Effects of Gd Addition on the Microstructure and Tensile Properties of Mg-4Al-5RE Alloy Produced by Three Different Casting Methods

doi:10.1007/s40195-021-01276-4

Abstract

Abstract:

This work deals with the effect of 0.67 wt% Gd addition on the microstructure and tensile properties of Mg-4Al-5RE (where RE represents La-Ce mischmetal) alloy produced by sand casting (SC), permanent mold casting (PMC), and high-pressure die casting (HPDC). The results show that Gd addition could refine the grains, but its efficiency decreases by increasing the cooling rate due to the shifting from SC to PMC and finally to the HPDC method. Meanwhile, the acicular Al₁₁RE₃ phase is modified into the short-rod or granular-like shape under the three casting conditions. Such refined and modified microstructures are due to the Al₂(Gd, RE) phases, which act as the nucleation sites in both the α-Mg matrix and Al₁₁RE₃ phase. Also, the weakening grain refinement effect in the increased cooling rates can be attributed to the narrow constitutional undercooling zone. After Gd addition, the 0.2% proof strength of the SC and PMC alloys increases by about 16.9% and 12.7%, respectively, while in the HPDC alloy, it decreases by about 5.9%. The main factor in the strength increment of the SC and PMC alloys is the grain boundary strengthening due to grain refinement which is proved by modeling the related mechanisms, whereas weak secondary phases and grain boundary strengthening mechanisms in the HPDC alloy lead to strength reduction. After Gd addition, the elongation to failure of the SC, PMC, and HPDC alloys is significantly enhanced by about 34.8%, 20.2%, and 12.3%, respectively, due to the crack resistance nature of the modified short-rod/granular Al₁₁(RE, Gd)₃ phase compared to the acicular one.

Key words: Magnesium alloy, Gadolinium, Grain refinement, Strengthening mechanism

Jie Wei, Qudong Wang, Li Zhang, Mahmoud Ebrahimi, Haiyan Jiang, Wenjiang Ding. Effects of Gd Addition on the Microstructure and Tensile Properties of Mg-4Al-5RE Alloy Produced by Three Different Casting Methods[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(10): 1361-1374.

Figures/Tables 19

Table 1 Chemical composition of the studied alloys (wt%)

	Al	La	Ce	Mn	Gd	Mg
Mg-4Al-5RE	3.936	2.607	2.967	0.219	-	Bal.
Mg-4Al-5RE-0.7Gd	3.745	2.311	2.707	0.225	0.667	Bal.

Fig. 1 Determination of average cooling rate: a cooling curves of the Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys during the SC, PMC, and HPDC processes, b methodology to determine the critical solidification characteristics. Note that the data for SC and PMC processes were acquired by experiment method, while for the HPDC case, they were by simulation

Table 2 Average cooling rate (°C/s) of the studied alloys during the three casting processes

	SC	PMC	HPDC
Mg-4Al-5RE	1.50	7.39	102.4
Mg-4Al-5RE-0.7Gd	0.95	7.04	101.2

Fig. 2 Grain characteristics of a-c Mg-4Al-5RE and d-f Mg-4Al-5RE-0.7Gd alloys produced by a, d SC, b, e PMC, c, f HPDC methods. Note that OM was used to observe the grains for the SC and PMC alloys, while EBSD was used for the HPDC alloys due to the fine grains

Fig. 3 Grain size of Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys produced by the three different casting methods

Fig. 4 Optical images of a-c Mg-4Al-5RE and d-f Mg-4Al-5RE-0.7Gd alloys produced by a, d SC, b, e PMC, c, f HPDC methods. The insets are magnified images of the rectangle overlaid regions

Fig. 5 Area fraction of the secondary phases of Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys produced by three different casting methods

Fig. 6 SEM images of a-c Mg-4Al-5RE and d-f Mg-4Al-5RE-0.7Gd alloys produced by a, d SC, b, e PMC, c, f HPDC methods

Fig. 7 XRD patterns of a Mg-4Al-5RE, b Mg-4Al-5RE-0.7Gd alloys produced by three different casting methods

Table 3 EDS results of the areas indicated in Fig. 6 (at.%)

	Casting method	Area	Mg K	Al K	La L	Ce L	Gd L	Al/(La + Ce + Gd)
Mg-4Al-5RE	SC	1	0.231	0.606	0.052	0.111	*	3.715
	SC	2	0.104	0.602	0.080	0.214	*	2.043
	PMC	3	0.740	0.206	0.019	0.035	*	3.769
	PMC	4	0.563	0.296	0.038	0.103	*	2.099
	HPDC	5	0.702	0.236	0.026	0.036	*	3.806
	HPDC	6	0.478	0.349	0.041	0.132	*	2.017
Mg-4Al-5RE-0.7Gd	SC	7	0.198	0.631	0.045	0.115	0.011	3.690
	SC	8	0.497	0.332	0.028	0.11	0.033	1.942
	PMC	9	0.198	0.632	0.056	0.101	0.013	3.718
	PMC	10	0.197	0.531	0.022	0.094	0.156	1.952
	HPDC	11	0.465	0.421	0.029	0.055	0.03	3.693
	HPDC	12	0.527	0.316	0.053	0.049	0.055	2.013

Fig. 8 Identification of the ORs between Al2(Gd, RE) and Mg matrix: a bright-field TEM image of the PMC Mg-4Al-5RE-0.7Gd alloy, b selected area diffraction patterns taken at the interface

Fig. 9 Engineering stress-strain curves of the studied samples

Fig. 10 Tensile properties of a Mg-4Al-5RE, b Mg-4Al-5RE-0.7Gd alloys produced by three different casting methods

Fig. 11 Strain hardening rates of a Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys produced by three different casting methods, b enlarged stage III with the fitting curve. Note that the fitting curve is an average fitting result for all samples

Fig. 12 Schematic diagram representation regarding the effect of cooling rate and solute on the constitutional undercooling formation ahead of the solid/liquid interface

Table 4 Mean value for the elastic constants of Mg [37, 38] and Al11Ce3 [39]

	Shear modulus,$\mu$	Poisson’s ratio,$v$
Mg	17.2	0.35
Al₁₁Ce₃	47.2	0.21

Fig. 13 Contribution of different strengthening mechanisms to the 0.2% proof strength of a Mg-4Al-5RE, b Mg-4Al-5RE-0.7Gd alloys produced by three different casting methods

Fig. 14 Contribution of proof strength and strain hardening in increasing UTS of Mg-4Al-5RE-0.7Gd alloy produced by three different casting methods

Fig. 15 SEM images of the longitudinal direction close to the fracture zone of a-c Mg-4Al-5RE and d-f Mg-4Al-5RE-0.7Gd alloys produced by a, d SC, b, e PMC, c, f HPDC methods

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