Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy

doi:10.11900/0412.1961.2018.00053

Acta Metall Sin

2019, Vol. 55

Issue (2): 202-212 DOI: 10.11900/0412.1961.2018.00053

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Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy

Yan YANG, Guangyu YANG(

), Shifeng LUO, Lei XIAO, Wanqi JIE

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China

Cite this article:

Yan YANG, Guangyu YANG, Shifeng LUO, Lei XIAO, Wanqi JIE. Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy. Acta Metall Sin, 2019, 55(2): 202-212.

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Abstract

As one of the most promising heat-resistant magnesium alloys, Mg-Gd series alloy has a wide application prospect in the industrial fields of aerospace, cars, and rail transit. There have been extensive researches on the performance improvement of Mg-Gd series alloys. As known, dendrites are the common solidification microstructures of castings of magnesium alloys, and solidification conditions have a significant effect on dendrite morphologies and growth orientation, which could strongly affect the mechanical properties of castings, thus it is critical to study the grain growth regularity for predicting the performance of magnesium castings. However, there are few studies on numerical simulation of dendrite growth process and growth orientation of magnesium alloys. Solidification behavior of magnesium alloys can be scientifically studied via directional solidification technology, and cellular automaton finite element (CAFE) method should be effective to simulate the dendrite growth process of magnesium alloys. In present work, microstructures and growth orientation of directionally solidified Mg-14.61Gd alloy under the temperature gradient G=30 K/mm and the withdrawal rate v=10~200 μm/s were investigated by EBSD measurement method and CAFE numerical simulation method. It was found that α-Mg primary phase presented unidirectional dendritic morphologies on longitudinal cross-section. The growth interface appearance of α-Mg changed from the protruding forward growth to the flat growth gradually and the dendritic arm spacing decreased gradually with the increasing v. when v increased from 10 μm/s to 100 μm/s, the main growth orientation of α-Mg changed from <1120> and <1010> to <1120>, and the deviation angle (θ) from solidification heat flow direction reduced from 11.0° to 5.7°, the reason for this lied mainly in the change of the heat flux. Further increasing v up to 200 μm/s, the main growth direction of α-Mg was still in <1120>, but the value of θ increased to 10.6°, and the anisotropy of the crystal was the dominant factor then. It was proved that the CAFE numerical simulation model could predict the grain structure and growth orientation reasonably for Mg alloy.

Key words: Mg-14.61Gd alloy directional solidification EBSD CAFE model growth orientation

Received: 05 February 2018

ZTFLH:

TG113.1

Fund: Supported by National Natural Science Foundation of China (Nos.51771152 and 51227001), National Key Research and Development Program of China (No.2018YFB1106800) and Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (No.138-QP-2015)

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	Yan YANG
	Guangyu YANG
	Shifeng LUO
	Lei XIAO
	Wanqi JIE

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https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00053 OR https://www.ams.org.cn/EN/Y2019/V55/I2/202

Fig.1 Enmeshment schematic representation of the Bridgman directional solidification device and the sample model

Table 1 Boundary conditions of the Bridgman directional solidification device and the sample model

Table 2 Thermo-physical parameters of the Mg-14.61Gd alloy

Parameter	Symbol	Value	Unit
Critical value of average nucleation supercooling degree	$Δ T ˉ$	6	K
Total supercooling degree	ΔT	1	K
Maximum nucleation density	n_max	5×10⁷	m^-2
Fitting polynomial coefficient	a₂	8.145×10^-7	ms^-1K^-2
Fitting polynomial coefficient	a₃	5.871×10^-7	ms^-1K^-3

Table 3 Calculation parameters used in nucleation model and simplified KGT model

Fig.2 Cooling curves (a) and the solidification temperature gradients (b) of Mg-14.61Gd alloy directionally solidified under temperature gradient G=30 K/mm and withdrawal rate v=100 μm/s by presetting the different heat transfer coefficients (h)

Fig.3 XRD spectra of the directionally solidified Mg-14.61Gd alloy under G=30 K/mm at different v

Fig.4 Transverse (a, c, e) and longitudinal (b, d, f) OM images of the directionally solidified Mg-14.61Gd alloy under G=30 K/mm at v=10 μm/s (a, b), v=100 μm/s (c, d) and v=200 μm/s (e, f)

Fig.5 Microstructure (a), EBSD image (b), inverse pole figure of α-Mg phase (c) and pole figures of α-Mg phase (d) of the directionally solidified Mg-14.61Gd alloy under G=30 K/mm and v=10 μm/s

Fig.6 Microstructure (a), EBSD image (b), inverse pole figure of α-Mg phase (c) and pole figures of α-Mg phase (d) of the directionally solidified Mg-14.61Gd alloy under G=30 K/mm and v=100 μm/s

Fig.7 Microstructure (a), EBSD image (b), inverse pole figure of α-Mg phase (c) and pole figures of α-Mg phase (d) of the directionally solidified Mg-14.61Gd alloy under G=30 K/mm and v=200 μm/s

v	Growth orientation of	θ
μms^-1	α-Mg dendrite
10	$[11 2 ? 0]$ , $[10 1 ? 0]$	11.0°
100	$[11 2 ? 0]$	5.7°
200	$[11 2 ? 0]$	10.6°

Table 4 Growth orientations of α-Mg dendrites in the directionally solidified Mg-14.61Gd alloy under G=30 K/mm at the different v analyzed by EBSD measurement method

Fig.8 Microstructures (a~e) and <100> pole figures of α-Mg phase (f~j) of the directionally solidified Mg-14.61Gd alloy calculated by CAFE model under G=30 K/mm at v=10 μm/s (a, f), v=40 μm/s (b, g), v=100 μm/s (c, h), v=150 μm/s (d, i) and v=200 μm/s (e, j) (α—deviation angle between the dendrites growth orientation and the projection axis)

v	(φ₁, Φ, φ₂)	Growth orientation of	α
μms^-1		α-Mg dendrite
10	(22.6, 148.3, 145.1)	$[2 ? 11 1 ?]$	10.886°
	(283.9, 6.3, 284.8)	$[0 1 ? 10]$
40	(91.5, 73.8, 166.7)	$[1 ? 1 ? 20]$	8.016°
	(115.7, 141.9, 224.5)	$[3 ? 122]$
100	(261.9, 78.7, 277.4)	$[1 2 ? 10]$	6.709°
	(87.2, 158.3, 86.1)	$[1 2 ? 10]$
150	(275.7, 90.8, 273.8)	$[1 2 ? 10]$	8.835°
	(261.8, 78.7, 277.3)	$[1 2 ? 10]$
200	(277.8, 62.2, 95.8)	$[1 2 ? 10]$	10.167°
	(52.56, 8.6, 303.2)	$[1 2 ? 10]$

Table 5 Growth orientations of α-Mg dendrites in the directionally solidified Mg-14.61Gd alloy under G=30 K/mm at the different v calculated by CAFE model

Fig.9 Variation of the grain orientation deviation with v for Mg-14.61Gd alloy

Fig.10 Relationship among dendritic growth direction, preferred orientation and heat flow direction(a) v≤100 μm/s (b) v>100 μm/s

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