稀有金属(英文版) 2019,38(04),343-349

As-cast microstructure of Al-Zn-Mg-Cu-Zr alloy containing trace amount of Sc

Yu Wang Bai-Qing Xiong Zhi-Hui Li Shu-Hui Huang Kai Wen Xi-Wu Li Yong-An Zhang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

作者简介：*Zhi-Hui Li,e-mail:lzh@grinm.com;

收稿日期：14 August 2017

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFB0300903);

As-cast microstructure of Al-Zn-Mg-Cu-Zr alloy containing trace amount of Sc

Yu Wang Bai-Qing Xiong Zhi-Hui Li Shu-Hui Huang Kai Wen Xi-Wu Li Yong-An Zhang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

Abstract：

It has been reported that the element scandium(Sc) is the most effective modificator which can significantly refine the grain size, prohibit recrystallization process and increase the strength. Adding trace of Sc in 7000 series aluminum alloys is considered to be an effective way to modify its micros tructure and promote mechanical properties. In order to study the effect of Sc element on ascast microstructure of Al-Zn-Mg-Cu-Zr alloy, ingots containing different amounts of Sc were prepared by ferrous-mold cast. Microstructures were characterized by means of differential scanning calorimeter(DSC), X-ray diffraction(XRD), optical microscope(OM) and scanning electrical microscope(SEM). The results indicate that when the Sc level exceeds a critical concentration,Al₃(Sc,Zr) primary phase would form in the melt and act as an efficient nucleant, resulting in very refined grain and an equiaxed grain structure. Sc element reduces the number of eutectic phases formed during solidification,coupled with an increase in the concentration of major alloying elements retained in the solute. This behavior suggests possible benefits in improving the integrated properties of terminal products.

Keyword：

Scandium; Aluminum alloy; As-cast; Microstructure; Nucleant;

Received： 14 August 2017

1 Introduction

Super high strength Al-Zn-Mg-Cu alloys have been widely used as structural materials in aerospace industry [ 1, 2, 3, 4, 5, 6] .With the needs of aircraft to reduce weight and high manufacturing cost related to the fabrication of riveted or fastened airframe structures,the research and development of weldable aluminum alloys attract more and more attention in recent years [ 7, 8, 9, 10] .However,the welding properties of 7000 series aluminum alloy are greatly affected by the total amounts of alloying elements,especially for Cu-containing alloys.For these alloys,high concentrations of alloying elements tend to show a wide freezing range and form eutectic phases with low melting point,resulting in solidification cracking,liquation cracking and poor corrosion performance of welded joint [ 11] .Adding trace of Sc in these alloys has been claimed to be a considerable way to modify its microstructure and promote welding property [ 12, 13, 14] .

The element Sc is one of the most powerful refiners and modificators of as-cast grain structure [ 15, 16, 17, 18, 19] .In Al-Sc binary alloy system,the nature of that effect of Sc is involved with the formation of Al₃Sc primary phase in the melt during solidification [ 20, 21] .According to the heterogeneous nucleation theory [ 22] ,the refinement level of the as-cast grain depends on two functions of the additive:the number of nucleated particles in the unit melt and the effective nucleation of these particles.The latter is determined by matching degree of the lattice constants and lattice types between nucleated particles andα-Al matrix.Similar crystal structure is the determined factor for grain refinement.The A1₃Sc phase possesses practically completely dimensional and structural agreement with the lattice ofα-Al matrix [ 23] .In higher order alloy system,Sc has also shown strong ability to refine grains when Sc content exceeds a critical level.However,due to the interaction with other alloying elements in this case,the critical value is quite different with that in binary alloy [ 24, 25] .Additionally,it was reported that the reduction in the quantity of eutectic phases formed on solidification for Sc-containing alloys was observed due to the increase in the grain boundaries area which disrupts the formation of eutectic films.Moreover,Sc also appears to increase the amount of solute retained in solid solution after freezing,which has the potential for increasing strength through enhancing the aging response [ 11, 25] ,although the mechanism is not yet understood well.

The purpose of this work is to investigate the effects of adding Sc to Al-Zn-Mg-Cu-Zr alloy in more detail,focusing on grain refinement,grain structure modification and solidification behavior.

2 Experimental

The ingots with different Sc contents used in this study were prepared by solidification in a trapezoid-shape mold with size of top length of 213 mm,bottom length of195 mm,width of 50 mm and height of 290 mm.Highpurity A1 (99.9 wt%),Zn (99.8 wt%),Mg (99.8 wt%),Al-50Cu,Al-2Sc and Mg-30Zr were used as raw materials.After all the raw materials were melted and mixed homogeneously,the molten was poured into the mold along the hypotenuse.The temperature of the molten alloy during casting was kept at 700-720°C.The mold was preheated to 200°C before casting initiation.The chemical composition and designation for each alloy are shown in Table 1.

Each ingot was cut off 20 mm at the bottom,and then,a cross section with 10 mm in thickness was sliced from the remainder.Specimens were taken from center of that cross section.To characterize the grain structure,the samples for optical metallography were prepared through a conventional mechanical polishing and followed by etching with Keller reagent (1 ml HF,2.5 ml HNO₃,1.5 ml HCl and95 ml H₂O).The metallographic observation was performed in Zeiss Axiovert 200 MAT optical microscope(OM).Grain size was measured according to an American national standard (ASTM El 12-2013).For scanning electron microscope (SEM) examination,samples were lightly polished and subsequently observed in JEOL JSM 7001F,operated at 10 kV.The intermetallic compounds were identified by energy-dispersive X-ray spectrometry (EDS).The microstructural characterization was carried out by X-ray diffractometer (XRD,Rigaku D/Max 2500) with0.5°step.Differential scanning calorimeter (DSC) measurements were taken using a Netzsch 403 at a scanning rate of 10℃·min^-1 in argon atmosphere.

3 Results and discussion

3.1 Solidification behaviors of Al-Zn-Mg-Cu-Sc-Zr alloy

Before considering the interaction of Sc with other alloying elements,it is necessary to learn the phase formation of simple binary Al-Sc system during the solidification.According to the Al-rich end of the Al-Sc equilibrium phase diagram,the eutectic reaction from L toα-Al and Al₃Sc occurs at the temperature of 660℃with the eutectic composition point of 0.55 wt%-0.60 wt% [ 20, 26] .The Al₃Sc phase has a crystalline lattice possessing a highfraction dimensional and structural match with the aluminum matrix.Therefore,it can be a very effective nucleant forα-Al grains.However,only the concentration of Sc exceeds the eutectic point,can the Al₃Sc phase form earlier in the melt thanα-Al solidification,resulting in grain refinement.

Al-Zn-Mg-Cu-Sc-Zr alloy is a complex aluminum system,which is formed by adding trace amount of Sc and Zr into the Al-Zn-Mg-Cu base material.Figure 1 shows the polythermal section of Al-8.0Zn-xMg-2.0Cu-0.3Zr-0.3Sc (x=0-8) phase diagram by using data of differential thermal analysis and the results of a structure investigation after annealing [ 27] .It can be clearly seen that there are many possibilities for different combinations of intermetallic phases existing in the room temperature microstructure.It is well known that both Mg and Zn can form constituents with A1 and Cu,like T phase and M phase,but they would not react with Sc or form a new phase.However,they will decrease the solute concentration of Sc in theα-Al matrix.As to the role of Cu,it can react with Sc and form a ternary phase W when the contents of Cu and Sc exceed critical levels,respectively [ 23] .It has been reported that W phase has detrimental effect on mechanical properties of terminal products [ 28] ,so both the concentrations of Cu and Sc should be carefully selected in order to avoid the formation of W phase.Zr has similar chemical characteristic to Sc.Zr can form Al₃Zr phase with Al,and Zr atoms can partially substitute Sc atoms in Al₃Sc phase,forming Al₃(Sc,Zr) phase which inherits most of the advancements of Al₃ Sc phase and displays much better thermal stability.Consequently,taking comprehensive aspects into consideration,Sc level may be chose among the range of 0.15 wt%-0.30 wt% [ 23] .

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Table 1 Chemical composition and designation of three alloys (wt%)

Fig.1 Polythermal section of Al-8.0Zn-(0-8)Mg-2.0Cu-0.3Zr-0.3Sc phase diagram [12]

3.2 Grain structure of Al-Zn-Mg-Cu-Sc-Zr alloy ingot

The as-cast microstructure of Sc-free and Sc-containing alloys is shown in Fig.2.The typical grain structure of Scfree (Alloy 1) consists of large columnar grains with a dendritic substructure,as shown in Fig.2a,b.The average grain size is measured to be about 100μm.For dilute addition of 0.15 wt%Sc (Alloy 2),shown in Fig.2d,its grain structure seems similar to that of Alloy 1,although the grain size is measured to be about 87μm averagely,showing little reduction compared with that of Sc-free alloy.However,Alloy 3 containing 0.25 wt%Sc exhibits a very different type of grain structure and much refiner grain size.As can be seen in Fig.2e,f,Alloy 3 displays typical fine equiaxed grains without substructures inside and the great refinement in grain size which is measured to be about 25μm.This behavior is attributed to the powerful grain refining effect of the Al₃(Sc,Zr) phase,leading to nucleation of a large number of grains at a low undercooling which then grow and impinge before their initially spherical growth fronts become unstable [ 21, 25, 29] .In this article,the fact that grain refinement is only clearly observed in Alloy 3 when primary Al₃(Sc,Zr) phase is present can provide a strong evidence to that view.

3.3 Phase formation during solidification

Figure 3 shows SEM images of the three alloys,from which phases formed during solidification can be identified.The large coarse columnar grains are found to have a continuous layer of eutectic along the grain boundaries for Alloy 1 (Fig.3a).XRD patterns in Fig.4 show that this alloy consists of two phases,α-Al and MgZn_2.DSC trace shown in Fig.5 displays a large peak at～475℃,which is most probably associated with the eutectic reaction among L,α-Al and MgZn₂.Therefore,the solidification process for this alloy may be described as theα-Al matrix first solidified with the melt cooling,and then,the eutectic phases with low melting point formed when the temperature reached the eutectic reaction.

Figure 3b shows SEM image of Alloy 2.Although the micro structure is slightly changed due to 0.15 wt%Sc addition,it can be seen that eutectic phases still form an almost continuous layer around the grains.XRD patterns and DSC curves show the similar results with those of Alloy 1.All the evidences indicate that when 0.15 wt%Sc were added,no new phase is detected and the solidification process may keep the same path with that of Sc-free alloy.

Fig.2 OM images of as-cast micro structure:a,b Alloy 1,c,d Alloy 2 and e,f Alloy 3

Fig.3 SEM images of as-cast microstructure:a Alloy 1,b Alloy 2 and c,d Alloy 3

However,the microstructure shows different characters when the content of Sc increases up to 0.25 wt%.Figure 3c,d exhibits the greatly refined equiaxed grain structure with less eutectic films around grain boundaries.A new type of phase with cubic facetted morphology is observed inside the grain or along the grain boundary,which is identified as Al₃(Sc,Zr) primary phase (Table 2)by EDS analysis,as the similar result reported in previous works [ 29, 30] .Additionally,more detailed information for the primary Al₃(Sc,Zr) particle is provided by TEM,as shown in Fig.6.It is first necessary to point out that EDS results show another clear evidence for the presence of the primary Al₃(Sc,Zr) particle (Fig.6c-i).L1₂ crystallographic structure is confirmed using selected area diffraction patterns (SADP) that reveal the presence of superlattice reflection (Fig.6b).Figure 6a displays the morphology of this particle with a square-shape feature and that different contrasts inside the particle are clearly observed as a result of composition fluctuation.

Fig.4 XRD patterns of as-cast alloys

Fig.5 DSC curves for as-cast alloys

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Table 2 EDS analysis for phases marked as A and B in Fig.3d (at%)

Except Al₃(Sc,Zr) primary phase,no additional new phase could be observed either in SEM image or on XRD patterns.Overall,apart from the formation of Al₃(Sc,Zr)primary phase,the solidification pathway of the modified alloy remains largely un-altered.However,the content of the eutectic phases reduces as Sc content increases.The width and height of the peak at 475℃of Alloy 3 shown in DSC curve are significantly reduced compared with those of other alloys.Therefore,the concentration of major alloying elements retained in solid solute should be increased.

Figure 7a shows the results of composition measurement by line scan across a typical grain of Alloy 1.The concentration of Mg inside the grain is about 1.98 wt%,distributing relatively uniformly.The solution content of Zn locates～4.71 wt%,and its distribution in the center of the grain is slightly lower than in the site close to the boundary.As well known,the degree of micro-segregation of zinc atom is larger than that of magnesium atom.Figure 7b displays the average concentrations of major alloying elements in solution for the three alloys,which were calculated by three random measurements depicted as Fig.7a.It can be clearly seen that there is a significantly difference among the three alloys.The concentration of Mg and Zn retained in solution after solidification increases with higher Sc level.The mechanism by which Sc increases the solubility of solute elements is not yet commonly understood.However,the increase in solid solution and the reduction in eutectic phases are both expected to result in more stronQer and more ductile as-solidified micro structures which may have useful implications as welding materials.

4 Conclusion

The effect of scandium addition to Al-Zn-Mg-Cu-Zr alloy on its as-cast microstructure was investigated.Experimental results show that adding trace amount of Sc about 0.25 wt%into the base material can lead to remarkable grain refinement and grain structure is transformed from columnar to equiaxed one.Al₃(Sc,Zr) primary phases will form first during the solidification process;then,they act as nucleant site ofα-Al.Subsequently,the eutectic reaction between L andα-Al+MgZn₂ produces amounts of eutectic structure distributed along the grain boundary and/or sub-grain boundary.The reduction in the number of eutectic phases formed during solidification,coupled with an increase in the concentration of major alloying elements retained in the solute,would produce a potential benefit for increasing strength and ductility.In the case of 7000 series aluminum alloys,these results are of interest to help find solutions to reduce the tendency for solidification cracking during welding operation.

Fig.6 TEM images of a primary Al₃(Sc,Zr) particle:a bright field and b SADP;c SEM image and EDS analysis:d Al,e Sc,f Zr,g Cu,h Mg and i Zn

Fig.7 a Composition for line scans across a typical grain for Alloy 1 (distance of adjacent points in x-axis being 1.3μm) and b average concentrations of Zn and Mg in solution for alloys

Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (No.2016YFB0300903).

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