MICROSTRUCTURE AND MECHANICAL PROPERTIES OF POWDER ALUMINIUM PREPARED BY SEVERE PLASTIC DEFORMATION

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1 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF POWR ALUMINIUM PREPARED BY SEVERE PLASTIC FORMATION Jiří DVOŘÁK 1a, Petr KRÁL 2a, Martin BALOG 3b, František SIMANČÍK 4b, Václav SKLENIČKA 5a a Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, Brno, Czech Republic, dvorak@ipm.cz b Institute of Materials & Machine Mechanics, Slovak Academy of Sciences, Račianská 75, Bratislava, Slovak Republic Abstract The presented paper describes properties of Al 99.7% produced by power metallurgy (PM). The Al powder was prepared by gas atomization in nitrogen atmosphere. Powder green bodies with 85% of theoretical bulk density were prepared via cold isostatic pressing (CIP) at the pressure of 200 MPa. The prepared powder about size less than 400 µm was compacted via conventional direct hot-extrusion () at temperature of 450 C to produce a rod of 40 mm in diameter. For comparison purposes, some extruded material was additionally processed by equal channel angular pressing (ECAP). The microstructural investigations were performed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an electron back-scatter diffraction (EBSD) unit. Examination of extruded powder compacts showed that during direct extrusion initial powder particles deformed into grains about size of 5 m. This material was further subjected to ECAP passes up to 2 passes by route B C. The mechanical properties of this material were investigated using hardness measurements. In addition, constant load creep tests in tension were conducted at temperature of 473 K and at the different applied stresses. The basic aim of this work is microstructure characterization and description of creep behaviour of powder aluminium prepared by direct extrusion. The further aim of this work is the evaluation of ECAP technique effect on mechanical properties of powder aluminium. 1. INTRODUCTION Powder metallurgy allows fabrication of bulk materials from metallic, composite or ceramic powders. One of the critical factors affecting the industrial application of PM is the consolidation process. When a material is fabricated through a powder metallurgy method, pores are generally existed in the materials. The pores are also microcrack initiators during deformation. Thus, a fabrication process with proper parameters is important to increase the interfacial bonding strength and to decrease the number of pores, and thus to improve the mechanical properties. Powder metallurgy applies usually die pressing for the cold compaction of powder materials. In the next production step sintering (heating to sufficiently high temperature for a sufficient time) is usually used for consolidation of powder compacts. Sintering of metals often occurs in a controlled atmosphere and it leads to creation of good bonding among powder grains. On the other hand a significant removal of pores does not take place during sintering in many cases. For cases that sintering does not lead to necessary removal of pores or it leads to drastic grain growth connected with the increase of the brittleness, sintering is replaced by hot isostatic pressing. In some cases, however, neither sintering nor hot isostatic pressing ensures a sufficient quality of the bulk material. It is well known that introduction of shear deformation, e.g. by direct extrusion, can result in significant refinement of the microstructure of metals and alloys [1]. However, this type of working is accompanied by extreme pressing loads and high extrusion temperatures.

2 In recent years, severe plastic deformation (SPD) processing has been developed as a new method of manufacturing bulk specimens having ultrafine/nano-crystalline grained structures [2,3]. Using the idea of this grain refinement effect, recently, bulk nano-structured materials processed by several methods of SPD, such as equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulated roll bonding, etc., were prepared [4]. The main advantage of SPD processed materials, compared to other nanostructured materials processed by gas condensation or ball milling with subsequent consolidation, is that it is possible to overcome a number of difficulties associated with residual defects and powder contaminations in the compacted samples. Furthermore, it is an effective method to consolidate the powder at relatively lower temperatures and loads than that used in conventional powders processing. The basic aim of this work is microstructure characterization and description of creep behaviour of powder aluminium prepared by direct extrusion. The further aim of this work is the evaluation of ECAP technique effect on mechanical properties of powder aluminium. 2. EXPERIMENTAL MATERIAL In this study, commercially pure aluminium powder was used as a raw material. Al powder of technical purity 99.7% about size less than 400 µm, supplied by company New materials development G.m.bH., was prepared by gas atomization in N 2 atmosphere. Powder green bodies with 85% of theoretical bulk density were prepared via cold isostatic pressing (CIP) at the pressure of 200 MPa. The prepared powder was than compacted via conventional direct hot-extrusion () at temperature of 450 C with extrusion ratio R = 11:1 to produce a rod of 40 mm in diameter. For comparison purpose, some extruded material was additionally processed by equal channel angular pressing (ECAP). The rods were cut into short billets having a length of 50 mm and a cross-section 10 x 10 mm 2. ECAP was conducted at room temperature with a die that had an internal angle of 90 between the two parts of the channel and an outer arc of curvature of 20, where these two parts intersect. It can be shown from earlier that these angles lead to an imposed strain of 1.15 in each passage of the sample. The pressing speed was 10 mm min -1. ECAP was applied by one and two passes using route B C where the rotation of the billet was always 90 in the same sense between successive passes. Boron nitride spray () and solution on the based of molybdenum disulfide (ECAP) were used as a lubricants during pressing. Following and ECAP pressing, samples were prepared for examination by means of optical microscopy and scanning electron microscope (SEM) equipped with an electron back scattering diffraction (EBSD) unit. All grain size measurements were made by the linear intercept method in the transversal direction. Creep tests in tension were performed at a temperature of 473K under constant applied load of 50 MPa. The flat creep specimens were cut out of and ECAP billets. Both tensile specimens were run to final fracture. True strain-time readings were continuously recorded by the PC-based data acquisition system. 3. RESULTS 3.1. Microstructure observation Fig. 1 gives an example of the EBSD microstructure in the cross-section perpendicular to the pressing direction.

3 Fig. 1a show EBSD microstructures observed for the samples subjected to. It was found that pressing by leads to reduction of grain size to 5 m in the cross section. Initial microstructure contained about 71 % of high-angle grain boundaries (HAGB) as it is demonstrated on Fig. 3a. Fig. 1. EBSD images of aluminium (misorientation Δ > 15 ) subjected by a) and b) subsequent 2 ECAP passes. From the inverse pole figure of the normal plane of cross section (Fig. 2) is evident that state has the texture. The crystallographic directions <100> and especially <111> are oriented in the pressing direction resp. are nearly perpendicular to the cross section. Fig. 3b shows the position of <111> in respect of pressing direction of direct extrusion. The investigation of surface by optical microscopy revealed a large amount of pores in material as visible on Fig. 4a. Fig. 1b demonstrated microstructure of the sample subjected to direct extrusion and subsequent 2 ECAP passes. Examination shown that ECAP method leads to a slowly decrease in the grain size to 3 m. The fraction of low-angle (θ < 15 ) grain boundary population has progressively increased as compared with state (Fig. 3a). Significant changes in the material s texture were also observed. It should be noted that the ECAP process produces changes in the crystallographic orientation (texture) which is statically random. The orientation of crystallographic directions <111> in structure after 2 ECAP passes is plotted in the Fig. 3b. Fig. 2. Inverse pole figure of the normal plane of cross section a) and b) subsequent 2 ECAP passes.

4 10 16 Number of boundaries [%] PM Al 99.7% Fraction of <111> directions [%] PM Al 99.7% Misorientation [degree] Angle of <111> directions Fig. 3. Aluminium subjected by and subsequent 2 ECAP passes: a) depending of number of boundaries on misorientation and b) angle of <111> directions with regard to rolling direction a) and b) subsequent 2 ECAP passes. Fig. 4. Optical microscopy of aluminium after a) and b) subsequent 2 ECAP passes Hardness tests Vickers microhardness was measured using a hardness testing machine Zwick for samples after and after plus processing by ECAP. The samples were cut to the direction parallel to the longitudinal axis. Each hardness value is the average of six data points measured for one specimen. It is shown that ECAP process leads to increasing of hardness already after a single pressing from value of 39 (only after ) to 50. The hardness measured reached after 1 ECAP pass is nearly unchanged with increasing number of passes. The hardness increase is probably caused due to grain refinement and work hardening at ECAP pressed material Creep results Representative standard creep curves me are shown in Fig. 5a. The creep testing was conducted on billet after direct extrusion and, for comparison purposes, on the same material additionally processed by ECAP method by route B (B Bc). All of these plots were obtained at an absolute temperature of 473 K ( 0.5 Tm) and at an applied tensile stress of 50 MPa. The creep tests in tension were running up to the final fracture of creep specimens. These standard ε vs. t creep curves can be easily replotted in the form of the instantaneous strain rate dε/dt versus time (as shown in Fig. 5b) and/or in the form of the instantaneous strain rate dε/dt versus strain ε (Fig. 5c).

5 STRAIN σ Al 99.7% 473 K, 50 MPa Tension +1ECAP pass CREEP RATE dε/dt [s -1 ] ECAP pass Al 99.7% 473 K, 50 MPa Tension TIME t [h] TIME t [h] 10-1 CREEP RATE dε/dt [s -1 ] Al 99.7% 473 K, 50 MPa Tension Fig. 5. Creep curves for samples after and additional 2 ECAP passes: (a) standard creep curves, (b) creep rate vs. time, (c) creep rate vs. strain ECAP pass STRAIN σ As demonstrated by figures, significant differences were found in the creep behaviour of the ECAP material when compared to its counterpart. First, the ECAPed material exhibits markedly longer creep life (Figs 5a) than its counterparts. The highest creep resistance is achieved already after first pass. However, successive ECAP pressing lead to a noticeable decrease in the creep properties. Second, the minimum creep rate for the ECAP material is slightly less than that of one. Third, the value of the minimum creep rate is achieved approximately in the middle of creep life and tertiary creep here represents dominant area of creep. On the contrary, the strain to fracture stays almost the same for both materials. 4. DISCUSSION It was reported earlier [5] that increasing the die-pressing pressure will decrease the number of pores and increase the interfacial bonding strength and density which will inevitably increase the strength and plasticity of the material. In our work, optical microscopy revealed less amount of pores in ECAP samples. It is likely that method ECAP can decrease volume fraction and density of the pores contained in material. This would be consistent with other reports of a decrease in level of porosity when processing by ECAP [6]. That

6 way we can decrease number of crack initiators which can be responsible for deterioration in the creep resistance. The highest creep resistance was achieved already after first pass and further pressing led to decrease in the creep properties. The same trend was also observed in an earlier investigation of pressed aluminium prepared by ingot metallurgy [7,8]. Material processed by only contains higher occurrence of HAGBs. After first ECAP pass, the boundaries were predominantly low-angle boundaries in character, and the population of high-angle boundaries had increased in materials following ECAP passes. This indicates that HAGBs have lower strengthening effect under creep than low-angle ones. The softening by HAGBs may be explained in terms of the indirect effect which grain boundaries exert on the creep resistance by influencing the evolution of the dislocation microstructure in modifying the rates of generation and annihilation of dislocations. 5. CONCLUSION In this paper, the effect of direct extrussion and following ECAP pressing on the microstructure and mechanical properties of powder metallurgy fabricated aluminium have been studied. It has been shown that leads to reduction of grain size. Further, extrusion by ECAP method does not lead to an additional marked decrease in the grain size. The highest creep resistance is achieved already after first ECAP pass. Successive ECAP pressing lead to a noticeable decrease in the creep properties. Nevertheless, ECAP process is suitable for decreasing of the pores and increase the density and interfacial bonding strength of this material, and thus can improve the creep properties. ACKNOWLEDGEMENTS Financial support for this work was provided by the Grant Agency of the Academy of Sciences of the Czech Republic under Grant KJB REFERENCES [1] GIL SEVILLANO, S. J., VAN HOUTTE, P., AERNOUDT, E. Large strain work hardening and textures,prog. Mater. Sci. Vol. 25, 1980, p [2] VALIEV, R.Z., ISLAMGALIEV, R.K., ALEXANDROV, I.V. Bulk nanostructured materials from severe plastic deformation. Prog. Mater. Sci. Vol. 45 (2000) p [3] IWAHASHI, Y., et al. An investigation of microstructural evolution during equal-channel angular pressing. Acta mater., Vol. 45, 1997, p [4] M. FURUKAWA, M., HORITA, Z., LANGDON, T.G. Factors Influencing Microstructural Development in Equal-Channel Angular Pressing. Met. Mater. Int. Vol. 9, 2003, p [5] SONG, M., HE, Y. Effects of die-pressure and extrusion on the microstructures and mechanical properties of SiC reinforced pure aluminium composites. Mat. And Design, Vol. 31, 2010, p [6] KAWASAKI, M., XU, C., Langdon, T.G., An investigation of cavity in superplastic aluminum alloy processed by ECAP. Acta Mater., Vol. 53, 2005, p [7] V. SKLENIČKA, V., DVOŘÁK, J., SVOBODA, M., KRÁL, P., VLACH, B. Effect of Processing route on Microstructure and mechanical Behaviour of ultrafine grained metals processed by severe plastic deformation. Mater. Sci. Forum, Vol. 482, 2005, p [8] V. SKLENIČKA, V., DVOŘÁK, J., KRÁL, P., STONAWSKÁ, Z., SVOBODA M. Creep processes in pure aluminium processed by equal-channel angular pressing. Mat. Sci. Eng. A , 2005, p