简介概要

Microstructure and mechanical properties of Mg-6Zn-0.5Y magnesium alloy prepared with ultrasonic treatment

来源期刊：Rare Metals2017年第8期

论文作者：Di Li Han-Song Xue Gang Yang Ding-Fei Zhang

文章页码：622 - 626

摘要：Ultrasonic effect on microstructure evolution and mechanical properties of Mg-6Zn-0.5Y magnesium alloy was investigated.The results show that ultrasonic treatment can significantly modify the as-cast microstructure of Mg-6Zn-0.5Y alloy.With the application of ultrasonic treatment,the primary a-Mg phase transforms from coarse dendrites to nearly fine equiaxed grains,and the continuous I-phase atα-Mg boundaries is modified into discontinuous one.Interestingly,the area fraction of I-phase decreases after ultrasonic treatment.Besides,the tensile properties of Mg-6Zn-0.5Y alloy are apparently improved after ultrasonic treatment.Compared to those of the untreated alloy,the ultimate tensile strength and elongation of Mg-6Zn-0.5Y alloy are improved by 67%and 56%,respectively.

稀有金属(英文版) 2017,36(08),622-626

Microstructure and mechanical properties of Mg-6Zn-0.5Y magnesium alloy prepared with ultrasonic treatment

Di Li Han-Song Xue Gang Yang Ding-Fei Zhang

College of Materials Science and Engineering,Chongqing University

National Engineering Research Center for Magnesium Alloys,Chongqing University

收稿日期：28 June 2014

基金：financially supported by the National Great Theoretic Research Project of China (No. 2013CB632200);the Sharing Fund of Chongqing University's Large-Scale Equipment (No.2013063009);

Microstructure and mechanical properties of Mg-6Zn-0.5Y magnesium alloy prepared with ultrasonic treatment

Di Li Han-Song Xue Gang Yang Ding-Fei Zhang

College of Materials Science and Engineering,Chongqing University

National Engineering Research Center for Magnesium Alloys,Chongqing University

Abstract：

Ultrasonic effect on microstructure evolution and mechanical properties of Mg-6Zn-0.5Y magnesium alloy was investigated.The results show that ultrasonic treatment can significantly modify the as-cast microstructure of Mg-6Zn-0.5Y alloy.With the application of ultrasonic treatment,the primary a-Mg phase transforms from coarse dendrites to nearly fine equiaxed grains,and the continuous I-phase atα-Mg boundaries is modified into discontinuous one.Interestingly,the area fraction of I-phase decreases after ultrasonic treatment.Besides,the tensile properties of Mg-6Zn-0.5Y alloy are apparently improved after ultrasonic treatment.Compared to those of the untreated alloy,the ultimate tensile strength and elongation of Mg-6Zn-0.5Y alloy are improved by 67%and 56%,respectively.

Keyword：

Magnesium alloy; Ultrasonic treatment; Microstructure; Mechanical properties;

Author： Han-Song Xue,e-mail:hsxue@sohu.com;

Received： 28 June 2014

1 Introduction

In order to satisfy the lightweight requirement in aeronautical and automobile industries,as the lightest constructional materials,magnesium alloys are widely utilized due to their high specific stiffness and strength,excellent machinability,good castability and easy recovery [ 1, 2, 3, 4] .The Mg-Zn-Y alloys seem particularly promising due to the formation of I-phase (Mg3Zn₆Y₁,icosahedral quasicrystalline structure),which possesses superior properties such as high strength and thermal conductivity,good hightemperature plasticity,low interfacial energy and friction coefficient [ 5, 6, 7] .However,the Mg-Zn-Y alloys prepared by ordinary ingot metallurgy process showed very low ductility due to the I-phase existing at grain boundaries in the form of continuous reticular [ 8] .The conventional method for microstructure modification is to add chemical elements.Various researches were done to investigate the effects of Mn [ 9] ,Al [ 10] ,Zr [ 11] and Re [ 12] on the I-phase modification.However,certain internal defects could exist,such as high cost,influencing the composition of alloy and polluting the environment [ 13] .Therefore,physical field for micro structure modification such as ultrasonic treatment is desire.

Numerous investigations show th at ultrasonic treatment is an effective physical method for modifying the microstructure of low-melting alloys [ 14, 15, 16] .However,previous investigations almost focus on aluminum alloys and Mg-Albased alloys.As to Mg-ZnY alloys,little work was done.Therefore,the main purpose of this paper is to investigate the ultrasonic effect on the microstructure evolution and mechanical properties of Mg-6Zn-0.5 Y alloy.

2 Experimental

Commercial highly pure Mg (purity>99.9 wt%),Zn (purity>99.9 wt%) and Mg (30 wt%)-Y master alloy were used to prepare the experimental alloys with nominal composition of Mg-6Zn-0.5Y (wt%).In the experiment,the alloy (about 0.75 kg) was melt in the stainless steel crucible protected by CO₂+0.5 vol%SF₆-mixed gas.The alloy was first heated to 750℃and maintained for 15 min at this temperature in order to get full homogenization.Then,the melt was poured into a steel mold at 680℃,and the ultrasonic horn set as 600 W preheated at 680℃was immersed into the melt for ultrasonic treatment until the melt was completely solidified.Figure 1 shows the schematic diagram of experimental devices.It mainly consisted of an ultrasonic generator system with fixed frequency of20 kHz and maximum power of 1 kW,a transducer with the size of 30 mm in diameter,a protection gas system,an electrical resistance furnace and a stainless steel crucible.

Fig.1 Schematic diagram of experimental devices for ultrasonic melt treatment

The cast specimens were sectioned,ground,mechanical polished and etched with 4%nital solution.The microstructure observation of the specimens was conducted on an optical metallographic microscope (OM,MDS) and a scanning electron microscope (SEM,TESCAN VEGAIlLMU) equipped with energy-dispersive spectrometer (EDS,OXFORD INCA Energy 350).Phase components of the specimens were characterized by X-ray diffractometer (XRD,D/Max 2500PC).The tensile tests were performed at a constant strain rate of3 mm·min^-1 on an electronic universal testing machine (Sans CMT 5105) at room temperature.The area fraction of I-phase was measured by software Image-Pro Plus.

3 Results and discussion

3.1 Microstructure

Figure 2 shows the micros true tures of Mg-6Zn-0.5Y alloys without and with ultrasonic treatment.As shown in Fig.2a,without ultrasonic melt treatment,the primaryα-Mg phase of the alloy exhibits developed dendritic structure which is proved to be dendrite growth mode.However,with the application of ultrasonic treatment,the primaryα-Mg phase exhibits nearly equiaxed grain structure (Fig.2b),indicating that a fine uniform microstructure is achieved after ultrasonic treatment.

Fig.2 OM images of Mg-6Zn-0.5Y alloy a without ultrasonic treatment and b with ultrasonic treatment

Previous studies showed that ultrasonic cavitation may occur when ultrasonic was introduced into the liquid medium.According to Refs. [ 17, 18] ,with ultrasonic treatment,when the melt is subjected to local tension stresses and further pulled cracks,the cavitation bubbles form.Then,the cavitation bubbles continue to expand until burst,thus creating high-pressure pulses in the melt.Under the high-pressure pulses,the dendrites are fragmentized and subsequently spread into the melt to grow as new grains.In addition,the collapse of cavitation bubbles induces high-speed flow in the melt and help to decrease the solute enrichment at the solidification front,thus the influence of constituent supercooling is reduced,enablingα-Mg grains to grow up consistently in all directions to form nearly equiaxed structure.

Figure 3 shows the XRD patterns of Mg-6Zn-0.5Y alloy without and with ultrasonic treatment.It can be seen that both the samples mainly consist ofα-Mg matrix and I-phase,which demonstrates that ultrasonic treatment does not change the phase constituent of the alloy.However,the diffraction peak of I-phase of the alloy with ultrasonic treatment is weakened compared to that of the untreated alloy,indicating that the amount of I-phase decreases.

It can be seen from Fig.4a that without ultrasonic treatment,a lot of coarse and reticular compounds distribute along the grain boundaries.At higher magnification(Fig.4b),it is clear that the reticular compounds are the typical lamella eutectic structure with composition of be Mg-21.53Zn-4.22Y (Fig.4c).Combined with the XRD results,the bright and black phases in eutectic structure are confirmed to be I-phase andα-Mg phase,respectively.As shown in Fig.4d,with the ultrasonic introduced into the melt,the thickness and continuity of I-phase decrease,and I-phase is mainly in the form of discontinuous lines and fine particles.Figure 5 shows the statistical value of the area fraction of I-phase,it is clear that after ultrasonic treatment,the area fraction of I-phase decreases.That is to say,ultrasonic treatment suppresses the formation of I-phase of Mg-6Zn-0.5Y alloy.

Fig.3 XRD patterns of Mg-6Zn-0.5Y alloy a without ultrasonic treatment and b with ultrasonic treatment

Fig.4 SEM images of Mg-6Zn-0.5Y alloy without ultrasonic treatment a and corresponding higher magnification b,EDS result of eutectic structure c,and SEM image of Mg-6Zn-0.5Y alloy with ultrasonic treatment d

I-phase can be fragmented under the impact force generated by bubbles collapse.Furthermore,the acoustic streaming decreases the enrichment of Zn and Y elements at the solidification front and disperses them into the remaining melt [ 19] ,which suppresses the formation of I-phase due to the lack of Zn and Y elements at the solidification front.

Fig.5 Area fraction of I-phase of Mg-6Zn-0.5Y alloy without and with ultrasonic treatment

3.2 Mechanical properties

The tensile tests at room temperature were conducted to study the ultrasonic effect on the tensile properties of Mg-6Zn-0.5 Y alloy.Figure 6a shows the engineering stress-strain curves of Mg-6Zn-0.5Y alloy without and with ultrasonic treatment.Figure 6b shows the room temperature tensile properties of the experimental alloys including ultimate tensile strength(UTS) and fracture elongation.As depicted in Fig.6,without ultrasonic treatment,the UTS and elongation are 110 MPa and 7.2%,respectively.With ultrasonic treatment,the room temperature tensile properties of the alloy are significantly improved,in which the UTS and fracture elongation are improved to 184 MPa and 11.2%,respectively.

The improvement of the mechanical properties of Mg-6Zn-0.5Y alloy can be mainly ascribed to two aspects:(1)achievement of fine uniform grains would obviously result in the improvement of tensile strength and elongation;(2)the continuous coarse I-phases atα-Mg boundaries are modified into discontinuous lines and fine particles after ultrasonic treatment,which can serve as stronger dispersion strengthening.On the other hand,the fine dispersed I-phase has little constraint onα-Mg matrix,which is beneficial to the enhanced plasticity of alloy.

Fig.7 SEM images of Mg-6Zn-0.5Y alloy at room temperature a without ultrasonic treatment and b with ultrasonic treatment

Figure 7 shows the SEM images of Mg-6Zn-0.5 Y alloy at room temperature without and with ultrasonic treatment.As shown in Fig.7a,obvious cleavage planes and few dimples are observed in the untreated alloy,indicating that the main fracture behavior of the untreated alloy is cleavage fracture.However,as shown in Fig.7b,with ultrasonic treatment,the tensile fracture surface of Mg-6Zn-0.5Y alloy exhibits more dimples,which is in good consistent with its relatively high elongation,with the main fracture characteristic being quasicleavage.

4 Conclusion

Ultrasonic treatment can significantly modify the microstructure of Mg-6Zn-0.5Y alloy,and the microstructure with fine uniform grains and dispersed I-phase is achieved after ultrasonic treatment.In the ultrasonic Mg-6Zn-0.5 Y alloy,the amount of I-phase at grain boundaries decreases.The tensile properties of Mg-6Zn-0.5Y alloy with ultrasonic treatment are significantly improved compared to those of the untreated alloy,which is mainly related to the fine uniform grains and the dispersed I-phase.

Fig.6 Room temperature tensile properties of Mg-6Zn-0.5 Y alloy without and with ultrasonic treatment:a engineering stress-strain curves and b UTS and elongation

Acknowledgments

This study was financially supported by the National Great Theoretic Research Project of China (No.2013CB632200) and the Sharing Fund of Chongqing University's Large-Scale Equipment (No.2013063009).