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Abstract: High-energy shot peening (HESP), a method to produce severe plastic deformation by high velocity flying balls, was applied on die cast magnesium ...
Journal of Wuhan University of Technolotgy-Mater. Sci. Ed. Aug.2009

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DOI 10.1007/s11595-009-4515-9

Surface Nanocrystallization of Magnesium Alloy AZ91D by High-Energy Shot Peening ZHANG Jin1, 2, OU Xingbin2, YANG Donghua2, SUN Zhifu3 (1. School of Materials Science & Engineering, University of Science and Technology Beijing, Beijing 100083, China; 2. College of Materials Science & Engineering, Chongqing University of Technology, Chongqing 400050, China; 3. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China)

Abstract: High-energy shot peening (HESP), a method to produce severe plastic deformation by high velocity flying balls, was applied on die cast magnesium alloy AZ91D. Effects of surface nanocrystallization by HESP and heat treatment at different temperatures were investigated. The microstructure evolution was conducted using X-ray diffraction (XRD) and field emission scanning electronic microscopy (FESEM). The hardness was measured by microhardness tester. The experimental results show that surface nanocrystrallization of AZ91D obtained by HESP would lead to the increase of microhardness. Low temperature heated at 100 ℃ for 1 h do not change the property obviously. However, both the microstructure and microhardness vary greatly after heat treatment at 400 ℃ for 1 h. Key words: AZ91D; surface nanocrystallization; HESP; microhardnss; corrosion resistance

1 Introduction The research on nanocrystalline (NC) materials has developed rapidly, owing to tremendous interest in this topic of scientific and technological importance since Herbert Gleiter first presented the concepts for developing NC materials with special properties. However, it is very difficult to prepare bulk NC materials for large scale industry application and it is also unnecessary under certain conditions to use bulk NC materials in order to prolong their service life[1]. In fact, in most cases materials failures such as wear and corrosion occurred on the surface. Therefore, optimization of the surface microstructures of materials may effectively enhance the combination performance of the materials in service. Surface nanocrystallization (SNC) is a new concept proposed of forming nanostructure only on materials surface layer[2]. It has been demonstrated that nanocrystallization structure of surface layers could be successfully achieved on different metallic materials via severe plastic deformation using surface mechanical attrition treatment (SMAT)[3], ultrasonic shot peening (USSP)[4], high-energy shot peening (HESP)[5], wire-brushing[6], etc. (Received: May 12, 2008; Accepted: Sep. 17, 2008) ZHANG Jin (张张):Prof. ; Ph D; E-mail: zhangjin@ mater. ustb.edu.cn Funded by the National Ministry of Education (No.207095) and Beijing Key Laboratory for Corrosion Erosion and Surface Technology

The various properties of surface layers, such as diffusion kinetics, microhardness, wear, fatigue, etc, have been enhanced greatly after SNC treatment[7]. Magnesium and its alloys are a kind of significant interesting materials due to their lightness, higher strength and stiffness, etc. In present work, die cast magnesium alloy AZ91D was chosen for HESP to explore the development of microstructure and properties of hardness in the surface layer.

2 Experimental The raw material used was magnesium alloy AZ91D with dimension of 140 mm×110 mm×20 mm. Whose chemical composition (wt%) was 8.5-9.5Al, 0.9-0.95Zn, 0.17-0.40Mn, ≤0.05Si, ≤0.001Ni, ≤0.004Fe, balance Mg. Fig.1 shows a schematic of the HESP. The stainless balls with a diameter 1.5 mm were accelerated by high pressure gas at room temperature and the distance between spray-gun and specimen was about 200-250 mm. During the process the entire surface of the sample was shot peened by the high velocity balls for 5 min. In order to make the deformation more uniform, the specimen could move left and right during the experiment. After HESP, heat treatment at 100 ℃ and 400 ℃ respectively were carried out in order to investigate whether the grain on the surface grows. The specimens were held for 1 h in

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an SX-2.5-12 electric resistance furnace without protected gas and cooled in air to room temperature. All samples codes and designation are shown in Table 1.

composed of α-Mg phase and β-Mg17Al12 phase shown in Fig.2c, which is the same as the as-received sample. There is not any obvious difference between the top surface layer and central matrix shown in the cross-sectional Fig.2b and Fig.2d. However, with the large magnification of FESEM, it could be seen clearly that very fine granular in nano scale appeared in the top surface layer as shown in Figs.2(e)-2(f). The average granular size is about 60-70 nm, which meant that surface nanocrystallization is realized on the top layer of magnesium alloy AZ91D after HESP.

Fig.1 Schematic illustration of the HESP technique Table 1 Specimen in the experiment Sample code Sample designation Experimental treatment 1 As-received Die cast 2 As-treated HESP 3 100 ℃×1 h HESP+100 ℃×1 h 4 400 ℃×1 h HESP+400 ℃×1 h

The X-ray diffraction analysis was carried out using a Rigaku D/max-2000 X-ray diffractometer with Cu target radiation (λKα1 = 1.54056Ǻ) in the continuous scanning mode. The experimental parameters was as follows: tube potential 40 kV, tube current 25 mA, scanning rang 30-90°, scanning velocity 0.06 °/s. The average grain size and microstrain were calculated in terms of the diffraction line broadening after removing the influence of instrument broadening, which was obtained by a standard AZ91D magnesium alloy specimen without any strain and grain refinement. The microstructure morphologies of samples were examined by using an OLYMPUS SZ61TR stereo microscope (SM), an OLYMPUS GX51F optical microscope (OM) and FEI Nova 400 field emission scanning electronic microscopy (FESEM). Before observation,the sample was mechanically polished firstly using silicon carbide paper to grade 1000, then on a polishing cloth with a liquid suspension of 0.04 μm alumina, and finally etched in a mixed solution consisting of 1 mL nitric acid, 1 mL acetic acid, 1 mL ethanedioic acid and 150 mL distilled water. Micro-hardness was measured on an HXS-1000 micro-hardness tester.

3 Results and Discussion 3.1 Microstructure observation HESP surface and cross-sectional morphologies of samples were observed with three different microscopes as shown in Fig.2 It could be found many spherical crown shape pits are produced on the treated sample surface (Fig.2a). The microstructure in the as-treated is still

Fig.2 Surface and cross-sectional morphologies of as-treated observed by three different microscopes: (a) surface morphology (SM), (b) cross-sectional morphology (SM), (c) surface morphology (OM), (d) cross-sectional morphology (OM), (e) surface morphology (FESEM) (f) cross-sectional morphology (FESEM)

The formation of nano grain is attributed to a great amount of deformation being introduced into samples’ surface in a very short time. And the original coarse grains were divided gradually by dislocation walls (DWs) and dislocation tangles (DTs), etc[7, 8]. The specific process of forming nanocrystalline varied from different kinds of materials. It is well known that magnesium alloys with HCP structure are very difficult to be deformed at room temperature and normally twinning plays an important role in the plastic deformation. However, it was reported in the Ref.[8] that no twinning was found at initial stage of plastic deformation. It still needs more research efforts to understand the mechanism and spe-

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Journal of Wuhan University of Technolotgy-Mater. Sci. Ed. Aug.2009

cific processes of forming nanocrystalline in magnesium alloys. 2.2 X-ray diffraction analysis The XRD patterns of AZ91D magnesium alloy before and after HESP and heat treatment at 100 ℃ and 400 ℃ are shown in Fig.3. It is observed that the as-received consists of α-Mg and β-Mg17Al12 phase, so do as other three specimens. In other words there are not any new phases formed after HESP or heat treatment. However, all positions of diffraction peaks in specimens of as-treated and heated left shifts to different extents compared to that of the as-received. The largest peak position shift and peak broadening occurs in as-treated specimen in Table 2. The peak position shift is made mainly by macro-stress during HESP. The peak broadening may be attributed to grain refinement, micro-strain and instrument broadening. In consideration of influences of micro-strain and instrument broadening, the average grain size and micro-stain of surface layer of as-treated sample were calculated according to the Sample

As-received

As-treated

100 ℃×1 h

400 ℃×1 h

Sample As-treated

100 ℃×1 h (micro-strain included) 100 ℃×1 h (no microstrain)

2θ/(°)

Scherrer and Wilson equation as shown in Table 3. The grain sizes for different crystallographic planes of α-Mg phase in the treated surface are slightly different and all of them are smaller than 100 nm, and the average is about 75.8 nm, which is very close to the results obtained by FESEM microscope. It is confirmed that surface nanocrystallization of magnesium alloy AZ91D is realized by HESP.

Fig.3 XRD patterns of AZ91D samples with different treatment (as indicated)

Table 2 Parameters and results of XRD Phase (hkl) d/Ǻ

Area/counts

FWHM/(°)

32.398

α-Mg

100

2.7611

5637

0.139

34.454

α-Mg

002

2.6009

3349

0.269

35.922

β-Mg17Al12

411

2.4979

329

0.204

36.8

α-Mg

101

2.4403

13259

0.305

32.169

α-Mg

100

2.7802

4628

0.335

34.35

α-Mg

002

2.6086

7193

0.319

35.772

β-Mg17Al12

411

2.508

2966

0.283

36.62

α-Mg

101

2.4519

32345

0.366

32.236 34 42 35.936

α-Mg β-Mg17Al12

100 002 411

2.7746 2 6029 2.497

2300 3145 1913

0.261 0 263 0.218

36.706

α-Mg

101

2.4464

16356

0.332

32.227 34 459 35.729

α-Mg β-Mg17Al12

100 002 411

2.7754 2 6005 2.511

2033 4009 522

0.194 0 165 0.07

36.746

α-Mg

101

2.4438

14241

0.186

Table 3 Micro-strain and grain size for as-treated and 100℃×1 h (hkl) Micro-strain β0hkl/rad β1hkl/rad 100

0.001368679

0.001579

002

0.001574199

101

0.001265926

100 002

Dhkl/nm

0.002279

62.62

0.001946

0.00144

99.68

0.001676

0.002216

65.16

0.004889356

0.005652

-0.00605

0.000768965

0.000953

-0.00148

101

0.002499693

0.003317

-0.00344

100

0

0

0.001989675

71.73

002

0

0

0.002391101

60.03

101

0

0

0.003211406

44.98

Note: β0 and β1represents the peak broadening inducing by micro-strain and grain refinement, respectively

Daverage/nm 75.82

58.91

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The XRD pattern of the 100 ℃×1 h is similar to that of as-treated except that the values of FWHM decrease slightly, and that the macro-stress might release as there is little deviation in XRD patterns from the as-received. According to the calculation results in Table 3, the average grain size after 100 ℃ heated is 58.9 nm, even smaller than that of as-treated. It shows strange phenomena for grains always grow up after heat treatment. This might be explained by the following two reasons. Firstly, the nanocrystalline do not grow when isothermally heated at 100 ℃ for 1 h and the calculation value is smaller if there exists calculation error. Secondly, generally speaking the grain boundary of nano grains is full of defects, such as dislocations, dislocation walls (DWS) and dislocation tangles (DTS), and these defects would arrange anew and disappear after heat treatment at low temperature so the boundary would become clear and smaller. From the calculation results, it could be found that the micro-strain is released after heat treatment at 100 ℃ for 1 hour, since the calculation values of peak broadening induced by grain refinement are negative if micro-strain is considered to be existed after heat treatment at 100 ℃. So the process of heating at 100 ℃ do not make the nanocrystalline grow while it releases the micro-strain in the surface. However, after heat at 400 ℃ for 1 h, the FWHM decreases greatly to the value of instrument broadening which might mean the nanocrystalline has grown dramatically. This conclusion could be proved by observation of metallurgy micrographs in Fig.4. The microstructure of the 100 ℃×1 h (Fig.4a) is almost the same as that of as-treated (Fig.2d), however after 400 ℃×1 h heat treatment the grain grows up and the grain size is about 50 μm on the top surface layer (Fig.4b). Due to the free energy stored in samples increases after HESP, the as-treated is in non-equilibrium state and tend to shift to equilibrium state spontaneously at elevated temperature, the process of which usually has two different stages termed recovery and recrystallinzation respectively. During recovery, some stored energy is relieved and the amount of dislocation reduce but the mechanical properties change little for the microstructure keeps the same as that of the original. However, during recrystallization a new set of strain-free and equiaxed grains are produced and the mechanical properties are restored to the pre-treated values. According to this perspective, it could be concluded that in the experimental only recovery occurs after the heat treatment at 100 ℃ for 1 h but recrystallization after 400 ℃×1 h heat treatment, since the microstructure and microhardness (in Fig.5b) on the treated surface change slightly at 100 ℃ but change greatly at 400 ℃.

Fig.4 Cross-sectional microstructure of the sample 100 ℃×1 h (a) and 400 ℃×1 h (b)

For pure metal, the lowest recrystallization temperature, which is defined as that recrystallization just reaches completion in 1 h at the temperature, is normally 0.35-0.4 Tm[9], where Tm is the absolute melting temperature. For pure magnesium metal, whose absolute melting temperature is 922 K, the lowest recystallization temperature is between 49.7 ℃and 95.8 ℃, which is lower than that of magnesium alloy AZ91D in the experimental. This is probably because of the influence of alloy elements in AZ91D on the recystallization temperature. Normally alloys have much higher recrystallalization temperature than that of pure metal, for alloy elements always segregate at the grain boundary and dislocation, which could impede dislocations movement and grain boundary motion so inhibited atoms diffusion and grain growth. Microhardness of different samples is shown in Fig.5. It can be found from Fig.5(a) that the microhardness greatly increases after HESP, which is almost twice larger than that of the as-received, and descends gradually from surface to center along the depth of layer. The slope coefficient become smaller with the distance increasing from the surface. Over 400 μm in depth, the hardness is the same as that of the matrix, which meant the plastic deformation region is about 400 μm. From the Fig.5(b), the microhardness on the top surface layer in the

Journal of Wuhan University of Technolotgy-Mater. Sci. Ed. Aug.2009

sample 100 ℃×1 h is almost the same as that of as-treated, however it decreases greatly after the 400 ℃×1 h heat treatment, which is close to that of as-received. The reasons for the micro-hardness increasing greatly in the surface may be attributed to the nanocrystalline and micro-stress formed in the surface by HESP. After the heat treatment at 100 ℃ for 1 h, the micro-hardness of the surface layer is almost the same as the treated, as shown in Fig.5 (b), for the grain size in the surface do not change much although the micro-strain releases after the heat treatment. So it could be concluded that it is just mainly because the nanocrystalline in the surface that the microhardness increases in the treated surface. However, with the increasing of heat-treatment temperature and grain size growing to that of the matrix, the microhardness of sample after 400 ℃×1 h heating become the same as that of the matrix.

519

micro-strain was also introduced into the surface by HESP. Due to the nanostructure and micro-strain formed on the surface, the micro-hardness of the surface was greatly enhanced, which was about twice as much as that of untreated. The micro-strain could be released by the heat treatment at 100 ℃ for 1 h, while the grain size and microhardness in the surface did not alter. However, the recrystallization and grain growing occurred and the grain size of the surface increased greatly after heat treatment at 400 ℃ for 1 h, which made the grain size and microhardness recover to the same condition of the as-received.

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Fig.5 Micro-hardness (a) along cross-section varied distance from surface in as-treated and (b) on the top surface in four different samples

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4 Conclusion

ment.[J] Journal of Alloys and Compounds, 2008, 452(2): 336-342

Surface nanocrystallization on die cast magnesium AZ91D could be realized by HESP and the average grain size on the treated surface layer were 60-70 nm. The

[9]

Cui Zhongqin. Physical Metallurgy and Heat Treatment [M]. Beijing: China Machine Press, 1988: 190-205