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Influence of B4C Particles on Processing and Strengthening Mechanisms in Aluminum Metal Matrix Composites - a Review
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 5356–5363
www.materialstoday.com/proceedings
ICMPC-2019
Influence of B4C Particles on Processing and Strengthening Mechanisms in Aluminum Metal Matrix Composites - a Review Sudipta Chanda, P. Chandrasekhara*, R.K. Sarangia, R.K. Nayaka a
KIIT Deemed to be University, Patia, Bhubaneswar, 751024, India
Abstract This paper reviews different experimental and theoretical studies related to B4C reinforced aluminum metal matrix composites (MMC). Various processes like solid sintering, powder processing through ball milling, and also various thermo-mechanical processing like heat treatment, quenching are discussed. The effects on the particle size of boron carbide (B4C) reinforcement on morphology evolution and microstructure are thoroughly studied. The strengthening mechanisms of B4C reinforced Al-MMC are thoroughly reviewed and the interfacial structure in between B4C and matrix material are discussed. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Boron Carbide; Aluminium MMC; Strengthening Mechanism
1. Introduction Metal matrix composites are popular due its high stiffness, high strength, wear resistance, elastic modulus and creep resistance. Among various MMC’s, particulate reinforced Al-MMC’s have high strength to weight ratio, low density, high stiffness, good wear resistance, and low cost[1–4]. These are used in various industries like automobile, aircraft, and defense sectors [5]. Boron carbide reinforcement is chosen as it has low density, high strength, high hardness (third hardest material), better thermo-chemical stability and has good neutron absorbing ability [5–8]. There are various process routes to produce B4C reinforced Al-MMC, like solid state consolidation (PM route), casting[9,10]pressure less infiltration(liquid phase method) [11] and in-situ compo casting [12]. Mechanical milling (MM) is a manufacturing procedure mainly used in the industries. This process is mainly used for blending of
* Corresponding author. Tel.: +91 7008199972 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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powders [13–15].This process is used in such a way that, during blending the microstructure of particle is not changed. Benjamin et al [16,17] studied that milling of aluminum or nickel powder in oxidizing atmosphere, the oxides of the particles are produced. They termed this process as mechanical alloying (MA), which resulted in a new material. This paper provides in-depth review of research done on mechanical milling in processing of B4C reinforced Al-MMC. The review also covers thermo-mechanical processing, the morphology evolution and the particle size effects on the microstructure of B4C reinforced Al-MMC. The strengthening mechanisms and the formation of interface between B4C particulates and Al matrix are also discussed. 2. Material Preparation Many researchers have used Al alloys and reinforced it with B4C particulates. In this paper the solid sintering, powder milling and thermo-mechanical processes have been discussed thoroughly. L.Jiang[18] stated that gas atomized 5083 Al matrix and spherical nano B4C particulates with average size 50 nm were cryomilled to produce mixed particles. 0.2 wt% of steric acid was mixed with these particles and was milled for 12h in liquid nitrogen. The milling speed was fixed at 180 RPM and ball-to-powder ratio of 30:1 was used. Then it was degassed for 20h at 500°C then it was was sintered by HIP followed by extrusion at 400°C. The preparation of nano composites is shown in Fig 1. Combining cryomilling and hot extrusion, the researchers fabricated the nanocomposites using 2.5 vol % nano B4C particles with Al 5083 and found out that yield strength was 761 MPa and a strain value of 2%.Various researchers have used this fabrication technique with the matrix 5083Al[1,2,19–22]. Z. Mo et al [23]used AA2024 powders with 10 vol % B4C particles and blended them. The blending time was for 30 minutes at a ball to powder ratio of 9:1. The mixture was heated under pure argon gas for about 60 minutes, followed by rolling and forming. The final product had strength of 480 MPa. It was achieved due to increase in relative density (0.9451) and decrease in porosity. Zhang et al. [24] blended the powders and then it was pressed using isostatic pressing, sintering and HIP/extrusion. The results showed that composites fabricated with HIP shows weak bonding and a low tensile strength (<300MPa) compared to hot extrusion process. Wu et al [25] fabricated 7075Al with different sizes of B4C particles.7075Al and 7.5wt% B4C were mixed in a mixer for 24h followed by sintering at a temperature of 530°C for three minutes. High relative density (>99.0%) was achieved in production of these nano composites. The yield compressive strength (469 MPa) and fracture compressive strength (701 MPa) was observed in composites which had smallest B4C particulate size (2µm). 6061Al with a particle size of 13µm and B4C particles of 7µm was studied by Li et. al[26]. These particles were mechanically mixed at 50 RPM for 8h taking 2:1 ball to powder ratio. Investigation was carried taking a varying B4C content (0-30wt %), different holding times and hot pressing temperatures (560-620). The optimal results were found at 580°C and 30 mins holding time. The tensile strength was 400MPa .It can be concluded that to get high tensile strength and good ductility we have to fabricate composites with high relative density which can achieved by hot vacuum degassed method before consolidated and vacuum or argon atmosphere with consolidating as shown in below Fig (1-4).
Fig:1 Material Preparation
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3. Morphology The mechanical milling process allows the incorporation of B4C reinforced phases and the aluminum matrix into the each of the powdered particle[14]. Therefore this process is very useful in synthesizing Al-B4C MMC. During the first stage the morphology and microstructure of the Al-B4C MMC is at original level. The B4C reinforced phase and the aluminum matrix is fractured and deformed with further milling. Further, the aluminum particles are cold welded and plastic deformation is seen[27,28].Fig. 2 shows the SEM micrographs of an Al-B4C powders mixture.B4C particles are also fractured into smaller size and kept decreasing till the stress caused due to the collision between the particles is equal or smaller to the fracture strength[14].
Fig 2.SEM of Al matrix mixedwith B4C particulates milled for different duration: (a) 0 h, (b) 4 h, (c) 8 h and (d) 16 h.[27]
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4. Strengthening Mechanism Different theories are available for predicting the strengthening behaviour of Al- B4C metal matrix composites. There are three stages in which we can divide the general failure process: yield stage, fracture and elastic stage[29]. However several strengthening mechanisms can be related to the increase of yield strength in B4C particles[4,29– 32]: (1) Load transfer from the matrix to the hard reinforcements in Al-B4C MMC; (2) The co-efficient of thermal expansion mismatch between Al matrix and the B4C particles;(3)Orowan Strengthening mechanisms in B4C reinforced Al-MMC;(4)Hall-Petch strengthening mechanisms in B4C reinforced Al-MMC;(5) Elastic modulus mismatch between Al matrix and B4C particles. Each strengthening method are discussed in the following sections. Orowan Strengthening 5. It is the effect of B4C particles on the strength of the metal matrix composites and its consists in interaction of B4C particles with dislocations. In the B4C nano particles the spacing between them is very less ,therefore it causes resistance to the passing of dislocations[10]. This type of strengthening mechanism is not applicable in the micro
Fig 3.Representation of various preparation methods of nano composites with different structures[18]
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sized B4C particulates due to large inter-particle spacing. The general expression for orowan strengthening is given below[30]. −
0.4Gb ln(d b) =M π (1 − υ )1 2 λ
σ orowan
2 d 3
−
d=
(2)
4 f −1 E G= 2(1 + υ ) −
λ = d
(1)
π
(3)
(4)
M is mean orientation factor, G is shear modulus of alluvium matrix, b is burgers vector, E is elastic modulus of aluminum matrix, υ is poisons ratio, λ is inter-particle spacing, and d is the diameter of B4C particle Load transfer effect This type of effect is applied when there is external application of load, such as hot rolling process, hot pressed process and other thermos mechanical processes. The load transfer takes place from Al matrix to the B4C reinforcements which contribute to the strengthening of Al matrix. In this process the yield stress is also improved which is strongly dependent on the volume fraction of boron carbide particulates. The general expression for load bearing contribution of B4C is given as[4,30]:
(l + t ) A (5) Δσ LT = v pσ m 4l v p is the volume fraction of B4C particulates, σ m is yield strength of aluminum matrix, l and t is the size of B4C particulate parallel and perpendicular to the load direction respectively, A = l / t is the B4C particulate aspect ratio,
For equiaxed B4C particle the expression is given below:
Δσ LT =
1 v pσ m 2
(6)
Hall petch strengthening This strengthening mechanism is based on the influence of grain size on the strength of MMC. In the Al matrix, the influence of grain size has a strong effect because the grain boundaries can obstruct the dislocation movement. The general expression for hall petch strengthening is given as[4,30]:
σ y = σ o + k y d −1 2
σ o is the original strength of B4C –Al MMC, k y is constant, and d
(7) is grain size of component
CTE and EM mismatch Co-efficient of thermal expansion (CTE) and the elastic modulus (EM) mismatch is related to the dislocation density and strength of the B4C reinforced Al-MMC. The difference of CTE and EM mismatch and the work hardening during hot rolling and hot pressing[30], causes the dislocations in B4C reinforced Al-MMC. Co-efficient
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of thermal expansion (CTE) and the elastic modulus (EM) mismatch is related to the dislocation density and strength of the B4C reinforced Al-MMC. The difference of CTE and EM mismatch and the work hardening during hot rolling and hot pressing[30], causes the dislocations in B4C reinforced Al-MMC. As reported by Jiang et al.[1] the strengthening mechanism was mainly subjected to orowan strengthening, load bearing and also dislocation strengthening. In Table(1) it is summarized that the structure, interfaces and the morphology of B4C Al-MMC with respect to the mechanical behaviour of the composite. When the particulate size of B4C is at micron level (>1000nm) the intergranular phases , and the strengthening mechanism was low dislocation strengthening by B4C reinforcement and high load bearing. When gradually B4C reinforcement size decreased to sub-micron level (1001000nm) it was observed that the strengthening mechanism was medium dislocation and medium load bearing in B4C particulate. Further when the reinforcement size decreased to nano level (<100nm) the strengthening mechanism observed was orowan strengthening for load bearing. The strengthening can also be contributed by grain refinement, dislocation strengthening and load bearing as stated by Jiang et al.(18). The calculation was done and they found out that the yield strength values of the theoretical results were matching with the experimental values. The CTE mismatch of B4C reinforced Al-MMC was proposed by Chen et.al (29) which stated that strength contribution can be attributed to CTE mismatch between B4C particles and 6061 Al. The existence of B4C particles and the orowan strengthening in the B4C reinforced Al-MMC will transfer the load from the aluminium matrix to the B4C particles. From the calculation it was clear that CTE mismatch values for B4C reinforced Al-MMC was 24.1 MPa , 34.23 MPa, 41.93 MPa and 48.41 MPa for 10%, 20%, 30% and 40% B4C reinforcement respectively. It was also stated that the yield stress of boron carbide reinforced aluminium metal matrix composites depends on volume fraction of boron carbide particulates. The load transfer values were calculated as 5.15 MPa, 10.30 MPa, 15.45 MPa, and 20.60 MPa for 10%, 20%, 30% and 40% B4C reinforcement respectively. The orowan strengthening values were calculated as 1.66 MPa, 2.55 MPa, 4.54 MPa and 10.21 MPa for 10%, 20%,30%, and 40% of B4C reinforcement respectively . 6. Interfacial Characteristics Interfacial Structure in B4C reinforced Al-MMC is influenced by length scale of B4C particulates[1]. Approximately 40% of B4C nano particles are located intragranularly where as B4C submicron particles are located at the Al grain boundaries, which created intergranular interfaces. These interfaces are incoherent where as intragranularnanoB4CAl interfaces are semi coherent. Fig 3.shows the interface orientation of nano B4C – Al intragranular interfaces. (1 1 1)Al//(0 2 4)B4C, 3.4° angle between (0 0 2)Al and (0 0 3)B4C, and 7.8° angle between (2 −2 0)Al and (0 2 1)B4C . To decrease stress location there is a presence of orientational relationship between aluminum matrix and the intragranularnano boron carbide particulates. Li et al.[26]found that during hot pressing of B4C/6061Al composites, the main reaction products were Al3BC and MgAL2O4 phases. The former increased with increase in temperature but interfacial reaction is still unclear as shown in below Fig 4(a-d).
Table 1.Strength and Ductitlity contribution on various B4C /Al interfaces[1] Interface
Strengthening contribution
Scale
Load bearing
Orowan strengthening
Dislocation strengthening
High
Unlikely
Low
Low
Low
Low
Medium
Unlikely
Medium
Low
Low
Medium
Low Low
Unlikely High
High High
Low High
Medium High
Medium High
Micron (>1000 nm) Submicron (100∼1000 nm) Nanoscale (<100 nm)
Ductility contribution Dislocation mobility Suppress near necking interface
Minimize stress localization
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Fig 4. 3D atomic models(a)B4C cells with cubic Al cells with various orientation;(b) B4C/Al with orientation of (1 1 1)Al//(0 2 4)B4C;(c)2D model of B4C/Al;(d)&(e) HRTEM images of B4C/Al[1]
Summary The strengthening mechanism, morphology evolution and the processing of B4C reinforced Al-MMC was reviewed. The effects of reinforcement size on the morphology and microstructure with respect to mechanical behaviour of Al matrix was reviewed. The review provided an overview on various strengthening mechanisms which can be applied to strengthen B4C reinforced Al –MMC.
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References [1] L. Jiang, et al. Acta Mater.(2015) 89 327–343. [2] M. Li, et al. Mater. Sci. Eng. A(2016) 656 241–248. [3] S. Rawal, JOM (2001) 53 14–17. [4] R. Casati, M. Vedani, Metals (2014) 4 65–83. [5] E. Mohammad Sharifi, F. Karimzadeh, M.H. Enayati, Mater. Des. (2011)32 3263–3271. [6] F. Toptan, A. Kilicarslan, A. Karaaslan, M. Cigdem, I. Kerti, Mater. Des.(2010) 31 87–91. [7] R.M. Mohanty, K. Balasubramanian, S.K. Seshadri, Mater. Sci. Eng. A (2008) 498 42–52. [8] I. Topcu, H.O. Gulsoy, N. Kadioglu, A.N. Gulluoglu, J. Alloys Compd. (2009) 482 516–521. [9] N. Tuncer, et al. Ceram. Int. (2011) 37 2861–2867. [10] B. Previtali, et al. Composites A (2008) 39 1606–1617. [11] M.F. Ibrahim, et al. Composites B (2015)79 83–94. [12] L. Jia, S.F. Li, H. Imai, B. Chen, Mater. Sci. Eng. A (2014) 614 129–135. [13] C. Suryanarayana, Prog. Mater. Sci. (2001) 46 1–184. [14] D.L. Zhang, Prog. Mater. Sci. (2004) 49 537–560. [15] H. Guo, Z. Zhang, et al. TMS 2017 146th Annual Meeting & Exhibition Supplemental Proceedings, 2017, 599–607. [16] J.S. Benjamin, Met. Trans. (1970) 1 2943–2951. [17] J.S. Benjamin, T.E. Vollin, Met. Trans. (1974) 5 1929–1934. [18] L. Jiang, et al. Acta Mater. (2016) 103 128–140. [19] J. Ye, et al. Scr. Mater. (2005) 53 481–486. [20] Z. Zhang, et al. Scr. Mater. (2011) 65 652–655. [21] Y. Li, et al. Acta Mater. (2011) 59 7206–7218. [22] H. Yang, et al. Mater. Sci. Eng. A (2014) 616 35–43. [23] Z. Mo, et al. Mater. Sci. Eng. A (2016) 652 305–314. [24] H. Zhang, et al. Mater. Sci. Eng. A (2006) 433 70–82. [25] C. Wu, et al. Mater. Sci. Eng. A (2016) 675 421–430. [26] Y.Z. Li, et al. Mater. Sci. Eng. A (2015) 620 445–453. [27] M. Khakbiz, F. Akhlaghi, J. Alloys Compd. (2009) 479 334–341. [28] S. Ozkaya, A. Canakci, Powder Technol. (2016) 297 8–16. [29] H.S. Chen, et al. J. Alloys Compd. (2015) 632 23–29. [30] A. Sanaty-Zadeh, Mater. Sci. Eng. A (2012) 531 112–118. [31] Z. Zhang, D.L. Chen, Scr. Mater. (2006) 54 1321–1326. [32] Z. Zhang, D.L. Chen, Mater. Sci. Eng. A (2008) 483 148–152.
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ScienceDirect Materials Today: Proceedings 18 (2019) 5356–5363
www.materialstoday.com/proceedings
ICMPC-2019
Influence of B4C Particles on Processing and Strengthening Mechanisms in Aluminum Metal Matrix Composites - a Review Sudipta Chanda, P. Chandrasekhara*, R.K. Sarangia, R.K. Nayaka a
KIIT Deemed to be University, Patia, Bhubaneswar, 751024, India
Abstract This paper reviews different experimental and theoretical studies related to B4C reinforced aluminum metal matrix composites (MMC). Various processes like solid sintering, powder processing through ball milling, and also various thermo-mechanical processing like heat treatment, quenching are discussed. The effects on the particle size of boron carbide (B4C) reinforcement on morphology evolution and microstructure are thoroughly studied. The strengthening mechanisms of B4C reinforced Al-MMC are thoroughly reviewed and the interfacial structure in between B4C and matrix material are discussed. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Boron Carbide; Aluminium MMC; Strengthening Mechanism
1. Introduction Metal matrix composites are popular due its high stiffness, high strength, wear resistance, elastic modulus and creep resistance. Among various MMC’s, particulate reinforced Al-MMC’s have high strength to weight ratio, low density, high stiffness, good wear resistance, and low cost[1–4]. These are used in various industries like automobile, aircraft, and defense sectors [5]. Boron carbide reinforcement is chosen as it has low density, high strength, high hardness (third hardest material), better thermo-chemical stability and has good neutron absorbing ability [5–8]. There are various process routes to produce B4C reinforced Al-MMC, like solid state consolidation (PM route), casting[9,10]pressure less infiltration(liquid phase method) [11] and in-situ compo casting [12]. Mechanical milling (MM) is a manufacturing procedure mainly used in the industries. This process is mainly used for blending of
* Corresponding author. Tel.: +91 7008199972 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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powders [13–15].This process is used in such a way that, during blending the microstructure of particle is not changed. Benjamin et al [16,17] studied that milling of aluminum or nickel powder in oxidizing atmosphere, the oxides of the particles are produced. They termed this process as mechanical alloying (MA), which resulted in a new material. This paper provides in-depth review of research done on mechanical milling in processing of B4C reinforced Al-MMC. The review also covers thermo-mechanical processing, the morphology evolution and the particle size effects on the microstructure of B4C reinforced Al-MMC. The strengthening mechanisms and the formation of interface between B4C particulates and Al matrix are also discussed. 2. Material Preparation Many researchers have used Al alloys and reinforced it with B4C particulates. In this paper the solid sintering, powder milling and thermo-mechanical processes have been discussed thoroughly. L.Jiang[18] stated that gas atomized 5083 Al matrix and spherical nano B4C particulates with average size 50 nm were cryomilled to produce mixed particles. 0.2 wt% of steric acid was mixed with these particles and was milled for 12h in liquid nitrogen. The milling speed was fixed at 180 RPM and ball-to-powder ratio of 30:1 was used. Then it was degassed for 20h at 500°C then it was was sintered by HIP followed by extrusion at 400°C. The preparation of nano composites is shown in Fig 1. Combining cryomilling and hot extrusion, the researchers fabricated the nanocomposites using 2.5 vol % nano B4C particles with Al 5083 and found out that yield strength was 761 MPa and a strain value of 2%.Various researchers have used this fabrication technique with the matrix 5083Al[1,2,19–22]. Z. Mo et al [23]used AA2024 powders with 10 vol % B4C particles and blended them. The blending time was for 30 minutes at a ball to powder ratio of 9:1. The mixture was heated under pure argon gas for about 60 minutes, followed by rolling and forming. The final product had strength of 480 MPa. It was achieved due to increase in relative density (0.9451) and decrease in porosity. Zhang et al. [24] blended the powders and then it was pressed using isostatic pressing, sintering and HIP/extrusion. The results showed that composites fabricated with HIP shows weak bonding and a low tensile strength (<300MPa) compared to hot extrusion process. Wu et al [25] fabricated 7075Al with different sizes of B4C particles.7075Al and 7.5wt% B4C were mixed in a mixer for 24h followed by sintering at a temperature of 530°C for three minutes. High relative density (>99.0%) was achieved in production of these nano composites. The yield compressive strength (469 MPa) and fracture compressive strength (701 MPa) was observed in composites which had smallest B4C particulate size (2µm). 6061Al with a particle size of 13µm and B4C particles of 7µm was studied by Li et. al[26]. These particles were mechanically mixed at 50 RPM for 8h taking 2:1 ball to powder ratio. Investigation was carried taking a varying B4C content (0-30wt %), different holding times and hot pressing temperatures (560-620). The optimal results were found at 580°C and 30 mins holding time. The tensile strength was 400MPa .It can be concluded that to get high tensile strength and good ductility we have to fabricate composites with high relative density which can achieved by hot vacuum degassed method before consolidated and vacuum or argon atmosphere with consolidating as shown in below Fig (1-4).
Fig:1 Material Preparation
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3. Morphology The mechanical milling process allows the incorporation of B4C reinforced phases and the aluminum matrix into the each of the powdered particle[14]. Therefore this process is very useful in synthesizing Al-B4C MMC. During the first stage the morphology and microstructure of the Al-B4C MMC is at original level. The B4C reinforced phase and the aluminum matrix is fractured and deformed with further milling. Further, the aluminum particles are cold welded and plastic deformation is seen[27,28].Fig. 2 shows the SEM micrographs of an Al-B4C powders mixture.B4C particles are also fractured into smaller size and kept decreasing till the stress caused due to the collision between the particles is equal or smaller to the fracture strength[14].
Fig 2.SEM of Al matrix mixedwith B4C particulates milled for different duration: (a) 0 h, (b) 4 h, (c) 8 h and (d) 16 h.[27]
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4. Strengthening Mechanism Different theories are available for predicting the strengthening behaviour of Al- B4C metal matrix composites. There are three stages in which we can divide the general failure process: yield stage, fracture and elastic stage[29]. However several strengthening mechanisms can be related to the increase of yield strength in B4C particles[4,29– 32]: (1) Load transfer from the matrix to the hard reinforcements in Al-B4C MMC; (2) The co-efficient of thermal expansion mismatch between Al matrix and the B4C particles;(3)Orowan Strengthening mechanisms in B4C reinforced Al-MMC;(4)Hall-Petch strengthening mechanisms in B4C reinforced Al-MMC;(5) Elastic modulus mismatch between Al matrix and B4C particles. Each strengthening method are discussed in the following sections. Orowan Strengthening 5. It is the effect of B4C particles on the strength of the metal matrix composites and its consists in interaction of B4C particles with dislocations. In the B4C nano particles the spacing between them is very less ,therefore it causes resistance to the passing of dislocations[10]. This type of strengthening mechanism is not applicable in the micro
Fig 3.Representation of various preparation methods of nano composites with different structures[18]
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sized B4C particulates due to large inter-particle spacing. The general expression for orowan strengthening is given below[30]. −
0.4Gb ln(d b) =M π (1 − υ )1 2 λ
σ orowan
2 d 3
−
d=
(2)
4 f −1 E G= 2(1 + υ ) −
λ = d
(1)
π
(3)
(4)
M is mean orientation factor, G is shear modulus of alluvium matrix, b is burgers vector, E is elastic modulus of aluminum matrix, υ is poisons ratio, λ is inter-particle spacing, and d is the diameter of B4C particle Load transfer effect This type of effect is applied when there is external application of load, such as hot rolling process, hot pressed process and other thermos mechanical processes. The load transfer takes place from Al matrix to the B4C reinforcements which contribute to the strengthening of Al matrix. In this process the yield stress is also improved which is strongly dependent on the volume fraction of boron carbide particulates. The general expression for load bearing contribution of B4C is given as[4,30]:
(l + t ) A (5) Δσ LT = v pσ m 4l v p is the volume fraction of B4C particulates, σ m is yield strength of aluminum matrix, l and t is the size of B4C particulate parallel and perpendicular to the load direction respectively, A = l / t is the B4C particulate aspect ratio,
For equiaxed B4C particle the expression is given below:
Δσ LT =
1 v pσ m 2
(6)
Hall petch strengthening This strengthening mechanism is based on the influence of grain size on the strength of MMC. In the Al matrix, the influence of grain size has a strong effect because the grain boundaries can obstruct the dislocation movement. The general expression for hall petch strengthening is given as[4,30]:
σ y = σ o + k y d −1 2
σ o is the original strength of B4C –Al MMC, k y is constant, and d
(7) is grain size of component
CTE and EM mismatch Co-efficient of thermal expansion (CTE) and the elastic modulus (EM) mismatch is related to the dislocation density and strength of the B4C reinforced Al-MMC. The difference of CTE and EM mismatch and the work hardening during hot rolling and hot pressing[30], causes the dislocations in B4C reinforced Al-MMC. Co-efficient
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of thermal expansion (CTE) and the elastic modulus (EM) mismatch is related to the dislocation density and strength of the B4C reinforced Al-MMC. The difference of CTE and EM mismatch and the work hardening during hot rolling and hot pressing[30], causes the dislocations in B4C reinforced Al-MMC. As reported by Jiang et al.[1] the strengthening mechanism was mainly subjected to orowan strengthening, load bearing and also dislocation strengthening. In Table(1) it is summarized that the structure, interfaces and the morphology of B4C Al-MMC with respect to the mechanical behaviour of the composite. When the particulate size of B4C is at micron level (>1000nm) the intergranular phases , and the strengthening mechanism was low dislocation strengthening by B4C reinforcement and high load bearing. When gradually B4C reinforcement size decreased to sub-micron level (1001000nm) it was observed that the strengthening mechanism was medium dislocation and medium load bearing in B4C particulate. Further when the reinforcement size decreased to nano level (<100nm) the strengthening mechanism observed was orowan strengthening for load bearing. The strengthening can also be contributed by grain refinement, dislocation strengthening and load bearing as stated by Jiang et al.(18). The calculation was done and they found out that the yield strength values of the theoretical results were matching with the experimental values. The CTE mismatch of B4C reinforced Al-MMC was proposed by Chen et.al (29) which stated that strength contribution can be attributed to CTE mismatch between B4C particles and 6061 Al. The existence of B4C particles and the orowan strengthening in the B4C reinforced Al-MMC will transfer the load from the aluminium matrix to the B4C particles. From the calculation it was clear that CTE mismatch values for B4C reinforced Al-MMC was 24.1 MPa , 34.23 MPa, 41.93 MPa and 48.41 MPa for 10%, 20%, 30% and 40% B4C reinforcement respectively. It was also stated that the yield stress of boron carbide reinforced aluminium metal matrix composites depends on volume fraction of boron carbide particulates. The load transfer values were calculated as 5.15 MPa, 10.30 MPa, 15.45 MPa, and 20.60 MPa for 10%, 20%, 30% and 40% B4C reinforcement respectively. The orowan strengthening values were calculated as 1.66 MPa, 2.55 MPa, 4.54 MPa and 10.21 MPa for 10%, 20%,30%, and 40% of B4C reinforcement respectively . 6. Interfacial Characteristics Interfacial Structure in B4C reinforced Al-MMC is influenced by length scale of B4C particulates[1]. Approximately 40% of B4C nano particles are located intragranularly where as B4C submicron particles are located at the Al grain boundaries, which created intergranular interfaces. These interfaces are incoherent where as intragranularnanoB4CAl interfaces are semi coherent. Fig 3.shows the interface orientation of nano B4C – Al intragranular interfaces. (1 1 1)Al//(0 2 4)B4C, 3.4° angle between (0 0 2)Al and (0 0 3)B4C, and 7.8° angle between (2 −2 0)Al and (0 2 1)B4C . To decrease stress location there is a presence of orientational relationship between aluminum matrix and the intragranularnano boron carbide particulates. Li et al.[26]found that during hot pressing of B4C/6061Al composites, the main reaction products were Al3BC and MgAL2O4 phases. The former increased with increase in temperature but interfacial reaction is still unclear as shown in below Fig 4(a-d).
Table 1.Strength and Ductitlity contribution on various B4C /Al interfaces[1] Interface
Strengthening contribution
Scale
Load bearing
Orowan strengthening
Dislocation strengthening
High
Unlikely
Low
Low
Low
Low
Medium
Unlikely
Medium
Low
Low
Medium
Low Low
Unlikely High
High High
Low High
Medium High
Medium High
Micron (>1000 nm) Submicron (100∼1000 nm) Nanoscale (<100 nm)
Ductility contribution Dislocation mobility Suppress near necking interface
Minimize stress localization
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Fig 4. 3D atomic models(a)B4C cells with cubic Al cells with various orientation;(b) B4C/Al with orientation of (1 1 1)Al//(0 2 4)B4C;(c)2D model of B4C/Al;(d)&(e) HRTEM images of B4C/Al[1]
Summary The strengthening mechanism, morphology evolution and the processing of B4C reinforced Al-MMC was reviewed. The effects of reinforcement size on the morphology and microstructure with respect to mechanical behaviour of Al matrix was reviewed. The review provided an overview on various strengthening mechanisms which can be applied to strengthen B4C reinforced Al –MMC.
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