Shear deformation behavior of Zircaloy-4 alloy plate

Journal Pre-proof Shear deformation behavior of Zircaloy-4 alloy plate Fusen Yuan, Geping Li, Fuzhou Han, Yingdong Zhang, Ali Muhammad, Wenbin Guo, Hengfei Gu PII:

S0921-5093(20)30006-X

DOI:

https://doi.org/10.1016/j.msea.2020.138914

Reference:

MSA 138914

To appear in:

Materials Science & Engineering A

Received Date: 14 May 2019 Revised Date:

31 December 2019

Accepted Date: 2 January 2020

Please cite this article as: F. Yuan, G. Li, F. Han, Y. Zhang, A. Muhammad, W. Guo, H. Gu, Shear deformation behavior of Zircaloy-4 alloy plate, Materials Science & Engineering A (2020), doi: https:// doi.org/10.1016/j.msea.2020.138914. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

1

Title: Shear deformation behavior of Zircaloy-4 alloy plate

2

Fusen Yuana,b, Geping Lia,*, Fuzhou Hana,b, Yingdong Zhanga,b, Ali Muhammada,b, Wenbin Guoa,b,

3

Hengfei Gua,c

4

a

5

Republic of China

6

b

7

Road, Baohe District, Hefei, Anhui 230026, People’s Republic of China

8

c

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s

School of Materials Science and Engineering, University of Science and Technology of China, 96 JinZhai

University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing, 100049, People’s Republic of China

9 10

Abstract

11

Zircaloy-4 alloy is widely used in light water reactors. During cold rolling, this alloy is prone to

12

cracking under shear stress. The microstructural characteristics after shear deformation have been

13

investigated in this study in order to provide technical support for avoiding failure of this alloy.

14

Shear tests for recrystallized Zircaloy-4 alloy plate were performed at room temperature using a

15

designed shear testing device. Shear fracture surface and microstructure were carefully

16

characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD)

17

techniques, respectively. Results showed that grains in the initial plate are strongly oriented with

18

their c-axis close to the transverse direction (TD). Shear fracture surface can be simply divided into

19

two typical zones based on the morphology features: (I) smooth zone and (II) rough zone. The plate

20

exhibited anisotropy of shear yield strength in the order given as: Normal direction (ND)
21

direction (RD)
22

the Schmid factor theory. The fracture shear strain increased with the increase area of zone (I).

23

Prismatic slip and 1012 < 1011 > tensile twinning were the predominant deformation

24

modes during the shear deformation. In addition, 1012 < 1011 > tensile twinning activity was

25

high in grains oriented with their c-axis perpendicular to the shear stress direction (SSD).

26

Furthermore, work hardening occurred due to slip and twinning in the shear deformation region

27

(SDR), which substantially increased the microhardness of the SDR (198 HV) as compared to that of

28

matrix (157 HV). Shear failure tended to occur on the RD-TD45° plane with external load.

29

Keywords: Shear test; Zircaloy-4; Shear fracture surface; Deformation mode; Work Hardening

30

Corresponding author: [email protected] (Geping.Li) 1

31

1. Introduction

32

Shear testing is an obvious way to study the mechanical behavior of metals because metals basically

33

deform plastically by shear. That is, deformation of metals and alloys is accomplished by

34

dislocations gliding between well-separated slip planes through shear stress [1, 2]. Moreover, in

35

practical applications, materials are often directly subjected to shear stress (punching, machining,

36

impact, etc.). Thus, shear is an essential deformation way in alloys fabrication processes and

37

subsequent mechanics services. Several shear test methods have been used: torsion test [3, 4], shear

38

test [5-7], shear punch test [8] and shear test with hat shaped specimen [9, 10]. Except for torsion test

39

method which induces a pure shear stress state in tube or bar specimens, most other shear test

40

techniques provide a mixed stress state in the plane of plate [6]. Shear punch test and shear test with

41

hat shaped specimen can impose high strain rate during the deformation. Shear test as a very efficient

42

technique which can evaluate the mechanical properties of plate samples [11]. Briefly, the device for

43

shear test is designed in order to induce a parallel displacement and shear deformation (SD) occurs in

44

the clearance between the two grips.

45

Zirconium-based alloys are widely used for structural products in chemical and nuclear industries

46

thanks to their outstanding corrosion resistance, good mechanical properties, high irradiation stability

47

and low neutron absorption [12, 13]. Zircaloy-4 (Zr-4) alloy is the most commonly used zirconium

48

alloy which belongs to a zirconium-tin alloy containing iron and chromium as the major elements

49

among the minor constituents. It has been used as a nuclear fuel cladding material in light water

50

reactors (LWRs) such as pressurized water and boiling water reactors (PWRs and BWRs) due to its

51

low thermal neutron absorption cross section, superior corrosion resistance over other materials, and

52

adequate mechanical properties [14, 15]. At room temperature it exhibits hexagonal close-packed

53

(HCP) α phase with a c/a ratio lower than ideal (c/a<1.633) [16, 17]. Due to the nature of HCP

54

crystal structure and its limited number of slip systems, this alloy offers inherently anisotropic

55

mechanical, physical and chemical properties. At room temperature the easiest slip system is

56

prismatic slip on 1010 planes with direction along the a-axis (< 1120 >). To accommodate

57

deformation along the c-axis, the likely mechanisms in this alloy are pyramidal < +

> slip,

58

1012 < 1011 > tensile twinning and 1122 < 1123 > compressive twinning [18-20]. The

59

relative role of each deformation mode strongly depends crystal orientation (texture), temperature,

60

and strain rate. Texture in the alloy influence the subsequent mechanical processing and has 2

61

significant effects on the in-service performance such as creep, stress corrosion cracking and hydride

62

formation [21]. Tensile, compression and fatigue behaviors of Zr-4 alloy have been extensively

63

studied [22-24]. However, the shear deformation behavior of this alloy has hardly been explored. In

64

practical applications, shear stress state runs through the whole process of the alloy fabrication (like

65

rolling) and subsequent service. For instance, some cracks were observed which were caused by

66

shear stress during the cold rolling process. So it is crucial and necessary to investigate the shear

67

deformation behavior to provide technical support for avoiding failure of this alloy.

68

In the present study, shear tests for recrystallized Zr-4 alloy plate were performed at room

69

temperature using a designed shear testing device. The shear testing device (see Fig. 2) was made of

70

H13 tool steel. It comprised symmetrical lower and upper grip and the region of the narrow clearance

71

between the two grips is subjected to shear stress. The device and corresponding specimen

72

preparation is very simple. In addition, the size along the shear direction of present specimen (2mm)

73

is lower than used

74

and guarantees the stability of the shear load. After shear failure, shear fracture surface was

75

systematically characterized and the shear yielding strength anisotropy was discussed by the Schmid

76

factor theory based on the initial texture. In addition, the microstructure evolution was analyzed by

77

means of electron backscatter diffraction (EBSD) technique through different strain variables.

78

Effects of active deformation modes, especially the prismatic slip and 1012 < 1011 >

79

tensile twinning were discussed in detail. Furthermore, the mechanism contributing to hardness of

80

the plate was also addressed.

in Ref. [7], which prevents possible rotation of the shear direction during the test

81 82

2. Experiments

83

The material used in the present study, a 10 mm thick Zr-4 (Zr-1.5Sn-0.2-Fe-0.1-Cr wt. %) alloy

84

plate, was manufactured by cold rolling and subsequent annealing treatment at 630 ℃ for 3h

85

followed by furnace cooling. A sample was cut from the plate for initial texture analysis. A standard

86

EBSD specimen preparation procedure was used involving grinding and mechanical polishing

87

followed by electro polishing in a solution containing perchloric acid and ethanol 1:9 in volume ratio.

88

The electro polishing was operated at -50 °C, with current of 0.6 A for about 3 min. Texture analysis

89

was carried out on a ZEISS MERLIN compact field emission scanning electron microscope

90

(FE-SEM) operating at 20kV accelerating voltage and equipped with an Instrument Aztec software 3

91

and the EBSD map was analyzed by HKL Channel 5 system.

92

The specific shear deformation state (SDS) was determined by shear stress direction (SSD) and shear

93

plane (SP). Here, five SDSs were prepared and numbered 1#, 2#, 3#, 4# and 5#. The corresponding

94

SSDs and SPs are listed in Table 2. Fig. 1 demonstrates the geometric relationship between

95

specimens and the plate. Specimens with SSD along normal direction (ND), rolling direction (RD)

96

and transverse direction (TD) are shown in Fig. 1a, 1b and 1c, respectively. In addition, SP of TD45°

97

(the angle between SP and TD is 45 °) (see Fig. 1d) was selected because this plane is subjected to

98

largest resolved shear stress during rolling and forging [25-27] and the crack was also on this plane

99

during cold rolling. The corresponding SSDs include TD45° (the angle between SSD and TD is 45 °)

100

and RD (see Figs. 1f and 1e). The specimens were cut from the plate. Subsequent grinding on 150,

101

800 and 2000 grit papers and mechanical polishing were performed until mirror surface was obtained.

102

The final dimensions of the specimen are listed in Table 1.

103

104 105

Fig. 1. Geometric relationship between specimens and the plate. (a), (b) and (c) SSD along ND, RD

106

and TD, respectively. The corresponding SPs were marked by red dashed rectangles. (e) The

107

RD-TD45° shear plane in the plate. (f) and (g) SSD along TD45° and RD on RD-TD45° SP,

108

respectively. The SSDs were marked by gray arrows for all of the specimens.

109 4

Table 1 Specimens information for the shear test

110

Sample Number 1# 2# 3# 4# 5#

SSD

SP

Shear strain rate /s-1

Specimen dimensions /mm

ND RD TD TD45° RD

TD-ND RD-TD TD-ND RD-TD45° RD-TD45°

0.1 0.1 0.1 0.1 0.1

40*8*2 40*8*2 40*8*2 10*8*2 10*8*2

111 112

Shear tests were performed at room with shear strain rate of 0.1s-1 using a designed shear testing

113

device.

114

The device and specimen used in this investigation are shown in Fig. 2. The device was installed on

115

INSTRON 5582 testing system (pink arrows shown in Fig. 2a). The lower grip was fixed and the

116

upper grip was loaded during the test and the clearance between the lower and upper grip was 70µm

117

(D=70µm). ∆ℎ represents the displacement of upper grip during SD (see Fig. 2b). Shear strain ( )

118

and engineering shear stress ( ) are calculated by following equations [6, 8]：

119

=

= ∆ℎ/

120

= /(ℎ )

(1) (2)

denotes shear load, ℎ and

121

where

122

The clearance between the lower and upper grip is shear deformation region (SDR) and the shear

123

angle ( ) in Fig. 2c is directly related to the shear strain. For numbered of 1#, 3#, 4# and 5#, two

124

specimens were prepared for each sample and the shear test was conducted until failure. For 2#

125

sample, five specimens were prepared. The first two specimens were tested until failure and the

126

average fracture shear strain ( ) was obtained. Then, the remained three specimens were subjected

127

to shear strain of

=25%,

=50% and

are the thickness and width of the specimen, respectively.

=75%.

128

5

129 130

Fig. 2. The designed shear testing device and specimen for the shear test. (a) Shear testing device. (b)

131

A schematic of the shear test. The lower grip is fixed and the upper grip is loaded,

132

clearance between the lower and upper grip, ∆ℎ denotes the displacement of the upper grip during

133

SD. The SDR is marked by red parallelogram and the specimen for EBSD analysis is marked by blue

134

dashed line. (c) Specimen for shear test and the equations for shear strain and shear stress.

represents the

135 136

Shear fracture surface was observed using FEI Inspect F50 field-emission scanning electron

137

microscope (SEM). The images were performed using secondary electron with an accelerating

138

voltage of 20 kV. Specimens for EBSD measurement were cut from the specimens with

139

=50% and

=25%,

=75%, as the blue dashed line marked in Fig. 2b. The preparation procedure of the

140

specimen was the same as the initial texture analysis. Vickers hardness test was carried out on

141

FM-600e micro-Vickers hardness test machine with load of 50gf and dwell time of 10 s.

142 143

3. Results

144

3.1. Initial texture

145

Fig. 3(a) and (b) show phase distribution map and inverse pole figure (IPF) map of the recrystallized

146

plate, respectively. It can be observed that the initial microstructure is equiaxed α phase with an

147

average grain size of 9.5 µm in diameter (measured by linear intercept method). In Fig. 3(b), the

148

color represents the crystal orientation of the grains. Most of the initial grains are colored in green

149

and blue, while few grains are in red, suggesting that most grains had their c-axis close to the TD. 6

150

The misorientation angle distribution (MAD) histogram corresponding to Fig. 3b is displayed in Fig.

151

3c. To facilitate analysis of the following results, grain boundaries with misorientation lower and

152

higher than 15° were categorized of low angle grain boundaries (LAGBs) and high angle grain

153

boundaries (HAGBs), respectively [28]. It is obvious that the frequency of HAGBs (88.6%) is

154

overwhelmingly larger than that of LAGBs (11.4%) in the initial plate. Fig. 3d is a grain boundary

155

(GB) map in which LAGBs, HAGBs and two common twin boundaries were marked by different

156

colors. 64

157

zirconium and its alloy, namely 1122 < 1123 > compressive twinning and 1012 < 1011 >

158

tensile twinning, respectively. None of the twin boundaries were clearly calibrated in Fig. 3d,

159

suggesting the absence of these twins in the initial plate. Pole figures of α-Zr and three-dimensional

160

(3-D) schematic diagrams are shown in Figs. 3(e) and 3(f), to show the geometric relationship

161

between the unit cell and the plate. In Fig. 3(e), it is obvious that the c-axis of majority of α-Zr grains

162

is parallel to the TD. In addition, most {1010} planes were perpendicular to the RD since the pole

163

(with intensity about 5.4) exists in the center of the {1010} pole figure [13].

2° < 1010 > and 85

2° < 1120 > denote two types of most common twins in

164 165

Fig. 3. Initial texture of the plate. (a) Phase distribution map. (b) IPF map and corresponding legend.

166

(c) MAD histogram. (d) GB map and corresponding legend. (e) {0001}, {1120} and {1010} pole 7

167

figures. (f) The 3-D schematic diagram showing the relationship between the dominant orientation of

168

grains and the plate.

169 170

3.2 Shear properties

171

The engineering shear stress-shear strain curve is obtained by using Eqs. (1) and (2). Fig. 4 shows the

172

corresponding curves of the five SDSs. Several apparent stages can be distinguished on the typical

173

shear stress-shear strain curves. These are: (a) initial linear stage, (b) parabolic stage (c) near linear

174

stage and (d) final fracture stage. The value of shear stress at which a deviation from linear to

175

parabolic deformation occurs on the curve has been used to define the shear yielding strength [29].

176

The specific shear properties are summarized in Table 2. It can be seen from Fig. 4 and Table 2 that

177

the specimens with different SDSs exhibit discrepant shear properties, indicating plastic deformation

178

anisotropy of the plate [6]. The sample 1# and 4# presented lowest (226.8MPa) and highest

179

(318.6MPa) shear yielding strength among the five SDSs, respectively. It means that the deformation

180

modes activated easily when the SSD was applied along the ND on TD-ND plane. However, when

181

the SSD is parallel to TD45° direction on RD-TD45° plane, the deformation modes could hardly

182

activate. In general, the yielding strength relationship of the four SSDs is ND
183

(Table 2). In addition, the fracture shear strain and ultimate shear strength of different SDSs also

184

vary greatly. These results revealed that the shear properties of the plate are highly anisotropic. The

185

anisotropy for shear yielding strength and fracture shear strain were studied in detail in the following

186

discussion section.

8

187 188

Fig. 4. Engineering shear stress-strain curves.

189

Table 2 Summary of shear properties (mean values) for the shear tests Number

Shear yielding strength (MPa)

Ultimate shear strength (MPa)

Fracture shear strain

1#

226.8

340.3

9.8

2#

261.7

399.6

10.6

3#

283.4

467.0

14.5

4#

318.6

506.7

12.8

5#

245.8

360.2

11.0

190 191

3.3 Shear fracture surface

192

The shear fracture surface has been hardly investigated systematically and its micro-morphology was

193

still unclear. Here, we study the entire morphologies and local micro morphology characteristics of

194

the shear fracture surface.

195

Figs. 5-9 show the shear fracture surface after shear tests. All of the entire morphologies exhibit a

196

similar characteristics. Shear fracture surface can be simply divided into two typical zones based on

197

the morphology features: (I) smooth zone, (II) rough zone (Figs. 5a, 6a, 7a, 8a and 9a). The smooth

198

zones were caused by rubbing between the fragment and the fracture surface [4, 30], resulting in

199

parallel lines along the SSD. The parallel lines defined as abrasion marks (AMs) [31, 32]. Zone (II)

200

showed apparent dimples. In addition, the dimples were elongated along the SSD, called as parabolic

201

dimples [33]. It means large plastic deformation has taken place [10]. Therefore, the zone (II) 9

202

belongs to the final fracture zone. Furthermore, the magnitude of the two zones for different SDSs

203

varied greatly. The area of zone (I) for sample 3# and 4# was larger than that of the other three SDSs

204

(Figs. 5b, 6b, 7b, 8b, and 9b). For sample 1#, AMs and PDs were the predominant characteristics in

205

the fracture surface and zone (II) occupied the main area of the surface, as shown in Fig. 5. For

206

sample 2#, AMs and PDs were present in the fracture surface. The AMs in zone (I) were more

207

obvious than sample 1# and dimples were less elongated than sample 1# (see Fig. 6). For sample 3#,

208

the AMs in zone (I) became more remarkable than sample 1# and 2#. In addition, secondary cracks

209

(SC) (see Fig. 7c and f) were observed in zone (I) which were perpendicular to the SSD. For sample

210

4#, there were the most remarkable AMs in zone (I) among the five samples, as shown in Fig. 8c and

211

f. In addition, the SCs were also detected (see Fig. 8d). For sample 5#, like sample 1#, zone (II)

212

occupied the main area of the surface, whereas, the PDs were less elongated than sample 1#, as

213

shown in Fig. 9.

214

215 216

Fig. 5. Shear fracture surface of sample 1#. (a) Entire surface. (b)-(f) Morphology of localized

217

regions at high magnification. AM and PD are marked by green and pink arrows, respectively.

218

Yellow arrow denotes the SSD.

10

219 220

Fig. 6. Shear fracture surface of sample 2#. (a) Entire surface. (b)-(f) Morphology of localized

221

regions at high magnification. AM and PD are marked by green and pink arrows, respectively.

222

Yellow arrow denotes the SSD.

223 224

Fig. 7. Shear fracture surface of sample 3#. (a) Entire surface. (b)-(f) Morphology of localized 11

225

regions at high magnification. AM, PD and SC are marked by green, pink and red arrows,

226

respectively. Yellow arrow denotes the SSD.

227

228 229

Fig. 8. Shear fracture surface of sample 4#. (a) Entire surface. (b)-(f) Morphology of localized

230

regions at high magnification. AM, PD and SC are marked by green, pink and red arrows,

231

respectively. Yellow arrow denotes the SSD.

12

232 233

Fig. 9. Shear fracture surface of sample 5#. (a) Entire surface. (b)-(f) Morphology of localized

234

regions at high magnification. AM and PD are marked by green and pink arrows, respectively.

235

Yellow arrow denotes the SSD.

236

4. Discussion

237

4.1 Shear properties anisotropy of the Zr-4 alloy plate

238

Zr and its alloys are characterized by pronounced mechanical anisotropy [34]. The mechanical

239

response of HCP α-Zr strongly depends on the deformation modes including slip and twining. For

240

slip systems of Zr, the direction of the dominant slip system is always < 1210 >, but the associated

241

slip plane may be either 0001 basal, 1010 prismatic or 1011 pyramidal [18, 19]. For

242

twinning in Zr, there are two dominant twins: tensile twinning ( 1012 < 1011 >) and compressive

243

twinning ( 1122 < 1123 >) [35, 36]. The relative contribution of these deformation modes

244

strongly depends on crystal orientation, temperature and strain rate [37, 38]. Deformation mode

245

having a higher Schmid factor value (#) can get a larger shear stress resolved from the external load

246

[28].

247

The anisotropy of shear yielding strength of the plate was discussed mainly through the initial texture,

248

because there was no variable of temperature and strain rate. Here three dominant slip systems (basal,

249

prismatic and pyramidal slip), tensile and compressive twinning were analyzed, while the 13

250

pyramidal slip was excluded in the consideration due to its high critical resolved shear stress

251

(CRSS) [39, 40]. The # of basal, prismatic and pyramidal slip and the two kinds of twins for

252

the grains were calculated based on EBSD results. Fig. 10 shows the distribution maps of the #

253

with the four SSDs. Each color represents a specific value of #. It is obvious that the #

254

significantly varies with the SSDs. Fig. 11 displays statistical results of # corresponding to those

255

given in Fig. 10. It can be identified that high frequency of high # valves existed for prismatic and

256

pyramidal slip and the two kinds of twins, while the SSD was along the ND, indicating that

257

majority of the grains in the plate were geometrically favorable for prismatic and pyramidal slip

258

and the two kinds of twins under this external load[13]. Similarly, prismatic and pyramidal slip

259

and tensile twinning were preferred while the SSD was along the RD. However, the relative

260

frequency of high # values along RD was lower than that along ND. When the SSD was along the

261

TD, in addition to the compressive twinning, the relative frequency of high # values for the

262

remaining three slips and tensile twinning was distributed below 6%, suggesting that few grains were

263

geometrically favorable for the initiation of dislocation and tensile twinning. When the SSD was

264

along TD45°, the relative frequency of high # values of the deformation modes was mostly lowest

265

among the four SSDs. Especially, the lowest frequency of high # values was found for prismatic

266

and pyramidal slips (the most easily active slip systems in zirconium) among the four SSDs.

267

Therefore, according to Fig. 11, the average # values for all the grains in the Zr-4 alloy plate

268

during the shear load can be described as: ND>RD>TD>TD45°. Thus, the yielding strength

269

relationship along the four SSDs is ND
270

results (Table 2).

14

271 272

Fig. 10. Schmid factor (#) value distribution maps of the recrystallized Zr-4 alloy plate. ND, RD,

273

TD and TD45° indicate the four SSDs.

15

274 275

Fig. 11. Relative frequency of # for the five deformation modes along the four loading directions.

276

(a) ND, (b) RD, (c) TD and (d) TD45°.

277 278

The fracture shear strain varied greatly in different SDSs (Table 2). According to the Eq. (1), the

279

shear strain is corresponded to the displacement of the upper grip (∆ℎ) due to the constant

280

the SD. In addition, there were two typical zones on the fracture surface and zone (II) belongs to the

281

final fracture zone. Therefore, ∆ℎ (shear strain) was mainly related to the area magnitude of zone (I).

282

Whereas, zone (II) made minor contribution to ∆ℎ. An outline on the area magnitude of zone (I) was

283

presented in Figs. 5-9 (marked by red dashed lines). The area of zone (I) of the five samples can be

284

expressed as: 3#>4#> (1#, 2# and 5#). This relationship is consistent with the shear strain (Table 2).

285

It is well known that the maximum resolved shear stress is located on the plane with its normal

286

direction 45° away from the compression direction [27]. Cracks are often observed on this plane

287

during cold rolling and forging. The designed RD-TD45° plane in the present study is exactly this

288

plane. By comparing these five SDSs, we can find that sample 5# (shear plane: RD-TD45°, shear 16

during

289

direction: RD) offered weak resistance to shear failure (including shear yielding strength, ultimate

290

shear strength and shear fracture strain) (see Table 2). Thus, based on the above results (RD-TD45°

291

plane having maximum resolved shear stress and weak resistance to shear failure), shear failure

292

tended to occur on the RD-TD45° plane during rolling process.

293 294

4.2 Microstructure evolution during the shear deformation process

295

SD with SSD along RD (sample 2#) with different shear strains was selected to reveal the

296

microstructure evolution during the SD process. Fig. 12 shows morphologies of the SDR. With

297

increasing the shear strain, the SDR (boxed by yellow lines in Fig. 12) became more and more

298

evident. The most serious SDR occurred at the top and bottom of the specimen (see pink arrows in

299

Fig. 12), in other words, at the contact interface between the specimen and device (see Fig. 2b).

300

Further observation of this region revealed a series of slip bands. In addition, a certain region

301

exhibited parallel slip bands (marked by blue arrows in Fig. 12d) and the size of this region was

302

exactly consistent with the average size of the α-grain (see Fig. 3b and green dashed circles in Fig.

303

12d). The slip band is the direct evidence for slip during the SD.

304

305 17

306

Fig. 12. Morphology of the SDR with SSD along RD (sample 2#) and corresponding SSD. (a), (b)

307

and (c) Optical morphology of the SDRs with shear stain

308

respectively. (d) SEM image of the region boxed by red lines in (c). The local parallel slip bands are

309

marked by blue arrows and the regions of green dash lines denote the initial α-Zr grains.

=25%,

=50% and

=75%,

310 311

To further reveal microstructural features of the SDRs during SD, EBSD examinations were

312

performed. The specimens for EBSD test were cut from the deformed specimens, as the blue dashed

313

lines shown in Fig. 2b. Fig. 13 shows IPF maps, GB maps and MAD histograms derived from the

314

EBSD data for different shear strains ( =25%,

315

texture, overwhelming majority of the grains should be green or blue as was before SD (see Fig. 3b).

316

However, from the IPF maps in Fig. 13, it can be observed that the color in SDRs (red) was quite

317

different from adjacent grains (green or blue), indicating a significant rotation of the grains [41].

318

According the legend, grains with red color means their c-axis lies close to the ND. Therefore the

319

c-axis of the grains was close to the TD and ND before and after SD, respectively, indicating the

320

grains in SDRs rotated about 90°. Careful analysis shows that some individual grains contain

321

different colors (yellow arrows marked in the IPF maps in Fig. 13). In addition, the GB maps show

322

these grains were subdivided into several parts separated by many LAGBs (yellow arrows marked in

323

the GB maps in Fig. 13) that were the product of dislocation slip [28]. Through comparison of the

324

GB maps for the three shear strains, it is obvious that a larger shear strain led to increased fraction of

325

LAGBs. A quantitative measurement on this trend is displayed in the MAD histograms. For the

326

slightly deformed specimen ( =25%), 20.2% of grain boundaries possessed misorientation angle

327

lower than 15°. This fraction of LAGBs is obviously higher than that of the recrystallized state

328

(11.6%) (see Fig. 3c), suggesting substantial initiation of dislocation slip [28]. As the shear strain

329

increased from

330

that prismatic slip is the most active one due to its low CRSS at room temperature [19, 42]. In

331

addition, when the load was perpendicular to the primary c-axis texture, prismatic slip was easy

332

to activate [43]. Furthermore, the high frequency of high # values for prismatic slip existed

333

while the SSD is along RD (SSD was almost perpendicular to the primary c-axis). Thus, the LAGBs

334

mainly came from the contribution of the prismatic slip. In other words, the predominant active

335

slip system was prismatic slip during the SD.

=25% to

=50% and

=75%). According to the initial

=75%, the fraction of LAGBs increased to 42.7%. It is well known

18

336

In addition to LAGBs, there were also some lamellae structures marked by red lines in the GB maps

337

indicating the presence of 1012 < 1011 > tensile twinning. Nevertheless no blue lines were

338

observed in Fig. 13 implying the absence of 1122 < 1123 > compressive twinning. Furthermore,

339

twin density increased with the shear strain increasing from

340

amount of these twins decreased with the subsequent larger shear strain ( =75%) (red arrows shown

341

in Fig. 13). It is attributed to the predominance of dislocation slip (LAGBs) with the shear strain

342

increasing from

343

in Fig. 13). Fig. 14 shows a schematic diagram of these twins based on the SD process. The black

344

lines delineate the grain boundary, the hexagonal prisms represent the crystal orientation of each

345

parent grain and the red (dashed) rectangles denote the tensile twinning. This can be described as the

346

twinning activity was high in grains oriented perpendicular to the SSD. In general, when the external

347

shear load was perpendicular to the primary c-axis texture, the dominant deformation modes were

348

prismatic slip and 1012 < 1011 > tensile twinning.

=50% to

=25% to

=50%. Whereas, the

=75%, making the fraction reduction of the twins (MAD histograms

349 19

350 351

Fig. 13. IPF maps, GB maps and MAD histograms of sample 2# with different shear strains ( =25%, =50% and

=75%).

352

353 354

Fig. 14. Schematic representation of the active twin systems during SD.

355 356

4.3 Work hardening and microhardness

357

The engineering shear stress-shear strain curve (Fig. 4) exhibited apparent work hardening after

358

yielding (stage (b) and (c)). The sample 2# was selected for detailed discussion. The engineering

359

shear stress-shear strain curve of sample 2# can be divided into two stages after yielding: stage (b)

360

with parabolic strain-hardening behavior and stage (c) with almost linear strain-hardening behavior.

361

The stage (b) is well-known with parabolic behavior due to slip hardening [44]. This result suggests

362

that slip is the predominant deformation mode in stage (b). The stage (c) containing almost linear

363

strain-hardening behavior is different from the stage (b). The EBSD results (Fig. 13) revealed that

364

twinning plays a significant role during this stage [44]. Thus, twinning and slip occurred

365

simultaneously in stage (c). In general, the slip increased the fraction of LAGBs, resulting in work

366

hardening (like the effects of refined crystalline strengthening). In addition, grain rotation and twin

367

boundaries induced by twinning also contributed to the work hardening. 20

= 75% were acquired and displayed in Fig. 15.

368

The microhardness values of the specimen with

369

The load for microhardness measurement was parallel to the SSD (see the inset of Fig. 15) and the

370

measured points were arranged along a line perpendicular to the SDR (red arrow in Fig. 15).

371

According to the magnitude of microhardness, the measurement trace can be divided into three

372

regions, including matrix, shear deformation affected region (SDAR) and SDR. The SDR is at the

373

center of the measurement trace. The microhardness of matrix and SDR was about 157 and 198 HV,

374

respectively. Apparently, the microhardness in the SDR was about 26% higher than that of the

375

matrix. This increase in the microhardness was essentially due to the work hardening induced by slip

376

and twinning activity in the SDR.

377

378 379

Fig. 15. Microhardness of the specimen with

380

made along red arrow while the load direction was parallel to the SSD.

= 75%. The inset denotes that the indents were

381 382

5. Conclusions

383

Shear tests with multi-direction were designed and performed on recrystallized Zr-4 alloy plate at

384

room temperature. The shear fracture surface of the Zr-4 alloy was studied systematically. In 21

385

addition, the shear properties anisotropy, microstructure evolution at the shear deformation region

386

and work hardening were discussed in detail. Major conclusions can be drawn as follows:

387 388

(1) Shear fracture surface can be simply divided into two typical zones based on the morphological

389

features: (I) smooth zone with predominant abrasion marks and (II) rough zone mainly

390

containing parabolic dimples. The zone (II) belonged to the final fracture zone and the fracture

391

shear strain increased with increasing the area of zone (I).

392

(2) Grains in the Zr-4 alloy plate were recrystallized in α phase, and strongly oriented with their

393

c-axis close to the TD. The plate exhibited anisotropy of shear yield strength in the order:

394

ND
395

different SSDs induced by initial texture.

396

(3) Prismatic slip and 1012 < 1011 > tensile twinning were the predominant deformation

397

modes during shear deformation. Meanwhile, tensile twinning activity was high in grains

398

oriented perpendicular to the SSD.

399

(4) Slip and twinning jointly work-hardened the alloy. Furthermore, the work hardening made the

400

microhardness within the SDR (198HV) significantly higher than that of the matrix (157HV).

401

(5) Shear failure tended to occur on the RD-TD45 plane can certainly be helpful in avoiding failure

402

of Zr-4 alloy during forming and applications.

403 404

Acknowledgements

405

We thank Dr. Jin Cai and Dr. Guangcai Ma for the shear tests and EBSD measurements,

406

respectively.

407 408

Author contributions

409

G.L. and F.Y. contributed to the design of the experiments. F.Y., C.L. and F.H. carried out the

410

experiments. G.L. and F.Y. wrote the first draft of the manuscript and all authors assisted in the

411

writing process and data analysis.

412 413 414

Declaration of Interest Statement

22

415

The authors declared that they have no conflicts of interest to this work.

416 417 418

References

419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457

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