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Acta Metall Sin  2016, Vol. 52 Issue (5): 567-574    DOI: 10.11900/0412.1961.2015.00333
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Wenhui WANG1,2,Di WU1(),Rongshi CHEN1,Changsheng LOU2
1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China
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Among the traditional cast magnesium alloys system, Mg-3Nd-1Zn alloy with high strength and heat resistance, has been widely applied in aeronautics such as in engine box and wing rib of airplane. In present research, the as-cast Mg-2.7Nd-0.6Zn-0.5Zr (NZ31) alloy was solution treated and then aged at temperatures ranging from 200 ℃ to 300 ℃. The microstructures and mechanical properties of the aged specimens especially at relatively high temperature (225~300 ℃) were systematically characterized by OM, SEM and TEM. A new kind of precipitates distributed in line was clearly found in the specimens aged at high temperature (225~275 ℃) for a short time (15~30 min), which corresponded to a significant enhancement of hardness and tensile strength at room temperature. The TEM results showed that the precipitates distributed in line had a composition of Mg12Nd and a granular shape, and mainly formed along the (0001)Mg basal plane. The special distribution of the granular Mg12Nd precipitates was effective barriers to the basal slips and may probably restrain the intergranular coordination during tensile deformation, leading to a large strengthening effect, that is, the yield strength and ultimate tensile strength, of the NZ31 alloy aged at 250~275 ℃ for 20~30 min, increased by about 70% and 29% respectively, in comparison with only the solutionized one.

Key words:  magnesium alloy      ageing treatment      precipitate      mechanical property     
Received:  26 June 2015     
Fund: Supported by National Natural Science Foundation of China (Nos.51301173 and 51531002) and National Basic Research Program of China (No.2013CB632202)

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Fig.1  Hardness evolution of the solutionized NZ31 magnesium alloy as a function of ageing time during isothermal ageing at 200 ℃ (a) and TEM image of NZ31 magnesium alloy after ageing at 200 ℃ for 12 h (Inset shows the SAED patter) (b)
Fig.2  Hardness evolution of the solutionized NZ31 magnesium alloy as a function of ageing temperature during equal time ageing for 30 min
Fig.3  OM (a~c, e, f) and SEM (d) images of NZ31 magnesium alloy in T4 condition (a) and ageing at 225 ℃ (b), 250 ℃ (c, d), 275 ℃ (e) and 300 ℃ (f) for 30 min (Thin straight lines in Figs.3b~e show precipitates distribution)
Fig.4  Hardness evolution of the solutionized NZ31 magnesium alloy as a function of ageing time during isothermal ageing at 275 ℃
Fig.5  OM images of the solutionized NZ31 magnesium alloy during isothermal ageing at 275 ℃ for 10 min (a), 20 min (b), 1 h (c) and 10 h (d)
Fig.6  Mechanical properties of the NZ31 magnesium alloy at room temperature in different conditions: solution treated (T4), ageing at 200 ℃ for 12 h (T6), ageing at 250 ℃for 30 min and ageing at 275 ℃ for 20 min after solution treatment
Fig.7  SEM image (a), TEM bright field image (b) and TEM dark field images (c, d) of precipitates after ageing at 275 ℃ for 20 min (Insets in Figs.7c and d show the SAED patterns of precipitates and Mg matrix taken with beam parallel to [011ˉ0]Mg zone axes)
Fig.8  SAED patterns show the precipitates of the NZ31 magnesium alloy aged at 275 ℃ for 20 min taken with beam parallel to [0001]Mg (a) and [112ˉ0]Mg (b) zone axes
Composition Mg Nd Zn Zr
Mass fraction / % 60.85 32.59 6.30 0.25
Atomic fraction / % 88.51 7.99 3.41 0.10
Error / % 0 0.01 0.04 1.64
Table 1  EDS results of the precipitate in NZ31 magnesium alloy aged at 275 ℃ for 20 min
Fig.9  Strength functional mechanism of precipitates in NZ31 magnesium alloy aged at high temperature for a short time
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