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Synthesis of nanostructured NiCr and Ni-Cr3C2 powders by an organic solution reaction method
Nrmstru~
Pergamon
Mataiak. Vol. 7. No. 8. pp. 857471.1996 Ekvia Science L.td Cop~iglt @ 1996 Acta MetallurgicaInc. RintcdinthcuSA. Allrightsreauved 0965-977396 $15.00 + .OO
PII 30965.9773(96)00057-g
SYNTHESIS OF NANOSTRUCTURED Ni/Cr AND Ni-C&z POWDERS BY AN ORGANIC SOLUTION REACTION METHOD T.D. Xhd2,
S. Torbad,
P.R. Struttl, and B.H. Kea?
1Advanced Technology Center for Precision Manufacturing, PMC, U- 119, University of Connecticut, Storm, CT 06269 2Current Address: US Nanocorp, 20 Washington Avenue, North Haven, CT 06473. 3Department of Ceramics, College of Engineering, Rutgers University, Piscataway, NJ 08855 (AcceptedSeptember 1996) Abstract - Nanostructurednickellchromium(NilCr) and nickel-chromiumcarbide (NiCr3C2)powder.swere prepared viaa chemicalroute. The techniqueinvolvesreductivedecompositionof an organic solutionof metalchlorideprecursors usingsodiumtriethylborohydride.After co-precipitationof a nanoscale mixedmetal(Ni+Cr) powder, lowtemperatureannealing is used toform nanostructuredNilCr alloypowder,or lowtemperaturegas phase carburizationtoform nanostructuredNi-Cr3Czcermetpowder.Thesepowderswere characterizedby x-ray diffraction (XRD), transmissionelectron microscopy (TEM), scanning electron microscopy @EM), and chemical analysis.The mechanismsof halide reductionand phaseformation are discussed. Copyright 0 1996 Acta Metallurgica Inc.
1. INTRODUCTION
At low Cr concentrations, Ni/Cr alloys are fee substitutional solid solutions (yphase). At higher Cr concentrations, exceeding the solubility limit in Ni, the equilibrium structure is a two-phase mixture of fee Ni/Cr solid solution (y-phase) and bee Cr (a-phase) (1). The corresponding chromium carbide-rich alloys contain a variety of carbide phases: Cr23C6, Cr& Cr2C or Cr&. Alloys of commercial interest have a high volume fraction of Cr3C2 carbide (typically > 60%) dispersed in aNi/Cr solid solution. Such alloys possess high strength, excellent hot corrosion resistance and superior wear properties, making them attractive candidate materials for a wide range of high temperature applications, e.g., as wear resistant coatings in the hot sections of gas turbine and diesel engines (2-7). In the past, Ni/Cr and Ni-C&2 materials were produced by melting and casting methods, as well as blending and mixing of constituent phases into a composite. More recently, rapid solidification processing technologies, including powder atomization and melt spinning, were introduced to achieve finer and more homogeneous structures. In order to produce materials with ultrafine or nanoscale structures, alternative processing routes are required. In this paper, we report on a new chemical method for the synthesis of nanostructured Ni/ Cr and Ni-Cr& materials. The method involves reductive decomposition of an organic solution 857
858
TD XIAO,S TORBAN, PR Smurr.
AND
BH KEAA
of metal chloride precursors using sodium triethyl-borohydride. After coprecipitation of a nanoscale mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured NiKr alloy powder, or low temperature gas phase carburization to form nanostructured Ni-CrjCz cermet powder. 2. EXPERIMENTAL The starting materials for the powder synthesis were nickel chloride (NiClz), chromium chloride (CrC13) and a 1.0 M toluene solution of sodium triethyl-borohydride (NaBEtsH), obtained from Aldrich Chemical Inc. The toluene solvent was first dried over calcium chloride, refluxed, distilled, and then deoxygenated. All manipulations were carried out under an atmosphere of ultrahigh purity nitrogen (99.99% purity) using standard Schlenk techniques, because of the oxygen and moisture sensitivities of the starting compounds and the resulting nanophase powders (8-9). Nanostructured Ni/Cr powders were prepared with the following compositions: 100% Ni, 80% NiKr, 50% Ni/Cr, 20% Ni/Cr and 100% Cr. In a typical experiment, e.g., for 20%Ni/Cr, 16.43 gm of CrCls (0.1038 mol), 3.36 gm of NiC12 (0.0259 mol) were dispersed in 400 ml of deoxygenated toluene to form a purple suspension. This was carried out in a 2-liter 3-neck flask, equipped with a magnetic stirrer, and gas inlet/outlet tubes. The stirred solution was fist bubbled with a stream of nitrogen for 2 hours at room temperature. Then 400 ml of 1.OM toluene solution of NaBEtsH was added slowly to the halide mixture via a pressureequalized addition funnel. The reaction was carried out at room temperature under nitrogen. A black suspension slowly formed, while hydrogen gas evolved. The suspension was stirred for 48 hours at room temperature to ensure complete reduction of the halides. The black powder was filtered and washed with deoxygenated toluene under a stream of nitrogen. At this stage, no attempt was made to remove the sodium chloride byproduct. The high surface area reactive powder was passivated with deoxygenated mineral oil and stored in a Schlenk flask under nitrogen. Other compositions of the nanophase Ni-Cr powders were prepated in a similar manner. Heat treatment was performed using a high temperature environmentally controlledLindburg tube furnace. About 2 grams of the powder mixture was placed in an alumina boat inside an aluminatubeofinnerdiameter32mm. Priorto heattreatment,thechamberwasevacuatedtoabout 10m5torr (1.33 x 10m3Pa), flushed with argon repeatedly, and backfilled with argon to near ambient pressure. Heat treatment was carried out at 300600°C with a fume gas (W%Ar-3%Hz) flow rate of about 1 L/min“. The sample was first slowly heated to 300°C in 1.5 hours, and then heated to the desired peak temperature at a heating rate of 25°C/min‘1, with a dwell time at the heat treatment temperature of 1 hour. After heat treatment, the fme powder was suspended in distilled water and then centrifuged to separate the sodium chlorideand water from the nanophase Ni/Cr alloy powder. Nanostructured Ni-CrQ powder was prepared via a gas phase carburization technique (10). About 2 grams of the powder mixture was transferred into a tubular carburization unit equipped with vacuum pumps, pressure gauges, and inlet/outlet gas tubes. The carburization unit was first evacuated to about 5 m torr, flushed with Ar, and backfilled with methane to 200 torr. Carburization was accomplished at a temperature of 880°C and a pressure of 200 torr. The heating rate and the dwell time at the reaction temperature was 60°C/min and 1 hour, respectively. After carburization, the powder was dispersed in distilled water and then centrifuged to remove sodium chloride and water from the nanophase cermet powder.
SWTHESIS OFNilCr AND Ni-Cr&
I
1 ’
1
40 2
50
I
60
THETA
’
I’
70
POWDERS
1 ‘I ’ 0 NICKEL 3.CHROMIUM
80
90
(DEGREE)
Figure 1. XRD patternsfor nanocrystalline Ni/Cralloys with differentcompositions afterheattreatmentat 600°C. (Note:the 80%NiKralloy is at 700°C).
859
TD XIAO,S TOREIAN, PR STRUT-~, AND BH KEAR
0
I,,,I,,,I,,,I,,,I,~~I,0
20
40
60
80
100
ATOMIC % CHROMIUM Figure 2. Particle size of the nanocrystalline Ni (‘y-phase) and Cr (o-phase) as a function of alloy concentration.
The as-synthesized nanostmctured Ni/Cr alloy and Ni-Cr$& powders were characterized by x-ray dilfraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and inductively coupled plasma spectroscopy (ICP). 3. RESULTS 3.1 NanostructuredNilCr Powders Reduction of the metal halides with a slight excess of NaBEtsH resulted in the formation of an amorphous, black Ni-Cr powder that contained crystalline NaCl. After heat treatment at > 3OO”C,the powder consists of a nanocrystalline mixture of Ni/Cr alloy and NaCl. After washing using distilled water, only nanocrystalline Ni/ Cr alloy was detected. Figure 1 shows XRD patterns of the nanophase Ni/Cr alloy powders with different compositions, after heat treatment at 600°C. With Cr concentration below 50 mole%, only Ni is detected. The peaks correspond to fee Ni/Cr. With Cr concentration > 50 mole%, both Ni and Cr are detected. The Ni peaks correspond to fee Ni/Cr, and the Cr peaks to bee Cr. Values of the lattice constants for Ni/Cr and Cr are listed in Table 1. The lattice parameter of the nanocrystalline Ni powder is estimated to be a = 3.5264 A, which agrees well with the known bulk value for Ni (a = 3.5238 A>.With increasing Cr concentration horn 20 to 80 mole%, the lattice parameter gradually increases: a = 3.5437 A for 2O%Cr, a = 3.5457 A for 5O%Cr,and 3.55 13 A for 8O%Cr. The lattice parameter of the nanocrystalline Cr powder is determined to be a = 2.8854 A,which again is in
661
SYNTHESIS OFNi/Cr AND Ni-Cr3C2POWDERS
TABLE 1
Lattice Parameters of Nanocrystalliue Nickel (y) and Chromium (a) Phases Cr lattice Parameter
Composition
Ni lattice Parameter
lW%Ni
3.5264 A
___
20%Cr
3.5431 A
-__
50%Cr
3.5451 A
2.8951 A
80%Cr
3.5513 A ___
2.8908 8,
100%Cr
2.8854 A
Figure 3. Bright-field ‘EM microgmph and corresponding electron diffraction pattern of the nanocrystalline 80%Ni/Cr alloy.
862
TD XIAO,S TOIWAN,PR STFIUTT.ANDBH KEAFI
good agreement with the known bulk value for pure Cr (a = 2.8839 A). As the concentration of Ni increases from 0 to 50%, there is a slight increase of the Cr lattice parameter: a = 2.8908 A for 20%Ni, and a = 2.8957 for 50%Ni. The crystallite size of the nanophase powder was determined from x-ray peak broadening. Figure 2 shows the variation of particle size for nanocrystalline Ni/Cr alloy as a function of Cr concentration. The particle size of Ni/Cr (y-phase) varied from 10 nm to 18 nm, for example, 10 nm for lO%Ni, 13 nm for 80%Ni/Cr, 10 nm for SO%Ni/Cr, and 18 nm for 20%Ni/Cr. The particle size of Cr (a-phase) is also shown in Figure 2. Theparticle sizes of the a-phase Cr were determined to be 14 nm for lOO%Cr, 29 nm for 8O%Cr, and 21 nm for 5O%Cr. A typical TEM micrograph for the 80%Ni/Cr alloy powder is shown in Figure 3. A particle size distribution determined by image analysis was performed (seeFigure4). The average particle size of the nanocrystalline alloy was 12 nm. In order to identify the specific size of different Ni/ Cr and Cr particles, further work by combined high resolution TEM and local probe analysis is needed. Selected area diffraction of the powders showed the presence of fee Ni and bee Cr. The effects of heat treatment on the crystallinity and phase formation of the as-synthesized powders were also investigated. Here, we describe XRD results for two representative alloy compositions with 80%Ni/Cr and 2O%Ni/Cr. Figure 5 shows XRDresults for the 80%Ni/Cr alloy. The as-synthesized Ni/Cr powder is amorphous, The 300°C annealed sample is also amorphous. Histogram of Current Image Feature r.Length Unit :llIll Classes t 20 Underflow: 0 Overflow : 0 Count 70
Frames
1
:
Objects : Max Cnt :
tin Val Max Val Tot Sum Mean Std Dev
: : :
I :
434 70
1.6444 32.383 5319.9 12.258 5.3271
60 50 40 30 20 10 0I 0
.
r-l,
32.3827
Length Figure 4. Particle size distribution of the 80%Ni/Cr alloy obtained by computer image analysis of the TEM micrograph.
SYNTHESIS OF NiiCr
80%
r
AND
Ni-Cr&
Ni 20%
863
POWDERS
Cr
Ni (111)
Ni (200) Ni (220) 700 c
600 C
30
40
50
60
70
80
90
100
2 THETA (DEGREE) Figure 5. XRD patterns showing the effect of annealing on the as-synthesized 8O%Ni/Cr powder. At 400°C the XRD peaks for Ni start to appear, superimposed on a very diffuse background intensity. As the annealing temperature increases, the diffraction peaks sharpen and the diffuse background decreases. At 500°C and 600°C, there is an observable background intensity in combination with very well-definedNi crystalline peaks. At 7OO”C,there is no diffuse background and the Ni phase peaks am sharp; but no Cr phase is detected in this particular composition. The crystallite size of the Ni phase increases from 4 nm to 13 nm when the temperature is increased from 300°C to 6OO”C,and is about 20 nm at 700°C. One should notice that at lower annealing temperatures, for example, below 600°C, otber broad peaks are evident. At 400°C, there are two very broad peaks centered at about 28 = 37” and 62”. Relative intensities of these two peaks decrease as the annealing temperature increases, and they finally disappear at a temperature of 7OO’C. Figure 6 shows XRD results for 20%Ni/Cr. Once again, the as-synthesized powder is amorphous. At 300°C. only Ni peaks are detected. At higher temperatures, however, peaks for both Ni and Cr phases are observed. At 5OO”C,no amorphous background is observed, but there is phase separation of Ni and Cr. At this temperature, the XRD signal of the Ni phase is strong, but the XRD signal of the Cr phase is weak. At 6OO”C,well-defined XRD peaks of the Cr phase are visible. The crystallite size of the Ni phase increases from 4 nm to 20 nm when the annealing
TDXIAO, S TOFIBAN, PR STRUTT, AND BH KEAR
80% Cr 20% Ni _
Ni (111) Cr (110)
50
60
70
80
90
2 THETA (DEGREE) Figure 6. XRD patterns showing the effect of annealing on the as-synthesized 20%Ni/Crpowder. temperature increasesfrom 300°Cto f5OO”C, while the Crphase increasesfrom 2 nm to 15nm when the temperature increases from 500°C to 600°C. The alloy powders are extremely fme after heat treatment and they can be readily dispersed in a solvent. Even after several months of storage, they show no evidence for settling. After washing in distilled water, they could not be separated from NaCl and water because all of the particles passed through the microporous filter. The separation process, however, may be accomplished by agglomerating the nanoparticlesinto micron-sizeagglomeratesviacentrifuging, followed by filtering. Morphological examination by SEM shows that these agglomerates are several tens of microns in size. From all of the above results, it is concluded that the powders after centrifugal processing are micron sizeagglomeratesconsistingof an assemblageof nanocrystabine Ni/Cr alloy particles.The agglomeratescan be easilybroken down into their individual entities via ultrasonic agitation. 3.2 Nanostructured Ni-Cr$‘2 Powders Preliminary studies have been carried out to convert the nanostructured Ni/Cr alloys into nickel-chromium carbide cermets by heat treatment in flowing methane gas at 880°C. Of
SYNTHESIS OFNi/Cr AND Ni-Cr.&
POWDERS
665
n NICKEL 0 CHROMIUM
CARBIDE
n
30
40
50
60
70
80
90
2 THETA (DEGREE) Figure 7. XRD pattern of the nanocrystalline 20%Ni-CrQ
composite powder.
commercial interest are the high volume fraction carbide cermets. In our studies, therefore, most of the carburization experiments were performed on the 20%Ni/Cr alloy. As shown by the XRD analysis, Figure 7, this alloy composition resulted in the formation of a nanophase mixture of Ni and Cr3C2. Morphological examination using SEM revealed that the cermet powder had a porous structure, but detailed information could not be obtained due to the very small particle size. A typical TEM micrograph of the powder is shown in Figure 8, which reveals a size distribution of particles ranging from a few nanometers to several tens of nanometers. Particle size analysis from the bright field micrograph via an image processing technique, Figure 9, indicates that the average particle size of the Ni-CrQ powder is about 34 nm. Using x-ray line broadening analysis, particle size of the Ni phase is 18 nm, and that of the Cr3C2 phase is 30 nm. This discrepancy in particle size may be attributed to (i) overlapping particles not being taken into consideration in the image analysis technique, and (ii) presence of amorphous regions in the particle grain boundaries. Electron diffraction analysis confumed the presence of Ni and C&2 phases. The individual particle sizes of the two different phases could not be determined from the phase contrast in the TEM studies. Further work using high resolution TEM is needed to study the grain boundaries, as well as to identify the presence of phases other than Ni and Cr3C2.
TD XIAO,S TORBAN,PR Smum, ANDBH KEAR
Figure 8. Bright-field TEM micrograph and corresponding electron diffraction pattern of the 20%Ni-Cr3C2 composite powder. 4. DISCUSSION 4.1 syinthesisand Processing ?‘he reduction of metal halides into metal clusters of Ni and Cr may be represented by the followi Ing equation: MCI, + xNal3EtsH + M+ xNaC1 + xBEt3 + (x/2)H2
111
where M = Ni for x = 2 and M = Cr for x = 3. According to previous studies on the reduction of similar metal halides (9,1 l- 15), this reaction either may result from direct hydride transfer to form
Swn-~~s~s OFNi/CrAND
867
Ni-Cr.& POWDERS
Histogram of Current Image Feature : Length :lllll Unit Classes : 20 Underflow: 0 Overflow : 0
Frames Objects Max Cnt nin Val Max Val Tot Sum Mean Std Dev
Count 61
:
1
:
292
: : :
3.2586: 94.277
:
9834.2
t
33.679 10.178
t
50 I 40 30 20 10 0I -0
94.2769 Length Figure 9. Particle size distribution obtained from computer image processing of the TEM micrograph of the 20%Ni-CrsCz composite powder. 10
20
30
50
60
70
ametal hydride intermediate, which subsequently reductively eliminates HZ, or occurs directly by an electron-transfer mechanism. In the preparation of ultrafine scale metal alloys, metal halides havealsobeenused(16-17)asprecursorstoreactwith sodiumborohydride. In thelatter,however, reactions were carried out in an aqueous medium instead of using an organic solvent. The high surface area metal powders obtained from this reduction process are extremely air and moisture sensitive. Initial experiments were done to separate the nanoparticle metal powders from NaCl via extensive washing using deoxygenated water, followed by the application of a protective coating of deoxygenated mineral oil. XRD analysis revealed that the washed powders were highly disordered or amorphous, since only very broad Ni and Cr peaks were observed. Attempts to convert them into nanocrystalline form via high temperature heat treatment, however, resulted in the formation of a greenish powder, which consisted of Ni and Cr203 phases. In the actual experiment, we observed that once the furnace temperature reached 60°C there was a sudden temperature increase up to 3OO’Cin a few minutes, although the heating rate was only set at a few degrees per minute. From these observations, combined with analytical results, it is concluded that the O-H groups of water molecules are initially physisorbed onto the metal surface. Upon slow heating, these O-H groups are gradually transformed into chemisorbed species, which then react with the metal powders to form Cr203, instead of being evaporated away in the heat treatment process, Figure 1Oa.
TD XIAO,S TOWAN,PR STRUTT, AND BH KEAFI
Ph
(a) phvsisorbed OH
chemisorbed OH
phvsisorbed C-H
chemisorbed C-H
carbide Iaver
Figure 10. Schematic diagram for the mechanisms of(i) oxidation of the Ni/Cr alloy powder after washing in deoxygenated Hz0 prior to heat treatment, and (ii) surface carburization of the Ni/Cr alloy after passivation with mineral oil prior to heat treatment.
Nickel
(weight
%I
40
60
80
I
I
I
40 Nickel
60 (atomic
80 %I
100 Ni
Figure 11. Equilibrium phase diagram of Ni/Cr alloy system (courtesy of D.R.F. West).
SYNTHESIS OFNi/Cr AND Ni-Cr.& PGWDERS
869
To avoid this oxidation problem, the as-synthesized powders were coated with deoxygenated mineral oil before washing. In this case, C-H groups derived from the mineral oil and residual toluene are first physisorbed onto the metal surfaces. As shown in Figure lob, these physisorbed C-H groups gradually transform into chemisorbed species during heat treatment, and then react with the metal surfaces to form thin layers of protective carbide phase. Since these layers are extremely thin, XRD analysis was not able to detect them. However, ICP analysis indicated that there was an appreciable carbon content in the sample. It should be noted here that during heat treatment of these materials, the initial heating rate should be extremely slow when the furnace temperature is below 300°C. Otherwise, the C-H groups will be partially evaporated, and this will create problems of slow burning of the sample upon exposure to air after heat treatment. Thus, it appears that the transformation of the physisorbed C-H groups into chemisorbed species is very important for the passivation of the nanocrystalline Ni/Cr alloys. 4.2 Phase Formation The established Ni-Cr phase diagram is shown in Figure 11. At a composition < 36 at.% Cr. only y-phase is present at 600°C. The pphase is at the Ni-rich region, where Cr atoms are substitutional. The a-phase is at the Cr-rich region, where only a very small amount (c 2%) of Ni solute atoms am substitutional. In the nanocrystalline Ni-Cr system, we detected only fee Ni XRD signals (see Figure 1) at lOO%Ni and 2O%Cr, with an increased lattice parameter for the 2O%Cr sample. This data agreed with the established phase diagram for conventional micron-sized Ni/ Cr, since these compositions are at the single phase field (y-phase). At higher concentrations of Cr. for example, 50%Cr and 8O%Cr, both y-phase and a-phase were evident, as predicted by the phase diagram. This data illustrates that our experimental findings for the nanocrystalline Ni-Cr system agree very well with the established phase diagram for the conventional microcrystalline Ni-Cr system. Figure 12 is a comparison of the lattice parameters for conventional microcrystalline Ni/Cr alloys (Floyd and Taylor) (18-19) and the corresponding nanocrystahine Ni/Cr alloys. In the experiments of Floyd and Taylor, the alloys of various compositions were annealed at 1100°C for six days. The samples were then cooled by (i) rapid quenching to room temperature, and (ii) slow cooling to room temperature by furnace cooling. The lattice parameter vs. composition plots for both sets of the samples had a similar slope up to 45%Cr, which is indicative of the presence of only one phase, the y-phase. In this study, it was found that at low concentrations of Cr, for example, O%Cr and 2O%Cr, the measured lattice parameters agreed well with the data obtained by Floyd and Taylor. However, a slope change was evident at higher concentrations of Cr. It seems that the nanophase alloy has already reached its solubility limit in the neighborhood of 20%Cr at a temperature of 600°C. Upon increasing the concentration of Cr, phase separation into y and a-phases occurs. This effect is not understood at the present time.
5. CONCLUSIONS We have developed a process for the synthesis of nanocrystalline Ni/Cr and Ni-CrsC2 powders by an organic solution reaction method. The technique involves reductive decomposition of an organic solution of metal chloride precursors using sodium triethyl-borohydride. After
TDXmo,S
870
TOR~AN.PRSTR~IT, AND BH KEAR
3.58 , A A
I9 0
0
‘9 @I
A Taylor & Floyd (Quenched)
l Nanocrystalline 0
Taylor & Floyd (Slow Cool)
Figure 12. Comparison of lattice parameters of nanocrystalline Ni/ Cr alloys with the data obtained by Taylor and Floyd for the conventional Ni/Cr alloys. co-precipitation of a nanoscale mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured Ni/Cr alloy powder, or low temperature gas phase carburization to form nanostructured NiCr3C2 cermet powder. The co-precipitated material is a mixture of amorphous Ni and Cr powders. After heat treatment at >3OO”C,nanocrystalline Ni/Cr alloy powders are obtained. Carburization of the as-synthesized powder in methane gas results in the formation of nanocrystalline Ni-CQQ. The results for the synthesis of nanocrystalline Ni/Cr alloys are consistent with the established phase diagram for Ni/Cr alloys. At low concentrations of Cr (below 2O%Cr), only fee Ni (y-phase) is detected. At higher concentrations of Cr, for example, 508Cr and 8O%Cr, both y-phase and u-phase are evident. Also, the crystal lattice of Ni increases with increasing Cr concentration. ACKNOWLEDGMENTS The authors would like to thank the Advanced Technology Center for Precision Manufacturing (PMC) of theuniversity of Connecticut for fmancial supportfor this project.Thanks are also due to Dr. R.K. Sadangi of the Department of Ceramics of Rutgers University for help with many of the experiments and valuable discussions on the carburization experiments.
SYNTHESIS OF NilCr ANDNi-Cr.& POWDERS
871
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
D.R.F. West, Ternary Equilibrium Diagrams, 2nd ed., Chapman & Hall, New York, p. 94-99 (1982). G.R. Kegg and J.M. Silcock, Metal Science Journal 6,47 (1972). J.J. Wert, D.M. Baker, and T.M. McKechnie, Surface & Coating Technology 3,245 (1987). D.L. Houck and R.F. Cheney, International Patent No. PCT/US82/ 0 1653 ( 198 1). B.J. Lee, Calphad fi (2), 121 (1992). H. Grammont and T. Puig, J. de Physique IV-Colloque C7, supplement au Journal de Physique III 1, c7-77 (1991). J.K. Dennis, S.T. Sheikh, and E.C. Silverstone, Trans. Institute of Metal Finishing s, 118 (1981). D.F. Shriver and M.A. Drezden, The Manipulation of Air-Sensitive Compounds, 2nd ed., Wiley-Interscience, New York, p. 78 (1986). D. Zeng and M.J. Hampden-Smith, Chem. Mater. 2 (5), 681 (1993). L.E. McCandlish, B.H. Kear and B.K. Kim, Nanostruct. Mater. 1,119 (1992). H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, and B. Korall, Angew. Chem. Int. Ed. Engl. a, 1312 (1991). R.D. Rieke and S.E. Bales, J. Am Chem. Sot. p6 (6), 1775 (1974). R.D. Rieke, Science m,l260 (1989). K.L. Tsai and J.L. Dye, J. Am. Chem. Sot. m, 1650 (1991). O.F. Schall, K. Robinson, J.L. Atwood, and G.W. Cokel, J. Am. Chem. Sot. m, 5962 (1993). K.E. Gonsalves, T.D. Xiao, G.M. Chow, and C.C. Law, Nanostruct. Mater. 4, 139 (1994). G.M. Chow, T. Ambrose, J.Q. Xiao, F. Kaotz, and A.M. Ervin, Nanostruct. Mater. 2, 131(1993). A. Taylor and R.W. Floyd, J. Inst. Met. a, 577 (1952). A. Taylor and K.G. Hinton, J. Inst. Met. 81, 169 (1952).
Pergamon
Mataiak. Vol. 7. No. 8. pp. 857471.1996 Ekvia Science L.td Cop~iglt @ 1996 Acta MetallurgicaInc. RintcdinthcuSA. Allrightsreauved 0965-977396 $15.00 + .OO
PII 30965.9773(96)00057-g
SYNTHESIS OF NANOSTRUCTURED Ni/Cr AND Ni-C&z POWDERS BY AN ORGANIC SOLUTION REACTION METHOD T.D. Xhd2,
S. Torbad,
P.R. Struttl, and B.H. Kea?
1Advanced Technology Center for Precision Manufacturing, PMC, U- 119, University of Connecticut, Storm, CT 06269 2Current Address: US Nanocorp, 20 Washington Avenue, North Haven, CT 06473. 3Department of Ceramics, College of Engineering, Rutgers University, Piscataway, NJ 08855 (AcceptedSeptember 1996) Abstract - Nanostructurednickellchromium(NilCr) and nickel-chromiumcarbide (NiCr3C2)powder.swere prepared viaa chemicalroute. The techniqueinvolvesreductivedecompositionof an organic solutionof metalchlorideprecursors usingsodiumtriethylborohydride.After co-precipitationof a nanoscale mixedmetal(Ni+Cr) powder, lowtemperatureannealing is used toform nanostructuredNilCr alloypowder,or lowtemperaturegas phase carburizationtoform nanostructuredNi-Cr3Czcermetpowder.Thesepowderswere characterizedby x-ray diffraction (XRD), transmissionelectron microscopy (TEM), scanning electron microscopy @EM), and chemical analysis.The mechanismsof halide reductionand phaseformation are discussed. Copyright 0 1996 Acta Metallurgica Inc.
1. INTRODUCTION
At low Cr concentrations, Ni/Cr alloys are fee substitutional solid solutions (yphase). At higher Cr concentrations, exceeding the solubility limit in Ni, the equilibrium structure is a two-phase mixture of fee Ni/Cr solid solution (y-phase) and bee Cr (a-phase) (1). The corresponding chromium carbide-rich alloys contain a variety of carbide phases: Cr23C6, Cr& Cr2C or Cr&. Alloys of commercial interest have a high volume fraction of Cr3C2 carbide (typically > 60%) dispersed in aNi/Cr solid solution. Such alloys possess high strength, excellent hot corrosion resistance and superior wear properties, making them attractive candidate materials for a wide range of high temperature applications, e.g., as wear resistant coatings in the hot sections of gas turbine and diesel engines (2-7). In the past, Ni/Cr and Ni-C&2 materials were produced by melting and casting methods, as well as blending and mixing of constituent phases into a composite. More recently, rapid solidification processing technologies, including powder atomization and melt spinning, were introduced to achieve finer and more homogeneous structures. In order to produce materials with ultrafine or nanoscale structures, alternative processing routes are required. In this paper, we report on a new chemical method for the synthesis of nanostructured Ni/ Cr and Ni-Cr& materials. The method involves reductive decomposition of an organic solution 857
858
TD XIAO,S TORBAN, PR Smurr.
AND
BH KEAA
of metal chloride precursors using sodium triethyl-borohydride. After coprecipitation of a nanoscale mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured NiKr alloy powder, or low temperature gas phase carburization to form nanostructured Ni-CrjCz cermet powder. 2. EXPERIMENTAL The starting materials for the powder synthesis were nickel chloride (NiClz), chromium chloride (CrC13) and a 1.0 M toluene solution of sodium triethyl-borohydride (NaBEtsH), obtained from Aldrich Chemical Inc. The toluene solvent was first dried over calcium chloride, refluxed, distilled, and then deoxygenated. All manipulations were carried out under an atmosphere of ultrahigh purity nitrogen (99.99% purity) using standard Schlenk techniques, because of the oxygen and moisture sensitivities of the starting compounds and the resulting nanophase powders (8-9). Nanostructured Ni/Cr powders were prepared with the following compositions: 100% Ni, 80% NiKr, 50% Ni/Cr, 20% Ni/Cr and 100% Cr. In a typical experiment, e.g., for 20%Ni/Cr, 16.43 gm of CrCls (0.1038 mol), 3.36 gm of NiC12 (0.0259 mol) were dispersed in 400 ml of deoxygenated toluene to form a purple suspension. This was carried out in a 2-liter 3-neck flask, equipped with a magnetic stirrer, and gas inlet/outlet tubes. The stirred solution was fist bubbled with a stream of nitrogen for 2 hours at room temperature. Then 400 ml of 1.OM toluene solution of NaBEtsH was added slowly to the halide mixture via a pressureequalized addition funnel. The reaction was carried out at room temperature under nitrogen. A black suspension slowly formed, while hydrogen gas evolved. The suspension was stirred for 48 hours at room temperature to ensure complete reduction of the halides. The black powder was filtered and washed with deoxygenated toluene under a stream of nitrogen. At this stage, no attempt was made to remove the sodium chloride byproduct. The high surface area reactive powder was passivated with deoxygenated mineral oil and stored in a Schlenk flask under nitrogen. Other compositions of the nanophase Ni-Cr powders were prepated in a similar manner. Heat treatment was performed using a high temperature environmentally controlledLindburg tube furnace. About 2 grams of the powder mixture was placed in an alumina boat inside an aluminatubeofinnerdiameter32mm. Priorto heattreatment,thechamberwasevacuatedtoabout 10m5torr (1.33 x 10m3Pa), flushed with argon repeatedly, and backfilled with argon to near ambient pressure. Heat treatment was carried out at 300600°C with a fume gas (W%Ar-3%Hz) flow rate of about 1 L/min“. The sample was first slowly heated to 300°C in 1.5 hours, and then heated to the desired peak temperature at a heating rate of 25°C/min‘1, with a dwell time at the heat treatment temperature of 1 hour. After heat treatment, the fme powder was suspended in distilled water and then centrifuged to separate the sodium chlorideand water from the nanophase Ni/Cr alloy powder. Nanostructured Ni-CrQ powder was prepared via a gas phase carburization technique (10). About 2 grams of the powder mixture was transferred into a tubular carburization unit equipped with vacuum pumps, pressure gauges, and inlet/outlet gas tubes. The carburization unit was first evacuated to about 5 m torr, flushed with Ar, and backfilled with methane to 200 torr. Carburization was accomplished at a temperature of 880°C and a pressure of 200 torr. The heating rate and the dwell time at the reaction temperature was 60°C/min and 1 hour, respectively. After carburization, the powder was dispersed in distilled water and then centrifuged to remove sodium chloride and water from the nanophase cermet powder.
SWTHESIS OFNilCr AND Ni-Cr&
I
1 ’
1
40 2
50
I
60
THETA
’
I’
70
POWDERS
1 ‘I ’ 0 NICKEL 3.CHROMIUM
80
90
(DEGREE)
Figure 1. XRD patternsfor nanocrystalline Ni/Cralloys with differentcompositions afterheattreatmentat 600°C. (Note:the 80%NiKralloy is at 700°C).
859
TD XIAO,S TOREIAN, PR STRUT-~, AND BH KEAR
0
I,,,I,,,I,,,I,,,I,~~I,0
20
40
60
80
100
ATOMIC % CHROMIUM Figure 2. Particle size of the nanocrystalline Ni (‘y-phase) and Cr (o-phase) as a function of alloy concentration.
The as-synthesized nanostmctured Ni/Cr alloy and Ni-Cr$& powders were characterized by x-ray dilfraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and inductively coupled plasma spectroscopy (ICP). 3. RESULTS 3.1 NanostructuredNilCr Powders Reduction of the metal halides with a slight excess of NaBEtsH resulted in the formation of an amorphous, black Ni-Cr powder that contained crystalline NaCl. After heat treatment at > 3OO”C,the powder consists of a nanocrystalline mixture of Ni/Cr alloy and NaCl. After washing using distilled water, only nanocrystalline Ni/ Cr alloy was detected. Figure 1 shows XRD patterns of the nanophase Ni/Cr alloy powders with different compositions, after heat treatment at 600°C. With Cr concentration below 50 mole%, only Ni is detected. The peaks correspond to fee Ni/Cr. With Cr concentration > 50 mole%, both Ni and Cr are detected. The Ni peaks correspond to fee Ni/Cr, and the Cr peaks to bee Cr. Values of the lattice constants for Ni/Cr and Cr are listed in Table 1. The lattice parameter of the nanocrystalline Ni powder is estimated to be a = 3.5264 A, which agrees well with the known bulk value for Ni (a = 3.5238 A>.With increasing Cr concentration horn 20 to 80 mole%, the lattice parameter gradually increases: a = 3.5437 A for 2O%Cr, a = 3.5457 A for 5O%Cr,and 3.55 13 A for 8O%Cr. The lattice parameter of the nanocrystalline Cr powder is determined to be a = 2.8854 A,which again is in
661
SYNTHESIS OFNi/Cr AND Ni-Cr3C2POWDERS
TABLE 1
Lattice Parameters of Nanocrystalliue Nickel (y) and Chromium (a) Phases Cr lattice Parameter
Composition
Ni lattice Parameter
lW%Ni
3.5264 A
___
20%Cr
3.5431 A
-__
50%Cr
3.5451 A
2.8951 A
80%Cr
3.5513 A ___
2.8908 8,
100%Cr
2.8854 A
Figure 3. Bright-field ‘EM microgmph and corresponding electron diffraction pattern of the nanocrystalline 80%Ni/Cr alloy.
862
TD XIAO,S TOIWAN,PR STFIUTT.ANDBH KEAFI
good agreement with the known bulk value for pure Cr (a = 2.8839 A). As the concentration of Ni increases from 0 to 50%, there is a slight increase of the Cr lattice parameter: a = 2.8908 A for 20%Ni, and a = 2.8957 for 50%Ni. The crystallite size of the nanophase powder was determined from x-ray peak broadening. Figure 2 shows the variation of particle size for nanocrystalline Ni/Cr alloy as a function of Cr concentration. The particle size of Ni/Cr (y-phase) varied from 10 nm to 18 nm, for example, 10 nm for lO%Ni, 13 nm for 80%Ni/Cr, 10 nm for SO%Ni/Cr, and 18 nm for 20%Ni/Cr. The particle size of Cr (a-phase) is also shown in Figure 2. Theparticle sizes of the a-phase Cr were determined to be 14 nm for lOO%Cr, 29 nm for 8O%Cr, and 21 nm for 5O%Cr. A typical TEM micrograph for the 80%Ni/Cr alloy powder is shown in Figure 3. A particle size distribution determined by image analysis was performed (seeFigure4). The average particle size of the nanocrystalline alloy was 12 nm. In order to identify the specific size of different Ni/ Cr and Cr particles, further work by combined high resolution TEM and local probe analysis is needed. Selected area diffraction of the powders showed the presence of fee Ni and bee Cr. The effects of heat treatment on the crystallinity and phase formation of the as-synthesized powders were also investigated. Here, we describe XRD results for two representative alloy compositions with 80%Ni/Cr and 2O%Ni/Cr. Figure 5 shows XRDresults for the 80%Ni/Cr alloy. The as-synthesized Ni/Cr powder is amorphous, The 300°C annealed sample is also amorphous. Histogram of Current Image Feature r.Length Unit :llIll Classes t 20 Underflow: 0 Overflow : 0 Count 70
Frames
1
:
Objects : Max Cnt :
tin Val Max Val Tot Sum Mean Std Dev
: : :
I :
434 70
1.6444 32.383 5319.9 12.258 5.3271
60 50 40 30 20 10 0I 0
.
r-l,
32.3827
Length Figure 4. Particle size distribution of the 80%Ni/Cr alloy obtained by computer image analysis of the TEM micrograph.
SYNTHESIS OF NiiCr
80%
r
AND
Ni-Cr&
Ni 20%
863
POWDERS
Cr
Ni (111)
Ni (200) Ni (220) 700 c
600 C
30
40
50
60
70
80
90
100
2 THETA (DEGREE) Figure 5. XRD patterns showing the effect of annealing on the as-synthesized 8O%Ni/Cr powder. At 400°C the XRD peaks for Ni start to appear, superimposed on a very diffuse background intensity. As the annealing temperature increases, the diffraction peaks sharpen and the diffuse background decreases. At 500°C and 600°C, there is an observable background intensity in combination with very well-definedNi crystalline peaks. At 7OO”C,there is no diffuse background and the Ni phase peaks am sharp; but no Cr phase is detected in this particular composition. The crystallite size of the Ni phase increases from 4 nm to 13 nm when the temperature is increased from 300°C to 6OO”C,and is about 20 nm at 700°C. One should notice that at lower annealing temperatures, for example, below 600°C, otber broad peaks are evident. At 400°C, there are two very broad peaks centered at about 28 = 37” and 62”. Relative intensities of these two peaks decrease as the annealing temperature increases, and they finally disappear at a temperature of 7OO’C. Figure 6 shows XRD results for 20%Ni/Cr. Once again, the as-synthesized powder is amorphous. At 300°C. only Ni peaks are detected. At higher temperatures, however, peaks for both Ni and Cr phases are observed. At 5OO”C,no amorphous background is observed, but there is phase separation of Ni and Cr. At this temperature, the XRD signal of the Ni phase is strong, but the XRD signal of the Cr phase is weak. At 6OO”C,well-defined XRD peaks of the Cr phase are visible. The crystallite size of the Ni phase increases from 4 nm to 20 nm when the annealing
TDXIAO, S TOFIBAN, PR STRUTT, AND BH KEAR
80% Cr 20% Ni _
Ni (111) Cr (110)
50
60
70
80
90
2 THETA (DEGREE) Figure 6. XRD patterns showing the effect of annealing on the as-synthesized 20%Ni/Crpowder. temperature increasesfrom 300°Cto f5OO”C, while the Crphase increasesfrom 2 nm to 15nm when the temperature increases from 500°C to 600°C. The alloy powders are extremely fme after heat treatment and they can be readily dispersed in a solvent. Even after several months of storage, they show no evidence for settling. After washing in distilled water, they could not be separated from NaCl and water because all of the particles passed through the microporous filter. The separation process, however, may be accomplished by agglomerating the nanoparticlesinto micron-sizeagglomeratesviacentrifuging, followed by filtering. Morphological examination by SEM shows that these agglomerates are several tens of microns in size. From all of the above results, it is concluded that the powders after centrifugal processing are micron sizeagglomeratesconsistingof an assemblageof nanocrystabine Ni/Cr alloy particles.The agglomeratescan be easilybroken down into their individual entities via ultrasonic agitation. 3.2 Nanostructured Ni-Cr$‘2 Powders Preliminary studies have been carried out to convert the nanostructured Ni/Cr alloys into nickel-chromium carbide cermets by heat treatment in flowing methane gas at 880°C. Of
SYNTHESIS OFNi/Cr AND Ni-Cr.&
POWDERS
665
n NICKEL 0 CHROMIUM
CARBIDE
n
30
40
50
60
70
80
90
2 THETA (DEGREE) Figure 7. XRD pattern of the nanocrystalline 20%Ni-CrQ
composite powder.
commercial interest are the high volume fraction carbide cermets. In our studies, therefore, most of the carburization experiments were performed on the 20%Ni/Cr alloy. As shown by the XRD analysis, Figure 7, this alloy composition resulted in the formation of a nanophase mixture of Ni and Cr3C2. Morphological examination using SEM revealed that the cermet powder had a porous structure, but detailed information could not be obtained due to the very small particle size. A typical TEM micrograph of the powder is shown in Figure 8, which reveals a size distribution of particles ranging from a few nanometers to several tens of nanometers. Particle size analysis from the bright field micrograph via an image processing technique, Figure 9, indicates that the average particle size of the Ni-CrQ powder is about 34 nm. Using x-ray line broadening analysis, particle size of the Ni phase is 18 nm, and that of the Cr3C2 phase is 30 nm. This discrepancy in particle size may be attributed to (i) overlapping particles not being taken into consideration in the image analysis technique, and (ii) presence of amorphous regions in the particle grain boundaries. Electron diffraction analysis confumed the presence of Ni and C&2 phases. The individual particle sizes of the two different phases could not be determined from the phase contrast in the TEM studies. Further work using high resolution TEM is needed to study the grain boundaries, as well as to identify the presence of phases other than Ni and Cr3C2.
TD XIAO,S TORBAN,PR Smum, ANDBH KEAR
Figure 8. Bright-field TEM micrograph and corresponding electron diffraction pattern of the 20%Ni-Cr3C2 composite powder. 4. DISCUSSION 4.1 syinthesisand Processing ?‘he reduction of metal halides into metal clusters of Ni and Cr may be represented by the followi Ing equation: MCI, + xNal3EtsH + M+ xNaC1 + xBEt3 + (x/2)H2
111
where M = Ni for x = 2 and M = Cr for x = 3. According to previous studies on the reduction of similar metal halides (9,1 l- 15), this reaction either may result from direct hydride transfer to form
Swn-~~s~s OFNi/CrAND
867
Ni-Cr.& POWDERS
Histogram of Current Image Feature : Length :lllll Unit Classes : 20 Underflow: 0 Overflow : 0
Frames Objects Max Cnt nin Val Max Val Tot Sum Mean Std Dev
Count 61
:
1
:
292
: : :
3.2586: 94.277
:
9834.2
t
33.679 10.178
t
50 I 40 30 20 10 0I -0
94.2769 Length Figure 9. Particle size distribution obtained from computer image processing of the TEM micrograph of the 20%Ni-CrsCz composite powder. 10
20
30
50
60
70
ametal hydride intermediate, which subsequently reductively eliminates HZ, or occurs directly by an electron-transfer mechanism. In the preparation of ultrafine scale metal alloys, metal halides havealsobeenused(16-17)asprecursorstoreactwith sodiumborohydride. In thelatter,however, reactions were carried out in an aqueous medium instead of using an organic solvent. The high surface area metal powders obtained from this reduction process are extremely air and moisture sensitive. Initial experiments were done to separate the nanoparticle metal powders from NaCl via extensive washing using deoxygenated water, followed by the application of a protective coating of deoxygenated mineral oil. XRD analysis revealed that the washed powders were highly disordered or amorphous, since only very broad Ni and Cr peaks were observed. Attempts to convert them into nanocrystalline form via high temperature heat treatment, however, resulted in the formation of a greenish powder, which consisted of Ni and Cr203 phases. In the actual experiment, we observed that once the furnace temperature reached 60°C there was a sudden temperature increase up to 3OO’Cin a few minutes, although the heating rate was only set at a few degrees per minute. From these observations, combined with analytical results, it is concluded that the O-H groups of water molecules are initially physisorbed onto the metal surface. Upon slow heating, these O-H groups are gradually transformed into chemisorbed species, which then react with the metal powders to form Cr203, instead of being evaporated away in the heat treatment process, Figure 1Oa.
TD XIAO,S TOWAN,PR STRUTT, AND BH KEAFI
Ph
(a) phvsisorbed OH
chemisorbed OH
phvsisorbed C-H
chemisorbed C-H
carbide Iaver
Figure 10. Schematic diagram for the mechanisms of(i) oxidation of the Ni/Cr alloy powder after washing in deoxygenated Hz0 prior to heat treatment, and (ii) surface carburization of the Ni/Cr alloy after passivation with mineral oil prior to heat treatment.
Nickel
(weight
%I
40
60
80
I
I
I
40 Nickel
60 (atomic
80 %I
100 Ni
Figure 11. Equilibrium phase diagram of Ni/Cr alloy system (courtesy of D.R.F. West).
SYNTHESIS OFNi/Cr AND Ni-Cr.& PGWDERS
869
To avoid this oxidation problem, the as-synthesized powders were coated with deoxygenated mineral oil before washing. In this case, C-H groups derived from the mineral oil and residual toluene are first physisorbed onto the metal surfaces. As shown in Figure lob, these physisorbed C-H groups gradually transform into chemisorbed species during heat treatment, and then react with the metal surfaces to form thin layers of protective carbide phase. Since these layers are extremely thin, XRD analysis was not able to detect them. However, ICP analysis indicated that there was an appreciable carbon content in the sample. It should be noted here that during heat treatment of these materials, the initial heating rate should be extremely slow when the furnace temperature is below 300°C. Otherwise, the C-H groups will be partially evaporated, and this will create problems of slow burning of the sample upon exposure to air after heat treatment. Thus, it appears that the transformation of the physisorbed C-H groups into chemisorbed species is very important for the passivation of the nanocrystalline Ni/Cr alloys. 4.2 Phase Formation The established Ni-Cr phase diagram is shown in Figure 11. At a composition < 36 at.% Cr. only y-phase is present at 600°C. The pphase is at the Ni-rich region, where Cr atoms are substitutional. The a-phase is at the Cr-rich region, where only a very small amount (c 2%) of Ni solute atoms am substitutional. In the nanocrystalline Ni-Cr system, we detected only fee Ni XRD signals (see Figure 1) at lOO%Ni and 2O%Cr, with an increased lattice parameter for the 2O%Cr sample. This data agreed with the established phase diagram for conventional micron-sized Ni/ Cr, since these compositions are at the single phase field (y-phase). At higher concentrations of Cr. for example, 50%Cr and 8O%Cr, both y-phase and a-phase were evident, as predicted by the phase diagram. This data illustrates that our experimental findings for the nanocrystalline Ni-Cr system agree very well with the established phase diagram for the conventional microcrystalline Ni-Cr system. Figure 12 is a comparison of the lattice parameters for conventional microcrystalline Ni/Cr alloys (Floyd and Taylor) (18-19) and the corresponding nanocrystahine Ni/Cr alloys. In the experiments of Floyd and Taylor, the alloys of various compositions were annealed at 1100°C for six days. The samples were then cooled by (i) rapid quenching to room temperature, and (ii) slow cooling to room temperature by furnace cooling. The lattice parameter vs. composition plots for both sets of the samples had a similar slope up to 45%Cr, which is indicative of the presence of only one phase, the y-phase. In this study, it was found that at low concentrations of Cr, for example, O%Cr and 2O%Cr, the measured lattice parameters agreed well with the data obtained by Floyd and Taylor. However, a slope change was evident at higher concentrations of Cr. It seems that the nanophase alloy has already reached its solubility limit in the neighborhood of 20%Cr at a temperature of 600°C. Upon increasing the concentration of Cr, phase separation into y and a-phases occurs. This effect is not understood at the present time.
5. CONCLUSIONS We have developed a process for the synthesis of nanocrystalline Ni/Cr and Ni-CrsC2 powders by an organic solution reaction method. The technique involves reductive decomposition of an organic solution of metal chloride precursors using sodium triethyl-borohydride. After
TDXmo,S
870
TOR~AN.PRSTR~IT, AND BH KEAR
3.58 , A A
I9 0
0
‘9 @I
A Taylor & Floyd (Quenched)
l Nanocrystalline 0
Taylor & Floyd (Slow Cool)
Figure 12. Comparison of lattice parameters of nanocrystalline Ni/ Cr alloys with the data obtained by Taylor and Floyd for the conventional Ni/Cr alloys. co-precipitation of a nanoscale mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured Ni/Cr alloy powder, or low temperature gas phase carburization to form nanostructured NiCr3C2 cermet powder. The co-precipitated material is a mixture of amorphous Ni and Cr powders. After heat treatment at >3OO”C,nanocrystalline Ni/Cr alloy powders are obtained. Carburization of the as-synthesized powder in methane gas results in the formation of nanocrystalline Ni-CQQ. The results for the synthesis of nanocrystalline Ni/Cr alloys are consistent with the established phase diagram for Ni/Cr alloys. At low concentrations of Cr (below 2O%Cr), only fee Ni (y-phase) is detected. At higher concentrations of Cr, for example, 508Cr and 8O%Cr, both y-phase and u-phase are evident. Also, the crystal lattice of Ni increases with increasing Cr concentration. ACKNOWLEDGMENTS The authors would like to thank the Advanced Technology Center for Precision Manufacturing (PMC) of theuniversity of Connecticut for fmancial supportfor this project.Thanks are also due to Dr. R.K. Sadangi of the Department of Ceramics of Rutgers University for help with many of the experiments and valuable discussions on the carburization experiments.
SYNTHESIS OF NilCr ANDNi-Cr.& POWDERS
871
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
D.R.F. West, Ternary Equilibrium Diagrams, 2nd ed., Chapman & Hall, New York, p. 94-99 (1982). G.R. Kegg and J.M. Silcock, Metal Science Journal 6,47 (1972). J.J. Wert, D.M. Baker, and T.M. McKechnie, Surface & Coating Technology 3,245 (1987). D.L. Houck and R.F. Cheney, International Patent No. PCT/US82/ 0 1653 ( 198 1). B.J. Lee, Calphad fi (2), 121 (1992). H. Grammont and T. Puig, J. de Physique IV-Colloque C7, supplement au Journal de Physique III 1, c7-77 (1991). J.K. Dennis, S.T. Sheikh, and E.C. Silverstone, Trans. Institute of Metal Finishing s, 118 (1981). D.F. Shriver and M.A. Drezden, The Manipulation of Air-Sensitive Compounds, 2nd ed., Wiley-Interscience, New York, p. 78 (1986). D. Zeng and M.J. Hampden-Smith, Chem. Mater. 2 (5), 681 (1993). L.E. McCandlish, B.H. Kear and B.K. Kim, Nanostruct. Mater. 1,119 (1992). H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, and B. Korall, Angew. Chem. Int. Ed. Engl. a, 1312 (1991). R.D. Rieke and S.E. Bales, J. Am Chem. Sot. p6 (6), 1775 (1974). R.D. Rieke, Science m,l260 (1989). K.L. Tsai and J.L. Dye, J. Am. Chem. Sot. m, 1650 (1991). O.F. Schall, K. Robinson, J.L. Atwood, and G.W. Cokel, J. Am. Chem. Sot. m, 5962 (1993). K.E. Gonsalves, T.D. Xiao, G.M. Chow, and C.C. Law, Nanostruct. Mater. 4, 139 (1994). G.M. Chow, T. Ambrose, J.Q. Xiao, F. Kaotz, and A.M. Ervin, Nanostruct. Mater. 2, 131(1993). A. Taylor and R.W. Floyd, J. Inst. Met. a, 577 (1952). A. Taylor and K.G. Hinton, J. Inst. Met. 81, 169 (1952).