TEM and HRTEM studies of ball milled 6061 aluminium alloy powder with Zr addition

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1 Journal of Microscopy, Vol. 237, Pt , pp Received 9 October 2008; accepted 13 July 2009 doi: /j x TEM and HRTEM studies of ball milled 6061 aluminium alloy powder with Zr addition L. LITYŃSKA-DOBRZYŃSKA, J. DUTKIEWICZ, W. MAZIARZ &Ł.ROGAL Institute of Metallurgy and Materials Science Polish Academy of Sciences, Cracow, Poland Key words. Aluminium alloys, ball milling, HRTEM, powder metallurgy, TEM. Summary The effect of mechanical alloying on the microstructure of atomized 6061 aluminium alloy powder and 6061 powder with a zirconium addition was studied in the work. The atomized 6061 aluminium alloy powder and 6061 powder with addition of 2 wt.% Zr were milled in a planetary ball mill and investigated using X-ray diffraction measurements, conventional and high-resolution electron microscopy (TEM/HRTEM) and high-angle annular dark field scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis. An increase of stresses was observed in milled powders after the refinement of crystallites beyond 100 nm. In the powder with zirconium addition, some part of the Zr atoms diffused in aluminium forming a solid solution containing up to 0.5 wt.% Zr. The remaining was found to form Zr-rich particles containing up to 88 wt.% Zr and were identified as face centred cubic (fcc) phase with lattice constant a = 0.48 nm. That fcc phase partially transformed into the L1 2 ordered phase. Eighty-hour milling brought an increase of microhardness (measured with Vickers method) from about 50 HV (168 MPa) for the initial 6061 powder to about 170 HV (552 MPa). The addition of zirconium had no influence on the microhardness. Introduction The 6000 series aluminium alloys have recently been increasingly used in automotive applications owing to the combination of their low density, good corrosion resistance, high strength and good formability. The microstructural refinement through an addition of transition metals like Zr, Ti, Sc is a well-known method used to increase the ductility and the strength of the aluminium alloys. Ball milling Correspondence to: L. Lityńska-Dobrzyńska, Institute of Metallurgy and Materials Science Polish Academy of Sciences, 25, Reymonta St., Cracow, Poland. Tel: ; fax: ; nmlityns@imim-pan. krakow.pl allows both to obtain a very fine microstructure and to extend the solid solubility limits of the elements added to the alloy (Suryanarayana, 2001). It was shown that the milling process increased three times the microhardness of the 6061 aluminium alloy powder in comparison with the as-received atomized powder (Fogagnolo et al., 2003) by the refinement of crystallites and a high degree of deformation. Although the maximum solubility of Zr in Al solid solution is only 0.07 at.% (Binary Alloy Phase Diagrams, 1990), it increases up to 6 at.% after the ball milling. Beyond this limit, the excess of Zr forms the L1 2 -Al 3 Zr metastable phase with Al (Desch et al., 1996). The ternary addition of Cu, Ni or Mn to the Al-Zr stabilizes the L1 2 -Al 3 Zr phase at high temperature in the milled powder (Moon et al., 2000). Three metastable phases were found after ball milling of Zr and Al powder blends over a wide composition range: (1) a nanocrystalline supersaturated solid solution of α(zr) for Al <15 at.%, (2) an amorphous phase for 15 at.% < Al < 40 at%, (3) a metastable face centred cubic (fcc) phase for the alloy containing 50 at.% of Al (Fecht et al., 1990). The addition of 5 at.% of Zr to Al-Mg alloy brings about the extension of the stability limit of the Al(Mg) solid solution, the formation of an Al-Zr phase after annealing, refining the structure and retarding grain growth (Al-Ageeli et al., 2005). The aim of the present investigation was to study the effect of mechanical alloying on the microstructure of atomized 6061 aluminium alloy powder and 6061 powder with a zirconium addition. Experimental details The atomized powder of 6061 aluminium alloy (containing wt% Mg, wt% Si, 0.7 wt% Fe, wt% Cu, 0.15 wt% Mn, 0.25 wt% Zn, 0.15 wt% Ti), with particle size below 63 μm supplied by ECKA GRANULATE VELDEN (Germany) and elemental zirconium powder (99.985% pure, particle size below 44 μm) were used as a starting materials. The ball milling of the pure 6061 alloy powder and a blend of the 6061 powder with the addition of 2 wt.% Zr was Journal compilation C 2010 The Royal Microscopical Society

2 TEM AND HRTEM STUDIES OF BALL MILLED 6061 ALLOYS 507 performed in a planetary high-energy ball mill Fritsch P5 for 80 h in an argon atmosphere using stainless steel balls with diameter 8 mm and a ball-to-powder weight ratio of 10:1. The microstructure of the powders was investigated by means of X-ray diffraction (XRD) using Philips PW 1710 with CoK α radiation, TEM/HRTEM using FEI Tecnai G 2 microscope at 200 kv equipped with EDAX energy dispersive X-ray and high-angle annular dark field scanning transmission electron microscopy (HAADF/STEM) detectors. The size of grains was calculated from the broadening of XRD reflections using a formula of PC-APD-3.5B Philips software, taking into account difference in profile widths of broadened and standard peaks. The accuracy of distance measurements in electron diffraction patterns was 2%. The camera length of 200 mm was chosen for the STEM mode, appropriate for Z-contrast HAADF imaging. In order to prepare the TEM samples, thin slices of the powders embedded in kit (composed of SPI-PON TM 812 epoxy resin, DDSA softener, NMA hardener, DMP-30 accelerator, prepared using a typical formula for hard materials) were cut with Leica EM UC6 ultramicrotome with a diamond knife (cutting speed 1 mm/s). Then the slices were placed on a carbon film supported by a copper grid. Microhardness Vickers measurements were performed using a CSM-Instruments microhardness tester. Results and discussion At the first stage of ball milling, the powder particles underwent deformation under ball impact and their morphology changed from the initial spherical shape to flattened one. Then the welding mechanism of flattened particles predominated and succeeded in building bigger, more equiaxial and harder particles, which eventually fractured after prolonged milling time. The mean size of the spherical particles was about 30 μm after 80 h of milling for both alloys. The XRD patterns the investigated powders showed a small broadening of α(al) reflections with increasing milling time, which might be a result of the deformation induced by the milling process and the grain refinement. The mean grain size, calculated by the Scherrer formula, decreased rapidly during milling down to nm after 40 h and did not change anymore up to 80 h of milling. The dimensions of the grains determined in that way were underestimated because of the contribution of the lattice distortion in peak broadening and were verified by the TEM investigation. In the 6061 powder, only α(al) XRD reflections were observed because of the small amounts of other phases present in the alloy. In the case of 6061+Zr powder, weak reflections of the α-zr (hexagonal structure) could be seen for the initial powder mixture (Fig. 1). After 20 h of milling, a new weak, broad reflection occurred at about 37.8 (marked by an arrow in Fig. 1) whose intensity grew as milling proceeded. The intensity of the α-zr reflections gradually decreased and completely disappeared after 40 h of milling. Fig. 1. XRD pattern sequence of mixed 6061 and Zr powders for various milling time. The reflections of α-al solid solution and α-zr phase are indicated. The arrow point on the reflection (111) attributed to an Al-Zr fcc phase that forms during milling. Fig. 2. TEM bright-field image of 6061 powder alloy mechanically alloyed for80handitscorrespondingdiffractionpattern reflectionsofα-alphase are indicated. The nanocrystalline structure of the milled powders was confirmed by the TEM investigation. The bright-field image of the alloy 6061 after 80 h milling (Fig. 2) showed a deformed microstructure of grains of size less than 100 nm with rough grain boundaries looking diffuse by a projection effect through the thin foil. The corresponding diffraction pattern consists of reflections lying on rings and belonging only to the α-al phase for both investigated alloys. HRTEM image of nanocrystals with [011] zone axis is shown in Fig. 3. High density of edge or

3 508 L. LITYŃSKA-DOBRZYŃSKA ET AL. Fig. 3. HRTEM image of the nanocrystallite of the 6061 powder alloy after 80 h of milling and as an insert Fourier filtered image showing the (111) lattice fringes the dislocations are marked by arrows. Fig. 5. (a) TEM bright-field image of wt% Zr powder ball milled for 80 h and (b), (c) electron diffraction patterns obtained with two tilt positions 45 apart from the Zr-rich particle. Reflections distances, angles and intensities correspond to an ordered fcc phase with zone axis close to [001] and [011] are described. Fig. 4. STEM-HAADF image of the wt% Zr powder ball milled for 80 h. complex dislocations with an edge component are well visible in the Fourier filtered part of the image showing (111) lattice fringes. Figure 4 shows a STEM-HAADF image of the 6061 powder with Zr additions after 80 h of milling, in which the Zr-rich particles are well visible with a bright contrast. The size of the Zr-rich particles ranges from a few nanometres up to 200 nm. The EDS chemical microanalysis performed for the particles lying on the edge of the thin foil (to avoid the influence of the surrounding matrix) showed that they contained wt% (44 69 at.%) of Zr. The mutual diffusion of elements at the interfaces of welded Zr and Al particles proceeded during the milling process and probably depended on the local conditions of milling. In consequence, the composition changed not only throughout the particles but also from one particle to another. The dark contrast could be noticed around the large Zr-rich particles in Fig. 4. Line profiles drawn through the interface of the particle and the matrix did not show the presence of any additional phase, which suggests that the contrast could be accounted for decohesion. Small areas with a contrast darker than the matrix are also visible in Fig. 4 and probable due to oxides, which formed on the surface of initial powder and were then crushed and distributed inside the matrix during milling process. Part of the Zr atoms diffused into Al to form a solid solution that contained 0.5 wt% Zr and 1.3 wt% Mg, 0.6 wt% Si and 0.1 wt% Fe. Figure 5 shows a bright-filed image of a Zr-rich particles lying near the thin foil edge and its diffraction patterns corresponding to two different tilt angles separated by 45. The reflections in the diffractograms are broad because the Zr-rich particle was not homogenous and consisted of the areas of different compositions and in consequence different lattice parameters. The areas were also mutually slightly disoriented. The ±100 and ±200 reflections in Fig. 5(b) are weaker compared to ±010 and ±020 because the diffraction pattern was taken a little away from the zone axis. The mean d hkl spacing s (0.24 nm and 0.28 nm) estimated from the diffraction patterns and angles measured between the reflections allowed to identify the Zr-rich compound as an fcc Zr-Al phase with a lattice constant 0.48 ± 0.01 nm. This fcc, probably metastable phase also explains the additional reflection at 37.8 in XRD patterns (Fig. 1) as the strong (111) reflection. A metastable fcc phase with the lattice constant 0.46 nm wasalreadyidentifiedinaballmilledpowderofpurezr(manna

4 TEM AND HRTEM STUDIES OF BALL MILLED 6061 ALLOYS 509 et al., 2002), whereas it is well known that zirconium exists as the α-(hcp) Zr or β-(bcc) Zr crystal structure below and above 863 C, respectively (Binary Alloy Phase Diagrams, 1990). A similar fcc phase has also been observed in ball milled mixture of Zr and Al powders (Fecht et al., 1990) and Al-Zr-Si alloy (Manna et al., 2003). Weak reflections (100) and (110) corresponding to the L1 2 ordering could also be seen in both diffraction patterns shown in Fig. 5. The HRTEM image and inverse fast Fourier transform obtained by using the superstructure reflections confirmed Table 1. Microhardness of atomized 6061 powder, 6061 and wt% Zr powder after 80 h of milling. Material Atomized ± h milling 174 ± Zr 80h milling 168 ± 28 Microhardness [HV 0.2N] that area of a few nanometres and rectangular shape with symmetry compatible with L1 2 exists in the fcc phase (Fig. 6). It was probably a phase transition preceding the formation of the ordered fcc phase that might be a precursor of the stable L1 2 -AlZr 3 (a = 0.43 nm) phase before the composition reached its exact stoichiometry. Table 1 presents the results of microhardness measurements of the atomized 6061 powder in the initial state and both the 6061 and 6061+Zr powders after 80 h of ball milling. The microhardness of powders increased from about 50 HV (initial powder) to about 170 HV after milling due to grain refinement and lattice deformation. The addition of zirconium had no significant influence on the microhardness. Conclusions (1) Ball milling of atomized powder of 6061 aluminium alloy and 6061 powder with addition of 2 wt.% Zr leads to the significant grain refinement down to 100 nm. It increases the microhardness from about 50 HV for initial 6061 to about 170 HV for powder after milling due to grain refinement. Addition of zirconium has no influence on the hardness. (2) The Zr-rich particles of maximum size 200 nm containing up to 88 wt.% (68 at.%) of Zr were distributed homogenously after 80 h of milling in the Al matrix containing less than 0.5 wt.% Zr in the solid solution. Their structure was identified as an fcc phase with a = 0.48 ± 0.01 nm which partially transformed to the ordered L1 2 structure and that phase was probably the precursor of the stable L1 2 -AlZr 3 (a = 0.43 nm) phase. Acknowledgements The authors would like to thank ECKA GRANULATE VELDEN (Germany) for supplying the atomized 6061 powder. Fig. 6. (a) HRTEM image of the nanocrystallite of the wt% Zr powder ball milled for 80 h. The insert is the Fourier Transform of the whole figure field. Both image and diffractogram exhibit a symmetry compatible with the L1 2 ordering; (b) the inverse Fast Fourier Transform obtained by using the L1 2 superstructure reflections highlights (partially) ordered nano-domains. References Al-Ageeli, N. Mendoza-Suarez, G. Labrie, A. & Drew, R.A.L. (2005) Phase evolution of Mg-Al-Zr nanophase alloys prepared by mechanical alloying.j. Alloys Compd. 400, Binary Alloy Phase Diagrams (1990) (ed. by T. Massalski) vol. 1, ASM- International, USA, pp

5 510 L. LITYŃSKA-DOBRZYŃSKA ET AL. Desch, P.B. Schwarz, R.B. & Nash, P. (1996) Mechanical alloying to produce L1 2 phases in the Al-Zr system. Scr. Mater. 34, Fecht, H.J. Han, G. Fu, Z. & Johnson, W.L. (1990) Metastable phase formation in the Zr-Al binary system induced by mechanical alloying. J. Appl. Phys. 67, Fogagnolo, J.B. Velasco, F. Robert, M.H. & Torralba, J.M. (2003) Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders. Mater. Sci. Eng. A 342, Manna, I. Chattopadhyay, P.P. Banhart, F. & Fecht, H.-J. (2002) Formation of face-centered-cubic zirconium by mechanical attrition. Appl. Phys. Lett. 81, Manna, I. Chattopadhyay, P.P. Banhart, F. & Fecht, H.-J. (2003) Solid state synthesis of Al-based amorphous and nonocystalline Al-Nb-Si and Al-Zr-Si alloys.z. Metallkd. 94, Moon, K.I. Chang, K.Y. & Lee, K.S. (2000) The effect of ternary addition on the formation and the thermal stability of L1 2 Al 3 Zr alloy with nanocrystalline structure by mechanical alloying. J. Alloys Compd. 312, Suryanarayana, C. (2001) Mechanical alloying and milling. Prog. Mater. Sci. 46,