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Composites from scrap? The future could be bright for MMCs
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Composites from scrap? The future could be bright for MMCs Using scrap steel and a slightly unusual processing route, Greek researchers have successfully produced nickel-iron-alumina composites that could challenge cost assumptions for future manufacture... etal and alloy matrix composites (MMCs) often show enhanced properties over unreinforced materials and represent an interesting class of composite for high-temperature applications [1-3]. Ceramic particle reinforced MMCs in particular also have the advantage of being generally isotropic. PM nickel alloys generally exhibit improved properties over conventional cast and wrought alloy products. Recently, nickeliron alloys have attracted much attention due to their mechanical and magnetic properties [4-7]. Bose et al have fabricated Ni3Fe-Y2O3 alloy matrix composites starting from the elemental Ni and carbonyl Fe powders [8]. More recently, interest has grown in the use of prealloyed powders produced by mechanical alloying for developing alloy matrix composites. Greek researchers working at the Chemical Engineering Department at the National Technical University in Athens looked at the commercial manufacturing potential of MMCs and took a novel approach to the raw material question, using powdered Ni3Fe ferro-nickel alloy
M
powder as the raw material instead of elemental powders. Ni3Fe is known for high ductility, its insensitivity toward testing environments, and its magnetic properties. In addition, there are ordering tendencies near this composition, generally improving mechanical performance [8-10]. It should be emphasised, they say, that the alloy powder used was produced from ferrous scrap, a widely available and comparatively low-cost waste material. The aim was to reduce the higher production cost of MMCs compared to conventional engineering materials, the final barrier for a broader expansion of their commercial application. The recovery of Ni3Fe in powder form from scrap is realised by a hydrometallurgical process with many advantages in its steps. Similarly produced Ni powder [11,12] has already been used in Ni-Al2O3 MMCs with encouraging results [13,14]. Composites were also prepared in the Greek study by mixing the appropriate amount of elemental Ni and Fe powders (3Ni+Fe: weight ratio of 3/1) of the same origin, to compare their physico-mechanical
properties with those of the composites developed starting from the Ni3Fe powder. From an economic point of view, if the production of comparable performance composites was achieved, the use of the metal powder mixture would be preferable to the corresponding alloy powder, taking into consideration the intrinsic parameters of the powder production method employed. The aim of making lower cost composites suggest the application of established, simple and economic PM techniques and the use of commercial grade Al2O3 powder as the ceramic reinforcement. α-Al2O3 is the most commonly used oxide for the production of MMCs because of its high hardness and specific stiffness, low density, electrical resistivity and thermal expansion coefficient and its stability, providing oxidation and corrosion resistance as well as high temperature mechanical properties. Ferrous scrap consisting of discarded cutting tools that could be classified as stainless steel 316, was the starting material that yielded metal and alloy powders for use in MMCs in the study. This waste
Figure 1. SEM micrographs of the Ni (left) Fe (centre) and Ni3Fe powders recovered from ferrous scrap.
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0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.
material was cut into smaller pieces and then treated by hydrometallurgy. The Ni3Fe powder production process employed is supplementary to previous research for the recovery of metals [11, 12]. According to this theory, Ni and Fe powders can be produced by reduction of the Ni and Fe chlorides with hydrogen, which results from the dissolution of the scrap with hydrochloric acid and are selectively extracted by Versatic Acid 6 (Shell Company Ltd.) from their acidic solution and then crystallised. Ni3Fe alloy powder was produced here from the Ni and Fe chlorides mixtures of the same origin in a similar way. The initial feeding was composed of FeCl2·4H2O and NiCl2·6H2O mixtures at weight proportions corresponding Fe/Ni=1:3. The parameters under investigation were the reduction temperature and time. A series of experiments
was carried out at the temperature range of 250oC - 450oC for two, four, six and eight hours. The optimum conditions proposed were T = 450oC and t = 6h. Measurements were carried out concerning the chemical composition, the particle shape and the size distribution of the produced powders. The particles were examined for cracks by mercury porosimetry. XRD analysis, SEM and WDXR analysis and DSC were used for the verification of the alloy. Commercial α-Al2O3 powder (corundum, 0.9 μm mean particle size, 99 per cent purity) from Aldrich Chemical Company, Inc was selected as the ceramic reinforcement. Two alloy ceramic composites, Ni3FeAl2O3 and (3Ni+Fe)-Al2O3, with a 100-65 wt% metallic content (0-45 vol% theoretically) were prepared through PM. The alloy or metal powders respectively were
prehomogenised with the ceramic powder in a laboratory mixer and then fully mixed in a ball mill for 20 min. The powder mixtures were uniaxially cold pressed at 750 MPa to form a series of disk shaped specimens. Compression difficulties concerning compacts' integrity and strength were observed with the rich-in-ceramic mixtures which appeared less sensitive to the increased compression load. These problems were more pronounced in the Ni3Febased samples, in which the particles’ plastic deformation during compression became even more difficult. This can be mainly attributed to the higher hardness of Ni3Fe powder (a permalloy) compared to Ni and Fe powders. From a practical point of view, compaction pressures even greater than those employed would be economically difficult to obtain. Actually, these hurdles were overcome by incorporating
Figure 2. Scrap steel may help the economics of metal matrix composite production.
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1.5 wt% zinc stearate powder in the mixtures. For measuring the Young's modulus and strength in particular, cylindrical composite bars were also formed, using CIP (2000 bar). In that case, polypropylene carbonate was added in the mixtures as a binder. All compacts were preheated at 400oC for one hour for additive burnout and then heated at 1200oC for four hours in nitrogen for consolidation. The sintering time and the relatively moderate sintering temperature were selected after optimisation, in order to achieve the best possible sintering results with the simple fabrication technique while restricting an excessive matrix grain growth and avoiding the formation of brittle interfacial reaction products which, otherwise, could result in a weak interface bond [15]. Finally, the specimens were gradually cooled to room temperature, also in nitrogen, to minimise oxidation and quenching. The phase identification of the sintered samples was performed by XRD measurements (Siemens, Diffractometer D 5000). The microstructural characterization was realised by SEM (Jeol JSM-6400) coupled with EDX analysis. The density of the sintered specimens was measured by the Archimedes' method. Then, relative densities were calculated using theoretical densities of the individual constituents. Electrical resistivity measurements were conducted on polished samples at room temperature using a four-point probe method (Programmable Current Source 224, Keithley UK; Multimeter 2000, Keithley UK; Sample rod SRH 9, Oxford Instruments UK). Hardness was measured on polished specimens using a Vickers microindentor (Shimadzu). The mean hardness values were calculated over five valid indentations per specimen. All measurements were performed on ten specimens of each composition and the average values were reported in the results. The Young's modulus and fracture strength were determined from bend measurements conducted on rectangular bars - machined from the fabricated cylindrical bars - in a load frame (Instron), using a three-point bend configuration with a crosshead speed of 0.1 m/min. All samples were loaded to failure, except for the pure metallic samples for which the plastic deformation exceeded 2 per cent. Typical SEM micrographs (Jeol JSM 35 CF) of the produced Ni, Fe and Ni3Fe
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powders are presented in Figure 1. The Ni3Fe powder exhibits distinguished structural units of the alloy, forming dendrites and agglomerates. WDXR analysis verified the proportion of Fe:Ni=1:3, so it can strongly be concluded that the agglomerate is attributed to Ni3Fe alloy known as Josephinite. No parti- Figure 3a. Apparent density as a function of metallic content. cle microcracks or crevices were revealed by mercury porosimetry. The characteristics of these powders are listed in Table 1. It can be seen that their values are generally similar to these of typical commercially available powders produced by atomisation - encouraging results for powder metallurgi- Figure 3b. Relative density as a function of metallic content. cal development. The XRD analysis results showed that metallographic observation which no phases other than the constituent were revealed small isolated pores in the matrix developed in the composites after sinter- and some large-sized pores or gaps, maining, which was expected at the sintering ly located at the interface, in the ceramictemperature and time applied. rich composites. The results obtained for The apparent density measurements porosity lead to the conclusion that the obtained for the sintered specimens are additive, although incorporated at only given in Figure 3a versus the metallic con- 1.5 wt%, did not only fill a fraction of tent (wt%). These results show that as the pores that otherwise would remain in the per cent amount of Al2O3 in the compos- bulk of the composites after compaction, ites increases, apparent density decreases but occupied a significant volume in these due to the lower density of the ceramic materials, thus creating new pores after its versus the metallic constituent. Values are removal during the compacts thermal even lower than those theoretically expect- treatment. From an economical aspect ed according to the rule of mixtures, however, a cost reduction is expected by because of residual porosity. The effect of producing objects of a reduced relative per cent reinforcement content on the rel- density. Besides, a porous microstructure ative density of the sintered composites offers advantages for specific applications, (or inversely the porosity) of the sintered e.g. with regard to thermal shock resisspecimens is illustrated in Figure 3b. An tance due to the improved expansion tolincrease in per cent ceramic clearly leads erance and a decrease in the modulus of to decreasing sintering degree (or inverse- elasticity (discussed below). The relationship between the electrical ly increasing porosity). In fact, the lower the amount of ceramic particles, the more resistivity of the composites and the continuous the contact between the metal- metallic content (wt%) is depicted in lic ones, allowing a more effective sinter- Figure 3a. It can be seen, that the lower the ing. This variation in consolidation degree per cent metallic content the greater the elecwith per cent Al2O3 was also verified during trical resistivity. The effect of incorporating
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only 10 wt% (about 20vol%) non-conductive ceramic phase on the composites electrical resistivity is relatively restricted, but above this per cent reinforcement there is a clear trend of increase in resistivity. All fabricated composites, even them with the higher Al2O3 content (35 wt% or about 55% vol%) remain conductive, which means that their metallic phases are continuous, verifying an accepted matrix grain connectivity degree. Therefore, the critical %vol ceramic for the transition from conductive to practically insulating behaviour was not surpassed. It should, however, be noted that resistivity values for the composites with the greater per cent Al2O3 rise at about three orders of magnitude higher than those of metals, which could be of importance for specific applications. Surely, these resistivity values indicate that many metallic particle contacts still remain in the aforementioned relative compositions, but it cannot necessarily be concluded that a fully interconnected metallic network has formed. For a better understanding of the resistivity values obtained, some microstructural parameters should also be taken into account, including mainly the residual porosity, the volume contiguity and even a possible electron scattering due to an increased dislocation density caused in the matrix by differential thermal residual stresses due to mismatch in the thermal expansion coefficient between metallic and ceramic phases [2]. (3Ni+Fe)-Al2O3 composites are more conductive than the Ni3Fe-based ones at each relative matrixreinforcement composition, because of a severe increase in resistivity that, generally, accompanies alloying in comparison to pure metals.
Figure 4a. Electrical resistivity values versus metallic content.
Figure 4b. Mean hardness values versus metallic content.
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The mean hardness values of the composites versus the metallic content (wt%) are plotted in Figure 4b. As the amount of the ceramic phase increases, the composites become harder, attaining relatively elevated hardness values at the higher per cent ceramic examined. This hardening effect of the reinforcement can be explained by an increase in the matrix strain hardening rate due to the development of a higher dislocation density. In fact, the Al2O3 particles act in the metal matrix of the composites as obstacles to dislocation movement and grain growth, thus increasing hardness. The characteristics of the used metal and alloy powders recovered from scrap, including their high chemical purity along with relatively small particle size, are beneficial to facilitating of sintering and binding of matrix at the relatively moderate temperatures chosen, taken into consideration the simplicity of the fabrication technique applied. Hence, they are considered to contribute to the improvement of hardness and other mechanical properties. In these composites, an increase in dislocation density, resulting in further increase of matrix hardening, may also be generated in the vicinity of the Al2O3 particles by the aforementioned thermal stresses developed at the metal-ceramic interfaces during and after processing. Attractive features of MMCs The mean hardness values achieved are clearly higher in the composites developed starting from the Ni3Fe powder, the harder of the matrix materials considered, than in the metal powder based ones, at each per cent Al2O3 content. Fig. 5a shows the modulus of elasticity of composites as a function of metallic content (wt%). Modulus (and therefore the stiffness) increases progressively with the increasing addition of stiffer Al2O3 particles. This modulus increase combined with the clear decrease in density lead to an improvement of specific modulus (modulus/density), that is broadly considered as one of the most attractive features of MMCs. The experimental values are lower than those theoretically predicted according to the rule of mixtures. This should be mainly attributed to residual porosity, and even some other microstructural features, such as possible microdamage accumulation in the bulk of the composites during
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Figure 5a. Young’s Modulus of Elasticity versus metallic content.
cooling, including microcracking within the Al2O3 particles, separation of contiguous ceramic particles, or metal-ceramic decohesion, reported in other research on Ni-Al2O3 composites [16]. Ni3Fe-Al2O3 composites slightly prevail in rigidity over (3Ni+Fe)-Al2O3 at a given per cent reinforcement amount. The difference in stiffness of the matrix materials used and a greater heterogeneity in the metal powder based MMCs - composed from three constituents instead of only two in the case of the alloy powder based ones - must be responsible for these results. In fact, this heterogeneity influences significant parameters for the composites’ elastic behaviour,
including porosity, the distribution of the reinforcement particles, the nature and strength of the interface and the development of microdamage. Figure 5a presents the fracture strength of composites versus metallic content (wt%). Strength exhibits slight increasing trends, when the per cent content of ceramic particles embedded in the matrix increases. Strengthening in these composites is actually achieved by two types of mechanisms. First, load transfer from matrix to reinforcement. Matrix strength maintains an important role, but strain-induced load transfer to reinforcement is appreciable in these discontinuously reinforced MMCs, given their relatively
Figure 5b. Strength versus metallic content.
metal-powder.net
Table 1. Comparison of Ni, Fe and Ni3Fe powders recovered from ferrous scrap with typical commercial grades.
high per cent ceramic [2]. Strengthening mechanisms concerning the reinforcement effect on the matrix deformation may come into play. Certainly, the residual porosity in these materials causes fracture to occur sooner after strain localisation begins, since pores are already present and do not have to grow significantly by plastic deformation during testing. On the other hand, porosity is a factor restricting matrix grain growth, but the benefits of this action cannot compensate the significant effects it has on failure mechanisms. The higher consolidation degree determined in the Ni3Fe-Al2O3 composites must count for the slight advantage in strength they present over (3Ni+Fe)-Al2O3 examples at a given per cent reinforcement. A higher relative density would normally lead to a further increase in strength but would also be more expensive to produce, and it must be noticed that for several PM applications a lower strength is not necessarily a disqualification criterion [17]. The testing results are encouraging, showing a tendency towards improved materials, with a clear density decrease, increased hardness values and improved
The authors THIS ARTICLE is based on Elaboration of MMCs Using Powders Recovered from Metallic Waste, a paper by V Karayannis, C Sotiriou, A Moutsatsou who work at the School of Chemical Engineering, National Technical University of Athens (NTUA). It was given at the EPMA's EuroPM2005 Conference and Exhibition in Prague.
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stiffness and fracture strength, as the percentage reinforcement increases, which are generally considered favourable and encouraging trends for structural applications. Electrical resistivity increases, but all MMCs, even them with the higher ceramic phase volume fraction, remain conductive. Both the Ni3Fe powder based composites and those produced from Ni and Fe powders
exhibit similar trends concerning their physico-mechanical properties with increasing per cent reinforcement content, but differences in matrix composition as well as in some microstructural features, mainly porosity, give a certain advantage in mechanical performance of Ni3Fe-Al2O3 over (3Ni+Fe)-Al2O3 MMCs at each per cent reinforcement content.
References [1] F Matthews and R Rawlings, Composite Materials: Engineering and Science, 2nd ed., Chapman & Hall, Oxford, 1995, p. 79. [2] T Clyne, P Withers, An Introduction to Metal Matrix Composites, Cambridge University Press, Cambridge, 1993, pp. 347,454. [3] S Suresh, A Mortensen, A Needleman, Fundamentals of Metal Matrix Composites, Butterworth-Heinemann, 1993, pp. 26, 297. [4] L Coutu, L Chaput, T Waeckerle, J Magn. Magn. Mater., 215-216 (2000), 237239. [5] H Li, F Ebrahimi Mat. Sci Eng., A347 (2003), 93-101. [6] H Li, F Ebrahimi, Acta mater., 51 (2003), 3905-3913. [7] F Ebrahimi, H Li, Rev. Adv. Mater. Sci., 5 (2003), 134-138. [8] A Bose, G Camus, R M German, D J Duquette, N.S. Stoloff, J. Mater. Res., 8 [3] (1993), 430-437. [9] J Wang, W J Chia, Y.W. Chung, C.T. Liu, Intermetallics, 8 (2000), 353-357. [10] C N Chinnasamy, A. Narayanasamy,
K. Chattopadhyay, N. Ponpandian, Nanostructured Mater., 12 [5-8] (1999), 951-954. [11] A Moutsatsou, G. Parissakis, in: Proceedings of the International Recycling Congress ReC'93, on "Advances in recovery and recycling", Geneva, Switzerland, Hexagon Ltd., 1993, pp. 273-277. [12] A Moutsatsou, S Tsivilis, S Tsimas, Hydrometallurgy, 38 (1995), 205-213. [13] V Karayannis, A Moutsatsou, in: Abstracts of the Metal Matrix Composites VII Conference, IoM, London, 1999. [14] A Moutsatsou, V Karayannis, Metal Powder Report, 55 (2000), 33-36. [15] P Lourdin, D Juve, D Treheux, J Europ. Ceram. Soc., 16 (1996), 745-752. [16] B H Rabin, R L Williamson, H.A. Bruck, X.-L. Wang, T.R. Watkins, Y.-Z. Feng, D.R. Clarke, J. Am. Ceram. Soc., 81 [6] (1998), 1541-49. [17] G Dowson, Powder Metallurgy: The Process and its Products, IOP Publishing Ltd., Adam Hilger, Bristol and New York, 1990, pp. 32, 46.
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Composites from scrap? The future could be bright for MMCs Using scrap steel and a slightly unusual processing route, Greek researchers have successfully produced nickel-iron-alumina composites that could challenge cost assumptions for future manufacture... etal and alloy matrix composites (MMCs) often show enhanced properties over unreinforced materials and represent an interesting class of composite for high-temperature applications [1-3]. Ceramic particle reinforced MMCs in particular also have the advantage of being generally isotropic. PM nickel alloys generally exhibit improved properties over conventional cast and wrought alloy products. Recently, nickeliron alloys have attracted much attention due to their mechanical and magnetic properties [4-7]. Bose et al have fabricated Ni3Fe-Y2O3 alloy matrix composites starting from the elemental Ni and carbonyl Fe powders [8]. More recently, interest has grown in the use of prealloyed powders produced by mechanical alloying for developing alloy matrix composites. Greek researchers working at the Chemical Engineering Department at the National Technical University in Athens looked at the commercial manufacturing potential of MMCs and took a novel approach to the raw material question, using powdered Ni3Fe ferro-nickel alloy
M
powder as the raw material instead of elemental powders. Ni3Fe is known for high ductility, its insensitivity toward testing environments, and its magnetic properties. In addition, there are ordering tendencies near this composition, generally improving mechanical performance [8-10]. It should be emphasised, they say, that the alloy powder used was produced from ferrous scrap, a widely available and comparatively low-cost waste material. The aim was to reduce the higher production cost of MMCs compared to conventional engineering materials, the final barrier for a broader expansion of their commercial application. The recovery of Ni3Fe in powder form from scrap is realised by a hydrometallurgical process with many advantages in its steps. Similarly produced Ni powder [11,12] has already been used in Ni-Al2O3 MMCs with encouraging results [13,14]. Composites were also prepared in the Greek study by mixing the appropriate amount of elemental Ni and Fe powders (3Ni+Fe: weight ratio of 3/1) of the same origin, to compare their physico-mechanical
properties with those of the composites developed starting from the Ni3Fe powder. From an economic point of view, if the production of comparable performance composites was achieved, the use of the metal powder mixture would be preferable to the corresponding alloy powder, taking into consideration the intrinsic parameters of the powder production method employed. The aim of making lower cost composites suggest the application of established, simple and economic PM techniques and the use of commercial grade Al2O3 powder as the ceramic reinforcement. α-Al2O3 is the most commonly used oxide for the production of MMCs because of its high hardness and specific stiffness, low density, electrical resistivity and thermal expansion coefficient and its stability, providing oxidation and corrosion resistance as well as high temperature mechanical properties. Ferrous scrap consisting of discarded cutting tools that could be classified as stainless steel 316, was the starting material that yielded metal and alloy powders for use in MMCs in the study. This waste
Figure 1. SEM micrographs of the Ni (left) Fe (centre) and Ni3Fe powders recovered from ferrous scrap.
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0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.
material was cut into smaller pieces and then treated by hydrometallurgy. The Ni3Fe powder production process employed is supplementary to previous research for the recovery of metals [11, 12]. According to this theory, Ni and Fe powders can be produced by reduction of the Ni and Fe chlorides with hydrogen, which results from the dissolution of the scrap with hydrochloric acid and are selectively extracted by Versatic Acid 6 (Shell Company Ltd.) from their acidic solution and then crystallised. Ni3Fe alloy powder was produced here from the Ni and Fe chlorides mixtures of the same origin in a similar way. The initial feeding was composed of FeCl2·4H2O and NiCl2·6H2O mixtures at weight proportions corresponding Fe/Ni=1:3. The parameters under investigation were the reduction temperature and time. A series of experiments
was carried out at the temperature range of 250oC - 450oC for two, four, six and eight hours. The optimum conditions proposed were T = 450oC and t = 6h. Measurements were carried out concerning the chemical composition, the particle shape and the size distribution of the produced powders. The particles were examined for cracks by mercury porosimetry. XRD analysis, SEM and WDXR analysis and DSC were used for the verification of the alloy. Commercial α-Al2O3 powder (corundum, 0.9 μm mean particle size, 99 per cent purity) from Aldrich Chemical Company, Inc was selected as the ceramic reinforcement. Two alloy ceramic composites, Ni3FeAl2O3 and (3Ni+Fe)-Al2O3, with a 100-65 wt% metallic content (0-45 vol% theoretically) were prepared through PM. The alloy or metal powders respectively were
prehomogenised with the ceramic powder in a laboratory mixer and then fully mixed in a ball mill for 20 min. The powder mixtures were uniaxially cold pressed at 750 MPa to form a series of disk shaped specimens. Compression difficulties concerning compacts' integrity and strength were observed with the rich-in-ceramic mixtures which appeared less sensitive to the increased compression load. These problems were more pronounced in the Ni3Febased samples, in which the particles’ plastic deformation during compression became even more difficult. This can be mainly attributed to the higher hardness of Ni3Fe powder (a permalloy) compared to Ni and Fe powders. From a practical point of view, compaction pressures even greater than those employed would be economically difficult to obtain. Actually, these hurdles were overcome by incorporating
Figure 2. Scrap steel may help the economics of metal matrix composite production.
metal-powder.net
April 2006 MPR
19
1.5 wt% zinc stearate powder in the mixtures. For measuring the Young's modulus and strength in particular, cylindrical composite bars were also formed, using CIP (2000 bar). In that case, polypropylene carbonate was added in the mixtures as a binder. All compacts were preheated at 400oC for one hour for additive burnout and then heated at 1200oC for four hours in nitrogen for consolidation. The sintering time and the relatively moderate sintering temperature were selected after optimisation, in order to achieve the best possible sintering results with the simple fabrication technique while restricting an excessive matrix grain growth and avoiding the formation of brittle interfacial reaction products which, otherwise, could result in a weak interface bond [15]. Finally, the specimens were gradually cooled to room temperature, also in nitrogen, to minimise oxidation and quenching. The phase identification of the sintered samples was performed by XRD measurements (Siemens, Diffractometer D 5000). The microstructural characterization was realised by SEM (Jeol JSM-6400) coupled with EDX analysis. The density of the sintered specimens was measured by the Archimedes' method. Then, relative densities were calculated using theoretical densities of the individual constituents. Electrical resistivity measurements were conducted on polished samples at room temperature using a four-point probe method (Programmable Current Source 224, Keithley UK; Multimeter 2000, Keithley UK; Sample rod SRH 9, Oxford Instruments UK). Hardness was measured on polished specimens using a Vickers microindentor (Shimadzu). The mean hardness values were calculated over five valid indentations per specimen. All measurements were performed on ten specimens of each composition and the average values were reported in the results. The Young's modulus and fracture strength were determined from bend measurements conducted on rectangular bars - machined from the fabricated cylindrical bars - in a load frame (Instron), using a three-point bend configuration with a crosshead speed of 0.1 m/min. All samples were loaded to failure, except for the pure metallic samples for which the plastic deformation exceeded 2 per cent. Typical SEM micrographs (Jeol JSM 35 CF) of the produced Ni, Fe and Ni3Fe
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powders are presented in Figure 1. The Ni3Fe powder exhibits distinguished structural units of the alloy, forming dendrites and agglomerates. WDXR analysis verified the proportion of Fe:Ni=1:3, so it can strongly be concluded that the agglomerate is attributed to Ni3Fe alloy known as Josephinite. No parti- Figure 3a. Apparent density as a function of metallic content. cle microcracks or crevices were revealed by mercury porosimetry. The characteristics of these powders are listed in Table 1. It can be seen that their values are generally similar to these of typical commercially available powders produced by atomisation - encouraging results for powder metallurgi- Figure 3b. Relative density as a function of metallic content. cal development. The XRD analysis results showed that metallographic observation which no phases other than the constituent were revealed small isolated pores in the matrix developed in the composites after sinter- and some large-sized pores or gaps, maining, which was expected at the sintering ly located at the interface, in the ceramictemperature and time applied. rich composites. The results obtained for The apparent density measurements porosity lead to the conclusion that the obtained for the sintered specimens are additive, although incorporated at only given in Figure 3a versus the metallic con- 1.5 wt%, did not only fill a fraction of tent (wt%). These results show that as the pores that otherwise would remain in the per cent amount of Al2O3 in the compos- bulk of the composites after compaction, ites increases, apparent density decreases but occupied a significant volume in these due to the lower density of the ceramic materials, thus creating new pores after its versus the metallic constituent. Values are removal during the compacts thermal even lower than those theoretically expect- treatment. From an economical aspect ed according to the rule of mixtures, however, a cost reduction is expected by because of residual porosity. The effect of producing objects of a reduced relative per cent reinforcement content on the rel- density. Besides, a porous microstructure ative density of the sintered composites offers advantages for specific applications, (or inversely the porosity) of the sintered e.g. with regard to thermal shock resisspecimens is illustrated in Figure 3b. An tance due to the improved expansion tolincrease in per cent ceramic clearly leads erance and a decrease in the modulus of to decreasing sintering degree (or inverse- elasticity (discussed below). The relationship between the electrical ly increasing porosity). In fact, the lower the amount of ceramic particles, the more resistivity of the composites and the continuous the contact between the metal- metallic content (wt%) is depicted in lic ones, allowing a more effective sinter- Figure 3a. It can be seen, that the lower the ing. This variation in consolidation degree per cent metallic content the greater the elecwith per cent Al2O3 was also verified during trical resistivity. The effect of incorporating
metal-powder.net
only 10 wt% (about 20vol%) non-conductive ceramic phase on the composites electrical resistivity is relatively restricted, but above this per cent reinforcement there is a clear trend of increase in resistivity. All fabricated composites, even them with the higher Al2O3 content (35 wt% or about 55% vol%) remain conductive, which means that their metallic phases are continuous, verifying an accepted matrix grain connectivity degree. Therefore, the critical %vol ceramic for the transition from conductive to practically insulating behaviour was not surpassed. It should, however, be noted that resistivity values for the composites with the greater per cent Al2O3 rise at about three orders of magnitude higher than those of metals, which could be of importance for specific applications. Surely, these resistivity values indicate that many metallic particle contacts still remain in the aforementioned relative compositions, but it cannot necessarily be concluded that a fully interconnected metallic network has formed. For a better understanding of the resistivity values obtained, some microstructural parameters should also be taken into account, including mainly the residual porosity, the volume contiguity and even a possible electron scattering due to an increased dislocation density caused in the matrix by differential thermal residual stresses due to mismatch in the thermal expansion coefficient between metallic and ceramic phases [2]. (3Ni+Fe)-Al2O3 composites are more conductive than the Ni3Fe-based ones at each relative matrixreinforcement composition, because of a severe increase in resistivity that, generally, accompanies alloying in comparison to pure metals.
Figure 4a. Electrical resistivity values versus metallic content.
Figure 4b. Mean hardness values versus metallic content.
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The mean hardness values of the composites versus the metallic content (wt%) are plotted in Figure 4b. As the amount of the ceramic phase increases, the composites become harder, attaining relatively elevated hardness values at the higher per cent ceramic examined. This hardening effect of the reinforcement can be explained by an increase in the matrix strain hardening rate due to the development of a higher dislocation density. In fact, the Al2O3 particles act in the metal matrix of the composites as obstacles to dislocation movement and grain growth, thus increasing hardness. The characteristics of the used metal and alloy powders recovered from scrap, including their high chemical purity along with relatively small particle size, are beneficial to facilitating of sintering and binding of matrix at the relatively moderate temperatures chosen, taken into consideration the simplicity of the fabrication technique applied. Hence, they are considered to contribute to the improvement of hardness and other mechanical properties. In these composites, an increase in dislocation density, resulting in further increase of matrix hardening, may also be generated in the vicinity of the Al2O3 particles by the aforementioned thermal stresses developed at the metal-ceramic interfaces during and after processing. Attractive features of MMCs The mean hardness values achieved are clearly higher in the composites developed starting from the Ni3Fe powder, the harder of the matrix materials considered, than in the metal powder based ones, at each per cent Al2O3 content. Fig. 5a shows the modulus of elasticity of composites as a function of metallic content (wt%). Modulus (and therefore the stiffness) increases progressively with the increasing addition of stiffer Al2O3 particles. This modulus increase combined with the clear decrease in density lead to an improvement of specific modulus (modulus/density), that is broadly considered as one of the most attractive features of MMCs. The experimental values are lower than those theoretically predicted according to the rule of mixtures. This should be mainly attributed to residual porosity, and even some other microstructural features, such as possible microdamage accumulation in the bulk of the composites during
22
MPR April 2006
Figure 5a. Young’s Modulus of Elasticity versus metallic content.
cooling, including microcracking within the Al2O3 particles, separation of contiguous ceramic particles, or metal-ceramic decohesion, reported in other research on Ni-Al2O3 composites [16]. Ni3Fe-Al2O3 composites slightly prevail in rigidity over (3Ni+Fe)-Al2O3 at a given per cent reinforcement amount. The difference in stiffness of the matrix materials used and a greater heterogeneity in the metal powder based MMCs - composed from three constituents instead of only two in the case of the alloy powder based ones - must be responsible for these results. In fact, this heterogeneity influences significant parameters for the composites’ elastic behaviour,
including porosity, the distribution of the reinforcement particles, the nature and strength of the interface and the development of microdamage. Figure 5a presents the fracture strength of composites versus metallic content (wt%). Strength exhibits slight increasing trends, when the per cent content of ceramic particles embedded in the matrix increases. Strengthening in these composites is actually achieved by two types of mechanisms. First, load transfer from matrix to reinforcement. Matrix strength maintains an important role, but strain-induced load transfer to reinforcement is appreciable in these discontinuously reinforced MMCs, given their relatively
Figure 5b. Strength versus metallic content.
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Table 1. Comparison of Ni, Fe and Ni3Fe powders recovered from ferrous scrap with typical commercial grades.
high per cent ceramic [2]. Strengthening mechanisms concerning the reinforcement effect on the matrix deformation may come into play. Certainly, the residual porosity in these materials causes fracture to occur sooner after strain localisation begins, since pores are already present and do not have to grow significantly by plastic deformation during testing. On the other hand, porosity is a factor restricting matrix grain growth, but the benefits of this action cannot compensate the significant effects it has on failure mechanisms. The higher consolidation degree determined in the Ni3Fe-Al2O3 composites must count for the slight advantage in strength they present over (3Ni+Fe)-Al2O3 examples at a given per cent reinforcement. A higher relative density would normally lead to a further increase in strength but would also be more expensive to produce, and it must be noticed that for several PM applications a lower strength is not necessarily a disqualification criterion [17]. The testing results are encouraging, showing a tendency towards improved materials, with a clear density decrease, increased hardness values and improved
The authors THIS ARTICLE is based on Elaboration of MMCs Using Powders Recovered from Metallic Waste, a paper by V Karayannis, C Sotiriou, A Moutsatsou who work at the School of Chemical Engineering, National Technical University of Athens (NTUA). It was given at the EPMA's EuroPM2005 Conference and Exhibition in Prague.
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stiffness and fracture strength, as the percentage reinforcement increases, which are generally considered favourable and encouraging trends for structural applications. Electrical resistivity increases, but all MMCs, even them with the higher ceramic phase volume fraction, remain conductive. Both the Ni3Fe powder based composites and those produced from Ni and Fe powders
exhibit similar trends concerning their physico-mechanical properties with increasing per cent reinforcement content, but differences in matrix composition as well as in some microstructural features, mainly porosity, give a certain advantage in mechanical performance of Ni3Fe-Al2O3 over (3Ni+Fe)-Al2O3 MMCs at each per cent reinforcement content.
References [1] F Matthews and R Rawlings, Composite Materials: Engineering and Science, 2nd ed., Chapman & Hall, Oxford, 1995, p. 79. [2] T Clyne, P Withers, An Introduction to Metal Matrix Composites, Cambridge University Press, Cambridge, 1993, pp. 347,454. [3] S Suresh, A Mortensen, A Needleman, Fundamentals of Metal Matrix Composites, Butterworth-Heinemann, 1993, pp. 26, 297. [4] L Coutu, L Chaput, T Waeckerle, J Magn. Magn. Mater., 215-216 (2000), 237239. [5] H Li, F Ebrahimi Mat. Sci Eng., A347 (2003), 93-101. [6] H Li, F Ebrahimi, Acta mater., 51 (2003), 3905-3913. [7] F Ebrahimi, H Li, Rev. Adv. Mater. Sci., 5 (2003), 134-138. [8] A Bose, G Camus, R M German, D J Duquette, N.S. Stoloff, J. Mater. Res., 8 [3] (1993), 430-437. [9] J Wang, W J Chia, Y.W. Chung, C.T. Liu, Intermetallics, 8 (2000), 353-357. [10] C N Chinnasamy, A. Narayanasamy,
K. Chattopadhyay, N. Ponpandian, Nanostructured Mater., 12 [5-8] (1999), 951-954. [11] A Moutsatsou, G. Parissakis, in: Proceedings of the International Recycling Congress ReC'93, on "Advances in recovery and recycling", Geneva, Switzerland, Hexagon Ltd., 1993, pp. 273-277. [12] A Moutsatsou, S Tsivilis, S Tsimas, Hydrometallurgy, 38 (1995), 205-213. [13] V Karayannis, A Moutsatsou, in: Abstracts of the Metal Matrix Composites VII Conference, IoM, London, 1999. [14] A Moutsatsou, V Karayannis, Metal Powder Report, 55 (2000), 33-36. [15] P Lourdin, D Juve, D Treheux, J Europ. Ceram. Soc., 16 (1996), 745-752. [16] B H Rabin, R L Williamson, H.A. Bruck, X.-L. Wang, T.R. Watkins, Y.-Z. Feng, D.R. Clarke, J. Am. Ceram. Soc., 81 [6] (1998), 1541-49. [17] G Dowson, Powder Metallurgy: The Process and its Products, IOP Publishing Ltd., Adam Hilger, Bristol and New York, 1990, pp. 32, 46.
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