Temperature dependence of mechanical strength in HPDC Mg–6Y–3Zn–1Al alloy with LPSO phase

Xin Yu a, Yafeng Li b, Yang Bai a, Wei Huang c, Bing Ye a, Xiangyang Kong d


The temperature dependence of mechanical strength including yield strength (YS) and ultimate tensile strength (UTS) in HPDC WZA631 alloy is investigated in a wide temperature range from room temperature (RT) to 350 °C. It is found that at 25–300 °C, YS and UTS do not drop markedly, from 173 MPa and 274 MPa at RT to 113 MPa and 170 MPa at 300 °C, respectively. While above 300 °C the flow stress falls rather rapidly, to 74 MPa for YS and to 108 MPa UTS at 350 °C. The (Al,Zn)2Y phase exhibits similar behavior between 150 °C and 350 °C, breaking up under stress and becoming the relatively stable crack source without catastrophic cracking. The LPSO phase, keeping steady and excellent critical resolved shear stress (CRSS) below 250 °C, remains unchanged with complete network structure at 150 °C and 250 °C. Therefore, the LPSO phase reinforces the alloy with slow flow stress reduction at RT-300 °C for WZA631 alloy. The tensile property deteriorates above 300 °C due to the destruction of the network structure as well as the reduction of CRSS for the prismatic slip of LPSO under tensile loading. Thus, the LPSO phase determines the mechanical strength evolution of WZA631 alloy, which is similar to the behavior of γ’ phase in the Ni-based superalloys. Besides, the relationship between strength and temperature is described by the Arrhenius equation, which may be reduced to an exponential model in the form of �=�+�� for HPDC WZA631 alloy.


Magnesium alloys are widely used in automobile, aerospace and biomedical areas due to advantages of low density, high specific strength and high specific stiffness [1,2]. Among the magnesium alloys with various preparation technologies, the high-pressure die-casting (HPDC) alloys are commonly-used owing to the merits of higher accuracy, excellent productivity and better economic benefit, which have great potential in weight saving for automobile [3]. However, only a small number of high-temperature components with the service condition below 175 °C are made of HPDC magnesium alloys [4]. For components such as engine block with working temperature around 200 °C or above, no HPDC Mg alloy is qualified. Up to now, the HPDC AE44 alloy has the best comprehensive properties among the existing commercial heat-resistant HPDC Mg alloys [5], but nevertheless, its service temperature is below 175 °C due to the deterioration of creep property at higher temperatures [6].

In recent years, magnesium alloys with long-period stacking ordered (LPSO) phase have received great attention due to superior mechanical properties and excellent microstructure stability [7]. Kawamura et al. reported that hot extruded Mg97Zn1Y2 alloy prepared by rapidly solidified powder metallurgy (RS P/M) exhibited high tensile strength at elevated temperature due to the dispersed nano-scale LPSO phase [8,9]. They also found similar results for Mg97Zn1RE2 alloys with LPSO phase where RE was other rare earth element [10]. Besides, the excellent microstructure stability is widely reported [10]. Bai et al. reported that LPSO phase in gravity casting Mg–Y–Zn alloy was coarse, and hence the tensile property was not remarkable [11]. Thus, it was the fine LPSO, rather than the coarse LPSO structure, that may improve the comprehensive properties of Mg alloys.

The HPDC process, which provides high cooling rate, is able to refine the LPSO phase and may further improve the contribution of LPSO to tensile property. However, the LPSO phase has been hardly reported in HPDC Mg alloys. Our group developed a novel HPDC Mg–6Y–3Zn–1Al (WZA631) alloy containing LPSO phase, which exhibited excellent tensile properties both at ambient and 200 °C [12,13]. The yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of the studied alloy at room temperature (RT) are 175 MPa, 281 MPa and 9.8 %, which is better compared with existing commercial HPDC Mg alloys [12]. At 200 °C, the alloy exhibits the highest UTS around 229 MPa among HPDC Mg alloys reported so far, even competing with A380 aluminum alloy. Besides, the minimum creep rate is 1.76 × 10−10 s−1 at 200 °C/70 MPa, which is one order of magnitude smaller than the best commercial HPDC AE44 alloy [13]. Yu et al. showed that the HPDC WZA631 alloy exhibited excellent microstructure thermal stability up to 500 °C through the grain growth kinetics study without external loading [14]. The grain size hardly changed up to 450 °C and the coarsening rate constant at 450–500 °C was around 2–4 orders lower than other typical Mg alloys [14]. Although HPDC WZA631 alloy shows superior mechanical performance at RT and 200 °C, it is still necessary to systematically explore the tensile properties at other temperatures. The investigation of strength evolution with temperature is fundamental and essential for the potential application at high temperatures, which has not been reported so far.

In the present work, the temperature dependence of mechanical strength in HPDC WZA631 alloy is investigated, which may provide more data for the exploration and application of HPDC WZA631 alloy.

Section snippets

Experimental procedure

The WZA631 alloy was prepared from pure Mg (99.9 %), pure Zn (99.9 %), pure Al (99.9 %) and Mg–30Y wt% master alloy by a solvent protection method. Pure Mg and Mg–30Y master alloy were melted at 700 °C in a graphite crucible before pure Zn and Al were added. The melt was refined at 740 °C and statically held for 20 min before the melt was poured into a 250-ton chamber die-casting machine. The HPDC trial was conducted at casting temperature of 740 °C, mold temperature of 200 °C, shot plunger

Tensile properties

The HPDC WZA631 alloy exhibits a fine equiaxed microstructure with the major intermetallic LPSO phase and minor (Al,Zn)2Y phase shown in Fig. 2. The LPSO phase formed a continues network structure along the grain boundaries and the (Al,Zn)2Y exhibited a block or an irregular shape. The details of LPSO and (Al,Zn)2Y phase identification were reported in previous work [12]. Fig. 3 shows the tensile properties of WZA631 alloy at ambient temperature and elevated temperatures. It shows in Fig. 3(a)


The temperature dependence of mechanical strength in HPDC LPSO-containing WZA631 alloy is investigated in a wide temperature range from RT to 350 °C. Based on the experimental results and model analysis, the conclusions obtained are summarized as follows.

  • (1)The curve of strength against temperature is divided into two stages. At 25–300 °C, the YS and UTS are relatively stable, from 173 MPa and 274 MPa at RT to 113 MPa and 170 MPa at 300 °C, respectively. While above 300 °C the flow stress falls

CRediT authorship contribution statement

Xin Yu: Data curation, Investigation, Writing – original draft. Yafeng Li: Investigation, Validation. Yang Bai: Resources, Validation. Wei Huang: Supervision. Bing Ye: Supervision, Validation, Writing – review & editing. Xiangyang Kong: Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


This work was supported by the National Key Research and Development Program of China [grant number 2016YFB0301001].

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