Effect of Heat Treatment on the Microstructure and Mechanical Property of Vacuum Die-casting NZ30K Mg-alloy

doi:10.11901/1005.3093.2018.289

Chinese Journal of Materials Research

2019, Vol. 33

Issue (1): 1-8 DOI: 10.11901/1005.3093.2018.289

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Effect of Heat Treatment on the Microstructure and Mechanical Property of Vacuum Die-casting NZ30K Mg-alloy

Jie WEI¹,Qudong WANG^1,²(

),Bing YE^1,²,Haiyan JIANG^1,²,Wenjiang DING^1,²

1. National Engineering Research Center of Light Alloy Net Forming and Key State Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2. Shanghai Innovation Institute for Materials, Shanghai 200240, China

Cite this article:

Jie WEI,Qudong WANG,Bing YE,Haiyan JIANG,Wenjiang DING. Effect of Heat Treatment on the Microstructure and Mechanical Property of Vacuum Die-casting NZ30K Mg-alloy. Chinese Journal of Materials Research, 2019, 33(1): 1-8.

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Abstract

Effect of heat treatment on the microstructure and mechanical property of vacuum die-casting (VDC) NZ30K Mg-alloy were systematically investigated by means of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), hardness test and tensile test. The results show that the as-cast alloy is composed of a surface zone and a central region. Fine α-Mg matrix and Mg₁₂Nd eutectic compounds were observed in the surface zone and the central region, besides, coarser externally solidified crystals (ESCs) existed in the central region. During solution treatment the grain growth of the central region was more significant than that of the surface zone, which can be explained by the grain growth model of unhomogenized structure, i.e.v=M₀ exp (-Q/RT) A (1/D₁-1/D₂). The optimized heat treatment of the alloy was 540^oC×6 h+200^oC×8 h. Compared with the as-cast alloy, the ultimate tensile strength and yield strength of the peak-aged alloy enhanced from 186.0±1.5 MPa to 223.6±4.1 MPa and from 131±2.5 MPa to 172.8±2.9 MPa respectively, with a decreased elongation (from 6.6±0.4 % to 4.2±0.3%). The strength enhancement may be mainly attributed to the plate-shaped β" precipitates, which could block the dislocation motion effectively. The fractography of surface zone exhibited ductile fracture pattern at different states. However, the fractography of central region showed quasi-cleavage, cleavage and quasi-cleavage fracture patterns for the as-cast, as-solutioned and peak-aged alloys, respectively.

Key words: metallic materials NZ30K alloy solution and aging treatment microstructure grain growth model mechanical property

Received: 24 April 2018

ZTFLH:

TG113

Fund: National Key Research and Development Program of China(2016YFB0301001);and the 111 Project(B16032)

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	Jie WEI
	Qudong WANG
	Bing YE
	Haiyan JIANG
	Wenjiang DING

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https://www.cjmr.org/EN/10.11901/1005.3093.2018.289 OR https://www.cjmr.org/EN/Y2019/V33/I1/1

Fig.1 Schematic illustrations of the tensile test bar and the microstructure observation plane

Table1 Process parameters of vacuum die casting

Table 2 Chemical composition of NZ30K alloy (mass frac-tion, %)

Fig.2 Macro-structure of as-cast alloy

Fig.3 SEM images of as-cast NZ30K alloy in (a) the surface layer and (b) the central region, with the inserted images exhibiting the corresponding grain size distribution and (c, d) giving the EDS analysis of Point 1 and Point 2

Fig.4 OM images of as-cast NZ30K alloy and after solid solution treatment at different temperature (a) as-cast, (b) 490℃×6 h, (c) 510℃×6 h, (d) 540℃×6 h

Fig.5 Curves of the grain size (a), the area friction of eutectic with different solution process (b)

Fig.6 Models of grain growth of the surface layer (a) and the central region (b), where P is the boundary driving force

Fig.7 Hardness evolution as a function of aging time during isothermal aging at 200℃ after different solution treatments (a) 510℃×6 h and (b) 540℃×6 h

Fig.8 SEM images of (a) the surface layer and (b) the central region of peak-aged NZ30K alloy

Fig.9 Mechanical properties (RT) of Vacuum die casting alloy at different heat treatment states

Fig.10 fractographies of as-cast state (a), as-solutioned state (b) and peak-aged state (c) alloys

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