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High cycle fatigue characterization of two die-cast magnesium alloys
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7 –5 7 0
High cycle fatigue characterization of two die-cast magnesium alloys Y. Yang a,⁎, Y.B. Liu b a
School of Materials Science and Engineering, Changchun University of Technology, 17 Yan'an Road, Changchun City, Jilin Province, Changchun 130012, PR China b Key Laboratory of Automobile Materials Ministry of Education, Jilin University, Changchun 130025, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
High cycle fatigue characteristics of two magnesium alloys including die-cast AZ91D and
Received 14 June 2006
AZ91D with 1%Ce(wt.%) addition were investigated. High cycle fatigue tests were carried out
Received in revised form
at a stress ratio(R) of 0.1 and a frequency of 90 Hz using cylindrical specimens at different
10 April 2007
maximum applied stress levels. Fatigue crack growth tests were conducted at R = 0.1 and a
Accepted 20 April 2007
frequency of 20 Hz using single edge V-notched plate specimens. The results showed that some new rod-like Al11Ce3 phases were observed, and the elongation, yield and tensile
Keywords:
strength of the materials are significantly improved in AZ91D alloy with 1%Ce(wt.%)
Die cast magnesium alloy
addition. Die-cast AZ91D magnesium alloy with 1%Ce(wt.%) addition exhibits higher fatigue
Cerium
performance. The superior fatigue behavior of the alloy containing 1%Ce(wt.%) is mostly due
High cycle fatigue
to the grain refinement, Al11Ce3 and fined β phases at the grain boundary diffusely and cast
Fatigue crack growth
characteristics improvement. The fractographs of two die-cast magnesium alloys shows basically quasi-cleavage fracture. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
Recently, magnesium alloys are very attractive materials due to their low density, high specific strength and easily recycled compared with other metals and alloys. As low-weight structural materials, there have been significant increase in usage of magnesium alloy for automobile, aerospace components, computer, mobile phones and household equipments. AZ91D alloy with about 9 wt.% Al and 1 wt.% Zn is the most common die-cast alloy in automotive applications due to excellent cast ability and good room-temperature mechanical properties [1–3]. However, low fatigue strength under service conditions have been an important factor in limiting the use of magnesium alloys in lowly stressed designs [4,5]. Cerium is an important alloying element in magnesium alloys, which can
improve their casting characteristics, high temperature properties, corrosion and wear resistance properties [6–8]. However, few researches on the effect of cerium on high cycle fatigue behavior of die-cast AZ91D magnesium alloy have been reported [9]. In the present study, the high cycle fatigue and fatigue crack growth behavior of two magnesium alloys including die-cast AZ91D and AZ91D containing 1%Ce(wt.%) were discussed.
2.
Experimental Procedures
Die-cast AZ91D alloy and the alloy with 1%Ce(wt.%) addition were used for tensile and fatigue studies. The chemical compositions of two test alloys are shown in Table 1. Uniaxial
⁎ Corresponding author. Tel.: +86 431 866 447 72; fax: +86 431 857 164 26. E-mail addresses: [email protected] (Y. Yang)., [email protected] (Y.B. Liu). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.04.016
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MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7–5 7 0
Table 1 – The chemical compositions of two test alloys (wt.%) Materials
Al
Zn Mn
Ce
Fe
Ni
Si
Mg
AZ91D 8.76 0.68 0.21 0 ≤ 0.006 ≤ 0.001 ≤ 0.05 Balance AZ91D-1% 8.82 0.64 0.22 0.95 ≤ 0.006 ≤ 0.001 ≤ 0.05 Balance Ce
tensile test was carried out at an initial strain rate 1 × 10− 4/s using cylindrical specimens with 6 mm gauge diameter and 20 mm gauge length on a MTS 810 machine. High cycle fatigue test with a stress-controlled constant amplitude was conducted on a PLG-20C high frequency fatigue testing machine in laboratory air at room temperature using a sine-wave form with a stress ratio of 0.1 and a frequency of 90 Hz. In the constant stress amplitude tests, the specimens were loaded until specimen failure or to a maximum number of cycles of 107. The fracture surfaces of the tested specimens were observed using a JSM-5600LV scanning electron microscope (SEM). The fatigue crack growth rates were studied using a single edge-notched specimen with a thickness, width, and length of 3 mm, 13 mm and 75 mm. All specimens were polished using fine aluminum oxide powder before testing. Fatigue crack growth tests were conducted in laboratory air, at a stress ratio of 0.1 with different maximum stress and a frequency of 20 Hz using a sevo-hydraulic material test system (Instron 8511). The tests were terminated at the stress intensity factor range value where no physical crack growth for 107 cycles of loading. The stress intensity factor range was calculated using the following equations [10]: pffiffiffiffiffiffiffiffiffi DK ¼ Kmax Kmin ¼ f ða=WÞDr ðpaÞ f ða=WÞ ¼ 1:12 0:23ða=WÞ þ 10:55ða=WÞ2 21:7ða=WÞ3 þ 30:38ða=WÞ4
ð1Þ
ð2Þ
where ΔK, Kmax, Kmin, Δσ, W, a are the stress intensity factor range, maximum stress intensity factor, minimum stress intensity factor, stress range, width of specimen and crack length, respectively.
3.
Fig. 1 – SEM micrographs of two die cast magnesium alloys (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition.
Results and Discussion
The SEM micrographs of two die cast magnesium alloys microstructures are shown in Fig. 1. In the die-cast AZ91D alloy, the β phase (Mg17Al12) can be seen at the grain boundary and primary α, eutectic α phase exist at the adjacent to β phase as shown in Fig. 1a. After adding 1%Ce(wt.%) in alloy, Fig. 1b showed that some fined β phase still remains at the grain boundaries, but some new rod-like Al11Ce3 phases are observed. The grain size of the alloy is slightly decreased from 25 μm (0%Ce) to 15 μm (1%Ce). The gas pore size of AZ91D alloy is also decreased and distributed evenly with the 1%Ce(wt.%) addition due to the excellent affinity of Ce with impurity elements O, S etc. The casting quality of alloys was improved greatly, the tendency of fatigue crack initiation reduced considering the influence of porosity on the crack formation [11].
Tensile properties of two magnesium alloys are shown in Table 2. It can be seen from the table that the elongation, tensile and yield strength of the material are significantly improved after 1%Ce(wt.%) addition. The mechanical properties improvement of Ce addition is attributed to the grain refinement and the change in the shape and distribution of β phase. The mechanical properties can be improved by grain refinement according to Hall–Petch relation. The grain growth can also be restricted by small particles (β phase) at the grain boundaries. The S–N data obtained from the high cycle fatigue test (Fig. 2) shows that the alloy with 1%Ce(wt.%) addition have improved fatigue strength at any given stress level compared to die-cast AZ91D alloy without Ce addition. The high fatigue performance of the AZ91D alloy with 1%Ce(wt.%) addition is mostly due to the amount of grain boundary increasing with the grain size decreasing, and grain boundary play a barriers role in dislocation movement plastic deformation. Furthermore, stress concentration in each grain could be reduced since the tensile stresses are undertaken by more grains and
Table 2 – Tensile properties of two test alloys Materials AZ91D AZ91D-1%Ce
0.2%Y.S (MPa)
UTS (MPa)
Elongation (%)
136 158
210 248
5.4 6.8
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7 –5 7 0
569
Fig. 2 – The S–N curves of two die-cast magnesium alloys.
improved cast quality after adding 1%Ce(wt.%) in the alloy [12]. The sameness and differences in the fatigue fracture behaviors between two alloys can be seen from the SEM micrographs in Figs. 3 and 4. Fracture surface of two alloys illustrates three representative regions, which are fatigue crack initiation, growth and collapse fracture regions, respectively. Fatigue cracks initiate at porosity and inclusions locations of alloy interior for two alloys, propagate along with grain boundary, and the direction of the crack propaga-
Fig. 4 – SEM micrographs of fatigue crack growth region of two magnesium alloys (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition, the direction of the crack propagation was represented by the arrows.
tion represented by the arrows. The fractographs, Fig. 3a and b, of die-cast AZ91D and AZ91D with 1%Ce(wt.%) addition alloys show predominantly quasi-cleavage fracture. It can be seen from Fig. 4 that the fatigue crack path changes to reveal a more faceted and serrated fracture surface. The facets are basically flat and smooth, facets sizes be nearly the same as (Fig. 4a), or smaller than (Fig. 4b) the average grain size.
Fig. 3 – SEM micrographs of fatigue fracture surface of two magnesium alloys indicating different fatigue regions (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition.
Fig. 5 – Variation of fatigue crack growth rate (da/dN) with stress intensity factor range (ΔK) for two die-cast magnesium alloys.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7–5 7 0
The typical relationship between fatigue crack growth rate, da/dN and stress intensity factor range ΔK for two alloys is shown in Fig. 5. The fatigue crack propagation (FCP) resistance of the alloy with 1%Ce(wt.%) addition is superior to that of diecast AZ91D alloy at any ΔK regions. The higher FCP resistance of with 1%Ce(wt.%) material is mostly due to rod-like Al11Ce3 and fined β phases at the grain boundary, which restrict fatigue crack propagation, and hence lower crack growth rate.
4.
Conclusions
The experimental studies on die-cast AZ91D and with 1%Ce (wt.%) addition alloys lead to the following conclusions: (a) The sizes of grain and β phase of die-cast AZ91D alloy with 1%Ce(wt.%) addition are decreased, and cast quality improved. (b) The elongation, yield and tensile strength are improved after adding 1%Ce(wt.%) in the AZ91D alloy. (c) Die-cast AZ91D magnesium alloy with 1%Ce(wt %) addition exhibits higher fatigue performance and lower fatigue crack growth rate.
REFERENCES [1] Froes FH, Eliezer D, Aghion E. The science, technology, and applications of magnesium. JOM September 1998:30–4.
[2] Polmear IJ. Magnesium alloys and applications. Mater Sci Technol 1994;10:1–16. [3] Mordike BL, Ebert T. Magnesium: properties applications potential. Mater Sci Eng A Struct Mater Prop Microstruct Process 2001;302:37–45. [4] Horstemeyer MF, Yang N, Ken G, McDowell DL, Fan J, Gullett PM. High cycle fatigue of a die cast AZ91E-T4 magnesium alloy. Acta Mater 2004;52:1327–36. [5] Zenner H, Renner F. Cyclic material behaviour of magnesium die castings and extrusions. Int J Fatigue 2002;24:1255–60. [6] Wei LY, Dunlop GL, Westengen H. Development of microstructure in cast Mg–Al-rare earth alloys. Mater Sci Technol 1996;12:741–50. [7] Pettersen G, Westengen H, Hoter R. Microstructure of a pressure die cast magnesium-4% aluminium alloy modified with rare earth additions. Mater Sci Eng A Struct Mater Prop Microstruct Process 1996;207:115–20. [8] Qi QJ, Liu YB, Yang XH. Friction and wear characteristics Mg–Al alloys containing rare earths. J Rare Earths 2003;21:157–62. [9] Gao HT, Wu GH, Ding WJ. Review on the fatigue behavior of magnesium alloys (in Chinese). Foundry Technol 2003;24:266–8. [10] Venkateswaran P, Ganesh Sundara Raman S, Pathak SD, Miyashita Y, Mutoh Y. Fatigue crack growth behavior of a die-cast magnesium alloy AZ91D. Mater Lett 2004;58:2525–9. [11] Mayer H, Papakyriacon M, Zettl B, Stanzl-Tschegg SE. Influence of porosity on the fatigue limit of die cast magnesium and aluminium alloys. Int J Fatigue 2003;25:245–56. [12] Lu YZ, Wang QD, Zeng XQ. Effects of rare earth on the microstructure, properties and fracture behavior of Mg–Al alloys. Mater Sci Eng A Struct Mater Prop Microstruct Process 2000;278:66–76.
High cycle fatigue characterization of two die-cast magnesium alloys Y. Yang a,⁎, Y.B. Liu b a
School of Materials Science and Engineering, Changchun University of Technology, 17 Yan'an Road, Changchun City, Jilin Province, Changchun 130012, PR China b Key Laboratory of Automobile Materials Ministry of Education, Jilin University, Changchun 130025, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
High cycle fatigue characteristics of two magnesium alloys including die-cast AZ91D and
Received 14 June 2006
AZ91D with 1%Ce(wt.%) addition were investigated. High cycle fatigue tests were carried out
Received in revised form
at a stress ratio(R) of 0.1 and a frequency of 90 Hz using cylindrical specimens at different
10 April 2007
maximum applied stress levels. Fatigue crack growth tests were conducted at R = 0.1 and a
Accepted 20 April 2007
frequency of 20 Hz using single edge V-notched plate specimens. The results showed that some new rod-like Al11Ce3 phases were observed, and the elongation, yield and tensile
Keywords:
strength of the materials are significantly improved in AZ91D alloy with 1%Ce(wt.%)
Die cast magnesium alloy
addition. Die-cast AZ91D magnesium alloy with 1%Ce(wt.%) addition exhibits higher fatigue
Cerium
performance. The superior fatigue behavior of the alloy containing 1%Ce(wt.%) is mostly due
High cycle fatigue
to the grain refinement, Al11Ce3 and fined β phases at the grain boundary diffusely and cast
Fatigue crack growth
characteristics improvement. The fractographs of two die-cast magnesium alloys shows basically quasi-cleavage fracture. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
Recently, magnesium alloys are very attractive materials due to their low density, high specific strength and easily recycled compared with other metals and alloys. As low-weight structural materials, there have been significant increase in usage of magnesium alloy for automobile, aerospace components, computer, mobile phones and household equipments. AZ91D alloy with about 9 wt.% Al and 1 wt.% Zn is the most common die-cast alloy in automotive applications due to excellent cast ability and good room-temperature mechanical properties [1–3]. However, low fatigue strength under service conditions have been an important factor in limiting the use of magnesium alloys in lowly stressed designs [4,5]. Cerium is an important alloying element in magnesium alloys, which can
improve their casting characteristics, high temperature properties, corrosion and wear resistance properties [6–8]. However, few researches on the effect of cerium on high cycle fatigue behavior of die-cast AZ91D magnesium alloy have been reported [9]. In the present study, the high cycle fatigue and fatigue crack growth behavior of two magnesium alloys including die-cast AZ91D and AZ91D containing 1%Ce(wt.%) were discussed.
2.
Experimental Procedures
Die-cast AZ91D alloy and the alloy with 1%Ce(wt.%) addition were used for tensile and fatigue studies. The chemical compositions of two test alloys are shown in Table 1. Uniaxial
⁎ Corresponding author. Tel.: +86 431 866 447 72; fax: +86 431 857 164 26. E-mail addresses: [email protected] (Y. Yang)., [email protected] (Y.B. Liu). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.04.016
568
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7–5 7 0
Table 1 – The chemical compositions of two test alloys (wt.%) Materials
Al
Zn Mn
Ce
Fe
Ni
Si
Mg
AZ91D 8.76 0.68 0.21 0 ≤ 0.006 ≤ 0.001 ≤ 0.05 Balance AZ91D-1% 8.82 0.64 0.22 0.95 ≤ 0.006 ≤ 0.001 ≤ 0.05 Balance Ce
tensile test was carried out at an initial strain rate 1 × 10− 4/s using cylindrical specimens with 6 mm gauge diameter and 20 mm gauge length on a MTS 810 machine. High cycle fatigue test with a stress-controlled constant amplitude was conducted on a PLG-20C high frequency fatigue testing machine in laboratory air at room temperature using a sine-wave form with a stress ratio of 0.1 and a frequency of 90 Hz. In the constant stress amplitude tests, the specimens were loaded until specimen failure or to a maximum number of cycles of 107. The fracture surfaces of the tested specimens were observed using a JSM-5600LV scanning electron microscope (SEM). The fatigue crack growth rates were studied using a single edge-notched specimen with a thickness, width, and length of 3 mm, 13 mm and 75 mm. All specimens were polished using fine aluminum oxide powder before testing. Fatigue crack growth tests were conducted in laboratory air, at a stress ratio of 0.1 with different maximum stress and a frequency of 20 Hz using a sevo-hydraulic material test system (Instron 8511). The tests were terminated at the stress intensity factor range value where no physical crack growth for 107 cycles of loading. The stress intensity factor range was calculated using the following equations [10]: pffiffiffiffiffiffiffiffiffi DK ¼ Kmax Kmin ¼ f ða=WÞDr ðpaÞ f ða=WÞ ¼ 1:12 0:23ða=WÞ þ 10:55ða=WÞ2 21:7ða=WÞ3 þ 30:38ða=WÞ4
ð1Þ
ð2Þ
where ΔK, Kmax, Kmin, Δσ, W, a are the stress intensity factor range, maximum stress intensity factor, minimum stress intensity factor, stress range, width of specimen and crack length, respectively.
3.
Fig. 1 – SEM micrographs of two die cast magnesium alloys (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition.
Results and Discussion
The SEM micrographs of two die cast magnesium alloys microstructures are shown in Fig. 1. In the die-cast AZ91D alloy, the β phase (Mg17Al12) can be seen at the grain boundary and primary α, eutectic α phase exist at the adjacent to β phase as shown in Fig. 1a. After adding 1%Ce(wt.%) in alloy, Fig. 1b showed that some fined β phase still remains at the grain boundaries, but some new rod-like Al11Ce3 phases are observed. The grain size of the alloy is slightly decreased from 25 μm (0%Ce) to 15 μm (1%Ce). The gas pore size of AZ91D alloy is also decreased and distributed evenly with the 1%Ce(wt.%) addition due to the excellent affinity of Ce with impurity elements O, S etc. The casting quality of alloys was improved greatly, the tendency of fatigue crack initiation reduced considering the influence of porosity on the crack formation [11].
Tensile properties of two magnesium alloys are shown in Table 2. It can be seen from the table that the elongation, tensile and yield strength of the material are significantly improved after 1%Ce(wt.%) addition. The mechanical properties improvement of Ce addition is attributed to the grain refinement and the change in the shape and distribution of β phase. The mechanical properties can be improved by grain refinement according to Hall–Petch relation. The grain growth can also be restricted by small particles (β phase) at the grain boundaries. The S–N data obtained from the high cycle fatigue test (Fig. 2) shows that the alloy with 1%Ce(wt.%) addition have improved fatigue strength at any given stress level compared to die-cast AZ91D alloy without Ce addition. The high fatigue performance of the AZ91D alloy with 1%Ce(wt.%) addition is mostly due to the amount of grain boundary increasing with the grain size decreasing, and grain boundary play a barriers role in dislocation movement plastic deformation. Furthermore, stress concentration in each grain could be reduced since the tensile stresses are undertaken by more grains and
Table 2 – Tensile properties of two test alloys Materials AZ91D AZ91D-1%Ce
0.2%Y.S (MPa)
UTS (MPa)
Elongation (%)
136 158
210 248
5.4 6.8
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7 –5 7 0
569
Fig. 2 – The S–N curves of two die-cast magnesium alloys.
improved cast quality after adding 1%Ce(wt.%) in the alloy [12]. The sameness and differences in the fatigue fracture behaviors between two alloys can be seen from the SEM micrographs in Figs. 3 and 4. Fracture surface of two alloys illustrates three representative regions, which are fatigue crack initiation, growth and collapse fracture regions, respectively. Fatigue cracks initiate at porosity and inclusions locations of alloy interior for two alloys, propagate along with grain boundary, and the direction of the crack propaga-
Fig. 4 – SEM micrographs of fatigue crack growth region of two magnesium alloys (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition, the direction of the crack propagation was represented by the arrows.
tion represented by the arrows. The fractographs, Fig. 3a and b, of die-cast AZ91D and AZ91D with 1%Ce(wt.%) addition alloys show predominantly quasi-cleavage fracture. It can be seen from Fig. 4 that the fatigue crack path changes to reveal a more faceted and serrated fracture surface. The facets are basically flat and smooth, facets sizes be nearly the same as (Fig. 4a), or smaller than (Fig. 4b) the average grain size.
Fig. 3 – SEM micrographs of fatigue fracture surface of two magnesium alloys indicating different fatigue regions (a) AZ91D (b) AZ91D with 1%Ce(wt.%) addition.
Fig. 5 – Variation of fatigue crack growth rate (da/dN) with stress intensity factor range (ΔK) for two die-cast magnesium alloys.
570
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 6 7–5 7 0
The typical relationship between fatigue crack growth rate, da/dN and stress intensity factor range ΔK for two alloys is shown in Fig. 5. The fatigue crack propagation (FCP) resistance of the alloy with 1%Ce(wt.%) addition is superior to that of diecast AZ91D alloy at any ΔK regions. The higher FCP resistance of with 1%Ce(wt.%) material is mostly due to rod-like Al11Ce3 and fined β phases at the grain boundary, which restrict fatigue crack propagation, and hence lower crack growth rate.
4.
Conclusions
The experimental studies on die-cast AZ91D and with 1%Ce (wt.%) addition alloys lead to the following conclusions: (a) The sizes of grain and β phase of die-cast AZ91D alloy with 1%Ce(wt.%) addition are decreased, and cast quality improved. (b) The elongation, yield and tensile strength are improved after adding 1%Ce(wt.%) in the AZ91D alloy. (c) Die-cast AZ91D magnesium alloy with 1%Ce(wt %) addition exhibits higher fatigue performance and lower fatigue crack growth rate.
REFERENCES [1] Froes FH, Eliezer D, Aghion E. The science, technology, and applications of magnesium. JOM September 1998:30–4.
[2] Polmear IJ. Magnesium alloys and applications. Mater Sci Technol 1994;10:1–16. [3] Mordike BL, Ebert T. Magnesium: properties applications potential. Mater Sci Eng A Struct Mater Prop Microstruct Process 2001;302:37–45. [4] Horstemeyer MF, Yang N, Ken G, McDowell DL, Fan J, Gullett PM. High cycle fatigue of a die cast AZ91E-T4 magnesium alloy. Acta Mater 2004;52:1327–36. [5] Zenner H, Renner F. Cyclic material behaviour of magnesium die castings and extrusions. Int J Fatigue 2002;24:1255–60. [6] Wei LY, Dunlop GL, Westengen H. Development of microstructure in cast Mg–Al-rare earth alloys. Mater Sci Technol 1996;12:741–50. [7] Pettersen G, Westengen H, Hoter R. Microstructure of a pressure die cast magnesium-4% aluminium alloy modified with rare earth additions. Mater Sci Eng A Struct Mater Prop Microstruct Process 1996;207:115–20. [8] Qi QJ, Liu YB, Yang XH. Friction and wear characteristics Mg–Al alloys containing rare earths. J Rare Earths 2003;21:157–62. [9] Gao HT, Wu GH, Ding WJ. Review on the fatigue behavior of magnesium alloys (in Chinese). Foundry Technol 2003;24:266–8. [10] Venkateswaran P, Ganesh Sundara Raman S, Pathak SD, Miyashita Y, Mutoh Y. Fatigue crack growth behavior of a die-cast magnesium alloy AZ91D. Mater Lett 2004;58:2525–9. [11] Mayer H, Papakyriacon M, Zettl B, Stanzl-Tschegg SE. Influence of porosity on the fatigue limit of die cast magnesium and aluminium alloys. Int J Fatigue 2003;25:245–56. [12] Lu YZ, Wang QD, Zeng XQ. Effects of rare earth on the microstructure, properties and fracture behavior of Mg–Al alloys. Mater Sci Eng A Struct Mater Prop Microstruct Process 2000;278:66–76.