A novel heat-resistant Al-Si-Cu-Ni-Mg base material synergistically strengthened by Ni-rich intermetallics and nano-AlNp microskeletons

doi:10.1016/j.jmst.2018.09.051

Abstract

Abstract:

In this work, nano-AlN particle (nano-AlN_p) microskeletons were introduced into an Al-Si-Cu-Ni-Mg alloy by Al-8AlN master alloy in which the nano-AlN_p reinforcements connect with each other to form three-dimensional networks. It is found that these nano-AlN_p microskeletons mainly distribute in the binary Al-Si eutectic zones resulting in flaky eutectic Si phases being modified to particulates. Meanwhile, the microskeletons strengthen the matrix synergistically with semi-continuous Ni-rich intermetallics in three dimensions. The tensile mechanical properties, micro-hardness and thermal expansion properties of the alloy at different temperatures are significantly improved. Especially, the ultimate tensile strength (UTS) at 350?°C increases from 85?MPa to 106?MPa, rising by 24.7%, which is ascribed to nano-AlN_p microskeletons assisting intermetallics with undertaking mechanical loading, and to the modification of eutectic Si phases to reduce the stress concentration at elevated temperatures.

Key words: Al-Si alloy, Heat-resistant Al alloy, Mechanical properties at high temperatures, Nano-AlN_p microskeletons

Kaiqi Hu, Qingfei Xu, Xia Ma, Qianqian Sun, Tong Gao, Xiangfa Liu. A novel heat-resistant Al-Si-Cu-Ni-Mg base material synergistically strengthened by Ni-rich intermetallics and nano-AlN_p microskeletons[J]. J. Mater. Sci. Technol., 2019, 35(3): 306-312.

Figures/Tables 10

Table 1 Chemical compositions of Al-Si-Cu-Ni-Mg alloy (wt%).

Si	Cu	Ni	Mg	Fe	Mn	Cr	P	Al
11.92	4.06	1.97	1.02	0.16	0.10	0.005	0.007	Bal.

Fig. 1. (a) Image of rod-like Al-8AlN master alloy, (b) SEI image of Al-8AlN master alloy, (c) SEI image of nano-AlNp extracted from Al-8AlN master alloy, showing three-dimensional morphologies and distribution of nano-AlNp and (d) XRD pattern of Al-8AlN master alloy.

Fig. 2. XRD patterns of B and BA alloys showing main phases in alloys.

Fig. 3. SEI images of B alloy showing semi-continuous Ni-rich intermetallics networks (a) and BA alloy showing distribution of nano-AlNp microskeletons (b).

Fig. 4. (a) Morphology and (b) EDS point analysis of modified eutectic Si phases and (c) EDS mapping results of networks of nano-AlNp microskeletons and Ni-rich intermetallics.

Fig. 5. (a) Bright field image of nano-AlNp microskeletons in BA alloy, (b) selected area electron diffraction pattern (SAED) showing diffraction rings of AlN and (c) HRTEM image indicating three connected AlN particles (the inset showing the diffraction pattern of the middle particle).

Table 2 Mechanical properties of B alloy and BA alloy at different temperatures.

Sample	Temperature (°C)	σ_b (MPa)	σ_s (MPa)	δ (%)
B alloy	25	255	240	0.5
	250	188	165	2
	300	157	146	3.5
	350	85	70	5
BA alloy	25	295	280	1
	250	205	185	1.2
	300	170	163	2.3
	350	106	90	3.5

Fig. 6. Engineering stress-strain curves of tensile test of B alloy and BA alloy at ambient temperature (a) and 350?°C (b), microhardness of B and BA alloy at different temperatures (c) and thermal expansion ratios of B and BA alloy with temperature increasing (d).

Fig. 7. Fractographs of tensile specimen at 350?°C of B alloy (a, b) and BA alloy (c, d) at low (a, c) and high (b, d) magnification, showing coarse Si phases and propagated cracks and distribution of nano-AlNp microskeletons, respectively (the inset exhibits the 3D morphology of nano-AlNp in BA alloy).

Fig. 8. (a) SEM image of BA alloy, (b, c) extracted Ni-rich intermetallics and nano-AlNp microskeletons from (a), with area fractions 5.6% and 3.2% calculated by Image J, (d, e) SEM images of deep etched BA alloy revealing three-dimensional networks of Ni-rich intermetallics and nano-AlNp microskeletons, respectively, (f, g) EDS analyses of points in (d) and (e).

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