- Email: [email protected]
M) Process
J. Mater. Sci. Technol., 2010, 26(2), 151-155.
Microstructure and Tensile Properties of ZK60 Alloy Fabricated by Simplified Rapid Solidification Powder Metallurgy (S-RS P/M) Process Zhenya Zhang, Huashun Yu† , Shaoqing Wang, Haitao Wang and Guanghui Min Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China [Manuscript received March 31, 2009, in revised form June 30, 2009]
This study investigated the microstructures and mechanical properties of ZK60 alloy prepared by a simplified rapid solidification powder metallurgy (RS P/M) processing system (S-RS P/M), which consists of warm press in dry air and hot extrusion. Microstructure characterizations showed that S-RS P/M alloy consisted of magnesium matrix and oxide stringers of ∼1 μm in width. TEM (transmission electron microscopy) observations illustrated nano-size magnesia particles (10–30 nm) constituted oxide stringer in detail. Due to a relatively higher volume of nano-size magnesia particle produced during S-RS P/M process, 0.2% yield strength of S-RS P/M ZK60 alloy was found to be as high as 382 MPa, which is 10% higher than that of RS P/M alloy. The improvement in mechanical properties is mainly attributed to the combination effects of Orowan mechanism and coefficient of thermal expansion (CTE) mismatch because of the approximately same average grain size. KEY WORDS: Microstructure; Mechanical properties; Rapid solidification powder metallurgy; Nano-size magnesia
1. Introduction Rapid solidification powder metallurgy (RS P/M) technology is one promising approach to design Mg alloy with excellent mechanical properties because of remarkable grain refinement and finely dispersed intermetallic strengthening phases. Conventional procedures preparing preforms in RS P/M Mg alloys have sought to encapsulation and vacuum degassing/hot press under vacuum before final consolidation. Examples of typical process can be found in Mg-ZnZr[1] , Mg-Zn-Y[2–4] , Mg-Al-Zn-RE[5] and Mg-RE[6] RS P/M alloys. Although above-mentioned strategies can effectively prevent the oxidation of powders, some disadvantages such as high cost and low production rate impede them scaled-up for industry † Corresponding author. Prof. Ph.D.; Tel: +86 531 88395639; Fax: +86 531 88395639; E-mail address: [email protected] (H.S. Yu).
application. Therefore, it is necessary to develop a high cost-effective process to promote wide application of RS P/M Mg alloys. To achieve this aim, it is important to clarify the influence of in-situ oxide particles on mechanical properties of RS P/M Mg alloys. However, to date, few literature can be referred. Therefore, in this work, we investigated the microstructures and mechanical properties of ZK60 alloy prepared by a simplified RS P/M processing system (S-RS P/M), which consists of warm press in dry air and hot extrusion. Particular attention has been paid to oxide distribution and morphology in extruded bars as well as the effect of oxide particles on tensile properties. 2. Experimental ZK60 extruded bars with chemical composition of Mg-5.20Zn-0.33Zr (wt pct) was obtained through a
152
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
powder metallurgical route. RS powders were produced by argon gas atomization and subsequently sieved into different size fractions. A double-action piston and a die with inner diameter of 48 mm were used to compact powders (100–150 μm) at 473 K in dry air. For comparison, a counterpart was prepared by using the same processing route but the system was protected under vacuum of 10−1 Pa. These resulting preforms were extruded at 523 K with an extrusion ratio of 25. The extruded bulk alloys produced from preforms under air and vacuum were termed SRS P/M alloy and RS P/M alloy, respectively. Microstructural characterizations of the extruded alloys were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM, FEI Nova NanoSEM 400) equipped with energy dispersive Xray spectroscopy (EDS) and transmission electron microscopy (TEM, H800). In order to trace in-situ oxide particles in S-RS P/M and RS P/M alloys, metallurgical preparation for SEM observation was performed according to standard metallographic polishing method and then electropolishing was carried out in commercial magnesium polishing solution AC-II for 60–70 s. The size and amount of oxide particles were estimated by using Image Pro Plus 5.0 software. Thin foils along extrusion direction (ED) for TEM observation were prepared by a twin jet electropolishing using the reactive mixture of HClO4 (5%), butanol (35%) and methanol (60%), and finally ion-beam milled. Tensile tests were performed in a universal tensile machine under a constant crosshead speed condition at an initial strain rate of 3.5×10−4 s−1 at room temperature. 3. Results and Discussion 3.1 Microstructural characterizations Figure 1 shows XRD patterns of the RS powders, RS P/M and S-RS P/M ZK60 alloys. Peaks of hexagonal close-packed (hcp) Mg and hcp MgZn2 phases are detected in all specimens. Nevertheless, diffraction peaks of MgZn2 phase in RS powders are not obvi-
Fig. 1 XRD patterns of the RS powders, RS P/M and S-RS P/M ZK60 alloys
ous because a high cooling rate brings about a form of supersaturated solid solution consisting of Zn and Zr elements. XRD analysis on the both bulk samples revealed the absence of oxide composite (i.e., MgO or MgO2 ), which can be attributed to the limitation of the filtered X-ray to detect phases with amount <2 vol. fraction[7] . Figure 2 shows SEM micrographs of RS powders, RS P/M and S-RS P/M ZK60 alloys. Rather small satellite particles adhere around primary particles, as can be seen in Fig. 2(a). The spherically shaped RS powder particles exhibit a twophase dendritic microstructure consisting of magnesium and β-MgZn2 phase in the interdendritic regions. Figure 2(b) and (c) show the cross-sectional back-scatter electron imaging (BEI) of RS P/M and S-RS P/M ZK60 alloys. After extrusion, the dendritic microstructure disappears and the second phase particles are homogeneously distributed in the magnesium matrix. However, it is noteworthy that S-RS P/M alloy with unidentified phase particles discontinuously locate at metal-to-metal bonding between powder particles (Fig. 2(c)), while in the RS P/M alloy powder particles are well bonded and no traces
Fig. 2 SEM micrograph of RS powders (a) and cross-sectional BEI of RS P/M (b) and S-RS P/M ZK60 (c) alloys
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
153
Fig. 3 Longitude-sectional SEI of: (a) RS P/M alloy, (b) enlarged micrograph of rectangular region in (a), (c) SRS P/M ZK60 alloy, (d) enlarged micrograph of rectangular region in (c)
of boundaries occur (Fig. 2(b)). Composition of this phase measured by EDS microanalysis is Mg-29.5±1.9 O-1.8±0.1 Zn (at. pct). Oxide distribution can further be identified in Fig. 3, which are the longitudesectional secondary electron imaging (SEI) of the RS P/M and S-RS P/M ZK60 alloys. As shown in Fig. 3(a) and (c), microstructure is fibrous regardless of the processing conditions because powder particles were elongated as they passed the intense shear zone during extrusion. Obviously, the band boundaries are decorated by oxide particles formed on powder surface and broken up during consolidation. The morphology and size of oxides are more distinct in enlarged micrographs of rectangular region in Fig. 3(b) and (d). A subtle difference between RS P/M and S-RS P/M alloy can be readily distinguished. That is to say, oxide particles of small submicrometre (∼300 nm) in RS P/M alloy (Fig. 3(b)) discontinuously embedded in the deformed particle boundaries; while in S-RS P/M alloy (Fig. 3(d)) oxide particles became coarsen and were inclined to form stringer. Further, based on multiple pictures analysis, the volume fraction of these oxides is estimated to be approximately 0.6% and 1.8% for RS P/M and S-
RS P/M alloy, respectively. From above observation, it demonstrates that S-RS P/M process inevitably introduces more oxides compared with RS P/M process. Thus, improved mechanical properties is expected because fine oxide particles have been very effective as obstacles to dislocation movement, assisting in the formation and stabilization of the substructure. TEM micrographs along extrusion direction were investigated to obtain more detailed information on the microstructural features identified by SEM. As shown in Fig. 4(a), RS P/M ZK60 alloy mainly consists of equiaxed grains with average size of 450 nm determined by analyzing the TEM micrographs with a line-intercept method. Small amount of spherically shaped strengthening particles located at grain boundaries are observed. The selected area electron diffraction (SAED) pattern indicates that spherically shaped particles constitute MgZn2 phase (Fig. 4(b)). In contrast, low-magnification image of S-RS P/M alloy (Fig. 4(c)) shows that there are two kinds of regions: matrix (marked I) and oxide stringer (marked II). A well-defined spherical particle structure (10– 20 nm) in region II is identified at higher magni fication in Fig. 4(d) and the associated SAED pat-
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
154
Fig. 4 TEM micrographs of: (a) RS P/M alloy, (b) inserted SAED indicating MgZn2 particles located at grain boundary, (c) S-RS P/M alloy, (d) morphology and (e) composite of oxide in region II Table 1 Tensile test results of S-RS P/M and RS P/M ZK60 alloys at room temperature Material Extruded alloy Rolled alloy RS P/M alloy S-RS P/M alloy
d/μm 13.5 21.5 0.45 0.45
σ0.2 /MPa 269 230 346 382
tern (Fig. 4(e)) shows a MgO ring diffraction pattern. These results illustrate that nano-size magnesia is the predominant oxide in S-RS P/M alloy. On the other hand, the microstructures of region I are similar to that of RS P/M alloy, which can be recognized. 3.2 Tensile properties Table 1 lists the tensile test results of S-RS P/M and RS P/M ZK60 alloys both at room temperature. It shows that both processes can offer improved mechanical properties for ZK60 alloy compared with those obtained from conventional thermomechanical treatment[8,9] . The main reason for the mechanical properties improvement is a refined primary microstructure gained by the RS process and further grain refinement by dynamic recrystallization (DRX) occurring in the extrusion process. Compared with RS P/M alloy, a further improvement in tensile properties is achieved by S-RS P/M process, e.g., yield strength (σ0.2 ) and ultimate tensile strength (σUTS ) of S-RS P/M alloy are over 10% and 7% higher than those of RS P/M alloy. The difference in mechanical properties is mainly attributed to the effects of nanosize magnesia because of similar grain sizes of each alloy. Since a relatively higher volume of nano-size magnesia particle is introduced in S-RS P/M process, the interaction between dispersed magnesia and dis-
σUTS /MPa 315 288 396 425
δ/% 12.3 10.3 19.4 15.0
Reference [8] [9] This work This work
locations can result a further strengthening. The role of nano-size magnesia phase can be explained by Orowan mechanism and coefficient of thermal expansion (CTE) mismatch. According to Orowan mechanism, residual dislocation loops formed around each particle after a dislocation bows out and bypasses it, lead to high work hardening rates and strengthen the material. The contribution to yield strength by Orowan strengthening can be expressed as[10] ΔσOrowan = M ·
¯ ln(d/b) 0.4 · G · b · ¯ πλ 1 − vMg
(1)
¯ 2/3d, M is the mean orientation factor for where d= magnesium, G is the shear modulus of the matrix, b is the Burgers vector, vMg is Poisson s ratio of the ma¯ is the mean inter-particle distance given trix, andλ ¯ d( ¯ π/4f − 1). by λ= Because of different CTE mismatch between the matrix and nanoscale magnesia particles, dislocations are also created from the relaxation of the thermal expansion mismatch between the matrix and the magnesia particles and may cause an increase in the yield stress, which is expressed as[11,12] 1/2
ΔσCTE = A · M · G · b · ρth
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
√ 12 2 · Δα · ΔT · f (2) ρth = b · d · (1 − f ) where constant A characterizes the transparency of the dislocation forest for basal-basal dislocations in magnesium at room temperature[13] . The dislocation density ρth can be estimated √ by assuming that dislocation loops of radius d/ 2 are punched by spherical particles of diameter d with volume fraction f to relax the thermal mismatch due to the difference in thermal expansion coefficients Δα for a temperature excursion ΔT . Based on the contributions by the abovementioned strengthening mechanisms, σ0.2 of the SRS P/M and RS P/M alloy increases with increasing volume fraction of reinforcement. However, when the volume fraction of nano-size magnesia particulates is increased from 0.6% to 1.8%, ductility of the S-RS P/M drops but remains higher compared with those obtained from conventional thermomechanical treatment[8,9] . This may be due to the agglomeration of nano-size magnesia particles with increasing volume fraction, which reduces the Orowan strengthening effect[14] . and
4. Conclusions (1) The microstructure of S-RS P/M ZK60 alloy consisted of magnesium grains/subgrains with average size of about 0.45 μm and fine MgZn2 particles located at grain boundaries. In-situ nano-size magnesia particles decorate deformed particle boundaries. (2) The S-RS P/M alloy exhibited higher strength than RS P/M alloy with sacrifice of some ductility. This is attributed to a relatively higher volume of
155
nano-size magnesia particle. The role of magnesia phase can be explained by Orowan mechanism and CTE mismatch. (3) S-RS P/M processing system is a cost-effective method for fabricating ZK60 alloy and further may be scaled-up for industry. REFERENCES [1 ] H. Watanabe, T. Mukai, M. Mabuchi and K. Higashi: Scripta Mater., 1999, 41, 209. [2 ] X.F. Guo and D. Shechtman: J. Mater. Process. Technol., 2007, 187-188, 640. [3 ] Y. Kawamura, K. Hayashi, A. Inoue and T. Masumoto: Mater. Trans., 2001, 42, 1172. [4 ] M. Nishida, Y. Kawamura and T. Yamamuro: Mater. Sci. Eng. A, 2004, 375, 1217. [5 ] T.S. Srivatsan, S. Vasudevan and M. Petraroli: J. Alloy. Compd., 2008, 461, 154. [6 ] K. Nakashima, H. Iwasaki, T. Mori, M. Mabuchi, M. Nakamura and T. Asahina: Mater. Sci. Eng. A, 2000, 293, 15. [7 ] B.D. Cullity: Elements of X-ray Diffraction, 3rd edn, Prentice Hall, London, 2001. [8 ] H. Somekawa, A. Singh and T. Mukai: J. Mater. Res., 2007, 22, 965. [9 ] W.J. Ding, D.Q. Li, Q.D. Wang and Q. Li: Mater. Sci. Eng. A, 2008, 483-484, 228. [10] B.Q. Han and D.C. Dunand: Mater. Sci. Eng. A, 2000, 277, 297. [11] W.L.E. Wong and M. Gupta: Compos. Sci. Technol., 2007, 67, 1541. [12] X.L. Zhong, W.L.E. Wong and M. Gupta: Acta Mater., 2007, 55, 5338. [13] F. Lavrentev: Mater. Sci. Eng. A, 1980, 46, 191. [14] D.J. Lloyd: Int. Mater. Rev., 1994, 39, 23.
Microstructure and Tensile Properties of ZK60 Alloy Fabricated by Simplified Rapid Solidification Powder Metallurgy (S-RS P/M) Process Zhenya Zhang, Huashun Yu† , Shaoqing Wang, Haitao Wang and Guanghui Min Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China [Manuscript received March 31, 2009, in revised form June 30, 2009]
This study investigated the microstructures and mechanical properties of ZK60 alloy prepared by a simplified rapid solidification powder metallurgy (RS P/M) processing system (S-RS P/M), which consists of warm press in dry air and hot extrusion. Microstructure characterizations showed that S-RS P/M alloy consisted of magnesium matrix and oxide stringers of ∼1 μm in width. TEM (transmission electron microscopy) observations illustrated nano-size magnesia particles (10–30 nm) constituted oxide stringer in detail. Due to a relatively higher volume of nano-size magnesia particle produced during S-RS P/M process, 0.2% yield strength of S-RS P/M ZK60 alloy was found to be as high as 382 MPa, which is 10% higher than that of RS P/M alloy. The improvement in mechanical properties is mainly attributed to the combination effects of Orowan mechanism and coefficient of thermal expansion (CTE) mismatch because of the approximately same average grain size. KEY WORDS: Microstructure; Mechanical properties; Rapid solidification powder metallurgy; Nano-size magnesia
1. Introduction Rapid solidification powder metallurgy (RS P/M) technology is one promising approach to design Mg alloy with excellent mechanical properties because of remarkable grain refinement and finely dispersed intermetallic strengthening phases. Conventional procedures preparing preforms in RS P/M Mg alloys have sought to encapsulation and vacuum degassing/hot press under vacuum before final consolidation. Examples of typical process can be found in Mg-ZnZr[1] , Mg-Zn-Y[2–4] , Mg-Al-Zn-RE[5] and Mg-RE[6] RS P/M alloys. Although above-mentioned strategies can effectively prevent the oxidation of powders, some disadvantages such as high cost and low production rate impede them scaled-up for industry † Corresponding author. Prof. Ph.D.; Tel: +86 531 88395639; Fax: +86 531 88395639; E-mail address: [email protected] (H.S. Yu).
application. Therefore, it is necessary to develop a high cost-effective process to promote wide application of RS P/M Mg alloys. To achieve this aim, it is important to clarify the influence of in-situ oxide particles on mechanical properties of RS P/M Mg alloys. However, to date, few literature can be referred. Therefore, in this work, we investigated the microstructures and mechanical properties of ZK60 alloy prepared by a simplified RS P/M processing system (S-RS P/M), which consists of warm press in dry air and hot extrusion. Particular attention has been paid to oxide distribution and morphology in extruded bars as well as the effect of oxide particles on tensile properties. 2. Experimental ZK60 extruded bars with chemical composition of Mg-5.20Zn-0.33Zr (wt pct) was obtained through a
152
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
powder metallurgical route. RS powders were produced by argon gas atomization and subsequently sieved into different size fractions. A double-action piston and a die with inner diameter of 48 mm were used to compact powders (100–150 μm) at 473 K in dry air. For comparison, a counterpart was prepared by using the same processing route but the system was protected under vacuum of 10−1 Pa. These resulting preforms were extruded at 523 K with an extrusion ratio of 25. The extruded bulk alloys produced from preforms under air and vacuum were termed SRS P/M alloy and RS P/M alloy, respectively. Microstructural characterizations of the extruded alloys were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM, FEI Nova NanoSEM 400) equipped with energy dispersive Xray spectroscopy (EDS) and transmission electron microscopy (TEM, H800). In order to trace in-situ oxide particles in S-RS P/M and RS P/M alloys, metallurgical preparation for SEM observation was performed according to standard metallographic polishing method and then electropolishing was carried out in commercial magnesium polishing solution AC-II for 60–70 s. The size and amount of oxide particles were estimated by using Image Pro Plus 5.0 software. Thin foils along extrusion direction (ED) for TEM observation were prepared by a twin jet electropolishing using the reactive mixture of HClO4 (5%), butanol (35%) and methanol (60%), and finally ion-beam milled. Tensile tests were performed in a universal tensile machine under a constant crosshead speed condition at an initial strain rate of 3.5×10−4 s−1 at room temperature. 3. Results and Discussion 3.1 Microstructural characterizations Figure 1 shows XRD patterns of the RS powders, RS P/M and S-RS P/M ZK60 alloys. Peaks of hexagonal close-packed (hcp) Mg and hcp MgZn2 phases are detected in all specimens. Nevertheless, diffraction peaks of MgZn2 phase in RS powders are not obvi-
Fig. 1 XRD patterns of the RS powders, RS P/M and S-RS P/M ZK60 alloys
ous because a high cooling rate brings about a form of supersaturated solid solution consisting of Zn and Zr elements. XRD analysis on the both bulk samples revealed the absence of oxide composite (i.e., MgO or MgO2 ), which can be attributed to the limitation of the filtered X-ray to detect phases with amount <2 vol. fraction[7] . Figure 2 shows SEM micrographs of RS powders, RS P/M and S-RS P/M ZK60 alloys. Rather small satellite particles adhere around primary particles, as can be seen in Fig. 2(a). The spherically shaped RS powder particles exhibit a twophase dendritic microstructure consisting of magnesium and β-MgZn2 phase in the interdendritic regions. Figure 2(b) and (c) show the cross-sectional back-scatter electron imaging (BEI) of RS P/M and S-RS P/M ZK60 alloys. After extrusion, the dendritic microstructure disappears and the second phase particles are homogeneously distributed in the magnesium matrix. However, it is noteworthy that S-RS P/M alloy with unidentified phase particles discontinuously locate at metal-to-metal bonding between powder particles (Fig. 2(c)), while in the RS P/M alloy powder particles are well bonded and no traces
Fig. 2 SEM micrograph of RS powders (a) and cross-sectional BEI of RS P/M (b) and S-RS P/M ZK60 (c) alloys
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
153
Fig. 3 Longitude-sectional SEI of: (a) RS P/M alloy, (b) enlarged micrograph of rectangular region in (a), (c) SRS P/M ZK60 alloy, (d) enlarged micrograph of rectangular region in (c)
of boundaries occur (Fig. 2(b)). Composition of this phase measured by EDS microanalysis is Mg-29.5±1.9 O-1.8±0.1 Zn (at. pct). Oxide distribution can further be identified in Fig. 3, which are the longitudesectional secondary electron imaging (SEI) of the RS P/M and S-RS P/M ZK60 alloys. As shown in Fig. 3(a) and (c), microstructure is fibrous regardless of the processing conditions because powder particles were elongated as they passed the intense shear zone during extrusion. Obviously, the band boundaries are decorated by oxide particles formed on powder surface and broken up during consolidation. The morphology and size of oxides are more distinct in enlarged micrographs of rectangular region in Fig. 3(b) and (d). A subtle difference between RS P/M and S-RS P/M alloy can be readily distinguished. That is to say, oxide particles of small submicrometre (∼300 nm) in RS P/M alloy (Fig. 3(b)) discontinuously embedded in the deformed particle boundaries; while in S-RS P/M alloy (Fig. 3(d)) oxide particles became coarsen and were inclined to form stringer. Further, based on multiple pictures analysis, the volume fraction of these oxides is estimated to be approximately 0.6% and 1.8% for RS P/M and S-
RS P/M alloy, respectively. From above observation, it demonstrates that S-RS P/M process inevitably introduces more oxides compared with RS P/M process. Thus, improved mechanical properties is expected because fine oxide particles have been very effective as obstacles to dislocation movement, assisting in the formation and stabilization of the substructure. TEM micrographs along extrusion direction were investigated to obtain more detailed information on the microstructural features identified by SEM. As shown in Fig. 4(a), RS P/M ZK60 alloy mainly consists of equiaxed grains with average size of 450 nm determined by analyzing the TEM micrographs with a line-intercept method. Small amount of spherically shaped strengthening particles located at grain boundaries are observed. The selected area electron diffraction (SAED) pattern indicates that spherically shaped particles constitute MgZn2 phase (Fig. 4(b)). In contrast, low-magnification image of S-RS P/M alloy (Fig. 4(c)) shows that there are two kinds of regions: matrix (marked I) and oxide stringer (marked II). A well-defined spherical particle structure (10– 20 nm) in region II is identified at higher magni fication in Fig. 4(d) and the associated SAED pat-
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
154
Fig. 4 TEM micrographs of: (a) RS P/M alloy, (b) inserted SAED indicating MgZn2 particles located at grain boundary, (c) S-RS P/M alloy, (d) morphology and (e) composite of oxide in region II Table 1 Tensile test results of S-RS P/M and RS P/M ZK60 alloys at room temperature Material Extruded alloy Rolled alloy RS P/M alloy S-RS P/M alloy
d/μm 13.5 21.5 0.45 0.45
σ0.2 /MPa 269 230 346 382
tern (Fig. 4(e)) shows a MgO ring diffraction pattern. These results illustrate that nano-size magnesia is the predominant oxide in S-RS P/M alloy. On the other hand, the microstructures of region I are similar to that of RS P/M alloy, which can be recognized. 3.2 Tensile properties Table 1 lists the tensile test results of S-RS P/M and RS P/M ZK60 alloys both at room temperature. It shows that both processes can offer improved mechanical properties for ZK60 alloy compared with those obtained from conventional thermomechanical treatment[8,9] . The main reason for the mechanical properties improvement is a refined primary microstructure gained by the RS process and further grain refinement by dynamic recrystallization (DRX) occurring in the extrusion process. Compared with RS P/M alloy, a further improvement in tensile properties is achieved by S-RS P/M process, e.g., yield strength (σ0.2 ) and ultimate tensile strength (σUTS ) of S-RS P/M alloy are over 10% and 7% higher than those of RS P/M alloy. The difference in mechanical properties is mainly attributed to the effects of nanosize magnesia because of similar grain sizes of each alloy. Since a relatively higher volume of nano-size magnesia particle is introduced in S-RS P/M process, the interaction between dispersed magnesia and dis-
σUTS /MPa 315 288 396 425
δ/% 12.3 10.3 19.4 15.0
Reference [8] [9] This work This work
locations can result a further strengthening. The role of nano-size magnesia phase can be explained by Orowan mechanism and coefficient of thermal expansion (CTE) mismatch. According to Orowan mechanism, residual dislocation loops formed around each particle after a dislocation bows out and bypasses it, lead to high work hardening rates and strengthen the material. The contribution to yield strength by Orowan strengthening can be expressed as[10] ΔσOrowan = M ·
¯ ln(d/b) 0.4 · G · b · ¯ πλ 1 − vMg
(1)
¯ 2/3d, M is the mean orientation factor for where d= magnesium, G is the shear modulus of the matrix, b is the Burgers vector, vMg is Poisson s ratio of the ma¯ is the mean inter-particle distance given trix, andλ ¯ d( ¯ π/4f − 1). by λ= Because of different CTE mismatch between the matrix and nanoscale magnesia particles, dislocations are also created from the relaxation of the thermal expansion mismatch between the matrix and the magnesia particles and may cause an increase in the yield stress, which is expressed as[11,12] 1/2
ΔσCTE = A · M · G · b · ρth
Z.Y. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(2), 151–155
√ 12 2 · Δα · ΔT · f (2) ρth = b · d · (1 − f ) where constant A characterizes the transparency of the dislocation forest for basal-basal dislocations in magnesium at room temperature[13] . The dislocation density ρth can be estimated √ by assuming that dislocation loops of radius d/ 2 are punched by spherical particles of diameter d with volume fraction f to relax the thermal mismatch due to the difference in thermal expansion coefficients Δα for a temperature excursion ΔT . Based on the contributions by the abovementioned strengthening mechanisms, σ0.2 of the SRS P/M and RS P/M alloy increases with increasing volume fraction of reinforcement. However, when the volume fraction of nano-size magnesia particulates is increased from 0.6% to 1.8%, ductility of the S-RS P/M drops but remains higher compared with those obtained from conventional thermomechanical treatment[8,9] . This may be due to the agglomeration of nano-size magnesia particles with increasing volume fraction, which reduces the Orowan strengthening effect[14] . and
4. Conclusions (1) The microstructure of S-RS P/M ZK60 alloy consisted of magnesium grains/subgrains with average size of about 0.45 μm and fine MgZn2 particles located at grain boundaries. In-situ nano-size magnesia particles decorate deformed particle boundaries. (2) The S-RS P/M alloy exhibited higher strength than RS P/M alloy with sacrifice of some ductility. This is attributed to a relatively higher volume of
155
nano-size magnesia particle. The role of magnesia phase can be explained by Orowan mechanism and CTE mismatch. (3) S-RS P/M processing system is a cost-effective method for fabricating ZK60 alloy and further may be scaled-up for industry. REFERENCES [1 ] H. Watanabe, T. Mukai, M. Mabuchi and K. Higashi: Scripta Mater., 1999, 41, 209. [2 ] X.F. Guo and D. Shechtman: J. Mater. Process. Technol., 2007, 187-188, 640. [3 ] Y. Kawamura, K. Hayashi, A. Inoue and T. Masumoto: Mater. Trans., 2001, 42, 1172. [4 ] M. Nishida, Y. Kawamura and T. Yamamuro: Mater. Sci. Eng. A, 2004, 375, 1217. [5 ] T.S. Srivatsan, S. Vasudevan and M. Petraroli: J. Alloy. Compd., 2008, 461, 154. [6 ] K. Nakashima, H. Iwasaki, T. Mori, M. Mabuchi, M. Nakamura and T. Asahina: Mater. Sci. Eng. A, 2000, 293, 15. [7 ] B.D. Cullity: Elements of X-ray Diffraction, 3rd edn, Prentice Hall, London, 2001. [8 ] H. Somekawa, A. Singh and T. Mukai: J. Mater. Res., 2007, 22, 965. [9 ] W.J. Ding, D.Q. Li, Q.D. Wang and Q. Li: Mater. Sci. Eng. A, 2008, 483-484, 228. [10] B.Q. Han and D.C. Dunand: Mater. Sci. Eng. A, 2000, 277, 297. [11] W.L.E. Wong and M. Gupta: Compos. Sci. Technol., 2007, 67, 1541. [12] X.L. Zhong, W.L.E. Wong and M. Gupta: Acta Mater., 2007, 55, 5338. [13] F. Lavrentev: Mater. Sci. Eng. A, 1980, 46, 191. [14] D.J. Lloyd: Int. Mater. Rev., 1994, 39, 23.