UDK 669.715:621.7 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 48(3)349(2014) MICROSTRUCTURE OF RAPIDLY SOLIDIFIED AND HOT-PRESSED Al-Fe-X ALLOYS MIKROSTRUKTURA HITRO STRJENIH IN VROČE STISKANIH ZLITIN Al-Fe-X Milena Voderova, Pavel Novak, Dalibor Vojtech Institute of Chemical Technology, Department of Metals and Corrosion Engineering, Technicka 5, 166 28 Prague 6, Czech Republic voderovm@vscht.cz Prejem rokopisa - received: 2013-06-07; sprejem za objavo - accepted for publication: 2013-08-28 Rapidly solidified aluminium alloys are promising materials for many technical applications. The main advantages of aluminium alloys alloyed with transition metals, such as Ni, Fe, Cr or Mn, are a good strength-to-weight ratio, a relatively low price, improved mechanical properties and a thermal stability. Mechanical properties of these alloys can be improved with rapid-solidification techniques. In this work, a production method employing melt spinning, cryogenic milling of melt-spun ribbons and the subsequent hot pressing was tested. The Al-11Fe, Al-7Fe-4Cr and Al-7Fe-4Ni alloys were processed in this way. The microstructure and mechanical properties of the prepared alloys were compared with the cast materials of the same composition. The results showed that the proposed technology can lead to a significant microstructure refinement and hardness improvement of the investigated alloys, when compared with the as-cast state. In the case of the Al-Fe alloy, a local coarsening of the Al13Fe4 particles was observed. Hot pressing is suggested to be a promising method for processing rapidly solidified alloys with respect to maintaining a fine structure and satisfactory hardness. Keywords: rapid solidification, aluminium alloys, hot pressing, microstructure Hitro strjene aluminijeve zlitine so obetajoč material za različno tehnično uporabo. Glavne prednosti aluminijevih zlitin, legiranih s prehodnimi kovinami, kot so Ni, Fe, Cr ali Mn, so ugodno razmerje med trdnostjo in maso, relativno nizka cena, izboljšane mehanske lastnosti in termična stabilnost. Mehanske lastnosti teh zlitin se lahko izboljša s tehniko hitrega strjevanja. Preizkušena je bila metoda izdelave z uporabo ulivanja tankega traku, kriogeno mletje tankega ulitega traku in končno vroče stiskanje prahu. Tako so bile izdelane zlitine Al-11Fe, Al-7Fe-4Cr in Al-7Fe-4Ni. Mikrostruktura in mehanske lastnosti izdelanih zlitin so bile primerjane z litim materialom z enako sestavo. Rezultati so pokazali, da predložena tehnologija, v primerjavi z litim stanjem, povzroča občutno drobnejšo mikrostrukturo in povišanje trdote. Pri zlitini Al-Fe je bilo opaženo lokalno povečanje delcev Al13Fe4. Domneva se, da je vroče stiskanje obetajoča metoda za obdržanje drobnozrnate strukture in zadovoljive trdote. Ključne besede: hitro strjevanje, aluminijeve zlitine, vroče stiskanje, mikrostruktura 1 INTRODUCTION temperatures and pressures. In this work, hot pressing as a less demanding alternative was tested. In recent years, Rapidly solidified aluminium alloys alloyed with the consumption of aluminium alloys in engineering has transition metals are promising materials for the techni- been rising and this is closely connected with the issue of cal applications requiring improved mechanical proper- a waste disposal. Aluminium scrap is often contaminated ties and a thermal stability.1,2 However, to be applicable, with a mixture of the elements coming from the steel the powders obtained with atomisation or the thin rib- parts, e.g., Fe, Ni, Cr, Mn, that are very difficult and bons obtained with melt spinning have to be compacted. costly to remove. However, these elements are often To produce a dense material, several methods are recom- recommended to improve the thermal stability of alumi- mended: hot isostatic pressing (HIP), hot extrusion, nium alloys. The idea that powder metallurgy can be spark-plasma sintering (SPS) or hot pressing (HP).3-6 used to recycle the aluminium contaminated with a mix- The main request about the manufacturing method for ture of elements was already mentioned in7. The alloys producing a bulk material is to keep the particle size at used in this experiment serve as a model leading to the the lowest possible level. All the listed methods require development of a process for recycling aluminium scrap relatively high temperatures and pressures. Although containing iron and stainless steel. HIP is beneficial in engineering, it is quite complicated for a laboratory use because of the complex equipment required. For the aluminium alloys investigated in this 2 EXPERIMENTAL WORK paper, hot extrusion or hot pressing are the most suitable compaction technologies. Recently, the extrusion of The samples of the Al-11Fe, Al-7Fe-4Ni and rapidly solidified aluminium alloys has been widely Al-7Fe-4Cr (mass fractions, w/%) alloys were prepared investigated.7,8 However, in the case of the alloys con- by conventional casting, melt spinning and hot pressing. taining high amounts of intermetallics, such as the The chemical compositions of the investigated alloys investigated Al-Fe-X alloys, hot extrusion requires high were chosen in order to model the metallic waste con- taining, e.g., the stainless steel including relatively high amounts of nickel and chromium. The alloys were prepared by melting the Al-11Fe master alloy and the master alloy with an addition of pure nickel or chromium. In the first step, the alloys were melted in an electric-resistance furnace at 1000 °C and cast into a non-preheated brass mould. After that, the alloys were subjected to a rapid solidification with the melt-spinning technique. In this process, a molten alloy was cast onto a copper-alloy wheel. The melting was carried out under argon protective atmosphere; the temperature of the melt was 1200 °C because of the high liquidus temperatures of the used alloys. The process yields aluminium-alloy ribbons, 30 pm thick. The rotation velocity of the cooling wheel in the melt-spinning process was 1420 r/min, which corresponds to the circumferential speed of 38 m/s. The rapidly solidified alloys were cryogenically milled in a planetary ball mill RETSCH PM 100 in liquid nitrogen in order to ensure an embrittlement of the ribbons and to simplify the milling. The milling was performed for 10 min at 400 r/min. For compacting the material, a univer- Figure 1: Microstructure of as-cast Al-11Fe (SEM) Slika 1: Mikrostruktura litega Al-11Fe (SEM) sal testing machine LabTest5.250SP1 was used. The milled powders were cold pre-pressed under the pressure of 220 MPa and then heated at 500 °C for 20 min to ensure temperature homogeneity. Hot pressing was performed for 5 min at 500 °C with the maximum pressure of 530 MPa. After hot pressing, a sample was cooled on air. The microstructures of all the samples were investigated with a TESCAN VEGA 3 LMU scanning electron microscope (SEM) equipped with an Oxford Instruments INCA 350 EDS analyser. The mechanical properties of the investigated alloys were examined by measuring the Vickers hardness and microhardness with the 5 kg (HV 5) and 0.005 kg (HV 0.005) loads at room temperature. The results are presented in the form of the average of ten values. The phase composition was determined with X- ray diffraction (XRD, PANalytical X'Pert Pro). 3 RESULTS AND DISCUSSION 3.1 Microstructure The microstructures of the as-cast alloys obtained with the scanning electron microscope are shown in the figures, acquired in the backscattered electron mode (BSE). A slow solidification rate causes the evolution of large amounts of irregularly shaped, coarse intermetallic phases. As seen in Figure 1, the conventionally cast Al-11Fe consists of large grains of a solid solution of Fe in Al and coarse Al13Fe4 intermetallics (also referred to as FeAl3 in9). The nickel added to the as-cast alloy (Figure 2) remains dissolved together with the iron in aluminium, while the iron forms coarse particles of Ali3Fe4. In an as-cast alloy, chromium is partly dissolved in aluminium or it forms Al5Cr and Al13Cr2 intermetallics together with stable Al13Fe4, as seen in the Figure 3. After increasing the solidification rate, the microstructures of the investigated alloys change significantly. The micro-structure of the rapidly solidified ribbon in a longitudinal cut is composed of two main areas; the wheel side and the free side. The wheel side is in the direct contact with Figure 2: Microstructure of as-cast Al-7Fe-4Ni (SEM) Slika 2: Mikrostruktura litega Al-7Fe-4Ni (SEM) Figure 3: Microstructure of as-cast Al-7Fe-4Cr (SEM) Slika 3: Mikrostruktura litega Al-7Fe-4Cr (SEM) Figure 4: Microstructure of melt-spun Al-11Fe (SEM) Slika 4: Mikrostruktura hitro strjenega traku iz Al-11Fe (SEM) the cooling wheel and it is usually composed of a supersaturated solid solution of the alloying elements in aluminium and low amounts of stable and metastable intermetallics and/or quasicrystals. The free side is cooled less intensely. This area contains more interme-tallics that are very fine and round-shaped. The micro-structure of the rapidly solidified Al-11Fe alloy in Figure 4 is composed of a supersaturated solid solution of Fe in Al and nanometre-sized intermetallic phases on the wheel side, while the stable Al13Fe4 and metastable Al6Fe phases are located on the free side. Both regions of the RS Al-Fe-Ni alloy are composed of a supersaturated solid solution of Ni and Fe in Al, quasicrystalline Al75Ni10Fe15 and stable Al13Fe4 and Al3Ni2 phases (Figure 5). The microstructure of the melt-spun alloy containing chromium in Figure 6 is composed of a supersaturated solid solution, Al13Cr2 and Al13Fe4 on both sides of the ribbon. 3.2 Milling of rapidly solidified alloys Rapidly solidified alloys were milled in liquid nitrogen to ensure the embrittlement of the ribbons. The optimum time for milling was determined as 10 min. After 5 min of milling, there was still a large amount of unmilled ribbons, while after 15 min all the nitrogen evaporated; the product became very hot, forming clusters instead of the required powder. The microstructure of milled ribbons is shown in Figure 7. From these micrographs it can be seen that the cryogenic milling of RS ribbons does not produce round, but irregularly shaped porous particles of various sizes. 3.3 Hot pressing of rapidly solidified milled powders The microstructures of the hot-pressed alloys are shown in Figures 8 to 10. It is obvious that the micro-structure of the hot-pressed Al-11Fe coarsened significantly. The metastable Al6Fe and supersaturated solution Figure 5: Microstructure of melt-spun Al-7Fe-4Ni (SEM) Slika 5: Mikrostruktura hitro strjenega traku iz Al-7Fe-4Ni (SEM) Figure 6: Microstructure of melt-spun Al-7Fe-4Cr (SEM) Slika 6: Mikrostruktura hitro strjenega traku iz Al-7Fe-4Cr (SEM) Figure 7: Al-11Fe powder obtained with cryogenic milling (SEM) Slika 7: Al-11Fe-prah, dobljen s kriogenim mletjem (SEM) were decomposed to form a large amount of Al13Fe4. Al-7Fe-4Ni has a fine microstructure including fine particles of Ali3Fe4 and Al4Ni3. The microstructure of the hot-pressed Al-7Fe-4Cr does not change significantly. It contains fine, homogeneously distributed particles of the gated alloys in the as-cast, rapidly solidified and hotsolid solution, and the Al13Fe4 and Al13Cr2 intermetallic pressed state are compared in Figures 11 to 13. In the phases. These results suggest a better thermal stability of case of Al-11Fe, the as-cast alloy contains a high amount the ternary alloys. The phase compositions of the investi- of Al13Fe4 which is transformed, due to a lack of diffusion time, into Al6Fe after rapid solidification. This metastable phase is then decomposed during hot pressing and a high fraction of Al13Fe4 is formed. The quasicry-stals of Al75Ni10Fe15 present in the rapidly solidified Al-7Fe-4Ni decompose after hot pressing. The phase Figure 8: Microstructure of hot-pressed Al-11Fe (SEM) Slika 8: Mikrostruktura vro~e stisnjene zlitine Al-11Fe (SEM) Figure 9: Microstructure of hot-pressed Al-7Fe-4Ni (SEM) Slika 9: Mikrostruktura vro~e stisnjene zlitine Al-7Fe-4Ni (SEM) SEffl HW 20 kV WO: 10.18 mm 111111 | \fiew field: 141 urn Del: BSC 20 urn SEM MAG: 1.00 kx : OatelmJdM: 03ri4M3 9000040000100000- AI-llFe as cast state 2 2 , 1 fJ 1,2 1-Al 1 2-AlijFei I 1; 1 AI-llFe melt spinning ................J 1,3 1-Al 2-AlijFe. 3-AUF« Ul....................A..................U.............................. AI-llFe hot press _____..... 1 1 2,4 tj 1-AI 2-AUFe4 2 4-FeAb 4.1 i k 10 20 30 40 50 60 70 80 90 Position Cmeta] Figure 11: XRD patterns of Al-11Fe prepared with different methods Slika 11: XRD-posnetki zlitine Al-11Fe, pripravljene z razli~nimi metodami 9000040000 100000 22500 I 10000 2500 AI-7Fe-4Ni as cast state 2 2 2 2 2 ......... .1 , IUA.«« >... • ,.„., , 1,2 1 1,2 WvU-------- l-Al [ 2-Ali3Fe. 1 1 . . .LJ Al-7Fe-4Ni melt spinning . ............... 1,2,4 1-Al 2-AluFei 3-AbNi2 1,2,3,4 4-Al75NuiFeE ä......................i...............Ix................... Al-7Fe-4Ni hot press .....................I.AaJ/I, , ...?J 1,5 1,2,5 1 [r-ßk...................^.J , l-Al 2-AliäF