- Email: [email protected]
nano–Al2O3 composite
Composites Science and Technology 69 (2009) 1070–1076
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Hot workability and deformation mechanisms in Mg/nano–Al2O3 composite Y.V.R.K. Prasad a, K.P. Rao a,*, M. Gupta b a b
Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 January 2009 Accepted 20 January 2009 Available online 31 January 2009 Keywords: A. Metal–matrix composites B. Plastic deformation C. Modelling D. Optical microscopy E. Extrusion
a b s t r a c t The response of extruded Mg/nano–Al2O3 (1 vol.%) composite to hot working in the temperature range 300–500 °C and strain rate range 0.0003–10 s1 has been characterized using processing map and kinetic analysis. The hot working window for the composite occurs at strain rates >0.1 s1 and the optimum range of temperature is 400–450 °C. In this window, the behavior of the composite is similar to that of the matrix and is controlled by the grain boundary self-diffusion. At lower strain rates, however, the composite exhibits much higher apparent activation energy than that for lattice self-diffusion unlike the matrix material. The deformed microstructures revealed that the prior particle boundaries decorated by the nano-Al2O3 particles, are stable and do not slide, rotate or migrate but kink after compressive deformation and as such contribute to the high temperature strength of the composite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium is the lightest of all the structural metals and its density is about two-thirds of that of aluminum. In view of this, magnesium materials are being actively considered for structural applications in automotive, aerospace, bicycle frames, computer hardware and portable electronic equipment [1]. Both cast and wrought magnesium alloys are being developed for several applications [2,3] but they have several limitations like poor corrosion resistance, creep strength and workability. One of the methods of enhancing the creep strength is through dispersion strengthening by hard ceramic particles. In recent years, metal matrix composites (MMCs) with discontinuous or continuous fibers or dispersoids are being developed for advanced applications [4,5]. In Mg MMCs, the matrix material is generally magnesium, Mg–Zn, Mg–Al–Si, or Mg– Zn–Zr. The dispersoids include SiC particles [6–9], SiC whiskers [10], Aluminum Borate whiskers [11], TiB2 [12], TiB2 + TiC [13] quasi-crystals [14], Ti–6Al–4 V particles [15] and nano-alumina, nano-yittria or nano-silica particles [16–21]. The mechanical properties of the composite are much better if the dispersoid particles are of nano-size since both the strength as well as the ductility may be improved simultaneously. The enhancement of mechanical properties in the novel nano particle reinforced MMCs has been reviewed recently [22]. Several techniques have been employed to prepare these composites and these include melt techniques, squeeze casting, and powder metallurgy. However, powder metallurgy appears to be the preferred process in view of its ability to * Corresponding author. Tel.: +852 2788 8409; fax: +852 2788 8423. E-mail address: [email protected] (K.P. Rao). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.01.032
give more uniform dispersions while hot extrusion is used for fabrication to take advantage of the compressive forces and the softening of the matrix at higher temperatures in enhancing the workability. As regards the mechanical properties, the reinforcements in general result in higher strength and hardness, often at the expense of some ductility. Out of all the dispersoids, two specific ones resulted in attractive mechanical properties: (1) the tensile strength and hardness of the aluminum borate whisker reinforce composite is doubled [12] although with some loss of ductility, and (2) both the strength and ductility of the alumina nano-composite improved by about 50%, which will result in a higher fracture toughness [17]. It may be noted that the mechanical properties are sensitive to the processing technique used to fabricate the material and considerable improvements may be achieved by applying science-based modeling techniques (described below) to optimize the processing procedure. The aim of the present investigation to evaluate the hot working behavior of Mg–1 vol.% nano-Al2O3 composite with a view to find optimum processing parameters and the high temperature deformation mechanisms. In order to bring out the effect of nano dispersion, the behavior of the composite is compared with that of the matrix material (extruded magnesium) under similar conditions. The hot working behavior has been characterized using processing maps as well as the standard kinetic approach. The technique of processing maps is based on the dynamic materials model, the principles of which are described earlier [23–25]. Briefly, the work-piece undergoing hot deformation is considered to be a dissipator of power and the strain rate sensitivity (m) of flow stress is the factor that partitions power between deformation heat and microstructural changes. The efficiency of power dissipation
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
occurring through microstructural changes during deformation (g) is derived by comparing the non-linear power dissipation occurring instantaneously in the work-piece with that of a linear dissipater for which the m value is unity, and is given by:
g ¼ 2m=ðm þ 1Þ
ð1Þ
The variation of efficiency of power dissipation with temperature and strain rate represents a power dissipation map which is generally viewed as an iso-efficiency contour map. Further, the extremum principles of irreversible thermodynamics as applied to continuum mechanics of large plastic flow [26] are explored to define a criterion for the onset of flow instability given by the equation for the instability parameter nðe_ Þ:
nðe_ Þ ¼
@ ln ½m=ðm þ 1Þ þm60 @ ln e_
ð2Þ
The variation of the instability parameter as a function of temperature and strain rate represents an instability map which delineates regimes of instability where n is negative. A superimposition of the instability map on the power dissipation map gives a processing map which reveals domains (efficiency contours converging towards a peak efficiency) where individual microstructural processes dominate and the limiting conditions for the regimes (bounded by a contour for n = 0) of flow instability. Processing maps help in identifying temperature–strain rate windows for hot working where the intrinsic workability of the material is maximum (e.g. dynamic recrystallization (DRX) or superplasticity) and also in avoiding the regimes of flow instabilities (e.g. adiabatic shear bands or flow localization). The processing map technique has been used earlier to study the hot deformation mechanisms in MMCs [23,27–30]. The standard kinetic rate equation relating the flow stress (r) to strain rate ðe_ Þ and temperature (T) is given by [31]:
e_ ¼ Arn exp
Q RT
1071
with a 0.8 mm diameter hole machined at mid height to reach the centre of the specimen. Isothermal uniaxial compression tests were conducted at constant true strain rates in the range 0.0003–10 s1 and temperature range 300–500 °C. Details of the test set-up and procedure are described in earlier publication [33]. Constant true strain rates during the tests were achieved using an exponential decay of actuator speed in the servo hydraulic machine. Graphite powder mixed with grease was used as the lubricant in all the experiments. The specimens were deformed up to a true strain of 0.7 and then quenched in water. The load–stroke data were converted into true stress–true strain curves using standard equations. The flow stress values were corrected for the adiabatic temperature rise. Processing maps were developed using the procedures described earlier [23] and with the flow stress data at different temperatures, strain rates and strains obtained from the above experiments as in-puts. The deformed specimens were sectioned in the center parallel to the compression axis and the cut surface was mounted, polished and etched for metallographic examination. All the specimens were etched with an aqueous solution containing 10% picric acid. 3. Results and discussion 3.1. Initial microstructure The starting microstructure of the extruded Mg–1% nano-Al2O3 composite is shown in Fig. 1a and b at lower and higher magnifica-
ð3Þ
where A = constant, n = stress exponent, Q = activation energy, and R = gas constant. The rate-controlling mechanisms are identified on the basis of the activation parameters n and Q.
2. Experimental Magnesium powder of 98.5% purity with a size range of 60– 300 lm (Merck, Germany) was used as the matrix material and alumina powder with a particulate size of 50 nm (Baikowski, USA) was used as the ceramic reinforcement phase. Magnesium nano-composite containing 1 vol.% of alumina powder was synthesized using powder metallurgy technique. The process involved blending pure magnesium powder and nano-sized Al2O3 powder in a RFTSCH PM-400 mechanical alloying machine at 200 rpm for 1 h. No balls or process control agent was used during the blending step. The blended powder mixture was cold compacted at a pressure of 97 bars (50 tons) to form billets of 35 mm diameter and 40 mm height using a 100 ton press. The compacted billets were sintered using an innovative hybrid microwave sintering technique described earlier [32]. Briefly, the billets were heated for 13 min to a temperature near the melting point of magnesium (640 °C) in a 900 W, 2.45 GHz SHARP microwave oven (multimode cavity). Sintered billets were soaked for 1 h at a temperature of 400 °C before extrusion and hot extruded at 350 °C using an extrusion ratio of 12.25:1 to produce rods of 10 mm diameter. Cylindrical specimens of 9.8 mm diameter and 15 mm height were machined from the extruded rods for compression testing along the direction of extrusion. For inserting a thermocouple to measure the specimen temperature as well as the adiabatic temperature rise during deformation, the specimens were provided
Fig. 1. Starting microstructure of extruded Mg/nano–Al2O3 composite at (a) low and (b) high magnification. The extrusion direction is vertical.
1072
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
tions. In these micrographs, the extrusion direction is vertical. The matrix magnesium particles are elongated in the extrusion direction and the prior particle boundaries (PPBs) are decorated by the nano-Al2O3 particles. The nano particles are not present within the matrix but distributed along the PPBs although there is agglomeration at some places (Fig. 1b).
90 Temperature
80
Mg/nano-Al2O3
o
Mg - Extruded
o
Mg/nano-Al2O3
o
Mg - Extruded
300 C 450 C
70
Material
o
300 C
450 C
3.2. Stress–strain behavior
3.3. Processing maps
60 Flow stress, MPa
The true stress–true strain curves obtained at 300 °C and 450 °C and at different strain rates are shown in Fig. 2a and b, respectively. For strain rates of 1 s1 and 10 s1, the flow stress reached a peak in the initial stages of deformation followed by a steady state at larger strains, the strain for the peak being lower at lower strain rates and higher temperatures. The variation of the flow stress at a strain of 0.5 with log (e_ ) at 300 °C and 450 °C, is plotted in Fig. 3 and compared with that in extruded Mg which is the matrix material. The composite exhibited higher flow stress than the matrix and the difference is higher at lower strain rates. This suggests that the nano dispersions are helpful in enhancing the high temperature strength properties, particularly at lower strain rates.
50
40
30
20
10
The processing map obtained at a strain of 0.5 (near steady state flow conditions) for the Mg/1 vol.% nano-Al2O3composite is shown
0 -4
-3 3
-2
-1
0
1
Log (Strain rate, s-1)
a 120
Fig. 3. Variation of flow stress at a true strain of 0.5 with logarithm of strain rate at 300 °C and 450 °C for Mg/nano–Al2O3 composite and matrix material (extruded magnesium).
MGNAL : 300 oC
True stress, MPa
100 80 Strain rate, s
in Fig. 4. The map exhibits a single domain in the temperature range 300–500 °C and strain rate range 0.1–10 s1 with a peak efficiency of about 34% occurring at about 420 °C and 10 s1, which are the optimum conditions for hot working this composite. It is significant to note that the efficiency of power dissipation is low at lower temperatures (300–400 °C) and strain rates (0.0003– 0.1 s1), which suggests that workability is not good under these conditions. As per Eq. (2), an instability regime occurs in the temperature range 400–500 °C and at strain rates below 0.01 s1,
-1
10
60
1
0.1
40
0.01 0.001 0.0003
20 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
True strain
b
60 MGNAL : 450 oC
True stress, MPa
50 Strain rate, s-1
40
10
30 1 0.1
20
0.01 0.001 0.0003
10 0 0.0
0.1
0.2
0.3
0.4 0.5 True strain
0.6
0.7
0.8
Fig. 2. (a) Stress–strain curves obtained in compression of Mg/nano–Al2O3 composite (MGNAL) at different strain rates and at a temperature of 300 °C. (b) Stress– strain curves obtained in compression of Mg/nano–Al2O3 composite (MGNAL) at different strain rates and at a temperature of 450 °C.
Fig. 4. Processing map at a strain of 0.5 for Mg/nano–Al2O3 composite (MGNAL). The numbers against the contours represent efficiency of power dissipation in percent. The dark line delineates the flow instability.
1073
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
3.4. Kinetics of hot deformation
Temperature
2.0
Mg/nano-Al2O3
o
Mg - Extruded
o
Mg/nano-Al2O3
o
Mg - Extruded
450 C
1.8
Material
o
350 C 350 C
450 C
m = 0.16 n = 6.25
1.6 m = 0.065 n = 15.4
1.4
1.2 m = 0.2 n = 5.0
1.0
0.8
0.6
0.4 -4.0
-3.0
-2.0
-1.0
0.0
1.0
-1
Log (Strain rate, s ) Fig. 6. Variation of flow stress at a strain of 0.5 with strain rate on a log–log scale for Mg/nano–Al2O3 composite and the matrix material (extruded magnesium).
-2.2
-2.4 n = 6.25 Q = 123 kJ/mole
-2.6
Log (Flow stress/Mu)
The kinetic rate equation (Eq. (3)) is generally obeyed within the domain and may be applied to evaluate the apparent activation energy. In Fig. 6, the variation of log (Flow stress) with log (Strain rate) has been plotted for the composite as well the matrix material. In the case of the composite, the variation was non-linear over the wide range of strain rate studied with a deviation occurring in the vicinity of about 0.1 s1. At higher strain rates (> about 0.1 s1), the stress exponent is about 6.25 while it is much higher at lower strain rates (n = 15.4). The higher strain rate–lower temperature behavior of the matrix material is strikingly similar (n = 6.06) to that of the composite, while at higher temperatures and lower strain rates, the stress exponent is about 5.0. The Arrhenius plot showing the variation of flow stress normalized with the shear modulus (l) with inverse of absolute temperature is shown in Fig. 7. In the higher strain rate regime of the composite, apparent activation energy of about 123 kJ/mol has been estimated and the value is strikingly similar for the matrix material. This apparent activation energy value is higher than grain boundary self-diffusion (92 kJ/mol) but less than that for lattice self-diffusion (135 kJ/mol) [35]. It may be noted that Mg matrix would have developed strong crystallographic texture during extrusion, which 0 > is generally a fiber texture with the basal planes or < 1 0 1 aligned with the extrusion direction [7]. Such a texture will not favor basal or prismatic slip but will facilitate pyramidal slip systems
2.2
Log (Flow stress, MPa)
which is delineated by a dark line in Fig. 4. Thus, the lower strain rate regime of the map is not favorable for hot working of this composite. The processing map obtained on the basis of the data published earlier [34] on extruded Mg, which is the matrix material, is shown in Fig. 5. The map exhibits three domains in the temperature and strain rate ranges given below: (1) 400–500 °C and 0.001–1.0 s1 with a peak efficiency of 36% occurring at 450 °C, (2) 320–440 °C and 1–10 s1, and (3) 470–500 °C and 1.0–10 s1 with a peak efficiency of 52% occurring at 500 °C and 10 s1. The third domain is interpreted to represent intercrystalline cracking, while the other two represent dynamic recrystallization. In the processing map for the composite (Fig. 4), domains (1) and (3) of the matrix material are absent and the temperature and strain rate range for domain (2) became significantly wider.
-2.8
n = 5.0 Q = 136 kJ/mole
-3.0
-3.2 Strain rate
-3.4
-3.6 1.25
Fig. 5. Processing map at a strain of 0.5 for extruded magnesium (matrix material). The numbers against the contours represent efficiency of power dissipation in percent. The dark line delineates the flow instability.
1.35
1.45
Material
10 s
-1
Mg/nano-Al2O3
10 s
-1
Mg - Extruded
0.1 s
-1
Mg/nano-Al2O3
0.1 s
-1
Mg - Extruded
1.55 1000 / T
1.65
1.75
Fig. 7. Arrhenius plot showing the variation of flow stress at a strain of 0.5 normalized with shear modulus for magnesium as a function of inverse of absolute temperature for Mg/nano–Al2O3 composite and the matrix material (extruded magnesium).
1074
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
which occur extensively at higher temperatures (>300 °C). Since these are ‘‘harder” slip systems, they give rise to higher back stress which would increase the apparent activation energy. In view of this, it is likely that the hot deformation in the higher strain rate regime is controlled by grain boundary self-diffusion. Such a mechanism is also reported in magnesium alloys deformed in similar strain rate regime [36–39]. The composite exhibited very high apparent activation energy (about 305 kJ/mol) at lower strain rates in comparison with that in the matrix (136 kJ/mol) which itself is close to that for lattice self-diffusion in magnesium [35]. This suggests that lattice self-diffusion in the matrix is considerably reduced in the nano-composite, which may be responsible for the increased strength particularly at lower strain rates since self-diffusion is one of the major factors for increasing high temperature creep rate or decreasing the creep strength. 3.5. Microstructures The microstructures recorded on the composite specimens deformed at different temperatures and strain rates are shown in Fig. 8. These reveal that the aspect ratio of the matrix grains, which was high in the initial extruded condition, is reduced due to compression parallel to the extrusion direction. The PPBs, which are decorated by the nano-alumina particles, are kinked due to the compressive deformation and do not slide, rotate or migrate. This suggests that the energy of PPBs is reduced by the presence of the nano-alumina particles and are therefore stable. Similar observations have been made in Mg–nano-SiC composite [20]. With a
view to examine the stability of the deformed microstructure, the specimen deformed at 450 °C and 10 s1 was annealed at 450 °C for 5 h and the microstructures of the specimen before and after annealing are shown in Fig. 9a and b, respectively. As a result of annealing, the PPBs have straightened and nearly resumed their alignment with the extrusion direction and the matrix exhibits recrystallization. 3.6. Mechanism of hot deformation The Mg/nano–Al2O3 composite exhibits a single window for hot working at strain rates higher than about 0.1 s1 and the optimum temperature is in the range 400–450 °C. This workability window is favorable for all the bulk metal working processes, particularly for hot extrusion which is a chosen process for magnesium materials. The behavior of the composite in this window is similar to that of the matrix material (extruded magnesium) and exhibits dynamic recrystallization during deformation with grain boundary self-diffusion as the rate-controlling mechanism. Due to the presence of nano-alumina particles at the PPBs, their energy is lowered and become very stable. When dynamic recrystallization occurs during hot deformation, the PPBs do not break-down and get reconstituted unlike other powder compacts [40]. They add to the high temperature strength of the composite since they do not slide, rotate or migration to cause softening. Depending on the direction of deformation, the PPBs may kink and store some elastic energy, which may be released by annealing. At lower strain rates, the apparent activation energy is a considerably higher than that for lattice self-diffusion, which is the rate-
Fig. 8. Microstructures of Mg/nano–Al2O3 composite deformed at different temperatures and strain rates. The compression direction is vertical.
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
1075
higher (305 kJ/mol) than that for hot deformation of the matrix material (135 kJ/mol) which itself is close to that for lattice self-diffusion. iv. The prior particle boundaries in the composite are stable during hot deformation and do not slide, rotate or migrate but only exhibit kinking during compression parallel to the original extrusion direction.
Acknowledgement The work presented in this paper has been fully supported by a Strategic Research Grant from City University of Hong Kong (Project Ref. No. 7002335). References
Fig. 9. Microstructures of Mg/nano–Al2O3 composite (a) deformed at 450 °C/10 s1 (b) deformed at 450 °C/10 s1 and annealed at 450 °C for 5 h.
controlling mechanism in the matrix material (extruded magnesium). This indicates that lattice self-diffusion is restricted by the presence of nano-alumina particles at the PPBs. One possibility is that since the energy of the PPBs is reduced substantially, they do not act as sources or sinks for vacancies which are essential for self-diffusion. As a result, the rate of thermal recovery is reduced and so nucleation of major softening processes like dynamic recrystallization does not take place during deformation. This major difference at lower strain rates in the behavior of the nanocomposite is actually beneficial in enhancing the creep strength of the composite material although not directly relevant to bulk metal working. 4. Conclusions The following conclusions are drawn from this investigation: i. The composite exhibits a workability window at strain rates >0.1 s1 with the optimum temperature in the range 400– 450 °C. ii. The behavior of the composite is similar to that of the matrix material in the workability window in which the apparent activation energy (123 kJ/mol) suggests grain boundary self-diffusion as the rate-controlling mechanism. iii. At lower strain rates, lattice self-diffusion is restricted by the presence of nano-alumina particles at the prior particle boundaries since the apparent activation energy is much
[1] Beals RS, Tissington C, Zhang X, Kainer K, Petrillo J, Verbrugge M. Magnesium global development: outcomes from the TMS 2007 annual meeting. J Met 2007;59(8):39–42. [2] Kainer KU, Dieringa H, Dietzel W, Hort N, Blawert C. The use of magnesium alloys: past, present and future. In: Pekguleryuz MO, Mackaenzie LWF, editors. Magnesium technology in the global age. Montreal: Canadian Institute of Mining, Metallurgy and Petroleum; 2006. p. 3–19. [3] Bettles CJ, Gibson MA. Current wrought magnesium alloys: strengths and weaknesses. J Met 2005;57:46–9. [4] Chawla N, Chawla KK. Metal matrix composites. New York: Springer; 2006. [5] Miracle DB. Metal matrix composites – from science to technological significance. Compos Sci Technol 2005;65:2526–40. [6] Trojanova Z, Szaraz Z, Labar J, Lukac P. Deformation behavior of an AS21 alloy reinforced by short saffil fibres and SiC particles. J Mater Proc Technol 2005;162–163:131–8. [7] Garces G, Perez P, Adeva P. Effect of extrusion texture on the mechanical behaviour of Mg–SiCp composites. Scripta Mater 2005;52:615–9. [8] El-Saeid Essa Y, Perez-Castellanos JL. Effect of the strain rate and temperature on the mechanical behavior of a Mg–5%Zn alloy reinforced with SiC particles. J Mater Proc Technol 2003;143–144:856–9. [9] Mukai T, Nieh TG, Iwasaki H, Higashi K. Superplasticity in doubly extruded magnesium composite ZK60/SiC/17p. Mater Sci Technol 1998;14:32–5. [10] Yan F, Wu K, Wu GL, Lee GL, Zhao M. Superplastic deformation behavior of a 19.7 vol.% b-SiCw/ZK60 composite. Mater Lett 2003;57:1992–6. [11] Zheng M, Wu K, Liang H, Kamado S, Kojima Y. Microstructure and mechanical properties of aluminum borate whisker-reinforced magnesium matrix composites. Mater Lett 2002;57:558–64. [12] Wang HY, Jiang QC, Wang Y, Ma BX, Zhao F. Fabrication of TiB2 particulate reinforced magnesium matrix composites by powder metallurgy. Mater Lett 2004;58:3509–13. [13] Xiuqing Z, Haowei W, Lihua L, Xinying T, Naiheng M. The mechanical properties of magnesium matrix composites reinforced with (TiB2 + TiC) ceramic particulates. Mater Lett 2005;59:2105–9. [14] Singh A, Watanabe M, Kato A, Tsai AP. Microstructure and strength of quasicrystal containing extruded Mg–Zn–Y alloys for elevated temperature application. Mater Sci Eng A 2004;385:382–96. [15] Xi YL, Chai DL, Zhang WX, Zhou JE. Ti–6Al–4V particle reinforced magnesium matrix composite by powder metallurgy. Mater Lett 2005;59:1831–5. [16] Hassan SF, Tan JJ, Gupta M. High-temperature tensile properties of Mg/Al2O3 nanocomposite. Mater Sci Eng A 2008;486:56–62. [17] Hassan SF, Gupta M. Development of high performance magnesium nanocomposites using nano-Al2O3 as reinforcement. Mater Sci Eng A 2005;392:163–8. [18] Wong WLE, Gupta M. Simultaneously improving strength and ductility of magnesium using nano-size SiC particulates and microwaves. Adv Eng Mater 2006;8:735–40. [19] Hassan SF, Gupta M. Development of nano-Y2O3 containing magnesium nanocomposites using solidification processing. J Alloy Compd 2007;429:176–83. [20] Ferkel H, Mordike BL. Magnesium strengthened by SiC nanoparticles. Mater Sci Eng A 2001;298:193–9. [21] Lu L, Lai MO, Liang W. Magnesium nanocomposite via mechanochemical milling. Compos Sci Technol 2004;64:2009–14. [22] Tjong SC. Novel nanoparticle-reinforced metal matrix composites with enhanced mechanical properties. Adv Eng Mater 2007;9:639–52. [23] Prasad YVRK, Sasidhara S, editors. Hot working guide: a compendium of processing maps. Materials Park (OH): ASM International; 1997. [24] Prasad YVRK, Seshacharyulu T. Modeling of hot deformation for microstructural control. Inter Mater Rev 1998;43:243–58. [25] Prasad YVRK. Processing maps – a status report. J Mater Eng Perform 2003;12:638–45. [26] Ziegler H. Some extremum principles in irreversible thermodynamics with applications to continuum mechanics. In: Sneddon IN, Hill R, editors. Progress in solid mechanics, vol. 4. New York: John Wiley; 1965. p. 91–193.
1076
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
[27] Cavaliere P, Cerri E, Leo P. Hot deformation and processing maps of a particulate reinforced 2618/Al2O3/20p metal matrix composite. Compos Sci Technol 2004;64:1287–91. [28] Ramanathan S, Karthikeyan R, Ganesan G. Development of processing maps for 2124 Al/SiCp composites. Mater Sci Eng A 2006;441:321–5. [29] Cerri E, Spigerelli S, Evangelista E, Cavaliere P. Hot deformation and processing maps of a particle reinforced 6061 + 20%Al2O3 composite. Mater Sci Eng A 2002;324:157–61. [30] Raghunath BK, Karthikeyan R, Ganesan G, Gupta M. An investigation of hot deformation response of particulate-reinforced magnesium + 9% titanium composite. Mater Des 2008;29:622–7. [31] Jonas JJ, Sellars CM, McG.Tegart WJ. Strength and structure under hot working conditions. Metall Rev 1969;14:1–24. [32] Gupta M, Wong WLE. Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering. Scripta Mater 2005;52:479–83. [33] Prasad YVRK, Rao KP. Mechanisms of high temperature deformation in electrolytic copper in extended ranges of temperature and strain rate. Mater Sci Eng A 2004;374:335–41.
[34] Sivakesavam O. Processing map for extruded magnesium. In: Prasad YVRK, Sasidhara S, editors. Hot working guide: a compendium of processing maps. Materials Park, OH: ASM International; 1997. p. 347–50. [35] Frost HJ, Ashby MF. Deformation-mechanism maps. Oxford: Pergamon; 1982. p. 44. [36] Prasad YVRK, Rao KP. Effect of crystallographic texture on the kinetics of hot deformation of rolled Mg–3Al–1Zn alloy plate. Mater Sci Eng A 2006;432:170–7. [37] Prasad YVRK, Rao KP. Hot deformation mechanisms and microstructural control in high temperature extruded AZ31 magnesium alloy. Adv Eng Mater 2007;9:558–65. [38] Rao KP, Prasad YVRK, Hort N, Kainer KU. Hot workability characteristics of cast and homogenized Mg–3Sn–1Ca alloy. J Mater Process Technol 2008;201:359–63. [39] Prasad YVRK, Rao KP. Processing maps for hot deformation of rolled AZ31 magnesium alloy plate: anisotropy of hot workability. Mater Sci Eng A 2008;487:316–27. [40] Gegel HL, Malas JC, Doraivelu SM, Barker DR, Morgan JT, Lark KA et al. Billet conditioning technique for manufacturing powder metallurgy performs. US Patent# 4, 762, 679; 1988.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Hot workability and deformation mechanisms in Mg/nano–Al2O3 composite Y.V.R.K. Prasad a, K.P. Rao a,*, M. Gupta b a b
Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 January 2009 Accepted 20 January 2009 Available online 31 January 2009 Keywords: A. Metal–matrix composites B. Plastic deformation C. Modelling D. Optical microscopy E. Extrusion
a b s t r a c t The response of extruded Mg/nano–Al2O3 (1 vol.%) composite to hot working in the temperature range 300–500 °C and strain rate range 0.0003–10 s1 has been characterized using processing map and kinetic analysis. The hot working window for the composite occurs at strain rates >0.1 s1 and the optimum range of temperature is 400–450 °C. In this window, the behavior of the composite is similar to that of the matrix and is controlled by the grain boundary self-diffusion. At lower strain rates, however, the composite exhibits much higher apparent activation energy than that for lattice self-diffusion unlike the matrix material. The deformed microstructures revealed that the prior particle boundaries decorated by the nano-Al2O3 particles, are stable and do not slide, rotate or migrate but kink after compressive deformation and as such contribute to the high temperature strength of the composite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium is the lightest of all the structural metals and its density is about two-thirds of that of aluminum. In view of this, magnesium materials are being actively considered for structural applications in automotive, aerospace, bicycle frames, computer hardware and portable electronic equipment [1]. Both cast and wrought magnesium alloys are being developed for several applications [2,3] but they have several limitations like poor corrosion resistance, creep strength and workability. One of the methods of enhancing the creep strength is through dispersion strengthening by hard ceramic particles. In recent years, metal matrix composites (MMCs) with discontinuous or continuous fibers or dispersoids are being developed for advanced applications [4,5]. In Mg MMCs, the matrix material is generally magnesium, Mg–Zn, Mg–Al–Si, or Mg– Zn–Zr. The dispersoids include SiC particles [6–9], SiC whiskers [10], Aluminum Borate whiskers [11], TiB2 [12], TiB2 + TiC [13] quasi-crystals [14], Ti–6Al–4 V particles [15] and nano-alumina, nano-yittria or nano-silica particles [16–21]. The mechanical properties of the composite are much better if the dispersoid particles are of nano-size since both the strength as well as the ductility may be improved simultaneously. The enhancement of mechanical properties in the novel nano particle reinforced MMCs has been reviewed recently [22]. Several techniques have been employed to prepare these composites and these include melt techniques, squeeze casting, and powder metallurgy. However, powder metallurgy appears to be the preferred process in view of its ability to * Corresponding author. Tel.: +852 2788 8409; fax: +852 2788 8423. E-mail address: [email protected] (K.P. Rao). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.01.032
give more uniform dispersions while hot extrusion is used for fabrication to take advantage of the compressive forces and the softening of the matrix at higher temperatures in enhancing the workability. As regards the mechanical properties, the reinforcements in general result in higher strength and hardness, often at the expense of some ductility. Out of all the dispersoids, two specific ones resulted in attractive mechanical properties: (1) the tensile strength and hardness of the aluminum borate whisker reinforce composite is doubled [12] although with some loss of ductility, and (2) both the strength and ductility of the alumina nano-composite improved by about 50%, which will result in a higher fracture toughness [17]. It may be noted that the mechanical properties are sensitive to the processing technique used to fabricate the material and considerable improvements may be achieved by applying science-based modeling techniques (described below) to optimize the processing procedure. The aim of the present investigation to evaluate the hot working behavior of Mg–1 vol.% nano-Al2O3 composite with a view to find optimum processing parameters and the high temperature deformation mechanisms. In order to bring out the effect of nano dispersion, the behavior of the composite is compared with that of the matrix material (extruded magnesium) under similar conditions. The hot working behavior has been characterized using processing maps as well as the standard kinetic approach. The technique of processing maps is based on the dynamic materials model, the principles of which are described earlier [23–25]. Briefly, the work-piece undergoing hot deformation is considered to be a dissipator of power and the strain rate sensitivity (m) of flow stress is the factor that partitions power between deformation heat and microstructural changes. The efficiency of power dissipation
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
occurring through microstructural changes during deformation (g) is derived by comparing the non-linear power dissipation occurring instantaneously in the work-piece with that of a linear dissipater for which the m value is unity, and is given by:
g ¼ 2m=ðm þ 1Þ
ð1Þ
The variation of efficiency of power dissipation with temperature and strain rate represents a power dissipation map which is generally viewed as an iso-efficiency contour map. Further, the extremum principles of irreversible thermodynamics as applied to continuum mechanics of large plastic flow [26] are explored to define a criterion for the onset of flow instability given by the equation for the instability parameter nðe_ Þ:
nðe_ Þ ¼
@ ln ½m=ðm þ 1Þ þm60 @ ln e_
ð2Þ
The variation of the instability parameter as a function of temperature and strain rate represents an instability map which delineates regimes of instability where n is negative. A superimposition of the instability map on the power dissipation map gives a processing map which reveals domains (efficiency contours converging towards a peak efficiency) where individual microstructural processes dominate and the limiting conditions for the regimes (bounded by a contour for n = 0) of flow instability. Processing maps help in identifying temperature–strain rate windows for hot working where the intrinsic workability of the material is maximum (e.g. dynamic recrystallization (DRX) or superplasticity) and also in avoiding the regimes of flow instabilities (e.g. adiabatic shear bands or flow localization). The processing map technique has been used earlier to study the hot deformation mechanisms in MMCs [23,27–30]. The standard kinetic rate equation relating the flow stress (r) to strain rate ðe_ Þ and temperature (T) is given by [31]:
e_ ¼ Arn exp
Q RT
1071
with a 0.8 mm diameter hole machined at mid height to reach the centre of the specimen. Isothermal uniaxial compression tests were conducted at constant true strain rates in the range 0.0003–10 s1 and temperature range 300–500 °C. Details of the test set-up and procedure are described in earlier publication [33]. Constant true strain rates during the tests were achieved using an exponential decay of actuator speed in the servo hydraulic machine. Graphite powder mixed with grease was used as the lubricant in all the experiments. The specimens were deformed up to a true strain of 0.7 and then quenched in water. The load–stroke data were converted into true stress–true strain curves using standard equations. The flow stress values were corrected for the adiabatic temperature rise. Processing maps were developed using the procedures described earlier [23] and with the flow stress data at different temperatures, strain rates and strains obtained from the above experiments as in-puts. The deformed specimens were sectioned in the center parallel to the compression axis and the cut surface was mounted, polished and etched for metallographic examination. All the specimens were etched with an aqueous solution containing 10% picric acid. 3. Results and discussion 3.1. Initial microstructure The starting microstructure of the extruded Mg–1% nano-Al2O3 composite is shown in Fig. 1a and b at lower and higher magnifica-
ð3Þ
where A = constant, n = stress exponent, Q = activation energy, and R = gas constant. The rate-controlling mechanisms are identified on the basis of the activation parameters n and Q.
2. Experimental Magnesium powder of 98.5% purity with a size range of 60– 300 lm (Merck, Germany) was used as the matrix material and alumina powder with a particulate size of 50 nm (Baikowski, USA) was used as the ceramic reinforcement phase. Magnesium nano-composite containing 1 vol.% of alumina powder was synthesized using powder metallurgy technique. The process involved blending pure magnesium powder and nano-sized Al2O3 powder in a RFTSCH PM-400 mechanical alloying machine at 200 rpm for 1 h. No balls or process control agent was used during the blending step. The blended powder mixture was cold compacted at a pressure of 97 bars (50 tons) to form billets of 35 mm diameter and 40 mm height using a 100 ton press. The compacted billets were sintered using an innovative hybrid microwave sintering technique described earlier [32]. Briefly, the billets were heated for 13 min to a temperature near the melting point of magnesium (640 °C) in a 900 W, 2.45 GHz SHARP microwave oven (multimode cavity). Sintered billets were soaked for 1 h at a temperature of 400 °C before extrusion and hot extruded at 350 °C using an extrusion ratio of 12.25:1 to produce rods of 10 mm diameter. Cylindrical specimens of 9.8 mm diameter and 15 mm height were machined from the extruded rods for compression testing along the direction of extrusion. For inserting a thermocouple to measure the specimen temperature as well as the adiabatic temperature rise during deformation, the specimens were provided
Fig. 1. Starting microstructure of extruded Mg/nano–Al2O3 composite at (a) low and (b) high magnification. The extrusion direction is vertical.
1072
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
tions. In these micrographs, the extrusion direction is vertical. The matrix magnesium particles are elongated in the extrusion direction and the prior particle boundaries (PPBs) are decorated by the nano-Al2O3 particles. The nano particles are not present within the matrix but distributed along the PPBs although there is agglomeration at some places (Fig. 1b).
90 Temperature
80
Mg/nano-Al2O3
o
Mg - Extruded
o
Mg/nano-Al2O3
o
Mg - Extruded
300 C 450 C
70
Material
o
300 C
450 C
3.2. Stress–strain behavior
3.3. Processing maps
60 Flow stress, MPa
The true stress–true strain curves obtained at 300 °C and 450 °C and at different strain rates are shown in Fig. 2a and b, respectively. For strain rates of 1 s1 and 10 s1, the flow stress reached a peak in the initial stages of deformation followed by a steady state at larger strains, the strain for the peak being lower at lower strain rates and higher temperatures. The variation of the flow stress at a strain of 0.5 with log (e_ ) at 300 °C and 450 °C, is plotted in Fig. 3 and compared with that in extruded Mg which is the matrix material. The composite exhibited higher flow stress than the matrix and the difference is higher at lower strain rates. This suggests that the nano dispersions are helpful in enhancing the high temperature strength properties, particularly at lower strain rates.
50
40
30
20
10
The processing map obtained at a strain of 0.5 (near steady state flow conditions) for the Mg/1 vol.% nano-Al2O3composite is shown
0 -4
-3 3
-2
-1
0
1
Log (Strain rate, s-1)
a 120
Fig. 3. Variation of flow stress at a true strain of 0.5 with logarithm of strain rate at 300 °C and 450 °C for Mg/nano–Al2O3 composite and matrix material (extruded magnesium).
MGNAL : 300 oC
True stress, MPa
100 80 Strain rate, s
in Fig. 4. The map exhibits a single domain in the temperature range 300–500 °C and strain rate range 0.1–10 s1 with a peak efficiency of about 34% occurring at about 420 °C and 10 s1, which are the optimum conditions for hot working this composite. It is significant to note that the efficiency of power dissipation is low at lower temperatures (300–400 °C) and strain rates (0.0003– 0.1 s1), which suggests that workability is not good under these conditions. As per Eq. (2), an instability regime occurs in the temperature range 400–500 °C and at strain rates below 0.01 s1,
-1
10
60
1
0.1
40
0.01 0.001 0.0003
20 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
True strain
b
60 MGNAL : 450 oC
True stress, MPa
50 Strain rate, s-1
40
10
30 1 0.1
20
0.01 0.001 0.0003
10 0 0.0
0.1
0.2
0.3
0.4 0.5 True strain
0.6
0.7
0.8
Fig. 2. (a) Stress–strain curves obtained in compression of Mg/nano–Al2O3 composite (MGNAL) at different strain rates and at a temperature of 300 °C. (b) Stress– strain curves obtained in compression of Mg/nano–Al2O3 composite (MGNAL) at different strain rates and at a temperature of 450 °C.
Fig. 4. Processing map at a strain of 0.5 for Mg/nano–Al2O3 composite (MGNAL). The numbers against the contours represent efficiency of power dissipation in percent. The dark line delineates the flow instability.
1073
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
3.4. Kinetics of hot deformation
Temperature
2.0
Mg/nano-Al2O3
o
Mg - Extruded
o
Mg/nano-Al2O3
o
Mg - Extruded
450 C
1.8
Material
o
350 C 350 C
450 C
m = 0.16 n = 6.25
1.6 m = 0.065 n = 15.4
1.4
1.2 m = 0.2 n = 5.0
1.0
0.8
0.6
0.4 -4.0
-3.0
-2.0
-1.0
0.0
1.0
-1
Log (Strain rate, s ) Fig. 6. Variation of flow stress at a strain of 0.5 with strain rate on a log–log scale for Mg/nano–Al2O3 composite and the matrix material (extruded magnesium).
-2.2
-2.4 n = 6.25 Q = 123 kJ/mole
-2.6
Log (Flow stress/Mu)
The kinetic rate equation (Eq. (3)) is generally obeyed within the domain and may be applied to evaluate the apparent activation energy. In Fig. 6, the variation of log (Flow stress) with log (Strain rate) has been plotted for the composite as well the matrix material. In the case of the composite, the variation was non-linear over the wide range of strain rate studied with a deviation occurring in the vicinity of about 0.1 s1. At higher strain rates (> about 0.1 s1), the stress exponent is about 6.25 while it is much higher at lower strain rates (n = 15.4). The higher strain rate–lower temperature behavior of the matrix material is strikingly similar (n = 6.06) to that of the composite, while at higher temperatures and lower strain rates, the stress exponent is about 5.0. The Arrhenius plot showing the variation of flow stress normalized with the shear modulus (l) with inverse of absolute temperature is shown in Fig. 7. In the higher strain rate regime of the composite, apparent activation energy of about 123 kJ/mol has been estimated and the value is strikingly similar for the matrix material. This apparent activation energy value is higher than grain boundary self-diffusion (92 kJ/mol) but less than that for lattice self-diffusion (135 kJ/mol) [35]. It may be noted that Mg matrix would have developed strong crystallographic texture during extrusion, which 0 > is generally a fiber texture with the basal planes or < 1 0 1 aligned with the extrusion direction [7]. Such a texture will not favor basal or prismatic slip but will facilitate pyramidal slip systems
2.2
Log (Flow stress, MPa)
which is delineated by a dark line in Fig. 4. Thus, the lower strain rate regime of the map is not favorable for hot working of this composite. The processing map obtained on the basis of the data published earlier [34] on extruded Mg, which is the matrix material, is shown in Fig. 5. The map exhibits three domains in the temperature and strain rate ranges given below: (1) 400–500 °C and 0.001–1.0 s1 with a peak efficiency of 36% occurring at 450 °C, (2) 320–440 °C and 1–10 s1, and (3) 470–500 °C and 1.0–10 s1 with a peak efficiency of 52% occurring at 500 °C and 10 s1. The third domain is interpreted to represent intercrystalline cracking, while the other two represent dynamic recrystallization. In the processing map for the composite (Fig. 4), domains (1) and (3) of the matrix material are absent and the temperature and strain rate range for domain (2) became significantly wider.
-2.8
n = 5.0 Q = 136 kJ/mole
-3.0
-3.2 Strain rate
-3.4
-3.6 1.25
Fig. 5. Processing map at a strain of 0.5 for extruded magnesium (matrix material). The numbers against the contours represent efficiency of power dissipation in percent. The dark line delineates the flow instability.
1.35
1.45
Material
10 s
-1
Mg/nano-Al2O3
10 s
-1
Mg - Extruded
0.1 s
-1
Mg/nano-Al2O3
0.1 s
-1
Mg - Extruded
1.55 1000 / T
1.65
1.75
Fig. 7. Arrhenius plot showing the variation of flow stress at a strain of 0.5 normalized with shear modulus for magnesium as a function of inverse of absolute temperature for Mg/nano–Al2O3 composite and the matrix material (extruded magnesium).
1074
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
which occur extensively at higher temperatures (>300 °C). Since these are ‘‘harder” slip systems, they give rise to higher back stress which would increase the apparent activation energy. In view of this, it is likely that the hot deformation in the higher strain rate regime is controlled by grain boundary self-diffusion. Such a mechanism is also reported in magnesium alloys deformed in similar strain rate regime [36–39]. The composite exhibited very high apparent activation energy (about 305 kJ/mol) at lower strain rates in comparison with that in the matrix (136 kJ/mol) which itself is close to that for lattice self-diffusion in magnesium [35]. This suggests that lattice self-diffusion in the matrix is considerably reduced in the nano-composite, which may be responsible for the increased strength particularly at lower strain rates since self-diffusion is one of the major factors for increasing high temperature creep rate or decreasing the creep strength. 3.5. Microstructures The microstructures recorded on the composite specimens deformed at different temperatures and strain rates are shown in Fig. 8. These reveal that the aspect ratio of the matrix grains, which was high in the initial extruded condition, is reduced due to compression parallel to the extrusion direction. The PPBs, which are decorated by the nano-alumina particles, are kinked due to the compressive deformation and do not slide, rotate or migrate. This suggests that the energy of PPBs is reduced by the presence of the nano-alumina particles and are therefore stable. Similar observations have been made in Mg–nano-SiC composite [20]. With a
view to examine the stability of the deformed microstructure, the specimen deformed at 450 °C and 10 s1 was annealed at 450 °C for 5 h and the microstructures of the specimen before and after annealing are shown in Fig. 9a and b, respectively. As a result of annealing, the PPBs have straightened and nearly resumed their alignment with the extrusion direction and the matrix exhibits recrystallization. 3.6. Mechanism of hot deformation The Mg/nano–Al2O3 composite exhibits a single window for hot working at strain rates higher than about 0.1 s1 and the optimum temperature is in the range 400–450 °C. This workability window is favorable for all the bulk metal working processes, particularly for hot extrusion which is a chosen process for magnesium materials. The behavior of the composite in this window is similar to that of the matrix material (extruded magnesium) and exhibits dynamic recrystallization during deformation with grain boundary self-diffusion as the rate-controlling mechanism. Due to the presence of nano-alumina particles at the PPBs, their energy is lowered and become very stable. When dynamic recrystallization occurs during hot deformation, the PPBs do not break-down and get reconstituted unlike other powder compacts [40]. They add to the high temperature strength of the composite since they do not slide, rotate or migration to cause softening. Depending on the direction of deformation, the PPBs may kink and store some elastic energy, which may be released by annealing. At lower strain rates, the apparent activation energy is a considerably higher than that for lattice self-diffusion, which is the rate-
Fig. 8. Microstructures of Mg/nano–Al2O3 composite deformed at different temperatures and strain rates. The compression direction is vertical.
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
1075
higher (305 kJ/mol) than that for hot deformation of the matrix material (135 kJ/mol) which itself is close to that for lattice self-diffusion. iv. The prior particle boundaries in the composite are stable during hot deformation and do not slide, rotate or migrate but only exhibit kinking during compression parallel to the original extrusion direction.
Acknowledgement The work presented in this paper has been fully supported by a Strategic Research Grant from City University of Hong Kong (Project Ref. No. 7002335). References
Fig. 9. Microstructures of Mg/nano–Al2O3 composite (a) deformed at 450 °C/10 s1 (b) deformed at 450 °C/10 s1 and annealed at 450 °C for 5 h.
controlling mechanism in the matrix material (extruded magnesium). This indicates that lattice self-diffusion is restricted by the presence of nano-alumina particles at the PPBs. One possibility is that since the energy of the PPBs is reduced substantially, they do not act as sources or sinks for vacancies which are essential for self-diffusion. As a result, the rate of thermal recovery is reduced and so nucleation of major softening processes like dynamic recrystallization does not take place during deformation. This major difference at lower strain rates in the behavior of the nanocomposite is actually beneficial in enhancing the creep strength of the composite material although not directly relevant to bulk metal working. 4. Conclusions The following conclusions are drawn from this investigation: i. The composite exhibits a workability window at strain rates >0.1 s1 with the optimum temperature in the range 400– 450 °C. ii. The behavior of the composite is similar to that of the matrix material in the workability window in which the apparent activation energy (123 kJ/mol) suggests grain boundary self-diffusion as the rate-controlling mechanism. iii. At lower strain rates, lattice self-diffusion is restricted by the presence of nano-alumina particles at the prior particle boundaries since the apparent activation energy is much
[1] Beals RS, Tissington C, Zhang X, Kainer K, Petrillo J, Verbrugge M. Magnesium global development: outcomes from the TMS 2007 annual meeting. J Met 2007;59(8):39–42. [2] Kainer KU, Dieringa H, Dietzel W, Hort N, Blawert C. The use of magnesium alloys: past, present and future. In: Pekguleryuz MO, Mackaenzie LWF, editors. Magnesium technology in the global age. Montreal: Canadian Institute of Mining, Metallurgy and Petroleum; 2006. p. 3–19. [3] Bettles CJ, Gibson MA. Current wrought magnesium alloys: strengths and weaknesses. J Met 2005;57:46–9. [4] Chawla N, Chawla KK. Metal matrix composites. New York: Springer; 2006. [5] Miracle DB. Metal matrix composites – from science to technological significance. Compos Sci Technol 2005;65:2526–40. [6] Trojanova Z, Szaraz Z, Labar J, Lukac P. Deformation behavior of an AS21 alloy reinforced by short saffil fibres and SiC particles. J Mater Proc Technol 2005;162–163:131–8. [7] Garces G, Perez P, Adeva P. Effect of extrusion texture on the mechanical behaviour of Mg–SiCp composites. Scripta Mater 2005;52:615–9. [8] El-Saeid Essa Y, Perez-Castellanos JL. Effect of the strain rate and temperature on the mechanical behavior of a Mg–5%Zn alloy reinforced with SiC particles. J Mater Proc Technol 2003;143–144:856–9. [9] Mukai T, Nieh TG, Iwasaki H, Higashi K. Superplasticity in doubly extruded magnesium composite ZK60/SiC/17p. Mater Sci Technol 1998;14:32–5. [10] Yan F, Wu K, Wu GL, Lee GL, Zhao M. Superplastic deformation behavior of a 19.7 vol.% b-SiCw/ZK60 composite. Mater Lett 2003;57:1992–6. [11] Zheng M, Wu K, Liang H, Kamado S, Kojima Y. Microstructure and mechanical properties of aluminum borate whisker-reinforced magnesium matrix composites. Mater Lett 2002;57:558–64. [12] Wang HY, Jiang QC, Wang Y, Ma BX, Zhao F. Fabrication of TiB2 particulate reinforced magnesium matrix composites by powder metallurgy. Mater Lett 2004;58:3509–13. [13] Xiuqing Z, Haowei W, Lihua L, Xinying T, Naiheng M. The mechanical properties of magnesium matrix composites reinforced with (TiB2 + TiC) ceramic particulates. Mater Lett 2005;59:2105–9. [14] Singh A, Watanabe M, Kato A, Tsai AP. Microstructure and strength of quasicrystal containing extruded Mg–Zn–Y alloys for elevated temperature application. Mater Sci Eng A 2004;385:382–96. [15] Xi YL, Chai DL, Zhang WX, Zhou JE. Ti–6Al–4V particle reinforced magnesium matrix composite by powder metallurgy. Mater Lett 2005;59:1831–5. [16] Hassan SF, Tan JJ, Gupta M. High-temperature tensile properties of Mg/Al2O3 nanocomposite. Mater Sci Eng A 2008;486:56–62. [17] Hassan SF, Gupta M. Development of high performance magnesium nanocomposites using nano-Al2O3 as reinforcement. Mater Sci Eng A 2005;392:163–8. [18] Wong WLE, Gupta M. Simultaneously improving strength and ductility of magnesium using nano-size SiC particulates and microwaves. Adv Eng Mater 2006;8:735–40. [19] Hassan SF, Gupta M. Development of nano-Y2O3 containing magnesium nanocomposites using solidification processing. J Alloy Compd 2007;429:176–83. [20] Ferkel H, Mordike BL. Magnesium strengthened by SiC nanoparticles. Mater Sci Eng A 2001;298:193–9. [21] Lu L, Lai MO, Liang W. Magnesium nanocomposite via mechanochemical milling. Compos Sci Technol 2004;64:2009–14. [22] Tjong SC. Novel nanoparticle-reinforced metal matrix composites with enhanced mechanical properties. Adv Eng Mater 2007;9:639–52. [23] Prasad YVRK, Sasidhara S, editors. Hot working guide: a compendium of processing maps. Materials Park (OH): ASM International; 1997. [24] Prasad YVRK, Seshacharyulu T. Modeling of hot deformation for microstructural control. Inter Mater Rev 1998;43:243–58. [25] Prasad YVRK. Processing maps – a status report. J Mater Eng Perform 2003;12:638–45. [26] Ziegler H. Some extremum principles in irreversible thermodynamics with applications to continuum mechanics. In: Sneddon IN, Hill R, editors. Progress in solid mechanics, vol. 4. New York: John Wiley; 1965. p. 91–193.
1076
Y.V.R.K. Prasad et al. / Composites Science and Technology 69 (2009) 1070–1076
[27] Cavaliere P, Cerri E, Leo P. Hot deformation and processing maps of a particulate reinforced 2618/Al2O3/20p metal matrix composite. Compos Sci Technol 2004;64:1287–91. [28] Ramanathan S, Karthikeyan R, Ganesan G. Development of processing maps for 2124 Al/SiCp composites. Mater Sci Eng A 2006;441:321–5. [29] Cerri E, Spigerelli S, Evangelista E, Cavaliere P. Hot deformation and processing maps of a particle reinforced 6061 + 20%Al2O3 composite. Mater Sci Eng A 2002;324:157–61. [30] Raghunath BK, Karthikeyan R, Ganesan G, Gupta M. An investigation of hot deformation response of particulate-reinforced magnesium + 9% titanium composite. Mater Des 2008;29:622–7. [31] Jonas JJ, Sellars CM, McG.Tegart WJ. Strength and structure under hot working conditions. Metall Rev 1969;14:1–24. [32] Gupta M, Wong WLE. Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering. Scripta Mater 2005;52:479–83. [33] Prasad YVRK, Rao KP. Mechanisms of high temperature deformation in electrolytic copper in extended ranges of temperature and strain rate. Mater Sci Eng A 2004;374:335–41.
[34] Sivakesavam O. Processing map for extruded magnesium. In: Prasad YVRK, Sasidhara S, editors. Hot working guide: a compendium of processing maps. Materials Park, OH: ASM International; 1997. p. 347–50. [35] Frost HJ, Ashby MF. Deformation-mechanism maps. Oxford: Pergamon; 1982. p. 44. [36] Prasad YVRK, Rao KP. Effect of crystallographic texture on the kinetics of hot deformation of rolled Mg–3Al–1Zn alloy plate. Mater Sci Eng A 2006;432:170–7. [37] Prasad YVRK, Rao KP. Hot deformation mechanisms and microstructural control in high temperature extruded AZ31 magnesium alloy. Adv Eng Mater 2007;9:558–65. [38] Rao KP, Prasad YVRK, Hort N, Kainer KU. Hot workability characteristics of cast and homogenized Mg–3Sn–1Ca alloy. J Mater Process Technol 2008;201:359–63. [39] Prasad YVRK, Rao KP. Processing maps for hot deformation of rolled AZ31 magnesium alloy plate: anisotropy of hot workability. Mater Sci Eng A 2008;487:316–27. [40] Gegel HL, Malas JC, Doraivelu SM, Barker DR, Morgan JT, Lark KA et al. Billet conditioning technique for manufacturing powder metallurgy performs. US Patent# 4, 762, 679; 1988.