In the frame of this work, Al-5Ni-1Co-3RE (RE-Rare Earth (Mischmetal)) rapidly solidified ribbons were manufactured and analyzed. The morphology of the as-cast structure, as well as the microstructural features were analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thermal stability has been investigated by combination of four point scanning electrical resistivity measurement (ER), differential scanning calorimetry (DSC) and microhardness measurement. From the results we can conclude, that Al-5Ni-1Co-3RE rapidly solidified alloys have good thermal stability due to very slow coarsening kinetics of precipitated particles

A. Nagode

A. Stoić

B. Karpe

B. Kosec

M. Bizjak

M. Dolenec

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Microstructure evolution and thermal stability of rapidly solidified Al-Ni-Co-RE alloy

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305METALURGIJA 52 (2013) 3, 305-308B. KARPE, A. NAGODE, B. KOSEC, A. STOIĆ, M. DOLENEC, M. BIZJAKMICROSTRUCTURE EVOLUTION AND THERMAL STABILITY OF RAPIDLY SOLIDIFIED Al-Ni-Co-RE ALLOYReceived – Prispjelo: 2012-09-20Accepted – Prihvaćeno: 2012-12-25Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 52(3) 305-308 (2013)UDC – UDK 621.746:672:536.21=111B. Karpe, A. Nagode, B. Kosec, M. Bizjak, Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia, A. Stoić, University of Applied Sciences, Slavonski Brod, Croatia.In the frame of this work, Al-5Ni-1Co-3RE (RE-Rare Earth (Mischmetal)) rapidly solidifi ed ribbons were manufactured and analyzed. The morphology of the as-cast structure, as well as the microstructural features were analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thermal stability has been inves-tigated by combination of four point scanning electrical resistivity measurement (ER), diff erential scanning calorim-etry (DSC) and microhardness measurement. From the results we can conclude, that Al-5Ni-1Co-3RE rapidly solidi-fi ed alloys have good thermal stability due to very slow coarsening kinetics of precipitated particles.Key words: rapid solidifi cation, metallic materials, heat transfer, precipitation, electrical resistivity measurementINTRODUCTIONAluminum alloys are commonly used as an engi-neering material for applications operating at lower temperatures [1,2]. For the synthesis of new aluminum alloys with good mechanical properties at elevated tem-peratures, a combination of powder metallurgy and rapid solidiﬁ cation is necessary. The largest hardening effect in Al based alloys is due to precipitation of ex-tremely small, uniformly and densely dispersed parti-cles in aluminum matrix [3]. Numerous studies have been carried out in this ﬁ eld. However, it is important to continually supplement our knowledge on the tempera-ture stability of these types of materials.The process of rapid solidiﬁ cation enables higher solubility of alloying elements in the αAl solid solution. This is important in the production of alloys containing elements with low equilibrium solid solubility such as the transition group metals and lanthanides. In Table 1 maximal equilibrium solubility at room and eutectic/peritectic temperature for some low soluble alloying el-ements is presented [4].Data concerning the extent and type of microstructural phases in microstructure of rapidly solidiﬁ ed aluminum alloys and their modiﬁ cations during heating are impor-tant for controlling their properties. In the present work, the initial as-cast microstructure of the rapidly solidiﬁ ed Al-Ni-Co-RE alloy ribbons and the resulting microstruc-ture after subsequent heat treatment as analyzed.EXPERIMENTAL METHODSAn investigated aluminum alloy, with the chemical composition given in Table 2, was melted in inductive furnace in argon atmosphere and then rapidly solidiﬁ ed in a single roller melt spinning apparatus at various cir-cumferential velocities.For the microstructure characterization of the rapid-ly solidiﬁ ed ribbons, transmission electron microscopy (TEM), and scanning electron microscopy (SEM), were used. The sequence of microstructural changes during heating was recorded by measuring the electric resist-ance of the ribbons and scanning calorimetry (DSC). Samples for SEM observation were prepared according to classical metallographic procedure and etched with Keller’s reagent. In order to observe the microstructure on surfaces parallel to the contact surface with the rotat-ing wheel, the samples were ground down to deﬁ ned thickness, polished and electrolytically etched. TEM observation samples were prepared by cutting out the disk-shaped specimens, which were then ion thinned and polished with Gatan PIPS M.691.RESULTS AND DISCUSSIONAll contemporary studies, concerning two or more component Al-TM rapidly solidiﬁ ed alloys, show dif-ferent solidiﬁ cation morphologies across the transverse cross-section of the ribbon with clear dependence on the thickness of solidiﬁ ed ribbon [5,6]. Rapidly solidi-ﬁ ed Al-Ni-Co-RE alloy ribbons also show different morphologies across the ribbon thickness. Main reason for the distinctive layers (zones) is the direct connection between ribbon thickness and solidiﬁ cation velocity. At circumferential velocities of 35, 30 and 20 m/s, ribbon thicknesses between 15-25, 20-40, and 35-60 µm, were obtained. Rapidly solidiﬁ ed ribbons having thickness of up to 25 µm have a single featureless globular layer, which does not etch in Keller’s reagent. Typical micro-structure (Figure 1) of ribbons having a thickness be-306  METALURGIJA 52 (2013) 3, 305-308B. KARPE et al.: MICROSTRUCTURE EVOLUTION AND THERMAL STABILITY OF RAPIDLY SOLIDIFIED Al-Ni-Co-RE ALLOYhigh-temperature intermetallic phase particles visible on the free surface of the ribbon (zone C).These particles can be seen in the center of columnar grains, as there grew radially outwards in a rosette like shape indicating that nucleation and growth started already in the melt. The morphology of ribbons having various thicknesses is shown in Figures l, Figure 3, Figure 4.Zone A is composed of tiny globular grains. The presence of this zone is a consequence of a large under-cooling, which occurs at the beginning of solidiﬁ cation through entire volume of the melt layer. Grains in this zone are between 2 and 5 µm in size, depending on the area being observed and the thickness of the ribbon.Figure 2 represents calculated cooling curves in Al ribbon as a function of ribbon thickness and contact time with the cooling wheel [7], and Figure 3 cooling rate as a function of the ribbon thickness and distance from the wheel contact surface [8].If the melt layer on the wheel surface is thin before the solidiﬁ cation is complete, the entire layer will be undercooled. This will result in a high solidiﬁ cation rate through the entire transverse cross-section of the rib-bon. Because of the small volume of the melt and con-Table 1  Maximal equilibrium solubility at room and eutectic/peritectic temperature for some low soluble alloying elements [4]Element Mass.% Te,TpNi 0 ÷ 0,24 639,9Co ≈ 0 657La 0 ÷ 0,05 640Ce ≈ 0 640Pr 0 ÷ 0,05 640Nd 0 ÷ 0,05 632Fe ≈ 0 655Zr 0 ÷ 0,16 660,8Table 2 Chemical composition of investigated alloyElement Mass.%Ni 4,6Co 0,8Re-Mish metal 3,0Fe+Zr+Mn+Si 1,0Al RestFigure 1  Morphologies of ribbons in a transverse cross-section: a) thickness 15 - 25 µm - rotation speed of 35 m/s, b) thickness 25-40 µm- rotation speed of 30 m/s, and c) thickness 35-60 µm - at a rotation speed of 20 m/sFigure 3  Cooling rate of Al ribbon as a function of its thickness and distance from the wheel contact surface [8]Figure 2  Al ribbon cooling curves as a function of its thickness [5]tween 20-40 µm is composed of two layers. Globular layer at the contact surface with the rotating wheel (zone A) and cellular microstructure layer (zone B). Ribbons thicker than 45 µm have, along with zones A and B, 307METALURGIJA 52 (2013) 3, 305-308B. KARPE et al.: MICROSTRUCTURE EVOLUTION AND THERMAL STABILITY OF RAPIDLY SOLIDIFIED Al-Ni-Co-RE ALLOYsequently small amount of crystallization heat released, the cooling wheel is able to absorb all the crystallization heat [9]. This can be seen on the calculated cooling curve diagram (Figure 2) for the 10 and 20 μm thick ribbons as an absence of the temperature arrest of the free surface, contrary to thicker ribbons.When the melt layer on the wheel surface is thicker, solidiﬁ cation at the contact surface starts before entire layer is undercooled. Some of the crystallization heat will be absorbed by the remaining melt, which will de-crease its cooling rate and change the conditions for the nucleation. Furthermore, when a layer of deﬁ ned thick-ness solidiﬁ es, thermal resistance in already solidiﬁ ed region of the melt layer becomes the limiting factor of the heat transfer, resulting in a decrease of the remain-ing melt cooling rate even further.Uniform cooling and solidifying rates through entire cross section of the ribbon can be achieved only when very thin (< 25 μm) ribbons are cast (Figure 4). In this case zone B of the rapidly solidiﬁ ed ribbons has a cellular microstructure. Cell walls are composed of intermetalic phase particles. The cells in this zone are between 100 and 200 nm in size. In this core the zone C has rosette-like grains between 5 and 15 µm in size, and the grain bounda-ries contain even more intermetalic phase particles than those found in zone B (Figure 5). The center of those grains is composed of a supersaturated solid solution αAl as the primary phase, while the particles and cell walls form in-termetalic phases rich in Al, Ni, Co and La. Intermetalic phase particles at the grain boundaries are richer in Ni, Co and La than the particles within the grains.Figure 6 shows differential scanning calorimetry (DSC) curve and electrical resistance curve (ER) during heating at a constant rate of 5 K/min up to 600 °C.Figure 4  Morphology of ribbons having a thickness between 25-45 μm in a transverse cross-section: a) contact surface b) zone B and c) free surface (electrolytically etched - SEM)Figure 6  Comparison of the electrical resistivity and DSC curves as a function of temperatureFigure 5  Microstructure of rapidly solidifi ed Al-Ni-Co-RE alloy ribbons having a thickness greater than 45 μm: a) cellular micro structure- zone B (TEM) b) rosette like shape crystals on the free surface- zone C (TEM)Electrical resistivity increases linearly all the way up to 400°C and there is no exothermic peak on DSC curve. This means that heating the ribbons up to 400 °C does not affect their microstructure at all. At higher temperatures the change of electrical resistivity becomes nonlinear. At 489,3 °C it is evident, that an exothermic peak on the DSC corresponds to the minimum of the electric resistance ﬁ rst derivative temperature curve (dR/dT). During heating above the Tm or rather the TM point, up to 510 °C, the cellular microstructure in zones B and C can still be ob-served. However, the transition to globular particles is also present. Larger differences can be observed on grain boundaries (Figure 7).As the microstructure during heating changes, so does the microhardness. This is conﬁ rmed by micro-hardness measurements made on ribbons of various thicknesses after rapid solidiﬁ cation and thermal an-nealing up to 500 °C. An average value was calculated from a series of microhardness measurements. Inﬂ u-308  METALURGIJA 52 (2013) 3, 305-308B. KARPE et al.: MICROSTRUCTURE EVOLUTION AND THERMAL STABILITY OF RAPIDLY SOLIDIFIED Al-Ni-Co-RE ALLOYence of temperature on the microhardness of ribbons having various thicknesses is presented in Figure 8.Higher microhardness of thinner ribbons is a conse-quence of microstructure evolution and increased solu-bility of alloying elements in aluminum matrix αAl.The microhardness of rapidly solidiﬁ ed Al-Ni-Co-RE alloy ribbons remains nearly constant during heat-ing at 5 K/min up to 400 °C. Decrease of the microhard-ness after heating above 400 °C indicates the precipita-tion of the alloying elements from the supersaturated αAl solid solution in the form of ﬁ ne intermetalic parti-cles [10]. Temperature dependency of the microhard-ness is in good agreement with the electrical resistivity temperature dependency, which also shows nonlinear change above that temperature [11,12]. This can be seen more precisely on the ﬁ rst derivate electrical resistivity curve (dR/dT).In comparison to the Al-Fe and Al-Fe-V alloys [12], the investigated Al-Ni-Co-RE alloy shows improved thermal stability in the temperature interval between 300 to 400 °C.Shape of the fracture surface, with a distinctly pitted surface and a large number of deep oval concave pits, indicates that investigated alloy is extremely ductile and classiﬁ es it as a promising potential candidate for hot extrusion process.CONCLUSIONSThe purpose of our investigation was to synthesize and investigate the temperature stability of aluminum alloy with addition of transition (Ni, Co) and lanthanide (RE-mischmetal) alloying elements beyond their maxi-mum equilibrium solubility. We further wanted to con-ﬁ rm appropriate measurements for tracking the precipi-tation sequence from supersaturated solid solution αAl.From the results of our investigation can be conclud-ed, that the rapid solidiﬁ cation by meltspinning tech-nique enables higher solid solubility of added alloying elements in the aluminum matrix. The as-cast micro-structure evolution is directly related to the thickness of the ribbon which governs the heat transfer and conse-quently nucleation event and solidiﬁ cation velocity. For higher supersaturation very thin (< 20 µm) ribbons need to be cast. Measurements of the electrical resistivity are closely linked to the microstructural changes in the rap-idly solidiﬁ ed alloys and are in good agreement with differential scanning calorimetry (DSC) and microhard-ness measurements. The precipitation of intermetalic particles from supersaturated αAl solid solution starts at temperature above 400 °C, when electrical resistivity becomes non-linear and last until 510 °C. The most in-tensive change in microstructure happened at ≈490 °C, where dR/dT minimum is detected. This is also in ac-cordance with the exothermic peak on DCS curve and microhardness decrease at 500 °C.REFERENCES[1]  H. H. Liebermann: Rapidly Solidiﬁ ed Alloys, Marcel Dek-ker, London, 1993.[2]  D. Klobčar, L. Kosec, A. Pietras, A. Smolej: Materials and Technology, 46(2012)5, 483-488.[3]  A. Inoue, H. Kimura: Solid State & Materials Science, 2(1997)3, 305-310.[4]  ASM International, Alloy Phase Diagrams Vol. 3, 1992.[5]  T. Haga, K. Inoue, H. Watani: Journal of Achievements in Materials and Manufacturing Engineering, 40(2010)2, 115-122.[6]  H. Jones: Journal of Materials Science, 19(1984)4, 1043-76.[7]  B. Karpe, B. Kosec, M. Bizjak, T. Kolenko: Metalurgija, 50(2011)1, 13-16.[8]  B. Karpe, M. Bizjak, B. Kosec: Journal of Achievements in Materials and Manufacturing Engineering, 46(2011)1, 88-94.[9]  G. X. Wang, E. F. Matthys: Modelling and Simulation in Material Science and Engineering, 10(2002), 35-55.[10] J. Tušek, D. Klobčar: Journal of Mechanical Engineering, 50 (2004) 2, 94-103.[11]  I. Budak, M. Soković, M. Barišić: Measurement, 44 (2011) 6, 1188-1200.[12]  M. Bizjak, L. Kosec: Zeitschrift fuer Metallkunde, 91 (2000), 160-164.Note:  The responsible translator for English language is U. Letonja. MOAR, Podgora, SloveniaFigure 8  Vickers microhardness as a function of ribbon thickness and temperatureFigure 7  Microstructure of rapidly solidifi ed Al-Ni-Co-RE alloy after heating at a constant rate of 5 K/min. up to 510 °C: a) zone B, b) zone C

Microstructure evolution and thermal stability of rapidly solidified Al-Ni-Co-RE alloy

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