Densification behaviour, microstructural characteristics and mechanical properties of liquid phase sintered W–2Ni–1Fe alloy

Journal Pre-proof Densification behaviour, microstructural characteristics and mechanical properties of liquid phase sintered W–2Ni–1Fe alloy Sanjib Majumdar, Jugal Kishor, Bhaskar Paul, Vivekanand Kain PII:

S0925-8388(19)34143-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.152897

Reference:

JALCOM 152897

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Journal of Alloys and Compounds

Received Date: 16 August 2019 Revised Date:

30 October 2019

Accepted Date: 1 November 2019

Please cite this article as: S. Majumdar, J. Kishor, B. Paul, V. Kain, Densification behaviour, microstructural characteristics and mechanical properties of liquid phase sintered W–2Ni–1Fe alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152897. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Densification behaviour, microstructural characteristics and mechanical properties of liquid phase sintered W-2Ni-1Fe alloy

Sanjib Majumdar*, Jugal Kishor, Bhaskar Paul, Vivekanand Kain High Temperature Materials Development Section, Materials Processing and Corrosion Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India Abstract Tungsten heavy alloy, W-2Ni-1Fe (wt.%) with higher tungsten content and higher density was successfully prepared by powder metallurgical processing sequences comprising of mechanical alloying, compaction and liquid phase sintering. The process parameters such as mechanical alloying conditions, compaction pressure, sintering atmosphere, and sintering schedule were optimized to achieve the desired sintered density. DTA studies were carried out to identify the onset temperature for formation of liquid phase, which eventually decided the temperature regime of liquid phase sintering. The synthesized alloy was characterized for the evolution of microstructure, phases, mechanical properties and texture using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), tensile testing, X-Ray diffraction (XRD), electron back scattered diffraction (EBSD) etc. The sintered alloy was found to comprise with γ-Ni matrix phase along with bcc W grains of size ranging 20-45 µm. Extensive, EBSD analysis revealed the preferred orientation in γ-Ni grains those undergone liquid to solid transition after cooling, whereas W grains were oriented randomly. Reprecipitation of fine size (~1 µm) W grains from the liquid phase was identified at the liquid / solid interfaces. Sintering atmosphere altered the density and microstructure of the alloy significantly. The tensile strength and ductility of the as-sintered W-2Ni-1Fe alloy was found to be 863 MPa and 12.5%, respectively. SEM characterization of the tensile-fracture surface 1

showed mixed mode of fracture, i.e. ductile and brittle fracture in γ-Ni and W grains, respectively. Keywords: Tungsten; Liquid phase sintering; Microstructure; Scanning electron microscopy; Electron back-scattered diffraction; Ductile fracture *

Corresponding author: [email protected], [email protected]

1. Introduction Tungsten heavy alloys (WHA), also known as, pseudo alloys are basically alloy of tungsten and Ni-Fe or Ni-Cu, with the high density ranging from 17 to 18.7 g/cm³. Tungsten heavy alloy having a composition of W-2Ni-1Fe (wt.%) is used as radiation shielding parts in cancer therapy machines, gamma radiation exposure devices etc, and it is a potential material for plasma facing components in fusion reactors [1-5], because of its density in higher side of the density range of WHAs. Amongst the other tungsten heavy alloys, W-2Ni-1Fe alloy contains the minimum amount (3 wt.%) of alloying elements and possesses highest density (18.2-18.5 g/cm3). There are different grades of tungsten heavy alloys with varying lower W content, i.e. higher alloying elements (Ni + Fe) with lower density. The diverse areas of application of tungsten heavy alloys include kinetic energy penetrators, heat sinks, balancing weight in cars, aerospace application, electrical contact material, collimators, rotors in watches etc. The efforts are being made to improve the strength of these heavy alloys by addition of other alloying elements, refining of tungsten grain sizes, and modifying the powder metallurgical processing sequences [6-19]. The promising potential of additive manufacturing technique and spark plasma sintering process for making tungsten heavy alloys is also reported elsewhere [20, 21]. However, liquid phase sintering is the industrially

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adopted process being used for production of these heavy alloys. A Ni-rich γ-phase containing Fe and some amount of W forms a liquid phase at the sintering temperature and flows by capillary action through the partially sintered skeleton already formed by W. Dissolution of W in liquid phase and re-precipitation causes grain growth of the W grains [22]. Different models are proposed considering the gravitational effect on the liquid flow behaviour during sintering process which might lead to the distortion of the shape of the sintered product [23-26]. Although liquid phase sintering is a conventional process for preparing WHA, however, the sintering of the alloy with density higher than 18 g/cm³ is a challenging task because of lower volume fraction of liquid phase.

Considering the

composition, the W-2Ni-1Fe alloy would produce the lowest amount of liquid phase content as compared to the other higher alloying added tungsten heavy alloys. While the higher amount of alloying element containing alloys are studied in more detail, there are limited literatures available on W-2Ni-1Fe alloy. In the present work, the effects of different process parameters were studied to achieve the desired density of W-2Ni-1Fe alloy for radiation shielding application in cancer therapy machines. The detailed characterization of the alloy was carried out after preparing the alloy through mechanical alloying and liquid phase sintering route. Based on the observations, the insights on the liquid phase sintering phenomenon, grain structure formation, and mechanical behaviour of the alloy were addressed.

2. Experimental Procedure Tungsten metal powder was produced by hydrogen reduction of tungsten tri-oxide at 1000°C. The purity of the as reduced W powder was maintained at 99.9%. High purity nickel (99.8%) and iron (99%) powder was procured from M/s Prabhat Chemicals, Gujarat, India. W-2Ni3

1Fe (wt.%) composition was produced by properly weighing the constituent powder and mixing them homogeneously using a turbo mixer. The mixed powder was subsequently placed inside the WC lined stainless steel pots of a planetary ball mill. The ball to powder (BPR) weight ratio was maintained at 3:1, using the WC balls. The time of milling was varied between 1 to 25 h with a uniform milling speed of 200 rpm. Intermediate cooling cycles were provided during the milling operation. The milling operation was conducted using flowing argon gas environment. After milling or mechanical alloying, the alloy powder was characterized using X-ray diffraction and scanning electron microscopy. The green compacts of the mechanically alloyed powder was prepared by uniaxial compaction technique using a 25mm diameter hole die made of D2 grade steel. The applied pressure was varied between 200 to 275 MPa. A small sample of green alloy compact was subjected to Differential thermal analysis (DTA) study to determine the liquid phase formation temperature using a Setaram make (Setsys Evolution) TG-DTA equipment. The heating rate was maintained at 15°C/min. The green alloy pellets were subjected to dilatometry to study the shrinkage behaviour using thermo mechanical analyzer (TMA) (Setsys Evolution TMA 1600, M/s SETARAM, France) in reducing atmosphere (He-8% H2, 20 ml/min). The green compacts were further sintered at different temperatures from 1450 to 1500°C for 2 h in a tungsten mesh heated furnace. Heating rate was maintained at 15°C/min followed by isothermal holding at maximum temperature for 2h and subsequent cooling at about 20°C/min. The sintering experiments were carried out at pure hydrogen, 8% H2-He mix gas and vacuum environments. The detailed characterization studies were carried out at every stages of the processed product. Particle size, shape and morphology of the feed powder were analysed using scanning electron microscopy (Camscan CS series microscope). The density of the sintered alloy was evaluated by both dimensional measurements and using Archimedes’ principle. The surface of the as-sintered alloy samples was analysed under SEM using both

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secondary electron and back scattered electron imaging. The energy dispersive spectrometric (EDS) analysis (Oxford X-max 80) of the constituent phases was conducted for finding out the elemental composition and their distribution in the individual phases. The sintered samples were metallographically ground using different grits of SiC embedded emery papers and subsequently polished to 1 µm diamond finish. The as polished samples were further characterized using SEM and EDS. Electron back scattered diffraction (EBSD) studies were carried out for sintered and heat treated alloy samples. For EBSD study, the samples were prepared by grinding up to 1 µm diamond finish and subsequent polishing with colloidal silica solution. The time of final polishing in colloidal silica varied between 4 to 8 h. The EBSD analysis of the properly polished samples were conducted using Oxford make (Model: NordlysNano) detector. The EBSD data was captured and analysed using Aztec and Channel 5 software. The presence of different phases, grain sizes, grain orientations etc. was characterized using EBSD analysis. Hardness was measured on the polished samples using Vickers diamond indenter applying 1000 g load. Tensile tests were performed at strain rate of 10−4/s at room temperature using a Zwick make screw driven 100kN universal testing machine (UTM). The plate-like miniature tensile samples were machined using EDM from the as-sintered alloy. Multiple specimens were tested at same conditions to check the repeatability. The fracture surfaces of the tensile samples were analysed in SEM using both secondary electron (SE) and back scattered electron (BSE) imaging techniques.

3. Results 3.1 Powder characterization Fig. 1(a)-(c) represents the secondary electron image showing the morphology of the feed powder, i.e. as-reduced tungsten, and nickel and iron powder. The W powder exhibited a 5

faceted morphology on predominantly round shape structure. The particle size of the powder varied in the range of 5-10 µm. Both Ni and Fe powders were having spherical morphology with average particle size ranged in 15-25 µm as shown in the Fig. 1(b) and 1(c), respectively. Figs. 1(d)-1(f) show the morphologies of mechanically alloyed powder milled for different durations. The particle size was found to reduce with increase in milling duration showing localized agglomeration tendencies. After 15 hour of milling (Fig. 1e), the identity of individual powder morphologies were found to be lost, and a new morphology was evolved with reduced particle size and localized soft agglomeration. The reduction of the particle size was also suggested by the XRD analysis with peak broadening with increase in milling time (Fig. 2). There was no peak shifting, and the peaks of Ni and Fe was not visible in the XRD pattern because of their very low concentration.

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Fig. 1 Secondary electron SEM images of the starting powder: (a) Hydrogen reduced tungsten, (b) Nickel, (c) Iron and (d) to (f) Mechanically alloyed W-2Ni-1Fe after 1h, 15hr and 25hr of milling

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Fig. 2: XRD pattern of mechanically alloyed W-2Ni-1Fe powder 3.2 Sintering studies 3.2.1 Effect of temperature Fig. 3 shows a DTA curve obtained during non-isothermal heating of the green W-2Ni-1Fe alloy compact indicating the presence of an endothermic peak at 1485°C, also highlighted in the inset. The endothermic peak corresponded to the melting of low melting phase during heating. The DTA curve revealed that the melting of W-2Ni-1Fe system can start at about 1445°C and therefore, the optimum temperature window for liquid phase sintering would be in 1450-1470°C.

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Fig. 3 Differential thermal analysis (DTA) plot obtained during heating of as W-2Ni-1Fe alloy compact up to 1600°C; heating rate 15°C/min

After knowing the temperature window, the green compacts were subjected to dilatometric studies for understanding the sintering behaviour. Fig. 4 shows the shrinkage profiles at different isothermal temperature of 1450 and 1470°C. The representative heating profile is shown in the inset of Fig. 4. The isothermal step at 900°C was given for removal of surface oxygen from the powder (in case partial oxidation have taken place during handling of the green compacts) to facilitate sintering with ease. From the shrinkage profiles, a thermal expansion of the green compact can be seen during the initial stage of heating followed by a substantial shrinkage, which starts at about 1200°C. The maximum shrinkage rate was found to be at about 1450°C. After 1450°C, the shrinkage rate was found to be decreased because at that temperature, liquid phase begins to form and start filling up the pores by the capillary action and forming a matrix while cooling. 2 1600

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3.2.2 Effect of compaction pressure and atmosphere Fig. 5 shows the effect of compaction pressure on the final sintered density, and Figs. 6(a)6(c) represent the microstructure of the sintered W-2Ni-1Fe alloy samples sintered at different atmospheric conditions for comparison. Compaction pressure plays a vital role in the case of W-2Ni-1Fe alloy, where the alloying elements, which are responsible for forming liquid phase, are very low. The large porosities formed in the case of lower compaction pressure cannot be filled by the liquid phase during the liquid phase sintering in such alloy system in contrast to the alloy system with higher concentration of alloying elements, as depicted in the representative microstructure in Fig. 6(a). A large number of porosities can be seen in the well developed desired microstructure, which clearly revealed that the compaction pressure plays an important role. 250 MPa was found to be the optimum compaction pressure for achieving the sintered density above 18.2 g/cm3. The inset of Fig. 5 shows the outlook of the sintered and machined W-2Ni-1Fe alloy disc compacted at 250 MPa and sintered at 1500°C, which yielded a sintered density of 18.2 g/ cm3. It is worth mentioning here that the tungsten heavy alloy showed superior machinability as compared to pure tungsten produced by hot pressing. Therefore, manufacturing the components of tungsten heavy alloy by machining of the sintered product would be feasible. The microstructure of the sintered W-2Ni-1Fe alloy, sintered at 1500°C in pure hydrogen atmosphere is shown in Fig. 6(b), indicating the formation of almost full dense microstructure. Fig. 6(c) shows the BSE image obtained from the polished surface of the W2Ni-1Fe alloy sintered at 1500°C under a vacuum level of 1×10-5 mbar using a green pellet, which was compacted at 250MPa. The formation of liquid phase at W grain boundaries could be observed. However, presence of larger size pores (indicated by arrows in Fig. 6c) was observed in the microstructures of the vacuum sintered alloy. Therefore, the density of the vacuum-sintered alloy was found to be lower (~17.6 g/cm3) than that obtained for the alloy

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sintered in hydrogen atmosphere. The evaporation loss of Ni and Fe from the surface of sintered W-2Ni-1Fe may be one of the reasons for formation of pores in vacuum. It is further mentioned here that the densification studies were also carried out using the green compacts prepared from the mixed primary powder (W, Ni, and Fe) without giving mechanical alloying treatment. However, the density of the hydrogen sintered alloy of the mixed powder compact was found to be low (~17.7 g/cm3), and comprising of porous microstructure similar to that presented in Fig. 6(c), which could be due to the presence of larger size particles and nonuniform distribution of the elements in the un-milled / mixed powder compact.

Fig. 5 Effect of compaction pressure on the sintered density. Inset shows the outlook of the liquid phase sintered and machined W-2Ni-1Fe alloy samples

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Fig. 6 BSE microstructure of the polished W-2Ni-1Fe sample sintered under different atmosphere

4. Characterization of W-2Ni-1Fe alloy 4.1 Microstructural Characterization The back-scattered SEM image captured from the top surface of the as-sintered (original) alloy is presented in Fig. 7. The microstructure consisted of a dark matrix phase network in which grains of bright phase were distributed. The EDS X-ray maps for the constituent elements corresponding to a particular area (Fig. 8) revealed that the brighter phase was composed of W and the darker phase consisted mainly of Ni and Fe. The network of the darker phase was formed by mainly Ni and Fe, as indicated by Figs. 8(c) and (d), respectively. Fig. 9(a) and 9(b) represent the BSE images (lower and higher magnification) obtained from the machined sintered alloy after metallographic polishing. The microstructures observed at the cross-section were similar to those presented in Fig. 7. Comparing the Figs. 7-9, it could be inferred that the fraction of the darker Ni-rich phase was less inside the sample as compared to that at the sintered surface. The size of the tungsten grains (bright phase in Fig. 9) is mostly non-uniform, and it varied between 20 and 45 µm. The appearance of Ni-rich phase was also found to be non-uniform (Fig. 9).

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Fig. 7 Backscattered electron SEM image obtained from the sintered surface of the W-2Ni1Fe alloy

Fig. 8(a) BSE image of the as-sintered surface of W-2Ni-1Fe alloy and the corresponding area EDS X-ray maps for the elements (b) W La, (c) Ni Ka and (d) Fe Ka

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Fig. 9 BSE images obtained from the polished W-2Ni-1Fe alloy sample produced by liquid phase sintering; (a) lower magnification and (b) higher magnification image, and (c) EDS Xray peaks obtained from the points 1 and 2 in figure b. Fig. 9(c) shows the EDS spectra taken at different locations indicated in Fig. 9(b). The darker phase showed the presence of Ni, Fe and W, whereas the bright phase was mainly composed with W. The elemental composition of the phases obtained by EDS analysis is presented in Table 1. The excitation area of EDS point analysis was about 3.5 µm × 3.5 µm. The darker matrix phase contains about 62 at.% Ni, 27 at. % Fe and 10.7 at.% W, and the brighter phase is composed with W only. The amount of interstitial elements such as oxygen and carbon was found to be very low (<0.1%).

4.2 EBSD analysis The microstructure of the tungsten heavy alloy mainly composed of two phases; a tungsten phase with larger grain sizes and volume fractions, and a nickel-rich phase distributed like thin network. These two phases possess different physical, chemical and electrochemical characteristics. Therefore, the preparation of suitable samples for EBSD analysis is difficult. The electro-polishing technique adopted for preparing the EBSD sample showed preferential etching of either W or Ni-rich grains. Therefore, final polishing with colloidal silica for sufficient time duration was carried out to obtain kikuchi patterns from both the phases within 14

the major portion of the scanned area of interest. Fig. 10(a) represents the BSE image captured in EBSD detector from W-2Ni-1Fe alloy sample placed on a 70° tilted holder. The EBSD phase map obtained from the corresponding area is presented in Fig. 10(b). The alloy comprised with two phase microstructure containing bcc W (red colour) and fcc Ni (blue colour in Fig. 10b) or γ phase.

Fig. 10 (a) BSE image from a selected area of W-2Ni-1Fe alloy, (b) EBSD phase contrast map, (c) band contrast image and (d) inverse pole figure image (inset: legend of IPF image) (e) BSE image at higher magnification, and (f) corresponding inverse pole figure image.

Fig. 10(c) is a band contrast image showing the quality of the EBSD scan, and also revealing the two separate phases. While the grain size of W grains varied between 20 to 45 µm, the thickness of the γ-Ni phase was calculated to be in the range of 1-9 µm. Fig. 10(d) represents the inverse pole figure (IPFZ) image indicating the orientation of the grains along different crystallographic directions as indicated by the legend shown in the inset. The W grains are oriented randomly and the γ-Ni grains show a preferred orientation along [111] in Fig. 10(d). 15

Figs. 10(e) and 10(f) show the EBSD maps captured at higher magnification from a different area of the sintered alloy. Formation of fine grains in the γ-Ni matrix was observed, and the presence of tiny (small size ~1 µm) W grains was also observed at the interface between the γ-Ni and big sized W grain. The presence of very fine size grains of W was also detected in Fig. 10(f) near the interfaces between γ-Ni and bcc W. All the EBSD images presented here were captured from the W-2Ni-1Fe alloy samples sintered under hydrogen atmosphere.

4.3 Mechanical Properties The micro-hardness values measured at different grains of W-2Ni-1Fe ranged between 295 and 315 HV at 1000 g applied load. Fig. 11 shows the representative engineering stress-strain curve of as-sintered W-2Ni-1Fe alloy. The yield strength, tensile strength and elongation were found to be 640 MPa, 863 MPa and 12.5%, respectively. The inset in Fig. 11 shows the diagram and the dimensions of the plate like tensile test sample prepared by electro-discharge machining. The SEM images obtained in secondary electron (SE) and back scattered electron (BSE) modes at different magnifications from the fractured surface of the tensile tested samples are presented in Fig. 12. Four type of fracture mechanisms are observed namely (a) ductile or dimple fracture in the Ni-rich binder phase, (b) cleavage or brittle fracture of the W grains, (c) cracking or breaking of the W grains and (d) delamination at the W/matrix interface. The mechanical properties such as tensile strength (863 MPa) and ductility (12.5%) values obtained from our as-sintered W-2Ni-1Fe alloy are comparable with those values reported for sintered and heat treated alloy [27]. The presence of Ni-rich binder phase is responsible for higher ductility of the tungsten heavy alloys, which was also observed from the fracture surface (Fig. 12) of the tensile tested specimens.

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Fig. 11 Engineering stress vs. strain plot obtained from tensile tests of as-sintered W-2Ni-1Fe alloy. The specimen geometry is shown in the inset.

Fig. 12 SEM images obtained from the fracture surface of the tensile tested specimens; (a) and (b) SE and BSE images, and (c) and (d) higher magnification SE and BSE images showing ductile and brittle fracture modes in binder and W grains, respectively.

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5. Discussion It was observed that during liquid phase sintering, Ni and Fe form a liquid phase in which W gets dissolved to a significantly higher concentration. About 27.5 wt.% tungsten (in Table 1) was found to be present in the matrix phase which was at liquid state at the sintering temperature of 1500°C. The fraction of W present at the liquid phase at the sintering temperature could be even higher, which led to the dissolution of the small size W particles in the liquid phase. The grain growth of the W grains further occurs due to re-precipitation of W from the liquid phase at the surface of existing larger grains. The classical phenomenon of Ostwald ripening which predicts the dissolution of smaller solid particle into the liquid followed by re-precipitation at the surfaces of the larger solid particles present in the liquid was found to be happening during the development of the microstructure of the W-2Ni-1Fe alloy. Mechanical alloying of the powder for sufficient duration was found to have a significant effect on densification behaviour of the tungsten heavy alloy. Mechanical milling treatment promotes intimate mixing of the constituent powder in the solid state. The size reduction caused by milling further enhances the densification kinetics. The major driving force for the process is the minimization of the total free energy of the green compact by elimination of the porosity and reduction of liquid-tungsten interfacial energy. The fraction of liquid phase present near the top surface of the hydrogen-sintered alloy was found to be more (in Figs. 7 and 8) than that present in the core (Fig. 9). This could be attributed to the gravitational effect [23, 24, 26] observed during liquid phase sintering. The gravitational effect causes distortion of the shape if the liquid phase fraction is high. However, as the liquid phase fraction formed in W-2Ni-1Fe alloy was lower, the shape distortion was not observed. The DTA plot shown in Fig. 3 confirms that the melting of the liquid phase starts at about 1445°C. Therefore, the liquid phase sintering of the W-2Ni-1Fe alloy could be carried out in the temperature range of 1450-1500°C. The effect of sintering time and temperature in this 18

temperature range on the grain size of the sintered alloy could be further studied to understand the grain growth mechanisms. Sintering atmosphere plays a very important role on densification. Sintering under vacuum creates porosities (Fig. 6c) in the microstructure of the tungsten heavy alloy, which could be attributed to the evaporation of the liquid phase [28]. EBSD analysis (Fig. 10) revealed certain interesting microstructural characteristics of the liquid phase sintered W-2Ni-1Fe alloy done in hydrogen environment. W grains were oriented in random direction, which was observed normally in the microstructures of the solid state sintered refractory metals and their alloys [29]. However, the γ-Ni grains formed after solidification of the liquid phase showed a preferred orientation. Mostly, the occurrence of [111] oriented γ-Ni grains was observed (Fig. 10d). A significant amount of the growth of the W grains was also detected in EBSD results. W-2Ni-1Fe alloy possesses higher tensile strength as compared to other WHAs, however, the observations on formation of dimples in γ-Ni grains after tensile facture of the alloy is being reported for the first time in this work.

6. Conclusions The process parameters such as milling time, compaction pressure, and sintering atmosphere, temperature and time were optimised to prepare W-2Ni-1Fe (wt.%) alloy with desired density and microstructure. The detailed characterization of the sintered alloy using SEM and EDS revealed the formation of Ni-rich liquid phase containing about 27.5 wt.% W as matrix with the distribution of pure W grains. The fraction of the liquid phase was found to be more on the surface as compared to the inner structure of the sintered alloy. The density of the alloy sintered in reducing atmosphere was found to be higher than that achieved by sintering in vacuum. EBSD analysis identified the formation of two phase microstructure comprising of 19

bcc W and fcc γ-Ni grains. The grain size of W grains varied between 20 to 45 µm, and the thickness of the liquid γ-Ni grains was found to be very low (1-9 µm). The orientation of the W grains was random, whereas a preferred texture of [111] was detected in fine sized γ-Ni grains. The phenomenon of re-precipitation of solid W from the liquid phase at the existing larger size W grains was observed in the EBSD maps, in which the presence of fine size W grains were detected at the liquid/solid interfaces. The yield strength, tensile strength and elongation of the as-sintered alloy were found to be 640 MPa, 863 MPa and 12.5%, respectively. Ductile and brittle fracture modes were observed in γ-Ni and W grains, respectively.

Acknowledgements Authors are thankful to Mr. P. B. Shelke, Mr. S. S. Molke, Mr. R. L. Vanneldas, and Mr. S. K. Gavai of High Temperature Materials Development Section of MP&CED, BARC for their consistent efforts in preparation of tungsten metal powder and fabrication of tungsten heavy alloy shapes. Special thanks to Shri Laxya Gupta of Materials Science Division, and Dr. Apu Sarkar and Shri Saurav Sunil of Mechanical Metallurgy Division for their contribution in density measurements, tensile testing etc. Authors also would like to thank Mr. Dheeraj Jain of Chemistry Division for his help in conducting dilatometric studies.

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Table 1: The EDS composition analysis data obtained from bright and dark phases respectively at spectrum 1 and 2 in Fig. 9. Analysis area / Element spot

Weight%

Atomic%

Spectrum 1 (bright phase)

W

100.0

100.0

Fe

21.2

27.0

Ni

51.3

62.3

W

27.5

10.7

Spectrum 2 (dark phase)

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Highlights • • • • •

Highest density of W-2Ni-1Fe alloy achieved by sintering in hydrogen atmosphere γ-Ni grains showed preferred orientation and W grains had random texture Re-precipitation of fine grains of W was detected at liquid / solid interfaces Sintered W-2Ni-1Fe alloy showed 863 MPa tensile strength and 12.5% ductility Ductile/dimple fracture in γ-Ni and cleavage in W grains was observed

Declaration of interests

√ ☐ The

authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: