- Email: [email protected]
Processing of Cr2Nb precursor through powder metallurgy route
Powder Technology 197 (2010) 177–183
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Processing of Cr2Nb precursor through powder metallurgy route G.P. Khanra, Abhay K. Jha ⁎, S. Girikumar, K. Thomas Tharian, Suresh Kumar Materials and Metallurgy Group, Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695 022, India
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 28 July 2009 Accepted 11 September 2009 Available online 28 September 2009 Keywords: Cr2Nb intermetallic Precursor Mechanical milling
a b s t r a c t Mechanical milling (MM) has been adopted to develop Cr2Nb intermetallic precursor, required for the processing of Cu–Cr–Nb alloy. Process parameters have been optimized through particle characterisation of powders produced by different milling time and conditions. It has been observed that particle morphology and size change with milling time. A milling time of 15 h, followed by annealing at 1400 °C has been found to be optimum condition to result pure Cr2Nb powders with desired properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Continuous efforts are underway to develop an alloy which can have an exceptional high temperature strength and good thermal conductivity for high heat flux applications such as regenerative cooled rocket motors, combustion chambers and heat exchangers. Performance of a regenerative cooled cryogenic engine is mainly governed by its combustion efficiency, which in turn depends upon the high temperature structural stability and high heat flux withstanding ability of thrust chamber liner. This calls for the use of high thermal conductive and creep resistant materials. Owing to excellent thermal conductivity and moderate strength, copper base alloys are widely used as thrust chamber inner liners for regenerative cooled liquid/cryo engines. A series of copper based alloys was developed by the addition of alloying element such as Cr, Co, Ag, Zr, Nb etc. in various combination at different laboratories. Some of these specific copper base alloys such as Cu–Cr– Zr–Ti and Cu–Ag–Zr have been effectively used for Vulcain, SSME and RD-120 engines by ESA, NASA and Russia respectively. In recent past, Cu–Cr–Nb alloy has emerged as a viable alternative to Cu–Cr, Cu–Ag–Zr alloy or other high conductivity material, where good mechanical properties are to be retained over a range of temperature which may be as high as 700 °C for long thermal exposure time, due to its stable microstructure at that temperature owing to finely distributed Cr2Nb precipitates/dispersoid in copper matrix. These particulates are extremely stable which act to pin the grain boundaries during thermal exposure to prevent grain coarsening as well as grain boundary sliding during creep. The absence of mutual solubility and very low diffusivity of Cr and Nb in copper prevents Oswald ripening of Cr2Nb particles, leading to high temperature stability of microstructure of the composite [1,2]. As
⁎ Corresponding author. Tel.: +91 471 2563628; fax: +91 471 2705048. E-mail address: [email protected] (A.K. Jha). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.09.012
per ternary phase diagram, for Cu–8 at.%Cr–4at% Nb composition, Cr and Nb go into liquid solution at high temperature and precipitate out as Cr2Nb in copper matrix during equilibrium cooling to provide high temperature strength. The full conversion of Cr and Nb into Cr2Nb yields 12.35 wt.% of Cr2Nb, an optimum amount of precipitate to provide highest strength. The high melting point intermetallic compound Cr2Nb, which is found to be stable and does not coarsen significantly during exposure up to 700°C, strengthen this new class of Cu-based alloy. This alloy has excellent oxidation resistance at high temperature [3]. However, realization of this alloy through melting route is difficult due to vast difference in melting points of alloying element Cu (1083 °C), Cr (1863 °C), Nb (2469 °C) and the absence of low melting eutectic formation of Cu–Cr or Cu–Nb, which pose difficulties to make master alloy. Few successful attempts were made elsewhere for melt spun-ribbon through induction melting and chill block melt spinning. However, processing the alloy through powder metallurgy route is the most viable route to make Cr2Nb precursors and consolidation with copper matrix. Mechanical alloying is a process that can be used to produce alloys with very fine microstructures without melting. This process can be effectively used to develop tailor made composites and amorphous/ microcrystalline systems depending upon applications [4]. This technique to produce intermetallic compounds of high melting points appears interesting mainly because (i) the compounds are very difficult or impossible to be produced by conventional casting techniques and (ii) these materials can have high strength owing to very fine grains obtained by mechanical alloying [5,6]. Intermetallic processing through mechanical alloying is less known as most of the recent works on mechanical alloying have been concentrated on oxide dispersed system (ODS) alloys, and other transition metals for amorphous and microcrystalline phase processing [7,8]. This attempt develops the most critical technology element for the composite i.e. Cr2Nb making through mechanical alloying involving solid state processing [9]. The process
178
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Table 1 Characteristics of Cr, Nb, Cr2Nb powder. Powder design
Purity (%)
Particle size (µm)
Density (g/cc)
Melting point (°C)
Crystal structure
Chromium Niobium Cr2Nb
99.8 99.9 100
<38 <75 <9
7.14 8.57 7.62
1863 2469 1770
bcc bcc hcp
consisted of blending of Cr and Nb powders, mixing, milling and annealing. This technical paper deals with efforts, which have been carried out towards intermetallic Cr2Nb precursor development by mechanical alloying. The evolution of the microstructure was obtained after milling the elemental powders for different periods of time to understand the mixing process and particle morphology change, which takes place prior to the formation of Cr2Nb through heat treatment. 2. Experimental The chemical analysis of elemental Cr and Nb powder was done through standard analytical method. According to the equilibrium diagram of Cr–Nb system, the intermetallic phase forms at a range of compositions between 48 and 52 wt.% Nb. The stoichiometric compositions (52 wt.% Cr–48 wt.% Nb) of the elemental powders were hand mixed and further mixed thoroughly in a roller mixer for 6 h. The salient physical properties of the powders are given in Table 1. Mechanical alloying of these mixed powders was performed by an attritor mill with hardened steel container and balls, keeping a weight ratio of balls to powder equaling five. Attritor mill model 01-HD (make: Union Press, USA) with container volume of 750 cm3 was used for carrying out the experiment. The hardened steel ball (3800 numbers) made of AISI 440C stainless steel and 4 mm diameter in size was used as a grinding medium. Milling was carried out continuously for different batches for a period of 15, 22 and 30 h under protective argon atmosphere at 400 rpm. Working rpm of 400 was adopted to avoid excessive heating of the system. Particle controlling agent (PCA) was not added considering non-seizure properties of chromium and niobium powders and also to avoid interstitial pick up by highly reactive Nb. Property evaluation for powders was carried out in different stages of milling as well as after annealing in order to correlate between process parameters and powder characteristics. Elemental purity, average particle size using FSSS/size distribution, particle shape and XRD analysis of powders were investigated for the optimization of process parameters. Fig. 2. Powder morphology after a) 15 h, b) 22 h and c) 30 h of milling.
Fig. 1. General morphology of mixed powders.
Fig. 3. Average particle size evolution during milling.
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
179
Table 2 Summary of milling experiment details. Milling procedure
Milling atmosphere/condition
Hours of milling
XRD analysis
Particle size of milled powder, microns (FSSS)
Remarks
Hand mixed Attritor mill Attritor mill Attritor mill Planetary ball mill
Air/dry Argon/dry Argon/dry Argon/dry Argon/dry
0 15 22 30 17
Cr, Cr, Cr, Cr, Cr,
15.5 9 7 6.4 11
Elemental powder remains unchanged Elemental powder remains unchanged Slight change in Cr peak height Slight indication of peak broadening of Nb Elemental powder remains unchanged
Nb Nb Nb Nb Nb
In another experimental attempt, stoichiometric composition of chromium and niobium was mixed in a roller mixer and charged in a planetary ball mill under argon atmosphere. The planetary ball mill model Pulverisette 5 (FRITSCH make) with container volume capacity of 250 cm3 was used for carrying out the experiment. Toughened zirconia ball of 10 mm diameter was used as a grinding medium. The milling was carried out for 17 h. The ratio for mass of grinding media to charge was kept 1:4. This was done as an additional attempt to see the effectiveness
of milling with other available equipment, however this data cannot be compared in totality to that of attritor mill experiment. The milled powders thus obtained through attritor milling as well as planetary milling were characterized by particle size measurement using Fisher Sub-sieve sizer (FSSS), their morphology using Scanning Electron Microscope (SEM) and phases present using X-ray Diffraction (XRD). The milled powders were further annealed at 1400 °C under high vacuum in high temperature sintering furnace to provide
Fig. 4. XRD pattern of the powder, milled for 15 h and 30 h.
180
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Fig. 5. XRD pattern of milled and heat treated (850 °C, H2 atms.) powder.
sufficient thermal energy, required to form Cr2Nb. Annealed powders with different milling procedures and milling time were analyzed for presence of Cr2Nb using XRD technique. 3. Results Chemical analysis of elemental Cr and Nb powder confirmed their purity up to 99.8 and 99.9% respectively. The elemental powder particles, while viewed under SEM were found to be angular (Fig. 1) while shape changed from angular to rounded after milling (Fig. 2). The particle size as measured under SEM, in the powder samples was in the range of 4–25 µm depending upon the milling condition (Fig. 2). Average particle size, measured using Fisher Sub-sieve sizer revealed that the average particle size of the milled powder decreases with milling time at a particular rpm (Fig. 3). However, trend was sluggish beyond milling time of 15 h. The average particle size of milled powder for different milling times (15 h, 22 h and 30 h) is plotted in Fig. 3. Milling of powder for 30 h has reduced average particle size from 15.5 µm to 6.4 µm, as per FSSS results. Particle size estimation under SEM was confined to individual particle, whereas FSSS analysis gave average particle size of bulk of powder based on surface area measurement technique. Hence, being a more reliable data of FSSS over SEM, average particle size of bulk of powder as estimated by FSSS has been taken as input to our experimental study (Table 2). XRD pattern of powder milled for 15 and 30 h, using CuKα radiation in Philips make XRD unit is shown in Fig. 4. Milled powder was treated at 850 °C under H2 atmosphere to provide activation energy for Cr2Nb transformation. XRD pattern revealed small peak of Cr2Nb (Fig. 5), indicating activation energy provided was not sufficient and hence annealing of this milled powder was carried out at 1400 °C under vacuum. Annealing resulted in point-to-point contact bonding (Fig. 6) and XRD pattern (Fig. 7) revealed complete transformation of Cr2Nb from intimate mixture of Cr and Nb. XRD pattern indicated relatively higher NbO in annealed powder milled for 30 h than that milled for 15 h. This was attributed to higher dissolved/adsorbed oxygen of powder milled in argon for 30 h. Experiment was repeated with milling in argon as well as under Toluene. Milling under Toluene resulted in finer powder (Fig. 8) but oxidation of Nb was observed (Fig. 9). Repeatability of the experiment was established.
Fig. 6. Powder morphology a) after 15 h of milling and annealing, b) after 22 h of milling and annealing and c) after 30 h of milling and annealing.
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
181
Fig. 7. XRD pattern of the powder, milled for 15 and 30 h and subsequently annealed.
4. Discussion The general morphology of the mixed powders revealed predominantly angular shape with rough surface. This morphology is favorable for attritor milling. It was found that the milling reduced the particle size and it was a function of milling time. Milling for 30 h has reduced the average particle size from 15.5 µm to 6.4 µm. It is evident that the reduction in average particle size beyond 15 h of milling is considerably sluggish. Oxygen pickup was found to be more for annealed powder milled for 30 h, due to longer exposure with commercial purity oxygen during milling and their finer size (6.4 µm). This oxygen, which remained with powder surface as adsorbed layer or dissolved reacted with Nb during annealing under vacuum and resulted in high NbO. Further intermixing of Cr and Nb is also not expected much after initial welding and fracturing process. This was the reason, 15 h of milling has been considered as optimum time for milling. The size, shape and surface morphological change of powders as a function of milling time, analyzed under SEM, revealed the distribution of particles in the range of 3.83 to 12.9 µm. Decrease in particle size and increase in surface roughness with milling time were evidenced from the study. The particles got rounded off during milling. The layered structure
as shown by arrows in Fig. 2a and b, of milled powder confirmed fracture and welding of particles during milling. Similar layered structure was found by Morris and Morris and reported elsewhere [9].
Fig. 8. Powder morphology after 15 h of milling in toluene and 1400 °C annealing.
182
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Fig. 9. XRD pattern of the powder, milled for 15 h under Toluene and annealed (1400 °C).
The annealed powder under SEM revealed localized particle-toparticle bonding at contact area, causing integration of particles. Hence the annealing temperature was restricted to 1400 °C to avoid particle–particle bonding. The XRD pattern of milled powder even up to 30 h could reveal little peak broadening (Fig. 4), indicating mechanical alloying of Cr and Nb. However, broadening was found to be insignificant. Equilibrium phase diagram indicated practically nil solubility of Cr in Nb or Nb in Cr at room temperature. However, in mechanical alloying, being a non-equilibrium process, little mechanical alloying can be expected. Morris and Morris [9] used high-energy ball mill and reported significant broadening due to mechanical alloying. Particle size reduction and microstructure refinement of particles associated with layered structure as observed in experimental study confirmed intimate mixing of the constituents during milling. Powder milled for 17 h in planetary ball mill followed by heating at 850 °C for 2 h in hydrogen atmosphere could reveal very small peak of Cr2Nb. This was an indication that thermal activation energy is necessary
to convert milled powder into Cr2Nb. However, for complete conversion of Cr2Nb from milled, Cr–Nb powder, obtained by mechanical milling (MM), annealing at temperature much higher than 850 °C was required. Hence annealing at 1400 °C, under 10− 5 Torr for 2 h was carried out. A strong peak corresponding to the Cr2Nb phase is quite evident on XRD pattern. Annealing under vacuum instead of H2 was decided due to appearance of extensive oxide peaks after annealing at 850 °C in H2. XRD pattern of annealed powders under vacuum is shown in Fig. 9. Summary of the interpretation of XRD pattern for milling experiments and the annealing treatment was given in Tables 3 and 4 respectively. Experimental details indicated that 15 h of dry milling under argon yielded best results with respect to purity and full conversion of Cr2Nb. The repeatability of the experimental study was also attempted and the formation of Cr2Nb phase was further studied under argon and toluene medium. Results were found to be repetitive in nature. However milling under toluene resulted in finer grains (Fig. 8) though oxygen pick up was more in the final powder (Fig. 9). On the other
Table 3 Summary of annealing treatment details. Milling procedure
Hours of milling
Annealing temperature (°C)
XRD analysis
Remarks
22
850 1400 850 1400 1400
Cr, Nb Cr, Cr2Nb, Nb Cr, Nb Cr2Nb Cr2Nb,Nb, NbO
Attritor mill
30
1400
Cr2Nb, Nb, NbO
Planetary ball mill
17
1400
Cr2Nb, Cr, Nb
No conversion to Cr2Nb Little conversion to Cr2Nb due to inadequate mixing of Cr and Nb Very less conversion to Cr2Nb Cr2Nb is the predominant phase Oxide of Nb due to (i) oxygen pick up from longer exposure with commercial purity Ar and (ii) finer powder More nos of NbO, Nb2O3 peaks due to (i) oxygen pick up from much longer exposure with commercial purity Ar and (ii) much finer powder Full conversion does not take place due to inadequate mixing of Cr and Nb
Hand mixed
0
Attritor mill
15
Attritor mill
Table 4 Summary of repeating trial run. Milling procedure
Hours of milling
Annealing temperature (°C)
XRD analysis
Remarks
Attritor mill (Toluene)
15
1400
Cr2Nb,Cr, NbO
Attritor mill (Argon)
15
1400
Cr2Nb, Cr
Oxide of Nb due to more oxygen pick up. Presence of Cr peak from excess Cr addition Absence of NbO. Presence of Cr peak from excess Cr addition
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
hand, processing under argon resulted in oxide free Cr2Nb. Hence the optimized process route was established to be mechanical milling for 15 h followed by annealing at 1400 °C. 5. Conclusions 1. Process for making Cr2Nb powder has been optimized as mechanical milling for 15 h followed by annealing at 1400 °C under vacuum yielded complete conversion to Cr2Nb. 2. 15 h of attritor milling yields high purity Cr2Nb with desired particle size owing to less exposure of Argon medium during milling. 3. Surface morphology of powder particles becomes roughened. Particles become rounded from irregular shape. Layered structure is formed during attritor milling. Acknowledgements The authors wish to express their sincere gratitude to Dr. P.P. Sinha, Deputy Director, VSSC for his constant encouragement and technical guidance during the experimental work. They are indebted
183
to Dr. K. Radhakrishnan, Director, VSSC for giving permission to publish the work.
References [1] D.L. Ellis, G.M. Michal, N.W. Orth, Scripta Metallurgica 24 (1990) 885. [2] D.L. Ellis, G.M. Michal, R.L. Dreshfield, A New Cu–8Cr–4Nb alloy for high temperature application, NASA tech. memo. (June 1995) 106910. [3] Linus U. Thomas-ogbuji, Donald L. Humphery, NASA/CR-2000-210369, Aug. 2000. [4] P.S. Gilman, J.S. Benjamin, Mechanical alloying, annual review, Materials Science 13 (1983) 279–300. [5] R. Sundaresan, F.H. Froes, Mechanical alloying, Jl of Metals 39 (1987) 22–27. [6] W.L. Johnson, Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials, Progress in Materials Science 30 (1986) 81–134. [7] R.B. Schwarz, C.C. Koch, Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and intermetallics, Applied Physics Letters 49 (1986) 146–148. [8] E. Hellstern, L. Schultz, Amorphization of transition metal Zr alloys by mechanical alloying, Applied Physics Letters 48 (1986) 124–126. [9] M.A. Morris and D.G. Morris, The evolution of microstructure during mechanical alloying to produce Cr2Nb, Institute of structural Metallurgy, University of Neuchatel, Switzerland, Solid state powder processing, The Minerals, Metals & Materials society, 1990, 299–314.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Processing of Cr2Nb precursor through powder metallurgy route G.P. Khanra, Abhay K. Jha ⁎, S. Girikumar, K. Thomas Tharian, Suresh Kumar Materials and Metallurgy Group, Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695 022, India
a r t i c l e
i n f o
Article history: Received 23 March 2009 Received in revised form 28 July 2009 Accepted 11 September 2009 Available online 28 September 2009 Keywords: Cr2Nb intermetallic Precursor Mechanical milling
a b s t r a c t Mechanical milling (MM) has been adopted to develop Cr2Nb intermetallic precursor, required for the processing of Cu–Cr–Nb alloy. Process parameters have been optimized through particle characterisation of powders produced by different milling time and conditions. It has been observed that particle morphology and size change with milling time. A milling time of 15 h, followed by annealing at 1400 °C has been found to be optimum condition to result pure Cr2Nb powders with desired properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Continuous efforts are underway to develop an alloy which can have an exceptional high temperature strength and good thermal conductivity for high heat flux applications such as regenerative cooled rocket motors, combustion chambers and heat exchangers. Performance of a regenerative cooled cryogenic engine is mainly governed by its combustion efficiency, which in turn depends upon the high temperature structural stability and high heat flux withstanding ability of thrust chamber liner. This calls for the use of high thermal conductive and creep resistant materials. Owing to excellent thermal conductivity and moderate strength, copper base alloys are widely used as thrust chamber inner liners for regenerative cooled liquid/cryo engines. A series of copper based alloys was developed by the addition of alloying element such as Cr, Co, Ag, Zr, Nb etc. in various combination at different laboratories. Some of these specific copper base alloys such as Cu–Cr– Zr–Ti and Cu–Ag–Zr have been effectively used for Vulcain, SSME and RD-120 engines by ESA, NASA and Russia respectively. In recent past, Cu–Cr–Nb alloy has emerged as a viable alternative to Cu–Cr, Cu–Ag–Zr alloy or other high conductivity material, where good mechanical properties are to be retained over a range of temperature which may be as high as 700 °C for long thermal exposure time, due to its stable microstructure at that temperature owing to finely distributed Cr2Nb precipitates/dispersoid in copper matrix. These particulates are extremely stable which act to pin the grain boundaries during thermal exposure to prevent grain coarsening as well as grain boundary sliding during creep. The absence of mutual solubility and very low diffusivity of Cr and Nb in copper prevents Oswald ripening of Cr2Nb particles, leading to high temperature stability of microstructure of the composite [1,2]. As
⁎ Corresponding author. Tel.: +91 471 2563628; fax: +91 471 2705048. E-mail address: [email protected] (A.K. Jha). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.09.012
per ternary phase diagram, for Cu–8 at.%Cr–4at% Nb composition, Cr and Nb go into liquid solution at high temperature and precipitate out as Cr2Nb in copper matrix during equilibrium cooling to provide high temperature strength. The full conversion of Cr and Nb into Cr2Nb yields 12.35 wt.% of Cr2Nb, an optimum amount of precipitate to provide highest strength. The high melting point intermetallic compound Cr2Nb, which is found to be stable and does not coarsen significantly during exposure up to 700°C, strengthen this new class of Cu-based alloy. This alloy has excellent oxidation resistance at high temperature [3]. However, realization of this alloy through melting route is difficult due to vast difference in melting points of alloying element Cu (1083 °C), Cr (1863 °C), Nb (2469 °C) and the absence of low melting eutectic formation of Cu–Cr or Cu–Nb, which pose difficulties to make master alloy. Few successful attempts were made elsewhere for melt spun-ribbon through induction melting and chill block melt spinning. However, processing the alloy through powder metallurgy route is the most viable route to make Cr2Nb precursors and consolidation with copper matrix. Mechanical alloying is a process that can be used to produce alloys with very fine microstructures without melting. This process can be effectively used to develop tailor made composites and amorphous/ microcrystalline systems depending upon applications [4]. This technique to produce intermetallic compounds of high melting points appears interesting mainly because (i) the compounds are very difficult or impossible to be produced by conventional casting techniques and (ii) these materials can have high strength owing to very fine grains obtained by mechanical alloying [5,6]. Intermetallic processing through mechanical alloying is less known as most of the recent works on mechanical alloying have been concentrated on oxide dispersed system (ODS) alloys, and other transition metals for amorphous and microcrystalline phase processing [7,8]. This attempt develops the most critical technology element for the composite i.e. Cr2Nb making through mechanical alloying involving solid state processing [9]. The process
178
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Table 1 Characteristics of Cr, Nb, Cr2Nb powder. Powder design
Purity (%)
Particle size (µm)
Density (g/cc)
Melting point (°C)
Crystal structure
Chromium Niobium Cr2Nb
99.8 99.9 100
<38 <75 <9
7.14 8.57 7.62
1863 2469 1770
bcc bcc hcp
consisted of blending of Cr and Nb powders, mixing, milling and annealing. This technical paper deals with efforts, which have been carried out towards intermetallic Cr2Nb precursor development by mechanical alloying. The evolution of the microstructure was obtained after milling the elemental powders for different periods of time to understand the mixing process and particle morphology change, which takes place prior to the formation of Cr2Nb through heat treatment. 2. Experimental The chemical analysis of elemental Cr and Nb powder was done through standard analytical method. According to the equilibrium diagram of Cr–Nb system, the intermetallic phase forms at a range of compositions between 48 and 52 wt.% Nb. The stoichiometric compositions (52 wt.% Cr–48 wt.% Nb) of the elemental powders were hand mixed and further mixed thoroughly in a roller mixer for 6 h. The salient physical properties of the powders are given in Table 1. Mechanical alloying of these mixed powders was performed by an attritor mill with hardened steel container and balls, keeping a weight ratio of balls to powder equaling five. Attritor mill model 01-HD (make: Union Press, USA) with container volume of 750 cm3 was used for carrying out the experiment. The hardened steel ball (3800 numbers) made of AISI 440C stainless steel and 4 mm diameter in size was used as a grinding medium. Milling was carried out continuously for different batches for a period of 15, 22 and 30 h under protective argon atmosphere at 400 rpm. Working rpm of 400 was adopted to avoid excessive heating of the system. Particle controlling agent (PCA) was not added considering non-seizure properties of chromium and niobium powders and also to avoid interstitial pick up by highly reactive Nb. Property evaluation for powders was carried out in different stages of milling as well as after annealing in order to correlate between process parameters and powder characteristics. Elemental purity, average particle size using FSSS/size distribution, particle shape and XRD analysis of powders were investigated for the optimization of process parameters. Fig. 2. Powder morphology after a) 15 h, b) 22 h and c) 30 h of milling.
Fig. 1. General morphology of mixed powders.
Fig. 3. Average particle size evolution during milling.
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
179
Table 2 Summary of milling experiment details. Milling procedure
Milling atmosphere/condition
Hours of milling
XRD analysis
Particle size of milled powder, microns (FSSS)
Remarks
Hand mixed Attritor mill Attritor mill Attritor mill Planetary ball mill
Air/dry Argon/dry Argon/dry Argon/dry Argon/dry
0 15 22 30 17
Cr, Cr, Cr, Cr, Cr,
15.5 9 7 6.4 11
Elemental powder remains unchanged Elemental powder remains unchanged Slight change in Cr peak height Slight indication of peak broadening of Nb Elemental powder remains unchanged
Nb Nb Nb Nb Nb
In another experimental attempt, stoichiometric composition of chromium and niobium was mixed in a roller mixer and charged in a planetary ball mill under argon atmosphere. The planetary ball mill model Pulverisette 5 (FRITSCH make) with container volume capacity of 250 cm3 was used for carrying out the experiment. Toughened zirconia ball of 10 mm diameter was used as a grinding medium. The milling was carried out for 17 h. The ratio for mass of grinding media to charge was kept 1:4. This was done as an additional attempt to see the effectiveness
of milling with other available equipment, however this data cannot be compared in totality to that of attritor mill experiment. The milled powders thus obtained through attritor milling as well as planetary milling were characterized by particle size measurement using Fisher Sub-sieve sizer (FSSS), their morphology using Scanning Electron Microscope (SEM) and phases present using X-ray Diffraction (XRD). The milled powders were further annealed at 1400 °C under high vacuum in high temperature sintering furnace to provide
Fig. 4. XRD pattern of the powder, milled for 15 h and 30 h.
180
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Fig. 5. XRD pattern of milled and heat treated (850 °C, H2 atms.) powder.
sufficient thermal energy, required to form Cr2Nb. Annealed powders with different milling procedures and milling time were analyzed for presence of Cr2Nb using XRD technique. 3. Results Chemical analysis of elemental Cr and Nb powder confirmed their purity up to 99.8 and 99.9% respectively. The elemental powder particles, while viewed under SEM were found to be angular (Fig. 1) while shape changed from angular to rounded after milling (Fig. 2). The particle size as measured under SEM, in the powder samples was in the range of 4–25 µm depending upon the milling condition (Fig. 2). Average particle size, measured using Fisher Sub-sieve sizer revealed that the average particle size of the milled powder decreases with milling time at a particular rpm (Fig. 3). However, trend was sluggish beyond milling time of 15 h. The average particle size of milled powder for different milling times (15 h, 22 h and 30 h) is plotted in Fig. 3. Milling of powder for 30 h has reduced average particle size from 15.5 µm to 6.4 µm, as per FSSS results. Particle size estimation under SEM was confined to individual particle, whereas FSSS analysis gave average particle size of bulk of powder based on surface area measurement technique. Hence, being a more reliable data of FSSS over SEM, average particle size of bulk of powder as estimated by FSSS has been taken as input to our experimental study (Table 2). XRD pattern of powder milled for 15 and 30 h, using CuKα radiation in Philips make XRD unit is shown in Fig. 4. Milled powder was treated at 850 °C under H2 atmosphere to provide activation energy for Cr2Nb transformation. XRD pattern revealed small peak of Cr2Nb (Fig. 5), indicating activation energy provided was not sufficient and hence annealing of this milled powder was carried out at 1400 °C under vacuum. Annealing resulted in point-to-point contact bonding (Fig. 6) and XRD pattern (Fig. 7) revealed complete transformation of Cr2Nb from intimate mixture of Cr and Nb. XRD pattern indicated relatively higher NbO in annealed powder milled for 30 h than that milled for 15 h. This was attributed to higher dissolved/adsorbed oxygen of powder milled in argon for 30 h. Experiment was repeated with milling in argon as well as under Toluene. Milling under Toluene resulted in finer powder (Fig. 8) but oxidation of Nb was observed (Fig. 9). Repeatability of the experiment was established.
Fig. 6. Powder morphology a) after 15 h of milling and annealing, b) after 22 h of milling and annealing and c) after 30 h of milling and annealing.
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
181
Fig. 7. XRD pattern of the powder, milled for 15 and 30 h and subsequently annealed.
4. Discussion The general morphology of the mixed powders revealed predominantly angular shape with rough surface. This morphology is favorable for attritor milling. It was found that the milling reduced the particle size and it was a function of milling time. Milling for 30 h has reduced the average particle size from 15.5 µm to 6.4 µm. It is evident that the reduction in average particle size beyond 15 h of milling is considerably sluggish. Oxygen pickup was found to be more for annealed powder milled for 30 h, due to longer exposure with commercial purity oxygen during milling and their finer size (6.4 µm). This oxygen, which remained with powder surface as adsorbed layer or dissolved reacted with Nb during annealing under vacuum and resulted in high NbO. Further intermixing of Cr and Nb is also not expected much after initial welding and fracturing process. This was the reason, 15 h of milling has been considered as optimum time for milling. The size, shape and surface morphological change of powders as a function of milling time, analyzed under SEM, revealed the distribution of particles in the range of 3.83 to 12.9 µm. Decrease in particle size and increase in surface roughness with milling time were evidenced from the study. The particles got rounded off during milling. The layered structure
as shown by arrows in Fig. 2a and b, of milled powder confirmed fracture and welding of particles during milling. Similar layered structure was found by Morris and Morris and reported elsewhere [9].
Fig. 8. Powder morphology after 15 h of milling in toluene and 1400 °C annealing.
182
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
Fig. 9. XRD pattern of the powder, milled for 15 h under Toluene and annealed (1400 °C).
The annealed powder under SEM revealed localized particle-toparticle bonding at contact area, causing integration of particles. Hence the annealing temperature was restricted to 1400 °C to avoid particle–particle bonding. The XRD pattern of milled powder even up to 30 h could reveal little peak broadening (Fig. 4), indicating mechanical alloying of Cr and Nb. However, broadening was found to be insignificant. Equilibrium phase diagram indicated practically nil solubility of Cr in Nb or Nb in Cr at room temperature. However, in mechanical alloying, being a non-equilibrium process, little mechanical alloying can be expected. Morris and Morris [9] used high-energy ball mill and reported significant broadening due to mechanical alloying. Particle size reduction and microstructure refinement of particles associated with layered structure as observed in experimental study confirmed intimate mixing of the constituents during milling. Powder milled for 17 h in planetary ball mill followed by heating at 850 °C for 2 h in hydrogen atmosphere could reveal very small peak of Cr2Nb. This was an indication that thermal activation energy is necessary
to convert milled powder into Cr2Nb. However, for complete conversion of Cr2Nb from milled, Cr–Nb powder, obtained by mechanical milling (MM), annealing at temperature much higher than 850 °C was required. Hence annealing at 1400 °C, under 10− 5 Torr for 2 h was carried out. A strong peak corresponding to the Cr2Nb phase is quite evident on XRD pattern. Annealing under vacuum instead of H2 was decided due to appearance of extensive oxide peaks after annealing at 850 °C in H2. XRD pattern of annealed powders under vacuum is shown in Fig. 9. Summary of the interpretation of XRD pattern for milling experiments and the annealing treatment was given in Tables 3 and 4 respectively. Experimental details indicated that 15 h of dry milling under argon yielded best results with respect to purity and full conversion of Cr2Nb. The repeatability of the experimental study was also attempted and the formation of Cr2Nb phase was further studied under argon and toluene medium. Results were found to be repetitive in nature. However milling under toluene resulted in finer grains (Fig. 8) though oxygen pick up was more in the final powder (Fig. 9). On the other
Table 3 Summary of annealing treatment details. Milling procedure
Hours of milling
Annealing temperature (°C)
XRD analysis
Remarks
22
850 1400 850 1400 1400
Cr, Nb Cr, Cr2Nb, Nb Cr, Nb Cr2Nb Cr2Nb,Nb, NbO
Attritor mill
30
1400
Cr2Nb, Nb, NbO
Planetary ball mill
17
1400
Cr2Nb, Cr, Nb
No conversion to Cr2Nb Little conversion to Cr2Nb due to inadequate mixing of Cr and Nb Very less conversion to Cr2Nb Cr2Nb is the predominant phase Oxide of Nb due to (i) oxygen pick up from longer exposure with commercial purity Ar and (ii) finer powder More nos of NbO, Nb2O3 peaks due to (i) oxygen pick up from much longer exposure with commercial purity Ar and (ii) much finer powder Full conversion does not take place due to inadequate mixing of Cr and Nb
Hand mixed
0
Attritor mill
15
Attritor mill
Table 4 Summary of repeating trial run. Milling procedure
Hours of milling
Annealing temperature (°C)
XRD analysis
Remarks
Attritor mill (Toluene)
15
1400
Cr2Nb,Cr, NbO
Attritor mill (Argon)
15
1400
Cr2Nb, Cr
Oxide of Nb due to more oxygen pick up. Presence of Cr peak from excess Cr addition Absence of NbO. Presence of Cr peak from excess Cr addition
G.P. Khanra et al. / Powder Technology 197 (2010) 177–183
hand, processing under argon resulted in oxide free Cr2Nb. Hence the optimized process route was established to be mechanical milling for 15 h followed by annealing at 1400 °C. 5. Conclusions 1. Process for making Cr2Nb powder has been optimized as mechanical milling for 15 h followed by annealing at 1400 °C under vacuum yielded complete conversion to Cr2Nb. 2. 15 h of attritor milling yields high purity Cr2Nb with desired particle size owing to less exposure of Argon medium during milling. 3. Surface morphology of powder particles becomes roughened. Particles become rounded from irregular shape. Layered structure is formed during attritor milling. Acknowledgements The authors wish to express their sincere gratitude to Dr. P.P. Sinha, Deputy Director, VSSC for his constant encouragement and technical guidance during the experimental work. They are indebted
183
to Dr. K. Radhakrishnan, Director, VSSC for giving permission to publish the work.
References [1] D.L. Ellis, G.M. Michal, N.W. Orth, Scripta Metallurgica 24 (1990) 885. [2] D.L. Ellis, G.M. Michal, R.L. Dreshfield, A New Cu–8Cr–4Nb alloy for high temperature application, NASA tech. memo. (June 1995) 106910. [3] Linus U. Thomas-ogbuji, Donald L. Humphery, NASA/CR-2000-210369, Aug. 2000. [4] P.S. Gilman, J.S. Benjamin, Mechanical alloying, annual review, Materials Science 13 (1983) 279–300. [5] R. Sundaresan, F.H. Froes, Mechanical alloying, Jl of Metals 39 (1987) 22–27. [6] W.L. Johnson, Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials, Progress in Materials Science 30 (1986) 81–134. [7] R.B. Schwarz, C.C. Koch, Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and intermetallics, Applied Physics Letters 49 (1986) 146–148. [8] E. Hellstern, L. Schultz, Amorphization of transition metal Zr alloys by mechanical alloying, Applied Physics Letters 48 (1986) 124–126. [9] M.A. Morris and D.G. Morris, The evolution of microstructure during mechanical alloying to produce Cr2Nb, Institute of structural Metallurgy, University of Neuchatel, Switzerland, Solid state powder processing, The Minerals, Metals & Materials society, 1990, 299–314.