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Preparation of TZM alloy by aluminothermic smelting and its characterization
Journal of Alloys and Compounds 393 (2005) 122–128
Preparation of TZM alloy by aluminothermic smelting and its characterization I.G. Sharma∗ , S.P. Chakraborty, A.K. Suri Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 8 June 2004; received in revised form 24 September 2004; accepted 29 September 2004 Available online 8 December 2004
Abstract Molybdenum based alloy having a nominal composition Mo–0.5Ti–0.08Zr–0.02C (wt.%) popularly known as TZM alloy is of immense interest amongst scientists worldwide for development in view of its excellent combination of metallurgical properties suitable for high temperature applications. In the present investigation, attempts were made to prepare this alloy by a non-conventional non-furnace process. The process essentially involves the use of cheap, indigenously available oxides of molybdenum, titanium and zirconium for their co-reduction with large excess of aluminium in presence of requisite amount of carbon in a simple experimental set-up. The charge composition was judiciously adjusted to achieve maximum alloy yield by utilizing the heat of the chemical reactions effectively. A typical charge constituting Al 10 wt.% over stoichiometric amount, MoO2 stoichiometric amount, ZrO2 , TiO2 two and five times the requisite amount, respectively and C 50% excess the required amount with an overall specific heat of the charge as 680 kcal kg−1 has resulted in an alloy yield of 92%. The as-reduced alloy after consolidation by remelting has exhibited the composition nearly matching the targeted composition. The alloy was further characterized with respect to composition, phases, microstructure and properties such as hardness, rolling, oxidation and coating to evaluate its suitability for high temperature applications. © 2004 Elsevier B.V. All rights reserved. Keywords: TZM alloy; Aluminothermic reduction; Smelting; Thermite process; Remelting
1. Introduction There is an increasing demand from aerospace, nuclear and electronic industries for materials that can maintain reliability under ever-increasing temperature conditions. Conventional super-alloys have nickel, cobalt or iron–nickel as the major constituents. Most of the nickel and cobalt based super-alloys can be used upto an operating temperature of about 1100 ◦ C, whereas iron–nickel alloys can be used upto a temperature of 800–850 ◦ C [1]. However, hot working environment requiring temperature above 1000 ◦ C with materials having adequate strength and deformation resistance demand for special type of metals or alloys. Steel and nickel alloys used for such jobs though possess adequate strength ∗ Corresponding author. Tel.: +91 22 25595313; fax: +91 22 25560750/25505151. E-mail address: [email protected] (I.G. Sharma).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.055
at RT, however, have poor high temperature strength and low thermal diffusivity. Molybdenum, on account of its excellent properties that meet these requirements, is experiencing an increasing demand. Molybdenum possesses exceptional strength and stiffness at high temperatures, good thermal conductivity, low thermal expansion, high corrosion resistance. An appropriate combination of these properties and characteristics predict increased usage of molybdenum in such applications as rocket nozzles, jet tabs, high temperature dies, electrodes, tools, heat shield and many other high temperature applications. Addition of small quantity of zirconium (0.012–0.06%), titanium (0.4–0.55%) and carbon (0.01–0.04%) to molybdenum renders a fine grained structure and the formation of TiC and ZrC in the grain boundaries of molybdenum inhibit grain growth and the related failure. Such type of alloy system containing Mo as the major constituent with small amount of Ti, Zr and C is known as TZM alloy. A typical TZM alloy of nominal composition
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Mo–0.5Ti–0.08Zr–0.02C (wt.%) is superior to other varieties in terms of strength and stress rupture behaviour [2]. TZM alloy is commercially prepared by either powder metallurgy or vacuum arc melting techniques. However, these methods are expensive in view of using pure components and high vacuum involving multiple steps. Melting operation is also a difficult job in view of large variation in the melting temperatures of various components. As there is no provision for stirring during melting, the alloying components do not mix well invariably leading to their segregation. As a result, final alloy is obtained as inhomogeneous mass. Repeated crushing and melting for a number of times can minimize the segregation problem, however, such attempts are time consuming and can escalate the cost. In the present investigation, an altogether different and alternative method has been employed to prepare TZM alloy of composition Mo–0.5Ti–0.08Zr–0.02C (wt.%). Here cheaper oxide intermediates such as MoO2 , TiO2 and ZrO2 in place of pure elemental components have been used as the starting materials for the preparation of TZM alloy. The alloy was synthesized from these oxides by their co-reduction with reductant aluminium in presence of requisite amount of carbon to make the TZM alloy. Heat of the reactions was judiciously utilized to achieve smelting and consolidation of the alloy product.
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reaction MoO3 + H2 = MoO2 + H2 O The weight and colour change before and after the reduction reaction were recorded. The product obtained after H2 reduction was confirmed as MoO2 by XRD. Titanium oxide (TiO2 ) and zirconium oxide (ZrO2 ): these oxides used in the present investigation were procured from Nuclear Fuel Complex, Hyderabad. The purity level of the oxides are more than 99.9% and the corresponding particle size lies in the range of −150 + 200 #. The as-received oxides were calcined at 750 ◦ C for 4–6 h to drive off moisture and other volatile matters. Reductant aluminium: aluminium powder (grade-C) of size −120 mesh ASTM supplied by M/s Indian Aluminium Company was used as a reductant. Carbon: the calcined petroleum coke powder obtained from M/s Coromondal Carbon, India was used as a source for carbon. It has a purity level better than 98%. The average particle size of the powder is −200 mesh # and its specific surface area is 4 m2 g−1 . Lime (CaO): laboratory grade lime received from M/s S.D.Fine Chemical, Boisar, India was used in the thermite experiments. 2.2. Equipment and procedure
2. Experimental 2.1. Reactant materials Molybdenum di-oxide (MoO2 ): as-received high grade molybdenite (MoS2 ) concentrate powder containing 50–53% molybdenum value was used as a source for MoO2 . This concentrate was obtained as a by-product during the processing of uranium ore by UCIL, Jadugoda, India. The concentrate received as black coloured powder has exhibited particle size of −150 + 200 mesh. As-received molybdenite concentrate (MoS2 ) was roasted at 600 ◦ C for 6–8 h on 500 g scale under dynamic flow of air to convert it into MoO3 as per the following reaction 2MoS2 + 7O2 = 2MoO3 + 4SO2 The reactant charge was kept in a salamander crucible and heated inside a vertical resistance-heating furnace. The change in weight and colour of the concentrate was monitored during and after roasting. The emitted gas was analyzed in situ to know about the sulphur content. The colour of the charge had changed from black to lemon yellow after the roasting. This colour corresponds to the colour of MoO3 . The roasting of the concentrate was repeated a number of times till the sulphur content in the roasted concentrate came down to below 0.7 wt.%. The roasted mass was confirmed to be MoO3 by XRD analysis. The MoO3 was then reduced by H2 at 600 ◦ C for 4–6 h at a flow rate of 500 ml/min as per the following
2.2.1. Thermite smelting and remelting The reactor set-up used for conducting thermite smelting experiments consists of a two end open cylindrical shaped mild steel reactor with length 0.25 m and i.d. 0.1 m. The reactor was internally lined with dead burnt magnesite powder. Lining was carried out using a paste of magnesite powder, binder (Na2 SiO3 ) and water. During lining, a slight tapering was provided at the bottom of the reactor to facilitate deposition of the molten alloy at the bottom. The lined reactor was then air dried for overnight and fired at 200 ◦ C for 3–4 h inside an oven to make it completely moisture free. The reactant mixture composed of oxide intermediates such as MoO2 , TiO2 and ZrO2 , reductant Al, requisite amount of C and flux CaO were thoroughly mixed and poured into the reactor cavity. The reactant mixture was then rammed mildly to improve particle-to-particle contact. A small amount of trigger mixture (containing potassium chlorate and aluminium taken in the ratio of 2:1 by weight) was kept over the charge at the center. The reaction was finally initiated by bringing a freshly pickled burning magnesium ribbon in contact with the trigger mixture. The reaction was quite vigorous, however well controlled. It went into completion very rapidly within a minute. The alloy button formed at the bottom of the charge was collected by breaking the top slag layer at an ambient temperature. As-reduced alloy samples which were obtained with reasonably good consolidation and separation from slag were further remelted in an arc melting furnace under an argon atmosphere using non-consumable tungsten electrode and a
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water cooled copper hearth. Altogether three meltings were carried out lasting for a duration of 8–10 min. The remelted samples were further encapsulated in a silica tube under vacuum and annealed at 1000 ◦ C for 6 h. 2.2.2. Characterization studies The products obtained from thermite smelting and arc melting were analyzed for components such as molybdenum, titanium, zirconium and aluminium present in the alloys using a combination of X-ray fluorescence (XRF), standard gravimetric and volumetric chemical analysis techniques. XRF technique was carried out using Si (Li) detector of area 30 mm × 3 mm having an energy resolution of 170 eV for 5.9 keV and employing an Am241 radioisotope as the source. Specimens for XRF analysis was prepared by making a cold pressed pellet of 3 mm diameter from the ground and sieved product samples using cellulose as the binder. Carbon in the alloys was analyzed by using a carbon analyzer. X-ray diffraction (XRD) was used for identifying and confirming various reactant and product phases using Cu K␣ radiation (wavelength = 0.154 nm) with a nickel filter and a secondary beam monochromator. Samples in the form of fine powder (20–25 m) were scanned from 10◦ to 150◦ with a scanning rate of 2◦ min−1 . The measured ‘d’ values with their corresponding intensities were compared with the data reported in the standard ASTM index card for identification of desired phases. Optical study of the microstructure of the TZM alloys was carried out using 5 mm × 10 mm rectangular samples mounted on quick setting epoxy resin. The mounted samples were polished with 60–600 grit papers followed by diamond polishing to ensure complete removal of all scratches. The polished samples were then etched with a chemical mixture containing lactic acid, nitric acid and hydrofluoric acid taken in the ratio of 6:2:1 by volume. The samples were then observed in an optical microscope under magnifications ranging from 80 to 200×. The hardness of TZM alloys was measured in an automatic micro hardness tester machine on the optically polished samples at five different locations in the transverse as well as longitudinal directions using a load of 0.5 kg for a duration of 10 s. The as-reduced and remelted TZM alloys were rolled in a two high rolling mill. Prior to rolling, the samples were cut into 15 mm × 10 mm × 5 mm rectangular specimens. These specimens were optically polished to have scratch free even surfaces with opposite surfaces parallel to each other. The initial thickness of the specimens was recorded prior to rolling. The specimens were subsequently jacketed with mild steel under vacuum and hot rolled at 950 ◦ C giving 10 min intermediate soaking. The specimens were repeatedly rolled until edge cracks appeared on the sample surface. After opening the mild steel jacket, final thickness of the specimens was recorded once again after rolling. The thermal stability of the TZM alloys was evaluated by heating them under isothermal heating condition in air,
oxygen and vacuum environments. Isothermal heating was carried out in a resistance-heating furnace by heating rectangular specimen of dimension 15 mm × 10 mm × 5 mm at different temperatures ranging from 200 to 1000 ◦ C for a duration of 6 h. The change in weight and the thickness of the specimen were recorded before and after the experiment. The remelted rectangular specimens of TZM alloy having dimension 15 mm × 10 mm × 5 mm were given a 50–70 m composite coating of Al and Si by pack cementation technique. Prior to applying the coating, the sharp edges of the specimen were smoothened.
3. Results and discussion 3.1. Thermodynamic and thermal feasibility of the reactions The reactions involved during the thermite smelting and their corresponding free energy (G◦ ) as well as heat (H◦ ) changes are indicated below for their considerations in the present thermite smelting campaign 3MoO2 + 4Al = 3Mo + 2Al2 O3 , G◦298 K = −72 kcal mol−1 , ◦ −1 H298 K = −220 kcal mol
(i)
3TiO2 + 4Al = 3Ti + 2Al2 O3 , G◦298 K = −18 kcal mol−1 , ◦ −1 H298 K = −124 kcal mol
(ii)
3ZrO2 + 4Al = 3Zr + 2Al2 O3 , G◦298 K = −3.5 kcal mol−1 , ◦ −1 H298 K = −15 kcal mol
(iii)
From thermodynamic point of view, combination of a fairly large amount of negative free energy and enthalpy changes (G◦ and H◦ ) are desirable for a chemical reaction to proceed autogeneously. It is evident from the above thermodynamic data that the reaction between MoO2 and Al (i) is reasonably favourable whereas the other two reactions of TiO2 and Al (ii), ZrO2 and Al (iii) are comparatively less favourable. In addition, two stage mechanisms of the aluminothermic reduction of TiO2 also reflect difficulties in attaining complete reduction, as discussed below 3TiO2 + 2Al = 3TiO + Al2 O3
(iv)
3TiO + 2Al = 3Ti + Al2 O3
(v)
The reaction (iv) proceeds easily due to favourable free energy change, however, reaction (v) is not so favourable. TiO
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being a strong base easily combines with Al2 O3 and an appreciable amount of titanium gets lost in the slag. In order to counteract this tendency and improve the extraction of titanium, lime(CaO) is added to the charge. Lime being a stronger base than TiO, combines preferentially with alumina and thereby facilitates reduction. Excess TiO2 is also added to compensate for the loss of any TiO2 in the slag. Unlike the reduction of TiO2 , aluminothermic reduction of ZrO2 does not involve formation of any lower oxides of zirconium. Formation of carbides of titanium (TiC, G◦ = −21.6 kcal mol−1 ) and zirconium (ZrC, G◦ = −23 kcal mol−1 ) are thermodynamically more favourable than their corresponding aluminides (G◦ of TiAl and Ti3 Al; 6.6–10.75 kcal mol−1 , G◦ of ZrAl and Zr3 Al; 10–13 kcal mol−1 ). The negative value of enthalpy shows that the above reactions are exothermic in nature and the combined enthalpy value of all the reactions work out to be 3000 kJ kg−1 . In the context of aluminothermic smelting, it would be appropriate to mention here that for a chemical reaction involving aluminothermic reduction of oxide to become autogeneous and for efficient slag-metal separation with higher metal yield, the heat generated in the process should be in the range of 2500–4500 kJ kg−1 as predicted by Dautzenberg [3] and Hall [4]. Based on the thermal calculation on the above reactions, it is observed that the overall heat value (3000 kJ kg−1 ) fits well in the recommended range. Therefore, the reactions are expected to be autogeneous for successful completion of thermite smelting.
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Fig. 1. Phase diagram of the CaO–AlO system.
it in an effective manner, the next experiment was conducted by placing 30% of the total charge at the bottom of the reactor for initiation of the reaction and rest of the charge was gradually added during the course of the reaction in a controlled manner. This mode of charging has helped the reaction to become less violent thereby aiding the reaction to proceed smoothly into completion. As a result, the alloy yield has improved to 60%. However, slag-metal separation still has remained poor as evident from the alloy buttons obtained in small globules form trapped in the slag layers. Therefore, in the third experiment and onwards, lime (CaO) was added to the thermite charge to improve the slag-metal separation by reducing the viscosity and melting point of the slag. This has resulted in marked improvement in the alloy yield from 60 to 75% (Run no. 3). In the present thermite smelting campaign, Al2 O3 is the predominantly main slag phase. It forms a number of compounds with lime at different compositions and under various temperatures as is evident from the phase diagram of Al2 O3 versus CaO, indicated in Fig. 1. As per the phase diagram, a slag phase corresponding to the composition 2CaO·Al2 O3 has the lowest melting temperature of 1400◦ C [5]. As a large addition of CaO increases the thermal burden on the charge, attempts were made to attain slag of melting
3.2. Studies on thermite smelting experiments A number of thermite smelting experiments were conducted on a 200 g scale by using different charge compositions for the preparation of TZM alloys. The results of the study are summarized in Table 1. The first experiment (Run no. 1) was conducted with a stoichiometric amount of aluminium. The reaction was found to be highly violent with poor slag-metal separation and only 40% alloy yield was achieved with this composition. The reason for poor yield can be attributed to the sputtering loss of the alloy due to generation of intense heat (3511 kJ kg−1 ) during the course of the reaction. In order to control the excess heat and utilize Table 1 Results of thermite smelting experiments Run no.
1 2 3 4 5 6 7 8 9
Charge composition (g)
X (wt.%)
MoO2
TiO2
ZrO2
132 132 132 132 132 132 132 132 660
0.42 0.42 0.42 0.42 0.42 0.42 0.84 1.68 4.20
0.06 0.06 0.06 0.06 0.18 0.24 0.36 0.36 1.50
CaO 0.0 0.0 14 14 14 14 14 14 70
C
Al
2.0 2.0 2.0 2.0 2.0 3.0 4.0 4.0 15.0
48 48 48 50.4 52.8 55.2 53.28 53.75 266.40
X: excess aluminium taken over stoichiometric amount, Q: specific heat of the charge, Y: alloy yield.
0.0 0.0 0.0 5.0 10 15 10 10 10
Q (kJ kg−1 )
Y (wt.%)
3511 3511 3043 2968 2893 2851 2843 2822 2842
40 60 75 85 90 91 90 89 92
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temperature much lower than that of Al2 O3 . In the successive three experiments (Run nos. 4–6) excess aluminium in the range of 5–15% over the stoichiometric amount was added to improve the alloy yield further. Consequently, the yield figure has gone up significantly giving a maximum yield of 91%. However, it was observed that more than 10% excess addition of aluminium in the charge has not contributed to any noticeable change in the alloy yield (from 90 to 91%) rather has contaminated the alloy product with further loading of aluminium. Composition analysis of the alloys obtained from all the above experiments (Run nos. 1–2) has shown poor loading of Ti, Zr and C. Although loading of Ti in the alloys improved significantly from Run no. 3 onwards due to addition of lime, however, the loading of other two components remained unaltered. This may be due to stoichiometric addition of their oxides and the requisite amount of carbon in the charge. Hence in the subsequent experiments (nos. 7–9) the addition of TiO2 , ZrO2 and C were varied from 50 to 500% excess over requisite amount in the thermite charge to improve the loading of the above elements in the resultant TZM alloy. After conducting several experiments, it was observed that the charge composition corresponding to the experiment no. 7 was found to be optimum. Based on the findings of this experiment, a scale-up operation (Run no. 9) was conducted on increasing the batch size by five-fold to substantiate the results. Alloy yield has further improved to 92% due to better heat output. TZM alloys made by the thermite process were cut into four pieces in the longitudinal and transverse directions by a corundum based abrasive cutter for examining defects such as blow holes, cracks, cavities, etc. Alloys with poor slagmetal separation were found to have traces of slag on the surface and presence of cavities and thermally induced cracks at the interior. On the contrary, alloys with good consolidation and good slag-metal separation were devoid of any such defects. In order to ensure complete elimination of defects with adequate consolidation and homogenization, the as-reduced alloys were further melted in a non-consumable arc melting furnace under an argon atmosphere. The arc melted samples were found to have excellent consolidation. During melting, alloy loss was found to be negligible and not more than 2% loss was registered. As a matter of interest, attempts were also made to prepare the TZM alloy by component melting using a direct arc melting technique under argon atmosphere. However, due to significant difference in the melting temperature and a large variation in the quantity of the constituent elements, the alloy button was obtained with poor uniformity in the composition. Therefore, in order to ensure adequate homogeneity of the alloy matrix, a number of melting were carried out resulting in large cumulative alloy loss (40–50%) by evaporation. It is therefore, evident that preparation of TZM alloy by a combination of aluminothermic reduction of mixed oxides of molybdenum, titanium, zirconium in presence of carbon is better suited than direct melting from pure components.
Table 2 Chemical analysis of TZM alloy Element
Target composition (wt.%)
Thermite alloy (wt.%)
Remelted alloy (wt.%)
Ti Zr C Mo
0.50 0.08 0.20 Balance
0.46 0.06 0.17 Balance
0.48 0.07 0.18 Balance
3.3. Compositional analysis of TZM alloy Qualitative analysis by XRF has confirmed the presence of Mo, Ti and Zr in the TZM alloy. Quantitative estimation was carried out by the combination of XRF and chemical analysis and the results of analysis of the as-reduced and remelted alloys obtained under optimum experimental condition is presented in Table 2. The remelted alloy composition is found to match nearly the designated alloy composition due to better consolidation and homogenization. 3.4. Identification of phases by X-ray diffraction Phase identification of TZM alloy was carried out by Xray diffraction technique. The diffraction pattern of the asreduced cum remelted alloy obtained under optimum experimental condition is indicated in Fig. 2. The plot exhibited three well defined and sharp peaks altogether. Amongst these, the tallest peak was confirmed to be of elemental molybdenum, the other two comparatively shorter peaks resembled carbides of titanium and zirconium. However, no such peak for elemental titanium, zirconium and aluminium nor a peak corresponding to their compounds was observed. 3.5. Microstructural study The optical microstructures of as-reduced and remelted TZM alloys are presented in Figs. 3 and 4. The microstructure
Fig. 2. XRD pattern of TZM alloy.
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127
Table 3 Results of hardness measurement on TZM alloy Type of alloy
Hardness value (VHN)
As-reduced
290 285 305 300 320 300 270 275 280 285 290 280
Average hardness Remelted
Average hardness
Fig. 3. Optical micrograph of as-reduced TZM alloy.
corresponding to the as-reduced alloy exhibits the presence of combination of large hexagonal to round shaped grains in the size range of 75–100 m. Dark small and round globules of TiC and ZrC were found to be distributed at random in the matrix. The microstructure of the remelted alloy revealed presence of comparatively smaller grains mostly round in shape in the size range of 20–25 m with well defined grain boundaries. Fine particles of titanium and zirconium carbides were found to be uniformly distributed all over the matrix. 3.6. Hardness measurement of TZM alloys Results of hardness measurement on TZM alloy are shown in Table 3. Average hardness value was found to be 300 VHN for as-reduced TZM alloy and 280 VHN for the remelted alloy. A large variation in hardness value (300–350 VHN) was observed at different locations of as-reduced alloy samples as compared to remelted alloy (270–290 VHN). Decrease in hardness value and comparatively lower variation in its values at different locations for the remelted alloy can be attributed to its better consolidation, refining and homogenization during remelting than the as-reduced alloys.
3.7. Evaluation of fabrication behaviour of TZM alloy by rolling Fabrication of TZM alloy into sheet from remelted alloy button was evaluated by measuring the percentage reduction in thickness during rolling as per the following relations: Eh =
Sf − Si × 100 Sf
where Eh is the % thickness reduction of the specimen, Sf and Si are the final and initial thickness of the specimen. The results of rolling study on TZM alloy are indicated in Table 4. It is observed that TZM alloy exhibits much better fabricability at elevated temperature (950 ◦ C) during rolling under the jacketed condition than at room temperature. Hot rolling has given better rolling ability of the TZM alloys due to better plastic deformation at high temperature. Rolling of the as-reduced alloy has resulted in early failure of the alloy by edge cracking due to inhomogeneous structure, however, the remelted alloy had shown excellent rolling characteristics at high temperature (950 ◦ C) due to better consolidation and homogeneity. A maximum 40% reduction in thickness was achieved for as-reduced alloy and 90% for remelted alloy at 950 ◦ C under jacketed condition. 3.8. Study on thermal stability of TZM alloy Thermal plots showing the oxidation pattern of TZM alloy under the atmospheres of air and oxygen are shown in Fig. 5. The plots exhibit gradual and slow weight gain up to 400 ◦ C due to the formation of dark brown MoO2 at a sluggish rate
Table 4 Results of fabrication study by rolling on TZM alloy
Fig. 4. Optical micrograph of remelted TZM alloy.
Type of alloy
Temperature (◦ C)
Reduction in thickness (%)
As-reduced Remelted As-reduced Remelted
RT RT 950 950
20 60 40 90
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4. Conclusion
Fig. 5. Oxidation pattern of TZM alloy.
on the specimen surface. At 400 ◦ C and onwards, there was a sharp increase in the weight gain till 750 ◦ C due to the formation of greenish yellow MoO3 layers at a rapid rate on the specimen surface. The cumulative weight gain was 70–80% which can be attributed to the formation of above oxide layers on the specimen surface. However, at 750 ◦ C onwards, there was a sharp decrease in the weight due to the volatilization of MoO3 . The cumulative weight loss was recorded to be 40–50%. It is observed that the cumulative effect on weight change was much higher under the atmosphere of oxygen than in air. However, there was neither any weight gain nor any weight loss when the TZM specimen was heated under vacuum or argon. An optimum composite coating of Al and Si in the range of 20–30 m on TZM specimens showed excellent resistance to oxidation. They did not show any weight change due to oxidation either in air or oxygen for the same duration of time (6 h). The coating remained consistently intact during and after heating.
The present investigation demonstrates the technical feasibility of preparation of TZM alloy of average composition Mo–0.48Ti–0.07Zr–0.18C nearly matching the nominal composition Mo–0.5Ti–0.08Zr–0.02C (wt.%) by direct aluminothermic co-reduction of mixed oxides of molybdenum, titanium and zirconium in the presence of excess aluminium and carbon. The alloy was made in a single step by judiciously utilizing the heat of the reactions and experimental parameters. A maximum alloy yield of 92% was achieved. The as-reduced alloys are found to have well-developed grains which became finer and round in shape on further remelting. The alloy showed excellent fabricability by rolling and has reasonably high hardness. The poor oxidation resistance of the alloy was surmounted at high temperature by a composite coating of Al and Si on the alloy surface. Acknowledgement The authors are grateful to Dr. S. Banerjee, Director, Materials Group, Bhabha Atomic Research Centre for his consistent support and encouragement during the course of above campaign.
References [1] C.K. Gupta, A.K. Suri, Extractive Metallurgy of Niobium, CRC Press, Boca Raton, FL, 1994, pp. 33–34. [2] Rembar Molybdenum Technical Information, The Rembar Company, Inc., NY, USA. [3] W. Dautzenberg, Ulmans Encyklopadie der Technischen Chemie, 4th ed., Verlag Chemie, Weinheim, 1974, pp. 351–361. [4] I.H. Hall, Ulmans Encyclopedia of Industrial Chemistry, vol. Al, 5th ed., Verlag Chemie, Weinheim, 1985, pp. 447–452. [5] S. Fillipov, The Theory of Metallurgical Processes, Mir Publishers, Moscow, 1975, p. 211.
Preparation of TZM alloy by aluminothermic smelting and its characterization I.G. Sharma∗ , S.P. Chakraborty, A.K. Suri Materials Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 8 June 2004; received in revised form 24 September 2004; accepted 29 September 2004 Available online 8 December 2004
Abstract Molybdenum based alloy having a nominal composition Mo–0.5Ti–0.08Zr–0.02C (wt.%) popularly known as TZM alloy is of immense interest amongst scientists worldwide for development in view of its excellent combination of metallurgical properties suitable for high temperature applications. In the present investigation, attempts were made to prepare this alloy by a non-conventional non-furnace process. The process essentially involves the use of cheap, indigenously available oxides of molybdenum, titanium and zirconium for their co-reduction with large excess of aluminium in presence of requisite amount of carbon in a simple experimental set-up. The charge composition was judiciously adjusted to achieve maximum alloy yield by utilizing the heat of the chemical reactions effectively. A typical charge constituting Al 10 wt.% over stoichiometric amount, MoO2 stoichiometric amount, ZrO2 , TiO2 two and five times the requisite amount, respectively and C 50% excess the required amount with an overall specific heat of the charge as 680 kcal kg−1 has resulted in an alloy yield of 92%. The as-reduced alloy after consolidation by remelting has exhibited the composition nearly matching the targeted composition. The alloy was further characterized with respect to composition, phases, microstructure and properties such as hardness, rolling, oxidation and coating to evaluate its suitability for high temperature applications. © 2004 Elsevier B.V. All rights reserved. Keywords: TZM alloy; Aluminothermic reduction; Smelting; Thermite process; Remelting
1. Introduction There is an increasing demand from aerospace, nuclear and electronic industries for materials that can maintain reliability under ever-increasing temperature conditions. Conventional super-alloys have nickel, cobalt or iron–nickel as the major constituents. Most of the nickel and cobalt based super-alloys can be used upto an operating temperature of about 1100 ◦ C, whereas iron–nickel alloys can be used upto a temperature of 800–850 ◦ C [1]. However, hot working environment requiring temperature above 1000 ◦ C with materials having adequate strength and deformation resistance demand for special type of metals or alloys. Steel and nickel alloys used for such jobs though possess adequate strength ∗ Corresponding author. Tel.: +91 22 25595313; fax: +91 22 25560750/25505151. E-mail address: [email protected] (I.G. Sharma).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.055
at RT, however, have poor high temperature strength and low thermal diffusivity. Molybdenum, on account of its excellent properties that meet these requirements, is experiencing an increasing demand. Molybdenum possesses exceptional strength and stiffness at high temperatures, good thermal conductivity, low thermal expansion, high corrosion resistance. An appropriate combination of these properties and characteristics predict increased usage of molybdenum in such applications as rocket nozzles, jet tabs, high temperature dies, electrodes, tools, heat shield and many other high temperature applications. Addition of small quantity of zirconium (0.012–0.06%), titanium (0.4–0.55%) and carbon (0.01–0.04%) to molybdenum renders a fine grained structure and the formation of TiC and ZrC in the grain boundaries of molybdenum inhibit grain growth and the related failure. Such type of alloy system containing Mo as the major constituent with small amount of Ti, Zr and C is known as TZM alloy. A typical TZM alloy of nominal composition
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Mo–0.5Ti–0.08Zr–0.02C (wt.%) is superior to other varieties in terms of strength and stress rupture behaviour [2]. TZM alloy is commercially prepared by either powder metallurgy or vacuum arc melting techniques. However, these methods are expensive in view of using pure components and high vacuum involving multiple steps. Melting operation is also a difficult job in view of large variation in the melting temperatures of various components. As there is no provision for stirring during melting, the alloying components do not mix well invariably leading to their segregation. As a result, final alloy is obtained as inhomogeneous mass. Repeated crushing and melting for a number of times can minimize the segregation problem, however, such attempts are time consuming and can escalate the cost. In the present investigation, an altogether different and alternative method has been employed to prepare TZM alloy of composition Mo–0.5Ti–0.08Zr–0.02C (wt.%). Here cheaper oxide intermediates such as MoO2 , TiO2 and ZrO2 in place of pure elemental components have been used as the starting materials for the preparation of TZM alloy. The alloy was synthesized from these oxides by their co-reduction with reductant aluminium in presence of requisite amount of carbon to make the TZM alloy. Heat of the reactions was judiciously utilized to achieve smelting and consolidation of the alloy product.
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reaction MoO3 + H2 = MoO2 + H2 O The weight and colour change before and after the reduction reaction were recorded. The product obtained after H2 reduction was confirmed as MoO2 by XRD. Titanium oxide (TiO2 ) and zirconium oxide (ZrO2 ): these oxides used in the present investigation were procured from Nuclear Fuel Complex, Hyderabad. The purity level of the oxides are more than 99.9% and the corresponding particle size lies in the range of −150 + 200 #. The as-received oxides were calcined at 750 ◦ C for 4–6 h to drive off moisture and other volatile matters. Reductant aluminium: aluminium powder (grade-C) of size −120 mesh ASTM supplied by M/s Indian Aluminium Company was used as a reductant. Carbon: the calcined petroleum coke powder obtained from M/s Coromondal Carbon, India was used as a source for carbon. It has a purity level better than 98%. The average particle size of the powder is −200 mesh # and its specific surface area is 4 m2 g−1 . Lime (CaO): laboratory grade lime received from M/s S.D.Fine Chemical, Boisar, India was used in the thermite experiments. 2.2. Equipment and procedure
2. Experimental 2.1. Reactant materials Molybdenum di-oxide (MoO2 ): as-received high grade molybdenite (MoS2 ) concentrate powder containing 50–53% molybdenum value was used as a source for MoO2 . This concentrate was obtained as a by-product during the processing of uranium ore by UCIL, Jadugoda, India. The concentrate received as black coloured powder has exhibited particle size of −150 + 200 mesh. As-received molybdenite concentrate (MoS2 ) was roasted at 600 ◦ C for 6–8 h on 500 g scale under dynamic flow of air to convert it into MoO3 as per the following reaction 2MoS2 + 7O2 = 2MoO3 + 4SO2 The reactant charge was kept in a salamander crucible and heated inside a vertical resistance-heating furnace. The change in weight and colour of the concentrate was monitored during and after roasting. The emitted gas was analyzed in situ to know about the sulphur content. The colour of the charge had changed from black to lemon yellow after the roasting. This colour corresponds to the colour of MoO3 . The roasting of the concentrate was repeated a number of times till the sulphur content in the roasted concentrate came down to below 0.7 wt.%. The roasted mass was confirmed to be MoO3 by XRD analysis. The MoO3 was then reduced by H2 at 600 ◦ C for 4–6 h at a flow rate of 500 ml/min as per the following
2.2.1. Thermite smelting and remelting The reactor set-up used for conducting thermite smelting experiments consists of a two end open cylindrical shaped mild steel reactor with length 0.25 m and i.d. 0.1 m. The reactor was internally lined with dead burnt magnesite powder. Lining was carried out using a paste of magnesite powder, binder (Na2 SiO3 ) and water. During lining, a slight tapering was provided at the bottom of the reactor to facilitate deposition of the molten alloy at the bottom. The lined reactor was then air dried for overnight and fired at 200 ◦ C for 3–4 h inside an oven to make it completely moisture free. The reactant mixture composed of oxide intermediates such as MoO2 , TiO2 and ZrO2 , reductant Al, requisite amount of C and flux CaO were thoroughly mixed and poured into the reactor cavity. The reactant mixture was then rammed mildly to improve particle-to-particle contact. A small amount of trigger mixture (containing potassium chlorate and aluminium taken in the ratio of 2:1 by weight) was kept over the charge at the center. The reaction was finally initiated by bringing a freshly pickled burning magnesium ribbon in contact with the trigger mixture. The reaction was quite vigorous, however well controlled. It went into completion very rapidly within a minute. The alloy button formed at the bottom of the charge was collected by breaking the top slag layer at an ambient temperature. As-reduced alloy samples which were obtained with reasonably good consolidation and separation from slag were further remelted in an arc melting furnace under an argon atmosphere using non-consumable tungsten electrode and a
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water cooled copper hearth. Altogether three meltings were carried out lasting for a duration of 8–10 min. The remelted samples were further encapsulated in a silica tube under vacuum and annealed at 1000 ◦ C for 6 h. 2.2.2. Characterization studies The products obtained from thermite smelting and arc melting were analyzed for components such as molybdenum, titanium, zirconium and aluminium present in the alloys using a combination of X-ray fluorescence (XRF), standard gravimetric and volumetric chemical analysis techniques. XRF technique was carried out using Si (Li) detector of area 30 mm × 3 mm having an energy resolution of 170 eV for 5.9 keV and employing an Am241 radioisotope as the source. Specimens for XRF analysis was prepared by making a cold pressed pellet of 3 mm diameter from the ground and sieved product samples using cellulose as the binder. Carbon in the alloys was analyzed by using a carbon analyzer. X-ray diffraction (XRD) was used for identifying and confirming various reactant and product phases using Cu K␣ radiation (wavelength = 0.154 nm) with a nickel filter and a secondary beam monochromator. Samples in the form of fine powder (20–25 m) were scanned from 10◦ to 150◦ with a scanning rate of 2◦ min−1 . The measured ‘d’ values with their corresponding intensities were compared with the data reported in the standard ASTM index card for identification of desired phases. Optical study of the microstructure of the TZM alloys was carried out using 5 mm × 10 mm rectangular samples mounted on quick setting epoxy resin. The mounted samples were polished with 60–600 grit papers followed by diamond polishing to ensure complete removal of all scratches. The polished samples were then etched with a chemical mixture containing lactic acid, nitric acid and hydrofluoric acid taken in the ratio of 6:2:1 by volume. The samples were then observed in an optical microscope under magnifications ranging from 80 to 200×. The hardness of TZM alloys was measured in an automatic micro hardness tester machine on the optically polished samples at five different locations in the transverse as well as longitudinal directions using a load of 0.5 kg for a duration of 10 s. The as-reduced and remelted TZM alloys were rolled in a two high rolling mill. Prior to rolling, the samples were cut into 15 mm × 10 mm × 5 mm rectangular specimens. These specimens were optically polished to have scratch free even surfaces with opposite surfaces parallel to each other. The initial thickness of the specimens was recorded prior to rolling. The specimens were subsequently jacketed with mild steel under vacuum and hot rolled at 950 ◦ C giving 10 min intermediate soaking. The specimens were repeatedly rolled until edge cracks appeared on the sample surface. After opening the mild steel jacket, final thickness of the specimens was recorded once again after rolling. The thermal stability of the TZM alloys was evaluated by heating them under isothermal heating condition in air,
oxygen and vacuum environments. Isothermal heating was carried out in a resistance-heating furnace by heating rectangular specimen of dimension 15 mm × 10 mm × 5 mm at different temperatures ranging from 200 to 1000 ◦ C for a duration of 6 h. The change in weight and the thickness of the specimen were recorded before and after the experiment. The remelted rectangular specimens of TZM alloy having dimension 15 mm × 10 mm × 5 mm were given a 50–70 m composite coating of Al and Si by pack cementation technique. Prior to applying the coating, the sharp edges of the specimen were smoothened.
3. Results and discussion 3.1. Thermodynamic and thermal feasibility of the reactions The reactions involved during the thermite smelting and their corresponding free energy (G◦ ) as well as heat (H◦ ) changes are indicated below for their considerations in the present thermite smelting campaign 3MoO2 + 4Al = 3Mo + 2Al2 O3 , G◦298 K = −72 kcal mol−1 , ◦ −1 H298 K = −220 kcal mol
(i)
3TiO2 + 4Al = 3Ti + 2Al2 O3 , G◦298 K = −18 kcal mol−1 , ◦ −1 H298 K = −124 kcal mol
(ii)
3ZrO2 + 4Al = 3Zr + 2Al2 O3 , G◦298 K = −3.5 kcal mol−1 , ◦ −1 H298 K = −15 kcal mol
(iii)
From thermodynamic point of view, combination of a fairly large amount of negative free energy and enthalpy changes (G◦ and H◦ ) are desirable for a chemical reaction to proceed autogeneously. It is evident from the above thermodynamic data that the reaction between MoO2 and Al (i) is reasonably favourable whereas the other two reactions of TiO2 and Al (ii), ZrO2 and Al (iii) are comparatively less favourable. In addition, two stage mechanisms of the aluminothermic reduction of TiO2 also reflect difficulties in attaining complete reduction, as discussed below 3TiO2 + 2Al = 3TiO + Al2 O3
(iv)
3TiO + 2Al = 3Ti + Al2 O3
(v)
The reaction (iv) proceeds easily due to favourable free energy change, however, reaction (v) is not so favourable. TiO
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being a strong base easily combines with Al2 O3 and an appreciable amount of titanium gets lost in the slag. In order to counteract this tendency and improve the extraction of titanium, lime(CaO) is added to the charge. Lime being a stronger base than TiO, combines preferentially with alumina and thereby facilitates reduction. Excess TiO2 is also added to compensate for the loss of any TiO2 in the slag. Unlike the reduction of TiO2 , aluminothermic reduction of ZrO2 does not involve formation of any lower oxides of zirconium. Formation of carbides of titanium (TiC, G◦ = −21.6 kcal mol−1 ) and zirconium (ZrC, G◦ = −23 kcal mol−1 ) are thermodynamically more favourable than their corresponding aluminides (G◦ of TiAl and Ti3 Al; 6.6–10.75 kcal mol−1 , G◦ of ZrAl and Zr3 Al; 10–13 kcal mol−1 ). The negative value of enthalpy shows that the above reactions are exothermic in nature and the combined enthalpy value of all the reactions work out to be 3000 kJ kg−1 . In the context of aluminothermic smelting, it would be appropriate to mention here that for a chemical reaction involving aluminothermic reduction of oxide to become autogeneous and for efficient slag-metal separation with higher metal yield, the heat generated in the process should be in the range of 2500–4500 kJ kg−1 as predicted by Dautzenberg [3] and Hall [4]. Based on the thermal calculation on the above reactions, it is observed that the overall heat value (3000 kJ kg−1 ) fits well in the recommended range. Therefore, the reactions are expected to be autogeneous for successful completion of thermite smelting.
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Fig. 1. Phase diagram of the CaO–AlO system.
it in an effective manner, the next experiment was conducted by placing 30% of the total charge at the bottom of the reactor for initiation of the reaction and rest of the charge was gradually added during the course of the reaction in a controlled manner. This mode of charging has helped the reaction to become less violent thereby aiding the reaction to proceed smoothly into completion. As a result, the alloy yield has improved to 60%. However, slag-metal separation still has remained poor as evident from the alloy buttons obtained in small globules form trapped in the slag layers. Therefore, in the third experiment and onwards, lime (CaO) was added to the thermite charge to improve the slag-metal separation by reducing the viscosity and melting point of the slag. This has resulted in marked improvement in the alloy yield from 60 to 75% (Run no. 3). In the present thermite smelting campaign, Al2 O3 is the predominantly main slag phase. It forms a number of compounds with lime at different compositions and under various temperatures as is evident from the phase diagram of Al2 O3 versus CaO, indicated in Fig. 1. As per the phase diagram, a slag phase corresponding to the composition 2CaO·Al2 O3 has the lowest melting temperature of 1400◦ C [5]. As a large addition of CaO increases the thermal burden on the charge, attempts were made to attain slag of melting
3.2. Studies on thermite smelting experiments A number of thermite smelting experiments were conducted on a 200 g scale by using different charge compositions for the preparation of TZM alloys. The results of the study are summarized in Table 1. The first experiment (Run no. 1) was conducted with a stoichiometric amount of aluminium. The reaction was found to be highly violent with poor slag-metal separation and only 40% alloy yield was achieved with this composition. The reason for poor yield can be attributed to the sputtering loss of the alloy due to generation of intense heat (3511 kJ kg−1 ) during the course of the reaction. In order to control the excess heat and utilize Table 1 Results of thermite smelting experiments Run no.
1 2 3 4 5 6 7 8 9
Charge composition (g)
X (wt.%)
MoO2
TiO2
ZrO2
132 132 132 132 132 132 132 132 660
0.42 0.42 0.42 0.42 0.42 0.42 0.84 1.68 4.20
0.06 0.06 0.06 0.06 0.18 0.24 0.36 0.36 1.50
CaO 0.0 0.0 14 14 14 14 14 14 70
C
Al
2.0 2.0 2.0 2.0 2.0 3.0 4.0 4.0 15.0
48 48 48 50.4 52.8 55.2 53.28 53.75 266.40
X: excess aluminium taken over stoichiometric amount, Q: specific heat of the charge, Y: alloy yield.
0.0 0.0 0.0 5.0 10 15 10 10 10
Q (kJ kg−1 )
Y (wt.%)
3511 3511 3043 2968 2893 2851 2843 2822 2842
40 60 75 85 90 91 90 89 92
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temperature much lower than that of Al2 O3 . In the successive three experiments (Run nos. 4–6) excess aluminium in the range of 5–15% over the stoichiometric amount was added to improve the alloy yield further. Consequently, the yield figure has gone up significantly giving a maximum yield of 91%. However, it was observed that more than 10% excess addition of aluminium in the charge has not contributed to any noticeable change in the alloy yield (from 90 to 91%) rather has contaminated the alloy product with further loading of aluminium. Composition analysis of the alloys obtained from all the above experiments (Run nos. 1–2) has shown poor loading of Ti, Zr and C. Although loading of Ti in the alloys improved significantly from Run no. 3 onwards due to addition of lime, however, the loading of other two components remained unaltered. This may be due to stoichiometric addition of their oxides and the requisite amount of carbon in the charge. Hence in the subsequent experiments (nos. 7–9) the addition of TiO2 , ZrO2 and C were varied from 50 to 500% excess over requisite amount in the thermite charge to improve the loading of the above elements in the resultant TZM alloy. After conducting several experiments, it was observed that the charge composition corresponding to the experiment no. 7 was found to be optimum. Based on the findings of this experiment, a scale-up operation (Run no. 9) was conducted on increasing the batch size by five-fold to substantiate the results. Alloy yield has further improved to 92% due to better heat output. TZM alloys made by the thermite process were cut into four pieces in the longitudinal and transverse directions by a corundum based abrasive cutter for examining defects such as blow holes, cracks, cavities, etc. Alloys with poor slagmetal separation were found to have traces of slag on the surface and presence of cavities and thermally induced cracks at the interior. On the contrary, alloys with good consolidation and good slag-metal separation were devoid of any such defects. In order to ensure complete elimination of defects with adequate consolidation and homogenization, the as-reduced alloys were further melted in a non-consumable arc melting furnace under an argon atmosphere. The arc melted samples were found to have excellent consolidation. During melting, alloy loss was found to be negligible and not more than 2% loss was registered. As a matter of interest, attempts were also made to prepare the TZM alloy by component melting using a direct arc melting technique under argon atmosphere. However, due to significant difference in the melting temperature and a large variation in the quantity of the constituent elements, the alloy button was obtained with poor uniformity in the composition. Therefore, in order to ensure adequate homogeneity of the alloy matrix, a number of melting were carried out resulting in large cumulative alloy loss (40–50%) by evaporation. It is therefore, evident that preparation of TZM alloy by a combination of aluminothermic reduction of mixed oxides of molybdenum, titanium, zirconium in presence of carbon is better suited than direct melting from pure components.
Table 2 Chemical analysis of TZM alloy Element
Target composition (wt.%)
Thermite alloy (wt.%)
Remelted alloy (wt.%)
Ti Zr C Mo
0.50 0.08 0.20 Balance
0.46 0.06 0.17 Balance
0.48 0.07 0.18 Balance
3.3. Compositional analysis of TZM alloy Qualitative analysis by XRF has confirmed the presence of Mo, Ti and Zr in the TZM alloy. Quantitative estimation was carried out by the combination of XRF and chemical analysis and the results of analysis of the as-reduced and remelted alloys obtained under optimum experimental condition is presented in Table 2. The remelted alloy composition is found to match nearly the designated alloy composition due to better consolidation and homogenization. 3.4. Identification of phases by X-ray diffraction Phase identification of TZM alloy was carried out by Xray diffraction technique. The diffraction pattern of the asreduced cum remelted alloy obtained under optimum experimental condition is indicated in Fig. 2. The plot exhibited three well defined and sharp peaks altogether. Amongst these, the tallest peak was confirmed to be of elemental molybdenum, the other two comparatively shorter peaks resembled carbides of titanium and zirconium. However, no such peak for elemental titanium, zirconium and aluminium nor a peak corresponding to their compounds was observed. 3.5. Microstructural study The optical microstructures of as-reduced and remelted TZM alloys are presented in Figs. 3 and 4. The microstructure
Fig. 2. XRD pattern of TZM alloy.
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Table 3 Results of hardness measurement on TZM alloy Type of alloy
Hardness value (VHN)
As-reduced
290 285 305 300 320 300 270 275 280 285 290 280
Average hardness Remelted
Average hardness
Fig. 3. Optical micrograph of as-reduced TZM alloy.
corresponding to the as-reduced alloy exhibits the presence of combination of large hexagonal to round shaped grains in the size range of 75–100 m. Dark small and round globules of TiC and ZrC were found to be distributed at random in the matrix. The microstructure of the remelted alloy revealed presence of comparatively smaller grains mostly round in shape in the size range of 20–25 m with well defined grain boundaries. Fine particles of titanium and zirconium carbides were found to be uniformly distributed all over the matrix. 3.6. Hardness measurement of TZM alloys Results of hardness measurement on TZM alloy are shown in Table 3. Average hardness value was found to be 300 VHN for as-reduced TZM alloy and 280 VHN for the remelted alloy. A large variation in hardness value (300–350 VHN) was observed at different locations of as-reduced alloy samples as compared to remelted alloy (270–290 VHN). Decrease in hardness value and comparatively lower variation in its values at different locations for the remelted alloy can be attributed to its better consolidation, refining and homogenization during remelting than the as-reduced alloys.
3.7. Evaluation of fabrication behaviour of TZM alloy by rolling Fabrication of TZM alloy into sheet from remelted alloy button was evaluated by measuring the percentage reduction in thickness during rolling as per the following relations: Eh =
Sf − Si × 100 Sf
where Eh is the % thickness reduction of the specimen, Sf and Si are the final and initial thickness of the specimen. The results of rolling study on TZM alloy are indicated in Table 4. It is observed that TZM alloy exhibits much better fabricability at elevated temperature (950 ◦ C) during rolling under the jacketed condition than at room temperature. Hot rolling has given better rolling ability of the TZM alloys due to better plastic deformation at high temperature. Rolling of the as-reduced alloy has resulted in early failure of the alloy by edge cracking due to inhomogeneous structure, however, the remelted alloy had shown excellent rolling characteristics at high temperature (950 ◦ C) due to better consolidation and homogeneity. A maximum 40% reduction in thickness was achieved for as-reduced alloy and 90% for remelted alloy at 950 ◦ C under jacketed condition. 3.8. Study on thermal stability of TZM alloy Thermal plots showing the oxidation pattern of TZM alloy under the atmospheres of air and oxygen are shown in Fig. 5. The plots exhibit gradual and slow weight gain up to 400 ◦ C due to the formation of dark brown MoO2 at a sluggish rate
Table 4 Results of fabrication study by rolling on TZM alloy
Fig. 4. Optical micrograph of remelted TZM alloy.
Type of alloy
Temperature (◦ C)
Reduction in thickness (%)
As-reduced Remelted As-reduced Remelted
RT RT 950 950
20 60 40 90
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4. Conclusion
Fig. 5. Oxidation pattern of TZM alloy.
on the specimen surface. At 400 ◦ C and onwards, there was a sharp increase in the weight gain till 750 ◦ C due to the formation of greenish yellow MoO3 layers at a rapid rate on the specimen surface. The cumulative weight gain was 70–80% which can be attributed to the formation of above oxide layers on the specimen surface. However, at 750 ◦ C onwards, there was a sharp decrease in the weight due to the volatilization of MoO3 . The cumulative weight loss was recorded to be 40–50%. It is observed that the cumulative effect on weight change was much higher under the atmosphere of oxygen than in air. However, there was neither any weight gain nor any weight loss when the TZM specimen was heated under vacuum or argon. An optimum composite coating of Al and Si in the range of 20–30 m on TZM specimens showed excellent resistance to oxidation. They did not show any weight change due to oxidation either in air or oxygen for the same duration of time (6 h). The coating remained consistently intact during and after heating.
The present investigation demonstrates the technical feasibility of preparation of TZM alloy of average composition Mo–0.48Ti–0.07Zr–0.18C nearly matching the nominal composition Mo–0.5Ti–0.08Zr–0.02C (wt.%) by direct aluminothermic co-reduction of mixed oxides of molybdenum, titanium and zirconium in the presence of excess aluminium and carbon. The alloy was made in a single step by judiciously utilizing the heat of the reactions and experimental parameters. A maximum alloy yield of 92% was achieved. The as-reduced alloys are found to have well-developed grains which became finer and round in shape on further remelting. The alloy showed excellent fabricability by rolling and has reasonably high hardness. The poor oxidation resistance of the alloy was surmounted at high temperature by a composite coating of Al and Si on the alloy surface. Acknowledgement The authors are grateful to Dr. S. Banerjee, Director, Materials Group, Bhabha Atomic Research Centre for his consistent support and encouragement during the course of above campaign.
References [1] C.K. Gupta, A.K. Suri, Extractive Metallurgy of Niobium, CRC Press, Boca Raton, FL, 1994, pp. 33–34. [2] Rembar Molybdenum Technical Information, The Rembar Company, Inc., NY, USA. [3] W. Dautzenberg, Ulmans Encyklopadie der Technischen Chemie, 4th ed., Verlag Chemie, Weinheim, 1974, pp. 351–361. [4] I.H. Hall, Ulmans Encyclopedia of Industrial Chemistry, vol. Al, 5th ed., Verlag Chemie, Weinheim, 1985, pp. 447–452. [5] S. Fillipov, The Theory of Metallurgical Processes, Mir Publishers, Moscow, 1975, p. 211.