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High damping functional materials prepared by spray deposition
Journal of Materials Processing Technology 106 (2000) 94±98
High damping functional materials prepared by spray deposition Yongchang Liu*, Jinfu Li, Gencang Yang, Yili Lu State Key Laboratory of Solidi®cation Processing, Northwestern Polytechnic University, Xian 710072, PR China
Abstract High damping materials with good mechanical properties suppress undesirable mechanical vibration and wave propagation, and have wide application in noise control and in the stability of vehicles and instruments. The effects of alloying and extrusion on the damping behavior and mechanical properties of as-spray-deposited alloy ZA27 were investigated to develop a new functional material possessing high damping capacity and good mechanical properties. Three levels of cerium content, 0.3, 0.5 and 0.7 wt.%, were adopted to evaluate the effectiveness of modi®cation. The damping capacities were measured at frequencies of 1 and 4 Hz over the 30±2008C temperature range. At temperatures below 808C, the as-spray-deposited materials appear to have virtually no frequency dependency. Above 808C, the materials become temperature sensitive, with the lower frequency exhibiting the higher damping. The extruded, as-spray-deposited material has the highest damping capacity and elongation values amongst all of them. The microstructure of the as-spray-deposited high silicon alloy ZA27 modi®ed by 0.5 wt.% cerium was made up of ®ne lamella eutectoid, pores, light dot-like phases and polygonal silicon-rich phases. The damping mechanisms are discussed in the light of the data obtained from characterization of the microstructure and the damping capacity. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Spray deposition; Damping capacity; Extrusion; Functional materials
1. Introduction The damping capacity of a material refers to its ability to convert mechanical vibration energy into thermal energy. The application of high damping materials may eliminate the need for special energy absorbers or dampers to attenuate undesirable noise and mechanical vibration. However, most of the frequently used metals and alloys usually exhibit a low damping capacity, which limits their application and performance in dynamic structures [1]. Accordingly, material researchers have sought to improve the damping capacity of metals and alloys as serious competitors to traditional engineering alloys through the use of innovative processing and alloying. Spray-deposition processing has received considerable attention for its potential to manufacture bulk rapidly solidi®ed materials. Zinc has been used traditionally to galvanize steel surfaces as protection against corrosion. Since the 1930s, diecast zinc alloys also have been in use. Zinc±aluminum alloys, however, as general-purpose castings, appeared more recently in industry for their good ambient temperature mechanical properties, damping capacity and wear resistance [2]. Yang et al. had developed a spray-deposited high silicon alloy ZA27 (5 wt.% silicon ) [3,4], which possesses *
Corresponding author. Tel.: 86-29-8491484; fax: 86-29-8491000 E-mail address: [email protected] (Y. Liu).
good high temperature properties and damping capacity. Here the cerium modi®cation and intrinsic damping mechanism of the as-spray deposited high silicon alloy ZA27 before and after extrusion are studied. 2. Experiment 2.1. Materials synthesis The alloy ZA27 utilized in the present study was of commercial grade. The melting process was carried out as follows. Firstly, the prepared alloy was melted in a resistance furnace, and at 5308C, 5 wt.% of silicon was added to it. Then, the melt was heated to 7008C, and some Al±18% Ce master-alloy was melted into it. For the purpose of composition homogenization the molten melt was superheated to 7808C and held at this temperature for 15±20 min, the temperature being ®nally reduced to 7408C and the melt transferred into the middle package of the spray deposition equipment to obtain the deposits. 2.2. Hot extrusion A 40-cm-diameter extrusion billet was removed from the spray-deposited materials and subsequently hot extruded at 2808C, using a reduction ratio of 6.9:1, to close the micro-
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 4 4 - 0
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
95
Table 1 Damping capacities of the as-spray deposited materials modi®ed by various cerium contents Cerium Qÿ1103 (1 Hz) content 358C 508C 658C 0.3 0.5 0.7
5.47 8.07 5.21
6.74 8.43 6.94
808C
10.41 16.79 12.54 18.91 11.11 17.25
958C
1108C 1258C 1408C
24.31 35.12 28.62 39.18 25.06 32.34
46.35 52.64 41.39
55.12 68.52 49.21
Fig. 1. Schematic diagram of the multifunction internal friction apparatus.
meter-sized pores that are normally present in as-spray deposited materials. The extruded sample was heat treated at 2808C for 2 h. 2.3. Damping capacity and mechanical property measurements Material damping is realized either by the decay of vibration amplitude in free vibration, or by the suppression of resonant amplitude and the phase lag of deformation behind the applied load in force vibration [5,6]. Commonly used units of measurement such as inverse quality factor, Qÿ1, and logarithmic decrement, d, are interchangeable with a proper conversion for the case of relatively low damping capacity. In this study, the measure of damping utilized is the inverse quality factor. The temperature and frequency range of interest are 35±2008C and 1±4 Hz, respectively. The damping capacities were determined using multifunction internal friction apparatus, its working principle being shown in Fig. 1. The damping specimens are rectangular bars with dimensions of 1 mm3 mm70 mm. The furnace temperature was increased at the rate of 28C minÿ1 from 30 to 2008C and the maximum surface strain amplitude produced in the specimens was maintained at 510ÿ5. During the temperature cycle, the sample was oscillated at the two discreet frequencies of 1 and 4 Hz. All of the experiments were carried out in a vacuum chamber maintained at 10ÿ1 Pa. The tension-test samples were machined from the deposits according to the GB 6397-86 R7 standard of China. Theses samples were pulled using a CSS-1101 universal tension test machine at a loading velocity of 1 mm minÿ1. 3. Experimental results and discussion 3.1. Damping capacity 3.1.1. Cerium modi®cation Three kinds of as-spray-deposited materials with cerium contents of 0.3, 0.5 and 0.7 wt.% were made to investigate the most effective modi®cation content on the condition of rapid solidi®cation. Their relationships between damping capacities and temperatures are all shown in Table 1. The experimental results show that the material modi®ed by 0.5 wt.% of cerium has the highest damping capacity.
3.1.2. Extrusion Typical sets of data corresponding to the as-cast alloy ZA27 (sample 1, Table 2) as-spray-deposited high silicon alloy ZA27 modi®ed by 0.5 wt.% cerium before (sample 2, Table 2) and after (sample 3, Table 2) extrusion are shown in Fig. 2. In each of the ®gures, the full symbols denote damping capacity at 4 Hz and the hollow symbols denote that at 1 Hz. Several interesting trends may be noted from Fig. 2, which are found to be characteristic of all the materials investigated. First, the extruded, as-spray-deposited material has the highest value of damping capacity, Next, at temperature below 808C, the as-sprayed material appears to have virtually no frequency dependence. Above 808C, the materials become temperature sensitive, with the lower frequency exhibiting the higher damping. No peak phenomena were observed for the specimens tested in the temperature range of interest. 3.2. Mechanical properties The result of the tensile tests are shown in Table 2. It is found that the ultimate tensile strengths of the as-sprayed high silicon alloy ZA27 modi®ed by 0.5 wt.% of cerium decreases much more than that of as-cast ZA27. After extrusion there is little difference of the ultimate tensile strength between the as-cast ZA27 and the as-sprayed high silicon alloy ZA27 and the fracture elongation of the extruded deposits increased greatly. This may be explained by the fracture morphology of the materials investigated (Fig. 3). The rupture morphology revealed a distribution of micrometer-sized pores in the as-sprayed deposits which were the sources of brittle failure. After extrusion, the extruded sample belongs to ductile rupture (the large number of dimples distributed in the fracture may establish this) so the elongation, and the ultimate tensile strength were improved greatly. Table 2 Mechanical properties of the materials investigated Sample number
Ultimate tensile strength (MPa)
Elongation (%)
1 2 3
403.33 279.50 373.00
6.26 1.20 15.99
96
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
silicon ZA27 alloy are made up of pores, ®ne lamella eutectoid, light dot-like phases and polygonal silicon-rich phases. Compared with the conventional cast high silicon ZA27 alloy, a large amount of lamella eutectoid phases appeared and the number and size increased and mini®cated. There are two main factors in¯uencing the form of lamella eutectoid, one is the atomizing and rapid solidi®cation process that increased the actual nucleating undercooling; the other is the effect of cerium modi®cation. The existence states of cerium in the alloy have two forms, solutionizing and forming high melting point Al11Ce3 phases [7]. These Al11Ce3 phases have a similar crystal structure to the primary a(Al) phases. This particular crystal structure promotes a(Al) phases to nucleate on the cerium-rich phases. At the ¯ying stage of droplets, a large cooling rate was achieved by the extensive heat exchange between the droplets and the ultrasonic gas, whilst after deposition on the substrate the heat conducted rate was limited by the heat dissipation of the substrate and deposited metal. The change of cooling rate from rapid to slow is bene®cial to the forming of ®ne lamella eutectoid phases. 3.4. Damping mechanisms
Fig. 2. Relationship between damping capacities and temperatures for the samples of Table 2: (a) 1; (b) 2; (c) 3.
3.3. Microstructure The variance of the damping capacity and mechanical properties relates to the difference of the microstructure (Fig. 4). The microstructures of the spray-deposited high
A variety of mechanisms may contribute to the overall damping behavior of metals and alloys. These may include the effects due to thermoelasticity, crystallographic defects, eddy current, Snoeck effects, stress-induced ordering, and others [7,8]. Defects that may be present in crystalline materials include point-, line-, surface- and bulk-defects. Commonly, a good damping capacity may be detrimental to the mechanical properties. The following discussion will address the factors that are thought to have been active in the extruded as-spray deposited materials. Discussions on grain boundary viscosity, relaxation, and anelasticity in polycrystalline metals by Keà et al. [9] have indicated that viscous ¯ow at grain boundaries will serve as a source of internal friction. The energy dissipated in the boundaries is depend on the temperature, the shear stress, and the anelastic shear strain. In view of this, the ®ne grain microstructure of the as-spray-deposited material before and after extrusion may play an important role in the dissipation of elastic strain energy in the upper range of the temperature investigated. The variance of damping capacities between the as-cast and as-deposited materials may con®rm this. Except for the ®ne grain boundaries, the very ®ne lamella interfaces and interfaces between the silicon-rich phase and the lamella eutectoid also play a primary role in the improvement of the damping property. The lamella eutectoids are composed of zinc-rich and aluminum-rich phases of which the crystal substructure are hcp and fcc, respectively. It is know that it is easier for dislocations to slide in the crystal structure of hcp than that of fcc. Thus, the interfaces between the lamella eutectoid belong to movable interfaces. With the fraction of lamella eutectoids increasing, much energy was
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
97
Fig. 3. Fracture morphology of the as-sprayed materials investigated: (a) 2; (b) 3.
Fig. 4. Microstructure of the materials investigated: (a) 1; (b) 2.
dissipated between the movable interfaces of the lamella eutectoids and the damping capacity was greatly improved. Interfaces may affect the damping behavior of the materials because the interfaces are two-dimensional defects where the crystal structure is distorted locally. Apart from the interfaces between the lamella eutectoids, the effect of phase interfaces between the silicon-rich phases and the matrix on damping may be rationalized on the basis of the Schoeck theory. Based on the Schoeck theory, internal friction is increased by interface relaxation and anelastic strain induced by dislocations in the vicinity of the interface. The internal friction is proportional to changes in the volume fraction of the silicon-rich phases and the local stress at the interfaces. By inspection of the equations that Schoeck used, the following equation may be used to predict their contribution to high temperature damping [8]: 1 8 1 ÿ n 1 X 3 0 2 ai p13 i p213 3p 2 ÿ n V 32 1 ÿ n X 4pa3i 0 2 p13 i 2 3v 3p13 2 ÿ n 32 1 ÿ n X 0 2 f p13 i 2 3p13 2 ÿ n
Qÿ1
where f is the volume fraction of the silicon-rich phases, p13 the external stress, n the Poisson's ratio, V the sample
volume, ai the radius of the oblate spheroid i, and (p0 13)i the component of p13 in the plane of the spheroid i which can be relaxed. The above equation suggests that the damping capacity is directly proportional to the volume fraction of the silicon-rich phases. According to the results of the microstructural analysis, spray deposition, cerium modi®cation and extrusion are bene®cial to the securing of a re®ned microstructure. After these processes the amount of lamella eutectoids and silicon-rich phases are increased and the size of these is greatly re®ned. so the as-spray-deposited high silicon alloy ZA27 modi®ed by cerium possesses a high damping capacity. 4. Conclusions The cerium modi®cation to as-spray deposited high silicon alloy ZA27 is bene®cial to re®ning the grain and lamella eutectoid. The microstructure of the spray-deposited high silicon alloy ZA27 was made up of pores, ®ne lamella eutectoid, light dot-like phases and polygonal silicon-rich phases. Two reasons for the forming of ®ne lamella phases are given, one being the in¯uence of cerium modi®cation to the solidi®cation processing, the other being the change of the cooling rate of the atomized droplets from rapid to slow in the course of atomization and deposition.
98
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
After extrusion, both the damping capacity and mechanical properties were greatly improved. According to analysis of the rupture morphology, a distribution of micrometersized pores in the as-sprayed deposits were the sources of brittle failure, whilst after extrusion, a large number of ductile dimples were distributed in the fracture, so the elongation and ultimate tensile strength were improved greatly. Apart from grain re®nement, the very ®ne lamella eutectoids and silicon-rich phases play a primary role in the improvement of the damping property. The lamella eutectoids are composed of zinc-rich and aluminum-rich phases of which the crystal substructures are hcp and fcc, respectively. The probable sliding system of substructure hcp is more than that of fcc so that the high damping capacities can be attributed primarily to phase interface thermoelastic damping. According to the Shoeck theory, the damping capacity is directly proportional to the volume fraction of the silicon-rich phases. No peak phenomena were observed for the specimens tested in the temperature range of interest.
Acknowledgements The authors would like to express their gratitude to the Natural Science Foundation for grant No. 59671026 and to the Aeronautical Science Foundation of China. References [1] J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 (2) (1994) 395±409. [2] M.A. Savas, S. Altintas, J. Mater. Sci. 28 (1993) 1775±1780. [3] L. Yang, Y. Liu, G. Yang, Y. Zhou, Acta Metall. Sinica (English Letters) 9 (2) (1996) 140±146. [4] L. Yang, L. Pang, G. Yang, Y. Zhou, Funct. Mater. 26 (4) (1995) 359± 361. [5] J. Zhang, R.J. Perez, C.R. Wong, E.J. Lavernia, Mater. Sci. Eng. R 13 (8) (1994) 330±332. [6] R.J. Perez, J. Zhang, M.N. Gungor, E.J. Lavernia, Metall. Trans. A 24 (1993) 701±709. [7] J.R. Davis, Aluminum and aluminum alloys, The Mater. Inform. Soc. (1993) 546. [8] J.D. Eshelby, Prog. Solid State Phys. 3 (1956) 79. [9] KeÃ, Lazan, Nowick, Zene.
High damping functional materials prepared by spray deposition Yongchang Liu*, Jinfu Li, Gencang Yang, Yili Lu State Key Laboratory of Solidi®cation Processing, Northwestern Polytechnic University, Xian 710072, PR China
Abstract High damping materials with good mechanical properties suppress undesirable mechanical vibration and wave propagation, and have wide application in noise control and in the stability of vehicles and instruments. The effects of alloying and extrusion on the damping behavior and mechanical properties of as-spray-deposited alloy ZA27 were investigated to develop a new functional material possessing high damping capacity and good mechanical properties. Three levels of cerium content, 0.3, 0.5 and 0.7 wt.%, were adopted to evaluate the effectiveness of modi®cation. The damping capacities were measured at frequencies of 1 and 4 Hz over the 30±2008C temperature range. At temperatures below 808C, the as-spray-deposited materials appear to have virtually no frequency dependency. Above 808C, the materials become temperature sensitive, with the lower frequency exhibiting the higher damping. The extruded, as-spray-deposited material has the highest damping capacity and elongation values amongst all of them. The microstructure of the as-spray-deposited high silicon alloy ZA27 modi®ed by 0.5 wt.% cerium was made up of ®ne lamella eutectoid, pores, light dot-like phases and polygonal silicon-rich phases. The damping mechanisms are discussed in the light of the data obtained from characterization of the microstructure and the damping capacity. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Spray deposition; Damping capacity; Extrusion; Functional materials
1. Introduction The damping capacity of a material refers to its ability to convert mechanical vibration energy into thermal energy. The application of high damping materials may eliminate the need for special energy absorbers or dampers to attenuate undesirable noise and mechanical vibration. However, most of the frequently used metals and alloys usually exhibit a low damping capacity, which limits their application and performance in dynamic structures [1]. Accordingly, material researchers have sought to improve the damping capacity of metals and alloys as serious competitors to traditional engineering alloys through the use of innovative processing and alloying. Spray-deposition processing has received considerable attention for its potential to manufacture bulk rapidly solidi®ed materials. Zinc has been used traditionally to galvanize steel surfaces as protection against corrosion. Since the 1930s, diecast zinc alloys also have been in use. Zinc±aluminum alloys, however, as general-purpose castings, appeared more recently in industry for their good ambient temperature mechanical properties, damping capacity and wear resistance [2]. Yang et al. had developed a spray-deposited high silicon alloy ZA27 (5 wt.% silicon ) [3,4], which possesses *
Corresponding author. Tel.: 86-29-8491484; fax: 86-29-8491000 E-mail address: [email protected] (Y. Liu).
good high temperature properties and damping capacity. Here the cerium modi®cation and intrinsic damping mechanism of the as-spray deposited high silicon alloy ZA27 before and after extrusion are studied. 2. Experiment 2.1. Materials synthesis The alloy ZA27 utilized in the present study was of commercial grade. The melting process was carried out as follows. Firstly, the prepared alloy was melted in a resistance furnace, and at 5308C, 5 wt.% of silicon was added to it. Then, the melt was heated to 7008C, and some Al±18% Ce master-alloy was melted into it. For the purpose of composition homogenization the molten melt was superheated to 7808C and held at this temperature for 15±20 min, the temperature being ®nally reduced to 7408C and the melt transferred into the middle package of the spray deposition equipment to obtain the deposits. 2.2. Hot extrusion A 40-cm-diameter extrusion billet was removed from the spray-deposited materials and subsequently hot extruded at 2808C, using a reduction ratio of 6.9:1, to close the micro-
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 4 4 - 0
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
95
Table 1 Damping capacities of the as-spray deposited materials modi®ed by various cerium contents Cerium Qÿ1103 (1 Hz) content 358C 508C 658C 0.3 0.5 0.7
5.47 8.07 5.21
6.74 8.43 6.94
808C
10.41 16.79 12.54 18.91 11.11 17.25
958C
1108C 1258C 1408C
24.31 35.12 28.62 39.18 25.06 32.34
46.35 52.64 41.39
55.12 68.52 49.21
Fig. 1. Schematic diagram of the multifunction internal friction apparatus.
meter-sized pores that are normally present in as-spray deposited materials. The extruded sample was heat treated at 2808C for 2 h. 2.3. Damping capacity and mechanical property measurements Material damping is realized either by the decay of vibration amplitude in free vibration, or by the suppression of resonant amplitude and the phase lag of deformation behind the applied load in force vibration [5,6]. Commonly used units of measurement such as inverse quality factor, Qÿ1, and logarithmic decrement, d, are interchangeable with a proper conversion for the case of relatively low damping capacity. In this study, the measure of damping utilized is the inverse quality factor. The temperature and frequency range of interest are 35±2008C and 1±4 Hz, respectively. The damping capacities were determined using multifunction internal friction apparatus, its working principle being shown in Fig. 1. The damping specimens are rectangular bars with dimensions of 1 mm3 mm70 mm. The furnace temperature was increased at the rate of 28C minÿ1 from 30 to 2008C and the maximum surface strain amplitude produced in the specimens was maintained at 510ÿ5. During the temperature cycle, the sample was oscillated at the two discreet frequencies of 1 and 4 Hz. All of the experiments were carried out in a vacuum chamber maintained at 10ÿ1 Pa. The tension-test samples were machined from the deposits according to the GB 6397-86 R7 standard of China. Theses samples were pulled using a CSS-1101 universal tension test machine at a loading velocity of 1 mm minÿ1. 3. Experimental results and discussion 3.1. Damping capacity 3.1.1. Cerium modi®cation Three kinds of as-spray-deposited materials with cerium contents of 0.3, 0.5 and 0.7 wt.% were made to investigate the most effective modi®cation content on the condition of rapid solidi®cation. Their relationships between damping capacities and temperatures are all shown in Table 1. The experimental results show that the material modi®ed by 0.5 wt.% of cerium has the highest damping capacity.
3.1.2. Extrusion Typical sets of data corresponding to the as-cast alloy ZA27 (sample 1, Table 2) as-spray-deposited high silicon alloy ZA27 modi®ed by 0.5 wt.% cerium before (sample 2, Table 2) and after (sample 3, Table 2) extrusion are shown in Fig. 2. In each of the ®gures, the full symbols denote damping capacity at 4 Hz and the hollow symbols denote that at 1 Hz. Several interesting trends may be noted from Fig. 2, which are found to be characteristic of all the materials investigated. First, the extruded, as-spray-deposited material has the highest value of damping capacity, Next, at temperature below 808C, the as-sprayed material appears to have virtually no frequency dependence. Above 808C, the materials become temperature sensitive, with the lower frequency exhibiting the higher damping. No peak phenomena were observed for the specimens tested in the temperature range of interest. 3.2. Mechanical properties The result of the tensile tests are shown in Table 2. It is found that the ultimate tensile strengths of the as-sprayed high silicon alloy ZA27 modi®ed by 0.5 wt.% of cerium decreases much more than that of as-cast ZA27. After extrusion there is little difference of the ultimate tensile strength between the as-cast ZA27 and the as-sprayed high silicon alloy ZA27 and the fracture elongation of the extruded deposits increased greatly. This may be explained by the fracture morphology of the materials investigated (Fig. 3). The rupture morphology revealed a distribution of micrometer-sized pores in the as-sprayed deposits which were the sources of brittle failure. After extrusion, the extruded sample belongs to ductile rupture (the large number of dimples distributed in the fracture may establish this) so the elongation, and the ultimate tensile strength were improved greatly. Table 2 Mechanical properties of the materials investigated Sample number
Ultimate tensile strength (MPa)
Elongation (%)
1 2 3
403.33 279.50 373.00
6.26 1.20 15.99
96
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
silicon ZA27 alloy are made up of pores, ®ne lamella eutectoid, light dot-like phases and polygonal silicon-rich phases. Compared with the conventional cast high silicon ZA27 alloy, a large amount of lamella eutectoid phases appeared and the number and size increased and mini®cated. There are two main factors in¯uencing the form of lamella eutectoid, one is the atomizing and rapid solidi®cation process that increased the actual nucleating undercooling; the other is the effect of cerium modi®cation. The existence states of cerium in the alloy have two forms, solutionizing and forming high melting point Al11Ce3 phases [7]. These Al11Ce3 phases have a similar crystal structure to the primary a(Al) phases. This particular crystal structure promotes a(Al) phases to nucleate on the cerium-rich phases. At the ¯ying stage of droplets, a large cooling rate was achieved by the extensive heat exchange between the droplets and the ultrasonic gas, whilst after deposition on the substrate the heat conducted rate was limited by the heat dissipation of the substrate and deposited metal. The change of cooling rate from rapid to slow is bene®cial to the forming of ®ne lamella eutectoid phases. 3.4. Damping mechanisms
Fig. 2. Relationship between damping capacities and temperatures for the samples of Table 2: (a) 1; (b) 2; (c) 3.
3.3. Microstructure The variance of the damping capacity and mechanical properties relates to the difference of the microstructure (Fig. 4). The microstructures of the spray-deposited high
A variety of mechanisms may contribute to the overall damping behavior of metals and alloys. These may include the effects due to thermoelasticity, crystallographic defects, eddy current, Snoeck effects, stress-induced ordering, and others [7,8]. Defects that may be present in crystalline materials include point-, line-, surface- and bulk-defects. Commonly, a good damping capacity may be detrimental to the mechanical properties. The following discussion will address the factors that are thought to have been active in the extruded as-spray deposited materials. Discussions on grain boundary viscosity, relaxation, and anelasticity in polycrystalline metals by Keà et al. [9] have indicated that viscous ¯ow at grain boundaries will serve as a source of internal friction. The energy dissipated in the boundaries is depend on the temperature, the shear stress, and the anelastic shear strain. In view of this, the ®ne grain microstructure of the as-spray-deposited material before and after extrusion may play an important role in the dissipation of elastic strain energy in the upper range of the temperature investigated. The variance of damping capacities between the as-cast and as-deposited materials may con®rm this. Except for the ®ne grain boundaries, the very ®ne lamella interfaces and interfaces between the silicon-rich phase and the lamella eutectoid also play a primary role in the improvement of the damping property. The lamella eutectoids are composed of zinc-rich and aluminum-rich phases of which the crystal substructure are hcp and fcc, respectively. It is know that it is easier for dislocations to slide in the crystal structure of hcp than that of fcc. Thus, the interfaces between the lamella eutectoid belong to movable interfaces. With the fraction of lamella eutectoids increasing, much energy was
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
97
Fig. 3. Fracture morphology of the as-sprayed materials investigated: (a) 2; (b) 3.
Fig. 4. Microstructure of the materials investigated: (a) 1; (b) 2.
dissipated between the movable interfaces of the lamella eutectoids and the damping capacity was greatly improved. Interfaces may affect the damping behavior of the materials because the interfaces are two-dimensional defects where the crystal structure is distorted locally. Apart from the interfaces between the lamella eutectoids, the effect of phase interfaces between the silicon-rich phases and the matrix on damping may be rationalized on the basis of the Schoeck theory. Based on the Schoeck theory, internal friction is increased by interface relaxation and anelastic strain induced by dislocations in the vicinity of the interface. The internal friction is proportional to changes in the volume fraction of the silicon-rich phases and the local stress at the interfaces. By inspection of the equations that Schoeck used, the following equation may be used to predict their contribution to high temperature damping [8]: 1 8 1 ÿ n 1 X 3 0 2 ai p13 i p213 3p 2 ÿ n V 32 1 ÿ n X 4pa3i 0 2 p13 i 2 3v 3p13 2 ÿ n 32 1 ÿ n X 0 2 f p13 i 2 3p13 2 ÿ n
Qÿ1
where f is the volume fraction of the silicon-rich phases, p13 the external stress, n the Poisson's ratio, V the sample
volume, ai the radius of the oblate spheroid i, and (p0 13)i the component of p13 in the plane of the spheroid i which can be relaxed. The above equation suggests that the damping capacity is directly proportional to the volume fraction of the silicon-rich phases. According to the results of the microstructural analysis, spray deposition, cerium modi®cation and extrusion are bene®cial to the securing of a re®ned microstructure. After these processes the amount of lamella eutectoids and silicon-rich phases are increased and the size of these is greatly re®ned. so the as-spray-deposited high silicon alloy ZA27 modi®ed by cerium possesses a high damping capacity. 4. Conclusions The cerium modi®cation to as-spray deposited high silicon alloy ZA27 is bene®cial to re®ning the grain and lamella eutectoid. The microstructure of the spray-deposited high silicon alloy ZA27 was made up of pores, ®ne lamella eutectoid, light dot-like phases and polygonal silicon-rich phases. Two reasons for the forming of ®ne lamella phases are given, one being the in¯uence of cerium modi®cation to the solidi®cation processing, the other being the change of the cooling rate of the atomized droplets from rapid to slow in the course of atomization and deposition.
98
Y. Liu et al. / Journal of Materials Processing Technology 106 (2000) 94±98
After extrusion, both the damping capacity and mechanical properties were greatly improved. According to analysis of the rupture morphology, a distribution of micrometersized pores in the as-sprayed deposits were the sources of brittle failure, whilst after extrusion, a large number of ductile dimples were distributed in the fracture, so the elongation and ultimate tensile strength were improved greatly. Apart from grain re®nement, the very ®ne lamella eutectoids and silicon-rich phases play a primary role in the improvement of the damping property. The lamella eutectoids are composed of zinc-rich and aluminum-rich phases of which the crystal substructures are hcp and fcc, respectively. The probable sliding system of substructure hcp is more than that of fcc so that the high damping capacities can be attributed primarily to phase interface thermoelastic damping. According to the Shoeck theory, the damping capacity is directly proportional to the volume fraction of the silicon-rich phases. No peak phenomena were observed for the specimens tested in the temperature range of interest.
Acknowledgements The authors would like to express their gratitude to the Natural Science Foundation for grant No. 59671026 and to the Aeronautical Science Foundation of China. References [1] J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 (2) (1994) 395±409. [2] M.A. Savas, S. Altintas, J. Mater. Sci. 28 (1993) 1775±1780. [3] L. Yang, Y. Liu, G. Yang, Y. Zhou, Acta Metall. Sinica (English Letters) 9 (2) (1996) 140±146. [4] L. Yang, L. Pang, G. Yang, Y. Zhou, Funct. Mater. 26 (4) (1995) 359± 361. [5] J. Zhang, R.J. Perez, C.R. Wong, E.J. Lavernia, Mater. Sci. Eng. R 13 (8) (1994) 330±332. [6] R.J. Perez, J. Zhang, M.N. Gungor, E.J. Lavernia, Metall. Trans. A 24 (1993) 701±709. [7] J.R. Davis, Aluminum and aluminum alloys, The Mater. Inform. Soc. (1993) 546. [8] J.D. Eshelby, Prog. Solid State Phys. 3 (1956) 79. [9] KeÃ, Lazan, Nowick, Zene.