STRUCTURE EVOLUTION OF AlCr5.5Fe2Ti1 ALLOY DURING ITS COMPACTIZATION

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1 STRUCTURE EVOLUTION OF AlCr5.5Fe2Ti1 ALLOY DURING ITS COMPACTIZATION Alena MICHALCOVÁ a,b, Dalibor VOJTĚCH a, Pavel NOVÁK a, Jan DRAHOKOUPIL c, Kamil KOLAŘÍK d a Institute of Chemical Technology, Prague, Department of Metals and Corrosion Engineering, Technicka 5, Prague, Czech Republic, michalca@vscht.cz b Institute of Chemical Technology, Prague, Department of Chemical Technology of Monuments Conservation, Technicka 5, Prague, Czech Republic c Institute of Physics Academy of Sciences CR, Na Slovance 2, Prague 8, Czech Republic d Faculty of Nuclear Science and Physical Engineering, CTU in Prague, Trojanova 13, Praha 2, Czech Republic Abstract Structure and properties of Al-Cr-Fe-Ti alloy produced by powder metallurgy are described in this paper. Rapidly solidified powder alloy was prepared by the pressure nitrogen melt atomization. The granulometric powder fraction of less than 45 µm was consequently hot-extruded. Microstructures of the initial powder and of the as-extruded material were observed by transmission electron microscope. The matrix fcc-al grains coarsened during compaction of the material. The fcc-al grains of the compacted material were recrystallized and neither residual stress nor texture were observed. No further grain coarsening took place by annealing of extruded material at 400 C. Keywords: powder metallurgy, nano-crystalline alloy, aluminium, Al-Cr alloy 1. INTRODUCTION Aluminium alloys prepared from rapidly solidified material by powder metallurgy are very promising materials. They are characterized by a refined and homogeneous structure positively affecting strength and thermal stability of mechanical properties [1,2]. Because of this reason, they have significantly higher mechanical properties and thermal stability than common aluminium alloy, which can be utilized up to approximately 250 C [3,4]. Powder metallurgy (PM) processing includes in general two steps: 1. powder preparation 2. powder compaction [5]. In the case of described material, the powder preparation was done by melt atomization. The compaction was done by hot extrusion at 450 C. It was documented that such temperature leads to significant changes in phase composition of Al-Cr-Fe rapidly solidified alloys [6]. The question is if the material keeps the superior properties during compaction. In this paper, the structure evolution, changes in phase composition and hardness of Al-Cr-Fe-Ti alloy are described. 2. EXPERIMENTAL An alloy with composition 91 wt.% Al-6 wt.% Cr-2 wt.%fe-1 wt.% Ti was prepared by melting in a resistance furnace. Afterwards, the alloy was remelted in an induction furnace in nitrogen atmosphere and then atomized by nitrogen. A granulometric fraction of <45 µm was separated for compaction by hot extrusion at 450 C. A rod of 6 mm in diameter was produced in this way. A common metallographic procedure (mounting, grinding, polishing, and etching in 0.5%HF) was used to characterize the microstructure of both (powder and compactized) materials. Light microscopy (LM-Neophot 2), scanning electron microscopy (SEM

2 Hitachi S-450 with EDS Noran), and transmission electron microscopy (for compactized material: TEM-JEOL JEM 1200 EX operating at 120 kv and for rapidly solidified powder: JEOL JEM 3010 operating at 300 kv) were used. Thin foils required for TEM were prepared from compactized material by electrolytic polishing in a solution of 75% methanol and 25% nitric acid at 20 C and 10 V. The TEM samples of rapidly solidified powder were prepared by mounting the powder in G2 epoxy in the brass tube with external diameter of 3 mm. Cylinders with high of mm were cut from this tube. Consequently, the cylinders were ground, dimpled by Gatan Dimple Grinder, Model 656 to the final dimple thickness of 10 µm. Consequently the samples were precision ion polished by Gatan PIPS, the Model 691. X-ray diffraction (XRD) (X Pert PRO PANalytical) was employed to determine phase composition, grain size, and biaxial residual stress of the alloy. Texture was investigated by the Debye Scherrer back-reflection diffraction patterns taken on an X-ray SEIFERT ID 3003 equipment. 3. RESULTS AND DISCUSSION The morphology of rapidly solidified powder particles is shown in Fig. 1. The particles have spherical shape typical for materials prepared by melt atomization. The structure of cross-sectioned particles is illustrated in Fig. 2. The structure of all powder particles is not the same. Mostly common is the structure with spherical intermetallics, in Fig. 2 marked by 1. The other plentiful part of particles has a flake like structure of intermetallics, in Fig. 2 marked by 2. Fig. 1. Morphology of rapidly solidified powder (SEM) Fig. 2. Structure of rapidly solidified powder (LM) Compact material with no observed porosity was produced by hot extrusion of rapidly solidified powder, see Fig. 3 and 4. In the cross section the material is very homogenous, as shown in Fig. 3. In the structure, the particles of initial powder are only slightly visible. On the other hand, the longitudinal section exhibits structure shaped parallel to extrusion, see Fig. 4. The stripes in structure are caused by different structure types in initial powder.

3 , Olomouc, Czech Republic, EU Fig. 3. Structure of extruded material, cross section Fig. 4. Structure of extruded material, longitudinal (LM) section (LM) TEM was used for more detailed observation. Fig. 5 shows the very fine structure of rapidly solidified powder. The structure of compacted material is documented in Fig. 6. All the fcc-al matrix grains in compacted material are recrystallized and are surrounded by particles of intermetallic phases. The matrix extruded materials grains also coarsened during compaction, but the compact material still has very fine microstructure. Perhaps, the grain growth stopped in the moment when all grain boundaries were locked by intermetallic particles. This idea is in agreement with our previous research, which showed that no grain coarsening took place during annealing of extruded material at 400 C [7]. Fig. 6. Structure of rapidly solidified powder (TEM) Fig. 6. Structure of extruded material, cross section (TEM)

4 The grain coarsening was quantified by image analysis and the results are given in the plot in Fig. 7. Fig. 7. Grain size distribution of rapidly solidified powder and extruded material Fig. 8. DSC curves of rapidly solidified powder and extruded material DSC heating curves of rapidly solidified powder and extruded material are shown in the graph in Fig. 8. DSC curve of powder exhibits an exothermal peak at temperature C. In the curve of extruded material, this peak was not observed. The reason is that temperature of 450 C is the temperature of hot extrusion. All metastable phases already decomposed during compaction. This idea was verified by XRD. The initial powder was formed by fcc-al matrix (1), metastable phase (2) and Al 13 Cr 2 intermetallic phase (3), see diffraction pattern in Fig. 9. The metastable phase is (according to XRD) probably decagonal quasicrystalline phase Al-Cr-Fe. The extruded material composed only of fcc-al (1) and Al 13 Cr 2 intermetallic phase (3). Fig. 9. XRD patterns of rapidly solidified powder and extruded material: 1 fcc-al, 2 metastable phase, 3 Al 13 Cr 2 Diffraction patterns obtained by XRD in Debye Scherrer back-reflection mode are given in Fig. 10. The intensity of powder diffraction pattern is lower, because of lower amount of material used for experiment.

5 Fig. 10. Debye Scherrer back-reflection diffraction patterns a) rapidly solidified powder and b) extruded material The diffraction circles of extruded material are absolutely homogenous. This means that no texture evolution took place during compaction. The grain recrystallization, which is perceptible from TEM micrograph in Fig 6, was confirmed by XRD residual stress measurements along the rod axis. The obtained stress value was 22 ± 11 MPa which, with respect to the experimental inaccuracy, corresponds to a strain-free material. 4. CONCLUSION Structural changes took place during the compaction of rapidly solidified alloy. The fcc-al matrix grains coarsened from approximatelly 210 nm to 960 nm. The metastable phase was decomposed during hot extrusion. On the other hand, the compact material still has very fine microstructure formed by recrystallized grains and Al 13 Cr 2 intermetallic phase particles, which inhibit the grain coarsening. The extruded material exhibits no texture and no residual stress. LITERATURE [1] Inoue A., Kimura H.: J. Light Met. 1 (2001) 31. [2] Inoue A., Kimura H., Zhang T.: Mater. Sci. Eng. A [3] Vojtěch D.: Aluminium 83 (2007) 83. [4] Michna Š., Lukáč I., Otčenášek V., Kořený R., Drápala J., Schneider H., Miškufová A.: Aluminium Materials and Technologies from A to Z, Adin, Prešov (2007). [5] Schatt W., K.P. Wieters: Powder Metallurgy: Processing and Materials.,EPMA, Shrewsbury (1997). [6] Galano M., Audebert F.,García Escorial A., Stone I.C., Cantor B., Journal of Alloys and Compounds 495 (2010) [7] Michalcova A., Vojtěch D., Novák P., Šittner P., Pilch J,, Int. J. Mat. Res. (formerly Z. Metallkd.) 101 (2010) 2