Effect of long-period stacking orderedphase on hot tearing susceptibility ofMg-Zn-xY alloys
Ye Zhou & Pingli Mao
School of Materials Science and Engineering, Shenyang Universityof Technology, Shenyang 110870, China
Introduction
Hot tearing, also known as hot crack, has been widely recognized as a major defect during solidification. It is a failure occurring in the mushy zone of a freezing alloy,
i.e. at the solid fraction of 0.9 < fs <0.99. Mg-Zn-Y ternary alloys have been attracted much attention due to its high strength performance. The content of Zinc (Zn),yttrium (Y), and Zn/ Y (mass fraction) ratio were regarded as the two most influential factors of Mg-Zn-Y ternary alloys hot tearing. The type of precipitated phase wasclosely related to the and Zn/ Y ratio, when Zn/ Y ratio > 1, Zn/ Y ratio ≈ 1, Zn/ Y ratio < 1, the precipitated phase of the alloys are I phase (or I phase and W phase), Wphase (or W phase and small amount of I phase), long-period stacking ordered (LOPS) phase (or small amount of W phase), respectively. Few studies have focused on
hot tearing of LPSO phase influential. Therefore, effect of long-period stacking ordered phase on hot tearing susceptibility of Mg-Zn-xY alloys and the influence of m (Zn)
/ m (Y) ratio on Mg-Zn-Y system alloys were investigated in the present work.

Experimental Procedures
MaterialResults(d)Mg-2.5Zn-4Y, and Mg-4.5Zn-6Y
Part 2. Mg-2.5Zn-4Y and Mg4.5Zn-6Y alloys (m(Zn)/m(Y) are
0.625 and 0.75, respectively)

Part 1. Mg-1ZnxY (x=1, 2, 3) alloys

Mg-1Zn-xY (x=1, 2, 3);

α-Mg

(d)
W PhaseText technology

Hot crack

Hot tearing text system(e)(e)
W PhaseHot crack

LPSO Phase(f)(f)LPSO Phase

Fig. 1 Schematic of experimental setup

Fig. 8 The cooling and force curves of casting Mg2.5Zn-4Y (a) and Mg-4.5Zn-6Y (b).

Thermal analysis system
(b)

10 μm
Fig. 3 Thermal analysis results of Mg-1Zn-xY alloys:(a) x=1, (b) x=2, (c) x=3; The corresponding
microstructure of Mg-1Zn-xY alloys: (d) x=1, (e) x=2,(f) x=3, marked with W phase and LPSO phase.

(a)Fig. 6 The contraction force curves
and microscopic samples of Mg1Zn-xY alloys ( (a) and (d) x=1, (b)and (e) x=2, (c) and (f) x=3).
(a)Fig. 2 Schematic diagram of thermal
analysis setup

Microstructure observation
XRD, SEM, EBSD

Fig. 4 The α-Mg, LPSO
phase, and W phasereaction temperature ofMg-1Zn-xY alloys.

(d)

1
2

100 μm(c)100 μm(d)Refilling

In the present study, The main
conclusions can be summarized asfollows:(1) The LPSO phase could reduce theHTS of Mg-Zn-xY (x= 1, 2, 3) alloys.
(2) When m (Zn) / m (Y) ＜ 1, LPSO
phase formed, and the content of LPSO
phase increased with increasing of Y
element for Mg-Zn-xY (x= 1, 2, 3)alloys.
(3) LPSO phase benefited to the liquidflow with certain strength, and increased
refilling traces at later stage ofsolidification.
(4) High m (Zn) / m (Y) exhibited low
HTS for Mg-2.5Zn-4Y and Mg-4.5Zn6Y alloys, and LPSO phase exists as astrengthening phase in Mg-Zn-Y alloys.

(c)Torn Bridge (b) Torn Liquid Flow

Tear

TearConclusions

HotTearing
Indicator(b)Refilling
100 μm

100 μmFig. 7 Fracture surface of hot tearing
and typical microstructure near hot
crack region of Mg-1Zn-1Y alloys(a) and (c) and Mg-1Zn-2Y (b) and(d).

Fig. 9 The crack volumes, microscopic samples andcorresponding numerical simulation results of Mg2.5Zn-4Y (m (Zn) / m (Y) is 0.625) and Mg-4.5Zn-6Y(m (Zn) / m (Y) is 0.75) alloys.

Table 1 The EDS results of point
1and 2 of Fig. 6 in atom percentage.

Fig. 5 EBSD maps of casting Mg-1Zn-1Y (a) and Mg1Zn-2Y (b) alloys.

PointMg

Zn

199.24

0.76

Y–278.45

16.90

4.65

Fig. 10 Microhardness of Mg-4.5Zn-6Y alloy on matrixand LPSO phase.

Acknowledgement
This project was supported by National Natural Sciences Foundation of China (NO.51571145), and Shenyang Scienceand Technology plan 2017 project: Key Technology for Automobile Lightweight and Development of New Aluminumand Magnesium Materials and Production (17-9-6-00).