods. Yellow and grey circles indicate the spots of Al and SiC, respectively.

Fig. 8. Grain size distributions of (a) 61SNC, (b) 65SNC and (c) 610SNC rods.

Fig. 9. Tensile engineering stress-strain curves of 61SNC, 65SNC and 610SNC rods.

Table 4 Tensile properties of as-extruded 61SNC, 65SNC and 610SNC rods

Rod	Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation (%)
61SNC	296 ± 10	445 ± 16	545 ± 13
65SNC	343 ± 16	496 ± 16	603 ± 18
610SNC	10.0 ± 1.4	4.4 ± 3.1	2.3 ± 0.7

Fig. 10. SEM images of the fracture surfaces of specimens cut from (a) 61SNC, (b) 65SNC and (c) 610SNC rods, and tested in tension.

Fig. 11. SEM images of the longitudinal sections below the fracture surfaces of specimens cut from (a) 61SNC, (b) 65SNC and (c) 610SNC rods, and tested in tension. The inset in (b) is the corresponding O element EDS map. The grey circle in (c) shows one of the particle boundaries.

Table 5 Parameters utilized in the calculation of the contributions of strengthening mechanisms

Rod	Average grain size, D (nm)	Particle size, d (nm)	Interparticle distance, λ (nm)	Dislocation density, ρ (m^-2)
61SNC	410.6	87.8	460.9	4.7 × 10¹³
65SNC	284.0	91	267.3	2.2 × 10¹⁴
610SNC	188.3	90.2	211.7	3.4 × 10¹⁴

Table 6 Calculated contributions of different strengthening mechanisms for different rods

Rod	Grain boundary strengthening (MPa)	Nanoparticle strengthening (MPa)	Dislocation strengthening (MPa)
61SNC	134	56	31
65SNC	161	113	66
610SNC	198	157	84

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