Microalloying Effect of Sn on Phase Transformation During Heat Treatment in Mg-Y-Zn-Zr Alloys

doi:10.1007/s40195-018-0826-1

Abstract

Abstract:

The microstructure and mechanical properties of the as-cast and heat-treated Mg-4.6Y-2.5Zn-0.6Zr-xSn (x = 0, 0.2 and 0.5 wt%) alloys were investigated in this work. The results showed that the eutectics have been refined with 0.2% Sn addition and it has no effect on the phase category of the alloys. However, Sn₃Y₅ phase was found in 0.5% Sn-added alloy. After heat treatment at 520°C, the transformation of the long-period stacking ordered (LPSO) phase takes place in the Mg-Y-Zn-Zr alloy, but the transition is not completed in the alloys containing Sn. In addition, during the heat treatment, the mechanical properties of Sn-free alloys are significantly improved, and the strength of alloys containing Sn does not change much. Through observation and analysis of the microstructure and mechanical properties, it is found that Sn addition hinders the process of α′-Mg→α-Mg+14H and the process is the key to the transition of 18H-LPSO to 14H-LPSO.

Key words: Long-period stacking ordered (LPSO) phase, Microstructure, Heat treatment, Phase transformation

Pei-Lin Zhang, Yu-Hong Zhao, Ruo-Peng Lu, Zhi-Bing Ding, Hua Hou. Microalloying Effect of Sn on Phase Transformation During Heat Treatment in Mg-Y-Zn-Zr Alloys[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(5): 550-558.

Figures/Tables 11

Table 1 Chemical compositions of studied alloys

Alloy	Composition (wt%)
A	Mg-4.6Y-2.5Zn-0.6Zr
B	Mg-4.6Y-2.5Zn-0.6Zr-0.2Sn
C	Mg-4.6Y-2.5Zn-0.6Zr-0.5Sn

Fig. 1 Optical microstructures of a-c as-cast and d-f solid solution treated a, d Mg-4.6Y-2.5Zn-0.6Zr, b, e Mg-4.6Y-2.5Zn-0.6Zr-0.2Sn, c, f Mg-4.6Y-2.5Zn-0.6Zr-0.5Sn alloys

Fig. 2 SDAS of as-cast alloys

Fig. 3 XRD patterns of a as-cast and b solid solution-treated alloys

Fig. 4 SEM micrographs of a-c as-cast and d-f solid solution treated a, d Mg-4.6Y-2.5Zn-0.6Zr, b, e Mg-4.6Y-2.5Zn-0.6Zr-0.2Sn, c, f Mg-4.6Y-2.5Zn-0.6Zr-0.5Sn alloys

Table 2 EDS analysis (at.%) of typical positions in Fig. 4

Location (point)	Mg	Y	Zn	Zr	Sn
Figure 4a, A	87.99	4.31	7.30	0.41	-
Figure 4a, B	91.34	4.01	4.27	0.39	-
Figure 4b, A	92.88	3.20	3.49	0.38	0.08
Figure 4b, B	84.45	5.34	9.79	0.34	0.08
Figure 4c, A	93.95	2.78	2.86	0.32	0.09
Figure 4c, B	91.92	3.27	4.25	0.43	0.14
Figure 4c, C	47.95	28.62	1.78	3.26	18.39
Figure 4d, A	90.72	4.43	4.49	0.36	-
Figure 4e, A	83.35	14.14	1.53	0.80	0.19
Figure 4e, B	79.28	6.94	13.15	0.48	0.16
Figure 4f, A	92.63	3.24	3.77	0.25	0.11
Figure 4f, B	91.88	3.34	4.43	0.39	0.06
Figure 4f, C	60.23	14.04	5.00	9.45	11.48

Fig. 5 Bright-field TEM image and corresponding SAED pattern of 14H-LPSO phase in alloy A after heat treatment at 520 °C

Fig. 6 Optical microstructure of alloy C after heat treatment at 520 °C for 20 h

Table 3 Tensile properties of Mg-4.6Y-2.5Zn-0.6Zr-xSn alloys in different states tested at room temperature

State	Alloy	YS (MPa)	UTS (MPa)	Elongation (%)
As cast	A	107	197	12.9
	B	124	231	11.1
	C	123	206	8.6
As solution treated	A	130	237	13.9
	B	125	230	13.4
	C	126	221	10.7

Fig. 7 Typical stress-strain curves of a as-cast and b solid solution-treated Mg-4.6Y-2.5Sn-0.6Zr-xSn alloys tested at ambient temperature

Fig. 8 Evolution processes of phase with addition of Sn in as-cast alloys: a Alloy A, b Alloy B, c Alloy C

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