- Email: [email protected]
AZ91 composites by RD-ECAP processing
Journal Pre-proof Enhancement of strength and ductility of SiCp/AZ91 composites by RD-ECAP processing Qiong Xu, Aibin Ma, Bassiouny Saleh, Yuhua Li, Yuchun Yuan, Jinghua Jiang, Chaoying Ni PII:
S0921-5093(19)31365-6
DOI:
https://doi.org/10.1016/j.msea.2019.138579
Reference:
MSA 138579
To appear in:
Materials Science & Engineering A
Received Date: 13 June 2019 Revised Date:
17 October 2019
Accepted Date: 19 October 2019
Please cite this article as: Q. Xu, A. Ma, B. Saleh, Y. Li, Y. Yuan, J. Jiang, C. Ni, Enhancement of strength and ductility of SiCp/AZ91 composites by RD-ECAP processing, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/j.msea.2019.138579. 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. © 2019 Published by Elsevier B.V.
1
Enhancement of strength and ductility of SiCp/AZ91 composites by
2
RD-ECAP processing
3
4
Qiong Xu a,b, Aibin Ma a,c,*, Bassiouny Saleh a, Yuhua Li a,c, Yuchun Yuana, Jinghua Jianga, Chaoying Ni b,* a
5 b
6
College of Mechanics and Materials, Hohai University, Nanjing 211100, China.
Department of Materials Science & Engineering, University of Delaware, DE 19716, USA c
7
Suqian Institute of Hohai University, Suqian 223800, China
8
9
10
11
12
13
14
15
16
17
18
19
* Correspondence
20
E-mail: [email protected] (A. MA); Tel: +86 25 8378 7239; Fax: +86 25 8378 6046
21
Email:[email protected] (C. Ni); Tel: +1 302 831 6359; Fax: + 302 831 4545
22
23
Abstract
24
In this research, as-cast SiCp/AZ91 composites were pre-homogenized at 420 ℃ for 20 h and then
25
processed by RD-ECAP at 250 ℃ for three different passes (4P, 8P, 16P). The effects of ECAP on the
26
microstructure and mechanical properties were investigated. Results showed that SiC particles were
27
more uniformly distributed, matrix grains were significantly refined, β-Al12Mg17 phases were
28
precipitated and homogenized, and texture was reformed to preferred crystal orientation after ECAP
29
processing. The mechanical tensile tests showed an effective increase in mechanical properties after
30
ECAP process. After 4P, 8P, 16P ECAP, the strength of the composites increased to 306 MPa, 288
31
MPa and 285 MPa, respectively; and the ductility increased to 6.4 %, 7.3 % and 8.2 %, respectively.
32
The strength of the composite was slightly decreased and the elongation was gradually increased
33
with the pass number increasing from 4~16P. The enhanced mechanical properties of the composites
34
in this study were ascribed to a combined effect of grain refinement, second phase precipitation,
35
dislocations and texture reforming.
36
Key words: magnesium metal matrix composites (Mg MMCs); RD-ECAP processing; strength and
37
ductility; grain refinement; second phase precipitation, texture.
38
1. Introduction
39
The overall world trend in the numerous engineering applications is that energy consumption
40
must be decreased by reducing the weight of products, therefore the automotive and aviation
41
industries develop rapidly. Due to the good properties of magnesium alloys, many researchers have
42
recently been interested in magnesium alloys. Magnesium alloys have lower densities, high specific
43
strength, precious resource and easy recycling [1-3]. However, magnesium alloy strength and
44
plasticity are usually less than aluminum alloy strength. Furthermore, magnesium alloy application is
45
limited due to the reduction in strength and the creep resistance when the temperature increases.
46
Researchers have shifted from the alloying system to the metal matrix composite system for
47
magnesium alloys to overcome these problems [4-6].
48
The proper reinforcement to form particle (such as SiC, B4C, TiC and Al2O3) reinforced
49
magnesium metal matrix composites (Mg MMCs) can provide superior characteristics such as high
50
strength and rigidity, reasonable wear and creep strength, and a lower thermo-expansion coefficient,
51
which are more suitable for advanced applications [3, 7-9]. There are several Mg MMCs applications,
52
such as automotive, aviation, communication, cutting tools, electronics, optoelectronics, etc., which
53
require unattainable properties in a single material [10]. Metal matrix composites (MMCs) can be
54
produced by many techniques such as solid-state techniques (like powder metallurgy and diffusion
55
bonding)[11], liquid-state techniques (e.g. stir casting, centrifugal casting and squeeze casting) [12,
56
13], in addition to semi-solid-state techniques and vapor deposition techniques (such as physical
57
vapor deposition and chemical vapor deposition) [14]. Great efforts have been made on Mg MMCs
58
after detailed positive studies on aluminum metal matrix composites (Al MMCs). Wu et al. [5, 6,
59
15-19] studied the properties of AZ91 alloy reinforced with SiC particles through using combined
60
method with heat treatment, forging and hot extrusion. In addition to microstructure evaluation, they
61
compared the effect of bimodal-size (micro+submicron) and the single-size (micro/submicron) SiC
62
particles on the mechanical properties of MMCs and obtained higher yield strength in the former one,
63
while the elongation was still poor.
64
Nie et al. [2] applied multi-pass forging to study mechanical properties of AZ91 alloy reinforced
65
with SiCp of 60 nm average size. The results showed that the yield strength improved, while the
66
ductility decreased as SiC nanoparticles coarsen the matrix in higher passes of forging. Niu et al. [20]
67
investigated the mechanical characteristics of AZ61 alloy reinforced with two different size
68
(micron+nano) of SiC particles with extrusion and study the synergistic effect of micron/nano SiCp
69
and submicron Mg17Al12 precipitates. These past investigations concluded that reinforcement size
70
has a significant effect on composite micro-structure and mechanical characteristics, as the
71
reinforcement mechanism differs in particle size composites [15, 21]. Ductility is adversely affected
72
by the large quantity of reinforcements, while micro-scale particles can achieve a highly specific
73
modulus and strength. Nano or bimodal strengthening can increase strength with little negative
74
impact on the ductility. Nevertheless, the nano-particles have a small refining effect only if they have
75
been pushed to the grain boundaries and agglomerated when solidified [22]. Micro-scale particles
76
can stimulate recrystallization, increase the strengthening of grain boundaries and transmit loads to
77
improve the refinement of grains efficiency and randomize the texture. Therefore, during
78
solidification and hot deformation, micron particles can obviously affect the microstructure, i.e.
79
improving strength and module.
80
According to previous researches, stir casting was the main processing method for manufacturing
81
Mg MMCs, including excellent mechanical properties, good machinability and low-cost benefits
82
[23-28]. Thermal-mechanical process often used to optimize the microstructure and properties for
83
Mg MMCs. Conventional secondary processing methods have been widely used, for example hot
84
rolling [4, 21, 29] , forging[2, 18, 26] in addition to hot extrusion [18, 20, 24, 30-33]. Today, severe
85
plastic deformation (SPD) in metallic materials has succeeded in obtaining a much finer structure of
86
grain. Among the SPD methods, equal channel angular pressing (ECAP) is more concentrated as
87
small grain sizes can be achieved and bulk materials are produced for practical applications [34, 35].
88
In the previous researches, there are reports of the successful use of ECAP on Al MMCs and other
89
composites [36-41], while the ECAP on MMCs has limited data and the enhanced mechanisms
90
during ECAP is far from readability. Chang et al. [42] conducted 1, 2 and 4 passes of ECAP on the
91
SiCp/AZ91 composites and investigated the mechanical properties at ambient and elevated
92
temperature. The results show that the grain size was reduced from ~20 µm to ~8 µm after 4P ECAP
93
and super-plasticity was achieved. Qiao et al. [43] studied the microstructure and mechanical
94
properties of AZ91 alloy reinforced with nano SiCp produced by extrusion and ECAP. The results
95
reveal that ECAP brought ~0.4 µm fine grains in the recrystallized region and led a uniform
96
distribution of the SiC particles; the strength of the composite was increased, whereas the ductility
97
was under ~2 %. Along with the other work [44, 45] on the SiCp/AZ91 composites, the above
98
researches also show that the characteristics of Mg MMCs are determined by and the interactions
99
between the comprehensive particle dispersion factors, grain refining, second phase, texture and
100
dislocations.
101
Our research group has developed a RD-ECAP equipment and has achieved grain refinement and
102
improvement of properties on various materials [41, 46-52]. RD-ECAP method can achieve high
103
ECAP deformation passes with one-time loading and unloading, protecting samples and improving
104
efficiency. Therefore, the current work is to improve the microstructure and mechanical properties in
105
addition to reducing casting defects of the AZ91 alloy reinforced with micro-SiCp by multi-pass
106
ECAP process (4, 8, 16P). The research focuses on the microstructure and mechanical properties of
107
the composite with the effect of RD-ECAP deformation, as also on strengthening mechanisms related
108
to grain refinement, second phase, dislocation and texture. The present study is of great significance
109
for the further application of MMCs and provides reference for the study of the deformation behavior
110
of highly deformed MMCs.
111
112
2. Experimental details
113
2.1 Materials and fabrication procedures
114
SiCp/AZ91 magnesium-metal matrix composite fabricated by semi-solid stirring casting was the
115
material used in the current work. The magnesium matrix alloy mainly contains 9.01 wt.% Al, 0.71
116
wt.% Zn, and 0.18 wt.% Mn. The reinforcement selected was SiC particles (5 vol. %) of density ~3.2
117
g/cm3, with an average size of ~10 µm. Samples with a dimension of 19.5 mm x 19.5 mm x 45 mm
118
were cut from the as-cast material and then homogenized at 420 ℃ for 20h. Then the
119
as-homogenized composites were processed at 250 ℃ for 4, 8, 16 passes through a 90 ° RD-ECAP
120
die, the operation principle of which can be found in our early work[50, 52-54]. Samples of five
121
different processing states were then collected, with the original casting material marked as as-cast,
122
the as-cast sample after solid solution marked as as-homogenized, the as-homogenized samples after
123
4, 8, 16 passes of ECAP process marked as 4P ECAP, 8P ECAP and 16P ECAP, respectively. The
124
mechanical properties of these ECAP composites were compared with as-cast and as-homogenized
125
metal composites, which is the main reference to the improvement of these properties.
126
2.2 Microstructure characterization
127
For all composites, microstructure analyzes were done to check particle distribution quality. Wire
128
electrical discharge machining (EDM) was used to cut metallographic samples with dimensions 10
129
mm x10 mm x2 mm. The specimens were polished to remove fine scratches and were etched in
130
acetic-picric solution. The microstructure and morphology were examined by an Olympus BX51M
131
optical microscopy and a scanning electron microscope (SEM, Sirion 200) equipped with an X-ray
132
energy dispersive spectrometer (EDS, Genesis 60S). The texture and phase analysis was carried out
133
by means of X-ray diffractometer (XRD, Cu-Kα, Bruker D8) with Cu Kα radiation. The electron
134
back-scatted diffraction (EBSD) analysis was conducted by an SEM (Hitachi S-3400N) equipped
135
with a HKL-EBSD system. TEM analysis was made with a FEI Tecnai-G2 thermo-emission
136
transmission electron microscope at an accelerating voltage of 200 kV.
137
2.3 Mechanical characterization
138
The size of tensile samples was selected according to ASTM E8 standard. The tensile test was
139
performed with a universal tensile testing machine at a strain rate of 0.5 mm/min. Dog-bone
140
geometry tensile specimens were cut along the plans that coincided with extrusion direction (0°),
141
with their length of 25 mm, 6 mm width and 6 mm shoulder radius. Five tensile samples were tested
142
and the median results were determined. Micro-hardness tests for measuring the hardness of all
143
composites were performed to check surface behavior. Ten points of each sample were tested, and
144
the average value of the Vickers hardness was calculated and plotted.
145
3. Results and discussion
146
The following sections explain the analysis of the microstructure of the composite specimens,
147
their mechanical properties and fracture morphology.
148
3.1 Microstructure analysis
149
Fig. 1 shows the microstructure of as-cast and as-homogenized SiCp/AZ91 composites. In the
150
as-cast composite shown in Fig. 1a and b, apart from the SiC particles, coarse second phases known
151
as β-Al12Mg17 are observed in the OM and SEM photographs (solid arrows in Fig. 1a and b). The
152
matrix is segregated by the SiC particles and those second phases. After homogenization, as shown
153
in Fig. 1c and d, almost no second phases are observed, indicating a complete dissolution of them. In
154
Fig. 1c, the matrix shows large grains with an average grain size ~150 µm, and the SiC particles are
155
clearly observed along the grain boundaries (dashed arrows in Fig. 1 c). The agglomeration of SiCp is
156
observed in some areas in both as-cast and as-homogenized composites according to the SEM
157
images, as marked in the dashed squares in Fig. 1b and d.
158
159 160 161 162 163
Fig. 1. Microstructure of SiCp/AZ91 composites: (a) and (b) OM and SEM for as-cast composite; (c) and (d) OM and SEM for as-homogenized composite.
Fig. 2 shows clearly the distribution and the morphology of the SiCp in the matrix of SiCp/AZ91
164
composites subjected to ECAP process. In the composite processed by 4P ECAP, it shows an obvious
165
SiCp flow along the extrusion direction and obvious agglomeration exists. After 8P ECAP, the SiCp
166
flow is “disordered” while it still can be detected. After 16P ECAP, the SiC particles are evenly
167
distributed almost in the whole version, as shown in Fig. 2c.
168 169
Fig. 2. SEM of the SiCp/AZ91 composites: (a) 4P ECAP; (b) 8P ECAP; (c) 16P ECAP.
170 171
Fig. 3 and Fig. 4 illustrate the grain refinement and second phases in SiCp/AZ91 composites
172
subjected to different passes of ECAP. Compared with the as-cast and as-homogenized composite in
173
Fig. 1, the grain structure of the ECAP composites is obviously refined with different degrees of
174
secondary phase precipitation. After 4P ECAP as shown in Fig. 3a, the matrix is composed of fine
175
grains and small amount of large grains that are highly deformed. Large numbers of fine second
176
phases were observed mostly around grain boundaries. The average grain size of the fine grains is
177
~1.5 µm, as illustrated in the inverse pole figure and the histogram in Fig. 4a and b. After 8P ECAP
178
as shown in Fig. 3b, almost the whole matrix is covered with fine grains and almost no large grains
179
remain, which suggests that dynamic recrystallization (DRX) is almost completed after 8P ECAP.
180
The composite processed by 16P ECAP has a more refined grain structure, and more small-scale
181
second phase precipitates are observed, as shown in Fig. 3c. These fine grains are equiaxed and the
182
average grain size is under 1 µm, as illustrated in Fig. 4c and d.
183 184
Fig. 3. OM of SiCp/AZ91 composites: (a) 4P ECAP; (b) 8P ECAP; (c) 16P ECAP.
(b) 10 Frequency(%)
9 8 7 6 5 4 3 2 1 0 0
1
2
3
4
5
6
7
8
9
10
Grain size(µm)
185
185 (d) 60 Frequency(%)
50 40 30 20 10 0 0
1
2
3
4
5
Grain size (µm)
187 188
Fig. 4. Inverse pole figure (a, c) and grain size histogram (b, d) of SiCp/AZ91 composites: (a) and (b) 4P ECAP; (c) and (d) 16P ECAP.
189 190
Fig. 5 shows the TEM photographs of SiCp/AZ91 composites processed by 4P ECAP and 16P
191
ECAP. After 4P ECAP as shown in Fig. 5a and b, DRX occurred; grains with high density of
192
dislocations are observed. After 16P ECAP, large numbers of second phases with grain sizes of ~200
193
nm to ~500 nm are observed around the grain boundaries, as shown in Fig. 5c and d. These fine
194
precipitates pin the grain boundaries and contribute to the grain refinement.
195
196 197
Fig. 5. TEM photographs of SiCp/AZ91 composites: (a) and (b) 4P ECAP; (c) and (d) 16P ECAP.
198 199
Fig. 6 shows the X-ray diffraction (XRD) patterns of the as-homogenized composite and the
200
composites obtained by different passes of ECAP process. In the XRD pattern in Fig. 6a, there is
201
only a weak Mg17Al12 phase diffraction peak, which means that only a small amount of
202
precipitation phases remain after the solid solution. After 4P ECAP, Mg17Al12 precipitation
203
occurred during the ECAP processing, showing obvious diffraction peaks, as shown in Fig 6b.
204
After 8P and 16P ECAP, the amounts of the Mg17Al12 phases further increase, as the intensity of
205
the diffraction peaks of the precipitation phases is stronger than that of the sample processed by
206
4P ECAP, as shown in Fig 6c and d. This observation in second phase change is consistent with
207
that in Fig. 3. Besides the change in the second phase precipitation, the XRD patterns in Fig.6
208
show that an obvious texture change occurs with the increase of the ECAP passes. The
209
as-homogenized composite shows a relatively strong basal (0002) texture and a weak 1010
210
texture, as shown in Fig. 6a. A clear deformation texture 1010
211
diffraction increases as shown in Fig. 6b-d, appears after ECAP processing.
212
from which maximum
213 214 215 216
Fig. 6. XRD patterns of SiCp/AZ91 composites: (a) As-homogenized; (b) 4P ECAP; (c) 8P ECAP; (d) 16P ECAP.
3.2 Mechanical properties
217
Fig. 7a shows the typical tensile stress-strain curves of the SiCp/AZ91 composites with different
218
processing states and Fig. 7b illustrates the yield strength (YS), ultimate tensile strength (UTS) and
219
elongation to failure. The as-cast SiCp/AZ91 composite possesses the lowest strength (YS of 78 MPa
220
and UTS of 132 MPa), and poor ductility (elongation of 4.7 %). After solid solution treatment, the
221
as-homogenized composite exhibits improved strength and ductility (elongation of 7.8 %), while the
222
improvement on the strength is not remarkable (YS of 86 MPa and UTS of 192 MPa). After ECAP
223
process, the composites exhibit simultaneously improved strength and ductility. After 4P, 8P and 16P
224
ECAP, the YS of the composite is 234 MP, 232 MPa and 225 MPa, respectively; the UTS is 306 MPa,
225
288 MPa and 285 MPa, respectively. The YS is almost tripled and the UTS is more than doubled
226
compared to that of the as-cast composite. Although the elongation is slightly decreased compared to
227
that of the as-homogenized composite (elongation of 7.8 %) in 4P ECAP and 8P ECAP composites,
228
the elongation increased with the increase of ECAP passes and is comparable to that of the
229
as-homogenized composite (elongation of 8.2 % after 16P ECAP). After 4P, 8P and 16P ECAP, the
230
increments of in the elongation of the composites are 36.2%, 55.3% and 74.5%, respectively,
231
compared to that of the as-cast composite. Table 1 shows the confidence intervals for tensile testing
232
results with confidence coefficient 0.95 and significance level 0.05.
(a) 350
(b) 350 as-cast as-homogenized 4P ECAP 8P ECAP 16P ECAP
Stress(MPa)
250
300
200 150
18 16
250
14 12
200
10 150
100
100
50
50
0
0
8 6
Elongation(%)
Stress (MPa)
300
20 YS UTS EL
4
0
2
4
6
8
10
12
14
16
18
20
2 0 as-cast as-homogenized 4PECAP
16P ECAP
Processing method
Strain (%)
233 234 235
8P ECAP
Fig. 7. (a) Stress-strain curves of the SiCp/AZ91 composites by different processing methods; (b) summary of yield strength, ultimate tensile strength and elongation.
236 237
Table 1 The confidence intervals for tensile testing results (EL (%), UTS and YS (MPa)). Processing Method
as-cast
as-homogenized EL
UTS
YS
4P ECAP EL
UTS
8P ECAP
Property
EL
UTS
YS
YS
Mean
4.92
136.36
78.20
8.26
190.58
85.40
6.66
301.42
232.80
Standard Deviation
1.13
18
9.55
1.19
14.61
2.88
0.84
15.52
3.90
Margin of Error
0.993
15.780
8.371
1.046
12.808
2.525
0.735
13.600
3.417
Point Estimate
4.92
136.36
78.20
8.26
190.58
85.40
6.66
301.42
Lower Limit
4.12
120.58
69.83
7.21
177.77
82.87
5.93
Upper Limit
6.11
152.14
86.57
9.31
203.39
87.93
7.39
EL
16P ECAP
UTS
YS
EL
UTS
YS
7.24
289.00
229.60
8.30
288.44
220.40
0.46
9.35
9.53
0.69
17.53
6.69
0.402
8.197
8.352
0.604
15.368
5.867
232.80
7.24
289.00
229.60
8.30
288.44
220.40
287.82
229.38
6.83
280.80
221.25
7.70
273.07
214.53
315.02
236.22
7.64
297.20
237.95
8.90
303.81
226.27
238 239
Fig. 8a shows the linear curve fitting of pass number and YS of SiCp /AZ91 composite; Fig. 8b
240
shows the linear curve fitting of pass number and EL of the composite. Equation (1) and (2) is
241
developed with a coefficient of determination (R2) of 98.6 % for YS and 92.9% for EL, respectively.
242
Also, equation (1) and (2) represent the empirical linear regression models that can be used to predict
243
the yield strength and the elongation of ECAP samples with different pass numbers from 4~16P. The
244
yield strength of the composite processed by ECAP is generally higher than the that of the as-cast
245
and as-homogenized composites (as shown in Fig. 7), while there is a slight decrease when the pass
246
number goes higher, as instructed by the linear curve in Fig. 8a and equation (1). The elongation of
247
the composites is gradually increased with the increase of the pass number from 4~16P, as shown in
248
Fig. 8b and equation (2). This illustrates that the mechanical property is determined by
249
comprehensive factors of grain refinement, second phases and texture. When processed by higher
250
passes of ECAP, a “soft orientation” is formed in the composite (basal texture is weakened and
251
plastic texture is intensified as shown in Fig. 6c-d), contributing to the above changes to the
252
253
mechanical property. = 237.5 − 0.7679
(1)
= 5.95 + 0.1446
(2)
Where: A is pass number. (a)
(b) 9.0
280
Experimental Linear Fit
260
8.0
Elongation (%)
240
YS (MPa)
Experimental Linear Fit
8.5
220 200
y=237.5-0.7679x 2 R =0.9856
180 160
7.5 7.0 6.5
y=5.95+0.1446x 2 R =0.9285
6.0 5.5 5.0
140 2
4
6
8
10
12
Pass number
14
16
18
2
4
6
8
10
12
14
16
18
Pass number
254 255
Fig. 8. Linear curve fitting for the SiCp /AZ91 composite: (a) pass number and YS; (b) pass number and EL.
256 257
Fig. 9 shows the effect of number of passes on micro-hardness values for the SiCp/AZ91
258
composites and matrix. Table 2 shows the confidence intervals for Hardness testing results with
259
confidence coefficient 0.95 and significance level 0.05. In comparison to the results obtained from
260
the conventional process, the micro-hardness values of samples deformed by ECAP processes after 4,
261
8 and 16 passes were all higher. After 4P ECAP, the increase in hardness is about 34% for composite,
262
from 85.96 HV to 115.25 HV. With an increase in the number of ECAP passes to 16, the hardness
263
values grow for both investigated cases. The hardness results achieved can be directly linked to the
264
microstructure as mentioned above, XRD and tensile investigation. ECAP modifies the
265
microstructure and mechanical characteristics of SiCp/AZ91 composites as described above in the
266
microstructure analysis. The grain refinement, second phase precipitation, and the enhanced bonding
267
between matrix and particles all contribute significantly to improving mechanical properties. The
268
formation of ECAP deformation texture is also a factor influencing the SiCp/AZ91composites. While
269
the grain refinement contributes to the significant incensement in strength, the weakening of (0002)
270
basal texture and the strengthening of 1010 deformation texture (Fig. 6) improve the ductility of
271
the alloys. As the ECAP processing was conducted at 250 ℃, which is below the eutectic point in the
272
Al-Mg phase diagram[55, 56], the second phases were precipitated and homogenized (Fig. 3 and Fig.
273
5). These second phases pin the grain boundaries and these hard phases also contributed to the
274
increase in hardness. 180
Matrix Composites
160
Hardness(HV)
140 120 100 80 60 40 20 0 as-homogenized
4P ECAP
8P ECAP
16P ECAP
Processing method
275 276 277 278
Fig. 9. Vickers micro-hardness evolution of the SiCp/AZ91 composites subjected to ECAP processing. Table 2 The confidence intervals for Hardness testing results. Property
Vickers Hardness (Matrix)
Processing Method
Vickers Hardness (Composites)
as-homogenized
4P ECAP
8P ECAP
16P ECAP
as-homogenized
4P ECAP
8P ECAP
16P ECAP
Mean
60.24
92.84
93.52
106.07
85.96
115.25
124.75
135.35
Standard Deviation
3.50
2.83
3.37
2.53
6.41
4.91
3.66
5.05
Significance Level
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
Margin of Error
2.17
1.75
2.09
1.57
3.97
3.04
2.27
3.13
Point Estimate
60.24
92.84
93.52
106.07
85.96
115.25
124.75
135.35
Lower Limit
58.07
91.09
91.43
104.50
81.99
112.21
122.48
132.22
Upper Limit
62.41
94.59
95.61
107.64
89.93
118.29
127.02
138.48
279 280
Fig. 10 shows the linear curve fitting of the pass number and the hardness of SiCp /AZ91
281
composites. It can be seen that Vickers hardness of composites increases with the increase of the pass
282
number. The equation (3) was developed with a coefficient of determination (R2) of 82.6 % for
283
Vickers hardness using this model. Also, equation (3) represent the empirical linear regression model
284
that can be used to predict the hardness of ECAP samples with different pass number from 0~16. ! "#
285
= 95.556 + 2.8245
Where: A is pass number.
(3)
150 Experimental Linear Fit
Hardness (HV)
140 130 120 110
y=95.556+2.8245x 2 R =0.826
100 90 80 0
2
4
6
8
10
12
14
16
18
20
Pass number
286 Fig. 10. Linear curve fitting for the pass number and the hardness of SiCp /AZ91 composite.
287 288
The linear regression model established using the achieved regression coefficients was validated
289
to be adequate in the confirmation experiments where new pass numbers have been used in the
290
model, and then the model results have been compared with the experimental results obtained for the
291
new set. Table 3 shows the new set of pass numbers for the adequacy checking, linear regression
292
model of mechanical properties and experimental results of mechanical properties. The percentage of
293
error was estimated based on a calculation of the hardness and yield strength results based on
294
experimental and linear regression models.
295
Table 3 Comparison of mechanical properties determined using experimental results and linear regression model. Hardness Results Pass
Experimental
number
Hardness (HV)
Yield strength Results
Linear Regression
Error
Experimental
Hardness
(%)
YS (MPa)
(HV)
Linear Regression YS (MPa)
Elongation Results
Error
Experimental
(%)
EL (%)
Linear Regression EL (%)
Error (%)
5
119.49
109.66
8.23
254.44
233.66
8.16
7.13
6.68
6.28
7
124.20
115.30
7.17
282.23
232.12
17.75
7.65
6.97
8.89
9
126.32
120.94
4.26
279.55
230.59
17.51
8.11
7.26
10.34
11
126.58
127.44
0.68
260.37
229.05
12.02
7.89
7.56
4.25
296 297
3.3 Fracture morphology
298
It is necessary to evaluate mechanical characteristics of composite material, while simultaneously
299
investigating mechanisms of deformation and failure and modes of fracture. So, the SEM was used
300
to analyze the fractural surface of the samples obtained through the tensile test. Fig. 11 illustrates the
301
fracture morphology of the as-cast composite and the composites processed by 4P and 16P ECAP.
302
The SEM results in Fig. 11a and b reveal that the as-cast composite shows a brittle fracture with
303
coarse grain structure, and the smooth surface of the coarse SiCp implies a relatively weak bonding
304
of the SiCp-AZ91 interface, which is coincident with the poor mechanical properties of the as-cast
305
composite. In the ECAP composites, there is brittle fracture of the composites with flat fracture
306
surface as shown in Fig. 11c and e. But also, the small dimples are found in the AZ91 matrix as
307
shown in Fig. 11d and f. The presence of dimples shows the typical matrix alloy ductile fracture. In
308
the 16P ECAP composite shown in Fig. 11f, the surface of the SiCp is quite rough and pits are
309
observed, which implies that the better interface bonding is achieved between the matrix and
310
particles. All of these illustrate that higher ECAP passes improve the microstructure and enhance the
311
mechanical properties of the SiCp/AZ91 composite.
312
313
314 315
Fig. 11. Fracture morphology of SiCp/AZ91 composites: (a) and (b) as-homogenized; (c) and (d) 4P ECAP; (e) and
(f) 16P ECAP.
316 317 318
4. Conclusions
319
In the present work, the as-cast SiCp/AZ91 composites were homogenized and subjected to high
320
passes of ECAP process with a 90˚ RD-ECAP die. The effects of the RD-ECAP process on the
321
microstructure and mechanical properties were studied, and the primary conclusions of this work
322
could be drawn as follows:
323
1) The high-pass RD-ECAP process effectively refined the matrix grain structure of the SiCp/AZ91
324
composites with intense second phase precipitation and reformed the texture to the preferred
325
orientation.
326
2) The RD-ECAP process largely eliminated the agglomeration of SiC particles and uniformed the
327
particle distribution. With the increase of the ECAP passe, the SiC particles were more evenly
328
distributed. At 4P and 8P ECAP, the SiC particles tended to be distributed along the extrusion
329
direction; at 16P ECAP, the SiC particles were evenly distributed in the matrix alloy.
330
3) The mechanical properties of the SiCp/AZ91 composites were effectively improved after
331
RD-ECAP process. The YS were almost tripped, and the UTS were more than doubled compared
332
to the as-cast composite. The elongation also increased after 4P, 8P and 16P ECAP, with an
333
increment of 36.2%, 55.3% and 74.5%, respectively, compared to that of the as-cast composite.
334
Micro-hardness increased with the increase of ECAP passes.
335
4) Fracture morphorlogy revealed the brittle fracture of the composites. The size of dimples in the
336
matrix of was smaller and SiCp-AZ91 interface bonding was better at higher ECAP passes than
337
lower ones, which indicated that higher passes of RD-ECAP improved the plastic property of the
338
SiCp/AZ91 composite.
339 340 341
5) For applications where mechanical efficiency is a significant demand, findings achieved using this SiCp/AZ91 composite can be effectively used. Acknowledgements
342
This study was supported by the National Natural Science Foundation of China (51774109 and
343
51501039), the Key Research and Development Project of Jiangsu Province (BE2017148) and the
344
Fundamental Research Funds for the Central Universities (2018B48414). Q.X. is grateful for the
345
support from the China Scholarship Council and the W. M. Keck Center for Advanced Microscopy
346
and Microanalysis at University of Delaware.
347
References
348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388
[1] M. Zha, H.M. Zhang, C. Wang, H.Y. Wang, E.B. Zhang, Q.C. Jiang, Prominent role of a high volume fraction of Mg17Al12 particles on tensile behaviors of rolled Mg-Al-Zn alloys, J. Alloy Compd. 728 (2017) 682-693. [2] K.B. Nie, J.G. Han, K.K. Deng, X.J. Wang, C. Xu, K. Wu, The Comparison in the microstructure and mechanical properties between AZ91 alloy and nano-SiCp/AZ91 composite processed by multi-pass forging under varying passes and temperatures, Materials 12 (2019) 625. [3] Y.S. Yi, R.F. Li, Z.Y. Xie, Y. Meng, S. Sugiyama, Effects of reheating temperature and isothermal holding time on the morphology and thixo-formability of SiC particles reinforced AZ91 magnesium matrix composite, Vacuum 154 (2018) 177-185. [4] W.Q. Liu, X.J. Wang, X.S. Hu, K. Wu, M.Y. Zheng, Effects of hot rolling on microstructure, macrotexture and mechanical properties of pre-extruded AZ31/SiC nanocomposite sheets, Mater. Sci. Eng.: A 683 (2017) 15-23. [5] K.K. Deng, K. Wu, Y.W. Wu, K.B. Nie, M.Y. Zheng, Effect of submicron size SiC particulates on microstructure and mechanical properties of AZ91 magnesium matrix composites, J. Alloy Compd. 504 (2010) 542-547. [6] K.K. Deng, C.J. Wang, J.Y. Shi, Y.W. Wu, K. Wu, Microstructure evolution mechanism of micron particle reinforced magnesium matrix composite at room temperature, Mater. Chem. Phys. 134 (2012) 581-584. [7] P. Xiao, Y.M. Gao, F.X. Xu, C.C. Yang, Y.F. Li, Z.W. Liu,et al., Tribological behavior of in-situ nanosized TiB2 particles reinforced AZ91 matrix composite, Tribol. Int. 128 (2018) 130-139. [8] S. Roy, G. Kannan, S. Suwas, M.K. Surappa, Effect of extrusion ratio on the microstructure, texture and mechanical properties of (Mg/AZ91)m-SiCp composite, Mater. Sci. Eng.: A 624 (2015) 279-290. [9] M.J. Shen, X.J. Wang, M.F. Zhang, X.S. Hu, M.Y. Zheng, K. Wu, Fabrication of bimodal size SiCp reinforced AZ31B magnesium matrix composites, Mater. Sci. Eng.: A 601 (2014) 58-64. [10] J. Singh, A. Chauhan, Characterization of hybrid aluminum matrix composites for advanced applications-A review, J. Mater. Res. Technol. 5 (2016) 159-169. [11] S.A. Lajevardi, T. Shahrabi, J.A. Szpunar, Synthesis of functionally graded nano Al2O3-Ni composite coating by pulse electrodeposition, Appl. Surf. Sci. 279 (2013) 180-188. [12] B. Saleh, J.H. Jiang, A.B. Ma, D. Song, D.H. Yang, Effect of main parameters on the mechanical and wear behaviour of functionally graded materials by centrifugal casting: A Review, Met. Mater. Int. (2019) 1-15. [13] I.M. El-Galy, M.H. Ahmed, B.I. Bassiouny, Characterization of functionally graded Al-SiCp metal matrix composites manufactured by centrifugal casting, J. Alexandria Eng. 56 (2017) 371-381. [14] N. Guo, M.C. Leu, Additive manufacturing: technology, applications and research needs, Front. of Mech. Eng. 8 (2013) 215-243. [15] K.K. Deng, X.J. Wang, C.J. Wang, J.Y. Shi, X.S. Hu, K. Wu, Effects of bimodal size SiC particles on the microstructure evolution and fracture mechanism of AZ91 matrix at room temperature, Mater. Sci. Eng.: A 553 (2012) 74-79. [16] K.K. Deng, C.J. Wang, X.J. Wang, K. Wu, M.Y. Zheng, Microstructure and elevated tensile properties of submicron SiCp/AZ91 magnesium matrix composite, Mater. Design 38 (2012) 110-114. [17] K.K. Deng, J.Y. Shi, C.J. Wang, X.J. Wang, Y.W. Wu, K.B. Nie, et al., Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite, Comps. Part A: Applied. S. 43 (2012) 1280-1284. [18] K. Wu, K.K. Deng, K.B. Nie, Y.W. Wu, X.J. Wang, X.S. Hu, et al., Microstructure and mechanical properties of SiCp/AZ91 composite deformed through a combination of forging and extrusion process, Mater. Design 31 (2010) 3929-3932. [19] K.K. Deng, K. Wu, X.J. Wang, Y.W. Wu, X.S. Hu, M.Y. Zheng, et al., Microstructure evolution and mechanical
389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432
properties of a particulate reinforced magnesium matrix composites forged at elevated temperatures, Mater. Sci. Eng.: A 527 (2010) 1630-1635. [20] X.F. Niu, G. Li, Z.Y. Zhang, P.W. Zhou, H.X. Wang, S.X. Zhang, et al., Simultaneously improving the strength and ductility of extruded bimodal size SiCp/AZ61 composites: Synergistic effect of micron/nano SiCp and submicron Mg17Al12 precipitates, Mater. Sci. Eng.: A 743 (2019) 207-216. [21] X.J. Wang, W.Q. Liu, X.S. Hu, K. Wu, Microstructural modification and strength enhancement by SiC nanoparticles in AZ31 magnesium alloy during hot rolling, Mater. Sci. Eng.: A 715 (2018) 49-61. [22] X.J. Wang, X.S. Hu, K.B. Nie, K. Wu, M.Y. Zheng, Hot extrusion of SiCp/AZ91 Mg matrix composites, T. Nonferr. Metal Soc. 22 (2012) 1912-1917. [23] L. Zhang, Q.D. Wang, G.P. Liu, W. Guo, H. Jiang, W. Ding, Effect of SiC particles and the particulate size on the hot deformation and processing map of AZ91 magnesium matrix composites[J]. Mater. Sci. Eng.: A 707 (2017) 315-324. [24] L. Zhang, Q.D. Wang, W.J. Liao, W. Guo, B. Ye, W.Z. Li, et al., Effects of cyclic extrusion and compression on the microstructure and mechanical properties of AZ91D magnesium composites reinforced by SiC nanoparticles, Mater. Charact. 126 (2017) 17-27. [25] X.J. Wang, X.S. Hu, W.Q. Liu, J.F. Du, K. Wu, Y.D. Huang, M.Y. Zheng, Ageing behavior of as-cast SiCp/AZ91 Mg matrix composites, Mater. Sci. Eng.: A 682 (2017) 491-500. [26] W. Guo, Q.D. Wang, W.Z. Li, H. Zhou, L. Zhang, W.J. Liao, Enhanced microstructure homogeneity and mechanical properties of AZ91-SiC nanocomposites by cyclic closed-die forging, J. Comps. Mater. 51 (2017) 681-686. [27] X.J. Wang, X.S. Hu, K. Wu, L.Y. Wang, Y.D. Huang, Evolutions of microstructure and mechanical properties for SiCp/AZ91 composites with different particle contents during extrusion, Mater. Sci. Eng.: A 636 (2015) 138-147. [28] S. Pawar, X. Zhou, T. Hashimoto, G.E. Thompson, G. Scamans, Z. Fan, Investigation of the microstructure and the influence of iron on the formation of Al8Mn5 particles in twin roll cast AZ31 magnesium alloy, J. Alloy Compd. 628 (2015) 195-198. [29] W.Q. Liu, X.S. Hu, X.J. Wang, K. Wu, M.Y. Zheng, Evolution of microstructure, texture and mechanical properties of SiC/AZ31 nanocomposite during hot rolling process, Mater. Design 93 (2016) 194-202. [30] M.J. Shen, X.J. Wang, T. Ying, K. Wu, W.J. Song, Characteristics and mechanical properties of magnesium matrix composites reinforced with micron/submicron/nano SiC particles, J. Alloy Compd. 686 (2016) 831-840. [31] X.J. Wang, K.B. Nie, X.S. Hu, Y.Q. Wang, X.J. Sa, K. Wu, Effect of extrusion temperatures on microstructure and mechanical properties of SiCp/Mg-Zn-Ca composite[J]. J. Alloy Compd. 532 (2012) 78-85. [32] M.J. Shen, X.J. Wang, M.F. Zhang, X.S. Hua, M.Y. Zheng, K.Wu, Effect of bimodal size SiC particulates on microstructure and mechanical properties of AZ31B magnesium matrix composites, Mater. Design (1980-2015) 52 (2013) 1011-1017. [33] H. Chang, C.J. Wang, K.K. Deng, K.B. Nie, K. Su, L. Liao, et al, Effects of SiCp Content on the microstructure and mechanical properties of SiCp/Mg-5Al-2Ca composites, Rare Metal Mater. Eng. 47 (2018) 1377-1384. [34] J. Xu, M. Shirooyeh, J. Wongsa-Ngam, D.B. Shan, B. Guo, T.G. Langdon, Hardness homogeneity and micro-tensile behavior in a magnesium AZ31 alloy processed by equal-channel angular pressing, Mater. Sci. Eng.: A 586 (2013) 108-114. [35] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, Y.T. Zhu, Producing bulk ultrafine-grained materials by severe plastic deformation, JOM 58 (2006) 33-39. [36] H. Zare, M. Jahedi, M.R. Toroghinejad, M. Meratian, M. Knezevic, Microstructure and mechanical properties of carbon nanotubes reinforced aluminum matrix composites synthesized via equal-channel angular pressing, Mater. Sci. Eng.: A 670 (2016) 205-216. [37] C.H. Qian, L. Ping, K.M. Xue, Influence of deformation passes on interface of SiCp/Al composites consolidated by
433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476
equal channel angular pressing and torsion, T. Nonferr. Metal Soc. 25 (2015) 1376-1382. [38] L. Ping, X. Zhang, K.M. Xue, L. Xiao, Effect of equal channel angular pressing and torsion on SiC-particle distribution of SiCp-Al composites, T. Nonferr. Metal Soc. 22 (2012) s402-s407. [39] M. Saravanan, R. Pillai, K. Ravi, B. Pai, M. Brahmakumar, Development of ultrafine grain aluminium-graphite metal matrix composite by equal channel angular pressing, Compos. Sci. Technol. 67 (2007) 1275-1279. [40] K. Ravi, M. Saravanan, R. Pillai, A. Mandal, B. Murty, M. Chakraborty,et al., Equal channel angular pressing of Al-5 wt% TiB2 in situ composite, J. Alloy Compd. 459 (2008) 239-243. [41] Q. Xu, A.M. Ma, J.J. Wang, J.P. Sun, J.H. Jiang, Y.H. Li, et al., Development of high-performance SiCp/Al-Si composites by equal channel angular pressing, Metals 8 (2018) 738. [42] H. Chang, X.J. Wang, M.Y. Zheng, K. Wu, Effect of ECAP on the microstructure and tensile property of SiCp/AZ91 magnesium matrix composite[C]//Key Engineering Materials. Trans. Tech. 353 (2007) 1342-1345. [43] X.G. Qiao, M.Y. Zheng, E.D.Wei, K.Wu, X.S. Hu,W.M.Gan, et al., Microstructure evolution and mechanical properties of nano-SiCp/AZ91 composite processed by extrusion and equal channel angular pressing (ECAP), Mater. Charact. 121 (2016) 222-230.. [44] B.N. Sahoo, F.K. Md, S. Babu, S.K. Panigrahi, G.D. Janaki Ram, Microstructural modification and its effect on strengthening mechanism and yield asymmetry of in-situ TiC-TiB2/AZ91 magnesium matrix composite, Mater. Sci. Eng.: A 724 (2018) 269-282. [45] S.J. Shang, K.K. Deng, K.B. Nie, J.C. Li, S.S. Zhou, F.J. Xu, et al., Microstructure and mechanical properties of SiCp/Mg-Al-Zn composites containing Mg17Al12 phases processed by low-speed extrusion, Mater. Sci. Eng.: A 610 (2014) 243-249. [46] Q. Xu, A.B. Ma, Y.H. Li, C.Y. Ni, Microstructure of a high strength AZ91 alloy processed by severe plastic deformation, Microsc. Microanal. 24 (2018) 2176-2177. [47] J.P. Sun, Z.Q.Yang, J. Han, H. Liu, D. Song, J.H. Jiang, High strength and ductility AZ91 magnesium alloy with multi-heterogenous microstructures prepared by high-temperature ECAP and short-time aging, Mater. Sci. Eng.: A 734 (2018) 485-490. [48] L.Y. Zhang, A.B. Ma, J.H. Jiang, X. Jie, Effect of processing methods on microhardness and acid corrosion behavior of low-carbon steel, Mater. Design (1980-2015), 65 (2015) 115-119. [49] C.C. Zhu, A.B. Ma, J.H. Jiang, X.B. Li, D. Song, D.H. Yang, et al., Effect of ECAP combined cold working on mechanical properties and electrical conductivity of Conform-produced Cu-Mg alloys. J. Alloy Compds. 582 (2014) 135-140. [50] Y.C. Yuan, A.B. Ma, J.H. Jiang, F.M. Lu, W.W. Jian, D. Song, et al., Optimizing the strength and ductility of AZ91 Mg alloy by ECAP and subsequent aging, Mater. Sci. Eng.: A 588 (2013) 329-334. [51] A.B. Ma, J.H. Jiang, N. Saito, I. Shigematsu, A. Watazu, Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die, T. Nonferr. Metal Soc. 17 (2007) 104-109. [52] A.B. Ma, Y. Nishida, K. Suzuki, I. Shigematsu, N. Saito, Characteristics of plastic deformation by rotary-die equal-channel angular pressing, Scripta Mater. 52 (2005) 433-437. [53] F.M. Lu, A.B. Ma, J.H. Jiang, D.H. Yang, D. Song, Y.C. Yuan, et al., Effect of multi-pass equal channel angular pressing on microstructure and mechanical properties of Mg97.1Zn1Gd1.8Zr0.1 alloy, Mater. Sci. Eng.: A 594 (2014) 330-333. [54] A.B. Ma, K. Suzuki, Y. Nishida, N. Saito, I. Shigematsu, M. Takagi, et al., Impact toughness of an ultrafine-grained Al-11mass% Si alloy processed by rotary-die equal-channel angular pressing, Acta Mater. 53 (2005) 211-220. [55] M. Mezbahul-Islam, A.O. Mostafa and M. Medraj, Essential magnesium alloys binary phase diagrams and their thermochemical data, J. Mater. 2014 (2014) 1-33. [56] R. Mola and A. Dziadoń, Formation of Mg eutecnic mixture layered composite, Arch. Foundry Eng., 8 (2008)
477
127-132.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0921-5093(19)31365-6
DOI:
https://doi.org/10.1016/j.msea.2019.138579
Reference:
MSA 138579
To appear in:
Materials Science & Engineering A
Received Date: 13 June 2019 Revised Date:
17 October 2019
Accepted Date: 19 October 2019
Please cite this article as: Q. Xu, A. Ma, B. Saleh, Y. Li, Y. Yuan, J. Jiang, C. Ni, Enhancement of strength and ductility of SiCp/AZ91 composites by RD-ECAP processing, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/j.msea.2019.138579. 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. © 2019 Published by Elsevier B.V.
1
Enhancement of strength and ductility of SiCp/AZ91 composites by
2
RD-ECAP processing
3
4
Qiong Xu a,b, Aibin Ma a,c,*, Bassiouny Saleh a, Yuhua Li a,c, Yuchun Yuana, Jinghua Jianga, Chaoying Ni b,* a
5 b
6
College of Mechanics and Materials, Hohai University, Nanjing 211100, China.
Department of Materials Science & Engineering, University of Delaware, DE 19716, USA c
7
Suqian Institute of Hohai University, Suqian 223800, China
8
9
10
11
12
13
14
15
16
17
18
19
* Correspondence
20
E-mail: [email protected] (A. MA); Tel: +86 25 8378 7239; Fax: +86 25 8378 6046
21
Email:[email protected] (C. Ni); Tel: +1 302 831 6359; Fax: + 302 831 4545
22
23
Abstract
24
In this research, as-cast SiCp/AZ91 composites were pre-homogenized at 420 ℃ for 20 h and then
25
processed by RD-ECAP at 250 ℃ for three different passes (4P, 8P, 16P). The effects of ECAP on the
26
microstructure and mechanical properties were investigated. Results showed that SiC particles were
27
more uniformly distributed, matrix grains were significantly refined, β-Al12Mg17 phases were
28
precipitated and homogenized, and texture was reformed to preferred crystal orientation after ECAP
29
processing. The mechanical tensile tests showed an effective increase in mechanical properties after
30
ECAP process. After 4P, 8P, 16P ECAP, the strength of the composites increased to 306 MPa, 288
31
MPa and 285 MPa, respectively; and the ductility increased to 6.4 %, 7.3 % and 8.2 %, respectively.
32
The strength of the composite was slightly decreased and the elongation was gradually increased
33
with the pass number increasing from 4~16P. The enhanced mechanical properties of the composites
34
in this study were ascribed to a combined effect of grain refinement, second phase precipitation,
35
dislocations and texture reforming.
36
Key words: magnesium metal matrix composites (Mg MMCs); RD-ECAP processing; strength and
37
ductility; grain refinement; second phase precipitation, texture.
38
1. Introduction
39
The overall world trend in the numerous engineering applications is that energy consumption
40
must be decreased by reducing the weight of products, therefore the automotive and aviation
41
industries develop rapidly. Due to the good properties of magnesium alloys, many researchers have
42
recently been interested in magnesium alloys. Magnesium alloys have lower densities, high specific
43
strength, precious resource and easy recycling [1-3]. However, magnesium alloy strength and
44
plasticity are usually less than aluminum alloy strength. Furthermore, magnesium alloy application is
45
limited due to the reduction in strength and the creep resistance when the temperature increases.
46
Researchers have shifted from the alloying system to the metal matrix composite system for
47
magnesium alloys to overcome these problems [4-6].
48
The proper reinforcement to form particle (such as SiC, B4C, TiC and Al2O3) reinforced
49
magnesium metal matrix composites (Mg MMCs) can provide superior characteristics such as high
50
strength and rigidity, reasonable wear and creep strength, and a lower thermo-expansion coefficient,
51
which are more suitable for advanced applications [3, 7-9]. There are several Mg MMCs applications,
52
such as automotive, aviation, communication, cutting tools, electronics, optoelectronics, etc., which
53
require unattainable properties in a single material [10]. Metal matrix composites (MMCs) can be
54
produced by many techniques such as solid-state techniques (like powder metallurgy and diffusion
55
bonding)[11], liquid-state techniques (e.g. stir casting, centrifugal casting and squeeze casting) [12,
56
13], in addition to semi-solid-state techniques and vapor deposition techniques (such as physical
57
vapor deposition and chemical vapor deposition) [14]. Great efforts have been made on Mg MMCs
58
after detailed positive studies on aluminum metal matrix composites (Al MMCs). Wu et al. [5, 6,
59
15-19] studied the properties of AZ91 alloy reinforced with SiC particles through using combined
60
method with heat treatment, forging and hot extrusion. In addition to microstructure evaluation, they
61
compared the effect of bimodal-size (micro+submicron) and the single-size (micro/submicron) SiC
62
particles on the mechanical properties of MMCs and obtained higher yield strength in the former one,
63
while the elongation was still poor.
64
Nie et al. [2] applied multi-pass forging to study mechanical properties of AZ91 alloy reinforced
65
with SiCp of 60 nm average size. The results showed that the yield strength improved, while the
66
ductility decreased as SiC nanoparticles coarsen the matrix in higher passes of forging. Niu et al. [20]
67
investigated the mechanical characteristics of AZ61 alloy reinforced with two different size
68
(micron+nano) of SiC particles with extrusion and study the synergistic effect of micron/nano SiCp
69
and submicron Mg17Al12 precipitates. These past investigations concluded that reinforcement size
70
has a significant effect on composite micro-structure and mechanical characteristics, as the
71
reinforcement mechanism differs in particle size composites [15, 21]. Ductility is adversely affected
72
by the large quantity of reinforcements, while micro-scale particles can achieve a highly specific
73
modulus and strength. Nano or bimodal strengthening can increase strength with little negative
74
impact on the ductility. Nevertheless, the nano-particles have a small refining effect only if they have
75
been pushed to the grain boundaries and agglomerated when solidified [22]. Micro-scale particles
76
can stimulate recrystallization, increase the strengthening of grain boundaries and transmit loads to
77
improve the refinement of grains efficiency and randomize the texture. Therefore, during
78
solidification and hot deformation, micron particles can obviously affect the microstructure, i.e.
79
improving strength and module.
80
According to previous researches, stir casting was the main processing method for manufacturing
81
Mg MMCs, including excellent mechanical properties, good machinability and low-cost benefits
82
[23-28]. Thermal-mechanical process often used to optimize the microstructure and properties for
83
Mg MMCs. Conventional secondary processing methods have been widely used, for example hot
84
rolling [4, 21, 29] , forging[2, 18, 26] in addition to hot extrusion [18, 20, 24, 30-33]. Today, severe
85
plastic deformation (SPD) in metallic materials has succeeded in obtaining a much finer structure of
86
grain. Among the SPD methods, equal channel angular pressing (ECAP) is more concentrated as
87
small grain sizes can be achieved and bulk materials are produced for practical applications [34, 35].
88
In the previous researches, there are reports of the successful use of ECAP on Al MMCs and other
89
composites [36-41], while the ECAP on MMCs has limited data and the enhanced mechanisms
90
during ECAP is far from readability. Chang et al. [42] conducted 1, 2 and 4 passes of ECAP on the
91
SiCp/AZ91 composites and investigated the mechanical properties at ambient and elevated
92
temperature. The results show that the grain size was reduced from ~20 µm to ~8 µm after 4P ECAP
93
and super-plasticity was achieved. Qiao et al. [43] studied the microstructure and mechanical
94
properties of AZ91 alloy reinforced with nano SiCp produced by extrusion and ECAP. The results
95
reveal that ECAP brought ~0.4 µm fine grains in the recrystallized region and led a uniform
96
distribution of the SiC particles; the strength of the composite was increased, whereas the ductility
97
was under ~2 %. Along with the other work [44, 45] on the SiCp/AZ91 composites, the above
98
researches also show that the characteristics of Mg MMCs are determined by and the interactions
99
between the comprehensive particle dispersion factors, grain refining, second phase, texture and
100
dislocations.
101
Our research group has developed a RD-ECAP equipment and has achieved grain refinement and
102
improvement of properties on various materials [41, 46-52]. RD-ECAP method can achieve high
103
ECAP deformation passes with one-time loading and unloading, protecting samples and improving
104
efficiency. Therefore, the current work is to improve the microstructure and mechanical properties in
105
addition to reducing casting defects of the AZ91 alloy reinforced with micro-SiCp by multi-pass
106
ECAP process (4, 8, 16P). The research focuses on the microstructure and mechanical properties of
107
the composite with the effect of RD-ECAP deformation, as also on strengthening mechanisms related
108
to grain refinement, second phase, dislocation and texture. The present study is of great significance
109
for the further application of MMCs and provides reference for the study of the deformation behavior
110
of highly deformed MMCs.
111
112
2. Experimental details
113
2.1 Materials and fabrication procedures
114
SiCp/AZ91 magnesium-metal matrix composite fabricated by semi-solid stirring casting was the
115
material used in the current work. The magnesium matrix alloy mainly contains 9.01 wt.% Al, 0.71
116
wt.% Zn, and 0.18 wt.% Mn. The reinforcement selected was SiC particles (5 vol. %) of density ~3.2
117
g/cm3, with an average size of ~10 µm. Samples with a dimension of 19.5 mm x 19.5 mm x 45 mm
118
were cut from the as-cast material and then homogenized at 420 ℃ for 20h. Then the
119
as-homogenized composites were processed at 250 ℃ for 4, 8, 16 passes through a 90 ° RD-ECAP
120
die, the operation principle of which can be found in our early work[50, 52-54]. Samples of five
121
different processing states were then collected, with the original casting material marked as as-cast,
122
the as-cast sample after solid solution marked as as-homogenized, the as-homogenized samples after
123
4, 8, 16 passes of ECAP process marked as 4P ECAP, 8P ECAP and 16P ECAP, respectively. The
124
mechanical properties of these ECAP composites were compared with as-cast and as-homogenized
125
metal composites, which is the main reference to the improvement of these properties.
126
2.2 Microstructure characterization
127
For all composites, microstructure analyzes were done to check particle distribution quality. Wire
128
electrical discharge machining (EDM) was used to cut metallographic samples with dimensions 10
129
mm x10 mm x2 mm. The specimens were polished to remove fine scratches and were etched in
130
acetic-picric solution. The microstructure and morphology were examined by an Olympus BX51M
131
optical microscopy and a scanning electron microscope (SEM, Sirion 200) equipped with an X-ray
132
energy dispersive spectrometer (EDS, Genesis 60S). The texture and phase analysis was carried out
133
by means of X-ray diffractometer (XRD, Cu-Kα, Bruker D8) with Cu Kα radiation. The electron
134
back-scatted diffraction (EBSD) analysis was conducted by an SEM (Hitachi S-3400N) equipped
135
with a HKL-EBSD system. TEM analysis was made with a FEI Tecnai-G2 thermo-emission
136
transmission electron microscope at an accelerating voltage of 200 kV.
137
2.3 Mechanical characterization
138
The size of tensile samples was selected according to ASTM E8 standard. The tensile test was
139
performed with a universal tensile testing machine at a strain rate of 0.5 mm/min. Dog-bone
140
geometry tensile specimens were cut along the plans that coincided with extrusion direction (0°),
141
with their length of 25 mm, 6 mm width and 6 mm shoulder radius. Five tensile samples were tested
142
and the median results were determined. Micro-hardness tests for measuring the hardness of all
143
composites were performed to check surface behavior. Ten points of each sample were tested, and
144
the average value of the Vickers hardness was calculated and plotted.
145
3. Results and discussion
146
The following sections explain the analysis of the microstructure of the composite specimens,
147
their mechanical properties and fracture morphology.
148
3.1 Microstructure analysis
149
Fig. 1 shows the microstructure of as-cast and as-homogenized SiCp/AZ91 composites. In the
150
as-cast composite shown in Fig. 1a and b, apart from the SiC particles, coarse second phases known
151
as β-Al12Mg17 are observed in the OM and SEM photographs (solid arrows in Fig. 1a and b). The
152
matrix is segregated by the SiC particles and those second phases. After homogenization, as shown
153
in Fig. 1c and d, almost no second phases are observed, indicating a complete dissolution of them. In
154
Fig. 1c, the matrix shows large grains with an average grain size ~150 µm, and the SiC particles are
155
clearly observed along the grain boundaries (dashed arrows in Fig. 1 c). The agglomeration of SiCp is
156
observed in some areas in both as-cast and as-homogenized composites according to the SEM
157
images, as marked in the dashed squares in Fig. 1b and d.
158
159 160 161 162 163
Fig. 1. Microstructure of SiCp/AZ91 composites: (a) and (b) OM and SEM for as-cast composite; (c) and (d) OM and SEM for as-homogenized composite.
Fig. 2 shows clearly the distribution and the morphology of the SiCp in the matrix of SiCp/AZ91
164
composites subjected to ECAP process. In the composite processed by 4P ECAP, it shows an obvious
165
SiCp flow along the extrusion direction and obvious agglomeration exists. After 8P ECAP, the SiCp
166
flow is “disordered” while it still can be detected. After 16P ECAP, the SiC particles are evenly
167
distributed almost in the whole version, as shown in Fig. 2c.
168 169
Fig. 2. SEM of the SiCp/AZ91 composites: (a) 4P ECAP; (b) 8P ECAP; (c) 16P ECAP.
170 171
Fig. 3 and Fig. 4 illustrate the grain refinement and second phases in SiCp/AZ91 composites
172
subjected to different passes of ECAP. Compared with the as-cast and as-homogenized composite in
173
Fig. 1, the grain structure of the ECAP composites is obviously refined with different degrees of
174
secondary phase precipitation. After 4P ECAP as shown in Fig. 3a, the matrix is composed of fine
175
grains and small amount of large grains that are highly deformed. Large numbers of fine second
176
phases were observed mostly around grain boundaries. The average grain size of the fine grains is
177
~1.5 µm, as illustrated in the inverse pole figure and the histogram in Fig. 4a and b. After 8P ECAP
178
as shown in Fig. 3b, almost the whole matrix is covered with fine grains and almost no large grains
179
remain, which suggests that dynamic recrystallization (DRX) is almost completed after 8P ECAP.
180
The composite processed by 16P ECAP has a more refined grain structure, and more small-scale
181
second phase precipitates are observed, as shown in Fig. 3c. These fine grains are equiaxed and the
182
average grain size is under 1 µm, as illustrated in Fig. 4c and d.
183 184
Fig. 3. OM of SiCp/AZ91 composites: (a) 4P ECAP; (b) 8P ECAP; (c) 16P ECAP.
(b) 10 Frequency(%)
9 8 7 6 5 4 3 2 1 0 0
1
2
3
4
5
6
7
8
9
10
Grain size(µm)
185
185 (d) 60 Frequency(%)
50 40 30 20 10 0 0
1
2
3
4
5
Grain size (µm)
187 188
Fig. 4. Inverse pole figure (a, c) and grain size histogram (b, d) of SiCp/AZ91 composites: (a) and (b) 4P ECAP; (c) and (d) 16P ECAP.
189 190
Fig. 5 shows the TEM photographs of SiCp/AZ91 composites processed by 4P ECAP and 16P
191
ECAP. After 4P ECAP as shown in Fig. 5a and b, DRX occurred; grains with high density of
192
dislocations are observed. After 16P ECAP, large numbers of second phases with grain sizes of ~200
193
nm to ~500 nm are observed around the grain boundaries, as shown in Fig. 5c and d. These fine
194
precipitates pin the grain boundaries and contribute to the grain refinement.
195
196 197
Fig. 5. TEM photographs of SiCp/AZ91 composites: (a) and (b) 4P ECAP; (c) and (d) 16P ECAP.
198 199
Fig. 6 shows the X-ray diffraction (XRD) patterns of the as-homogenized composite and the
200
composites obtained by different passes of ECAP process. In the XRD pattern in Fig. 6a, there is
201
only a weak Mg17Al12 phase diffraction peak, which means that only a small amount of
202
precipitation phases remain after the solid solution. After 4P ECAP, Mg17Al12 precipitation
203
occurred during the ECAP processing, showing obvious diffraction peaks, as shown in Fig 6b.
204
After 8P and 16P ECAP, the amounts of the Mg17Al12 phases further increase, as the intensity of
205
the diffraction peaks of the precipitation phases is stronger than that of the sample processed by
206
4P ECAP, as shown in Fig 6c and d. This observation in second phase change is consistent with
207
that in Fig. 3. Besides the change in the second phase precipitation, the XRD patterns in Fig.6
208
show that an obvious texture change occurs with the increase of the ECAP passes. The
209
as-homogenized composite shows a relatively strong basal (0002) texture and a weak 1010
210
texture, as shown in Fig. 6a. A clear deformation texture 1010
211
diffraction increases as shown in Fig. 6b-d, appears after ECAP processing.
212
from which maximum
213 214 215 216
Fig. 6. XRD patterns of SiCp/AZ91 composites: (a) As-homogenized; (b) 4P ECAP; (c) 8P ECAP; (d) 16P ECAP.
3.2 Mechanical properties
217
Fig. 7a shows the typical tensile stress-strain curves of the SiCp/AZ91 composites with different
218
processing states and Fig. 7b illustrates the yield strength (YS), ultimate tensile strength (UTS) and
219
elongation to failure. The as-cast SiCp/AZ91 composite possesses the lowest strength (YS of 78 MPa
220
and UTS of 132 MPa), and poor ductility (elongation of 4.7 %). After solid solution treatment, the
221
as-homogenized composite exhibits improved strength and ductility (elongation of 7.8 %), while the
222
improvement on the strength is not remarkable (YS of 86 MPa and UTS of 192 MPa). After ECAP
223
process, the composites exhibit simultaneously improved strength and ductility. After 4P, 8P and 16P
224
ECAP, the YS of the composite is 234 MP, 232 MPa and 225 MPa, respectively; the UTS is 306 MPa,
225
288 MPa and 285 MPa, respectively. The YS is almost tripled and the UTS is more than doubled
226
compared to that of the as-cast composite. Although the elongation is slightly decreased compared to
227
that of the as-homogenized composite (elongation of 7.8 %) in 4P ECAP and 8P ECAP composites,
228
the elongation increased with the increase of ECAP passes and is comparable to that of the
229
as-homogenized composite (elongation of 8.2 % after 16P ECAP). After 4P, 8P and 16P ECAP, the
230
increments of in the elongation of the composites are 36.2%, 55.3% and 74.5%, respectively,
231
compared to that of the as-cast composite. Table 1 shows the confidence intervals for tensile testing
232
results with confidence coefficient 0.95 and significance level 0.05.
(a) 350
(b) 350 as-cast as-homogenized 4P ECAP 8P ECAP 16P ECAP
Stress(MPa)
250
300
200 150
18 16
250
14 12
200
10 150
100
100
50
50
0
0
8 6
Elongation(%)
Stress (MPa)
300
20 YS UTS EL
4
0
2
4
6
8
10
12
14
16
18
20
2 0 as-cast as-homogenized 4PECAP
16P ECAP
Processing method
Strain (%)
233 234 235
8P ECAP
Fig. 7. (a) Stress-strain curves of the SiCp/AZ91 composites by different processing methods; (b) summary of yield strength, ultimate tensile strength and elongation.
236 237
Table 1 The confidence intervals for tensile testing results (EL (%), UTS and YS (MPa)). Processing Method
as-cast
as-homogenized EL
UTS
YS
4P ECAP EL
UTS
8P ECAP
Property
EL
UTS
YS
YS
Mean
4.92
136.36
78.20
8.26
190.58
85.40
6.66
301.42
232.80
Standard Deviation
1.13
18
9.55
1.19
14.61
2.88
0.84
15.52
3.90
Margin of Error
0.993
15.780
8.371
1.046
12.808
2.525
0.735
13.600
3.417
Point Estimate
4.92
136.36
78.20
8.26
190.58
85.40
6.66
301.42
Lower Limit
4.12
120.58
69.83
7.21
177.77
82.87
5.93
Upper Limit
6.11
152.14
86.57
9.31
203.39
87.93
7.39
EL
16P ECAP
UTS
YS
EL
UTS
YS
7.24
289.00
229.60
8.30
288.44
220.40
0.46
9.35
9.53
0.69
17.53
6.69
0.402
8.197
8.352
0.604
15.368
5.867
232.80
7.24
289.00
229.60
8.30
288.44
220.40
287.82
229.38
6.83
280.80
221.25
7.70
273.07
214.53
315.02
236.22
7.64
297.20
237.95
8.90
303.81
226.27
238 239
Fig. 8a shows the linear curve fitting of pass number and YS of SiCp /AZ91 composite; Fig. 8b
240
shows the linear curve fitting of pass number and EL of the composite. Equation (1) and (2) is
241
developed with a coefficient of determination (R2) of 98.6 % for YS and 92.9% for EL, respectively.
242
Also, equation (1) and (2) represent the empirical linear regression models that can be used to predict
243
the yield strength and the elongation of ECAP samples with different pass numbers from 4~16P. The
244
yield strength of the composite processed by ECAP is generally higher than the that of the as-cast
245
and as-homogenized composites (as shown in Fig. 7), while there is a slight decrease when the pass
246
number goes higher, as instructed by the linear curve in Fig. 8a and equation (1). The elongation of
247
the composites is gradually increased with the increase of the pass number from 4~16P, as shown in
248
Fig. 8b and equation (2). This illustrates that the mechanical property is determined by
249
comprehensive factors of grain refinement, second phases and texture. When processed by higher
250
passes of ECAP, a “soft orientation” is formed in the composite (basal texture is weakened and
251
plastic texture is intensified as shown in Fig. 6c-d), contributing to the above changes to the
252
253
mechanical property. = 237.5 − 0.7679
(1)
= 5.95 + 0.1446
(2)
Where: A is pass number. (a)
(b) 9.0
280
Experimental Linear Fit
260
8.0
Elongation (%)
240
YS (MPa)
Experimental Linear Fit
8.5
220 200
y=237.5-0.7679x 2 R =0.9856
180 160
7.5 7.0 6.5
y=5.95+0.1446x 2 R =0.9285
6.0 5.5 5.0
140 2
4
6
8
10
12
Pass number
14
16
18
2
4
6
8
10
12
14
16
18
Pass number
254 255
Fig. 8. Linear curve fitting for the SiCp /AZ91 composite: (a) pass number and YS; (b) pass number and EL.
256 257
Fig. 9 shows the effect of number of passes on micro-hardness values for the SiCp/AZ91
258
composites and matrix. Table 2 shows the confidence intervals for Hardness testing results with
259
confidence coefficient 0.95 and significance level 0.05. In comparison to the results obtained from
260
the conventional process, the micro-hardness values of samples deformed by ECAP processes after 4,
261
8 and 16 passes were all higher. After 4P ECAP, the increase in hardness is about 34% for composite,
262
from 85.96 HV to 115.25 HV. With an increase in the number of ECAP passes to 16, the hardness
263
values grow for both investigated cases. The hardness results achieved can be directly linked to the
264
microstructure as mentioned above, XRD and tensile investigation. ECAP modifies the
265
microstructure and mechanical characteristics of SiCp/AZ91 composites as described above in the
266
microstructure analysis. The grain refinement, second phase precipitation, and the enhanced bonding
267
between matrix and particles all contribute significantly to improving mechanical properties. The
268
formation of ECAP deformation texture is also a factor influencing the SiCp/AZ91composites. While
269
the grain refinement contributes to the significant incensement in strength, the weakening of (0002)
270
basal texture and the strengthening of 1010 deformation texture (Fig. 6) improve the ductility of
271
the alloys. As the ECAP processing was conducted at 250 ℃, which is below the eutectic point in the
272
Al-Mg phase diagram[55, 56], the second phases were precipitated and homogenized (Fig. 3 and Fig.
273
5). These second phases pin the grain boundaries and these hard phases also contributed to the
274
increase in hardness. 180
Matrix Composites
160
Hardness(HV)
140 120 100 80 60 40 20 0 as-homogenized
4P ECAP
8P ECAP
16P ECAP
Processing method
275 276 277 278
Fig. 9. Vickers micro-hardness evolution of the SiCp/AZ91 composites subjected to ECAP processing. Table 2 The confidence intervals for Hardness testing results. Property
Vickers Hardness (Matrix)
Processing Method
Vickers Hardness (Composites)
as-homogenized
4P ECAP
8P ECAP
16P ECAP
as-homogenized
4P ECAP
8P ECAP
16P ECAP
Mean
60.24
92.84
93.52
106.07
85.96
115.25
124.75
135.35
Standard Deviation
3.50
2.83
3.37
2.53
6.41
4.91
3.66
5.05
Significance Level
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
Margin of Error
2.17
1.75
2.09
1.57
3.97
3.04
2.27
3.13
Point Estimate
60.24
92.84
93.52
106.07
85.96
115.25
124.75
135.35
Lower Limit
58.07
91.09
91.43
104.50
81.99
112.21
122.48
132.22
Upper Limit
62.41
94.59
95.61
107.64
89.93
118.29
127.02
138.48
279 280
Fig. 10 shows the linear curve fitting of the pass number and the hardness of SiCp /AZ91
281
composites. It can be seen that Vickers hardness of composites increases with the increase of the pass
282
number. The equation (3) was developed with a coefficient of determination (R2) of 82.6 % for
283
Vickers hardness using this model. Also, equation (3) represent the empirical linear regression model
284
that can be used to predict the hardness of ECAP samples with different pass number from 0~16. ! "#
285
= 95.556 + 2.8245
Where: A is pass number.
(3)
150 Experimental Linear Fit
Hardness (HV)
140 130 120 110
y=95.556+2.8245x 2 R =0.826
100 90 80 0
2
4
6
8
10
12
14
16
18
20
Pass number
286 Fig. 10. Linear curve fitting for the pass number and the hardness of SiCp /AZ91 composite.
287 288
The linear regression model established using the achieved regression coefficients was validated
289
to be adequate in the confirmation experiments where new pass numbers have been used in the
290
model, and then the model results have been compared with the experimental results obtained for the
291
new set. Table 3 shows the new set of pass numbers for the adequacy checking, linear regression
292
model of mechanical properties and experimental results of mechanical properties. The percentage of
293
error was estimated based on a calculation of the hardness and yield strength results based on
294
experimental and linear regression models.
295
Table 3 Comparison of mechanical properties determined using experimental results and linear regression model. Hardness Results Pass
Experimental
number
Hardness (HV)
Yield strength Results
Linear Regression
Error
Experimental
Hardness
(%)
YS (MPa)
(HV)
Linear Regression YS (MPa)
Elongation Results
Error
Experimental
(%)
EL (%)
Linear Regression EL (%)
Error (%)
5
119.49
109.66
8.23
254.44
233.66
8.16
7.13
6.68
6.28
7
124.20
115.30
7.17
282.23
232.12
17.75
7.65
6.97
8.89
9
126.32
120.94
4.26
279.55
230.59
17.51
8.11
7.26
10.34
11
126.58
127.44
0.68
260.37
229.05
12.02
7.89
7.56
4.25
296 297
3.3 Fracture morphology
298
It is necessary to evaluate mechanical characteristics of composite material, while simultaneously
299
investigating mechanisms of deformation and failure and modes of fracture. So, the SEM was used
300
to analyze the fractural surface of the samples obtained through the tensile test. Fig. 11 illustrates the
301
fracture morphology of the as-cast composite and the composites processed by 4P and 16P ECAP.
302
The SEM results in Fig. 11a and b reveal that the as-cast composite shows a brittle fracture with
303
coarse grain structure, and the smooth surface of the coarse SiCp implies a relatively weak bonding
304
of the SiCp-AZ91 interface, which is coincident with the poor mechanical properties of the as-cast
305
composite. In the ECAP composites, there is brittle fracture of the composites with flat fracture
306
surface as shown in Fig. 11c and e. But also, the small dimples are found in the AZ91 matrix as
307
shown in Fig. 11d and f. The presence of dimples shows the typical matrix alloy ductile fracture. In
308
the 16P ECAP composite shown in Fig. 11f, the surface of the SiCp is quite rough and pits are
309
observed, which implies that the better interface bonding is achieved between the matrix and
310
particles. All of these illustrate that higher ECAP passes improve the microstructure and enhance the
311
mechanical properties of the SiCp/AZ91 composite.
312
313
314 315
Fig. 11. Fracture morphology of SiCp/AZ91 composites: (a) and (b) as-homogenized; (c) and (d) 4P ECAP; (e) and
(f) 16P ECAP.
316 317 318
4. Conclusions
319
In the present work, the as-cast SiCp/AZ91 composites were homogenized and subjected to high
320
passes of ECAP process with a 90˚ RD-ECAP die. The effects of the RD-ECAP process on the
321
microstructure and mechanical properties were studied, and the primary conclusions of this work
322
could be drawn as follows:
323
1) The high-pass RD-ECAP process effectively refined the matrix grain structure of the SiCp/AZ91
324
composites with intense second phase precipitation and reformed the texture to the preferred
325
orientation.
326
2) The RD-ECAP process largely eliminated the agglomeration of SiC particles and uniformed the
327
particle distribution. With the increase of the ECAP passe, the SiC particles were more evenly
328
distributed. At 4P and 8P ECAP, the SiC particles tended to be distributed along the extrusion
329
direction; at 16P ECAP, the SiC particles were evenly distributed in the matrix alloy.
330
3) The mechanical properties of the SiCp/AZ91 composites were effectively improved after
331
RD-ECAP process. The YS were almost tripped, and the UTS were more than doubled compared
332
to the as-cast composite. The elongation also increased after 4P, 8P and 16P ECAP, with an
333
increment of 36.2%, 55.3% and 74.5%, respectively, compared to that of the as-cast composite.
334
Micro-hardness increased with the increase of ECAP passes.
335
4) Fracture morphorlogy revealed the brittle fracture of the composites. The size of dimples in the
336
matrix of was smaller and SiCp-AZ91 interface bonding was better at higher ECAP passes than
337
lower ones, which indicated that higher passes of RD-ECAP improved the plastic property of the
338
SiCp/AZ91 composite.
339 340 341
5) For applications where mechanical efficiency is a significant demand, findings achieved using this SiCp/AZ91 composite can be effectively used. Acknowledgements
342
This study was supported by the National Natural Science Foundation of China (51774109 and
343
51501039), the Key Research and Development Project of Jiangsu Province (BE2017148) and the
344
Fundamental Research Funds for the Central Universities (2018B48414). Q.X. is grateful for the
345
support from the China Scholarship Council and the W. M. Keck Center for Advanced Microscopy
346
and Microanalysis at University of Delaware.
347
References
348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388
[1] M. Zha, H.M. Zhang, C. Wang, H.Y. Wang, E.B. Zhang, Q.C. Jiang, Prominent role of a high volume fraction of Mg17Al12 particles on tensile behaviors of rolled Mg-Al-Zn alloys, J. Alloy Compd. 728 (2017) 682-693. [2] K.B. Nie, J.G. Han, K.K. Deng, X.J. Wang, C. Xu, K. Wu, The Comparison in the microstructure and mechanical properties between AZ91 alloy and nano-SiCp/AZ91 composite processed by multi-pass forging under varying passes and temperatures, Materials 12 (2019) 625. [3] Y.S. Yi, R.F. Li, Z.Y. Xie, Y. Meng, S. Sugiyama, Effects of reheating temperature and isothermal holding time on the morphology and thixo-formability of SiC particles reinforced AZ91 magnesium matrix composite, Vacuum 154 (2018) 177-185. [4] W.Q. Liu, X.J. Wang, X.S. Hu, K. Wu, M.Y. Zheng, Effects of hot rolling on microstructure, macrotexture and mechanical properties of pre-extruded AZ31/SiC nanocomposite sheets, Mater. Sci. Eng.: A 683 (2017) 15-23. [5] K.K. Deng, K. Wu, Y.W. Wu, K.B. Nie, M.Y. Zheng, Effect of submicron size SiC particulates on microstructure and mechanical properties of AZ91 magnesium matrix composites, J. Alloy Compd. 504 (2010) 542-547. [6] K.K. Deng, C.J. Wang, J.Y. Shi, Y.W. Wu, K. Wu, Microstructure evolution mechanism of micron particle reinforced magnesium matrix composite at room temperature, Mater. Chem. Phys. 134 (2012) 581-584. [7] P. Xiao, Y.M. Gao, F.X. Xu, C.C. Yang, Y.F. Li, Z.W. Liu,et al., Tribological behavior of in-situ nanosized TiB2 particles reinforced AZ91 matrix composite, Tribol. Int. 128 (2018) 130-139. [8] S. Roy, G. Kannan, S. Suwas, M.K. Surappa, Effect of extrusion ratio on the microstructure, texture and mechanical properties of (Mg/AZ91)m-SiCp composite, Mater. Sci. Eng.: A 624 (2015) 279-290. [9] M.J. Shen, X.J. Wang, M.F. Zhang, X.S. Hu, M.Y. Zheng, K. Wu, Fabrication of bimodal size SiCp reinforced AZ31B magnesium matrix composites, Mater. Sci. Eng.: A 601 (2014) 58-64. [10] J. Singh, A. Chauhan, Characterization of hybrid aluminum matrix composites for advanced applications-A review, J. Mater. Res. Technol. 5 (2016) 159-169. [11] S.A. Lajevardi, T. Shahrabi, J.A. Szpunar, Synthesis of functionally graded nano Al2O3-Ni composite coating by pulse electrodeposition, Appl. Surf. Sci. 279 (2013) 180-188. [12] B. Saleh, J.H. Jiang, A.B. Ma, D. Song, D.H. Yang, Effect of main parameters on the mechanical and wear behaviour of functionally graded materials by centrifugal casting: A Review, Met. Mater. Int. (2019) 1-15. [13] I.M. El-Galy, M.H. Ahmed, B.I. Bassiouny, Characterization of functionally graded Al-SiCp metal matrix composites manufactured by centrifugal casting, J. Alexandria Eng. 56 (2017) 371-381. [14] N. Guo, M.C. Leu, Additive manufacturing: technology, applications and research needs, Front. of Mech. Eng. 8 (2013) 215-243. [15] K.K. Deng, X.J. Wang, C.J. Wang, J.Y. Shi, X.S. Hu, K. Wu, Effects of bimodal size SiC particles on the microstructure evolution and fracture mechanism of AZ91 matrix at room temperature, Mater. Sci. Eng.: A 553 (2012) 74-79. [16] K.K. Deng, C.J. Wang, X.J. Wang, K. Wu, M.Y. Zheng, Microstructure and elevated tensile properties of submicron SiCp/AZ91 magnesium matrix composite, Mater. Design 38 (2012) 110-114. [17] K.K. Deng, J.Y. Shi, C.J. Wang, X.J. Wang, Y.W. Wu, K.B. Nie, et al., Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite, Comps. Part A: Applied. S. 43 (2012) 1280-1284. [18] K. Wu, K.K. Deng, K.B. Nie, Y.W. Wu, X.J. Wang, X.S. Hu, et al., Microstructure and mechanical properties of SiCp/AZ91 composite deformed through a combination of forging and extrusion process, Mater. Design 31 (2010) 3929-3932. [19] K.K. Deng, K. Wu, X.J. Wang, Y.W. Wu, X.S. Hu, M.Y. Zheng, et al., Microstructure evolution and mechanical
389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432
properties of a particulate reinforced magnesium matrix composites forged at elevated temperatures, Mater. Sci. Eng.: A 527 (2010) 1630-1635. [20] X.F. Niu, G. Li, Z.Y. Zhang, P.W. Zhou, H.X. Wang, S.X. Zhang, et al., Simultaneously improving the strength and ductility of extruded bimodal size SiCp/AZ61 composites: Synergistic effect of micron/nano SiCp and submicron Mg17Al12 precipitates, Mater. Sci. Eng.: A 743 (2019) 207-216. [21] X.J. Wang, W.Q. Liu, X.S. Hu, K. Wu, Microstructural modification and strength enhancement by SiC nanoparticles in AZ31 magnesium alloy during hot rolling, Mater. Sci. Eng.: A 715 (2018) 49-61. [22] X.J. Wang, X.S. Hu, K.B. Nie, K. Wu, M.Y. Zheng, Hot extrusion of SiCp/AZ91 Mg matrix composites, T. Nonferr. Metal Soc. 22 (2012) 1912-1917. [23] L. Zhang, Q.D. Wang, G.P. Liu, W. Guo, H. Jiang, W. Ding, Effect of SiC particles and the particulate size on the hot deformation and processing map of AZ91 magnesium matrix composites[J]. Mater. Sci. Eng.: A 707 (2017) 315-324. [24] L. Zhang, Q.D. Wang, W.J. Liao, W. Guo, B. Ye, W.Z. Li, et al., Effects of cyclic extrusion and compression on the microstructure and mechanical properties of AZ91D magnesium composites reinforced by SiC nanoparticles, Mater. Charact. 126 (2017) 17-27. [25] X.J. Wang, X.S. Hu, W.Q. Liu, J.F. Du, K. Wu, Y.D. Huang, M.Y. Zheng, Ageing behavior of as-cast SiCp/AZ91 Mg matrix composites, Mater. Sci. Eng.: A 682 (2017) 491-500. [26] W. Guo, Q.D. Wang, W.Z. Li, H. Zhou, L. Zhang, W.J. Liao, Enhanced microstructure homogeneity and mechanical properties of AZ91-SiC nanocomposites by cyclic closed-die forging, J. Comps. Mater. 51 (2017) 681-686. [27] X.J. Wang, X.S. Hu, K. Wu, L.Y. Wang, Y.D. Huang, Evolutions of microstructure and mechanical properties for SiCp/AZ91 composites with different particle contents during extrusion, Mater. Sci. Eng.: A 636 (2015) 138-147. [28] S. Pawar, X. Zhou, T. Hashimoto, G.E. Thompson, G. Scamans, Z. Fan, Investigation of the microstructure and the influence of iron on the formation of Al8Mn5 particles in twin roll cast AZ31 magnesium alloy, J. Alloy Compd. 628 (2015) 195-198. [29] W.Q. Liu, X.S. Hu, X.J. Wang, K. Wu, M.Y. Zheng, Evolution of microstructure, texture and mechanical properties of SiC/AZ31 nanocomposite during hot rolling process, Mater. Design 93 (2016) 194-202. [30] M.J. Shen, X.J. Wang, T. Ying, K. Wu, W.J. Song, Characteristics and mechanical properties of magnesium matrix composites reinforced with micron/submicron/nano SiC particles, J. Alloy Compd. 686 (2016) 831-840. [31] X.J. Wang, K.B. Nie, X.S. Hu, Y.Q. Wang, X.J. Sa, K. Wu, Effect of extrusion temperatures on microstructure and mechanical properties of SiCp/Mg-Zn-Ca composite[J]. J. Alloy Compd. 532 (2012) 78-85. [32] M.J. Shen, X.J. Wang, M.F. Zhang, X.S. Hua, M.Y. Zheng, K.Wu, Effect of bimodal size SiC particulates on microstructure and mechanical properties of AZ31B magnesium matrix composites, Mater. Design (1980-2015) 52 (2013) 1011-1017. [33] H. Chang, C.J. Wang, K.K. Deng, K.B. Nie, K. Su, L. Liao, et al, Effects of SiCp Content on the microstructure and mechanical properties of SiCp/Mg-5Al-2Ca composites, Rare Metal Mater. Eng. 47 (2018) 1377-1384. [34] J. Xu, M. Shirooyeh, J. Wongsa-Ngam, D.B. Shan, B. Guo, T.G. Langdon, Hardness homogeneity and micro-tensile behavior in a magnesium AZ31 alloy processed by equal-channel angular pressing, Mater. Sci. Eng.: A 586 (2013) 108-114. [35] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, Y.T. Zhu, Producing bulk ultrafine-grained materials by severe plastic deformation, JOM 58 (2006) 33-39. [36] H. Zare, M. Jahedi, M.R. Toroghinejad, M. Meratian, M. Knezevic, Microstructure and mechanical properties of carbon nanotubes reinforced aluminum matrix composites synthesized via equal-channel angular pressing, Mater. Sci. Eng.: A 670 (2016) 205-216. [37] C.H. Qian, L. Ping, K.M. Xue, Influence of deformation passes on interface of SiCp/Al composites consolidated by
433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476
equal channel angular pressing and torsion, T. Nonferr. Metal Soc. 25 (2015) 1376-1382. [38] L. Ping, X. Zhang, K.M. Xue, L. Xiao, Effect of equal channel angular pressing and torsion on SiC-particle distribution of SiCp-Al composites, T. Nonferr. Metal Soc. 22 (2012) s402-s407. [39] M. Saravanan, R. Pillai, K. Ravi, B. Pai, M. Brahmakumar, Development of ultrafine grain aluminium-graphite metal matrix composite by equal channel angular pressing, Compos. Sci. Technol. 67 (2007) 1275-1279. [40] K. Ravi, M. Saravanan, R. Pillai, A. Mandal, B. Murty, M. Chakraborty,et al., Equal channel angular pressing of Al-5 wt% TiB2 in situ composite, J. Alloy Compd. 459 (2008) 239-243. [41] Q. Xu, A.M. Ma, J.J. Wang, J.P. Sun, J.H. Jiang, Y.H. Li, et al., Development of high-performance SiCp/Al-Si composites by equal channel angular pressing, Metals 8 (2018) 738. [42] H. Chang, X.J. Wang, M.Y. Zheng, K. Wu, Effect of ECAP on the microstructure and tensile property of SiCp/AZ91 magnesium matrix composite[C]//Key Engineering Materials. Trans. Tech. 353 (2007) 1342-1345. [43] X.G. Qiao, M.Y. Zheng, E.D.Wei, K.Wu, X.S. Hu,W.M.Gan, et al., Microstructure evolution and mechanical properties of nano-SiCp/AZ91 composite processed by extrusion and equal channel angular pressing (ECAP), Mater. Charact. 121 (2016) 222-230.. [44] B.N. Sahoo, F.K. Md, S. Babu, S.K. Panigrahi, G.D. Janaki Ram, Microstructural modification and its effect on strengthening mechanism and yield asymmetry of in-situ TiC-TiB2/AZ91 magnesium matrix composite, Mater. Sci. Eng.: A 724 (2018) 269-282. [45] S.J. Shang, K.K. Deng, K.B. Nie, J.C. Li, S.S. Zhou, F.J. Xu, et al., Microstructure and mechanical properties of SiCp/Mg-Al-Zn composites containing Mg17Al12 phases processed by low-speed extrusion, Mater. Sci. Eng.: A 610 (2014) 243-249. [46] Q. Xu, A.B. Ma, Y.H. Li, C.Y. Ni, Microstructure of a high strength AZ91 alloy processed by severe plastic deformation, Microsc. Microanal. 24 (2018) 2176-2177. [47] J.P. Sun, Z.Q.Yang, J. Han, H. Liu, D. Song, J.H. Jiang, High strength and ductility AZ91 magnesium alloy with multi-heterogenous microstructures prepared by high-temperature ECAP and short-time aging, Mater. Sci. Eng.: A 734 (2018) 485-490. [48] L.Y. Zhang, A.B. Ma, J.H. Jiang, X. Jie, Effect of processing methods on microhardness and acid corrosion behavior of low-carbon steel, Mater. Design (1980-2015), 65 (2015) 115-119. [49] C.C. Zhu, A.B. Ma, J.H. Jiang, X.B. Li, D. Song, D.H. Yang, et al., Effect of ECAP combined cold working on mechanical properties and electrical conductivity of Conform-produced Cu-Mg alloys. J. Alloy Compds. 582 (2014) 135-140. [50] Y.C. Yuan, A.B. Ma, J.H. Jiang, F.M. Lu, W.W. Jian, D. Song, et al., Optimizing the strength and ductility of AZ91 Mg alloy by ECAP and subsequent aging, Mater. Sci. Eng.: A 588 (2013) 329-334. [51] A.B. Ma, J.H. Jiang, N. Saito, I. Shigematsu, A. Watazu, Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die, T. Nonferr. Metal Soc. 17 (2007) 104-109. [52] A.B. Ma, Y. Nishida, K. Suzuki, I. Shigematsu, N. Saito, Characteristics of plastic deformation by rotary-die equal-channel angular pressing, Scripta Mater. 52 (2005) 433-437. [53] F.M. Lu, A.B. Ma, J.H. Jiang, D.H. Yang, D. Song, Y.C. Yuan, et al., Effect of multi-pass equal channel angular pressing on microstructure and mechanical properties of Mg97.1Zn1Gd1.8Zr0.1 alloy, Mater. Sci. Eng.: A 594 (2014) 330-333. [54] A.B. Ma, K. Suzuki, Y. Nishida, N. Saito, I. Shigematsu, M. Takagi, et al., Impact toughness of an ultrafine-grained Al-11mass% Si alloy processed by rotary-die equal-channel angular pressing, Acta Mater. 53 (2005) 211-220. [55] M. Mezbahul-Islam, A.O. Mostafa and M. Medraj, Essential magnesium alloys binary phase diagrams and their thermochemical data, J. Mater. 2014 (2014) 1-33. [56] R. Mola and A. Dziadoń, Formation of Mg eutecnic mixture layered composite, Arch. Foundry Eng., 8 (2008)
477
127-132.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: