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Preparation and characterization of iron aluminide based intermetallic alloy from titaniferous magnetite ore
Journal of Alloys and Compounds 359 (2003) 159–168
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Preparation and characterization of iron aluminide based intermetallic alloy from titaniferous magnetite ore S.P. Chakraborty, I.G. Sharma*, P.R. Menon, A.K. Suri Refractory Metals Development Section, Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 19 December 2002; received in revised form 27 January 2003; accepted 27 January 2003
Abstract Intermetallic alloys such as iron aluminide (Fe 3 Al) display an attractive combination of physical and mechanical properties. It is a potential candidate for application demanding high strength, superior oxidation, sulphidation and corrosion resistance. However, the utilization of this alloy is limited owing to problems of its preparation by conventional techniques and poor room temperature ductility. The present paper is, therefore, an attempt to prepare an Fe 3 Al-based alloy (iron aluminide alloy) of composition Fe–17.24Al–5.54Cr– 1V–0.05C (wt%) by a non-conventional non-furnace process. The process essentially involves the use of cheap, indigenously available titaniferous magnetite ore containing oxides of iron and vanadium and oxide of chromium that are subsequently co-reduced by aluminium in the presence of excess aluminium and a requisite amount of carbon in a specially designed water-cooled copper reactor. The charge composition is judiciously adjusted to attain the alloy composition by utilizing the exothermicity of the overall reactions. The alloy is further characterized with respect to composition, phases and microstructure and properties such as fabricability, hardness, strength and oxidation resistance were studied to indicate its suitability for high temperature applications. 2003 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys; Solid state reactions; Powder metallurgy; Metallography
1. Introduction Intermetallic alloys or compounds display an attractive combination of physical and mechanical properties that has led to their consideration for application in many structural and non-structural areas. Amongst the various intermetallics, iron aluminide (Fe 3 Al) is being considered as a potential candidate for application in the areas of nuclear, aerospace, chemical and thermal power plants as structural components, as heating elements in toasters, oven and furnaces for the following favourable properties. Iron aluminide possesses low density, superior oxidation, sulphidation and corrosion resistance, high strength to weight ratio, high melting point and electrical resistivity, combined with its ability to retain strength and stiffness at elevated temperatures. However, this alloy has not yet come to the stage of commercialization owing to problems of synthesis and poor fabricability [1]. Synthesis of intermetallics is a difficult job for the following reasons. Intermetallics are generally formed over *Corresponding author. Fax: 191-22-2556-0750 / 2550-5151. E-mail address: [email protected] (I.G. Sharma). 0925-8388 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00197-X
a narrow range of composition. Consequently, during the stages of formation, apart from the desired phase, some other unintended phases may also form, resulting in contamination and deterioration in the properties of the parent alloy. Intermetallic alloys are invariably made of alloying constituents having large difference in melting temperature and density leading to segregation of components during formation. Such alloys are apt to have poor room temperature ductility [2]. Efforts were made in various laboratories to prepare these materials by using conventional melting and powder processing approaches. Both these techniques employ pure components for melting. Conventional melting is usually carried out either by arc melting or by induction melting techniques under air, vacuum or controlled atmosphere. However, conventional melting encounters a host of problems related to melting. Firstly, due to large difference in melting temperature and density of iron and aluminium, homogeneous melting is not achieved. Secondly, adding aluminium to molten iron causes a large temperature rise of the molten bath. Such a sudden rise in temperature is dangerous for operators and results in melt oxidation, longer holding time prior to pouring and a potential for
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missing the target chemistry because of oxidation of alloying elements [3]. Under the powder processing route, two techniques such as mechanical alloying and self-propagating high temperature reaction synthesis (SPHTS) were attempted by various researchers to prepare iron aluminide alloys. Mechanical alloying is carried out by attrition ball milling of extremely fine high-purity crystalline powders of the alloying components iron and aluminium, together for a continuous period of 100 h or so. Repeated fracture and re-welding of the powder particles results in the alloy formation. However, handling of the fine powder gives rise to an increase in the level of interstitial impurities in the resultant alloy. Sometimes, the alloys are also found to contain traces of unalloyed components due to an incomplete process of alloying which can damage the mechanical properties of the final alloy. Stringent specification of the starting materials, their high cost and prolonged duration of mechanical alloying restricts the process for commercial exploitation [4]. The SPHTS process works on the principle of alloying fine powders of iron and aluminium by utilizing the heat evolved during their synthesis. However, heat evolution during the synthesis reaction (DH5216 kcal) is inadequate for carrying out any bulk melting. Therefore, the process has so far only been applied for localized melting jobs, e.g., joining of two iron aluminide alloys [5]. A vast reserve of titaniferous magnetite ore (TMO) estimated to be around 3.2 MT is available in the Masanikere region of Karnataka, India. The ore contains a high amount (75.4%) of iron oxide (Fe 2 O 3 ) and good amount of V2 O 5 (1.43%). In spite of the high iron content in the ore, it is scarcely utilized by iron and steel industries for pig iron production due to the presence of high amounts of titania (TiO 2 ) in the ore. Easy availability of titania-free hematite ore is also another cause for the steel producers to prefer hematite in place of TMO. Consequently, the TMO remains mostly unutilized. As the oxide intermediates of iron and vanadium present in TMO are easily amenable to chemical reduction by aluminium with good evolution of heat, a programme, therefore, has been initiated to prepare an iron aluminide (Fe 3 Al)-based alloy of composition Fe–17.24Al–5.54Cr–1V–0.05C (wt%) from TMO with external addition of Cr 2 O 3 by their co-reduction with aluminium in the presence of excess aluminium and the requisite amount of carbon. The alloying addition of chromium in the iron aluminide alloy is intended to improve its ductility by formation of a passive layer of Cr 2 O 3 , while the presence of vanadium is expected to improve its strength by formation of fine precipitates of vanadium carbides. Heat generated from the chemical reduction reactions between oxides and Al is utilized for smelting and consolidation. Reduction smelting was carried out under protected environment of argon in a watercooled copper reactor (WCCR). The reactor was specially designed to conduct the smelting experiments in a closed
environment so that the resultant iron aluminide alloy does not pick up moisture from surrounding air and also to obtain a clean alloy button by avoiding lining material as usually needed thermal insulation. The slag, enriched with a high content of titania (TiO 2 ), can be a potential byproduct for titania production for ultimate use in the paint industry [6]. The as-reduced alloys were further remelted by nonconsumable tungsten arc melting for consolidation and annealed for homogenization. Subsequently the alloy products were characterized with respect to chemical composition, microstructure, homogeneity, phases, etc., and the properties such as fabricability, hardness, strength and oxidation behavior were evaluated to examine its suitability for commercial application.
2. Experimental
2.1. Reactant materials Titaniferous magnetite ore (TMO) supplied by Bhadravati Iron and Steel Company was used as the source for Fe 2 O 3 and V2 O 5 . The as-received ore having particle sizes varying from 30 to 40 mm was first crushed into smaller dimensions (4–5 mm) by employing a Jaw crushing machine. Then the crushed ore was further ground to 21501200 [ mesh size by attrition grinding. The ground ore was subsequently calcined at 600 8C for 6 h before use. Technical-grade chromium oxide of purity 99% (wt%) was used. The oxide was calcined at 700 8C for 6 h before use. Aluminium powder (grade-C) of purity 99% and mesh size 2120[ ASTM supplied by Indian Aluminium (INDALCO) was used as a reducing agent and component for alloying. The reductant has been procured from Indian Aluminium. A calcined petroleum coke powder obtained from Coromondal Carbon India with purity level better than 98% and size 200[ ASTM was used. Laboratory-grade lime received from S.D. Fine Chemicals (Boisar, India), was used. It was calcined at 800 8C prior to use. Chemical analysis of the above oxides, reductant aluminium and carbon is presented in Table 1.
2.2. Equipment and procedure The reactions involved during the preparation of the iron aluminide alloy and subsequent phase transformation of the alloys were studied by differential thermal analysis (DTA) technique. The samples were prepared by mixing TMO powder, Cr 2 O 3 , V2 O 5 , C and Al in their desired ratios and cold pressing the mixture into small pellets weighing around 20 mg. During the operation, the samples
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Table 1 Chemical analysis of charge constituents Materials
Amount (wt%) Oxide
TMO Cr 2 O 3 CaO Al C
Al
75.4 2.48 (Fe 2 O 3 ) 99.38 – 99.0 0.02 99.4 – Ash content |1.0 max.
Ti
Si
Mg
V
Mn
S
P
Cu
Ni
Fe
Ca
7.13
3.04
–
0.8
–
–
–
–
–
–
–
0.07 0.10 –
0.03 0.40 0.15
– – – – – – 0.12 0.06 – Heavy metals |0.005
0.08 – –
0.08 0.08 – – – – Silica1Fe,0.01
0.07 0.01 0.04
0.14 0.02 0.10
– – –
were heated to 1700 8C from room temperature at a heating rate of 108 / min. The calcined reactants such as TMO, Cr 2 O 3 , and CaO were first classified into 21501200[ mesh size using standard Tyler series sieves for subsequent use as the reactant oxides for thermite smelting. Classified materials were then mixed together along with reductant aluminium and carbon in their desired ratio thoroughly in an attrition ball mill keeping the charge to ball ratio at 10:1 for 1 h. Experiments were conducted in a water-cooled copper reactor. A schematic diagram of a water-cooled copper reactor is shown in Fig. 1. The reactor assembly essentially consists of a one end open, double-walled cylindrical vessel made of copper. The top end of the reactor is fitted with a copper flange having a provision for an ‘O’ ring arrangement for isolating the reactor from the surrounding environment. The double wall is provided for circulating the cooling water around the reactor. The reactor used for small-scale experiments has dimensions of 45 mm i.d., 50 mm o.d. and 200 mm length, whereas the reactor used for large-scale experiments has dimensions of 100 mm i.d. and 425 mm length. All the reactors are provided with a slight tapering at the bottom to facilitate the collection of the
Fig. 1. A schematic diagram of water-cooled copper reactor.
reduced alloy button. The reactor is also provided with an alumina tube inserted via a Wilson seal arrangement inside the reactor near the charge to add glycerine to the charge for triggering the reaction. The other end of the tube is connected to a funnel for storing glycerine. After mixing, the charge is then placed inside the reactor cavity and moderately rammed. The reactor is then bolted from top by means of the copper flange. The unit is then tested for leaks, evacuated and back-filled with argon several times and kept under argon flow during the period of smelting. Cooling water supply to the reactor is ensured prior to initiation of the reaction. The thermite reaction is initiated by pouring glycerine over the small amount of KMnO 4 crystals kept over the trigger mixture (KClO 3 1 Al) lying on the top of the reacting charge. The reaction, once initiated, proceeds briskly to completion. An instant rise in the outlet water temperature indicates the commencement of the reaction. The flow rate of the outgoing gas is also found to increase significantly during the course of the reaction. The flow of argon and cooling water are continued until the reaction product attains room temperature. Then, at room temperature, a well-consolidated alloy button collected beneath the slag layer is recovered on breaking the top slag layer. It was then centrally sectioned in the longitudinal and transverse directions into four pieces to physically examine the presence of any defects such as cracks, blow holes, cavities, etc. The as-reduced alloy buttons were further remelted by arc melting under argon atmosphere in a water-cooled copper hearth for consolidation. The remelted alloy buttons were again cut in the longitudinal and transverse directions to physically examine the presence of any cavity or holes. After remelting, the alloy samples were encapsulated in an evacuated quartz tube and then further isothermally heated at 900 8C for 6 h for homogenization. Compositional analysis of the as-reduced and remelted / homogenized alloys was evaluated by standard gravimetric wet analytical techniques and further substantiated by induction-coupled plasma atomic emission (ICP-AES) technique. Carbon was analyzed using a gas analysis technique using a Leco gas analyzer. The optically polished and etched specimens were also used for the EPMA studies. Pure elements such as iron, aluminium, chromium, vanadium and carbon were used as
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the standards. The EPMA was operated at an accelerating voltage of 15 kV with a stabilized beam current of 100 nA. The Ka radiations of Fe, Cr, and V were diffracted by a LiF crystal, whereas a TAP crystal was used to diffract Ka radiation of Al. Identification of different phases present in the as-reduced and remelted / homogenized iron aluminide alloy samples was carried out by X-ray diffraction (XRD) using Cu Ka radiation ( l50.154 nm) with a nickel filter and a secondary beam monochromator. One mg of each alloy sample taken in powder form was scanned from 108 to 1508 at a scanning rate of 28 / min. The measured ‘d’ values with their corresponding intensities were compared with the data reported in the standard ASTM card index for the identification of the desired phases. Iron aluminide alloys obtained from different batches were cut into 5310-mm rectangular pieces and then investigated by standard metallography. Polished samples were etched with an etchant wt% H 2 O, 42 wt% CH 3 COOH, 25 wt% HCl and 25 wt% HNO 3 . Subsequently, the specimens were examined in an optical microscope in magnification range of 380–200. The hardness of as-reduced and remelted / homogenized iron aluminide alloys was measured on optically polished samples in the transverse as well as longitudinal directions at several different places using an Automatic Vickers Microhardness tester. Before choosing a location for hardness measurement, the corresponding microstructure in that location was observed in an auto-microhardness tester. An optically polished sample was used. A load of 1 kg was applied on the sample surface for 15 s duration. Rolling of as-reduced and remelted / homogenized iron aluminide samples was carried out in a two-high rolling mill. Prior to rolling, the samples were cut into 40320-mm rectangular specimens having 10-mm thickness. Two sets of specimens were used for rolling. The first set has been rolled directly, whereas the second set has been jacketed with mild steel prior to rolling. Rolling was carried out under both cold and hot working conditions. For hot rolling, before commencing the rolling operation, the specimens were heated in a resistance heating furnace at 950 8C and soaked for about 20 min duration. During every course of rolling, the specimens were given four to five passes and at each interval of the rolling operation, the specimens were given 10 min soaking at 950 8C. The same steps were repeated until cracks were observed on either surface of the specimen or jacket. After the rolling operation, the thickness of the rolled samples was recorded. The oxidation study on as-reduced and remelted / homogenized iron aluminide alloys was carried out in an oxidizing atmosphere under isothermal as well as nonisothermal heating conditions. For the isothermal study, rectangular specimens with dimensions 203530.5 mm were fabricated from the alloy buttons by an electrodischarge machining technique. The specimens were then
optically polished to remove surface roughness and washed with acetone to clean them of grease and dirt. The specimens were then heated at constant temperatures of 800, 1000 and 1200 8C in a resistance-heating furnace for a time duration varying from 2 to 24 h. The oxidation study under non-isothermal heating condition was carried out by a thermogravimetric analysis (TGA) technique. A thin slice of a specimen weighing 16 mg was heated from room temperature to 1200 8C at a heating rate of 10 8C / min in an oxidizing atmosphere. As-reduced and remelted / homogenized samples were first examined by radiographic tests to ensure that they were free from solidification defects. Then the samples were fabricated into tensile specimens by an electro-discharge machining technique having gauge dimensions 12.533.531.5 mm. The specimen surfaces were further mechanically polished using diamond paste. Tensile strength measurement was carried out in a universal tensile machine (Instron) at RT under argon atmosphere. Strain rates in the order of 0.004–0.006 s 21 were applied to the specimens until fracture occurred.
3. Results and discussion
3.1. Thermodynamic and thermal feasibility of the reactions Aluminothermic reduction smelting for the preparation of present iron aluminide alloy involves a number of chemical reactions. These reactions take place when their 0 0 standard free energy (DG ) and enthalpy values (DH ) are sufficiently negative. Hence, the reactions and their above thermodynamic data are indicated below for the consideration of thermite smelting. Fe 2 O 3 1 2Al 5 2Fe 1 Al 2 O 3 DG 0298 5 2 280 kJ mol 21 , DH 0 5 2 852 kJ mol 21 Cr 2 O 3 1 2Al 5 2Cr 1 Al 2 O 3 DG 0298 5 2 174 kJ mol 21 , DH 0 5 2 534 kJ mol 21
(1)
(2)
3V2 O 5 1 10Al 5 6V 1 5Al 2 O 3 DG 0298 5 2 230.24 kJ mol 21 , DH 0298 5 2 3536 kJ mol 21 (3) The free energy of each of the above reactions is found to be highly negative indicating that the reactions are thermodynamically feasible. The enthalpy value of each of the above reactions shows that the reactions are highly exothermic and the combined specific heat value of all the reactions works out to be 3100 kJ kg 21 . In an aluminothermic reaction, the heat of the reaction is the main guiding force in determining whether the smelting operation will take place or not. In this context, it would be appropriate to mention here that for an aluminothermic reaction to
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endothermic peak at 1540 8C shows the congruent melting and formation of Fe 3 Al alloy and the sixth exotherm in the cooling curve corresponds to the solidification of the same Fe 3 Al alloy. Phase analysis by XRD of the DTA-treated sample further confirmed the presence of the Fe 3 Al alloy phases.
3.2. Studies on thermite smelting experiments
Fig. 2. DTA profile of a mixture of TMO, Cr 2 O 3 , C and Al.
become self-sustaining resulting in efficient slag-metal separation with higher metal yield, the specific heat generated in the process should be in the range of 2500– 4000 kJ kg 21 , as predicted by Hall [7] and Dautzenberg [8]. In the present case, the combined specific heat value of the above reactions is found to be 3100 kJ kg 21 that fits well in the recommended range. Thermal characteristics of the above reactions are also substantiated by differential thermal analysis (DTA) studies, as shown in Fig. 2. The thermal plot shows five peaks in the heating curve and one peak in the cooling curve. The first endotherm at a temperature of 660 8C was for the melting of aluminium. The second exothermic peak at 870 8C corresponds to the reduction reaction of Cr 2 O 3 by Al. Walton and Poulos [9] have reported this temperature to be around 882 8C. The third intense and large exothermic peak at 900 8C corresponds to the reaction between Fe 2 O 3 and Al. Biswas et al. [10] have reported this temperature to be around 900 8C that is close to the present value. The fourth small peak at 1050 8C refers to the reaction between V2 O 5 and Al. Sharma and Kale [11] have reported this temperature to be 1000 8C. The fifth
A number of thermite smelting experiments were conducted on 0.2–1 kg scale by using different charge compositions for the preparation of present iron aluminide alloy. The results are summarized in Table 2. It is observed that addition of excess aluminium over stoichiometric amount in the charge improves the alloy yield considerably (78–93 wt%) as is evident in experiment nos. 1–4. However, more than 10 wt% excess of aluminium in the charge contaminated the alloy product without improving the yield significantly (Expt. no. 4). Carbon in the charge was varied from 50 to 200 wt% for its proper loading in the final alloy. It is, however, observed that 100 wt% excess of carbon in the charge is adequate to maintain the desired level of carbon in the as-reduced alloy. Addition of lime (CaO) was intended to control the thermite reactions by reducing the excess heat output and to lower the viscosity and melting point of the slag for better slag– metal separation. Incorporation of 10–20 g lime in the charge (Expt. nos. 6–8) was found to be sufficient for achieving good slag–metal separation. However, more than 10 g lime in the charge led to poor alloy yield due to additional thermal burden on the charge. In experiment no. 7, the optimum charge composition containing 10 wt% excess aluminium, requisite amount of Cr 2 O 3 , 100 wt% carbon and 10 g CaO in the charge has led to the satisfactory alloy yield of 93 wt% with excellent slag– metal separation and alloy consolidation. A scale-up experiment on 1 kg charge was conducted to substantiate the results obtained in experiment no. 7, maintaining the
Table 2 Results of thermite smelting experiments Run no.
1 2 3 4 5 6 7b 8
Charge composition TMO a (g)
Cr 2 O 3 (g)
Al (g)
X1 (wt%)
X2 (wt%)
X3 (wt%)
Q (kcal kg 21 )
Y (wt%)
144 144 144 144 144 144 144 720
8.15 8.15 8.15 8.15 8.15 8.15 8.15 40.75
58.0 60.0 62.12 64.16 62.12 62.12 62.12 310.60
0 5 10 15 10 10 10 10
0 50 100 200 100 100 100 100
0 0 0 0 10 20 10 10
825 820 818 815 822 753 788 790
78.0 86.0 92.0 93.0 92.8 88.0 93.0 93.5
X1 , excess over stoichiometric amount of aluminium, wt%; X2 , excess over requisite amount of carbon, wt%; X3 , excess over requisite amount of CaO, wt%; Q, specific heat of overall thermite smelting charge, kcal / kg; Y, alloy yield, wt%. a Titaniferous magnetite ore (taken 163 g for Expts. 1–7 and 815 g for Expt. 8). b Charge composition corresponding to optimum experimental condition.
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same experimental conditions and the yield has further improved to 93.5 wt% (Expt. no. 8)
3.3. Compositional analysis of iron aluminide alloys Results of analysis of the as-reduced and remelted / equilibrated alloy samples obtained under optimum experimental condition (Expt. no. 7, Table 3) is presented in Table 3. The remelted / equilibrated alloy composition is found to match nearly the designed alloy composition. A slight increase in the elemental concentration in remelted / equilibrated alloy compared to the as-reduced alloy is due to better consolidation of the alloy and uniform distribution of the alloying components.
3.4. Study of distribution of components in the alloys The EPMA study on iron aluminide alloys reveals that the as-reduced thermite alloys contain imperfections such as solidification cavities, voids, thermally induced cracks and compositional inhomogeneity. These imperfections are more pronounced in the poorly developed alloys; however, the well-consolidated alloys with clean slag–metal separation contain significantly less such imperfections. As per the equilibrium phase diagram of iron and aluminium as shown in Fig. 3, the Fe 3 Al phase should conform to an intermetallic alloy phase containing around 11–20 wt% Al [12]. The desired Fe 3 Al-based alloy has a theoretical composition of Fe–17.24Al–5.54Cr–1V–0.05C (wt%). The EPMA scanning study shows that above composition with minor variation was obtained in only well consolidated and clean as-reduced iron aluminide alloy at some selected areas. The same alloy after remelting and homogenization treatment shows that the above composition with slight variation is maintained throughout the matrix due to better distribution of alloying components and dissolution of cracks and voids on remelting.
Fig. 3. Binary phase diagram of iron and aluminium.
3.5. Identification of phases by X-ray diffraction The diffraction patterns of the as-reduced thermite alloy obtained under optimum experimental condition and the corresponding remelted / homogenized iron aluminide alloy are presented in Figs. 4 and 5, respectively. The broadened intensity peaks in Fig. 4 at 2u angles of 35, 45 and 658 are found to correspond to the Fe 3 Al, CrVC 2 and VC 2 phases. The sharp and well-defined intensity peaks in Fig. 5 at 2u angles of 30, 45 and 658 correspond again to the Fe 3 Al, CrVC 2 and VC 2 phases, respectively. However, phases
Table 3 Compositional analysis of iron a aluminide alloys made from TMO Element
Al Cr V C Mn Ni O Fe a
Theoretical composition (wt%)
Actual composition (wt%) As-reduced
Remelted
17.24 5.54 1.01 0.05 – – – Balance
16.45 5.28 0.94 0.046 0.015 0.015 ,0.02 Balance
16.89 5.34 0.98 0.048 ,0.010 ,0.010 ,0.02 Balance
Corresponding to the optimum experimental condition (Expt. no. 7, Table 2).
Fig. 4. XRD pattern showing the phases present in as-reduced iron aluminide alloy.
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3.6. Microstructural study The optical microstructures of as-reduced thermite and remelted / homogenized alloys are presented in Figs. 6 and 7. The microstructure of as-reduced alloy shows a combination of large and medium grains (size range, 50–100 mm) from hexagonal to irregular shape. The remelted / homogenized alloy shows a considerably refined microstructure having nearly equiaxed grains (30–50 mm) distributed uniformly throughout the matrix and smaller in size than the thermite alloys. Carbides of chromium and vanadium are found to be distributed at random in the matrix of the as-reduced alloy in the form of dark round spots. However, the same is distributed selectively at the grain boundaries in the remelted / homogenized alloy. This can be attributed to the isothermal annealing treatment given to the remelted alloy.
3.7. Hardness measurement of iron aluminide alloys
Fig. 5. XRD pattern showing the phases present in remelted iron aluminide alloy.
such as FeAl, TiAl, Ti 3 Al, Al and silicides of aluminium were not observed in either of the two alloys. In the remelted / homogenized alloy, the reason for the intensity peaks being well-defined and sharp can be attributed to the better formation of the above phases in this alloy due to the consolidation and homogenization treatment given to the iron aluminide alloy.
Results of hardness measurement on iron aluminide alloys are shown in Table 4. The average hardness value for the as-reduced iron aluminide alloy was found to be 396 VHN and for the corresponding remelted / homogenized alloy 300 VHN. The remelted alloy shows comparatively less hardness (300 VHN) than the as-reduced alloy (396 VHN) in view of its better consolidation, refining and homogenization. A large variation in hardness value (370– 420 VHN) was found at different locations of the asreduced sample as compared to the remelted alloy (290– 310 VHN). This again confirms that the remlted alloy is better homogenized than the as-reduced alloy.
Fig. 6. Optical microstructure of as-reduced iron aluminide alloy.
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Fig. 7. Optical microstructure of remelted iron aluminide alloy.
3.8. Evaluation of fabricability of iron aluminide alloys by rolling The fabricability of iron aluminide alloys was evaluated by monitoring their reduction in thickness during rolling. Results of rolling on iron aluminide alloys are indicated in Table 5. It is observed that both as-reduced and remelted / homogenized samples have undergone much better reduction in thickness during hot rolling under jacketed condition than at room temperature under unjacketed condition. Hot rolling has given better rolling ability of the alloys due to better plastic deformation at high temperature, whereas jacketing has provided protection against oxidation and superior heat retaining capacity of the underlying alloys. Rolling under unjacketed condition has Table 4 Results of hardness measurement on iron aluminide alloy Type of alloy
Hardness value (VHN)
As-reduced
370 380 390 405 410 420 396 290 295 300 305 310 300
Average hardness Remelted
Average hardness
resulted in early failure of the alloys by edge cracking at elevated temperature (950 8C) due to faster cooling of the alloys. A maximum 40% thickness reduction for the asreduced alloy and 80% for the remelted / homogenized alloy were achieved under jacketed condition.
3.9. Oxidation study on iron aluminide alloys An evaluation of the oxidation resistance of iron aluminide alloys was carried out under isothermal and non-isothermal heating conditions by monitoring their change in weight with time. Plots showing oxidation pattern of the above alloys are shown in Figs. 8 and 9. It is observed from the plots that the oxidation at the initial stage takes place rapidly for a 5–10-h heating cycle. However, the weight gain is very small, varying from 0.005 to 0.008 mg for the as-reduced alloys (5310 25 –83 10 25 mg / mm 2 ) and (2310 25 25310 25 mg / mm 2 ) for the remelted alloys. This rapid weight gain at the initial stage is due to the formation of passive layer of Al 2 O 3 on the Table 5 Results of rolling study on iron aluminide alloys Type of alloy
Temp. (8C)
Reduction in thickness (wt%)
Unjacketed as-reduced Unjacketed remelted Jacketed as-reduced Jacketed remelted Unjacketed as-reduced Unjacketed remelted Jacketed as-reduced Jacketed remelted
RT RT RT RT 950 950 950 950
10 20 15 30 20 50 40 80
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Table 6 Results of tensile strength measurement on iron aluminide alloys Type of alloy
Strength (MPa)
Elongation (wt%)
As-reduced
550 560 570 580 590 600 600 610 620 630 640 650
20 18 17 16 16 15 35 30 27 25 23 20
Remelted
Fig. 8. Oxidation pattern of as-reduced iron aluminide alloy at various temperatures.
alloy surface. However, during the 10–24 h of heating cycle, the oxidation rate becomes sluggish with hardly any noticeable weight gain. As the layer of the oxide scale increases, further oxidation of the underlying metal proceeds by a process of diffusion of O 2 through the oxide layer of Al 2 O 3 . As a result, the oxidation rate has become sluggish during 10–24 h heating cycle. However, the cumulative weight gain for the entire oxidation study is found to be nominal as described below. The cumulative weight gain for as-reduced alloys at 800 8C is found to vary from 0.005 to 0.008 mg, at 1000 8C from 0.005 to 0.01 mg and at 1200 8C from 0.008 to 0.012 mg, respectively. Correspondingly, the cumulative weight gain for remelted alloys is found to vary at 800 8C from 0.002 to 0.003 mg, at 1000 8C from 0.004 to 0.005 mg and at
1200 8C from 0.006 to 0.008 mg, respectively. Non-isothermal heating of either as-reduced or remelted iron aluminide alloys has not shown any weight gain due to oxidation as no peak corresponding to weight gain was observed in the temperature profile of the TGA study.
3.10. Tensile strength measurement Results of the tensile strength measurements are indicated in Table 6. It is observed during tensile strength measurements that iron aluminide alloys possess high tensile strength at room temperature lying in the range of 550–650 MPa with corresponding elongation in the range of 15–35%. The as-reduced alloys have exhibited comparatively lower strength values (550–600 MPa) and lower elongations (15–20%) than the remelted / homogenized alloys (600–650 MPa) and elongation (20–30%). The remelted / homogenized sample has exhibited better strength and ductility which can be attributed to its having a better homogenized structure with adequate grain refinement and less or no defects due to porosity.
4. Conclusion
Fig. 9. Oxidation pattern of annealed iron aluminide alloy at various temperatures.
The above studies demonstrate the technical feasibility of preparing an iron aluminide alloy of composition Fe– 17.24Al–5.54Cr–1V–0.05C (wt%) from indigenously available cheap titaniferous magnetite ore by a direct aluminothermic co-reduction technique in a specially designed water cooled copper reactor. By optimizing the process parameters such as reductant aluminium, carbon, fluxing agent lime and heat of the reaction, it has been possible to obtain well-consolidated alloys of compositions close to the nominal compositions. The as-reduced alloy after a remelting and homogenization treatment has exhibited a homogenized composition, refined microstructure, excellent fabricability, high strength and hardness with adequate oxidation resistance suitable for high-temperature applications.
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References [1] N.S. Stoloff, C.T. Liu, S.C. Deevi, Intermetallics 8 (2000) 1313– 1320. [2] C.G. Mckamey, J.H. DeVan, P.F. Tortorelli, V.K. Sikka, J. Mater. Res. 6 (8) (1991) 6–12. [3] V.K. Sikka, in: Proceedings of the Symposium on Processing, Properties and Applications of Iron Aluminides. The Minerals, Metals and Materials Society, 1994, pp. 8–10. [4] J.S. Benjamin, Metal. Trans. 1 (1970) 1–9. [5] J.H. DeVan, Metall. Trans. A 22 (1991) 267–275. [6] H.Y. Sohn, O.N. Carlson, J.T. Smith (Eds.), Extractive Metallurgy of Refractory Metals, The Metallurgical Society of AIME, Warrendale, PA, 1981, pp. 127–145.
[7] I.H. Hall, Aluminothermic process, in: 5th Edition, Ulmans Encyclopedia of Industrial Chemistry, Vol. A1, Verlag Chemie, Weinheim, 1985, pp. 447–452. [8] W. Dautzenberg, Aluminothermic, in: Ulmans Encyklopadie der Technischen Chemie, 4th Edition, Verlag Chemie, Weinheim, 1974, pp. 351–361. [9] J.D. Walton, N.E. Poulos, J. Am. Ceram. Soc. 42 (1959) 40–45. [10] A. Biswas, I.G. Sharma, G.B. Kale, D.K. Bose, Met. Trans. B–29 (1998) 309–315. [11] I.G. Sharma, G.B. Kale, Trans Indian Inst. Met. 48 (3) (1995) 197–201. [12] F. Lihl, H. Ebel, in: A Handbook of Lattice Spacings and Structure of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967, pp. 550–560.
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Preparation and characterization of iron aluminide based intermetallic alloy from titaniferous magnetite ore S.P. Chakraborty, I.G. Sharma*, P.R. Menon, A.K. Suri Refractory Metals Development Section, Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 19 December 2002; received in revised form 27 January 2003; accepted 27 January 2003
Abstract Intermetallic alloys such as iron aluminide (Fe 3 Al) display an attractive combination of physical and mechanical properties. It is a potential candidate for application demanding high strength, superior oxidation, sulphidation and corrosion resistance. However, the utilization of this alloy is limited owing to problems of its preparation by conventional techniques and poor room temperature ductility. The present paper is, therefore, an attempt to prepare an Fe 3 Al-based alloy (iron aluminide alloy) of composition Fe–17.24Al–5.54Cr– 1V–0.05C (wt%) by a non-conventional non-furnace process. The process essentially involves the use of cheap, indigenously available titaniferous magnetite ore containing oxides of iron and vanadium and oxide of chromium that are subsequently co-reduced by aluminium in the presence of excess aluminium and a requisite amount of carbon in a specially designed water-cooled copper reactor. The charge composition is judiciously adjusted to attain the alloy composition by utilizing the exothermicity of the overall reactions. The alloy is further characterized with respect to composition, phases and microstructure and properties such as fabricability, hardness, strength and oxidation resistance were studied to indicate its suitability for high temperature applications. 2003 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys; Solid state reactions; Powder metallurgy; Metallography
1. Introduction Intermetallic alloys or compounds display an attractive combination of physical and mechanical properties that has led to their consideration for application in many structural and non-structural areas. Amongst the various intermetallics, iron aluminide (Fe 3 Al) is being considered as a potential candidate for application in the areas of nuclear, aerospace, chemical and thermal power plants as structural components, as heating elements in toasters, oven and furnaces for the following favourable properties. Iron aluminide possesses low density, superior oxidation, sulphidation and corrosion resistance, high strength to weight ratio, high melting point and electrical resistivity, combined with its ability to retain strength and stiffness at elevated temperatures. However, this alloy has not yet come to the stage of commercialization owing to problems of synthesis and poor fabricability [1]. Synthesis of intermetallics is a difficult job for the following reasons. Intermetallics are generally formed over *Corresponding author. Fax: 191-22-2556-0750 / 2550-5151. E-mail address: [email protected] (I.G. Sharma). 0925-8388 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00197-X
a narrow range of composition. Consequently, during the stages of formation, apart from the desired phase, some other unintended phases may also form, resulting in contamination and deterioration in the properties of the parent alloy. Intermetallic alloys are invariably made of alloying constituents having large difference in melting temperature and density leading to segregation of components during formation. Such alloys are apt to have poor room temperature ductility [2]. Efforts were made in various laboratories to prepare these materials by using conventional melting and powder processing approaches. Both these techniques employ pure components for melting. Conventional melting is usually carried out either by arc melting or by induction melting techniques under air, vacuum or controlled atmosphere. However, conventional melting encounters a host of problems related to melting. Firstly, due to large difference in melting temperature and density of iron and aluminium, homogeneous melting is not achieved. Secondly, adding aluminium to molten iron causes a large temperature rise of the molten bath. Such a sudden rise in temperature is dangerous for operators and results in melt oxidation, longer holding time prior to pouring and a potential for
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missing the target chemistry because of oxidation of alloying elements [3]. Under the powder processing route, two techniques such as mechanical alloying and self-propagating high temperature reaction synthesis (SPHTS) were attempted by various researchers to prepare iron aluminide alloys. Mechanical alloying is carried out by attrition ball milling of extremely fine high-purity crystalline powders of the alloying components iron and aluminium, together for a continuous period of 100 h or so. Repeated fracture and re-welding of the powder particles results in the alloy formation. However, handling of the fine powder gives rise to an increase in the level of interstitial impurities in the resultant alloy. Sometimes, the alloys are also found to contain traces of unalloyed components due to an incomplete process of alloying which can damage the mechanical properties of the final alloy. Stringent specification of the starting materials, their high cost and prolonged duration of mechanical alloying restricts the process for commercial exploitation [4]. The SPHTS process works on the principle of alloying fine powders of iron and aluminium by utilizing the heat evolved during their synthesis. However, heat evolution during the synthesis reaction (DH5216 kcal) is inadequate for carrying out any bulk melting. Therefore, the process has so far only been applied for localized melting jobs, e.g., joining of two iron aluminide alloys [5]. A vast reserve of titaniferous magnetite ore (TMO) estimated to be around 3.2 MT is available in the Masanikere region of Karnataka, India. The ore contains a high amount (75.4%) of iron oxide (Fe 2 O 3 ) and good amount of V2 O 5 (1.43%). In spite of the high iron content in the ore, it is scarcely utilized by iron and steel industries for pig iron production due to the presence of high amounts of titania (TiO 2 ) in the ore. Easy availability of titania-free hematite ore is also another cause for the steel producers to prefer hematite in place of TMO. Consequently, the TMO remains mostly unutilized. As the oxide intermediates of iron and vanadium present in TMO are easily amenable to chemical reduction by aluminium with good evolution of heat, a programme, therefore, has been initiated to prepare an iron aluminide (Fe 3 Al)-based alloy of composition Fe–17.24Al–5.54Cr–1V–0.05C (wt%) from TMO with external addition of Cr 2 O 3 by their co-reduction with aluminium in the presence of excess aluminium and the requisite amount of carbon. The alloying addition of chromium in the iron aluminide alloy is intended to improve its ductility by formation of a passive layer of Cr 2 O 3 , while the presence of vanadium is expected to improve its strength by formation of fine precipitates of vanadium carbides. Heat generated from the chemical reduction reactions between oxides and Al is utilized for smelting and consolidation. Reduction smelting was carried out under protected environment of argon in a watercooled copper reactor (WCCR). The reactor was specially designed to conduct the smelting experiments in a closed
environment so that the resultant iron aluminide alloy does not pick up moisture from surrounding air and also to obtain a clean alloy button by avoiding lining material as usually needed thermal insulation. The slag, enriched with a high content of titania (TiO 2 ), can be a potential byproduct for titania production for ultimate use in the paint industry [6]. The as-reduced alloys were further remelted by nonconsumable tungsten arc melting for consolidation and annealed for homogenization. Subsequently the alloy products were characterized with respect to chemical composition, microstructure, homogeneity, phases, etc., and the properties such as fabricability, hardness, strength and oxidation behavior were evaluated to examine its suitability for commercial application.
2. Experimental
2.1. Reactant materials Titaniferous magnetite ore (TMO) supplied by Bhadravati Iron and Steel Company was used as the source for Fe 2 O 3 and V2 O 5 . The as-received ore having particle sizes varying from 30 to 40 mm was first crushed into smaller dimensions (4–5 mm) by employing a Jaw crushing machine. Then the crushed ore was further ground to 21501200 [ mesh size by attrition grinding. The ground ore was subsequently calcined at 600 8C for 6 h before use. Technical-grade chromium oxide of purity 99% (wt%) was used. The oxide was calcined at 700 8C for 6 h before use. Aluminium powder (grade-C) of purity 99% and mesh size 2120[ ASTM supplied by Indian Aluminium (INDALCO) was used as a reducing agent and component for alloying. The reductant has been procured from Indian Aluminium. A calcined petroleum coke powder obtained from Coromondal Carbon India with purity level better than 98% and size 200[ ASTM was used. Laboratory-grade lime received from S.D. Fine Chemicals (Boisar, India), was used. It was calcined at 800 8C prior to use. Chemical analysis of the above oxides, reductant aluminium and carbon is presented in Table 1.
2.2. Equipment and procedure The reactions involved during the preparation of the iron aluminide alloy and subsequent phase transformation of the alloys were studied by differential thermal analysis (DTA) technique. The samples were prepared by mixing TMO powder, Cr 2 O 3 , V2 O 5 , C and Al in their desired ratios and cold pressing the mixture into small pellets weighing around 20 mg. During the operation, the samples
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Table 1 Chemical analysis of charge constituents Materials
Amount (wt%) Oxide
TMO Cr 2 O 3 CaO Al C
Al
75.4 2.48 (Fe 2 O 3 ) 99.38 – 99.0 0.02 99.4 – Ash content |1.0 max.
Ti
Si
Mg
V
Mn
S
P
Cu
Ni
Fe
Ca
7.13
3.04
–
0.8
–
–
–
–
–
–
–
0.07 0.10 –
0.03 0.40 0.15
– – – – – – 0.12 0.06 – Heavy metals |0.005
0.08 – –
0.08 0.08 – – – – Silica1Fe,0.01
0.07 0.01 0.04
0.14 0.02 0.10
– – –
were heated to 1700 8C from room temperature at a heating rate of 108 / min. The calcined reactants such as TMO, Cr 2 O 3 , and CaO were first classified into 21501200[ mesh size using standard Tyler series sieves for subsequent use as the reactant oxides for thermite smelting. Classified materials were then mixed together along with reductant aluminium and carbon in their desired ratio thoroughly in an attrition ball mill keeping the charge to ball ratio at 10:1 for 1 h. Experiments were conducted in a water-cooled copper reactor. A schematic diagram of a water-cooled copper reactor is shown in Fig. 1. The reactor assembly essentially consists of a one end open, double-walled cylindrical vessel made of copper. The top end of the reactor is fitted with a copper flange having a provision for an ‘O’ ring arrangement for isolating the reactor from the surrounding environment. The double wall is provided for circulating the cooling water around the reactor. The reactor used for small-scale experiments has dimensions of 45 mm i.d., 50 mm o.d. and 200 mm length, whereas the reactor used for large-scale experiments has dimensions of 100 mm i.d. and 425 mm length. All the reactors are provided with a slight tapering at the bottom to facilitate the collection of the
Fig. 1. A schematic diagram of water-cooled copper reactor.
reduced alloy button. The reactor is also provided with an alumina tube inserted via a Wilson seal arrangement inside the reactor near the charge to add glycerine to the charge for triggering the reaction. The other end of the tube is connected to a funnel for storing glycerine. After mixing, the charge is then placed inside the reactor cavity and moderately rammed. The reactor is then bolted from top by means of the copper flange. The unit is then tested for leaks, evacuated and back-filled with argon several times and kept under argon flow during the period of smelting. Cooling water supply to the reactor is ensured prior to initiation of the reaction. The thermite reaction is initiated by pouring glycerine over the small amount of KMnO 4 crystals kept over the trigger mixture (KClO 3 1 Al) lying on the top of the reacting charge. The reaction, once initiated, proceeds briskly to completion. An instant rise in the outlet water temperature indicates the commencement of the reaction. The flow rate of the outgoing gas is also found to increase significantly during the course of the reaction. The flow of argon and cooling water are continued until the reaction product attains room temperature. Then, at room temperature, a well-consolidated alloy button collected beneath the slag layer is recovered on breaking the top slag layer. It was then centrally sectioned in the longitudinal and transverse directions into four pieces to physically examine the presence of any defects such as cracks, blow holes, cavities, etc. The as-reduced alloy buttons were further remelted by arc melting under argon atmosphere in a water-cooled copper hearth for consolidation. The remelted alloy buttons were again cut in the longitudinal and transverse directions to physically examine the presence of any cavity or holes. After remelting, the alloy samples were encapsulated in an evacuated quartz tube and then further isothermally heated at 900 8C for 6 h for homogenization. Compositional analysis of the as-reduced and remelted / homogenized alloys was evaluated by standard gravimetric wet analytical techniques and further substantiated by induction-coupled plasma atomic emission (ICP-AES) technique. Carbon was analyzed using a gas analysis technique using a Leco gas analyzer. The optically polished and etched specimens were also used for the EPMA studies. Pure elements such as iron, aluminium, chromium, vanadium and carbon were used as
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the standards. The EPMA was operated at an accelerating voltage of 15 kV with a stabilized beam current of 100 nA. The Ka radiations of Fe, Cr, and V were diffracted by a LiF crystal, whereas a TAP crystal was used to diffract Ka radiation of Al. Identification of different phases present in the as-reduced and remelted / homogenized iron aluminide alloy samples was carried out by X-ray diffraction (XRD) using Cu Ka radiation ( l50.154 nm) with a nickel filter and a secondary beam monochromator. One mg of each alloy sample taken in powder form was scanned from 108 to 1508 at a scanning rate of 28 / min. The measured ‘d’ values with their corresponding intensities were compared with the data reported in the standard ASTM card index for the identification of the desired phases. Iron aluminide alloys obtained from different batches were cut into 5310-mm rectangular pieces and then investigated by standard metallography. Polished samples were etched with an etchant wt% H 2 O, 42 wt% CH 3 COOH, 25 wt% HCl and 25 wt% HNO 3 . Subsequently, the specimens were examined in an optical microscope in magnification range of 380–200. The hardness of as-reduced and remelted / homogenized iron aluminide alloys was measured on optically polished samples in the transverse as well as longitudinal directions at several different places using an Automatic Vickers Microhardness tester. Before choosing a location for hardness measurement, the corresponding microstructure in that location was observed in an auto-microhardness tester. An optically polished sample was used. A load of 1 kg was applied on the sample surface for 15 s duration. Rolling of as-reduced and remelted / homogenized iron aluminide samples was carried out in a two-high rolling mill. Prior to rolling, the samples were cut into 40320-mm rectangular specimens having 10-mm thickness. Two sets of specimens were used for rolling. The first set has been rolled directly, whereas the second set has been jacketed with mild steel prior to rolling. Rolling was carried out under both cold and hot working conditions. For hot rolling, before commencing the rolling operation, the specimens were heated in a resistance heating furnace at 950 8C and soaked for about 20 min duration. During every course of rolling, the specimens were given four to five passes and at each interval of the rolling operation, the specimens were given 10 min soaking at 950 8C. The same steps were repeated until cracks were observed on either surface of the specimen or jacket. After the rolling operation, the thickness of the rolled samples was recorded. The oxidation study on as-reduced and remelted / homogenized iron aluminide alloys was carried out in an oxidizing atmosphere under isothermal as well as nonisothermal heating conditions. For the isothermal study, rectangular specimens with dimensions 203530.5 mm were fabricated from the alloy buttons by an electrodischarge machining technique. The specimens were then
optically polished to remove surface roughness and washed with acetone to clean them of grease and dirt. The specimens were then heated at constant temperatures of 800, 1000 and 1200 8C in a resistance-heating furnace for a time duration varying from 2 to 24 h. The oxidation study under non-isothermal heating condition was carried out by a thermogravimetric analysis (TGA) technique. A thin slice of a specimen weighing 16 mg was heated from room temperature to 1200 8C at a heating rate of 10 8C / min in an oxidizing atmosphere. As-reduced and remelted / homogenized samples were first examined by radiographic tests to ensure that they were free from solidification defects. Then the samples were fabricated into tensile specimens by an electro-discharge machining technique having gauge dimensions 12.533.531.5 mm. The specimen surfaces were further mechanically polished using diamond paste. Tensile strength measurement was carried out in a universal tensile machine (Instron) at RT under argon atmosphere. Strain rates in the order of 0.004–0.006 s 21 were applied to the specimens until fracture occurred.
3. Results and discussion
3.1. Thermodynamic and thermal feasibility of the reactions Aluminothermic reduction smelting for the preparation of present iron aluminide alloy involves a number of chemical reactions. These reactions take place when their 0 0 standard free energy (DG ) and enthalpy values (DH ) are sufficiently negative. Hence, the reactions and their above thermodynamic data are indicated below for the consideration of thermite smelting. Fe 2 O 3 1 2Al 5 2Fe 1 Al 2 O 3 DG 0298 5 2 280 kJ mol 21 , DH 0 5 2 852 kJ mol 21 Cr 2 O 3 1 2Al 5 2Cr 1 Al 2 O 3 DG 0298 5 2 174 kJ mol 21 , DH 0 5 2 534 kJ mol 21
(1)
(2)
3V2 O 5 1 10Al 5 6V 1 5Al 2 O 3 DG 0298 5 2 230.24 kJ mol 21 , DH 0298 5 2 3536 kJ mol 21 (3) The free energy of each of the above reactions is found to be highly negative indicating that the reactions are thermodynamically feasible. The enthalpy value of each of the above reactions shows that the reactions are highly exothermic and the combined specific heat value of all the reactions works out to be 3100 kJ kg 21 . In an aluminothermic reaction, the heat of the reaction is the main guiding force in determining whether the smelting operation will take place or not. In this context, it would be appropriate to mention here that for an aluminothermic reaction to
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endothermic peak at 1540 8C shows the congruent melting and formation of Fe 3 Al alloy and the sixth exotherm in the cooling curve corresponds to the solidification of the same Fe 3 Al alloy. Phase analysis by XRD of the DTA-treated sample further confirmed the presence of the Fe 3 Al alloy phases.
3.2. Studies on thermite smelting experiments
Fig. 2. DTA profile of a mixture of TMO, Cr 2 O 3 , C and Al.
become self-sustaining resulting in efficient slag-metal separation with higher metal yield, the specific heat generated in the process should be in the range of 2500– 4000 kJ kg 21 , as predicted by Hall [7] and Dautzenberg [8]. In the present case, the combined specific heat value of the above reactions is found to be 3100 kJ kg 21 that fits well in the recommended range. Thermal characteristics of the above reactions are also substantiated by differential thermal analysis (DTA) studies, as shown in Fig. 2. The thermal plot shows five peaks in the heating curve and one peak in the cooling curve. The first endotherm at a temperature of 660 8C was for the melting of aluminium. The second exothermic peak at 870 8C corresponds to the reduction reaction of Cr 2 O 3 by Al. Walton and Poulos [9] have reported this temperature to be around 882 8C. The third intense and large exothermic peak at 900 8C corresponds to the reaction between Fe 2 O 3 and Al. Biswas et al. [10] have reported this temperature to be around 900 8C that is close to the present value. The fourth small peak at 1050 8C refers to the reaction between V2 O 5 and Al. Sharma and Kale [11] have reported this temperature to be 1000 8C. The fifth
A number of thermite smelting experiments were conducted on 0.2–1 kg scale by using different charge compositions for the preparation of present iron aluminide alloy. The results are summarized in Table 2. It is observed that addition of excess aluminium over stoichiometric amount in the charge improves the alloy yield considerably (78–93 wt%) as is evident in experiment nos. 1–4. However, more than 10 wt% excess of aluminium in the charge contaminated the alloy product without improving the yield significantly (Expt. no. 4). Carbon in the charge was varied from 50 to 200 wt% for its proper loading in the final alloy. It is, however, observed that 100 wt% excess of carbon in the charge is adequate to maintain the desired level of carbon in the as-reduced alloy. Addition of lime (CaO) was intended to control the thermite reactions by reducing the excess heat output and to lower the viscosity and melting point of the slag for better slag– metal separation. Incorporation of 10–20 g lime in the charge (Expt. nos. 6–8) was found to be sufficient for achieving good slag–metal separation. However, more than 10 g lime in the charge led to poor alloy yield due to additional thermal burden on the charge. In experiment no. 7, the optimum charge composition containing 10 wt% excess aluminium, requisite amount of Cr 2 O 3 , 100 wt% carbon and 10 g CaO in the charge has led to the satisfactory alloy yield of 93 wt% with excellent slag– metal separation and alloy consolidation. A scale-up experiment on 1 kg charge was conducted to substantiate the results obtained in experiment no. 7, maintaining the
Table 2 Results of thermite smelting experiments Run no.
1 2 3 4 5 6 7b 8
Charge composition TMO a (g)
Cr 2 O 3 (g)
Al (g)
X1 (wt%)
X2 (wt%)
X3 (wt%)
Q (kcal kg 21 )
Y (wt%)
144 144 144 144 144 144 144 720
8.15 8.15 8.15 8.15 8.15 8.15 8.15 40.75
58.0 60.0 62.12 64.16 62.12 62.12 62.12 310.60
0 5 10 15 10 10 10 10
0 50 100 200 100 100 100 100
0 0 0 0 10 20 10 10
825 820 818 815 822 753 788 790
78.0 86.0 92.0 93.0 92.8 88.0 93.0 93.5
X1 , excess over stoichiometric amount of aluminium, wt%; X2 , excess over requisite amount of carbon, wt%; X3 , excess over requisite amount of CaO, wt%; Q, specific heat of overall thermite smelting charge, kcal / kg; Y, alloy yield, wt%. a Titaniferous magnetite ore (taken 163 g for Expts. 1–7 and 815 g for Expt. 8). b Charge composition corresponding to optimum experimental condition.
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same experimental conditions and the yield has further improved to 93.5 wt% (Expt. no. 8)
3.3. Compositional analysis of iron aluminide alloys Results of analysis of the as-reduced and remelted / equilibrated alloy samples obtained under optimum experimental condition (Expt. no. 7, Table 3) is presented in Table 3. The remelted / equilibrated alloy composition is found to match nearly the designed alloy composition. A slight increase in the elemental concentration in remelted / equilibrated alloy compared to the as-reduced alloy is due to better consolidation of the alloy and uniform distribution of the alloying components.
3.4. Study of distribution of components in the alloys The EPMA study on iron aluminide alloys reveals that the as-reduced thermite alloys contain imperfections such as solidification cavities, voids, thermally induced cracks and compositional inhomogeneity. These imperfections are more pronounced in the poorly developed alloys; however, the well-consolidated alloys with clean slag–metal separation contain significantly less such imperfections. As per the equilibrium phase diagram of iron and aluminium as shown in Fig. 3, the Fe 3 Al phase should conform to an intermetallic alloy phase containing around 11–20 wt% Al [12]. The desired Fe 3 Al-based alloy has a theoretical composition of Fe–17.24Al–5.54Cr–1V–0.05C (wt%). The EPMA scanning study shows that above composition with minor variation was obtained in only well consolidated and clean as-reduced iron aluminide alloy at some selected areas. The same alloy after remelting and homogenization treatment shows that the above composition with slight variation is maintained throughout the matrix due to better distribution of alloying components and dissolution of cracks and voids on remelting.
Fig. 3. Binary phase diagram of iron and aluminium.
3.5. Identification of phases by X-ray diffraction The diffraction patterns of the as-reduced thermite alloy obtained under optimum experimental condition and the corresponding remelted / homogenized iron aluminide alloy are presented in Figs. 4 and 5, respectively. The broadened intensity peaks in Fig. 4 at 2u angles of 35, 45 and 658 are found to correspond to the Fe 3 Al, CrVC 2 and VC 2 phases. The sharp and well-defined intensity peaks in Fig. 5 at 2u angles of 30, 45 and 658 correspond again to the Fe 3 Al, CrVC 2 and VC 2 phases, respectively. However, phases
Table 3 Compositional analysis of iron a aluminide alloys made from TMO Element
Al Cr V C Mn Ni O Fe a
Theoretical composition (wt%)
Actual composition (wt%) As-reduced
Remelted
17.24 5.54 1.01 0.05 – – – Balance
16.45 5.28 0.94 0.046 0.015 0.015 ,0.02 Balance
16.89 5.34 0.98 0.048 ,0.010 ,0.010 ,0.02 Balance
Corresponding to the optimum experimental condition (Expt. no. 7, Table 2).
Fig. 4. XRD pattern showing the phases present in as-reduced iron aluminide alloy.
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3.6. Microstructural study The optical microstructures of as-reduced thermite and remelted / homogenized alloys are presented in Figs. 6 and 7. The microstructure of as-reduced alloy shows a combination of large and medium grains (size range, 50–100 mm) from hexagonal to irregular shape. The remelted / homogenized alloy shows a considerably refined microstructure having nearly equiaxed grains (30–50 mm) distributed uniformly throughout the matrix and smaller in size than the thermite alloys. Carbides of chromium and vanadium are found to be distributed at random in the matrix of the as-reduced alloy in the form of dark round spots. However, the same is distributed selectively at the grain boundaries in the remelted / homogenized alloy. This can be attributed to the isothermal annealing treatment given to the remelted alloy.
3.7. Hardness measurement of iron aluminide alloys
Fig. 5. XRD pattern showing the phases present in remelted iron aluminide alloy.
such as FeAl, TiAl, Ti 3 Al, Al and silicides of aluminium were not observed in either of the two alloys. In the remelted / homogenized alloy, the reason for the intensity peaks being well-defined and sharp can be attributed to the better formation of the above phases in this alloy due to the consolidation and homogenization treatment given to the iron aluminide alloy.
Results of hardness measurement on iron aluminide alloys are shown in Table 4. The average hardness value for the as-reduced iron aluminide alloy was found to be 396 VHN and for the corresponding remelted / homogenized alloy 300 VHN. The remelted alloy shows comparatively less hardness (300 VHN) than the as-reduced alloy (396 VHN) in view of its better consolidation, refining and homogenization. A large variation in hardness value (370– 420 VHN) was found at different locations of the asreduced sample as compared to the remelted alloy (290– 310 VHN). This again confirms that the remlted alloy is better homogenized than the as-reduced alloy.
Fig. 6. Optical microstructure of as-reduced iron aluminide alloy.
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Fig. 7. Optical microstructure of remelted iron aluminide alloy.
3.8. Evaluation of fabricability of iron aluminide alloys by rolling The fabricability of iron aluminide alloys was evaluated by monitoring their reduction in thickness during rolling. Results of rolling on iron aluminide alloys are indicated in Table 5. It is observed that both as-reduced and remelted / homogenized samples have undergone much better reduction in thickness during hot rolling under jacketed condition than at room temperature under unjacketed condition. Hot rolling has given better rolling ability of the alloys due to better plastic deformation at high temperature, whereas jacketing has provided protection against oxidation and superior heat retaining capacity of the underlying alloys. Rolling under unjacketed condition has Table 4 Results of hardness measurement on iron aluminide alloy Type of alloy
Hardness value (VHN)
As-reduced
370 380 390 405 410 420 396 290 295 300 305 310 300
Average hardness Remelted
Average hardness
resulted in early failure of the alloys by edge cracking at elevated temperature (950 8C) due to faster cooling of the alloys. A maximum 40% thickness reduction for the asreduced alloy and 80% for the remelted / homogenized alloy were achieved under jacketed condition.
3.9. Oxidation study on iron aluminide alloys An evaluation of the oxidation resistance of iron aluminide alloys was carried out under isothermal and non-isothermal heating conditions by monitoring their change in weight with time. Plots showing oxidation pattern of the above alloys are shown in Figs. 8 and 9. It is observed from the plots that the oxidation at the initial stage takes place rapidly for a 5–10-h heating cycle. However, the weight gain is very small, varying from 0.005 to 0.008 mg for the as-reduced alloys (5310 25 –83 10 25 mg / mm 2 ) and (2310 25 25310 25 mg / mm 2 ) for the remelted alloys. This rapid weight gain at the initial stage is due to the formation of passive layer of Al 2 O 3 on the Table 5 Results of rolling study on iron aluminide alloys Type of alloy
Temp. (8C)
Reduction in thickness (wt%)
Unjacketed as-reduced Unjacketed remelted Jacketed as-reduced Jacketed remelted Unjacketed as-reduced Unjacketed remelted Jacketed as-reduced Jacketed remelted
RT RT RT RT 950 950 950 950
10 20 15 30 20 50 40 80
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Table 6 Results of tensile strength measurement on iron aluminide alloys Type of alloy
Strength (MPa)
Elongation (wt%)
As-reduced
550 560 570 580 590 600 600 610 620 630 640 650
20 18 17 16 16 15 35 30 27 25 23 20
Remelted
Fig. 8. Oxidation pattern of as-reduced iron aluminide alloy at various temperatures.
alloy surface. However, during the 10–24 h of heating cycle, the oxidation rate becomes sluggish with hardly any noticeable weight gain. As the layer of the oxide scale increases, further oxidation of the underlying metal proceeds by a process of diffusion of O 2 through the oxide layer of Al 2 O 3 . As a result, the oxidation rate has become sluggish during 10–24 h heating cycle. However, the cumulative weight gain for the entire oxidation study is found to be nominal as described below. The cumulative weight gain for as-reduced alloys at 800 8C is found to vary from 0.005 to 0.008 mg, at 1000 8C from 0.005 to 0.01 mg and at 1200 8C from 0.008 to 0.012 mg, respectively. Correspondingly, the cumulative weight gain for remelted alloys is found to vary at 800 8C from 0.002 to 0.003 mg, at 1000 8C from 0.004 to 0.005 mg and at
1200 8C from 0.006 to 0.008 mg, respectively. Non-isothermal heating of either as-reduced or remelted iron aluminide alloys has not shown any weight gain due to oxidation as no peak corresponding to weight gain was observed in the temperature profile of the TGA study.
3.10. Tensile strength measurement Results of the tensile strength measurements are indicated in Table 6. It is observed during tensile strength measurements that iron aluminide alloys possess high tensile strength at room temperature lying in the range of 550–650 MPa with corresponding elongation in the range of 15–35%. The as-reduced alloys have exhibited comparatively lower strength values (550–600 MPa) and lower elongations (15–20%) than the remelted / homogenized alloys (600–650 MPa) and elongation (20–30%). The remelted / homogenized sample has exhibited better strength and ductility which can be attributed to its having a better homogenized structure with adequate grain refinement and less or no defects due to porosity.
4. Conclusion
Fig. 9. Oxidation pattern of annealed iron aluminide alloy at various temperatures.
The above studies demonstrate the technical feasibility of preparing an iron aluminide alloy of composition Fe– 17.24Al–5.54Cr–1V–0.05C (wt%) from indigenously available cheap titaniferous magnetite ore by a direct aluminothermic co-reduction technique in a specially designed water cooled copper reactor. By optimizing the process parameters such as reductant aluminium, carbon, fluxing agent lime and heat of the reaction, it has been possible to obtain well-consolidated alloys of compositions close to the nominal compositions. The as-reduced alloy after a remelting and homogenization treatment has exhibited a homogenized composition, refined microstructure, excellent fabricability, high strength and hardness with adequate oxidation resistance suitable for high-temperature applications.
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