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Microstructural evolution in mechanically alloyed Al-heavy-metal oxide composites
Materials Chemistry and Physics 81 (2003) 387–389
Microstructural evolution in mechanically alloyed Al-heavy-metal oxide composites L. Blaz a,∗ , J. Kaneko b , M. Sugamata b a
Faculty of Non-ferrous Metals, University of Mining and Metallurgy, Al. Mickiewicza 30, Cracow 30-069, Poland b Nihon University, Narashino, Chiba 275-8575, Japan
Abstract Structural observations of mechanically alloyed Al–MnO2 , Al–HfO2 and Al–WO3 composites were performed by means of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The distribution of the constituent elements was tested in X-ray energy-dispersive spectroscopy (EDS). Samples of the as-extruded material were annealed at 873 K for various times. The microstructure changed due to a solid-state reaction between the Al-matrix and heavy-metal oxides. The distribution of oxygen was analyzed by EDS across oxide/matrix boundaries in order to test an Al-matrix oxidation nearby pre-existing oxide particles. X-ray diffraction analysis revealed an intermetallic grain growth after long annealing times. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; Al–MnO2 ; Al–HfO2 ; Al–WO3 ; Metallic composites
1. Introduction Mechanical alloying is a method for the production of materials containing components, which are immiscible both in solid and liquid state. For example, hot extrusion of mechanically alloyed Al-based alloys, containing additions of PbO, Sb2 O3 , SnO or some other metal oxides (MeO), was found to be an effective method for the production of fine-grained composites [1,2]. An extremely fine-grained structure of the composites results in high strength and hardness of the material. A higher affinity of the oxygen to the aluminum than to the Me element leads to a decomposition of the Me-oxides in the Al-matrix during high-temperature annealing. Me-particle development within the Al-matrix was accompanied by an oxidation of the neighboring Al-matrix. A chemical reaction of the components was also found to result in an increased porosity of the material. A local volume reduction was related to the density reduction of substrates taking place in the reaction. In the present work, the addition of manganese, tungsten and hafnium oxides to Al-based composites was used. Experiments on heat treatment of the materials were performed in order to test the structural features of a chemical reaction between the Al-matrix and the alloying compo∗ Corresponding author. Tel.: +48-12-617-2648; fax: +48-12-632-5615. E-mail address: [email protected] (L. Blaz).
nents. In result of Me-oxide reduction at high temperatures and diffusion of the released heavy metal into Al-matrix, new structural components may develop, i.e. intermetallic phase grains. Transmission electron microscopy (TEM) observations were undertaken in order to test the material structure, which had become transformed as a result of the high-temperature annealing.
2. Experimental details Mechanical alloying of high-purity aluminum powder with addition of approximately 10 wt.% oxide powders was performed in argon atmosphere by means of an Attritor ball mill. The alloyed powders were then compressed at room temperature and hot extruded at 673 K with cross-section reduction of 1/25. A set of samples was annealed at 873 K for 30, 60, 360 and 1440 min. Vickers hardness tests and structure observations were performed on samples sectioned along the extrusion axis. Hardness tests were performed under a load of 9.8 N. The samples for TEM observations were cut from as-extruded and annealed rods of 7 mm in diameter. Thin foils for TEM observations were prepared by means of the Gatan’s PIPS 691-type precision ion thinning machine. Structure analysis was performed by means of a JEM 2010 ARP analytical electron microscope equipped with a scanning transmission electron microscopy (STEM) device
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00028-2
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L. Blaz et al. / Materials Chemistry and Physics 81 (2003) 387–389
and Oxford-Pentafet X-ray detector operating under ISIS computer program.
3. Results A TEM micrograph of the as-extruded Al–HfO2 composite is shown in Fig. 1. It should be stressed that there were not any particular differences in TEM micrographs taken from the other as-extruded alloys, i.e. Al–WO3 and Al–MnO2 . The fine-grained material did not practically contain any voids. Some retained aluminum grains, 0.5–1 m in size, were observed within the heavily refined mixture of the components. Refined Al- and Me-oxide grains, 5–300 nm in size, were uniformly distributed within the matrix. The effect of annealing time at 873 K on the hardness of tested materials is shown in Fig. 2. The initial hardness of the as-extruded composites was also marked in the figure. X-ray diffraction analysis was performed in order to detect intermetallic phases developed in the materials annealed for 1440 min at 873 K. The results are shown in Fig. 3. The identified intensity peaks are marked in the figure. Annealing experiments were performed for various annealing times. A typical structure of the samples annealed at 873 K for 24 h, is shown in Fig. 4. The distribution of elements was tested by STEM mapping and EDS analyses for as-extruded
Fig. 3. X-ray diffraction analysis results for samples annealed at 873 K for 24 h: (a) Al–HfO2 , (b) Al–MnO2 , (c) Al–WO3 . Identified peaks of the intermetallic phases are marked in the figure.
and annealed samples. An example of EDS analysis for an Al–MnO2 sample annealed at 873 K for 6 h is shown in Fig. 5. The EDS analyses were performed at a set of points displaced along the line marked in the figure. Fig. 1. TEM micrograph of as-extruded Al–HfO2 composite; Al: remaining Al-grains, Hf: HfO2 particle.
4. Discussion
Fig. 2. Effect of annealing time at 873 K on the hardness of Al–MnO2 , Al–WO3 and Al–HfO2 composites.
TEM observations revealed a very fine structure of both as-extruded and annealed Al–HfO2 , Al–MnO2 and Al–WO3 composites. A mixture of mechanically refined alloying components, 5–300 nm in size, seemed to be similar for all materials tested by TEM. Very few remaining Al-grains, 0.5–1 m in size, could be distinguished within the matrix. STEM mapping and EDS analyses revealed a practically uniform distribution of fine oxide particles. Voids were rarely observed (Fig. 1). The fine-grained structure is responsible for the high hardness of the materials (Fig. 2). Structural softening processes, i.e. recovery and/or recrystallization, were very limited. Thus, instead of softening, the hardness increase, at
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Fig. 5. Element distribution across a pre-existing MnO2 particle in an Al–MnO2 sample annealed at 873 K for 6 h. EDS analyses were performed at points placed along the line marked in the figure.
Analysis of the oxygen distribution in the neighborhood of heavy-metal oxides revealed the creation of an Al-oxide skin at the beginning of the chemical reaction between the particle and Al-matrix. An oxygen-enriched layer was also observed within the intermetallic grains that probably marked the initial position of the pre-existing heavy-metal oxide (Fig. 5). Similar effects were observed for the other tested materials.
5. Conclusions • Fine-grained Al-based composites, containing Hf, Mn and W oxides, can be produced by mechanical alloying and hot-extrusion method. • Annealing of the composites results in a chemical reaction of the components and growth of very fine Al-oxide particles and new intermetallic grains.
Acknowledgements Fig. 4. STEM pictures of samples annealed at 873 K for 24 h: (a) Al–HfO2 , (b) Al–MnO2 , (c) Al–WO3 ; O: oxide, IP: intermetallic phase.
the initial annealing. Annealing of the composites at 873 K promoted a chemical reaction between aluminum matrix and heavy-metal oxides. As a result, needle-like Al-oxide particles and intermetallic grains were grown. Thus, the hardness increases due to the development of fine structural components. The hardness maximum was followed by a hardness decrease at long aging times, which might result from grain/particle coarsening.
TEM observations were performed under grant no. 4T08A 00122. One of the authors (LB) is indebted to the Foundation for Polish Science for supplying the JEM electron microscope. References [1] J. Kaneko, M. Sugamata, L. Blaz, R. Kamei, Key Eng. Mater. 188 (2000) 73. [2] L. Blaz, J. Kaneko, M. Sugamata, R. Kamei, Mater. Sci. Eng., in press.
Microstructural evolution in mechanically alloyed Al-heavy-metal oxide composites L. Blaz a,∗ , J. Kaneko b , M. Sugamata b a
Faculty of Non-ferrous Metals, University of Mining and Metallurgy, Al. Mickiewicza 30, Cracow 30-069, Poland b Nihon University, Narashino, Chiba 275-8575, Japan
Abstract Structural observations of mechanically alloyed Al–MnO2 , Al–HfO2 and Al–WO3 composites were performed by means of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The distribution of the constituent elements was tested in X-ray energy-dispersive spectroscopy (EDS). Samples of the as-extruded material were annealed at 873 K for various times. The microstructure changed due to a solid-state reaction between the Al-matrix and heavy-metal oxides. The distribution of oxygen was analyzed by EDS across oxide/matrix boundaries in order to test an Al-matrix oxidation nearby pre-existing oxide particles. X-ray diffraction analysis revealed an intermetallic grain growth after long annealing times. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; Al–MnO2 ; Al–HfO2 ; Al–WO3 ; Metallic composites
1. Introduction Mechanical alloying is a method for the production of materials containing components, which are immiscible both in solid and liquid state. For example, hot extrusion of mechanically alloyed Al-based alloys, containing additions of PbO, Sb2 O3 , SnO or some other metal oxides (MeO), was found to be an effective method for the production of fine-grained composites [1,2]. An extremely fine-grained structure of the composites results in high strength and hardness of the material. A higher affinity of the oxygen to the aluminum than to the Me element leads to a decomposition of the Me-oxides in the Al-matrix during high-temperature annealing. Me-particle development within the Al-matrix was accompanied by an oxidation of the neighboring Al-matrix. A chemical reaction of the components was also found to result in an increased porosity of the material. A local volume reduction was related to the density reduction of substrates taking place in the reaction. In the present work, the addition of manganese, tungsten and hafnium oxides to Al-based composites was used. Experiments on heat treatment of the materials were performed in order to test the structural features of a chemical reaction between the Al-matrix and the alloying compo∗ Corresponding author. Tel.: +48-12-617-2648; fax: +48-12-632-5615. E-mail address: [email protected] (L. Blaz).
nents. In result of Me-oxide reduction at high temperatures and diffusion of the released heavy metal into Al-matrix, new structural components may develop, i.e. intermetallic phase grains. Transmission electron microscopy (TEM) observations were undertaken in order to test the material structure, which had become transformed as a result of the high-temperature annealing.
2. Experimental details Mechanical alloying of high-purity aluminum powder with addition of approximately 10 wt.% oxide powders was performed in argon atmosphere by means of an Attritor ball mill. The alloyed powders were then compressed at room temperature and hot extruded at 673 K with cross-section reduction of 1/25. A set of samples was annealed at 873 K for 30, 60, 360 and 1440 min. Vickers hardness tests and structure observations were performed on samples sectioned along the extrusion axis. Hardness tests were performed under a load of 9.8 N. The samples for TEM observations were cut from as-extruded and annealed rods of 7 mm in diameter. Thin foils for TEM observations were prepared by means of the Gatan’s PIPS 691-type precision ion thinning machine. Structure analysis was performed by means of a JEM 2010 ARP analytical electron microscope equipped with a scanning transmission electron microscopy (STEM) device
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00028-2
388
L. Blaz et al. / Materials Chemistry and Physics 81 (2003) 387–389
and Oxford-Pentafet X-ray detector operating under ISIS computer program.
3. Results A TEM micrograph of the as-extruded Al–HfO2 composite is shown in Fig. 1. It should be stressed that there were not any particular differences in TEM micrographs taken from the other as-extruded alloys, i.e. Al–WO3 and Al–MnO2 . The fine-grained material did not practically contain any voids. Some retained aluminum grains, 0.5–1 m in size, were observed within the heavily refined mixture of the components. Refined Al- and Me-oxide grains, 5–300 nm in size, were uniformly distributed within the matrix. The effect of annealing time at 873 K on the hardness of tested materials is shown in Fig. 2. The initial hardness of the as-extruded composites was also marked in the figure. X-ray diffraction analysis was performed in order to detect intermetallic phases developed in the materials annealed for 1440 min at 873 K. The results are shown in Fig. 3. The identified intensity peaks are marked in the figure. Annealing experiments were performed for various annealing times. A typical structure of the samples annealed at 873 K for 24 h, is shown in Fig. 4. The distribution of elements was tested by STEM mapping and EDS analyses for as-extruded
Fig. 3. X-ray diffraction analysis results for samples annealed at 873 K for 24 h: (a) Al–HfO2 , (b) Al–MnO2 , (c) Al–WO3 . Identified peaks of the intermetallic phases are marked in the figure.
and annealed samples. An example of EDS analysis for an Al–MnO2 sample annealed at 873 K for 6 h is shown in Fig. 5. The EDS analyses were performed at a set of points displaced along the line marked in the figure. Fig. 1. TEM micrograph of as-extruded Al–HfO2 composite; Al: remaining Al-grains, Hf: HfO2 particle.
4. Discussion
Fig. 2. Effect of annealing time at 873 K on the hardness of Al–MnO2 , Al–WO3 and Al–HfO2 composites.
TEM observations revealed a very fine structure of both as-extruded and annealed Al–HfO2 , Al–MnO2 and Al–WO3 composites. A mixture of mechanically refined alloying components, 5–300 nm in size, seemed to be similar for all materials tested by TEM. Very few remaining Al-grains, 0.5–1 m in size, could be distinguished within the matrix. STEM mapping and EDS analyses revealed a practically uniform distribution of fine oxide particles. Voids were rarely observed (Fig. 1). The fine-grained structure is responsible for the high hardness of the materials (Fig. 2). Structural softening processes, i.e. recovery and/or recrystallization, were very limited. Thus, instead of softening, the hardness increase, at
L. Blaz et al. / Materials Chemistry and Physics 81 (2003) 387–389
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Fig. 5. Element distribution across a pre-existing MnO2 particle in an Al–MnO2 sample annealed at 873 K for 6 h. EDS analyses were performed at points placed along the line marked in the figure.
Analysis of the oxygen distribution in the neighborhood of heavy-metal oxides revealed the creation of an Al-oxide skin at the beginning of the chemical reaction between the particle and Al-matrix. An oxygen-enriched layer was also observed within the intermetallic grains that probably marked the initial position of the pre-existing heavy-metal oxide (Fig. 5). Similar effects were observed for the other tested materials.
5. Conclusions • Fine-grained Al-based composites, containing Hf, Mn and W oxides, can be produced by mechanical alloying and hot-extrusion method. • Annealing of the composites results in a chemical reaction of the components and growth of very fine Al-oxide particles and new intermetallic grains.
Acknowledgements Fig. 4. STEM pictures of samples annealed at 873 K for 24 h: (a) Al–HfO2 , (b) Al–MnO2 , (c) Al–WO3 ; O: oxide, IP: intermetallic phase.
the initial annealing. Annealing of the composites at 873 K promoted a chemical reaction between aluminum matrix and heavy-metal oxides. As a result, needle-like Al-oxide particles and intermetallic grains were grown. Thus, the hardness increases due to the development of fine structural components. The hardness maximum was followed by a hardness decrease at long aging times, which might result from grain/particle coarsening.
TEM observations were performed under grant no. 4T08A 00122. One of the authors (LB) is indebted to the Foundation for Polish Science for supplying the JEM electron microscope. References [1] J. Kaneko, M. Sugamata, L. Blaz, R. Kamei, Key Eng. Mater. 188 (2000) 73. [2] L. Blaz, J. Kaneko, M. Sugamata, R. Kamei, Mater. Sci. Eng., in press.