Microwave and Conventional Sintering of Premixed and Prealloyed Tungsten Heavy Alloys

A. Mondal, A. Upadhyaya

Indian Institute of Technology Kanpur, Uttar Pradesh, India

D. Agrawal

The Pennsylvania State University, State College, Pennsylvania, USA

Key words: Microwave Sintering; Conventional Sintering; Tungsten Heavy Alloys.

Abstract

Microwave sintering is fundamentally different from conventional sintering. In the microwave sintering process, the material is heated internally and volumetrically unlike in a conventional process where heat originates from an external heating source. Sintering cycle time for microwave sintering is much shorter as compared with the conventional sintering cycle. The rapid heating rate of microwave sintering greatly restricts tungsten grain coarsening, which resulted in fine tungsten grains distributed in Ni-Cu matrix. The aim of the present investigation is to study the sintering behavior of premixed and prealloyed 90W-7Ni-3Cu powders in both microwave and conventional furnaces. A comparative analysis has been made based on the sintered density, densification parameter, hardness and microstructures of the samples. The present investigation also includes the variation of matrix composition as a function of temperature by EPMA analysis.

Introduction

Tungsten-nickel-copper alloys are widely used for ordnance application and for electrical contacts of switches, radiation shielding, mass balances etc. Tungsten heavy alloys have been processed through powder metallurgy route since 1930’s [1]. These heavy alloys contain mainly pure tungsten as principal phase in association with a binder phase comprising of transition metals (Ni, Fe, Cu, Co) [2]. Price et al. [3] were the first to propose Ni-Cu as the binder for tungsten heavy alloys. They had shown that highly dense W-Ni-Cu heavy alloys could only be obtained by liquid phase sintering.

Subsequently, researchers studied the fully dense W-Ni-Cu composites fabricated by solid state sintering. Solid state activated sintering of tungsten powder with various Ni-Cu additions was first reported by Brophy et al. [4]. Their study was mainly confined to densification mechanism. They proposed that shrinkage occurs during the initial two stages of sintering: (i) rearrangement and (ii) solution-reprecipitation stages. The role of phase relationships on the activated sintering of tungsten was studied by Prill et al. [5]. They proposed that at lower Ni:Cu content, the acceleration of sintering of tungsten diminishes. Kothari [6] studied the densification and grain growth kinetics of W-Ni-Cu heavy alloys with different Ni:Cu ratios. The rate constant for the sintering process was evaluated from the volume change

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Paradigm Shift in the Metals Industry

as a function of sintering period. The activation energy was found to be independent of binder composition. Grain growth rate was proportional to the sintering period. Makarov et al. [7] studied the coalescence phenomenon in W-Ni-Cu alloys during liquid phase sintering. The effect of tungsten and copper powder size variation on the properties of W-Ni-Cu heavy alloys was carried out by Srikanth et al. [8, 9]. They found enhanced sintered properties with finer particle size of the constituent with increasing nickel content in the binder. The effect of composition and sintering temperature on the densification and microstructure of W-Ni-Cu heavy alloys was studied by Ramakrishanan et al. [10]. Kuzmic [11] proposed that rapid cooling from the sintering temperature prevents the formation of brittle phase in order to obtain good mechanical properties. Ariel et al. [12] correlated the mechanical properties of W-Ni-Cu system with sintered microstructure. Their study showed that the mechanical properties are a function of mean free path between tungsten grains, volume fraction of tungsten grains and the contiguity of tungsten spheroids.

The solubility of tungsten in the liquid binder plays a dominant role in sintered W-Ni-Cu alloys. The solubility of W in the Ni-Cu additive can be regulated by an appropriate selection of Ni:Cu ratio. Nowadays, W-Ni-Cu alloys are also being consolidated by employing prealloyed powders [13]. Use of prealloyed powder improves homogeneity. The homogenization process accelerates sintering and promotes densification. The alloy formation during sintering decreases the material viscosity and, hence, stimulates material flow under the action of capillary forces [14]. In the binary Ni-W system either solid tungsten or a Ni-based solid solution can co-exist in equilibrium with a liquid phase [15, 16], which depends upon temperature and relative amount of components in the system.

Despite widespread application, difficulties still exist in the manufacturing of liquid phase sintered tungsten heavy alloys. Large dimensional changes take place during sintering. The linear shrinkage can be as large as 20%, which causes slumping. The distortion behavior of 80W-20 (Ni, Cu) alloy was studied by Upadhyaya [17] who found that high dihedral angle minimizes compact slumping.

In order to avoid thermal shock, processing of tungsten heavy alloys in conventional furnace involves heating at a slower rate (<10°C/min) and with an isothermal hold at intermittent temperatures. This not only increases the process time, but also results in significant microstructural coarsening during sintering, leading to the degradation of mechanical properties. This problem is further aggravated when the initial powder size is extremely fine. Hence, it is envisaged that a fast heating rate would mitigate this problem. One of the techniques to achieve fast and relatively uniform sintering is through microwaves [18]. Microwave heating involves energy conversion whereas conventional heating involves energy transfer. Microwave heating is much more uniform at a rapid rate resulting in reduction of processing time and energy consumption. Also rapid heating leads to finer microstructure enhancing the mechanical properties. Clark and Sutton [19] cited many other benefits of the process such as precise and controlled heating, environmentally friendly etc. Microwave heating is a very sensitive function of the material being processed and depends on several factors, such as sample size, its mass and geometry [20]. Though there have been attempts to explain microwave heating of metal powders, still there is not yet any consensus on a comprehensive theory to explain the mechanism [21].

Applicability of microwave sintering to metals was ignored due to the fact that they reflect microwaves. Roy et al. [22] reported that particulate metals can be heated rapidly in microwaves. This has led to the use of microwaves to consolidate a range of particulate metals

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and alloys [23-25]. While researchers have reported microstructural refinement due to rapid heating in microwaves, the effect of microwave sintering on the microstructural homogeneity and mechanical properties seem to be system specific [23-25]. Researchers have also attempted to consolidate refractory materials such as pure W and Mo, W-Ni-Fe and W-Ni-Cu [26-29] using microwave energy and reported significant reduction in process time, elimination of brittle intermetallic formation and superior mechanical properties. The aim of the present investigation is to study the sintering behavior of premixed and prealloyed 90W-7Ni-3Cu powders in both microwave and conventional furnaces. The comparative analysis has been based on the sintered density, densification parameter, hardness and microstructures of the samples.

Experimental Procedure The as-received powders were characterized for their size, size distribution and

morphology. Table 1 summarizes the characteristics of the as-received elemental W, Ni, Cu and prealloyed Ni-Cu powder used for this study. Figure 1 shows the scanning electron micrographs of the powders in as-received condition. The chemically reduced tungsten powder has irregular (faceted polyhedral) morphology. Nickel powder, which was produced by carbonyl decomposition, resulted in spherical shape; but with a spiky appearance. The alloy compositions were prepared by mixing the required proportions of each powder in a turbula mixer (model: T2C Nr 921266, supplier: Bachofen AG, Switzerland) for 60 min. Mixing ensures complete homogeneity in the powder mixture. For investigating the densification response and compaction studies of various compositions, cylindrical pellets (diameter: 1.6 cm and height 0.8 cm) were pressed at 200 MPa using a uniaxial semi-automatic hydraulic press (model: CTM 50, supplier: FIE, India) of 50 T capacity. To facilitate compaction and subsequent removal of compacted samples, zinc-stearate powder was applied as die-wall lubricant. The as-pressed green compact density varied from 56% to 58% of the theoretical density of the alloy, where the theoretical density, Tρ , was calculated using the inverse rule of mixtures as follows:

∑=

=N

i i

i

T

w1

1ρρ

(1)

Where, N is the number of elements in the mixture, iw is the weight fraction of the ith component, and iρ the theoretical density of the ith element. The formula assumes that the individual components do not react and are not soluble in each other.

To study the densification behavior; the green (as-pressed) compacts were sintered using conventional and microwave furnaces. The conventional sintering was carried out in a MoSi2heated horizontal tubular furnace (model: OKAY 70T-7, supplier: Bysakh, Kolkata, India) at a constant heating rate of 5°C/min. To ensure uniform temperature distribution during heating, intermittent isothermal hold for 30 min. was provided at 500°C, and 900°C. The sintering temperatures selected for the liquid phase sintering of 90W-7Ni-3Cu alloys at 1300°C and 1450°C. Sintering was carried out for 60 min. in flowing H2 atmosphere with dew point -35°C. Microwave sintering was carried out using a multi-mode. 2.45 GHz, 6 kW microwave furnace. The temperature of the sample was monitored using an optical pyrometer (Leeds & Northrup Co., Philadelphia, PA) through a small quartz window at the top of the furnace. The sintered density was obtained by both dimensional measurements as well as Archimedes’ density measurement technique. To take into account the influence of the initial as-

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pressed density, the compact sinterability was also expressed in terms of densification parameter which is calculated as follows:

Densification parameter = (sintered density - green density)(theoretical density - green density)

(2)

Table 1: Powder characteristics of as received elemental W, Ni, Cu and prealloyed Ni-Cu powder

Property W Ni Cu 69Ni-31Cu

Supplier Widia Inco ACu Powder International, LLC Ametek

Processing techniquechemical reduction carbonyl process gas atomization gas atomization

Powder shape irregular spiky spherical spherical

powder size, µm

D10 2.0 3.8 19.4 8.2

D50 4.2 11.0 47.0 17.3

D90 6.2 31.8 96.3 40.4

To compare the densification response of various compositions, the sintered densities were normalized with respect to the theoretical density. The formulae used for calculation of axial ( Aδ ) and radial shrinkage ( Rδ ) of the sintered cylindrical compacts are given below.

%1001 ×

−=

g

sA h

hδ (3)

%1001 ×

−=

g

sR r

rδ (4)

Where, gh and sh are heights of green and sintered compact; gr and sr are radii of the green and sintered compact, respectively. An Anter 1161V vertical dilatometer was used for measuring axial shrinkage and shrinkage rate during constant heating rate sintering of premixed and prealloyed 90W-7Ni-3Cu compacts. It measures dimensional changes over the entire sintering cycle with a precision of 1µm. The dilatometery studies were done at constant heating and cooling rate of 10°C/min. Metallographic techniques were employed on the sintered samples. The sintered samples were wet polished in a manual polisher (model: Lunn Major, supplier: Struers, Denmark) using a series of 6 µm, 3 µm and 1 µm diamond paste, followed by cloth polishing using a suspension of 0.04 µm colloidal SiO2 suspension. The scanning electron micrographs of as-polished samples were obtained by a scanning electron microscope (model: FEI quanta, Netherlands) in the secondary electron (SE) mode. In the present study, quantitative analysis was carried out on selected specimens using an EPMA (model: JXA-8600 SX Super-Probe, supplier: JEOL, Japan) equipment. Phase determination and phase evolution, if any were studied for all the samples with the help of X-Ray Diffractometer (model: Rich. Seifert & Co., GmbH & Co. KG, Germany).

Bulk hardness measurements were performed on polished surfaces of sintered cylindrical compacts at a load of 5 kg using Vickers hardness tester (model: V100-C1, supplier: Leco, Japan). The load was applied for 30 s. Micro hardness tester (model: 8299, supplier: Leitz, Germany) was used to evaluate the hardness of tungsten grains and the matrix phase. The loads

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applied on tungsten grain and matrix phase during the micro hardness measurement were 50 g and 15 g, respectively.

Figure: 1 SEM micrograph of as received elemental W, Ni, Cu and prealloyed Ni-Cu powder respectively.

The bulk hardness and the micro hardness values of tungsten grain and matrix phase for each sample were an average of five readings taken at different locations on respective phases.Transverse rupture strength (TRS) measurements of premixed and prealloyed samples were performed following the procedure described in MPIF Standard 41 [30].

Results and Discussion

Heating Profiles

Figure 2 compares the thermal profiles for both the compositions heated in both conventional and microwave furnaces. From Fig. 2, it is quite evident that both the premixed and prealloyed 90W-7Ni-3Cu samples couple with the microwave quite well and do get heated up fast. Interestingly, the prealloyed samples start to heat up slightly faster than the premixed samples, which could possibly be attributed to different levels of coupling of the samples due to the ‘anisothermal heating’. Roy et al. [31] attributed the ‘microwave effect’ to anisothermal heating in the case where the system comprises of more than two different components having different microwave absorption factors that causes the local temperature gradients leading to

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enhanced reaction/heating rates. Hence anisothermal heating may play a major role in enhancing the sintering kinetics in multi-phase system component.

Figure: 2 Change in thermal profile as a function of heating mode of premixed and prealloyed 90W-7Ni-3Cu alloys after sintering in H2 atmosphere.

In the case of premixed samples the mixed elements, W, Ni and Cu, would possibly cause ‘anisothermal’ heating leading to changes in the sintering kinetics. In the case of prealloyed samples (W + Ni-Cu) the Ni and Cu elements are alloyed and hence the sintering kinetics would be different than the premixed alloy and would be much higher than conventionally sintered samples. In case of microwave heating, temperature could only be measured from 700°C onwards. However, it takes about 20 to 30 min. depending on initial power settings and compositions to heat up the compacts from room temperature to 700°C. The overall heating rates in microwave furnace were 22°C/min. Taking into consideration the slow heating rate (5°C/min) and isothermal holds at intermittent temperatures in conventional furnace, there is about 70 to 80% reduction in the process time during sintering of all the compositions in microwave furnace. Despite such a fast heating rate, no micro- or macro-cracking was observed in all microwave-sintered samples. This underscores the efficacy of volumetric heating associated with microwaves. Furthermore, for both the heating modes, no distortion was observed in the sintered samples.

Densification Response

Sintering of premixed and prealloyed 90W-7Ni-3Cu alloys

90W-7Ni-3Cu samples were sintered at 1300°C and 1450°C, in flowing H2 atmosphere in both conventional as well as microwave furnaces. For both the compositions densification is achieved without any concomitant shape distortion.

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Figure: 3 Change in sintered density and densification parameter as a function of heating mode of premixed and prealloyed 90W-7Ni-3Cu alloys after sintering in H2 atmosphere.

Figures 3a and b show the effect of heating mode upon the sintered density and densification parameter of both premixed and prealloyed compacts. For all the temperatures, the sintered density of prealloyed compacts is slightly higher than that of premixed ones. Radial shrinkage is always higher than the shrinkage in axial direction as observed in this experiment. Anisotropic shrinkage can be attributed as the die compaction involves uniaxial straining. Although necks do develop in the transverse direction, their size is smaller than the size of the necks in the axial direction. Such difference in neck size will result in substantially different grain boundary fluxes at the tips of the two necks and different shrinkage strains. Although anisotropy in sintering can originate from other sources including prior compaction, elongated particles, pore gas pressure, interface porosity and crystallographic texture [32]. Densification parameter measurement shows the similar trend as of densification.

Dilatometry

Dilatometry experiments were performed to identify the temperatures at which significant variations in densification take place. Figure 4 shows compact shrinkage as a function of temperature for both premixed and prealloyed 90W-7Ni-3Cu compact at a constant heating rate of 10°C/min. A slight expansion was observed between 850°C and 1000°C in the dilatometry traces corresponding to the prealloyed 90W-7Ni-3Cu compositions. It happened between 600ºC and 800ºC for W-Cu. Although its origin has not been investigated in this work, a previous research [33] reported the same effect for W-Cu composites fabricated with atomized Cu powder. According to their explanation, the water atomized Cu powder contains some internal porosity and these pores are filled with a gas that includes a certain amount of oxygen. When the compact is heated in hydrogen, the H2 diffuses inside the Cu particles and reacts with the O2 producing water vapour, which cannot diffuse out easily. The generated overpressure is

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able to plastically deform the Cu particles increasing the size of the pores, of the Cu particles, and hence of the whole compact.

Figure: 4 Change in linear a) shrinkage and b) shrinkage rate as a function of temperature of premixed and prealloyed 90W-7Ni-3Cu alloys compacts sintered to 1400°C at a rate of 10°C/min in H2 atmosphere.

This explanation may also hold for the prealloyed 90W-7Ni-3Cu composition. Consequently, further research must be undertaken to ascertain the cause of swelling. Additionally, shrinkage by solid state sintering is observed in both the premixed and prealloyed 90W-7Ni-3Cu sample between ~1050°C and ~1282°C. The dilatometry curves exhibit an increase in slope around 1282ºC for the both 90W-7Ni-3Cu sample, which could be due to the beginning of the additive phase melting. The actual solidus and liquidus temperatures of 69Ni-31Cu powder measured by DTA are ~1295°C and ~1355°C, respectively [13]. The lower experimental values can be attributed to the impurities that the powder contains. The melting of the additive speeds up all the processes that were already occurring in the solid state. As W is soluble in the liquid, the W-W necks can be broken by dissolution and rearrangement of W particles can take place, which accounts for the greater shrinkage rate observed in the dilatometry graphs. Now, the amount of liquid is increasing continuously with temperature. Additionally, the liquid moves very rapidly by viscous flow under the action of capillary forces, so its spreading on the W particle surfaces is also accelerated, increasing the W-liquid interface area and reducing the diffusion distances. Moreover, the diffusion coefficient of the W in the additive is dramatically augmented from ~10-

10cm2/s in the solid additive, to ~10-5cm2s in the liquid at ~1350°C for pure Ni, [34, 35] providing a quicker path for mass transportation.

Macro and Microstructure Study

Visual inspection as well as dimensional measurement at various locations ensures that there was no evidence of distortion or slumping of the compacts in both heating modes. In spite of rapid heating in microwave there was no evidence of cracks in the sintered samples. Microstructural observation was made in order to confirm the effect of heating mode on microstructural homogeneity and grain coarsening.

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Figure: 5 SEM micrograph of premixed and prealloyed 90W-7Ni-3Cu a, b) conventional and c, d) microwave sintered at 1300°C, 60 min, in H2.

Figure 5 represents sintered microstructures of conventionally and microwave sintered premixed and prealloyed 90W-7Ni-3Cu samples. Significant microstructural differences have been observed between conventional and microwave sintered samples at 1300°C for both premixed and prealloyed compositions. From the microstructures, it was noticed that in conventional samples W particles maintain their original faceted morphology at 1300°C and there was no spheroidisation of W particles. Conventionally sintered micrographs show a non homogeneous distribution and higher residual porosity. Whereas microwave sintered samples show a major change in the shape of W particles in the microstructure for both premixed as well as prealloyed compositions. Figures 5c and d show spherical morphology of W particles distributed in the Ni-Cu matrix, which is quite desirable to achieve better mechanical properties for these alloys. This significant difference in microstructure can be attributed to the fact that tungsten spheroid formation is manifested only under those conditions wherein the melt formation occurs in sufficient quantity. In case of microwave sintering the measured surface temperature during heating of a multi-phase system is an average temperature of all the phases, where as the actual temperature of a particular phase which is Ni-Cu in this case, may be much higher than the average value due to anisothermal heating as explained earlier. Because of this phenomenon it is quite possible that the amount of liquid phase is higher in microwave sintering (which helps the tungsten spheroid formation) than in the conventional sintering.

Figure 6 shows the micrographs of premixed and prealloyed compositions sintered at 1450°C. At this temperature no significant differences have been observed except grain size. All the micrographs show spheroidal solid grain embedded in a liquid matrix. Microwave sintered samples show a finer particle size distributed in the matrix. There was no significant difference in grain size observed between premixed and prealloyed compositions.

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Figure: 6 SEM micrograph of premixed and prealloyed 90W-7Ni-3Cu a, b) conventional and c, d) microwave sintered at 1450°C, 60 and 30 min, in H2 respectively.

Electron Probe Micro Analysis (EPMA)

Table 2 shows the EPMA analysis of the matrix of conventional and microwave sintered premixed and prealloyed 90W-7Ni-3Cu samples. In conventionally sintered alloys the matrix contains higher wt. % W. The matrix of premixed samples is richer in tungsten content than prealloyed matrix.

Table 2: Variation in matrix composition as a function of heating mode of premixed and prealloyed 90W-7Ni-3Cu compacts after sintering at 1450°C in H2 atmosphere.

Heating Mode 90W-7Ni-3Cu-PM 90W-7Ni-3Cu-PA

Conventional 28.6±2.4 25.0±1.3

Microwave 24.2±1.8 20.6±1.0

XRD Analysis

Figure 7 compares the XRD patterns of both the compositions. But it is interesting to note that although the sample size and other parameters were constant during the experiments still relative intensities were lower in microwave sintered samples than conventional one in case of premixed composition. Further, in microwave samples sintered at 1450°C, no crystalline Ni-Cu phase was observed and this phenomenon is more predominating in prealloyed composition. This means that Ni-Cu remains in amorphous state after microwave sintering. This is no definite

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explanation for this observation except that due to short cycle time there is not enough time for the liquid phase to crystallize while cooling.

Figure: 7 XRD pattern of conventionally and microwave sintered of premixed and prealloyed 90W-7Ni-3Cu alloys.

Properties

Table 3 summarizes the bulk hardness measurement of both the compositions. Vickers hardness measurement of all the samples showed that irrespective of the composition microwave sintered samples exhibit higher hardness than their respective conventionally sintered counterparts. This can be attributed to the relatively higher sintered density noted in microwave sintering and smaller grain size.

Table 3: Bulk hardness of sintered compacts as a function of heating mode of premixed and prealloyed 90W-

7Ni-3Cu alloys 90W-7Ni-3Cu-PM 90W-7Ni-3Cu-PA

Heating Mode 1300°C 1450°C 1300°C 1450°C

Conventional 276±21 301±16 290±19 330±13

Microwave 309±17 328±13 330±18 350±12 Table 4: Micro hardness of the matrix as a function of heating mode of premixed and prealloyed 90W-7Ni-

3Cu alloys

Tables 4 and 5 summarize the micro hardness of the matrix phase and transverse rupture strength of both the compositions. Lower micro hardness in the matrix phase in case of microwave at 1450oC can be attributed to reduced amount of W solubility in the Ni-Cu matrix which was observed in EPMA analysis, which ultimately results in lower solid-solution

90W-7Ni-3Cu-PM 90W-7Ni-3Cu-PA Heating Mode

1300°C 1450°C 1300°C 1450°C

Conventional 260±20 333±13 250±25 320±16

Microwave 290±23 312±15 280±21 290±12

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hardening. Higher transverse rupture strength in microwave sintered samples can be attributed with the spheroidal solid grain embedded in the liquid matrix which ultimately provides higher mean free path for crack propagation.

Table 5: TRS of the sintered compacts as a function of heating mode of premixed and prealloyed 90W-7Ni-3Cu alloys

Conclusions

Compacts prepared by using both premixed and prealloyed 90W-7Ni-3Cu alloys were sintered in a microwave furnace for temperatures corresponding to liquid phase sintering. The conclusions of the results are as the following.

As compared to conventional sintering, both premixed and prealloyed 90W-7Ni-3Cu alloys were microwave sintered in significantly less time. Microwave heating lowers the sintering temperature. Hardness of the premixed and prealloyed microwave sintered samples is higher than the corresponding conventional samples. In spite of higher heating rate in microwave sintering, no micro- or macro-cracking was observed in all microwave-sintered samples. This underscores the efficacy of volumetric heating associated with microwaves. Finer particle size and more uniform microstructure can be obtained by using microwave energy for both the compositions. The transverse rupture strength was significantly higher in microwave sintered samples than the conventional samples due to superior microstructure obtained by microwave heating.

Acknowledgement

This collaborative research was done as a part of the Center for Development of Metal-Ceramic Composites through Microwave Processing established under the Indo-US Science and Technology Forum (IUSSTF), New Delhi, and partially supported by Halliburton Corp. and Microwave Processing and Engineering Center, Penn State University, USA.

References

1. S.G. Caldwell, Tungsten Heavy Alloys, ASM Hand Book, P.W. Lee and R. Lacocca (eds.), Powder Metal Technologies and Applications, Materials Park, Ohio, USA, v. 7, 1998, p. 914 –921.

2. A. Upadhyaya, Processing Strategy for Consolidating Tungsten Heavy Alloys for Ordnance Application, Met. Chem. Phy., n. 67, 2001, p.101-110.

3. G.H.S. Price, C.J. Smithells, and S.V. Williams, Sintered alloys. Part I. Copper-Nickel-Tungsten Alloys Sintered with a Liquid Phase Present, J. Inst. Met, vol. 62, 1938, p. 239-264.

90W-7Ni-3Cu-PM 90W-7Ni-3Cu-PA Heating Mode

1300°C 1450°C 1300°C 1450°C

Conventional 484±20 1346±24 710±16 1338±19

Microwave 1507±23 - 1670±17 -

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4. J.H. Brophy and A.L. Prill, A Reanalysis of Data on the Solution-Reprecipitation Stage of Liquid Phase Sintering, Transaction AIME, vol. 236, 1966, p. 85-91.

5. A.L. Prill, The Role of Phase Relationships in the Activated Sintering of Tungsten, Transaction AIME, vol. 230, 1964, p. 769-772.

6. N.C. Kothari, Densification and Grain Growth during Liquid Phase Sintering of Tungsten-Nickel-Copper alloys, J. Less-Comm. Met, vol. 13, 1967, p. 457-461.

7. R. Makarov, O.K. Teodorovich, and I.N. Fruntsevich, The Coalescence Phenomena in Liquid Phase Sintering in the Systems W-Ni-Fe and W-Ni-Cu, Sov. Powder Metall. Met. Ceram., vol. 4, 1965, p. 554-559.

8. V. Srikanth and G.S. Upadhyaya, Properties of Sintered Heavy Alloys, Int. J. Refract. Hard Met., vol. 2, 1983, no 3. p. 49-54.

9. V. Srikanth and G.S. Upadhyaya, Effect of Tungsten Particle Size on Sintered Properties of Heavy Alloys, Powder Technology, vol. 39, 1984, p. 61-65.

10. K.N. Ramakrishnan and G.S. Upadhyaya, Effect of Composition and Sintering on the Densification and Microstructure of Heavy Alloys Containing Copper and Nickel, J. Mate. Sci. Lets., vol. 9, 1990, p. 456-459.

11. J.F. Kuzmic, Modern Development in Powder Metallurgy, Ed. H.H. Hausner, Plenum Press New York, vol. 3 , 1966, p. 166-171

12. E. Ariel, J. Batra, and D. Brandon, Activated Sintering, Powder Metall. Inter., vol. 5, 1973, p 125-128.

13. J. M. Martin, John L. Johnson, Randall M. German, and F. Castro, Microstructural Evolution of Tungsten Heavy Alloys during Heating, San Diego 2006.

14. A. P. Savitskii, Scientific Approaches to Problems of Mixtures Sintering, Sci. Sintering,v. 37, 2005, p. 03-17.

15. M. Hansen and K.P. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York 1957.

16. V.N. Eremenko, R.V. Minakova, and M.M. Churakov, Solubility of Tungsten in Copper-Nickel Melts, Sov. Powder Metall. Met. Ceram., vol. 16, 1977, p. 283-286.

17. A. Upadhyaya, A Microstructure-Based Model for Shape Distortion during Liquid Phase Sintering, Ph.D.Thesis, Penn State, USA, 1998.

18. K.J. Rao and P.D. Ramesh, Use of Microwaves for the Synthesis and Processing of Materials, Bull. Mater. Sci., 1995, v.18, n.4, p. 447-465.

19. D.E. Clark and W. H. Sutton, Microwave Processing of Materials, Ann. Rev. Mater. Sci.,v. 26, 1996, p. 299-331.

20. R. M. Hutcheon, M. S. De Jong, F. P. Adams, Microw: Theor. Appl. Mater. Proc. III, Ceram. T., Am. Ceram. Soc., vol. 59, 1995, p. 215.

21. M. W. Porada, Microw: Theor. Appl. Mater. Proc. III, Ceram. T., Am. Ceram. Soc, vol. 80, 1997, p. 153.

22. R. Roy, D.K. Agrawal, J.P. Cheng, and S. Gedevanishvili, Full Sintering of Powdered Metals Using Microwaves, Nature, v. 399, n.17, 1999, p. 668-670.

23. G. Sethi, A. Upadhyaya, and D. Agrawal, Microwave and Conventional Sintering of Premixed and Pre-alloyed Cu-12Sn Bronze, Sci. Sintering, v. 35, 2003, p. 49-65.

24. R.M. Anklekar, D.K. Agrawal, and R. Roy, Microwave Sintering and Mechanical Properties of P/M Copper Steel, Powder Metall., v. 44, n. 4, 2001, p. 355-362.

25. K. Saitou, Microwave Sintering of Iron, Cobalt, Nickel, Copper and Stainless Steel Powders, Scripta Mater., v. 54, n. 5, 2006, p. 875-879.

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26. M. Jain, G. Skandan, K. Martin, K. Cho, B. Klotz, R. Dowding, D. Kapoor, D. Agarwal, and J. Chang, Microwave Sintering: A New Approach to Fine-Grain Tungsten-I, Int. J. Powder Metall., v. 42, n. 2, 2006, p. 45-50.

27. P. Chhillar, D. Agrawal, and J. H. Adair, Sintering of molybdenum metal powder using microwave energy, Powder Metall., vol. 51, no.2, 2008, p. 182-187.

28. A. Upadhyaya, S. K. Tiwari, and P. Mishra, Microwave Sintering of W-Ni-Fe Alloys, Scripta Mater., v.56, 2007, p. 5-8.

29. A. Mondal, D. Agarwal, and A. Upadhyaya, Microwave Sintering of Tungsten Based Alloys, Int. Conf. Refract. Hard Met., Washington DC, 2008.

30. MPIF Standard 41, Determination of Transverse Rupture Strength of Powder Metallurgy Materials, Standard Test Methods for Metal Powders and Powder Metallurgy Products,MPIF, Princeton, New Jersey, USA, 2002, p. 55-57.

31. R. Roy, D. K. Agrawal, J. P. Cheng, Microw: Theor. Appl. Mater. Proc. V, Ceram. T. III, 2000, p. 459.

32. A. Zavaliangos, J.M. Missiaen, and D. Bouvard, Anisotropy in Shrinkage during Sintering, Sci. Sintering, vol. 38, 2006, p. 13-25.

33. A. Upadhyaya and R. M. German, Densification and Dilation of Sintered W-Cu Alloys, Int. J. Powder Metall., vol. 34, no 2 , 1998, p. 43-55.

34. G. E. Murch and C. M. Bruff, Chemical Diffusion in Inhomogeneous Binary Alloys, Diffusion in Solid Metals and Alloys, Landolt-Börnstein, Numerical Data and Functional Relationships, New Series, Group III, compiled by H. Mehrer, Spring-Verlag, Germany, vol. 26, 1990, p. 332.

35. J. P. Leonard, T. J. Renk, M. O. Thompson, and M. J. Aziz, Solute Diffusion in Liquid Nickel Measured by Pulsed Ion Beam Melting, Metall. Mater. Trans. A, vol. 35A, no. 9, 2004, p. 2803-2807.

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