- Email: [email protected]
The effects of Cr and Zr addition on the microstructure and mechanical properties of rapidly solidified Al-20Si-5Fe alloys
MATERIALS SelEWeE & ENGINEERING ELSEVIER
Materials Science and Engineering A226-228
A
(1997) 878482
The effects of Cr and Zr addition on the microstructure and mechanical properties of rapidly solidified Al-20Si- 5Fe alloys S.J. Hong a, T.S. Kim a, W.T. Kim b,*, B.S. Chun a a Engineering
Research
Center for b Department
Rapidly
Materials, Department of Met. Eng., Chungnam National University, Taejeon 305-764, South Korea Chotigiu UniueAy, 36 Naeaokaong, Chongiu 360-764, Sorrtk Korea
Soli@ed
of Physics,
220 Gungco17g,
Abstract The effectsof Cr and Zr addition on mechanicalpropertiesof Al-20Si-5Fe alloys, manufacturedby gasatomization followed by :degassingand hot extrusion, were studied by a combination of scanning electron microscopy, transmissionelectron microscopy,energy dispersiveX-ray microanalysis,X-ray diffractometry, tensiletesting and wear testing. The microstructure of extruded bars showeda homogeneous distribution of two different sizesof monoclinic ,B-Al,FeSi and Si particles embeddedin an a-Al matrix. Tensilestrengthsof Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys at room temperaturewere 375 and 387
MPa, respectively, which were about 12% higher than that of the Al-20Si-5Fe
alloy. However, the effects of Cr or Zr addition
on the wear resistancewere not significant.Coefficientsof thermal expansionof Al-2OSi-SFe-2Cr and Al-20Si-5Fe-2Zr were 16.6x 10P6 and 17.5x 10m6K-l, respectively,which are smallerthan that of the Al-20Si alloy. 0 1997Elsevier ScienceS.A. Keywords:
Coefficient of thermal expansion; Rapid solidification; Wear-resistant alloy
1. Introduction Hypereutectic Al-Si
alloys are used in many appli-
cation areas in automotive and electronic industries due to their good wear resistance and low coefficient of thermal expansion. These properties are attributed to the high volume fraction of hard Si particles embedded in an Al matrix. However, the alloys show poor machinability, low thermal resistance and low fatigue strength. It is common to increase the Si content to improve the wear resistance and the mechanical strength. However, primary Si crystals become coarser, with the increase of Si content above the eutectic composition, resulting in deteriorating properties. Several different techniques have been applied to refine the primary Si crystals such as modification treatment [1,2], addition of ternary alloying element [3] or rapid solidification technique [4-111. Due to the recent development of technique for consolidation of Al alloy powders [7,8], rapid solidification of
hypereutectic Al-Si alloy seems to be the most promising route to improve wear resistance and strength. It has been reported that Fe is also effective in improving wear resistance by forming an Al-SiFe intermetallic compound [9] and high-temperature strength by forming line dispersoids in an Al matrix. However, large acicular 6-Al,FeSi, particles formed during the gas atomization of the ternary alloy system may cause a problem in consolidating powders. Therefore we have tried to modify microstructure by adding quaternary elements. Cr and Zr were selected as candidates. Zr may act as a grain refiner. Cr was reported to reduce primary Si size during the rapid solidification [lo]. This study was carried out to find out the effects of Cr and Zr addition on the microstructure and mechanical properties of Al-20SiLSFe alloys by using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray microanalysis (EDX), X-ray diffractometry
(XRD),
temperatures, * Corresponding author. 0921~5093/97/%17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(96)10811-X
tensile test at room
and high
and wear testing. The coefficients of
thermal expansion of the alloys were also measured.
S.J. Hong et al. /Materials
2. Experimental
Science and Engineering A226-228
procedure
Master alloys of Al-20Si-5Fe-2X (X = Cr, Zr) were manufactured by induction melting of commercial pure metals in graphite crucibles in air. Al20Si-5Fe-2X alloy powders were manufactured by remelting the master alloy in a graphite crucible 200 K above the liquidus temperature of each alloy, pouring into a tundish and then bottom pouring through a graphite melt delivery nozzle of 3 nun in diameter to an annulus N2 gas atomizer operating with a pressure of 0.8 MPa. The melt flow rate, estimated from operating time and from the weight of atomized melt, was about 1.2 kg min- ‘. The gas flow rate, calculated from the gas consumption rate, was about 0.8 N*m3 min-‘. Size distribution of alloy powders was measured by conventional sieving analysis. Alloy powders with size between 61 and 120 pm were selected for consolidation. Each alloy powder was consolidated by cold compaction, canning, degassing for 1 h at 673 K down to 10v3 torr and then by hot extrusion after 1 h holding at 673 K with a reduction ratio of 25:l into a bar with a diameter of 10 mm. None of the extruded bars showed blistering or cracking on the surface. The microstructure of extruded bars was examined by using a scanning electron microscope after conventional procedure of polishing and etching in Keller’s reagent. For TEM, thin foil specimens were prepared by ion beam milling after mechanical grinding of extruded bars. The microstructure of the specimens was observed by using a CM20 transmission electron microscope operating at 200 keV. The crystal structures of the constituting phases in the alloy powders and extruded bars were determined by XRD. XRD traces were obtained with monochromatic CuKcl radiation over a 20 range of 10-70”. Mechanical properties of the extruded bars were evaluated by tensile and wear tests. Alloy bars were machined into small tensile specimens in accordance with ASTMA370 and tensile testing was performed at 300, 473 and 573 K with a cross head speed of 0.2 mm min- ’ using an Instron 4206 machine. For wear testing, plate type specimens were polished and mounted on an Ohgoshi-type wear testing machine. In Ohgoshi wear testing, ring-shaped material (AISI 1045) is pressed and rotated against a specimen’s surface, forming a wear track. During the test, the sliding distance and load were kept constant at 100 m and 2.1 kg, respectively. Also coefficients of thermal expansion of each alloys were measured using a dilatometer during heating from room temperature to 673 K with a heating rate of 10 K min- ‘.
(1997) 878-882
879
3. Results and discussion 3.1. Microstructure Fig. l(a) and (b) shows typical scanning electron micrographs of Al-20Si-5Fe-2Zr and Al-20Si-5Fe2Cr alloys, respectively. Both alloys show a microstructure consisting of two different sizes of both Si particles and elongated particles distributed in an Al matrix. Typical sizes of large Si and small Si particles are about 3 and 0.7 pm, respectively. The typical length of large elongated particles is about 3 pm. Due to severe shear deformation during hot extrusion, the large elongated intermetallic 6 -Al,FeSi, compound observed in as-solidified powder was fragmented onto smaller elongated particles [9]. During gas atomization of the Al-20Si-5Fe alloy, a liquid stream of molten metal was disintegrated into small particles by high-velocity gas jet and then solidified rapidly -by a surrounding gas. With decreasing powder size, the cooling rate increases and the microstructure becomes finer. For powders with the size of about X0 pm, solidification occurs in the following sequences PI: (1) nucleation and growth of
Fig. 1. SEM images of (a) Al-20Si-5Fe-2Zr 2Cr alloys.
and (b) Al-20Si-5Fe-
880
S.J. Hong
et al. /Materials
Science
,....
*--to “=. ..
20
30
40 50 60 28(Deg.) _~ _~-_.._~~_
Fig. 2. XRD trace taken from Al-20Si-5Fe-2Cr analyzed results.
70
So
A226-228
(1997)
878-882
Figs. 4(a) and (b) show EDX traces from elongated particles in the Al-20Si-5Fe-2Cr and Al-20Si-5Fe2Zr alloy bars, respectively. EDX trace from elongated particle in the Al-20Si-5Fe-2Cr alloy consists of peaks corresponding to Al, Si, Fe and Cr, indicating that some of the Cr is dissolved into the intermetallic compound. However, EDX trace from particle in the Al-20Si-5Fe-2Zr alloy shows that Zr is not dissolved into the intermetallic compound, as can be seen in Fig. 4(b)3.2. Tensile properties
alloy bar with
metastable 6-Al,FeSi, particles; (2) nucleation and growth of primary Si; and finally (3) metastable eutectic solidification of remaining liquid into a-Al + 5Al,FeSi, + Si. The large Si particles in extruded bars correspond to primary Si in as-solidified powders. The small Si particles are the product of coarsening of Si in the eutectic structure during degassing and hot extrusion. Fig. 2 shows an XRD trace taken from the Al-20Si5Fe-2Cr alloy bar with analyzed results. Most diffraction peaks in the XRD pattern could be analyzed into a mixture of f,c.c. a-Al, diamond cubic Si and monoclinic /3-Al,FeSi phases. The XRD analysis indicated that tetragonal 6-Al,FeSi, transformed into a stable monoclinic ,8-Al,FeSi during degassing and the hot extrusion process. TEM examination reveals that the microstructure of extruded alloys is grossly inhomogeneous, due to difference in initial powder size and microstructural variation within an as-solidified powder. Fig. 3(a) and (b) shows bright-field images of TEM micrographs taken from coarse and tie scale regions in the Al-20Si-5Fe-2Zr alloy bar, respectively. The TEM micrograph from the coarse region shows a microstructure consisting of elongated large p-Al,FeSi particles as marked by A and about 0.7 ym sized Si particles. The TEM micrograph from the fine scale region shows a microstructure consisting of about 1.5 pm sized a-Al grains, about 0.2 ym sized particles with internal planar defects as marked by arrows and 0.05 pm sized fine particles distributed in a-Al matrix. Selected area electron diffraction patterns from particles with planar defects could be analyzed into monoclinic ,8-A1,FeSi. The small ,6 particles seem to be the product transformed from particles in Al-SiFe metastable eutectic structure in as-solidified microstructure during degassing and hot extrusion, A similar microstructure was observed in Al-20Si-5FeCr alloys. The alloy microstructure showed two differ-
ent sizes of ,8 and Si particles distributed
and Engineering
in Al matrix.
Fig. 5 shows room-temperature and high-temperature tensile properties of the Al-20Si-5Fe-2X alloy bars. The tensile properties of Al-20Si and Al-20Si5Fe alloys, manufactured by the same process l9], was also included for comparison. By adding 5 wt.% Fe to an Al-20Si alloy, the tensile strength increased from 215 to 337 MPa at 300 K, from 112 to 205 MPa at 473 K and from 80 to 130 MPa at 573 K. Cr and Zr addition to Al-20Si-5Fe alloys increased tensile strength further with losing some elongation. Tensile
Fig. 3. Bright-field images of TEM micrographs taken from (a) coarse scale and (b) fine scale regions in Al-20Si-5Fe-2Zr alloy.
S.J. Hong
et al. /Materials
Science
and Engineering
A226-228
(1997)
881
878482
-5.0 -
r Fe
1
10
8
1
2
3
Sliding speed (m&c) Fig. 6. Variations of specific wear of Al-20Si-5Fe-2X Zr) alloys as a function of sliding speed.
Energy
(eV)
Fig. 4. EDX traces from particles with internal planar defects in (a) Al-20Si-5Fe-2Cr and (b) Al-20Si-5Fe-2Zr alloy bars.
strengths of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys decreased from 375 to 160 MPa and from 387 to 155 MPa, respectively, with increasing testing temperature from 300 to 573 K. 3.3. Wear property and themal
expansion
Fig. 6 shows variations of specific wear results of Al-20Si-5Fe-2X alloys as a function of sliding speed from 0.5 to 3.6 m s-l. The wear resistance of Al-20Si and Al-20Si-5Fe alloys, manufactured by the same process [9], was also included for comparison. By adding 5 wt.% Fe to the Al-20Si alloy, the specific wear decreased significantly at all sliding speeds. The increase in wear resistance results from P-Al,FeSi particles embedded in Al matrix.
1
*I
(X = 0, -.,
The specific wear increased gradually with increasing sliding speed in all alloys due to softening of material arising from temperature increase. The effects of Cr or Zr addition on the wear resistance were not significant, even though sliding speed dependence of specific wear in quaternary alloys containing Cr or Zr was weaker than in Al-20Si and Al-20Si-5Fe alloys. In this study, the Ohgoshi wear test was used to evaluate wear properties. During the test, the contact area between specimen and counterpart material was large. Fine particles therefore formed in quaternary alloys may not contribute to increase in wear resistance even though they can improve tensile strength. However, if the pin test is used to test wear property, the quaternary alloy may show better wear properties. In reality, wear resistance is dependent very much on the type of test and test conditions. Table 1 shows coefficients of thermal expansion (CTE) of Al-20Si-5Fe-2X (X = Cr and Zr) alloys. For comparison, the CTE of Al-20Si alloy, manufactured by the same process, was included. The CTE values of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr were 16.6 x 10m6 and 17.5 x 10W6 K-l, respectively, both of which are smaller than that of the Al-20Si alloy.
4. Conclusion
The microstructure geneous distribution
of extruded bars showed a homoof two different sizes of both ,8
Table 1 Coefficients of thermal expansion (CTE) of extruded bars L
Fig. 5. Room-temperature and high-temperature tensile properties of extruded bars: A, 3, C and D correspond to Al-20Si, Al-20Si-SFe, Al-20Si-5Fe-2Zr and Al-20Si-5Fe-2Cr, respectively.
Alloy
CTE (10W6 K-‘)
Al-20Si Al-20Si-5Fe-2Cr Al-20Si-5Fe-2Zr
18.2 16.6 17.5
882
S.J. Hong et al. /Materials
Science and Engineering A226-228
and Si particles embedded in an a-Al matrix. Tensile strengths of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys were 380 and 390 MPa, respectively, at room temperature, which were 12% higher than that of the Al-20Si-5Fe alloy. However, the effects of Cr or Zr addition on the wear resistance were not significant. The coefficients of thermal expansion of Al-20Si5Fe-2Cr and Al-20Si-5Fe-2Zr were 16.6 x 10m6 and 17.5 x 10V6 K-l, respectively, both of which are smaller than that of the Al-20Si alloy.
References [l] M.D. Hanna, S. Lu and A. Hellawell, MerulE. Trans. A, 15 (1984) 459.
[2] [3] [4] [5]
(1997) 878-882
F. Yilmaz and R. Elliot, J. Mater. Sk, 24 (1989) 2065. H.M. Skelly and C.F. Dixon, Powder Metall., 10 (1976) 232. J.L. Estrada and J, Duszczyk, J. Filer, Sci., 25 (1990) 886. Z. Zhou, J. Duszczyk and B.M. Korevaar, J. Mater. Sci., 26 (1991) 824. [6] S. Das, A.H. Yegneswaran and P.K. Rohatgi, J. Mater. Sci., 22 (1987) 3173. [7] K. Kondoh, T. Kaji, T. Hayashi and Y. Takeda, Advances in Powder Metallurgy and Particulate Materials, Vol. 2, Metal Powder Ind. Federation, 1992, p. 339. [8] Y. Takeda, T. Hayashi, Y. Odani, N. Amano and N. Kuroishi, Modern Dev. Powder MetaN., 18-21 (1988) 533. 191 B.S. Chun, S.S. Cho, S.J. Hong, T.S. Kim, W.T. Kim and C.W. Won, in F.H. Froes (Ed.), Synthesis-Processing of Lightweight Metallic Materials, TMS, Warrendale, PA, 1995, p. 285. [lo] N. Amano et al., Metal Powder Rep. (Mar. 1983) 186. [ll] R.M. Gomes, T. Sate, H. Tezuka and A. Kamio, Met. Sci. Forum, 217-222 (1966) 789.
Materials Science and Engineering A226-228
A
(1997) 878482
The effects of Cr and Zr addition on the microstructure and mechanical properties of rapidly solidified Al-20Si- 5Fe alloys S.J. Hong a, T.S. Kim a, W.T. Kim b,*, B.S. Chun a a Engineering
Research
Center for b Department
Rapidly
Materials, Department of Met. Eng., Chungnam National University, Taejeon 305-764, South Korea Chotigiu UniueAy, 36 Naeaokaong, Chongiu 360-764, Sorrtk Korea
Soli@ed
of Physics,
220 Gungco17g,
Abstract The effectsof Cr and Zr addition on mechanicalpropertiesof Al-20Si-5Fe alloys, manufacturedby gasatomization followed by :degassingand hot extrusion, were studied by a combination of scanning electron microscopy, transmissionelectron microscopy,energy dispersiveX-ray microanalysis,X-ray diffractometry, tensiletesting and wear testing. The microstructure of extruded bars showeda homogeneous distribution of two different sizesof monoclinic ,B-Al,FeSi and Si particles embeddedin an a-Al matrix. Tensilestrengthsof Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys at room temperaturewere 375 and 387
MPa, respectively, which were about 12% higher than that of the Al-20Si-5Fe
alloy. However, the effects of Cr or Zr addition
on the wear resistancewere not significant.Coefficientsof thermal expansionof Al-2OSi-SFe-2Cr and Al-20Si-5Fe-2Zr were 16.6x 10P6 and 17.5x 10m6K-l, respectively,which are smallerthan that of the Al-20Si alloy. 0 1997Elsevier ScienceS.A. Keywords:
Coefficient of thermal expansion; Rapid solidification; Wear-resistant alloy
1. Introduction Hypereutectic Al-Si
alloys are used in many appli-
cation areas in automotive and electronic industries due to their good wear resistance and low coefficient of thermal expansion. These properties are attributed to the high volume fraction of hard Si particles embedded in an Al matrix. However, the alloys show poor machinability, low thermal resistance and low fatigue strength. It is common to increase the Si content to improve the wear resistance and the mechanical strength. However, primary Si crystals become coarser, with the increase of Si content above the eutectic composition, resulting in deteriorating properties. Several different techniques have been applied to refine the primary Si crystals such as modification treatment [1,2], addition of ternary alloying element [3] or rapid solidification technique [4-111. Due to the recent development of technique for consolidation of Al alloy powders [7,8], rapid solidification of
hypereutectic Al-Si alloy seems to be the most promising route to improve wear resistance and strength. It has been reported that Fe is also effective in improving wear resistance by forming an Al-SiFe intermetallic compound [9] and high-temperature strength by forming line dispersoids in an Al matrix. However, large acicular 6-Al,FeSi, particles formed during the gas atomization of the ternary alloy system may cause a problem in consolidating powders. Therefore we have tried to modify microstructure by adding quaternary elements. Cr and Zr were selected as candidates. Zr may act as a grain refiner. Cr was reported to reduce primary Si size during the rapid solidification [lo]. This study was carried out to find out the effects of Cr and Zr addition on the microstructure and mechanical properties of Al-20SiLSFe alloys by using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray microanalysis (EDX), X-ray diffractometry
(XRD),
temperatures, * Corresponding author. 0921~5093/97/%17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(96)10811-X
tensile test at room
and high
and wear testing. The coefficients of
thermal expansion of the alloys were also measured.
S.J. Hong et al. /Materials
2. Experimental
Science and Engineering A226-228
procedure
Master alloys of Al-20Si-5Fe-2X (X = Cr, Zr) were manufactured by induction melting of commercial pure metals in graphite crucibles in air. Al20Si-5Fe-2X alloy powders were manufactured by remelting the master alloy in a graphite crucible 200 K above the liquidus temperature of each alloy, pouring into a tundish and then bottom pouring through a graphite melt delivery nozzle of 3 nun in diameter to an annulus N2 gas atomizer operating with a pressure of 0.8 MPa. The melt flow rate, estimated from operating time and from the weight of atomized melt, was about 1.2 kg min- ‘. The gas flow rate, calculated from the gas consumption rate, was about 0.8 N*m3 min-‘. Size distribution of alloy powders was measured by conventional sieving analysis. Alloy powders with size between 61 and 120 pm were selected for consolidation. Each alloy powder was consolidated by cold compaction, canning, degassing for 1 h at 673 K down to 10v3 torr and then by hot extrusion after 1 h holding at 673 K with a reduction ratio of 25:l into a bar with a diameter of 10 mm. None of the extruded bars showed blistering or cracking on the surface. The microstructure of extruded bars was examined by using a scanning electron microscope after conventional procedure of polishing and etching in Keller’s reagent. For TEM, thin foil specimens were prepared by ion beam milling after mechanical grinding of extruded bars. The microstructure of the specimens was observed by using a CM20 transmission electron microscope operating at 200 keV. The crystal structures of the constituting phases in the alloy powders and extruded bars were determined by XRD. XRD traces were obtained with monochromatic CuKcl radiation over a 20 range of 10-70”. Mechanical properties of the extruded bars were evaluated by tensile and wear tests. Alloy bars were machined into small tensile specimens in accordance with ASTMA370 and tensile testing was performed at 300, 473 and 573 K with a cross head speed of 0.2 mm min- ’ using an Instron 4206 machine. For wear testing, plate type specimens were polished and mounted on an Ohgoshi-type wear testing machine. In Ohgoshi wear testing, ring-shaped material (AISI 1045) is pressed and rotated against a specimen’s surface, forming a wear track. During the test, the sliding distance and load were kept constant at 100 m and 2.1 kg, respectively. Also coefficients of thermal expansion of each alloys were measured using a dilatometer during heating from room temperature to 673 K with a heating rate of 10 K min- ‘.
(1997) 878-882
879
3. Results and discussion 3.1. Microstructure Fig. l(a) and (b) shows typical scanning electron micrographs of Al-20Si-5Fe-2Zr and Al-20Si-5Fe2Cr alloys, respectively. Both alloys show a microstructure consisting of two different sizes of both Si particles and elongated particles distributed in an Al matrix. Typical sizes of large Si and small Si particles are about 3 and 0.7 pm, respectively. The typical length of large elongated particles is about 3 pm. Due to severe shear deformation during hot extrusion, the large elongated intermetallic 6 -Al,FeSi, compound observed in as-solidified powder was fragmented onto smaller elongated particles [9]. During gas atomization of the Al-20Si-5Fe alloy, a liquid stream of molten metal was disintegrated into small particles by high-velocity gas jet and then solidified rapidly -by a surrounding gas. With decreasing powder size, the cooling rate increases and the microstructure becomes finer. For powders with the size of about X0 pm, solidification occurs in the following sequences PI: (1) nucleation and growth of
Fig. 1. SEM images of (a) Al-20Si-5Fe-2Zr 2Cr alloys.
and (b) Al-20Si-5Fe-
880
S.J. Hong
et al. /Materials
Science
,....
*--to “=. ..
20
30
40 50 60 28(Deg.) _~ _~-_.._~~_
Fig. 2. XRD trace taken from Al-20Si-5Fe-2Cr analyzed results.
70
So
A226-228
(1997)
878-882
Figs. 4(a) and (b) show EDX traces from elongated particles in the Al-20Si-5Fe-2Cr and Al-20Si-5Fe2Zr alloy bars, respectively. EDX trace from elongated particle in the Al-20Si-5Fe-2Cr alloy consists of peaks corresponding to Al, Si, Fe and Cr, indicating that some of the Cr is dissolved into the intermetallic compound. However, EDX trace from particle in the Al-20Si-5Fe-2Zr alloy shows that Zr is not dissolved into the intermetallic compound, as can be seen in Fig. 4(b)3.2. Tensile properties
alloy bar with
metastable 6-Al,FeSi, particles; (2) nucleation and growth of primary Si; and finally (3) metastable eutectic solidification of remaining liquid into a-Al + 5Al,FeSi, + Si. The large Si particles in extruded bars correspond to primary Si in as-solidified powders. The small Si particles are the product of coarsening of Si in the eutectic structure during degassing and hot extrusion. Fig. 2 shows an XRD trace taken from the Al-20Si5Fe-2Cr alloy bar with analyzed results. Most diffraction peaks in the XRD pattern could be analyzed into a mixture of f,c.c. a-Al, diamond cubic Si and monoclinic /3-Al,FeSi phases. The XRD analysis indicated that tetragonal 6-Al,FeSi, transformed into a stable monoclinic ,8-Al,FeSi during degassing and the hot extrusion process. TEM examination reveals that the microstructure of extruded alloys is grossly inhomogeneous, due to difference in initial powder size and microstructural variation within an as-solidified powder. Fig. 3(a) and (b) shows bright-field images of TEM micrographs taken from coarse and tie scale regions in the Al-20Si-5Fe-2Zr alloy bar, respectively. The TEM micrograph from the coarse region shows a microstructure consisting of elongated large p-Al,FeSi particles as marked by A and about 0.7 ym sized Si particles. The TEM micrograph from the fine scale region shows a microstructure consisting of about 1.5 pm sized a-Al grains, about 0.2 ym sized particles with internal planar defects as marked by arrows and 0.05 pm sized fine particles distributed in a-Al matrix. Selected area electron diffraction patterns from particles with planar defects could be analyzed into monoclinic ,8-A1,FeSi. The small ,6 particles seem to be the product transformed from particles in Al-SiFe metastable eutectic structure in as-solidified microstructure during degassing and hot extrusion, A similar microstructure was observed in Al-20Si-5FeCr alloys. The alloy microstructure showed two differ-
ent sizes of ,8 and Si particles distributed
and Engineering
in Al matrix.
Fig. 5 shows room-temperature and high-temperature tensile properties of the Al-20Si-5Fe-2X alloy bars. The tensile properties of Al-20Si and Al-20Si5Fe alloys, manufactured by the same process l9], was also included for comparison. By adding 5 wt.% Fe to an Al-20Si alloy, the tensile strength increased from 215 to 337 MPa at 300 K, from 112 to 205 MPa at 473 K and from 80 to 130 MPa at 573 K. Cr and Zr addition to Al-20Si-5Fe alloys increased tensile strength further with losing some elongation. Tensile
Fig. 3. Bright-field images of TEM micrographs taken from (a) coarse scale and (b) fine scale regions in Al-20Si-5Fe-2Zr alloy.
S.J. Hong
et al. /Materials
Science
and Engineering
A226-228
(1997)
881
878482
-5.0 -
r Fe
1
10
8
1
2
3
Sliding speed (m&c) Fig. 6. Variations of specific wear of Al-20Si-5Fe-2X Zr) alloys as a function of sliding speed.
Energy
(eV)
Fig. 4. EDX traces from particles with internal planar defects in (a) Al-20Si-5Fe-2Cr and (b) Al-20Si-5Fe-2Zr alloy bars.
strengths of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys decreased from 375 to 160 MPa and from 387 to 155 MPa, respectively, with increasing testing temperature from 300 to 573 K. 3.3. Wear property and themal
expansion
Fig. 6 shows variations of specific wear results of Al-20Si-5Fe-2X alloys as a function of sliding speed from 0.5 to 3.6 m s-l. The wear resistance of Al-20Si and Al-20Si-5Fe alloys, manufactured by the same process [9], was also included for comparison. By adding 5 wt.% Fe to the Al-20Si alloy, the specific wear decreased significantly at all sliding speeds. The increase in wear resistance results from P-Al,FeSi particles embedded in Al matrix.
1
*I
(X = 0, -.,
The specific wear increased gradually with increasing sliding speed in all alloys due to softening of material arising from temperature increase. The effects of Cr or Zr addition on the wear resistance were not significant, even though sliding speed dependence of specific wear in quaternary alloys containing Cr or Zr was weaker than in Al-20Si and Al-20Si-5Fe alloys. In this study, the Ohgoshi wear test was used to evaluate wear properties. During the test, the contact area between specimen and counterpart material was large. Fine particles therefore formed in quaternary alloys may not contribute to increase in wear resistance even though they can improve tensile strength. However, if the pin test is used to test wear property, the quaternary alloy may show better wear properties. In reality, wear resistance is dependent very much on the type of test and test conditions. Table 1 shows coefficients of thermal expansion (CTE) of Al-20Si-5Fe-2X (X = Cr and Zr) alloys. For comparison, the CTE of Al-20Si alloy, manufactured by the same process, was included. The CTE values of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr were 16.6 x 10m6 and 17.5 x 10W6 K-l, respectively, both of which are smaller than that of the Al-20Si alloy.
4. Conclusion
The microstructure geneous distribution
of extruded bars showed a homoof two different sizes of both ,8
Table 1 Coefficients of thermal expansion (CTE) of extruded bars L
Fig. 5. Room-temperature and high-temperature tensile properties of extruded bars: A, 3, C and D correspond to Al-20Si, Al-20Si-SFe, Al-20Si-5Fe-2Zr and Al-20Si-5Fe-2Cr, respectively.
Alloy
CTE (10W6 K-‘)
Al-20Si Al-20Si-5Fe-2Cr Al-20Si-5Fe-2Zr
18.2 16.6 17.5
882
S.J. Hong et al. /Materials
Science and Engineering A226-228
and Si particles embedded in an a-Al matrix. Tensile strengths of Al-20Si-5Fe-2Cr and Al-20Si-5Fe-2Zr alloys were 380 and 390 MPa, respectively, at room temperature, which were 12% higher than that of the Al-20Si-5Fe alloy. However, the effects of Cr or Zr addition on the wear resistance were not significant. The coefficients of thermal expansion of Al-20Si5Fe-2Cr and Al-20Si-5Fe-2Zr were 16.6 x 10m6 and 17.5 x 10V6 K-l, respectively, both of which are smaller than that of the Al-20Si alloy.
References [l] M.D. Hanna, S. Lu and A. Hellawell, MerulE. Trans. A, 15 (1984) 459.
[2] [3] [4] [5]
(1997) 878-882
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