Strengthening Mechanisms in Nanostructured Al/Sicp Composite Manufactured by Accumulative

1 Strengthening mechanisms in nanostructured Al/SiCp composite

2 manufactured by accumulative press bonding

3 4 Sajjad Amirkhanlou a,b,c,*, Mehdi Rahimian c,d, Mostafa Ketabchi a, Nader Parvin a, Parisa

5 Yaghinali a, Fernando Carreño b

6 7 a Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran,

8 Iran

9 b Department of Physical Metallurgy, CENIM-CSIC, Av. Gregorio del Amo 8, 28040 Madrid, Spain

10 c Institute of Materials and Manufacturing, Brunel University London, London UB8 3PH, United

11 Kingdom

12 d IMDEA Materials Institute, C/Eric Kandel 2, 28906, Getafe, Madrid, Spain

13 14 * Corresponding author: Email: [email protected]; [email protected] 15

17 Abstract

18 The strengthening mechanisms in nanostructured Al/SiCp composite deformed to

19 high strain by a novel severe plastic deformation process, accumulative press

20 bonding (APB), was investigated. The composite exhibited yield strength of 148

21 MPa which was 5 and 1.5 times higher than that of raw aluminum (29 MPa) and

22 aluminum-APB (95 MPa) alloys, respectively. A remarkable increase was also

23 observed in the ultimate tensile strength of Al/SiCp-APB composite, 222 MPa,

24 which was 2.5 and 1.2 times greater than the obtained values for raw aluminum

25 (88 MPa) and aluminum-APB (180 MPa) alloys, respectively. Analytical models

26 well described the contribution of various strengthening mechanisms. The

27 contribution of grain boundary, strain hardening, thermal mismatch, Orowan,

28 elastic mismatch and load-bearing strengthening mechanisms to the overall

29 strength of the Al/SiCp micro-composite were 64.9, 49, 6.8, 2.4, 5.4 and 1.5 MPa,

30 respectively. Whereas Orowan strengthening mechanism was considered as the 2

31 most dominating strengthening mechanism in Al/SiCp nanocomposites, it was

32 negligible for strengthening of the micro-composite. Al/SiCp nanocomposite

33 showed good agreement with quadratic summation model; however, experimental

34 results exhibited a good accordance with arithmetic and compounding summation

35 models in the micro-composite. While average grain size of the composite reached

36 380 nm, it was less than 100 nm in the vicinity of SiC particles as a result of

37 particle stimulated nucleation mechanism.

38 39 Keywords: Accumulative press bonding (APB); Severe plastic deformation (SPD);

40 Strengthening mechanisms; Analytical models; Metal matrix composites;

41 Nanostructured materials

43 1. Introduction

44 Aluminum matrix composites (AMCs), reinforced with particulate reinforcement,

45 have attracted considerable attention in automotive and aerospace industries, due

46 to their low weight and high mechanical properties [1, 2]. Silicon carbide (SiCp) is

47 considered as a typical cost effective particulate reinforcement used widely in

48 AMCs because of its high strength and modulus [3, 4]. Traditional processing

49 routes for fabrication of Al/SiCp composite, including casting, powder metallurgy

50 and spray forming encounter various shortcomings. The main drawbacks of those

51 liquid state techniques [5, 6] can be referred as SiCp agglomeration, weak

52 adhesion and undesirable chemical reaction occurred between Al and SiCp [7, 8].

53 However, manufacturing techniques in solid state can overcome the above

54 problems [9-11]. Microstructure and mechanical properties of Al/SiCp composite,

55 manufactured by accumulative roll bonding (ARB) as a solid-state process, was 3

56 evaluated by Jamaati et al. [12-14]. Accumulative press bonding (APB), introduced

57 for the first time in our previous works, is another severe plastic deformation

58 process [15, 16] enabling us to fabricate particle reinforced AMCs. Uniform

59 distribution of reinforcement, nano/ultra-fine structure and high mechanical

60 properties are obtained using APB process [17-20]. Many researches were focused

61 on the fabrication and characterization Al/SiCp composites prepared by metal

62 forming processes [21, 22]. However, individual contributions of various

63 micromechanics strengthening factors in AMCs deformed to high strain were not

64 investigated in previous studies. In this study the novel APB process was utilized

65 for fabrication of Al/SiCp composite and the effect and proportion of various

66 strengthening mechanisms on the final yield strength was assessed. Moreover,

67 advanced microstructural characterization techniques were employed to verify

68 each strengthening mechanism.

70 2. Experimental procedure

71 As-received AA1050 aluminum sheets, chemical composition is given in Table 1,

72 and SiC particles with an average size of 10 m were used as raw materials.

73 Aluminum sheets with the dimensions of 100 mm  50 mm  1.5 mm were

74 annealed at 623 K (350 ºC) for 1 h. The accumulative press bonding (APB)

75 process for manufacturing of the Al/10 vol.% SiCp composite was schematically

76 reported in ref. [23, 24]. The aluminum sheets were degreased in acetone bath

77 followed by scratch brushing with 0.4 mm wire diameter and peripheral speed of

78 2800 rpm. The reinforcement particles were uniformly spread between surfaces by

79 a hand sprayer. A hydraulic press machine was utilized to form a mechanical bond

80 between two stacked sheets, in a channel die, where the thickness of sheets 4

81 reduced by 50%. The APB process was performed at ambient temperature. The

82 fabricated sheet was cut in two pieces and the whole mentioned process was

83 repeated 5 times in order to increase SiC particles to 10 vol.%. Thereafter, the

84 above process was repeated 7 times but without any reinforcement addition. The

85 same process was employed for the production of the monolithic aluminum in

86 which the aluminum sheets were processed by APB without adding any SiCp

87 powder through the process.

88 Tensile tests were performed according to ASTM E8 standard at a rate of 1.6 by

89 a Houndsﬁeld H50KS machine. The gauge width, thickness and length of

90 specimens were 6, 1.5 and 25 mm, respectively. Various microstructural aspects

91 of specimens were investigated by transmission electron microscopy (TEM, JEOL

92 JEM 2000 FX II, JEOL Ltd. Tokyo, Japan) operating at 200 kV and field-emission

93 scanning transmission electron microscopy (FE-STEM, HITACHI S-4800, Hitachi

94 Ltd., Tokyo, Japan) operating at 10 kV complemented by energy-dispersive

95 spectroscopy (EDS, 10mm2 SDD Detector X-ACT, Oxford instrument, Oxford,

96 England). Also the grain boundary characterization was performed by electron

97 backscattered diffraction (EBSD, JEOL JSM 6500 F) adjusted at 20 kV with a

98 working distance of 15 mm, step size of 80 nm and tilt angle of 70º. Thin foils

99 required for EBSD, TEM and STEM investigations were mechanically ground and

100 punched into 3 mm discs with an average thickness of less than 100 μm. The discs

101 were subsequently thinned to perforation using a twin-jet electropolishing facility

102 (TenuPol-5, Struers) with a solution of 30% nitric acid and 70% methanol at 11 V

103 and 245 K (−28 ºC). The X-ray pattern of the manufactured Al/SiCp composite was

104 recorded with an X-ray diffractometer (XRD). The XRD experiment was conducted by a

105 Philips X’PERT MPD X-ray diffractometer with CuKα radiation in the range of using a 5

106 step size of and a counting time of 1 s per step. Consequently, XRD patterns were

107 analyzed via X’Pert HighScore software.

109 3. Results and discussion

110 The stress-strain curves of annealed aluminum (Al), monolithic aluminum (Al-APB)

111 and Al/SiCp-APB composite are shown in Figure 1. According to the Figure 1, the

112 yield strength of the aluminum, which is 29 MPa, was improved by 5 times, as it

113 increases to 148 MPa. A remarkable increase was also observed in the ultimate

114 tensile strength of Al/SiCp-APB composite, 222 MPa, which was 2.5 and 1.2 times

115 greater than the obtained values for raw aluminum (88 MPa) and aluminum-APB

116 (180 MPa) alloys, respectively. Although this study has not been done previously,

117 relevant composites fabricated via other production processes are summarized

118 in Table 2. The superior strength of the produced composite through APB process

119 is obtained mainly due to the uniform distribution of particles, formation of ultra-fine

120 structure and low level of porosity. The enhancement of composite’s strength can

121 be described by different mechanisms. In following sections, microstructural

122 evidences and theoretical models are employed to explain each strengthening

123 mechanism.

125 3.1- Grain boundary

126 Figure 2 shows STEM micrographs of Al/SiCp composite after various cycles of

127 APB process. It is observed that gradual grain refining occurred during process

128 and grains are slightly elongated in the longitudinal direction. Average grain size

129 reduced to 380 nm after 14 cycles of APB, Figure 2e. Grain refining is the most

130 desirable strengthening mechanism because it is only mechanism which leads to 6

131 simultaneous increment of strength and toughness [25, 26]. The formation

132 mechanism of nano grains by the APB process is considered as continuous

133 dynamic recovery (CDR). In CDR the size of small (sub) grains remains constant,

134 whereas grains misorientation increases. In fact, there isn’t any nucleation and

135 growth of deformed nuclei in CDR, because the dislocations glide directly from one

136 side of grain to the other side resulting in the increment of grains misorientation.

137 This is the most equilibrated way of obtaining the finest and sharpest histogram of

138 grain sizes, which leads to the highest misorientation for the given processing

139 conditions. The grain refinement mechanisms of pure aluminum under APB

140 process were discussed in our previous studies [19, 20]. However, two other

141 factors encourage CDR of Al/SiCp composite including severe shear deformation

142 and micro-size particles. In fact, finer grain size can be obtained in APB process

143 on account of the present of non-deformable reinforcements. Figure 3 displays the

144 interface of the SiC particle and aluminum matrix. The finer grain sizes are

145 recognized in the vicinity of SiC particles where the average grain size measured

146 less than 100 nm. When the composite is exposed by deformation during the

147 process, the existence of non-deformable particles induces strain to their vicinity.

148 As a result, the vicinity of particles is fertilized to form new boundaries due to the

149 introduction of a high dislocation density, referred as particle stimulated nucleation

150 (PSN) [27, 28]. The accumulation of dislocations in the vicinity of particles

151 facilitated the formation of fine grains by continuous dynamic recovery mechanism.

152 Consequently, the average grain size of the composite, 380 nm, is finer than that

153 of monolithic aluminum which is 450 nm [19]. Other factor, considered for grain

154 refinement of pure aluminum and the composite, is severe shear deformation.

155 TEM micrographs of surface and center of the monolithic aluminum after one APB 7

156 cycle are shown in Figure 4. Comparison of Figure 4a and b demonstrates the

157 higher density of dislocation tangle zones on the surface. This observation is

158 attributed to the severe shear strain exists between the sample and press anvil. In

159 each APB cycle, the surface containing higher dislocation density is moved toward

160 the center resulting in homogeneous distribution of dislocation through the bulk

161 material. Therefore, dislocations formed because of severe shear contribute to the

162 final grain refinement. Grain boundary strengthening () can be explained by well-

163 known Hall-Petch equation (Eq. 1) [29]. Higher fractions of grain boundaries

164 existing in finer grain structures increase the number of obstacles against

165 dislocation movement.

166 (1)

167 where is average grain size, is constant and typically equal to 40 MPa for

168 aluminum alloys [19, 30]. While the grain boundary strengthening was calculated

169 5.2 MPa for Al [20], it increased to 59.6 MPa and 64.9 , for Al-APB and Al/SiCp

170 composite, respectively.

172 3.2- Thermal mismatch (TM)

173 Discrepancy of thermal expansion coefficient (CTE) between matrix and

174 reinforcement acts as a dislocation generation source [31, 32]. Since, thermal

175 expansion coefficient of the matrix, , differs from the SiCp reinforcement, , strain is

176 induced to the matrix around the particles resulting in dislocation formation, as

177 shown in Figure 5a. Multi-directional thermal stresses at the particle/matrix

178 interface, which are induced by the difference of thermal expansion between

179 aluminum and SiC particles, result in mismatch strain around the particles. The

180 system makes an attempt to reduce internal energy, mismatch strain, via 8

181 introducing new dislocations [33, 34]. High dislocation density in the vicinity of

182 particles, observed in Figure 5a, can arrange and form new grain boundaries via

183 continuous dynamic recovery during APB process, as shown in Figure 5b.

184 Strengthening effect of thermal mismatch () can be expressed by the following

185 equations [35, 36]:

186 (2)

187 where is shear modulus (~25.4 GPa for aluminum) and is the average value of

188 dislocation strengthening efﬁciency (∼1 for pure metals [37]) and is the Burgers

189 vector (=0.286 nm [38]). Dislocation density, resulted from CTE mismatch, is

190 governed by particles volume fraction, , difference between processing and

191 ambient temperature, [39], and variation between CTE of particles and matrix, .

192 Dislocation density induced by thermal mismatch can be calculated by [40]:

193 (3)

194 The amount of is calculated around 6.8 MPa for Al/SiCp composite, while this

195 mechanism is not taken into account for Al and Al-APB alloys.

197 3.3- Elastic mismatch (EM)

198 The difference of elastic modulus between matrix and reinforcement introduce an

199 additional dislocation into the composite in order to reduce induced plastic strain.

200 The density of generated dislocation due to elastic modulus mismatch can be 9

201 estimated by Eq. (4). These dislocations induce additional strength to the

202 composite which is expressed by Eq. (5) [41]:

203 (4)

204 (5)

205 where is yield strain (0.2%) and is density of dislocations caused by elastic

206 mismatch [42]. Whereas, due to absence of reinforcement in Al and Al-APB, there

207 is no elastic mismatch strengthening effect, it is calculated around 5.4 for Al/SiCp

208 composite.

210 3.4- Strain hardening

211 Figure 6 displays EBSD/orientation imaging microscopy (OIM) and grain boundary

212 maps of Al/SiCp composite. The red/gray lines correspond to the low angle grain

213 boundaries (LAGBs) having misorientations 2-15º, and the high angle grain

214 boundaries (HAGBs) are shown as black lines which have misorientations above

215 15º. The fraction of high angle grain boundaries () and the mean misorientation

216 angle of the boundaries () for the Al/SiCp composite were 73% and 35º,

217 respectively. According to EBSD results, it is obvious that APB process had a

218 significant effect on the development of an ultra-fine grain structure surrounded

219 mainly by high-angle boundaries. Formation of the well-developed high angle

220 boundary during APB process is attributed to the rearrangement of the

221 dislocations via short-range diffusion [43-46]. As a result of mechanical

222 deformation, dislocations will be generated resulting in the increment of strength. It

223 is well known that dislocations tend to array and form low angle grain boundaries

224 during severe plastic deformation process. Therefore, low angle grain boundaries

225 can be considered as a dislocation resource. In other word, HAGBs contribute to 10

226 the grain boundary strengthening mechanism which is determined by Hall-Petch

227 relation, whereas dislocation strengthening mechanism is related to LAGBs, as

228 explained by Hansen et al. [47]. The strength imposed by LAGBs to the system is

229 expressed by:

230 (6)

231 where  is the dislocation strengthening efﬁciency (the average value = 0.24) and

232 M is the Taylor factor (for aluminum is 3.06). Following equation shows the density

233 of dislocations introduced by LAGBs to the system [48, 49]:

234 (7)

235 where , and are the mean misorientation of LAGBs, volume fraction of HAGBs and

236 average LAGBs spacing that is measured from EBSD results. is 8, 47 and 49

237 MPa for initial aluminum, Al-APB and Al/SiCp composite processed by APB,

238 respectively.

240 3.5- Orowan strengthening

241 Orowan mechanism corresponds to the interaction of the particles and dislocations

242 in which nano particles pin dislocations resulting in bowing dislocation around

243 particles and create Orowan rings. Increment of yield strength, in polycrystalline

244 materials, induced by Orowan mechanisms can be calculated by [41, 50]:

245 (8)

246 where is the Poisson’s ratio (0.33). A small contribution of Orowan strengthening

247 mechanism, , in Al/SiCp micro-composite can be interpreted by large distance of

248 micro-size particles.

250 3.6- Load-bearing 11

251 FE-SEM micrographs of Al/SiCp composite after several APB cycles are shown in

252 then Figure 7. With increasing number of cycles, the laminar structure is converted

253 into the homogeneous structure. The formation mechanism of this structure is

254 explained comprehensively in our previous study [17, 18]. It should be briefly

255 pointed out that aluminum plastic flow, because of applied stress during APB, led

256 to refinement and dispersion of SiCp clusters. The high pressures associated with

257 APB resulted in the squeezing of the Al-matrices within the SiCp clusters producing

258 homogenous structure. Formation of strong bond between the particles and matrix

259 due to extensive pressure can be another advantage of current process. Since, in

260 the tensile test a fraction of stress is transferred to particles, having higher

261 modulus and strength compared with matrix, composite can withstand higher load

262 than monolithic aluminum. In order to achieve maximum potential of load-bearing

263 effect, homogeneous distributed particles having strong bond with matrix are

264 required. Figure 8 displays SEM micrographs of Al/SiCp composite produced by

265 APB together with its EDS and X-ray maps. Al4C3 phase, observed usually in the

266 cast Al/SiCp composites, exhibits detrimental effect on interfacial bonding and

267 mechanical properties on account of its brittle nature [51]. The X-ray maps (Figure

268 8b-f) and X-ray diffraction pattern (Figure 9) show that there is no evidence of

269 undesired phase such as Al4C3 in the microstructure considered as the advantage

270 of solid state fabrication of Al/SiCp composite by the current process.

271 Well distributed particles endure a proportion of applied force imposed directly by

272 tensile test. The contribution of load-bearing mechanism in increasing of yield

273 strength is expressed by Eq. 9, which is the modification of shear-lag model:

274 (9) 12

275 where and are referred to volume fraction of particles and matrix yield strength,

276 respectively. is 1.5 MPa for Al/SiCp composite.

277 The total yield strength is calculated by three well-known models referred as

278 arithmetic summation (Eq. 10), quadratic summation (Eq. 11) and compounding

279 methods (Eq. 12) [41, 52, 53]:

280 (10)

281 (11)

282 (12)

283 Contribution of the various strengthening mechanisms as well as yield strength,

284 obtained by various models and tensile tests, are displayed in Table 3. The

285 influence of each factor on yield strength of micro-composite is evaluated against

286 that of nanocomposite, which was investigated in our previous study [20].

287 Matrix flow through micro-particles is easier than nanoparticles so nanocomposite

288 is associated with smaller grain (280 nm) compare with composites reinforced with

289 micro-particles (380 nm). Therefore, improvement of yield strength due to grain

290 boundary mechanism is 75.6 MPa for nanocomposite, while this value is 64.9 for

291 Al/SiCp micro-composite. Although grain boundaries strengthening mechanism has

292 conquered the second place enhancing mechanical properties in the

293 nanocomposite, it is promoted to first place in the micro-size composite. By

294 decreasing volume fraction of reinforcement/matrix interfaces in the macro-

295 composite compare with the nanocomposite, dislocation density formed in the

296 matrix of micro-composite due to thermal and elastic mismatch is significantly

297 decreased. Whereas Orowan strengthening mechanism was considered as the

298 most important strengthening mechanism in nanocomposites, it is negligible for

299 strengthening of the micro-size composites. As a result of large size and distance 13

300 of reinforcement, grains and subgrains interact with dislocations instead of

301 interacting with SiC particles. Strain hardening and grain boundary strengthening

302 mechanisms are considered as the two most effective strengthening mechanisms

303 in Al/SiCp micro-composite. The load transfer effect in both composites is

304 negligible because of particulate shape and low volume fraction of reinforcement.

305 Since the number of active strengthening mechanisms in Al/SiCp nanocomposite is

306 considerably higher than the micro-composite, the final experimental yield strength

307 of the nanocomposite increased up to 210 MPa. Based on the result of

308 calculations performed by each model, it is understood that experimental result

309 exhibits a good accordance with arithmetic summation and compounding models

310 in micro-composite. However, nanocomposite shows good agreement with

311 quadratic summation model, as demonstrated in previous study [20]. Short

312 dislocation gliding distance in the nanocomposites imposed by well distributed

313 nanoparticles and concomitant very fine grains results in the overestimating of

314 calculated results compared with experimental one. In other words, the first

315 obstacle on the way of dislocation movement, which can be LAGBs, HAGBs or

316 nanoparticles, leads to the strengthening of nanocomposite. Therefore, it is

317 expected that considering the contribution of strain hardening (LAGBs), grain

318 boundaries (HAGBs) and Orowan (nanoparticles) mechanisms together in

319 strengthening of nanocomposite, exhibiting an overestimation of the resistance of

320 the alloy.

322 4. Conclusions

323 In the present investigation, the micromechanics strengthening in nanostructured

324 Al/SiCp composite deformed to high strain by a novel severe plastic deformation 14

325 process, accumulative press bonding (ARB), was investigated. The improvement

326 in yield strength of Al/SiCp composite was described by various strengthening

327 mechanisms. Advanced microstructural techniques were employed to present

328 evidences of each strengthening mechanism. The conclusions drawn from the

329 results can be summarized as follows:

330 1) Homogeneous distribution of SiC particles (with average particle size of 10

331 µm) was successfully achieved after 14 cycles of APB process.

332 2) The EDS maps and X-ray diffraction pattern showed that there was no

333 evidence of detrimental phases in the microstructure of Al/SiCp composite

334 considered as the advantage of solid state fabrication process.

335 3) Nanostructured Al/SiCp composite with the average grain size of 380 nm and

336 well-developed high-angle grain boundaries (73% high angle boundaries and

337 35° average misorientation angle) was obtained by performing 14 cycles of

338 APB process.

339 4) As a result of particle stimulated nucleation mechanism, grain size of the

340 composite was less than 100 nm in the vicinity of SiC particles.

341 5) The yield strength of the aluminum, being 29 MPa, was improved by 5 times,

342 as it increased to 148 MPa.

343 6) The contribution of grain boundary, strain hardening, thermal mismatch,

344 Orowan, elastic mismatch and load-bearing strengthening mechanisms were

345 64.9, 49, 6.8, 2.4, 5.4 and 1.5 MPa, respectively. Clearly, strain hardening and

346 grain boundary mechanisms demonstrate higher contribution to the overall

347 strength of the Al/SiCp composite.

348 7) Al/SiCp nanocomposite showed good agreement with quadratic summation

349 model, however, based on the result of calculations performed by each model,

350 it is understood that experimental result exhibits a good accordance with

351 arithmetic and compounding summation models in micro-composite. 15

353 Acknowledgment

354 The authors acknowledge ﬁnancial support from CICYT (Spain) under program

355 MAT2012-38962-C03-01, and the Ministry of Science, Research and Technology

356 of Iran.

358 16

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458 20

459 Table captions:

460 Table 1. Chemical composition of AA1050 sheets.

461 Table 2. Summary of AMCs strength from literatures.

462 Table 3. Contribution of strengthening mechanisms and yield strength obtained by

463 theoretical models and experiment in Al/SiCp composites.

465 Figure captions:

466 Figure 1. Engineering stress-strain curves of annealed aluminum (Al), monolithic

467 aluminum (Al-APB) and Al/10 vol.% SiCp composite produced by APB process.

468 Figure 2. STEM micrographs of Al/SiCp composite after different APB cycles; (a) 2,

469 (b) 5, (c) 7 and (d) 10 and (e) 14 cycles.

470 Figure 3. STEM micrograph of aluminum/SiCp interface.

471 Figure 4. TEM micrographs of aluminum after one cycle of APB process; (a)

472 surface (b) center of specimen.

473 Figure 5. TEM micrographs of Al/SiCp interface after 14 cycle of APB process.

474 Figure 6. Al/SiCp composite after 14 APB cycle: (a) EBSD/OIM and (b) grain

475 boundary maps.

476 Figure 7. FE-SEM micrographs of Al/SiCp composite after (a) 1, (b) 3, (c) 5 and (d)

477 10 and (e) 14 APB cycles.

478 Figure 8. (a) SEM micrograph of Al/SiCp composite along with its (b) aluminum, (c)

479 silicon and (d) carbon X-ray maps. EDS analysis of points (e) 1 and (f) 2.

480 Figure 9. X-ray diffraction (XRD) pattern of Al/SiCp composite.

481 21

483 Tables:

484 Table 1. Chemical composition of AA1050 sheets.

Element Al Si Fe Mn Cu Mg Zn Ti wt.% Bal. 0.2 0.22 0.02 0.01 0.01 0.01 0.01 485

486 22

488 Table 2. Summary of AMCs strength from literatures. AMCs Methods Reinforceme YS (MPa) UTS (MPa) Reference nt particle size

Al/5 wt.% Al2O3 Casting 20 µm ~112 ~157 [54]

Al/5 vol.% SiCp Casting 8 µm ~80 ~115 [55]

Al/3 wt.% Al2O3 Casting 50 nm ~107 ~162 [54]

Al/20 vol.% Al2O3 Casting+extrusion 12 µm ~175 ~220 [56]

Al/2 vol.% SiCp Friction stir welding 15 nm ~130 ~108 [57]

Al/20 vol.% SiCp Powder metallurgy 17 µm ~87 ~107 [58]

Al-5Cu/13vol.% SiCp Powder metallurgy 10 µm ~134 ~175 [59]

Al/10vol.% SiCp Accumulative press 10 µm 180 222 Present work bonding 489

490 23

491 Table 3. Contribution of strengthening mechanisms and yield strength obtained by

492 theoretical models and experiment in Al/SiCp composites.

Strengthening mechanisms and yield Al/SiCp micro-composite Al/SiCp nano-composite strength

Grain boundary () 64.9 75.6 Thermal mismatch () 6.8 39.6 Elastic mismatch () 5.4 34.4 Strain hardening () 49 42 Orowan looping () 2.4 172 Load-bearing () 1.5 0.3 Experimental yield strength () 148 210 Calculated arithmetic yield strength () 159 393 Calculated quadratic yield strength () 111 228 144 Calculated compounding yield strength () 289