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Microstructure, microtexture and grain boundary character evolution in microwave sintered copper
Materials Characterization 157 (2019) 109921
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microstructure, microtexture and grain boundary character evolution in microwave sintered copper
T
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G.N. Felege , N.P. Gurao, Anish Upadhyaya Department of Materials Science & Engineering, Indian Institute of Technology Kanpur, 208016 U.P., India
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper Microwave sintering Microtexture Microstructure EBSD
In the present investigation, the evolution of microstructure and microtexture of commercially available copper powder during solid state sintering using microwave furnace has been studied. The powder was compacted using 300 MPa uniaxial die pressure and the green compacts were sintered at 610 °C, 880 °C and 1020 °C in argon atmosphere at 20 min and 45 min isothermal holding times using multi-mode microwave furnace which has output power of 3 kW and operated at 2.45 GHz frequency. The temperatures were selected to obtain different dominant densification processes. For sintered samples comparative analysis has been made based on densification behavior, microstructural and microtexture evolution and mechanical properties. Electron backscatter diffraction (EBSD) study indicated that there is a distinct evolution of microtexture and microstructure in terms of evolution of grain boundary character distribution, misorientation, size, shape and morphology of grains and pores. Attributed to high temperature driven volume diffusion process, 1020 °C sintered samples showed better sintering between copper particles, grain growth, grain coalescence, reduction in porosity area fraction and increased pore rounding. For all microwave sintered samples, the highest fraction of CSL boundary fraction found to be Σ3 boundary with a common (111) grain boundary plane which indicated coherent twin boundaries with low energy. More Σ3 twin population is found on intermediate sintering temperature range whereas significant reduction is observed higher temperature sintering which might be the result of grain coalescence and volume diffusion at higher sintering temperature that reduced high angle grain boundaries. Microwave sintered copper samples brought random microtexture evolution with variation of sintering temperature and holding time. This might indicate the presence of non-conventional sintering mechanisms during microwave sintering.
1. Introduction
reduce time and energy consumption and to achieve improved properties, microwave processing technique has been applied for sintering of metals [1–3]. Opposing to conventional sintering that heats the sample through radiation and conduction, microwave sintering heats the sample through conversion of absorbed electromagnetic energy into thermal energy throughout the volume of the metal powder compact. Consequently, the heating is uniform and fast [1,2]. Microwaves do not interact with bulk metals due to small microwave penetration depth. Hence, microwave interaction with the bulk metal is restricted to the surface only. This is because, metals are good conductors hence there will not be any induced internal electric field inside the bulk metal, instead the charge induced will be on the surface as a result the incident microwave will be reflected. Contrary to this, a metal powder compact absorb microwave. This is because metal powders have high surface area as compared to bulk metals, subsequently most of the powder body couples with microwave and heating takes place. If the particle size is coarser, the center of the particle heats up
Powder metallurgy is important processing route for producing different near net shaped objects from metals and alloys as well as ceramics. In powder metallurgy sintering is the key step which provides most of the final properties of the material. Sintering in pure metal powder compact occurs through solid state diffusion of material at atomic level. This will result microstructural evolution and densification on the compacts. Most commonly sintering is carried out in conventional electric furnace, where the heating is carried out through radiation from the electric heat source to the sample and hence the sample heats from surface to the inner core through conduction. This causes non-uniformity on the sinter density of the sintered components. To alleviate this problem slow heating rate and intermittent holding employed during the sintering cycle, as a result long processing time and more energy is required during conventional sintering. In recent years, to
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Corresponding author at: Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, 208016, India. E-mail addresses: [email protected] (G.N. Felege), [email protected] (N.P. Gurao), [email protected] (A. Upadhyaya).
https://doi.org/10.1016/j.matchar.2019.109921 Received 4 May 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 06 September 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 157 (2019) 109921
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through heat conduction from the surface to the inner core of the particle. Recent studies reported that the source of microwave heating effect varies with the material. In ceramics, the heating comes from electric field part of the microwave through the dielectric loss mechanisms include electronic polarization, dipolar polarization, ionic polarization and interfacial polarization. In case of pure metal powders and carbide samples, the heating mainly comes from magnetic component of microwave through magnetic loss which includes hysteresis, eddy currents, magnetic resonance and domain wall oscillations [3,4]. The sinterability of copper in microwave furnace has been reported by many investigators. Sheinberg et al. [5] used microwave sintering to consolidate dispersion-strengthened copper by compacting copper powder samples which were exposed to air to form thin layer of copper oxide at 650 °C using a 2.45 GHz microwave oven operating at maximum power of 700 W. The work by Cheng et al. [3], showed using single mode microwave sintering furnace that bulk copper do not get heated up in both electrical and magnetic field but copper powder compacts heated up in both fields. The shrinkage behavior of copper powder compacts in microwave and conventional sintering furnaces studied by Saitou et al. [6]. They found out the shrinkage parameter for microwave sintering is higher than that of conventional sintering at low temperature. In addition, based on activation energy calculation they concluded microwave sintering did not brought change on sintering mechanism different from conventional one. On the contrary, Demirskyi et al. [7–9] in their series of investigation studied neck formation and growth and densification kinetics of copper powder and compacts under single-mode and multimode microwave sintering at 2.45 GHz. At initial stage of sintering they observed traces of plasma discharging on the fracture surface of porous sintered copper samples which is an indication of enhanced sintering. On their neck growth study, they have proposed that surface diffusion is the predominant mechanism in the initial stages of microwave sintering. On the intermediate stage they observed some abnormal neck growth rate and they attributed it to non-conventional diffusion mechanisms. In cases where long sintering time is employed volume diffusion can become active at this stage. At higher sintering stage in similar fashion as that of conventional sintering, volume diffusion is predominant diffusion mechanism. In addition, they attributed the abnormality to melt formation and evaporation of matter between interparticle contacts may exist due to eddy current development on surface or over heating in contact zone that can facilitates particle rearrangement via viscous flow interparticle sliding and accommodation and hence accelerated shrinkage occurs on the compact. On their related study they analyzed densification kinetics using densification parameter and obtained faster sintering kinetics on single-mode applicator than multimode ones and they attributed this to partial melting of matter. Takayama et al. [10] proved the possibility of microwave sintering of copper powder compacts in the absence of protective atmosphere. The effect of particle size and initial porosity level on microwave heating rate of copper powder compacts was studied by Avijit et al. [11]. The study indicated smaller particle size and high initial porosity caused high microwave heating rate. Although the microwave sinterability of copper reported by many researchers, evolution of microstructure, microtexture and grain boundary character of copper in microwave sintering is not yet explored. In view of this, the present study investigates the evolution of microstructure, microtexture and grain boundary character of microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times.
Table 1 Cu powder characteristics. Characteristics
Cu
Apparent density, (g/ cm3)
Tap density, (g/ cm3)
Flow time, (sec/ 50 g)
Particle size, μm D10
D50
D90
4.58
5.35
6.67
13.4
28.4
57.4
received powder was characterized for particle morphology (size and shape) and flow behavior as per the Metal Powder Industries Federation standard, [12] and the data are shown in Table 1. To study the morphology of the powder a scanning electron microscope (Zeiss Evo 50, UK) was used and the morphology of the powders was obtained to be spherical with various particle size. A laser-scattering particle size analyzer (Fritsch, Germany) was used for the particle size determination. The powder was weighed and put into cylindrical die, then pressed using 300 MPa compaction pressure at room temperature to make cylindrical pellets (height ~6 mm, and diameter ~16.08 mm) using a uniaxial semi-automatic hydraulic press (model CTM-100, Blue star, India). The die was cleaned with acetone and lubricated with zinc stearate prior to each powder compaction to minimize friction. The green compacts of cylindrical pellets (as pressed) were sintered at 610 °C, 880 °C and 1020 °C in argon atmosphere using microwave furnace (Supplier: Enerzi Microwave Systems Pvt. Ltd., model PTF 2730, Bangalore-10, India) which has output power of 3 kW and operated at 2.45 GHz frequency at two isothermal holding times. The sintered density which is expressed in grams per cubic centimeter was obtained by dimensional measurements and counter checked using Archimedes displacement method. The sintered samples were polished in a series of SiC emery papers (paper grades 220, 350, 500 and 1000), followed by cloth polishing using a suspension of 1 μm, 0.3 μm alumina diluted with water and then colloidal silica polishing (0.04 μm), finally light electro polishing was carried out applying 6 V DC supply in an electrolyte having composition of 25%H3PO4 + 50% H2O + 25% Ethanol, for 50 s.
2.2. Characterization An optical microscope (Leica, DM2500, Germany) was used to obtain the micrographs of sintered samples taken along the surface perpendicular to uniaxial compaction direction. Micro hardness of the sintered samples was measured using Vickers hardness tester (Bareiss, Germany). Hardness measurements were taken on mirror finish polished sample surface perpendicular to compaction direction by applying 0.1 kg of load for 10 s. EBSD experiments were carried out to study the evolution of microstructure and microtexture of the sintered samples using Nova Nano SEM 450 Field-Emission scanning electron microscope with 1 nm ultimate resolution. The samples were mounted on 70° pre-tilted EBSD sample holder from horizontal sample stage and the working distance was set to 15 mm away from pole piece. On each sample EBSD scans were performed on an area of ~900 × 900 μm2 scan area with a step size 0.7 μm at 20 kV operating voltage. Kikuchi patterns were collected and automatically indexed using Orientation Imaging Microscopy (OIM™) software (V. 6). Analysis of collected EBSD data was carried out using OIM ™ Analysis V7.3 software from TSL. For analyzing the distribution of crystallographic orientation in terms of pole figures and inverse pole figures Orientation Distribution (OD) of crystallites was calculated using Harmonic Series Expansion method with Series rank 22, Gaussian Half Width is 5°, axial sample symmetry, and 5° Gaussian smoothing. Primary recrystallization twin (Σ3) in sintered copper samples is obtained by considering 60° misoreintation about a 〈111〉 crystal axis within 8.6° tolerance Brandon's criterion. Secondary recrystallization twin (Σ9) in sintered copper samples is obtained by
2. Experimental procedure 2.1. Material and processing For the present investigation, gas atomized copper powder (Metal Powder Company Ltd., India) is used as starting material. The as 2
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Fig. 1. Thermal profiles followed for microwave sintering of copper at 100 °C/min heating rate for (a) 20 min and (b) 45 min isothermal holding time.
considering 38.9° misoreintation about a 〈110〉 crystal axis within 5° tolerance Brandon's criterion.
temperatures compared to 20 min holding time sintered copper samples. This is due to prolonged holding time that gave more time for the material transport. Fig. 3 shows SEM micrographs obtained from the surface perpendicular to uniaxial compaction direction for all microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times. The microstructure and grain size distribution (Figs. 2 and 3) clearly indicates grain coarsening and higher grain size for copper samples sintered at 1020 °C temperature for 45 min holding time. This is because, higher sintering temperature causes volume diffusion and longer holding time gives more time for volume diffusion to take place. With increasing sintering temperature for both holding times pore size reduces, and porosity shape changes significantly. Fig. 4 compares percentage pore area for samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times. The bar graphs indicate a decreasing trend of pore fraction for both isothermal holding times. As compared to 20 min holding time, 45 min holding time sintered copper samples show less pore area fraction with an increase in sintering temperature from 610 °C to 1020 °C. The reduced pore area fraction is attributed to the longer holding time that allows more material diffusion to occur during sintering process. Grain boundary character distribution (GBCD) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal
3. Results 3.1. Starting material and processing The characteristics of the as received powder are shown in Table 1. The median particle size (D50) is found to be 28.4 μm and arithmetic mean is 36.8 μm. The powder has spherical shape. Fig. 1 compares the thermal profiles of microwave sintered copper samples for 20 min and 45 min holding time. The diagram indicated 75–80% overall sintering time reduction as compared to conventional resistance electric furnace sintering. Because of this a lot of energy being saved. 3.2. Evolution of microstructure The average grain size for all microwave sintered copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 min and 45 min holding times is given in Fig. 2. The diagrams indicated that as sintering temperature increases for both holding times grain size increases. Bigger grain size is observed for 45 min holding time for all sintering
Fig. 2. Plot of grain size distribution of cumulative area fraction for all microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min holding times. 3
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Fig. 3. SEM micrographs of microwave sintered copper at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times. Micrographs are taken along the surface perpendicular to uniaxial compaction direction.
holding times. On the contrary, the CSL boundary fraction for copper sample sintered at 610 °C and 1020 °C for 45 min holding time slightly decreases as compared to 20 min holding time sintered samples. Fraction of the random high angle boundaries (HAGB) declines for longer holding time sintered samples attributed to grain coarsening effect, due to grain growth and coalescence. The reduction in HAGB is more pronounced on the long holding time sintered samples. Out of all CSL boundaries considered, Σ3 primary twin and Σ9 secondary twin boundaries show the highest fraction. Primary and secondary recrystallization twin fraction of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is shown in Fig. 6a & b. Sample sintered at 20 min holding time shows an increase in Σ3 boundary fraction from 610 °C to 880 °C sintering temperature and then slightly decreases towards 1020 °C sintered samples. Whereas, 45 min holding time sintered sample shows an increase in Σ3 boundary fraction from 610 °C to 880 °C sintering temperature and then decreases by large fraction towards 1020 °C sintering temperature. The first increment is due to growth of twins and the later one is due to reduction of the total grain boundary including Σ3 twin boundary that occurs at high sintering temperature. The reduction of the boundaries is due to grain coarsening, boundary migration and grain coalescence that comes from volume diffusion at higher sintering temperature coupled with long holding time. Distribution of the Σ3 grain boundary plane orientations of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is given in Fig. 7. The Σ3 boundary planes distribution is obtained at a misorientation of 60°/[111]. For all cases, the largest distribution density of the Σ3 boundary plane orientations is at the axis [111]. This implies that Σ3 boundary plane has higher probability to be distributed on the (111) plane, hence large fraction of the Σ3 boundary is coherent twin boundary. In addition, the results
Fig. 4. Percentage pore area for samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times.
holding times is shown in Fig. 5. As can be seen from the plot, 20 min holding time microwave sintered samples showed very small fraction of low angle boundaries and high fraction of high angle boundaries as compared to 45 min holding time sintered samples. The other group of grain boundaries is coincident site lattice (CSL) boundaries. These are boundaries that contain common atomic positions called coincident sites between neighboring grains. Sigma (Σ) values are used to denote fraction of coincident sites. Sample sintered at 880 °C shows constant CSL boundary fraction regardless of different 4
Materials Characterization 157 (2019) 109921
G.N. Felege, et al.
Fig. 5. Plot of grain boundary character distribution of microwave copper samples sintered at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min holding times.
indicate that the Σ3 twin boundary has a higher probability distribution density in for 880 °C sintered copper samples at for both holding times and the least probability distribution density is on 1020 °C sintered copper sample with long holding time sintering.
from the texture observed in copper samples sintered at 610 °C and 880 °C temperature. Texture strength is higher for 1020 °C sintered samples as compared to 610 °C and 880 °C sintered copper samples for both holding times. With increasing holding time from 20 to 45 min, the texture strength shows marginal increment.
3.3. Evolution of microtexture 3.4. Densification and mechanical properties Fig. 8 indicates grain coarsening with increasing sintering temperature for both sintering conditions. Most of the coarser grains in copper sample sintered at 1020 °C shows single orientation as a result of coalescence. This is due to preferential growth of certain grains with single orientation during elevated temperature sintering. The overall crystal orientation maps show different orientation for different grains in the microstructure. Inverse pole figure (IPF) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is shown in Fig. 9. Microwave sintered copper samples shows different texture as a function of sintering temperature. Copper sample sintered at 610 °C, shows weak fiber texture near 〈101〉 for 20 min and near 〈310〉 for 45 min holding time. Copper sample sintered at 880 °C shows near 〈332〉 and 〈321〉 weak fiber texture for 20 min and 45 min holding times respectively. At 1020 °C sintered copper sample shows the same 〈001〉 oriented fiber texture for both holding times, and this is different
Fig. 10 shows variation of sinter density against sintering temperature and holding time. For short holding time, the bar graph indicates a slight decrease in sinter density of copper samples with the increasing sintering temperature, whereas the long holding sintered samples shows slightly more sinter density which is not affected by sintering temperatures variation as compared to low holding time sintered samples. Variation of Vickers micro-hardness (HV0.1) of copper samples, with sintering temperature and holding time is shown in Fig. 11. The hardness value for 610 °C and 880 °C at both holding times is almost similar. But 1020 °C for longer holding time showed significant increment in the hardness. This is because, at lower temperature sintering even if the holding time is different the amount of sintering is very less. That is why there is no difference in Vickers hardness. Whereas at longer holding time difference in holding time brings pronounced effect on the
Fig. 6. Plot of percentage Σ3 and Σ9 twin boundary fraction of copper samples sintered at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min isothermal holding times. 5
Materials Characterization 157 (2019) 109921
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Fig. 7. Distribution of the Σ3 grain boundary plane orientations of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times. The distribution is plotted in a standard stereographic projection, and the units are multiples of a random distribution.
4. Discussion
sintering as compared to lower temperature sintering. Hence longer holding time at 1020 °C facilitates more material transport, better densification and reduction in pore number and size. Consequently, better hardness is observed.
The results of the present work indicated evolution of distinct microstructure and microtexture in microwave sintered pure copper
Fig. 8. Crystal orientation maps of copper samples sintered at 610 °C, 880 °C, and 1020 °C in microwave furnace. 6
Materials Characterization 157 (2019) 109921
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Fig. 9. Inverse pole figure (IPF) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times.
samples upon increasing sintering temperature from 610 °C, to 880 °C and then to 1020 °C and increasing holding time from 20 min to 45 min. 4.1. Evolution of microstructure SEM micrographs, grain size and pore area fraction plots clearly indicate microstructural evolutions that took place during microwave sintering of pure copper at different sintering temperature and different isothermal holding time. Copper sample sintered in microwave furnace at 1020 °C for both holding times results an increase in neck size (Fig. 3), grain growth (Fig. 2), coalescence, reduction in porosity and pore rounding. In addition, it shows an increase of the low angle misorientation fraction and a decrease in Σ3 twin boundary fraction. These changes are more pronounced on the copper samples sintered 45 min holding time due to extra time for diffusion to take place. All the microstructural changes observed are mainly due to the high sintering temperature that stimulates random atomic jumps or motion and the long holding time which increases the volume of jumps. This brings diffusion of atoms within the particle and between particles. Diffusion of atoms takes place along grain boundaries, over the surfaces, and through the crystalline lattice [13]. These diffusion processes cause reduction of surface area of the particles and fraction of grain boundaries, and hence reduction in misorientation angle. Because of this, the total free energy of the system reduces and the sintered bond between the particles grows, and the size and number fraction of porosity reduces. Moreover, due to coalescence grain growth occurs. With exception to the above trend, in the present study sintered copper sample at 880 °C sintering temperature shows insignificant change in CSL fraction for both isothermal holding times. At this temperature range, neck size grows, and bigger particles consume small particles, but the amount of coalescence between big particles is not much. Hence most of CSL and high angle boundaries are preserved.
Fig. 10. Sinter density variation of microwave sintered copper samples at 610 °C, 880 °C and 1020 °C sintering temperature for 20 min and 45 min isothermal holding times.
4.2. Evolution of microtexture Inverse pole figures of the sintered copper samples show a distinct microtexture evolution at different sintering temperatures for both isothermal holding times. At 610 °C, the ⟨101⟩ weak fiber texture is observed for 20 min holding time. The observed ⟨101⟩ fiber texture on 610 °C microwave sintered samples is probably due to rotation of grains towards the compression axis during compaction [14]. In addition to that, at this temperature range surface diffusion is dominant mechanism for diffusion [15]. The work by Hackerman et al. [16] on rates of surface self-diffusion over the principal planes of a single crystal of
Fig. 11. Vickers micro-hardness (HV0.1) variation of microwave sintered copper samples with sintering temperature and holding time.
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copper indicates diffusion coefficient is more along 〈110〉 for (100) and (110) planes. Hence this might be the reason for 〈101⟩ fiber texture at this temperature range. However, in case of microwave sintered samples at 610 °C for 45 min holding time, near 〈310⟩ fiber texture is detected which might be because of longer holding time. At 880 °C sintered copper sample shows near 〈332⟩ and 〈321⟩ fiber texture for 20 min and 45 min holding time respectively. In addition to the difference in holding time during sintering, in microwave sintering particularly in intermediate sintering range as discussed by Demirskyi et al. [8] anomalies can happen, which might destroys or randomize texture that acquired during compaction and low-temperature sintering. As a result, the intermediate temperature shows random type of texture as compared to other sintering temperature ranges. Copper samples sintered at 1020 °C for 20 and 45 min holding time show identical 〈001⟩ fiber texture and the latter one exhibits more texture strength. As compared to samples sintered at 610 °C and 880 °C, copper sample microwave sintered at 1020 °C shows higher 〈001⟩ fiber texture strength for both holding times. This type of fiber texture is a characteristics texture during solidification of cubic metals [17]. This is because, during solidification atoms which are in liquid phase prefer to adhere to less closely packed plane of the solid crystal, since it easy to get comfortable locations. That is why 〈100〉 is the fastest growth direction for cubic metals. This might be the case for showing a similar 〈001〉 fiber texture in samples sintered at 1020 °C that might involve temporary melting, and then growth of grains.
5. Conclusion In the present study, gas atomized spherical copper powder was cold compacted at 300 MPa. The green compacts were sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times in an argon gas atmosphere using multi-mode microwave furnace, which has output power of 3 kW and operated at 2.45 GHz frequency. Evolution of the microstructure, microtexture and grain boundary character in microwave sintered copper powder compacts were investigated using scanning electron microscope and EBSD. The conclusions drawn from results of the present study are summarized below. Higher sintering temperature coupled with long isothermal holding time in microwave sintered copper samples brought more grain growth and particle coalescence due to high temperature driven volume diffusion process, but did not affected the microtexture evolution of the sintered samples significantly. Random microtexture evolution with variation of sintering temperature and holding time is observed, which is an indication of non-conventional sintering mechanisms during microwave sintering. In all microwave sintered samples, high fraction of Σ3 boundary with (111) common boundary plane is found out of all CSL boundaries. This indicates coherent twin boundaries with relatively low energies. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
4.3. Densification and mechanical property Acknowledgments Long holding time shows better sinter density as compared to short holding time samples. This is the result of having more time for volume diffusion to take place and reduce porosities in size and number. The bar graphs also indicate, a slight decrease in the sinter density of copper samples with an increase in sintering temperature for short holding time. This might be due to elastic expansion of samples in compaction direction. Expansion occurs when samples have sorbed gases during compaction. During sintering with having closed pores on the compact pressure of the trapped gas causes expansion of the sample [18]. Despite the difference in the sinter density, there is no significant difference in hardness for both sintering routes.
The authors would like to thank Department of Materials Science and Engineering at Indian Institute of Technology Kanpur for microscope facility. References [1] R. Roy, D. Agrawal, J. Cheng, S. Gedevanlshvili, Full sintering of powdered-metal bodies in a microwave field, Nature 399 (6737) (1999) 668–670. [2] R.M. Anklekar, D.K. Agrawal, R. Roy, Microwave sintering and mechanical properties of PM copper steel, Powder Metall. 44 (4) (2001) 355–362. [3] J. Cheng, R. Roy, D. Agrawal, Radically different effects on materials by separated microwave electric and magnetic fields, Mater. Res. Innov. 5 (3–4) (2002) 170–177. [4] M. Gupta, E.W.W. Leong, Microwaves and Metals, Wiley, 2008. [5] H. Sheinberg, T.T. Meek, R.D. Blake, Microwaving of Normally Opaque and Semiopaque Substances, Los Alamos National Laboratory (LANL), Los Alamos, NM, 1990. [6] K. Saitou, Microwave sintering of iron, cobalt, nickel, copper and stainless steel powders, 54 (2006) 875–879. [7] D. Demirskyi, D. Agrawal, A. Ragulya, Neck formation between copper spherical particles under single-mode and multimode microwave sintering, Mater. Sci. Eng. A 527 (7–8) (2010) 2142–2145. [8] D. Demirskyi, D. Agrawal, A. Ragulya, Neck growth kinetics during microwave sintering of nickel powder, J. Alloys Compd. 509 (5) (2011) 1790–1795. [9] D. Demirskyi, D. Agrawal, A. Ragulya, Densification kinetics of powdered copper under single-mode and multimode microwave sintering, Mater. Lett. 64 (13) (2010) 1433–1436. [10] S. Takayama, Y. Saito, M. Sato, T. Nagasaka, T. Muroga, Y. Ninomiya, Sintering behavior of metal powders involving microwave-enhanced chemical reaction, Jpn. J. Appl. Phys. 45 (3R) (2006) 1816. [11] A. Mondal, D. Agrawal, A. Upadhyaya, Microwave heating of pure copper powder with varying particle size and porosity, J. Microw. Power Electromagn. Energy. 43 (1) (2008) 5–10. [12] MPIF, Standard Test Methods for Metal Powders and Powder Metallurgy Products Standard 3, 46, 48, Princeton, NJ, USA, MPIF, 1991. [13] R.M. German, Sintering concept, Powder Metallurgy and Particulate Materals Processing, MPIF, 2005, pp. 219–236. [14] W.F. Hosford, Lattice rotation in compression, Mechanical Behavior of Materials, Cambridge University Press, 2010, p. 131. [15] D. Demirskyi, D. Agrawal, A. Ragulya, Neck growth kinetics during microwave sintering of copper, Scr. Mater. 62 (8) (2010) 552–555. [16] N. Hackerman, N.H. Simpson, Rates of surface self-diffusion over the principal planes of a single crystal of copper, Trans. Faraday Soc. 52 (1956) 628–633. [17] U.F. Kocks, C.N. Tomé, H.-R. Wenk, A.J. Beaudoin, Texture and Anisotropy: Preferred Orientations in Polycrystals and Their Effect on Materials Properties, Cambridge university press, 2000.
4.4. Heating mode comparison In comparison to conventionally sintered copper samples which showed 〈101〉 fiber texture at intermediate and higher sintering temperature in earlier work [19], microwave sintered samples show random texture at lower and intermediate and 〈001〉 fiber texture at the higher sintering temperature. This randomization might be due the nature of microwave heating which involves different sintering mechanisms like plasma discharging [20], evaporation of matter and even melting may occur at particle contact areas [15] in addition to conventional diffusion mechanisms. This might randomizes the orientation of grains through particle rearrangement. For both heating modes Σ3 twin boundary population shows a decrease during transition from intermediate to higher sintering temperature in account of grain growth and coalescence of grains. Pore area fraction also decreases from lower sintering temperature to higher sintering temperature for both heating modes. As compared to conventional sintering, microwave sintering produced equivalent grain size at 45 min isothermal holding time than in a lesser time. The overall sintering time got reduced by 75–80% in case of microwave sintering, as a result a lot of energy is saved. This is because of volumetric heating of the compact during microwave sintering. This avoids the need for slow heating rate and intermittent holding times that directly contribute to more energy consumption during conventional sintering. 8
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[20] G. Veltl, F. Petzoldt, P. Pueschner, “Effects of Microwaves on Sintering Processes,” Presented at the European Powder Metallurgy Conference (EURO PM2004-Vienna, Austria), (2004), pp. 106–111.
[18] V.A. Invenson, Densification of Metal Powders During Sintering, Springer US, 1973. [19] N.G. Felege, N.P. Gurao, A. Upadhyaya, Evolution of microtexture and microstructure during sintering of copper, Metall. Mater. Trans. A 50 (9) (2019) 4193–4204, https://doi.org/10.1007/s11661-019-05317-7.
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Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microstructure, microtexture and grain boundary character evolution in microwave sintered copper
T
⁎
G.N. Felege , N.P. Gurao, Anish Upadhyaya Department of Materials Science & Engineering, Indian Institute of Technology Kanpur, 208016 U.P., India
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper Microwave sintering Microtexture Microstructure EBSD
In the present investigation, the evolution of microstructure and microtexture of commercially available copper powder during solid state sintering using microwave furnace has been studied. The powder was compacted using 300 MPa uniaxial die pressure and the green compacts were sintered at 610 °C, 880 °C and 1020 °C in argon atmosphere at 20 min and 45 min isothermal holding times using multi-mode microwave furnace which has output power of 3 kW and operated at 2.45 GHz frequency. The temperatures were selected to obtain different dominant densification processes. For sintered samples comparative analysis has been made based on densification behavior, microstructural and microtexture evolution and mechanical properties. Electron backscatter diffraction (EBSD) study indicated that there is a distinct evolution of microtexture and microstructure in terms of evolution of grain boundary character distribution, misorientation, size, shape and morphology of grains and pores. Attributed to high temperature driven volume diffusion process, 1020 °C sintered samples showed better sintering between copper particles, grain growth, grain coalescence, reduction in porosity area fraction and increased pore rounding. For all microwave sintered samples, the highest fraction of CSL boundary fraction found to be Σ3 boundary with a common (111) grain boundary plane which indicated coherent twin boundaries with low energy. More Σ3 twin population is found on intermediate sintering temperature range whereas significant reduction is observed higher temperature sintering which might be the result of grain coalescence and volume diffusion at higher sintering temperature that reduced high angle grain boundaries. Microwave sintered copper samples brought random microtexture evolution with variation of sintering temperature and holding time. This might indicate the presence of non-conventional sintering mechanisms during microwave sintering.
1. Introduction
reduce time and energy consumption and to achieve improved properties, microwave processing technique has been applied for sintering of metals [1–3]. Opposing to conventional sintering that heats the sample through radiation and conduction, microwave sintering heats the sample through conversion of absorbed electromagnetic energy into thermal energy throughout the volume of the metal powder compact. Consequently, the heating is uniform and fast [1,2]. Microwaves do not interact with bulk metals due to small microwave penetration depth. Hence, microwave interaction with the bulk metal is restricted to the surface only. This is because, metals are good conductors hence there will not be any induced internal electric field inside the bulk metal, instead the charge induced will be on the surface as a result the incident microwave will be reflected. Contrary to this, a metal powder compact absorb microwave. This is because metal powders have high surface area as compared to bulk metals, subsequently most of the powder body couples with microwave and heating takes place. If the particle size is coarser, the center of the particle heats up
Powder metallurgy is important processing route for producing different near net shaped objects from metals and alloys as well as ceramics. In powder metallurgy sintering is the key step which provides most of the final properties of the material. Sintering in pure metal powder compact occurs through solid state diffusion of material at atomic level. This will result microstructural evolution and densification on the compacts. Most commonly sintering is carried out in conventional electric furnace, where the heating is carried out through radiation from the electric heat source to the sample and hence the sample heats from surface to the inner core through conduction. This causes non-uniformity on the sinter density of the sintered components. To alleviate this problem slow heating rate and intermittent holding employed during the sintering cycle, as a result long processing time and more energy is required during conventional sintering. In recent years, to
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Corresponding author at: Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, 208016, India. E-mail addresses: [email protected] (G.N. Felege), [email protected] (N.P. Gurao), [email protected] (A. Upadhyaya).
https://doi.org/10.1016/j.matchar.2019.109921 Received 4 May 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 06 September 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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through heat conduction from the surface to the inner core of the particle. Recent studies reported that the source of microwave heating effect varies with the material. In ceramics, the heating comes from electric field part of the microwave through the dielectric loss mechanisms include electronic polarization, dipolar polarization, ionic polarization and interfacial polarization. In case of pure metal powders and carbide samples, the heating mainly comes from magnetic component of microwave through magnetic loss which includes hysteresis, eddy currents, magnetic resonance and domain wall oscillations [3,4]. The sinterability of copper in microwave furnace has been reported by many investigators. Sheinberg et al. [5] used microwave sintering to consolidate dispersion-strengthened copper by compacting copper powder samples which were exposed to air to form thin layer of copper oxide at 650 °C using a 2.45 GHz microwave oven operating at maximum power of 700 W. The work by Cheng et al. [3], showed using single mode microwave sintering furnace that bulk copper do not get heated up in both electrical and magnetic field but copper powder compacts heated up in both fields. The shrinkage behavior of copper powder compacts in microwave and conventional sintering furnaces studied by Saitou et al. [6]. They found out the shrinkage parameter for microwave sintering is higher than that of conventional sintering at low temperature. In addition, based on activation energy calculation they concluded microwave sintering did not brought change on sintering mechanism different from conventional one. On the contrary, Demirskyi et al. [7–9] in their series of investigation studied neck formation and growth and densification kinetics of copper powder and compacts under single-mode and multimode microwave sintering at 2.45 GHz. At initial stage of sintering they observed traces of plasma discharging on the fracture surface of porous sintered copper samples which is an indication of enhanced sintering. On their neck growth study, they have proposed that surface diffusion is the predominant mechanism in the initial stages of microwave sintering. On the intermediate stage they observed some abnormal neck growth rate and they attributed it to non-conventional diffusion mechanisms. In cases where long sintering time is employed volume diffusion can become active at this stage. At higher sintering stage in similar fashion as that of conventional sintering, volume diffusion is predominant diffusion mechanism. In addition, they attributed the abnormality to melt formation and evaporation of matter between interparticle contacts may exist due to eddy current development on surface or over heating in contact zone that can facilitates particle rearrangement via viscous flow interparticle sliding and accommodation and hence accelerated shrinkage occurs on the compact. On their related study they analyzed densification kinetics using densification parameter and obtained faster sintering kinetics on single-mode applicator than multimode ones and they attributed this to partial melting of matter. Takayama et al. [10] proved the possibility of microwave sintering of copper powder compacts in the absence of protective atmosphere. The effect of particle size and initial porosity level on microwave heating rate of copper powder compacts was studied by Avijit et al. [11]. The study indicated smaller particle size and high initial porosity caused high microwave heating rate. Although the microwave sinterability of copper reported by many researchers, evolution of microstructure, microtexture and grain boundary character of copper in microwave sintering is not yet explored. In view of this, the present study investigates the evolution of microstructure, microtexture and grain boundary character of microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times.
Table 1 Cu powder characteristics. Characteristics
Cu
Apparent density, (g/ cm3)
Tap density, (g/ cm3)
Flow time, (sec/ 50 g)
Particle size, μm D10
D50
D90
4.58
5.35
6.67
13.4
28.4
57.4
received powder was characterized for particle morphology (size and shape) and flow behavior as per the Metal Powder Industries Federation standard, [12] and the data are shown in Table 1. To study the morphology of the powder a scanning electron microscope (Zeiss Evo 50, UK) was used and the morphology of the powders was obtained to be spherical with various particle size. A laser-scattering particle size analyzer (Fritsch, Germany) was used for the particle size determination. The powder was weighed and put into cylindrical die, then pressed using 300 MPa compaction pressure at room temperature to make cylindrical pellets (height ~6 mm, and diameter ~16.08 mm) using a uniaxial semi-automatic hydraulic press (model CTM-100, Blue star, India). The die was cleaned with acetone and lubricated with zinc stearate prior to each powder compaction to minimize friction. The green compacts of cylindrical pellets (as pressed) were sintered at 610 °C, 880 °C and 1020 °C in argon atmosphere using microwave furnace (Supplier: Enerzi Microwave Systems Pvt. Ltd., model PTF 2730, Bangalore-10, India) which has output power of 3 kW and operated at 2.45 GHz frequency at two isothermal holding times. The sintered density which is expressed in grams per cubic centimeter was obtained by dimensional measurements and counter checked using Archimedes displacement method. The sintered samples were polished in a series of SiC emery papers (paper grades 220, 350, 500 and 1000), followed by cloth polishing using a suspension of 1 μm, 0.3 μm alumina diluted with water and then colloidal silica polishing (0.04 μm), finally light electro polishing was carried out applying 6 V DC supply in an electrolyte having composition of 25%H3PO4 + 50% H2O + 25% Ethanol, for 50 s.
2.2. Characterization An optical microscope (Leica, DM2500, Germany) was used to obtain the micrographs of sintered samples taken along the surface perpendicular to uniaxial compaction direction. Micro hardness of the sintered samples was measured using Vickers hardness tester (Bareiss, Germany). Hardness measurements were taken on mirror finish polished sample surface perpendicular to compaction direction by applying 0.1 kg of load for 10 s. EBSD experiments were carried out to study the evolution of microstructure and microtexture of the sintered samples using Nova Nano SEM 450 Field-Emission scanning electron microscope with 1 nm ultimate resolution. The samples were mounted on 70° pre-tilted EBSD sample holder from horizontal sample stage and the working distance was set to 15 mm away from pole piece. On each sample EBSD scans were performed on an area of ~900 × 900 μm2 scan area with a step size 0.7 μm at 20 kV operating voltage. Kikuchi patterns were collected and automatically indexed using Orientation Imaging Microscopy (OIM™) software (V. 6). Analysis of collected EBSD data was carried out using OIM ™ Analysis V7.3 software from TSL. For analyzing the distribution of crystallographic orientation in terms of pole figures and inverse pole figures Orientation Distribution (OD) of crystallites was calculated using Harmonic Series Expansion method with Series rank 22, Gaussian Half Width is 5°, axial sample symmetry, and 5° Gaussian smoothing. Primary recrystallization twin (Σ3) in sintered copper samples is obtained by considering 60° misoreintation about a 〈111〉 crystal axis within 8.6° tolerance Brandon's criterion. Secondary recrystallization twin (Σ9) in sintered copper samples is obtained by
2. Experimental procedure 2.1. Material and processing For the present investigation, gas atomized copper powder (Metal Powder Company Ltd., India) is used as starting material. The as 2
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Fig. 1. Thermal profiles followed for microwave sintering of copper at 100 °C/min heating rate for (a) 20 min and (b) 45 min isothermal holding time.
considering 38.9° misoreintation about a 〈110〉 crystal axis within 5° tolerance Brandon's criterion.
temperatures compared to 20 min holding time sintered copper samples. This is due to prolonged holding time that gave more time for the material transport. Fig. 3 shows SEM micrographs obtained from the surface perpendicular to uniaxial compaction direction for all microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times. The microstructure and grain size distribution (Figs. 2 and 3) clearly indicates grain coarsening and higher grain size for copper samples sintered at 1020 °C temperature for 45 min holding time. This is because, higher sintering temperature causes volume diffusion and longer holding time gives more time for volume diffusion to take place. With increasing sintering temperature for both holding times pore size reduces, and porosity shape changes significantly. Fig. 4 compares percentage pore area for samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min holding times. The bar graphs indicate a decreasing trend of pore fraction for both isothermal holding times. As compared to 20 min holding time, 45 min holding time sintered copper samples show less pore area fraction with an increase in sintering temperature from 610 °C to 1020 °C. The reduced pore area fraction is attributed to the longer holding time that allows more material diffusion to occur during sintering process. Grain boundary character distribution (GBCD) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal
3. Results 3.1. Starting material and processing The characteristics of the as received powder are shown in Table 1. The median particle size (D50) is found to be 28.4 μm and arithmetic mean is 36.8 μm. The powder has spherical shape. Fig. 1 compares the thermal profiles of microwave sintered copper samples for 20 min and 45 min holding time. The diagram indicated 75–80% overall sintering time reduction as compared to conventional resistance electric furnace sintering. Because of this a lot of energy being saved. 3.2. Evolution of microstructure The average grain size for all microwave sintered copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 min and 45 min holding times is given in Fig. 2. The diagrams indicated that as sintering temperature increases for both holding times grain size increases. Bigger grain size is observed for 45 min holding time for all sintering
Fig. 2. Plot of grain size distribution of cumulative area fraction for all microwave sintered copper samples at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min holding times. 3
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Fig. 3. SEM micrographs of microwave sintered copper at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times. Micrographs are taken along the surface perpendicular to uniaxial compaction direction.
holding times. On the contrary, the CSL boundary fraction for copper sample sintered at 610 °C and 1020 °C for 45 min holding time slightly decreases as compared to 20 min holding time sintered samples. Fraction of the random high angle boundaries (HAGB) declines for longer holding time sintered samples attributed to grain coarsening effect, due to grain growth and coalescence. The reduction in HAGB is more pronounced on the long holding time sintered samples. Out of all CSL boundaries considered, Σ3 primary twin and Σ9 secondary twin boundaries show the highest fraction. Primary and secondary recrystallization twin fraction of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is shown in Fig. 6a & b. Sample sintered at 20 min holding time shows an increase in Σ3 boundary fraction from 610 °C to 880 °C sintering temperature and then slightly decreases towards 1020 °C sintered samples. Whereas, 45 min holding time sintered sample shows an increase in Σ3 boundary fraction from 610 °C to 880 °C sintering temperature and then decreases by large fraction towards 1020 °C sintering temperature. The first increment is due to growth of twins and the later one is due to reduction of the total grain boundary including Σ3 twin boundary that occurs at high sintering temperature. The reduction of the boundaries is due to grain coarsening, boundary migration and grain coalescence that comes from volume diffusion at higher sintering temperature coupled with long holding time. Distribution of the Σ3 grain boundary plane orientations of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is given in Fig. 7. The Σ3 boundary planes distribution is obtained at a misorientation of 60°/[111]. For all cases, the largest distribution density of the Σ3 boundary plane orientations is at the axis [111]. This implies that Σ3 boundary plane has higher probability to be distributed on the (111) plane, hence large fraction of the Σ3 boundary is coherent twin boundary. In addition, the results
Fig. 4. Percentage pore area for samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times.
holding times is shown in Fig. 5. As can be seen from the plot, 20 min holding time microwave sintered samples showed very small fraction of low angle boundaries and high fraction of high angle boundaries as compared to 45 min holding time sintered samples. The other group of grain boundaries is coincident site lattice (CSL) boundaries. These are boundaries that contain common atomic positions called coincident sites between neighboring grains. Sigma (Σ) values are used to denote fraction of coincident sites. Sample sintered at 880 °C shows constant CSL boundary fraction regardless of different 4
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Fig. 5. Plot of grain boundary character distribution of microwave copper samples sintered at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min holding times.
indicate that the Σ3 twin boundary has a higher probability distribution density in for 880 °C sintered copper samples at for both holding times and the least probability distribution density is on 1020 °C sintered copper sample with long holding time sintering.
from the texture observed in copper samples sintered at 610 °C and 880 °C temperature. Texture strength is higher for 1020 °C sintered samples as compared to 610 °C and 880 °C sintered copper samples for both holding times. With increasing holding time from 20 to 45 min, the texture strength shows marginal increment.
3.3. Evolution of microtexture 3.4. Densification and mechanical properties Fig. 8 indicates grain coarsening with increasing sintering temperature for both sintering conditions. Most of the coarser grains in copper sample sintered at 1020 °C shows single orientation as a result of coalescence. This is due to preferential growth of certain grains with single orientation during elevated temperature sintering. The overall crystal orientation maps show different orientation for different grains in the microstructure. Inverse pole figure (IPF) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times is shown in Fig. 9. Microwave sintered copper samples shows different texture as a function of sintering temperature. Copper sample sintered at 610 °C, shows weak fiber texture near 〈101〉 for 20 min and near 〈310〉 for 45 min holding time. Copper sample sintered at 880 °C shows near 〈332〉 and 〈321〉 weak fiber texture for 20 min and 45 min holding times respectively. At 1020 °C sintered copper sample shows the same 〈001〉 oriented fiber texture for both holding times, and this is different
Fig. 10 shows variation of sinter density against sintering temperature and holding time. For short holding time, the bar graph indicates a slight decrease in sinter density of copper samples with the increasing sintering temperature, whereas the long holding sintered samples shows slightly more sinter density which is not affected by sintering temperatures variation as compared to low holding time sintered samples. Variation of Vickers micro-hardness (HV0.1) of copper samples, with sintering temperature and holding time is shown in Fig. 11. The hardness value for 610 °C and 880 °C at both holding times is almost similar. But 1020 °C for longer holding time showed significant increment in the hardness. This is because, at lower temperature sintering even if the holding time is different the amount of sintering is very less. That is why there is no difference in Vickers hardness. Whereas at longer holding time difference in holding time brings pronounced effect on the
Fig. 6. Plot of percentage Σ3 and Σ9 twin boundary fraction of copper samples sintered at 610 °C, 880 °C and 1020 °C for (a) 20 min and (b) 45 min isothermal holding times. 5
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Fig. 7. Distribution of the Σ3 grain boundary plane orientations of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times. The distribution is plotted in a standard stereographic projection, and the units are multiples of a random distribution.
4. Discussion
sintering as compared to lower temperature sintering. Hence longer holding time at 1020 °C facilitates more material transport, better densification and reduction in pore number and size. Consequently, better hardness is observed.
The results of the present work indicated evolution of distinct microstructure and microtexture in microwave sintered pure copper
Fig. 8. Crystal orientation maps of copper samples sintered at 610 °C, 880 °C, and 1020 °C in microwave furnace. 6
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Fig. 9. Inverse pole figure (IPF) of copper samples sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times.
samples upon increasing sintering temperature from 610 °C, to 880 °C and then to 1020 °C and increasing holding time from 20 min to 45 min. 4.1. Evolution of microstructure SEM micrographs, grain size and pore area fraction plots clearly indicate microstructural evolutions that took place during microwave sintering of pure copper at different sintering temperature and different isothermal holding time. Copper sample sintered in microwave furnace at 1020 °C for both holding times results an increase in neck size (Fig. 3), grain growth (Fig. 2), coalescence, reduction in porosity and pore rounding. In addition, it shows an increase of the low angle misorientation fraction and a decrease in Σ3 twin boundary fraction. These changes are more pronounced on the copper samples sintered 45 min holding time due to extra time for diffusion to take place. All the microstructural changes observed are mainly due to the high sintering temperature that stimulates random atomic jumps or motion and the long holding time which increases the volume of jumps. This brings diffusion of atoms within the particle and between particles. Diffusion of atoms takes place along grain boundaries, over the surfaces, and through the crystalline lattice [13]. These diffusion processes cause reduction of surface area of the particles and fraction of grain boundaries, and hence reduction in misorientation angle. Because of this, the total free energy of the system reduces and the sintered bond between the particles grows, and the size and number fraction of porosity reduces. Moreover, due to coalescence grain growth occurs. With exception to the above trend, in the present study sintered copper sample at 880 °C sintering temperature shows insignificant change in CSL fraction for both isothermal holding times. At this temperature range, neck size grows, and bigger particles consume small particles, but the amount of coalescence between big particles is not much. Hence most of CSL and high angle boundaries are preserved.
Fig. 10. Sinter density variation of microwave sintered copper samples at 610 °C, 880 °C and 1020 °C sintering temperature for 20 min and 45 min isothermal holding times.
4.2. Evolution of microtexture Inverse pole figures of the sintered copper samples show a distinct microtexture evolution at different sintering temperatures for both isothermal holding times. At 610 °C, the ⟨101⟩ weak fiber texture is observed for 20 min holding time. The observed ⟨101⟩ fiber texture on 610 °C microwave sintered samples is probably due to rotation of grains towards the compression axis during compaction [14]. In addition to that, at this temperature range surface diffusion is dominant mechanism for diffusion [15]. The work by Hackerman et al. [16] on rates of surface self-diffusion over the principal planes of a single crystal of
Fig. 11. Vickers micro-hardness (HV0.1) variation of microwave sintered copper samples with sintering temperature and holding time.
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copper indicates diffusion coefficient is more along 〈110〉 for (100) and (110) planes. Hence this might be the reason for 〈101⟩ fiber texture at this temperature range. However, in case of microwave sintered samples at 610 °C for 45 min holding time, near 〈310⟩ fiber texture is detected which might be because of longer holding time. At 880 °C sintered copper sample shows near 〈332⟩ and 〈321⟩ fiber texture for 20 min and 45 min holding time respectively. In addition to the difference in holding time during sintering, in microwave sintering particularly in intermediate sintering range as discussed by Demirskyi et al. [8] anomalies can happen, which might destroys or randomize texture that acquired during compaction and low-temperature sintering. As a result, the intermediate temperature shows random type of texture as compared to other sintering temperature ranges. Copper samples sintered at 1020 °C for 20 and 45 min holding time show identical 〈001⟩ fiber texture and the latter one exhibits more texture strength. As compared to samples sintered at 610 °C and 880 °C, copper sample microwave sintered at 1020 °C shows higher 〈001⟩ fiber texture strength for both holding times. This type of fiber texture is a characteristics texture during solidification of cubic metals [17]. This is because, during solidification atoms which are in liquid phase prefer to adhere to less closely packed plane of the solid crystal, since it easy to get comfortable locations. That is why 〈100〉 is the fastest growth direction for cubic metals. This might be the case for showing a similar 〈001〉 fiber texture in samples sintered at 1020 °C that might involve temporary melting, and then growth of grains.
5. Conclusion In the present study, gas atomized spherical copper powder was cold compacted at 300 MPa. The green compacts were sintered at 610 °C, 880 °C and 1020 °C for 20 and 45 min isothermal holding times in an argon gas atmosphere using multi-mode microwave furnace, which has output power of 3 kW and operated at 2.45 GHz frequency. Evolution of the microstructure, microtexture and grain boundary character in microwave sintered copper powder compacts were investigated using scanning electron microscope and EBSD. The conclusions drawn from results of the present study are summarized below. Higher sintering temperature coupled with long isothermal holding time in microwave sintered copper samples brought more grain growth and particle coalescence due to high temperature driven volume diffusion process, but did not affected the microtexture evolution of the sintered samples significantly. Random microtexture evolution with variation of sintering temperature and holding time is observed, which is an indication of non-conventional sintering mechanisms during microwave sintering. In all microwave sintered samples, high fraction of Σ3 boundary with (111) common boundary plane is found out of all CSL boundaries. This indicates coherent twin boundaries with relatively low energies. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
4.3. Densification and mechanical property Acknowledgments Long holding time shows better sinter density as compared to short holding time samples. This is the result of having more time for volume diffusion to take place and reduce porosities in size and number. The bar graphs also indicate, a slight decrease in the sinter density of copper samples with an increase in sintering temperature for short holding time. This might be due to elastic expansion of samples in compaction direction. Expansion occurs when samples have sorbed gases during compaction. During sintering with having closed pores on the compact pressure of the trapped gas causes expansion of the sample [18]. Despite the difference in the sinter density, there is no significant difference in hardness for both sintering routes.
The authors would like to thank Department of Materials Science and Engineering at Indian Institute of Technology Kanpur for microscope facility. References [1] R. Roy, D. Agrawal, J. Cheng, S. Gedevanlshvili, Full sintering of powdered-metal bodies in a microwave field, Nature 399 (6737) (1999) 668–670. [2] R.M. Anklekar, D.K. Agrawal, R. Roy, Microwave sintering and mechanical properties of PM copper steel, Powder Metall. 44 (4) (2001) 355–362. [3] J. Cheng, R. Roy, D. Agrawal, Radically different effects on materials by separated microwave electric and magnetic fields, Mater. Res. Innov. 5 (3–4) (2002) 170–177. [4] M. Gupta, E.W.W. Leong, Microwaves and Metals, Wiley, 2008. [5] H. Sheinberg, T.T. Meek, R.D. Blake, Microwaving of Normally Opaque and Semiopaque Substances, Los Alamos National Laboratory (LANL), Los Alamos, NM, 1990. [6] K. Saitou, Microwave sintering of iron, cobalt, nickel, copper and stainless steel powders, 54 (2006) 875–879. [7] D. Demirskyi, D. Agrawal, A. Ragulya, Neck formation between copper spherical particles under single-mode and multimode microwave sintering, Mater. Sci. Eng. A 527 (7–8) (2010) 2142–2145. [8] D. Demirskyi, D. Agrawal, A. Ragulya, Neck growth kinetics during microwave sintering of nickel powder, J. Alloys Compd. 509 (5) (2011) 1790–1795. [9] D. Demirskyi, D. Agrawal, A. Ragulya, Densification kinetics of powdered copper under single-mode and multimode microwave sintering, Mater. Lett. 64 (13) (2010) 1433–1436. [10] S. Takayama, Y. Saito, M. Sato, T. Nagasaka, T. Muroga, Y. Ninomiya, Sintering behavior of metal powders involving microwave-enhanced chemical reaction, Jpn. J. Appl. Phys. 45 (3R) (2006) 1816. [11] A. Mondal, D. Agrawal, A. Upadhyaya, Microwave heating of pure copper powder with varying particle size and porosity, J. Microw. Power Electromagn. Energy. 43 (1) (2008) 5–10. [12] MPIF, Standard Test Methods for Metal Powders and Powder Metallurgy Products Standard 3, 46, 48, Princeton, NJ, USA, MPIF, 1991. [13] R.M. German, Sintering concept, Powder Metallurgy and Particulate Materals Processing, MPIF, 2005, pp. 219–236. [14] W.F. Hosford, Lattice rotation in compression, Mechanical Behavior of Materials, Cambridge University Press, 2010, p. 131. [15] D. Demirskyi, D. Agrawal, A. Ragulya, Neck growth kinetics during microwave sintering of copper, Scr. Mater. 62 (8) (2010) 552–555. [16] N. Hackerman, N.H. Simpson, Rates of surface self-diffusion over the principal planes of a single crystal of copper, Trans. Faraday Soc. 52 (1956) 628–633. [17] U.F. Kocks, C.N. Tomé, H.-R. Wenk, A.J. Beaudoin, Texture and Anisotropy: Preferred Orientations in Polycrystals and Their Effect on Materials Properties, Cambridge university press, 2000.
4.4. Heating mode comparison In comparison to conventionally sintered copper samples which showed 〈101〉 fiber texture at intermediate and higher sintering temperature in earlier work [19], microwave sintered samples show random texture at lower and intermediate and 〈001〉 fiber texture at the higher sintering temperature. This randomization might be due the nature of microwave heating which involves different sintering mechanisms like plasma discharging [20], evaporation of matter and even melting may occur at particle contact areas [15] in addition to conventional diffusion mechanisms. This might randomizes the orientation of grains through particle rearrangement. For both heating modes Σ3 twin boundary population shows a decrease during transition from intermediate to higher sintering temperature in account of grain growth and coalescence of grains. Pore area fraction also decreases from lower sintering temperature to higher sintering temperature for both heating modes. As compared to conventional sintering, microwave sintering produced equivalent grain size at 45 min isothermal holding time than in a lesser time. The overall sintering time got reduced by 75–80% in case of microwave sintering, as a result a lot of energy is saved. This is because of volumetric heating of the compact during microwave sintering. This avoids the need for slow heating rate and intermittent holding times that directly contribute to more energy consumption during conventional sintering. 8
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[20] G. Veltl, F. Petzoldt, P. Pueschner, “Effects of Microwaves on Sintering Processes,” Presented at the European Powder Metallurgy Conference (EURO PM2004-Vienna, Austria), (2004), pp. 106–111.
[18] V.A. Invenson, Densification of Metal Powders During Sintering, Springer US, 1973. [19] N.G. Felege, N.P. Gurao, A. Upadhyaya, Evolution of microtexture and microstructure during sintering of copper, Metall. Mater. Trans. A 50 (9) (2019) 4193–4204, https://doi.org/10.1007/s11661-019-05317-7.
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