Dynamic compressive property and failure behavior of extruded Mg-Gd-Y alloy under high temperatures and high strain rates

Full length article

Dynamic compressive property and failure behavior of extruded Mg-Gd-Y

alloy under high temperatures and high strain rates

Jin-cheng Yu

*

, Zheng Liu, Yang Dong, Zhi Wang

School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China Received 19 May 2014; revised 10 December 2014; accepted 20 April 2015

Available online 15 May 2015

Abstract

For the purpose of investigating the dynamic deformational behavior and failure mechanisms of magnesium under high strain rates, the Split Hopkinson Pressure Bar (SHPB) was used for investigating dynamic mechanical properties of extruded Mg-Gd-Y Magnesium alloy at ambient temperature (300 K), 200
C (473 K) and 300
C (573 K) temperature. The samples after compression were analyzed by scanning electron microscope (SEM) and metallographic microscope. Dynamic mechanical properties, crack performance and plastic deformation mechanism of extruded Mg-Gd-Y Magnesium alloy along the extrusion direction (ED) were discussed. The results show that, extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive strength which is 535 MPa at ambient temperature (300 K) and strain rate of 2826 s1. When temperature increases, dynamic compressive strength decreases, while ductility increases. The dynamic compression fracture mechanism of extruded Mg-Gd-Y Magnesium alloy is multi-crack propagation and intergranular quasi-cleavage fracture at both ambient temperature and high temperature. The dynamic compressive deformation mechanism of extruded Mg-Gd-Y Magnesium alloy is a combination of twinning, slipping and dynamic recrystallization at both ambient temperature and high temperature.

Copyright 2015, National Engineering Research Center for Magnesium Alloys of China, Chongqing University. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Extruded Mg-Gd-Y magnesium alloy; Split Hopkinson Pressure Bar; Dynamic compressive property; Failure behavior; High strain rates; High temperature

1. Introduction

Magnesium alloys are the lowest density of the metal structure material which have been widely applied to the automotive field due to the advantages of high specific strength, high specific stiffness, good damping and

machin-ability [1,2]. In recent years, magnesium alloys used at the

temperature above 200
C are all magnesium rare-earth.

Mg-Gd-Y alloy system has become as one of the high strength cast

and wrought magnesium systems [3]. However, there have

been small amount of studies highlighting the dynamic

mechanical property of magnesium rare-earth and it is a lim-itation to use magnesium rare-earth complying with the actual usage.

JI W et al.[4]found that, the shear bands in Mg-Gd-Y alloy

during the course of ballistic were white under the optical microscope. The white bands were only formed in the stable plastic penetration stage. During the high temperature SHPB compression, significant plastic deformation bands were only

formed when T¼ 735 K. ZHANG F et al.[5]found that,

Mg-Gd-Y alloy showed a brittle cleavage fracture characteristics. They found ASBs in the specific location were phase transition bands. Strain rates affected the fracture mechanism by improving the thermal softening effect of Mg-Gd-Y alloy.

In the author's laboratory, the dynamic compressive

prop-erties and microstructure evolution of extruded Mg-Gd-Y alloy at ambient temperature along ED, TD and ND directions were * Corresponding author. Tel./fax: þ86 0 24 25496166.

E-mail address:[email protected](J.-c. Yu).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China, Chongqing University.

ScienceDirect

Journal of Magnesium and Alloys 3 (2015) 134e141 www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567

http://dx.doi.org/10.1016/j.jma.2015.04.003.

studied[6,7]. Dynamic mechanical properties of extruded Mg-Gd-Y alloy do not exhibit strong anisotropy and along ED direction are relatively optimal. Microstructures of extruded Mg-Gd-Y Magnesium alloy along ED, TD and ND compres-sion directions are sensitive to strain rates. When strain rate

reaches 2500 s1, the adiabatic shear band (ASB) begins to

form.

In this paper, Split Hopkinson Pressure Bar apparatus

(SHPB) with heating apparatus [8] was used to research the

dynamic compressive property of extruded Mg-Gd-Y Mag-nesium alloy along ED direction. The failure mechanism and deformation behavior of extruded Mg-Gd-Y Magnesium alloy were discussed at high strain rates and high temperatures. The fracture surfaces were analyzed by SEM and the microstruc-tures were observed using the optical microscopy.

2. Material and specimens

Extruded Mg-Gd-Y magnesium alloy (EW75M) as pro-vided by Beijing General Research Institute for Non-ferrous

Metals, which nominal compositions are listed in Table 1.

The extrusion ratio is 20 and the extrusion temperature is

400
C.Researchers from Beijing General Research Institute

for Non-ferrous Metals found that the strength and ductility of Mg-Gd-Y alloy were greatly improved with the increase of

extrusion ratio[9]. Specimens of dynamic compression were

cut from the Mg-Gd-Y magnesium extrusion (Fig. 1(a)) along

ED direction as shown inFig. 1(b) and the size of specimen

wasf8 mm  6 mm.

Split Hopkinson Pressure Bar (SHPB) was used under different strain rates at ambient temperature (300 K),

200
C(473 K) and 300
C(573 K) temperature. The details of

experimental device and methods can be found in papers

[10e12]. SHPB system is shown inFig. 2. The fractures were observed by SEM and microstructures of section perpendic-ular to loading direction, microstructures of section parallel with loading direction and microstructure of diagonal section were observed in broken specimens by metallographic microscope.

3. Results and discussion

3.1. Dynamic compressive properties

The dynamic compressive true stress-true strain curves of extruded Mg-Gd-Y magnesium alloy at different temperatures

are shown inFig. 3.Fig. 3(a) shows the true stress-true strain

curves of extruded Mg-Gd-Y Magnesium alloy at ambient

temperature (300 K) and the strain rates from 949 s1 to

2826 s1. It can be found that extruded Mg-Gd-Y magnesium

alloy yields continuously, and has a clearly positive strain rate effect with increasing strain rates. When strain rate comes to

2826 s1, the specimen cracks. Dynamic compressive strength

of extruded Mg-Gd-Y magnesium alloy is 535 MPa and strain

to failure is 15% at ambient temperature (300 K). Fig. 3(b)

shows the true stress-true strain curves of extruded Mg-Gd-Y

magnesium alloy at 200
C(473 K) and the strain rates from

1078 s1to 3010 s1. It can be seen that the curves firstly fall

down and then rise up with increased strain rates at a higher

temperature 200
C(473 K). It is because there is a competition

between temperature and strain rate. High temperatures cause a soften effect while high strain rates cause a harden effect. At

strain rates of 1406 s1and 2187 s1, the soften effect plays a

leading role, thus the curves fall down. But the specimens are

not broken. When strain rate comes to 3010 s1, the specimen

cracks. Dynamic compressive strength of extruded Mg-Gd-Y

magnesium alloy at 200
C(473 K) is 434 MPa and strain to

failure is 10%.Fig. 3(c) shows the true stress-true strain curves

of extruded Mg-Gd-Y magnesium alloy at 300
C(573 K) and

the strain rates from 1320 s1to 3344 s1. It can be found that

extruded Mg-Gd-Y magnesium alloy yields continuously, and has a clearly positive strain rate effect with increasing strain

rates either at 300
C(573 K). When strain rate comes to

3344 s1, the specimen cracks. Dynamic compressive strength

of extruded Mg-Gd-Y magnesium alloy is only 308 MPa but

strain to failure is 20% at 300
C(573 K).

Fig. 3(c) shows the true stress-true strain curves of extruded

Mg-Gd-Y magnesium alloy at 300
C(573 K) and at the strain

rates from 1320 s1 to 3344 s1. It can be found that at

300
C(573 K) extruded Mg-Gd-Y magnesium alloy yields

continuously, and has a clearly positive strain rate effect with

increased strain rates. When strain rate comes to 3344 s1, the

specimen cracks. Dynamic compressive strength of extruded

Mg-Gd-Y magnesium alloy at 300
C(573 K) is only 308 MPa

but strain to failure is 20%. Above all, at ambient temperature Table 1

Nominal compositions of extruded Mg-Gd-Y alloy (mass fraction, %).

Gd Y Nd Zr Mg

7.09 4.56 1.31 0.52 Bal.

(300 K) and the strain rate of 2826 s1, extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive

strength which is 535 MPa.Fig. 3(d) shows the comparison of

the dynamic compressive true stress-true strain curves at

similar strain rate at ambient temperature (300 K),

200
C(473 K) and 300
C(573 K) temperature. It can be

found that when temperature increases, the dynamic

compressive strength of extruded Mg-Gd-Y magnesium alloy decreases, while the ductility increases. Extruded Mg-Gd-Y magnesium alloy has the largest dynamic compressive strength at ambient temperature (300 K) and the strain rate of

2826 s1. Temperature affects the strength and ductility of

extruded Mg-Gd-Y magnesium alloy under dynamic

compression.

3.2. Observation of fracture surfaces

Fig. 4 shows the dynamic compressive fractures of extruded Mg-Gd-Y magnesium alloy at different

tempera-tures. It can be seen in Fig. 4(a)e(c) that the specimens

gradually deform with increased strain rates and eventually fail. When specimens break up, the fractures are along certain

directions.Fig. 4(d) indicates the approximate fracture mode.

According to the law of Schmid, 45
direction is the direction

of maximal orientation factor when specimen is dynamic compressed. The shear stress is the maximum along

45
direction. Thus when the specimen is compressed under

high strains, main cracks appear along two 45
shear

di-rections. InFig. 4(a) and (b) there are two 45
direction main

cracks andFig. 4(c) there is only one 45
direction main crack.

The amount of main crack depends on experimental condi-tions and specimens themselves. When shear stress increases with increasing strain rates, main cracks propagate and make the specimen broken in the end.

Fig. 5 shows the SEM graphs of dynamic compressive fractures of extruded Mg-Gd-Y magnesium alloy at different temperatures. It is observed that there are a lot of steps on the gentle fracture section. Some small secondary cracks and a

great deal of winding torn edges (inFig. 5(a), (c) and (e)) are

observed along the steps (inFig. 5(a)). There are flat

tongue-shaped pattern on the steps (in Fig. 5(a), (e) and (f)). These

results show that plastic deformation happens in localized Fig. 2. Schematic view of SHPB system.

0.00 0.04 0.08 0.12 0.16 0.20 0 100 200 300 400 500 2187s-1 1406s-1 3010s-1 1078s-1 (b) T rue Stress/Mpa T rue Stress/Mpa True Strain 473K 0.000 0.04 0.08 0.12 0.16 0.20 100 200 300 400 500 600 (a) 2826s-1 1992s-1 1392s-1 949s-1 True Strain 300K 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 50 100 150 200 250 300 350 (c) 1978s-1 2292s-1 3344s-1 1320s-1 True S tre ss/M pa True Strain 573K 0 100 200 300 400 500 600 1992s-1 -300K 2187s-1 -473K 2292s-1 -573K (d) True Stress/Mpa True Strain 0.00 0.05 0.10 0.15 0.20 0.25

Fig. 3. Dynamic compressive true stressdtrue strain curves of Mg-Gd-Y magnesium alloy: (a) 300 K; (b) 473 K; (c) 573 K; (d) comparison at similar strain rate of three temperatures.

place and cracks perform along certain crystallographic plane. Thus, the fracture performances of extruded Mg-Gd-Y mag-nesium alloy under dynamic compressive are all quasi-cleavage fracture at high temperatures and the type of frac-ture mechanism is not sensitive to temperafrac-tures.

Furthermore, some melting areas are found along the crack in Fig. 5(b)e(d). HUANG H et al. [13] in the compressive

broken specimen of AZ31 under the strain rate of 2300 s1

and LIU T M et al.[14]in AZ31 in compressive test all found

the special melting areas. It is due to under high strain rate the transient temperature is over the melting point when most plastic deformation work produces heat and adiabatic heating happens. In Taylor impact test of Tie6Ale4V alloy, REN Y

[15]thinks that these serious melting regions can be observed

on the upper side of the smooth and smeared surface, which illustrates that a significant melting of the original shear area material takes place prior to shear fracture of the specimen. The melting areas are also proofs of thermal softening effect in localized place.

3.3. Microstructure observation

Fig. 6 shows the original microstructures of section perpendicular to loading direction along ED direction. Orig-inal microstructure of extruded Mg-Gd-Y magnesium alloy along ED direction is typical extrusion state microstructure. There are a great amount of uniform equiaxed big-sized

grains. The size is about 10 mm. A small amount of

small-sized recrystallization grains which lie between the big-small-sized grains can be seen in black striped area. In additional, no twins are observed in both of the big and small grain sizes.

Fig. 7 shows the compressive microstructures of extruded Mg-Gd-Y magnesium alloy along ED direction at 300 K under

strain rate of 2826 s1. Fig. 7(a) is the microstructure of

section perpendicular to loading direction. It can be observed that twinning happens in only several grains which are both big size grains and small size grains. Original recrystallized grains become more and grow up and the recrystallization

bands become wide and continuous in a row.Fig. 7(b) is the

microstructure of section parallel with loading direction. The results show that a continuous white band appears and its

average width is 3 mm. The white band is parallel with the

band of recrystal grains and separates equiaxed grains from

recrystal grains.Fig. 7(c) and (d) show the microstructures of

diagonal section after dynamic compression. There are many paralleled cracks in the broken specimen. It indicates that the dynamic failure mechanism of magnesium is muti-crack propagation mechanism and cracks extend along the grain boundary.

Fig. 8 shows the compressive microstructures of extruded Mg-Gd-Y magnesium alloy along ED direction at 473 K under

strain rate of 3010 s1. Fig. 8(a) is the microstructure of

section perpendicular to loading direction. It can be observed that twinning happens in only several grains which are both big size grains and small size grains.

It indicates that twinning is a kind of dynamic compressive deformation mechanisms of extruded Mg-Gd-Y magnesium alloy at both ambient temperature and high temperature. The other deformation mechanism is slip. The higher strain rate is, the more percentage of slip is. This is because magnesium

alloys have less slip systems at room temperature[16] and it

causes a strong stress concentration near the grain boundaries. The strong stress concentration can improve the formation of twins and coordinate the plastic deformation. When strain rates and temperatures increase, cross slip and non-basal slip can start although the critical shear stress decreases which twinning needs. The amount of starting slip is increasing so that the stress concentration is weakened and it is against the formation of twins. As a result, twinning makes less contri-bution to the plastic deformation at high strain rates. On the contrary non-basal slip which has thermal activation charac-teristics can play important role during the release of stress concentration and the coordination of plastic deformation. In addition, during the plastic deformation twinning will choose to appear in advantageously oriented grains. It explains that twinning happens in only several grains which orientations are advantaged. Original recrystallized grains become more and Fig. 4. (a) Photograph of specimens after dynamic compression: (a) 300 K; (b)

grow up and the recrystallization bands become wide and

continuous in a row.Fig. 8(b) is the microstructure of section

parallel with loading direction. There is a continuous white band and twinning appears in it. The average width of the

white band is 3mm. The white band is parallel with the band

of recrystal grains and separates equiaxed grains from

recrystal grains.Fig. 8(c) shows the microstructure of diagonal

section after dynamic compression. Original recrystallization bands become more and original grains are broken into small grains after dynamic compression. In summary, the dynamic compressive deformation mechanism of extruded Mg-Gd-Y magnesium alloy under high stain rate compression is a

combination of twinning, slipping and dynamic

recrystallization.

Fig. 9 shows the compressive microstructures of extruded Mg-Gd-Y magnesium alloy along ED direction at 573 K under

strain rate of 3344 s1. Fig. 9(a) is the microstructure of

Fig. 5. SEM graphs of Mg-Gd-Y magnesium alloy under dynamic compression: (a) and (b) 300 K; (c) and (d) 473 K; (e) and (f) 573 K.

Fig. 6. Original microstructure of extruded Mg-Gd-Y magnesium alloy along ED direction.

section perpendicular to loading direction. It can be observed that there are some abnormal big grains in the microstructure. This is because under high strain rates, the whole deformation is in a very short time and a large part (about 90%) of plastic work turns into heat. Heat concentration leads several grains growing up abnormally by grain boundary migration to absorb adjacent grains. The big grains decrease the material strength. It indicates that adiabatic temperature rising causes the

thermal softening[17].Fig. 9(b) and (c) are the microstructure

of section parallel with loading direction. The results show that a continuous white band appears and parallel with the band of recrystal grains. In other area, recrystallized grains become more and grow up and the recrystallization bands

become wide. Ion[18]thought that magnesium alloys can be

easily dynamically recrystallized: (1) There are fewer slip systems of magnesium alloys and piling up of dislocations Fig. 7. Microstructures of extruded Mg-Gd-Y magnesium alloy at 300 K under strain rate of 2826 s1: (a) microstructure of section perpendicular to loading direction; (b) microstructure of section parallel with loading direction; (c) and (d) microstructure of diagonal section.

Fig. 8. Microstructures of extruded Mg-Gd-Y magnesium alloy at 473 K under strain rate of 3010 s1: (a) microstructure of section perpendicular to loading direction; (b) microstructure of section parallel with loading direction; (c) microstructure of diagonal section.

Fig. 9. Microstructures of extruded Mg-Gd-Y magnesium alloy at 573 K under strain rate of 3344 s1: (a) microstructure of section perpendicular to loading direction; (b) and (c) microstructure of section parallel with loading direction; (d) and (e) microstructure of diagonal section.

happens easily so that dislocation density can meet the stan-dard for dynamic recrystallization; (2) Magnesium and its alloys have lower stacking fault energy and extended dislo-cation is hard to strand. Therefore, it is difficult to slip and climb,. and dynamically recover slowly but improve recrys-tallization; (3) Magnesium alloys have higher speed of the grain boundary diffusion and the dislocation energy piled up on the subgrain boundaries can be absorbed by the boundaries which can speed up the dynamic recrystallization. The grains of dynamic recrystallization are relevant not only to the size of deformation degree, but also to the temperature of deformation and strain rate. Higher the temperature is, more sufficient dynamic recrystallization is and more homogeneous micro-structure is.

Fig. 9(d) shows the microstructures of diagonal section after dynamic compression. It is shown that there are many broken grain boundaries bands in the specimen where the

small cracks appear and grow up, as shown inFig. 9(d). When

the strain rate and temperature increase, the grain boundaries become weak and then broken into bands at last. And the cracks choose the least resistance way to grow up, as seen in

Fig. 9(e). The process of crack propagation is also the process of impact energy attenuation. Thus, it is indicates that the dynamic failure mechanism of magnesium is intergranular muti-crack propagation mechanism and cracks extend along the grain boundary.

4. Conclusions

1) At ambient temperature (300 K) and the strain rate of

2826 s1, extruded Mg-Gd-Y magnesium alloy has the

largest dynamic compressive strength which is 535 MPa.

When temperature increases, dynamic compressive

strength of extruded Mg-Gd-Y magnesium alloy de-creases, while ductility of extruded Mg-Gd-Y Magnesium alloy increases.

2) The dynamic compression fracture mechanisms of

extruded Mg-Gd-Y magnesium alloy at both ambient temperature and high temperature are multi-crack propa-gation and intergranular quasi-cleavage fracture.

3) The dynamic compressive deformation mechanisms of extruded Mg-Gd-Y magnesium alloy at both ambient temperature and high temperature are a combination of twinning, slipping and dynamic recrystallization.

Acknowledgments

The authors would like to acknowledge the financial sup-port from the National Key Basic Research Program (973 Program), Project (2013CB632205).

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