Altered age-hardening behavior in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

doi:10.1016/j.jmst.2020.11.017

Abstract

Abstract:

To gain insight into the ageing behavior of ultrafine grain (UFG) structure, the precipitation phenomena and microstructural evolutions of Mg-6Zn-1Y-0.4Ce-0.5 Zr (wt.%) alloy processed by sliding friction treatment (SFT) were systematically studied using hardness texting, transmission electron microscopy (TEM) equipped with high-angle annular dark-field scanning (HADDF-STEM), X-ray diffraction (XRD) and XRD line broadening analysis. The microhardness of the SFT-processed (SFTed) sample initially decreases from 109.6 HV to 104.8 HV at ageing for 8 h, and then increases to the peak-ageing point of 115.4 HV at 16 h. Subsequently, it enters the over-aged period. The un-SFTed sample, as the counterpart, follows a regular ageing behavior that increases from 89.9 HV to 99.6 HV when ageing for 12 h, and then drops. A multi-mechanistic model is established to describe the strengthening due to grain refinement, dislocation accumulation, precipitation etc. The analysis reveals that the temperature sensitive UFG structure has an obvious grain coarsening effect, which arouses the soft phenomenon in the early ageing stage. But precipitation hardening provides an excellent hardness enhancement for overcoming the negative influence and helping to reach the peak-aged point. In our microstructural observations, a lot of equilibrium ultrafine MgZn2 precipitates precipitate along dislocations because defects can provide the favorable conditions for the migration and segregation of solute atoms.

Key words: Ageing behavior, Ultrafine-crystalline grain, Nano precipitates, Sliding friction treatment

Chunquan Liu, Xianhua Chen, Yuan Yuan, Wei Zhang, Yusheng Zhang, Fusheng Pan. Altered age-hardening behavior in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment[J]. J. Mater. Sci. Technol., 2021, 78: 20-29.

Figures/Tables 13

Fig. 1. Age-hardening curves of the SFTed layer and un-SFTed layer.

Fig. 2. TEM observations before the ageing treatment: (a, c) the SFTed layer; (b, d) the un-SFTed layer.

Fig. 3. (a) Bright-field TEM image of the SFTed layer aged for 8 h, (b) the dark-field from the yellow frame in (a), (c) HAADF-STEM image of the SFTed layer aged for 8 h, (d) bright-field TEM image of the SFTed layer aged for 16 h, (e) the dark-field from the yellow frame in (d), (f) HAADF-STEM image of the SFTed layer aged for 16 h.

Fig. 4. (a) Bright-field TEM image of the precipitates inside a grain, (b) corresponding SAED patterns, (c) HRTEM image of framed area c in (a), (d) HRTEM image of interface between Mg3Zn3Y2 phase and Mg matrix.

Fig. 5. (a-c) Bright and dark-field TEM image of the un-SFTed layer aged 12 h, (d) magnified image from the yellow frame in (a), (e) the dark-field image of (d), (f) HAADF-STEM image of the DRXed area in un-SFTed layer aged 12 h.

Fig. 6. (a, d) Bright-field and dark-field TEM micrographs of the precipitates near the dislocations, (b, e) magnified images of the yellow frames in (a) and (d), (c) HRTEM image of the precipitates near the dislocation, (f) the magnified image of the frame in (c) and corresponding FFT patterns.

Fig. 7. (a) Bright-field TEM micrographs of SFTed layer aged for 60 h, (b) HAADF-STEM image of the SFTed area aged for 60 h, (c, d) bright-field TEM micrographs of un-SFTed layer aged for 60 h.

Table 1 Summary of position, average size, inter-particle spacing of secondary phase and corresponding contribution for hardness at varies ageing states.

Sample ID	Position	Average size (nm)	Inter-particle spacing (nm)	Contribution (HV)
SFTed-0 h	Intragranular	22.1	44.5	17.6
SFTed-8 h	Intragranular	25.3	30.2	26.9
SFTed-16 h	Intragranular	40.6	25.2	36.4
SFTed-60 h	Intragranular	42.5	24.3	38.1
un-SFTed-0 h	unDRXed area	89.6	185.4	22.6
un-SFTed-0 h	DRXed area	26.4	49.1	22.6
un-SFTed-12 h	unDRXed area	103.2	155.8	34.8
un-SFTed-12 h	DRXed area	29.5	30.6	34.8
un-SFTed-60 h	unDRXed area	106.3	152.3	36.7
un-SFTed-60 h	DRXed area	32.1	29.4	36.7

Fig. 8. Overview of XRD patterns aged at different times: (a) the un-SFTed layer; (b) the SFTed layer.

Table 2 The crystallite size, dislocation density and corresponding contribution for hardness at different ageing states.

Samples	Grain size, d (μm)	Contribution (HV)	ρ (×10¹⁴ m^-2)	Contribution (HV)
SFTed-0 h	0.192	27.9	3.31	24.1
SFTed-8 h	0.739	14.2	2.71	21.8
SFTed-16 h	0.848	13.3	2.32	20.2
SFTed-60 h	1.412	10.3	1.26	14.9
un-SFTed-0 h	2.361	7.9	3.62	25.2
un-SFTed-12 h	3.156	6.9	3.24	23.8
un-SFTed-60 h	3.564	6.5	1.31	15.2

Fig. 9. The contribution of each mechanism to hardness in un-SFTed layer under different ageing states.

Fig. 10. The contribution of each mechanism to hardness in SFTed layer under different ageing states.

Fig. 11. Comparison between calculated hardness and measured hardness. (a) un-SFTed layer; (b) SFTed layer.

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