- Email: [email protected]
Investigation of the aluminium-rich corner in the Al-Si-W phase diagram
Scripta Metallurgica
et Materialia, Vol. 32, No. 9. pp. 1423-1428.1995 Copyright Q 1995 Elsewier Science Ltd Printed in the USA. AU rights reserved 0956-716X/95 $9.50 + .OO
0956-716X(95)00182-4
INVESTIGATION
OF THE AL UMINIUM-RICH
CORNER IN THE Al-Si-W PHASE DIAGRAM
H.-H.Angermann, 0. Van Der Biest, L.Froyen Department Metallurgy and Materials Engineering, KU. Leuven, de Croylaan 2, B-3001 Heverlee, Belgium (Received September 21, 1994) (Revised November 23, 1994) Introduction Aluminium alloys are interesting materials because of their high specific physical and mechanical properties /I/. Discontinuous reinforced aluminium composites have the additional advantage that they offer to the materials designer the possibility to tailor the properties by selecting the volume fraction, type and/or morphology of the reinforcement phase. However, one hindrance for their broad application, e.g. in the automotive industry, are the high costs for t.he processing of the alloys. The most costeffective way to produce composites in high volumes is via the molten-metal route /2,3/. Especially interesting is the manufacture of discontinuous reinforcements in situ in the molten aluminium /4/. This can principally be achieved by heating a mechanical mixture of elementary components in the desired stochiometry and let the reinforcing particles form. However, in order to have control of the microstructure via temperature and alloy composition the phase diagram must be known. In the present work the Al-rich comer of the Al-Si system with the refractory metal W was investigated at 1OOO’C. The information available for this ternary system refers to 1500°C and to higher Si-and W-contents /5/. The phases which had fomted after holding at 1000°C were used to construct an isothermal section of the phase diagram at this temperature. Also the compounds which precipitate from the Al-melt during cooling are described. Possible applications of the alloys are discussed. Exnerimental Procedure As initial components, aluminiun. in block-form ( Pechiney, France, 99.999% ), silicon in wafer form ( IMEC, Belgium, 99.999% ) and the refractory metal tungsten in powder form ( Reframet Hoboken, grade 425-A, 99.99% ) were used. Seven different compositions of Al-Si-W were chosen which are compiled in atom and weight percent in TABLE 1. Sample 1 to 4 contained a constant amount of 3.5 a/o W while the silicon contents stepwise increased. For the alloys 5~to 7 the W- and/or the Si-fraction has been considerably increased. TABLE 1 : Composition of the Seven Alloys in Atom and Weight Percent ( bal : balance ) alloy
t
1 2 3 4 5 6 7
a/O
Al bal bal bal bal bal bal bal
Si 1.5 5.2 8.1 17.5 34.1 16.2 3.0
1423
WI0
W 3.5 3.5 3.5 3.5 12.7 7.0 12.7
Al bal bal bal bal bal bal bal
Si 1.3 4.5 7.0 15.0 20.0 11.9 1.8
W 20.0 20.0 20.0 20.0 49.8 33.7 49.8
1424
Al-%-W PHASE DIAGRAM
Vol. 32, No. 9
For heat treatment the elementary components were introduced into an Al203-tube with a 10 mm inside diameter. About 3 g of material was used for every experiment. The alloys were inductively heated within 30 minutes to 1000°C and held there for two hours under a protective argon atmosphere. The tip of a thermocouple, protected by a thin A1203-tube, was located in the melt about 1 mm near the bottom of the crucible. After the heat treatment the alloys were air cooled in the crucible. For metallographic investigation, the polished samples were etched 5s with Kellers reagent. A microprobe ( Jeol Superprobe 733 ) was used to quantify and/or identify the phases ( Al K-, Si K- and W L-lines; ZAP correction )_ Because the alloy compositions were located in the Al-rich comer of the ternary AI-Si-W system, the amount of phases was often small so that only the ternary T-phase W(AlxSil_x)2, the binary W(AlxSil_x)2 with WSi2-structure and Al4W could be determined by X-ray diffraction ( Phillips PW 1010 ). Light microscopy provided an overview about the morphology of the microstructure. Results and Discussion Morphology of the Alloys The alloys 1 to 4 each contained 3.5 a/o of tungsten and exhibited roughly the same microstructure. W-containing reaction products formed a cluster at the bottom of the crucibles. This effect was caused by the large density difference between the liquid aluminium and the W-powder and/or W-containing sedimentations. The region covered about 15 to 25% of the sample volume. Above the cluster there was an Al-matrix with some precipitates which formed during cooling down from 1000°C. It is probable that the cooling down was so rapid that these precipitates could not settle at the bottom of the crucible. For sample 5 and 6, which contained 12.7 and 7.0 a/o tungsten, respectively, the W containing precipitates within the cluster covered about 50% of the sample volume. In contrast, alloy 7 with 12.7 a/o W but much less Si as the alloys 5 and 6, was almost totally composed of a sintered network of a tungsten-containing compound with Al filling the voids. The reason for the formation of particles at 1000°C was an oversaturation of the Al-melt with the transition metal W. In the binary, liquid Al can dissolve at 1000°C only about 2 a/o W. Regarding the chosen amount of at least 3.5 a/o W and the fact that the addition of a third element normally lowers the solubility of the second one in the melt, it can be assumed that the solubility limit of the Al-melt was exceeded with respect to tungsten. In contrast, the solubility of Si in Al at 1000°C amounts to about 46 a/o whereas maximum 34.1 a/o Si were added in the ternary system. Metallographic investigation of the samples showed also no indication for a primary Si-precipitation. The identity and the composition of the W-containing particle cluster was used as data for construction of the Al-rich comer of the Al-Si-W system at 1OOO’C. Figure la to If depict the transition region between the particle cluster at the bottom and the Al-matrix above for the samples 1 to 6. Figure Ig presents the continuous network of particles obtained for alloy 7. There could be three different precipitates found in the Al-matrix : a plate shaped and an angular one, both containing W, and needle to flake shaped Si particles from the binary AI-B eutectic. The Al-Si-W Phase Diagram in the Al-Rich Comer In Figure 2, the Al-rich comer of the Al-Si-W phase diagram is shown at 1000°C as deducted from the experiments. The location of the samples 1 to 7 is indicated. T represents the ternary W(Al,Sil_x)2-phase with CrSi2-crystal structure and E the binary A14W /5/. W(AlxSil_,)2 depicts the binary WSi2 compound, where some of the Si is substituted by Al. The 6- and the y-phases, which stand for A15W and All2W. respectively, may form during cooling down for temperatures ~871 and <697”C, respectively /6/. In the following, the microstructure of the samples 1 to 7 will be discussed by means of Figure 2.
Vol. 32, No. 9
AI-S-W PHASE DIAGRAM
1425
Alloy 1 ( A11.5Si3.5W, Figure la ) was the alloy which contained the least silicon. The particles at the bottom, which were formed during the holding time at lOOO”C, were solely composed of the coarse angular a-phase Al4W. The particles were strongly sintered together and formed a homogeneous network with Al filling the voids. It is concluded that alloy 1 falls into a two-phase region where E is in equilibrium with the Al-melt. In alloy 2 (A15.2Si3.5W, Figure lb ) and alloy 7 (A13.OSi12.7W, Figure lg ) there were additionally a few small areas where particles were grouped together in a small cluster of particles having a smaller size as Al4W. They were identified by microprobe as the ternary r-phase W(AlxSil_x)2 with CrSi2-structure. The x-value amounted to x=0.60. The formation of this compound was obviously caused by the increased silicon content in both alloys with respect to alloy 1. Alloy 2 and 7 fall into a 3-phase region where the T-phase with x=0.60 is equilibrium with Al4W and the Al-melt. Because of the small amount of r-phase it is concluded that the separation line between the twoand three phase region is close to the compositions of alloy 2 and 7. It should be noted that in sample 7 with its considerably increased W-content the network of Al4W occupied practically the whole sample volume.
In the Al-matrix above the particle clusters in alloy 1, 2 and 7 there were some angular isolated Al.,W-precipitates and plate shape’d 5 to 15 pm thin compounds. The latter were determined by microprobe to be composed of (86 f 1 ) a/o and (14 + 1) a/o W. It is suggested that the platelike phase is identical with A&W. The 3 a/o overshoot of Al measured by microprobe was probably due to a co-penetration of the surrounding Al-matrix by the e--beam. According to the Al-W-equilibrium diagram Al5W forms via a peritectic reaction from the melt and Al4W. However, the metallographic investigation of the samples did not indicate a formation of the AlgW-plates through peritectic reaction. It is assumed that the air cooling was so fast ( from 1000 to 600°C in about 90s ) that the peritectic reaction was suppressed or that the reaction zone was too small to be detectable. In alloy 3 (Al8 lSi3SW, Figure lc) the silicon content was raised huther. The cluster of sedimentations at the bottom of the tube was now solely composed of the ternary r-phase again with x=0.60. In contrast to AhW the W(Al,Si&-particles were not or only little sintered together. This was probably a reflection of a much higher currently unknown melting point of the W(AIXSi& r-phase than of AldW (perhectic decomposition at 1327’C). The r-phase was reported to be stable up to at least 155O’C /7/. In the Al-matrix above the cluster there were again plateshaped A&W &precipitates and angular particles which proved to be the r-phase (x=0.53). Because of the presence of the r-phase and the absence of A&W-particles in the cluster at the bottom of the sample it is believed that the composition of sample 3 falls within a two-phase r-Al-melt region in the phase diagram (Figure 2). The location of the #separating line between the 3-phase r-~-Al and 2-phase &-Al region is not totally clear, but because of the x-value of 0.60 of the r-phase, which corresponded to alloy 2, it is assumed that the nominal composition of sample 3 is close to the separation line and falls just into the two phase region. Alloy 4 ( Al17.5Si3.5W, Figure Id ) contained the most silicon of the samples 1 to 4 which had a constant Wcontent of 3.5 :1/o. As for sample 3 the cluster at the bottom was composed of W(Al,Sil_,)2 z-phase but the xvalue shifted to 0.38, i.e. towards higher Si-contents. In the Al-matrix above the cluster there was one angular Wcontaining precipitate which was identified as All2W y-phase. There were no longer any plateshaped AlgWparticles. A112W was more homogeneously distributed in the matrix than the other precipitates due to its lower Wcontent and therefore lower density. A similar microstructure was found in alloy 6 (Al16.2Si7.OW, Figure If). The cluster was comlposed also of the z-phase with x-values between 0.38 and 0.49. In the Al-matrix there were angular particles which proved to be the r-phase with x=0.41. Both alloy 4 and alloy 6 were located within the two-phase rAl-region of the phase-diagram ( Figure 2 ) just like alloy 3. Alloy 5 ( A134.l.Si12.7W, Figure le ) had the highest W-and Si-content of all alloys studied here. The cluster at the bottom of the sample was composed of 10 to 50 urn small angular particles which were slightly sintered together.
1426
AM-W PHASEDIAGRAM
Vol. 32. No. 9
With microprobe they were identified as W(AIxSil_x)2 with x=0.1 1. This phase was crystallographically identical with the binary WSi2, but about 8 a/o of the Si were substituted by aluminium. In the Al-matrix there were angular W(AlxSiI_x)2 particles ( WSi2-structure) with x=0.15 ( 10 a/o Al). Alloy 5 was located within a two-phase W(Al,SiI_,)2 ( WSi2-structure )-Al-melt region. Comments on the Application of the Alloys The W-content of the Al-Si-W alloys in this work was chosen > 3.5a/o. The purpose was to exceed the solubility limit of W so that precipitates had formed after annealing at 1000°C from which the phase diagram could be derived. If a materials designer is aiming at discontinuous reinforced Al-Si-W alloys for structural application he has to chose lower W-contents or higher annealing temperatures so that the W is thoroughly dissolved in the Al melt. The particles form during cooling down. The experiments in this work showed that these precipitates do not sediment during air cooling. Higher W-contents must be avoided in order to maintain the density advantage of Alalloys. Another application of alloys from the Al-rich comer of the Al-Si-W system may be as wear resistant material. The ternary z-phase W(Al,Sil_,)2 could function as the phase with high wear resistance because it combines high hardness with little tendency to coarsen and to sinter together during processing. The particles have to be precipitated during cooling down of the melt. The microstructure is similar to a commercially refined hypereutectic Al-Si alloy /8/, where the Si-particles would be replaced by the z-phase. Conclusions The Al-rich comer of the Al-Si-W phase diagram was investigated up to maximum 12.7 a/o, respectively. The alloys were inductively heated in alumina crucibles to hours and subsequently air cooled. Due to the low solubility of W in the Al-melt, had formed at the bottom of the crucibles after the holding time. Their identity construct the Al-rich comer of the Al-Si-W phase diagram at 1OOO’C.
Si-and W-contents of 34.1 and 1000°C in argon, held for two W-containing particle clusters and composition was used to
Nearest to the Al-comer of the Al-Si-W phase diagram and the molten Al phase was a two-phase region composed of the A14W a-phase and the Al-melt. For more elevated Si-and W-contents an alloy was located either in a three phase &-z-Al melt or two-phase r-Al melt region. The r-phase was the ternary W(AlxSil_x)2-compound with CrSi2 crystal structure. The r-phase was angular like AlqW, but not so course and tended much less to sinter together to a particle network. This was associated with a probably much higher melting point of the r-phase with respect to E. The maximum value of x for the W(Al,SiI_,)2 r-phase in the two-and three-phase equilibria with E and the Almelt at 1000°C amounted to 0.60. For higher Si-contents above about 18 a/o and W above 2 a/o the pseudobinary W(Al,Sil_,)2 with WSi2-structure was formed. The particles which were formed during air cooling through precipitation from the Al-melt were also investigated. Angular Al4W, platelike AlgW, the angular r-phase and W(AlxSil_x)2 with WSi2 were found according to the respective Si-and W-content of the alloy. These particles were fairly well distributed in the Al-matrix. The All2Wparticles with their lowest W-content of all precipitations exhibited the most homogeneous distribution in the Almatrix. A possible commercial application of the alloys of the Al-Si-W system is as discontinuous reinforced material for structural application or as wear resistant material. However, the amount of W added must be limited in order to maintain the low density advantage of the Al-based alloy.
Vol. 32. No. 9
1. 2.
3. 4. 5. 6. I. 8.
Al-S-W PHASEDIAGRAM
1427
Y.Kurih.ara, J. of Mat. 46 5, 12, ( 1994 ) J.E.Alli:jon and GSCole, J. of Mat. 45 1, 19, ( 1993 ) TSritharan, K. Xia, J. Heathcock and J. Mihelich, in Met. & Cer. Matrix Comp. : Processing, Modelling PSahoo and M.J.Koczak, Mat. Sci. and Eng., A131.69 ( 1991 ) Ternary Alloys Vol.8, ed. by G.Petzow and G.Effenberg; VCH Verlagsgesellschaft, Weinheim, Germany ( 1993 ) Binary ,411oys Phase Diagram ed.-in-chief T.B.Massalski; ASM Intern., Materials Park, USA, ( 1990) H.Nowotny, C.Brukl and F.Benesovsky, Monatsh. Chemie 92, 116,( 1961 ) ASM Handbook, Vol. 18 Friction, Lubrication and Wear Technology volume chairman : P.J.Blau; p.786, ASM Intern., Materials Park, USA ( 1990 )
FIG.1 The transition region between the cluster of W-containing reaction products at the bottom of the sample and the Al-matrix above for alloy 1 ( Figure la ) to alloy 6 ( Figure If) ( r : ternary W(AI,Sil_,)2, E : AlqW, 6 : AlgW, ‘y : All2W, W(AlxSil_x)2 : binary WSi2 with some Si substituted by Al ). Figure lg represents the particle network obtained with alloy 7. The little amount of r-phase for alloy 2 and 7 is not visible in Figure lb and lg, respectively.
1428
AI-%-W PHASE DIAGRAM
Vol. 32, No. 9
FIG.2 The Al-rich comer of the Al-Si-W phase diagram at 1000°C; z : ternary W(Al,Sil_,)2, E : A4W, 6 : AlgW, y : Al 12W. 6 and y may form during cooling down at T-z87 1“C and T<697”C, respectively.
et Materialia, Vol. 32, No. 9. pp. 1423-1428.1995 Copyright Q 1995 Elsewier Science Ltd Printed in the USA. AU rights reserved 0956-716X/95 $9.50 + .OO
0956-716X(95)00182-4
INVESTIGATION
OF THE AL UMINIUM-RICH
CORNER IN THE Al-Si-W PHASE DIAGRAM
H.-H.Angermann, 0. Van Der Biest, L.Froyen Department Metallurgy and Materials Engineering, KU. Leuven, de Croylaan 2, B-3001 Heverlee, Belgium (Received September 21, 1994) (Revised November 23, 1994) Introduction Aluminium alloys are interesting materials because of their high specific physical and mechanical properties /I/. Discontinuous reinforced aluminium composites have the additional advantage that they offer to the materials designer the possibility to tailor the properties by selecting the volume fraction, type and/or morphology of the reinforcement phase. However, one hindrance for their broad application, e.g. in the automotive industry, are the high costs for t.he processing of the alloys. The most costeffective way to produce composites in high volumes is via the molten-metal route /2,3/. Especially interesting is the manufacture of discontinuous reinforcements in situ in the molten aluminium /4/. This can principally be achieved by heating a mechanical mixture of elementary components in the desired stochiometry and let the reinforcing particles form. However, in order to have control of the microstructure via temperature and alloy composition the phase diagram must be known. In the present work the Al-rich comer of the Al-Si system with the refractory metal W was investigated at 1OOO’C. The information available for this ternary system refers to 1500°C and to higher Si-and W-contents /5/. The phases which had fomted after holding at 1000°C were used to construct an isothermal section of the phase diagram at this temperature. Also the compounds which precipitate from the Al-melt during cooling are described. Possible applications of the alloys are discussed. Exnerimental Procedure As initial components, aluminiun. in block-form ( Pechiney, France, 99.999% ), silicon in wafer form ( IMEC, Belgium, 99.999% ) and the refractory metal tungsten in powder form ( Reframet Hoboken, grade 425-A, 99.99% ) were used. Seven different compositions of Al-Si-W were chosen which are compiled in atom and weight percent in TABLE 1. Sample 1 to 4 contained a constant amount of 3.5 a/o W while the silicon contents stepwise increased. For the alloys 5~to 7 the W- and/or the Si-fraction has been considerably increased. TABLE 1 : Composition of the Seven Alloys in Atom and Weight Percent ( bal : balance ) alloy
t
1 2 3 4 5 6 7
a/O
Al bal bal bal bal bal bal bal
Si 1.5 5.2 8.1 17.5 34.1 16.2 3.0
1423
WI0
W 3.5 3.5 3.5 3.5 12.7 7.0 12.7
Al bal bal bal bal bal bal bal
Si 1.3 4.5 7.0 15.0 20.0 11.9 1.8
W 20.0 20.0 20.0 20.0 49.8 33.7 49.8
1424
Al-%-W PHASE DIAGRAM
Vol. 32, No. 9
For heat treatment the elementary components were introduced into an Al203-tube with a 10 mm inside diameter. About 3 g of material was used for every experiment. The alloys were inductively heated within 30 minutes to 1000°C and held there for two hours under a protective argon atmosphere. The tip of a thermocouple, protected by a thin A1203-tube, was located in the melt about 1 mm near the bottom of the crucible. After the heat treatment the alloys were air cooled in the crucible. For metallographic investigation, the polished samples were etched 5s with Kellers reagent. A microprobe ( Jeol Superprobe 733 ) was used to quantify and/or identify the phases ( Al K-, Si K- and W L-lines; ZAP correction )_ Because the alloy compositions were located in the Al-rich comer of the ternary AI-Si-W system, the amount of phases was often small so that only the ternary T-phase W(AlxSil_x)2, the binary W(AlxSil_x)2 with WSi2-structure and Al4W could be determined by X-ray diffraction ( Phillips PW 1010 ). Light microscopy provided an overview about the morphology of the microstructure. Results and Discussion Morphology of the Alloys The alloys 1 to 4 each contained 3.5 a/o of tungsten and exhibited roughly the same microstructure. W-containing reaction products formed a cluster at the bottom of the crucibles. This effect was caused by the large density difference between the liquid aluminium and the W-powder and/or W-containing sedimentations. The region covered about 15 to 25% of the sample volume. Above the cluster there was an Al-matrix with some precipitates which formed during cooling down from 1000°C. It is probable that the cooling down was so rapid that these precipitates could not settle at the bottom of the crucible. For sample 5 and 6, which contained 12.7 and 7.0 a/o tungsten, respectively, the W containing precipitates within the cluster covered about 50% of the sample volume. In contrast, alloy 7 with 12.7 a/o W but much less Si as the alloys 5 and 6, was almost totally composed of a sintered network of a tungsten-containing compound with Al filling the voids. The reason for the formation of particles at 1000°C was an oversaturation of the Al-melt with the transition metal W. In the binary, liquid Al can dissolve at 1000°C only about 2 a/o W. Regarding the chosen amount of at least 3.5 a/o W and the fact that the addition of a third element normally lowers the solubility of the second one in the melt, it can be assumed that the solubility limit of the Al-melt was exceeded with respect to tungsten. In contrast, the solubility of Si in Al at 1000°C amounts to about 46 a/o whereas maximum 34.1 a/o Si were added in the ternary system. Metallographic investigation of the samples showed also no indication for a primary Si-precipitation. The identity and the composition of the W-containing particle cluster was used as data for construction of the Al-rich comer of the Al-Si-W system at 1OOO’C. Figure la to If depict the transition region between the particle cluster at the bottom and the Al-matrix above for the samples 1 to 6. Figure Ig presents the continuous network of particles obtained for alloy 7. There could be three different precipitates found in the Al-matrix : a plate shaped and an angular one, both containing W, and needle to flake shaped Si particles from the binary AI-B eutectic. The Al-Si-W Phase Diagram in the Al-Rich Comer In Figure 2, the Al-rich comer of the Al-Si-W phase diagram is shown at 1000°C as deducted from the experiments. The location of the samples 1 to 7 is indicated. T represents the ternary W(Al,Sil_x)2-phase with CrSi2-crystal structure and E the binary A14W /5/. W(AlxSil_,)2 depicts the binary WSi2 compound, where some of the Si is substituted by Al. The 6- and the y-phases, which stand for A15W and All2W. respectively, may form during cooling down for temperatures ~871 and <697”C, respectively /6/. In the following, the microstructure of the samples 1 to 7 will be discussed by means of Figure 2.
Vol. 32, No. 9
AI-S-W PHASE DIAGRAM
1425
Alloy 1 ( A11.5Si3.5W, Figure la ) was the alloy which contained the least silicon. The particles at the bottom, which were formed during the holding time at lOOO”C, were solely composed of the coarse angular a-phase Al4W. The particles were strongly sintered together and formed a homogeneous network with Al filling the voids. It is concluded that alloy 1 falls into a two-phase region where E is in equilibrium with the Al-melt. In alloy 2 (A15.2Si3.5W, Figure lb ) and alloy 7 (A13.OSi12.7W, Figure lg ) there were additionally a few small areas where particles were grouped together in a small cluster of particles having a smaller size as Al4W. They were identified by microprobe as the ternary r-phase W(AlxSil_x)2 with CrSi2-structure. The x-value amounted to x=0.60. The formation of this compound was obviously caused by the increased silicon content in both alloys with respect to alloy 1. Alloy 2 and 7 fall into a 3-phase region where the T-phase with x=0.60 is equilibrium with Al4W and the Al-melt. Because of the small amount of r-phase it is concluded that the separation line between the twoand three phase region is close to the compositions of alloy 2 and 7. It should be noted that in sample 7 with its considerably increased W-content the network of Al4W occupied practically the whole sample volume.
In the Al-matrix above the particle clusters in alloy 1, 2 and 7 there were some angular isolated Al.,W-precipitates and plate shape’d 5 to 15 pm thin compounds. The latter were determined by microprobe to be composed of (86 f 1 ) a/o and (14 + 1) a/o W. It is suggested that the platelike phase is identical with A&W. The 3 a/o overshoot of Al measured by microprobe was probably due to a co-penetration of the surrounding Al-matrix by the e--beam. According to the Al-W-equilibrium diagram Al5W forms via a peritectic reaction from the melt and Al4W. However, the metallographic investigation of the samples did not indicate a formation of the AlgW-plates through peritectic reaction. It is assumed that the air cooling was so fast ( from 1000 to 600°C in about 90s ) that the peritectic reaction was suppressed or that the reaction zone was too small to be detectable. In alloy 3 (Al8 lSi3SW, Figure lc) the silicon content was raised huther. The cluster of sedimentations at the bottom of the tube was now solely composed of the ternary r-phase again with x=0.60. In contrast to AhW the W(Al,Si&-particles were not or only little sintered together. This was probably a reflection of a much higher currently unknown melting point of the W(AIXSi& r-phase than of AldW (perhectic decomposition at 1327’C). The r-phase was reported to be stable up to at least 155O’C /7/. In the Al-matrix above the cluster there were again plateshaped A&W &precipitates and angular particles which proved to be the r-phase (x=0.53). Because of the presence of the r-phase and the absence of A&W-particles in the cluster at the bottom of the sample it is believed that the composition of sample 3 falls within a two-phase r-Al-melt region in the phase diagram (Figure 2). The location of the #separating line between the 3-phase r-~-Al and 2-phase &-Al region is not totally clear, but because of the x-value of 0.60 of the r-phase, which corresponded to alloy 2, it is assumed that the nominal composition of sample 3 is close to the separation line and falls just into the two phase region. Alloy 4 ( Al17.5Si3.5W, Figure Id ) contained the most silicon of the samples 1 to 4 which had a constant Wcontent of 3.5 :1/o. As for sample 3 the cluster at the bottom was composed of W(Al,Sil_,)2 z-phase but the xvalue shifted to 0.38, i.e. towards higher Si-contents. In the Al-matrix above the cluster there was one angular Wcontaining precipitate which was identified as All2W y-phase. There were no longer any plateshaped AlgWparticles. A112W was more homogeneously distributed in the matrix than the other precipitates due to its lower Wcontent and therefore lower density. A similar microstructure was found in alloy 6 (Al16.2Si7.OW, Figure If). The cluster was comlposed also of the z-phase with x-values between 0.38 and 0.49. In the Al-matrix there were angular particles which proved to be the r-phase with x=0.41. Both alloy 4 and alloy 6 were located within the two-phase rAl-region of the phase-diagram ( Figure 2 ) just like alloy 3. Alloy 5 ( A134.l.Si12.7W, Figure le ) had the highest W-and Si-content of all alloys studied here. The cluster at the bottom of the sample was composed of 10 to 50 urn small angular particles which were slightly sintered together.
1426
AM-W PHASEDIAGRAM
Vol. 32. No. 9
With microprobe they were identified as W(AIxSil_x)2 with x=0.1 1. This phase was crystallographically identical with the binary WSi2, but about 8 a/o of the Si were substituted by aluminium. In the Al-matrix there were angular W(AlxSiI_x)2 particles ( WSi2-structure) with x=0.15 ( 10 a/o Al). Alloy 5 was located within a two-phase W(Al,SiI_,)2 ( WSi2-structure )-Al-melt region. Comments on the Application of the Alloys The W-content of the Al-Si-W alloys in this work was chosen > 3.5a/o. The purpose was to exceed the solubility limit of W so that precipitates had formed after annealing at 1000°C from which the phase diagram could be derived. If a materials designer is aiming at discontinuous reinforced Al-Si-W alloys for structural application he has to chose lower W-contents or higher annealing temperatures so that the W is thoroughly dissolved in the Al melt. The particles form during cooling down. The experiments in this work showed that these precipitates do not sediment during air cooling. Higher W-contents must be avoided in order to maintain the density advantage of Alalloys. Another application of alloys from the Al-rich comer of the Al-Si-W system may be as wear resistant material. The ternary z-phase W(Al,Sil_,)2 could function as the phase with high wear resistance because it combines high hardness with little tendency to coarsen and to sinter together during processing. The particles have to be precipitated during cooling down of the melt. The microstructure is similar to a commercially refined hypereutectic Al-Si alloy /8/, where the Si-particles would be replaced by the z-phase. Conclusions The Al-rich comer of the Al-Si-W phase diagram was investigated up to maximum 12.7 a/o, respectively. The alloys were inductively heated in alumina crucibles to hours and subsequently air cooled. Due to the low solubility of W in the Al-melt, had formed at the bottom of the crucibles after the holding time. Their identity construct the Al-rich comer of the Al-Si-W phase diagram at 1OOO’C.
Si-and W-contents of 34.1 and 1000°C in argon, held for two W-containing particle clusters and composition was used to
Nearest to the Al-comer of the Al-Si-W phase diagram and the molten Al phase was a two-phase region composed of the A14W a-phase and the Al-melt. For more elevated Si-and W-contents an alloy was located either in a three phase &-z-Al melt or two-phase r-Al melt region. The r-phase was the ternary W(AlxSil_x)2-compound with CrSi2 crystal structure. The r-phase was angular like AlqW, but not so course and tended much less to sinter together to a particle network. This was associated with a probably much higher melting point of the r-phase with respect to E. The maximum value of x for the W(Al,SiI_,)2 r-phase in the two-and three-phase equilibria with E and the Almelt at 1000°C amounted to 0.60. For higher Si-contents above about 18 a/o and W above 2 a/o the pseudobinary W(Al,Sil_,)2 with WSi2-structure was formed. The particles which were formed during air cooling through precipitation from the Al-melt were also investigated. Angular Al4W, platelike AlgW, the angular r-phase and W(AlxSil_x)2 with WSi2 were found according to the respective Si-and W-content of the alloy. These particles were fairly well distributed in the Al-matrix. The All2Wparticles with their lowest W-content of all precipitations exhibited the most homogeneous distribution in the Almatrix. A possible commercial application of the alloys of the Al-Si-W system is as discontinuous reinforced material for structural application or as wear resistant material. However, the amount of W added must be limited in order to maintain the low density advantage of the Al-based alloy.
Vol. 32. No. 9
1. 2.
3. 4. 5. 6. I. 8.
Al-S-W PHASEDIAGRAM
1427
Y.Kurih.ara, J. of Mat. 46 5, 12, ( 1994 ) J.E.Alli:jon and GSCole, J. of Mat. 45 1, 19, ( 1993 ) TSritharan, K. Xia, J. Heathcock and J. Mihelich, in Met. & Cer. Matrix Comp. : Processing, Modelling PSahoo and M.J.Koczak, Mat. Sci. and Eng., A131.69 ( 1991 ) Ternary Alloys Vol.8, ed. by G.Petzow and G.Effenberg; VCH Verlagsgesellschaft, Weinheim, Germany ( 1993 ) Binary ,411oys Phase Diagram ed.-in-chief T.B.Massalski; ASM Intern., Materials Park, USA, ( 1990) H.Nowotny, C.Brukl and F.Benesovsky, Monatsh. Chemie 92, 116,( 1961 ) ASM Handbook, Vol. 18 Friction, Lubrication and Wear Technology volume chairman : P.J.Blau; p.786, ASM Intern., Materials Park, USA ( 1990 )
FIG.1 The transition region between the cluster of W-containing reaction products at the bottom of the sample and the Al-matrix above for alloy 1 ( Figure la ) to alloy 6 ( Figure If) ( r : ternary W(AI,Sil_,)2, E : AlqW, 6 : AlgW, ‘y : All2W, W(AlxSil_x)2 : binary WSi2 with some Si substituted by Al ). Figure lg represents the particle network obtained with alloy 7. The little amount of r-phase for alloy 2 and 7 is not visible in Figure lb and lg, respectively.
1428
AI-%-W PHASE DIAGRAM
Vol. 32, No. 9
FIG.2 The Al-rich comer of the Al-Si-W phase diagram at 1000°C; z : ternary W(Al,Sil_,)2, E : A4W, 6 : AlgW, y : Al 12W. 6 and y may form during cooling down at T-z87 1“C and T<697”C, respectively.