THERMAL STABILITY OF RAPIDLY SOLIDIFIED Al-Fe-X ALLOYS Milena VODĚROVÁ, Pavel NOVÁK, Alena MICHALCOVÁ, Dalibor VOJTĚCH Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic, voderovm@vscht.cz Abstract Aluminium alloys prepared by rapid solidification are perspective materials for many special applications. In this work, microstructure of three rapidly solidified aluminium alloys has been studied and compared. Alloys of following chemical composition were studied: Al-11Fe, Al-7Fe-4Ni and Al-7Fe-4Cr (wt. %). Nickel and chromium were chosen as alloying elements due to the low solubility and diffusivity in aluminium matrix which improves mechanical properties and thermal stability at elevated temperatures. Samples in the form of thin ribbons were prepared by melt spinning technique. Microstructure of investigated alloys was studied by scanning electron microscopy (SEM). Phase composition was determined by x- ray diffraction (XRD). Vickers hardness (HV 0,005) was measured to estimate mechanical properties as well as thermal stability after long term annealing (500 h) at 300 and 400 C. Rapidly solidified alloys consist predominantly of supersaturated solid solution of alloying elements in aluminium on the wheel side and fine particles of intermetallic phases on the free side. Material hardness decreases with increasing temperature. Chromium improves the thermal stability more than nickel. Keywords: Aluminium alloys, rapid solidification, transition metals, melt spinning 1. INTRODUCTION Aluminium alloys are well known as construction materials used especially in automotive or aerospace industry. These alloys are dominantly used for light components due to relatively low price connected with good strength-to-weight ratio. However, thermal stability of aluminium alloyed by Zn, Cu or Mg manufactured by conventional casting is unsatisfactory. Applicability limits of common Al alloys do not exceed 150-200 C. It is already mentioned in literature [1, 2, 3] that thermal stability can be improved by alloying aluminium by transition metals (TM), e.g. Fe, Ni, Cr or Mn. Transition metals are able to stabilize the alloy properties at elevated temperatures due to low solubility and diffusivity in aluminium matrix. The other way how to improve thermal stability and mechanical properties is to refine the microstructure by increasing cooling rate. In technical praxis, atomisation, melt spinning and re-melting the surface by electron beam or laser beam are commonly used. Atomisation of the melt by inert gas or liquid produces metallic powder; the solidification speed varies in 10 2-10 4 K.s -1. Melt spinning process allows reaching solidification rates even higher, about 10 4-10 6 K.s -1.[4] In this process, molten alloy is cast onto intensively cooled wheel. Thin metallic ribbon is formed on the wheel s surface. Rapid solidification (RS) allows formation of supersaturated solid solutions, metastable and quasicrystalline or amorphous phases. [5, 6, 7] This has significant effect on microstructure of the alloy, which becomes very fine and fine microstructure signifies better mechanical properties or thermal stability. When compared with traditional aluminium alloys, it is possible to use these materials in wide range of industrial branches and in special applications. Expansion of automotive industry results in increasing production of aluminium alloys. But increasing production is closely related to dealing with waste. Aluminium scrap is often contaminated by mixture of metals and metalloids (Fe, Si, Cr, Ni, Cu or Mg), which are very difficult to be separated. Generally, there is no effective and economically suitable method for this waste treatment. Higher amounts of iron can be separated by magnetic separation, it is also possible to dilute aluminium scrap by pure aluminium, but this is
very costly and increases the material price. Producing the master alloys including transition metals from aluminium scrap seems to be very perspective. These master alloys can be further processed, e.g. by mentioned rapid solidification. Manufactured alloys have better properties such as hardness, thermal stability, better ductility connected to low density. Due to this unique combination they can probably replace titanium alloys in some lower-demanding applications, while the price and density are lower. Aim of this work was to describe the microstructure and properties of Al-Fe, Al-Fe-Ni and Al- Fe- Cr alloys prepared by melt spinning. Chemical composition of investigated alloys was chosen as a model of iron- and stainless steel containing Al scrap. 2. EXPERIMENTAL PART Investigated samples were composed of Al-11Fe, Al-7Fe-4Ni and Al-7Fe-4Cr (wt. %). Chemical composition of investigated alloys was chosen in order to model the metallic waste containing e.g. stainless steel containing relatively high amount of nickel and chromium. Alloys were prepared by melting of the AlFe11 master alloy and by melting the master alloy with pure nickel and chromium. Samples in the form of thin ribbons were prepared by melt spinning. This process allows solidification rates about 10 4-10 6 K.s -1. The molten alloy was cast onto high- frequency rotating copper alloy wheel. Melting was obtained under argon protective atmosphere; temperature of the melt was 1200 C. The process yields aluminium alloy ribbons of approx. 30 μm of thickness. Melt spinning was performed by Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Bratislava. Microstructure of all the samples was investigated by Olympus PME3 light microscope and TESCAN VEGA 3 LMU scanning electron microscope (SEM) equipped with Oxford Instruments INCA 350 EDS analyser. Mechanical properties of investigated alloys were examined by room temperature Hanemann hardness measurements with a 0,005 kg (HV 0,005) load. Phase composition was determined by x- ray diffraction (XRD, X Pert Pro). Thermal stability was investigated by Hanemann hardness measurement after annealing at 300 and 400 C for 500 h. 3. RESULTS AND DISCUSSION 3.1 Microstructure Microstructure of melt spun ribbons obtained by scanning electron microscope is shown in following micrographs (fig. 1-3). Rapidly solidified sample is usually composed of supersaturated solid solution of alloying elements in aluminium on the wheel side where the cooling rate is more intensive. Air side of the ribbon consists of fine stable and metastable intermetallic particles. There is also significant transition zone between solid solution and intermetallic zone. Particle size increases while moving from wheel side to the air side.
Fig. 1 Microstructure of Al-11Fe (SEM) Fig. 2 Microstructure of Al-7Fe-4Ni (SEM) Fig. 3 Microstructure of Al-7Fe-4Cr (SEM) XRD analysis proved that wheel side of the ribbon is dominantly composed of supersaturated solid solution and nanocrystalline intermetallic phases, mainly metastable Al 6 Fe, while nickel or chromium remain dissolved in the aluminium matrix. Air side of Al-11Fe consists of fine Al 6 Fe and negligible amount of Al 13 Fe 4. In the structure of Al-7Fe-4Ni, both wheel side and air side are composed of supersaturated solid solution of iron and nickel in aluminium, Al 3 Ni 2, Al 13 Fe 4 and quasicrystalline phase Al 75 Ni 10 Fe 15. The amount of intermetallics and particle size differs when moving from wheel side to less intensively cooled part of the ribbon. Wheel side of Al-7Fe-4Cr is composed of chromium and iron dissolved in solid solution, air side contains low amount of FeAl 3 and quasicrystalls of Al 80 Cr 13,5 Fe 6,5.
3.2 Thermal stability Results obtained by long term annealing at 300 400 C are shown in fig. 4-5. Samples of Al-Fe annealed at 300 C exhibit oscillating values of microhardness caused probably by precipitation of intermetallics and grain coarsening. Chromium and nickel containing alloy are more stable, microhardness of those materials does not exceed 150 HV. Increase of annealing temperature to 400 C results in dramatic decrease of hardness of Al-Fe and Al-Fe-Ni. Al-Fe-Cr stands at the value about 100 HV. It was evaluated that Al-Fe and Al-Fe-Ni alloys are not suitable for using at elevated temperatures because of their unsatisfactory hardness, changing with annealing time. Fig. 4 Long- term annealing at 300 C Fig. 5 Long- term annealing at 400 C 4. CONCLUSIONS This study was focused on microstructure and thermal stability of Al- TM alloys prepared by melt spinning. Microstructure of melt spun ribbons is composed of supersaturated solid solutions of transition metals in aluminium matrix and very fine stable, metastable and quasicrystalline intermetallics. Transition elements block the intermetallics growth and that results in very fine microstructure. On the other hand, Al- Fe alloy
has reached the highest value of microhardness in default state which is caused presumably by large amount of fine metastable phase Al 6 Fe. Chromium has significantly better effect on improving thermal stability. At 300 C, hardness of Al-Fe-Cr lies in the average of Al- Fe and Al-Fe-Ni values, but at 400 C remains at almost same values, in contrary to distinctly lower values for two other alloys. This fact can be explained by decomposition of quasicrystalline phases into more stable fine intermetallics. Low values of hardness for Al- Fe and Al- Fe- Ni are caused by easier coarsening of intermetallic phases. Unsatisfactory thermal stability determines these two alloys to be used only for thermally non-stressed components. Hardness of Al- Fe- Cr kept above 100 HV for both temperatures is promising. ACKNOWLEDGEMENT This research was financially supported by Czech Science Foundation, project No. P108/12/G043. LITERATURE [1] KATGERMAN, l., Dom, F. Rapidly solidified aluminium alloys by meltspinning. Materials Science and Engineering A, 2004, vol. 375-377, p. 1212-1216. [2] NAYAK, S., et al. Formation of metastable phases and nanocomposite structures in rapidly solidified Al- Fe alloys, Materials Science and Engineering A, 2011, vol. 528, p. 5967-5973. [3] ROZENAK, P. Deformation and fracture of Al-8Fe rapidly solidified alloys. Journal of Materials Science, 1996, vol. 31, p. 6351-6359. [4] VOJTĚCH, D., et al. Rychlé chlazení kovů význam, technologie a využití. Chem. Listy, 2004, vol. 98, p. 180-184. [5] TASHLYKOVA- BUSHKEVICH, I., et al. Structural and phase analysis of rapidly solidified Al- Fe alloys. Journal of Surface Investigation. X- ray, Synchrotron and Neutron Techniques, 2008, vol. 2, p. 310-316. [6] KARAKÖSE, E., et al. Structural investigations of mechanical properties of Al based rapidly solidified alloys. Mater. Des., 2011, vol. 32, p. 4970-4979. [7] SIVTSOVA, P.A., Shepelevich V.G. A study of rapidly solidified foils of an alloy of the Al- Fe- Cr- system. Metal Science and Heat Treatment, 2007, vol.49, p. 284-287.