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Viscous flow assisted sintering of AlCoCrFeNi high entropy alloy powder
Materials Letters 256 (2019) 126668
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Viscous flow assisted sintering of AlCoCrFeNi high entropy alloy powder S. Rohila, Rahul B. Mane, S. Naskar, Bharat B. Panigrahi ⇑ Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana 502285, India
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 12 September 2019 Accepted 13 September 2019 Available online 13 September 2019 Keywords: Powder technology Sintering Diffusion Thermal analysis Grain boundaries
a b s t r a c t Present investigation attempts to evaluate the sintering kinetics of mechanically alloyed AlCoCrFeNi high entropy alloy powder. Dilatometric study shows an accelerated densification above 1150 °C during nonisothermal sintering. The process was characterized with an activation energy of 166 ± 13 kJ/mol. Sintering was found to be controlled by the viscous flow mechanisms and atomic diffusion of constituent elements in the melt phase. This was attributed to the formation of some Cr-rich viscous phase, which had a good wettability on the solid. Results were further confirmed by an endothermic peak during differential scanning calorimetry and the solidified phases around the grains, in the quenched microstructure. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
High entropy alloys (HEAs) are known for their high strength, good corrosion and wear resistance over a wide range of temperatures [1,2]. Processing routes may affect the phase composition and properties of the alloy significantly [1–3]. Among the fundamental understanding about HEAs, diffusion is one issue, which has not been fully resolved so far. Initial studies have suggested, the diffusions are sluggish in nature [2]; however, many investigators contradict this [4]. Alloying order (number of alloying elements) was found to affect the sintering kinetics of HEA powders (CoFeNi, CoFeNiCr, CoFeNiCrMn and CoFeNiCrMnAl alloys) [5]. Densification rate decreases and activation energy increases, with increasing number of elements [5]. AlCoCrFeNi alloy is of a particular interest due to its attractive properties. Recently mechanically alloyed AlCoCrFeNi powder was sintered to nearly full density (over 98%) through pressureless sintering at 1275 °C [6]. This densification was attributed to the formation of some Cr-rich liquid phase at higher temperatures. However, the sintering kinetics have not yet been fully analyzed. Present investigation aims to study the dilatometric sintering behavior of AlCoCrFeNi HEA powder and attempts to evaluate densification kinetics during non-isothermal heating.
Equiatomic AlCoCrFeNi alloy powder was synthesized through mechanical alloying from elemental powders (Al, Co, Cr, Fe and Ni) as reported previously [6]. Powder was compacted using a steel die of about 7 mm diameter at a uniaxial pressure of about 100 MPa to a relative green density of about 53%. Green pellet was placed on a vertical dilatometer (Theta Industries Inc., USA). Sample was heated at a rate of about 10 °C/min up to about 1250 °C under high purity Ar gas. To capture the high temperature phases, few samples were quenched from 1000 °C and 1250 °C in to water. Alloy powder was subjected to differential scanning calorimetry (DSC) while heated at the same rate. Samples were characterized using x-ray diffraction (Rigaku Ultima IV) and scanning electron microscope (SEM JSM-7800F, JEOL) equipped with EDS (Octane Elite).
⇑ Corresponding author. E-mail address: [email protected] (B.B. Panigrahi). https://doi.org/10.1016/j.matlet.2019.126668 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
3. Results and discussion Alloy powder contains BCC phase as a major fraction, along with some FCC and B2 phases (Fig. 1). The ratio of BCC and FCC phases was reported to vary with increasing temperatures [7]. During the dilatometric study (Fig. 2), initially, length increases almost. At about 464 °C, the shrinkage commences, which was continued till about 1100 °C. Above 1100 °C, sample undergoes a rapid shrinkage. Relative sintered density was found to be about 87% at 1250 °C. At lower temperature range, no significant change was observed during DSC analysis (Fig. 2). However, at the higher
2
S. Rohila et al. / Materials Letters 256 (2019) 126668
temperature range small endothermic peak was observed, indicating formation of some viscous phase. SEM micrograph (Fig. 3a) of the sample quenched from 1000 °C shows acicular grains. In contrast, the sample quenched from 1250 °C contains multiple phases (Fig. 3b). Grey colour grains are surrounded by bright layers at the edges, attributed to the solidified viscous phase. Bright region shows Cr-rich phase (Fig. 3c), compared to grey regions (Fig. 3d). Sample quenched from 1000 °C shows FCC as a major fraction (Fig. 1). Sample quenched from 1250 °C shows BCC as the major phase along with FCC and B2 phases. Further, high temperature shrinkage data was analyzed for different sintering mechanisms, such as grain boundary diffusion (GBD), volume diffusion (VD) and viscous flow (VF) mechanisms, using non-isothermal sintering model [8], as shown in Eq. (1).
ln½Tp ðdY=dTÞ ¼ lnC ½Q =ðn þ 1ÞRT Fig. 1. XRD patterns of powder and quenched samples.
Fig. 2. Dilatometer curve of AlCoCrFeNi compact. DSC curve of milled powder has been shown in the inset.
ð1Þ
where T is the temperature, Y is the axial shrinkage, Q is activation energy (AE) of given mechanism, R is the universal gas constant and C is the constant. Values of n are 0, 1, 2 and that of P are 1, 3/2 and 5/3 for VF, VD and GBD respectively. From the shrinkage data, Arrhenius plots were produced using Eq. (1) for different mechanisms (Fig. 4a). AE values were found to be 166 ± 13 kJ/mol, 522 ± 39 kJ/mol and 344 ± 26 kJ/mol, for VF, GBD and VD mechanisms, respectively. Estimated AEs were compared with the literature data [9,10]. It was found that the AEs estimated in the present work for GBD and VD mechanisms were relatively larger than the literature data range (Fig. 4b). This comparison suggests, the possibility of domination of GBD or VD mechanisms are unlikely in the current system. VF mechanism generally acts in glossy phase system or liquid phase system. To verify this, it needs viscosity related data for this alloy, which have been shown in Fig. 4(c). AEs for VF above melting temperature, for Al, Co, Fe and Ni were reported to be 16.5, 44.4, 41.4 and 50.2 kJ/mol respectively [11]. These values are much lower than the estimated AE in the present system (Fig. 4c). Interestingly, AE for VF for Cr melt, was reported to be unusually high (185 kJ/mol). Recently, Ding et al. [12] reported the ab-initio molecular dynamics (AIMD) simulation on melts of CrCoNi, CrFe-
Fig. 3. SEM images of: (a) sample quenched from 1000 °C, (b) Sample quenched from 1250 °C, (c) and (d) are the EDS pattern at Point 1 and Point 2 in (b).
S. Rohila et al. / Materials Letters 256 (2019) 126668
3
observed AE, endothermic peak during DSC analysis and microstructural evidences strongly suggest, the densification at higher temperature is controlled by viscous flow mechanism and diffusion of constituent atoms in melt phase. 4. Conclusions Sintering behavior of the mechanically alloyed AlCoCrFeNi HEA powder was studied through the dilatometric experiments. At higher temperature, sample exhibited a rapid shrinkage during non-isothermal heating and yield a sintered density of about 87%. Sintering was found to be characterized with an activation energy of 166 ± 13 kJ/mol and attributed to the viscous flow mechanism and atomic diffusion in liquid state. Viscous phase was found to be a Cr-rich phase, which had formed during mechanical alloying stage. Utilization of presence of small fraction of low melting phase, could be one of the alternate route to sinter high entropy alloy powders. Declaration of Competing Interest 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. Acknowledgement Fig. 4. (a) Arrhenius plots for different mechanisms, (b) comparison of present data with the range of activation energies reported on literature for GBD & VD [9,10] and VF [11,12], and (c) comparison of present VF activation energy with literature data [11,12].
DST-SERB (Govt. of India) is gratefully acknowledged for FIST-I funding for SEM to MSME, IITH. References
CoNi and CrMnFeCoNi to understand the characteristic of the melt phase. Though the composition of the present alloy (Al containing) is slightly different than the above alloys; it appears that the AE obtained for VF mechanism (166 ± 13 kJ/mol) is nearly the same (Fig. 4c). In the additive sintering or liquid phase sintering; two factors are very important: a) grain boundary wetting, and b) solubility of base alloy in to additive phase or liquid phase. A good wetting of the grain boundaries, enables grain sliding and rearrangements, i.e. densification. In addition, if the liquid phase has a good solubility for the base alloy, it accelerates the mass transport process. In the current HEA, all elements have capability of forming solid solution to a great extent; that means, a good solubility would be there in the viscous phase too. It appears that Cr seems to be segregated at the grain boundaries and diffused more to the viscous phase, hence, the fraction of secondary phase increased during sintering (Fig. 3d). During cooling, the solidified phase also has a good wetting, hence, it formed a network (bright phase) around the solid grains (Fig. 3b). Segregation and wetting phenomena was reported earlier [13] on a low carbon ferritic steel also, where a network of cementite (Fe3C) was formed due to grain boundary wetting around a-Fe (ferrite) grains. In another study [14], on the ultrafine grained Fe-C alloy produced through the high pressure torsion, segregation of carbon at the grain boundaries yielded the network of cementite around the ferrite grains. In the present HEA powder,
[1] B.S. Murty, J.W. Yeh, S. Ranganathan, High-Entropy Alloys, 1st ed., Butterworth-Heinemann, London, 2014, doi.org/10.1016/C2013-0-14235-3. [2] D.B. Miracle, O.N. Senkov, Acta Mater. 122 (2017) 448–511, https://doi.org/ 10.1016/j.actamat.2016.08.081. [3] M. Vaidya, G.M. Muralikrishna, B.S. Murty, J. Mater. Res. 34 (2019) 664–686, https://doi.org/10.1557/jmr.2019.37. [4] S.V. Divinski, A.V. Pokoev, N. Esakkiraja, A. Paul, Diffusion Foundations 17 (2018) 69–104, https://doi.org/10.4028/www.scientific.net/df.17.69. [5] R.B. Mane, B.B. Panigrahi, Mater. Lett. 217 (2018) 131–134, https://doi.org/ 10.1016/j.matlet.2018.01.092. [6] S. Rohila, R.B. Mane, G. Ummethala, B.B. Panigrahi, J. Mater. Res. 34 (2019) 777–786, https://doi.org/10.1557/jmr.2019.9. [7] A. Munitz, S. Salhov, S. Hayun, N. Frage, J. Alloys Compd. 683 (2016) 221–230, https://doi.org/10.1016/j.jallcom.2016.05.034. [8] J. Han, A.M. Senos, P. Mantas, J. Eur. Ceram. Soc. 19 (1999) 1003–1006, https:// doi.org/10.1016/s0955-2219(98)00362-8. [9] R.B. Mane, Y. Rajkumar, B.B. Panigrahi, Powder Metall. 61 (2018) 131–138, https://doi.org/10.1080/00325899.2018.1433268. [10] R.B. Mane, B.B. Panigrahi, J. Mater. Res. 33 (2018) 3321–3329, https://doi.org/ 10.1557/jmr.2018.225. [11] L. Battezzati, A.L. Greer, Acta Metallurgica 37 (1989) 1791–1802, https://doi. org/10.1016/0001-6160(89)90064-3. [12] J. Ding, M. Asta, R.O. Ritchie, Appl. Phys. Lett. 113 (2018), https://doi.org/ 10.1063/1.5045216. [13] B.B. Straumal, Y.O. Kucheev, L.I. Efron, A.L. Petelin, J.D. Majumdar, I. Manna, J. Mater. Eng. Perform. 21 (2012) 667–670, https://doi.org/10.1007/s11665-0120130-6. [14] B.B. Straumal, S.V. Dobatkin, A.O. Rodin, S.G. Prostasova, A.A. Mazilin, D. Goll, B. Baretzky, Adv. Eng. Mater. 13 (2011) 463–469, https://doi.org/10.1002/ adem.201000312.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Viscous flow assisted sintering of AlCoCrFeNi high entropy alloy powder S. Rohila, Rahul B. Mane, S. Naskar, Bharat B. Panigrahi ⇑ Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana 502285, India
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 12 September 2019 Accepted 13 September 2019 Available online 13 September 2019 Keywords: Powder technology Sintering Diffusion Thermal analysis Grain boundaries
a b s t r a c t Present investigation attempts to evaluate the sintering kinetics of mechanically alloyed AlCoCrFeNi high entropy alloy powder. Dilatometric study shows an accelerated densification above 1150 °C during nonisothermal sintering. The process was characterized with an activation energy of 166 ± 13 kJ/mol. Sintering was found to be controlled by the viscous flow mechanisms and atomic diffusion of constituent elements in the melt phase. This was attributed to the formation of some Cr-rich viscous phase, which had a good wettability on the solid. Results were further confirmed by an endothermic peak during differential scanning calorimetry and the solidified phases around the grains, in the quenched microstructure. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
High entropy alloys (HEAs) are known for their high strength, good corrosion and wear resistance over a wide range of temperatures [1,2]. Processing routes may affect the phase composition and properties of the alloy significantly [1–3]. Among the fundamental understanding about HEAs, diffusion is one issue, which has not been fully resolved so far. Initial studies have suggested, the diffusions are sluggish in nature [2]; however, many investigators contradict this [4]. Alloying order (number of alloying elements) was found to affect the sintering kinetics of HEA powders (CoFeNi, CoFeNiCr, CoFeNiCrMn and CoFeNiCrMnAl alloys) [5]. Densification rate decreases and activation energy increases, with increasing number of elements [5]. AlCoCrFeNi alloy is of a particular interest due to its attractive properties. Recently mechanically alloyed AlCoCrFeNi powder was sintered to nearly full density (over 98%) through pressureless sintering at 1275 °C [6]. This densification was attributed to the formation of some Cr-rich liquid phase at higher temperatures. However, the sintering kinetics have not yet been fully analyzed. Present investigation aims to study the dilatometric sintering behavior of AlCoCrFeNi HEA powder and attempts to evaluate densification kinetics during non-isothermal heating.
Equiatomic AlCoCrFeNi alloy powder was synthesized through mechanical alloying from elemental powders (Al, Co, Cr, Fe and Ni) as reported previously [6]. Powder was compacted using a steel die of about 7 mm diameter at a uniaxial pressure of about 100 MPa to a relative green density of about 53%. Green pellet was placed on a vertical dilatometer (Theta Industries Inc., USA). Sample was heated at a rate of about 10 °C/min up to about 1250 °C under high purity Ar gas. To capture the high temperature phases, few samples were quenched from 1000 °C and 1250 °C in to water. Alloy powder was subjected to differential scanning calorimetry (DSC) while heated at the same rate. Samples were characterized using x-ray diffraction (Rigaku Ultima IV) and scanning electron microscope (SEM JSM-7800F, JEOL) equipped with EDS (Octane Elite).
⇑ Corresponding author. E-mail address: [email protected] (B.B. Panigrahi). https://doi.org/10.1016/j.matlet.2019.126668 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
3. Results and discussion Alloy powder contains BCC phase as a major fraction, along with some FCC and B2 phases (Fig. 1). The ratio of BCC and FCC phases was reported to vary with increasing temperatures [7]. During the dilatometric study (Fig. 2), initially, length increases almost. At about 464 °C, the shrinkage commences, which was continued till about 1100 °C. Above 1100 °C, sample undergoes a rapid shrinkage. Relative sintered density was found to be about 87% at 1250 °C. At lower temperature range, no significant change was observed during DSC analysis (Fig. 2). However, at the higher
2
S. Rohila et al. / Materials Letters 256 (2019) 126668
temperature range small endothermic peak was observed, indicating formation of some viscous phase. SEM micrograph (Fig. 3a) of the sample quenched from 1000 °C shows acicular grains. In contrast, the sample quenched from 1250 °C contains multiple phases (Fig. 3b). Grey colour grains are surrounded by bright layers at the edges, attributed to the solidified viscous phase. Bright region shows Cr-rich phase (Fig. 3c), compared to grey regions (Fig. 3d). Sample quenched from 1000 °C shows FCC as a major fraction (Fig. 1). Sample quenched from 1250 °C shows BCC as the major phase along with FCC and B2 phases. Further, high temperature shrinkage data was analyzed for different sintering mechanisms, such as grain boundary diffusion (GBD), volume diffusion (VD) and viscous flow (VF) mechanisms, using non-isothermal sintering model [8], as shown in Eq. (1).
ln½Tp ðdY=dTÞ ¼ lnC ½Q =ðn þ 1ÞRT Fig. 1. XRD patterns of powder and quenched samples.
Fig. 2. Dilatometer curve of AlCoCrFeNi compact. DSC curve of milled powder has been shown in the inset.
ð1Þ
where T is the temperature, Y is the axial shrinkage, Q is activation energy (AE) of given mechanism, R is the universal gas constant and C is the constant. Values of n are 0, 1, 2 and that of P are 1, 3/2 and 5/3 for VF, VD and GBD respectively. From the shrinkage data, Arrhenius plots were produced using Eq. (1) for different mechanisms (Fig. 4a). AE values were found to be 166 ± 13 kJ/mol, 522 ± 39 kJ/mol and 344 ± 26 kJ/mol, for VF, GBD and VD mechanisms, respectively. Estimated AEs were compared with the literature data [9,10]. It was found that the AEs estimated in the present work for GBD and VD mechanisms were relatively larger than the literature data range (Fig. 4b). This comparison suggests, the possibility of domination of GBD or VD mechanisms are unlikely in the current system. VF mechanism generally acts in glossy phase system or liquid phase system. To verify this, it needs viscosity related data for this alloy, which have been shown in Fig. 4(c). AEs for VF above melting temperature, for Al, Co, Fe and Ni were reported to be 16.5, 44.4, 41.4 and 50.2 kJ/mol respectively [11]. These values are much lower than the estimated AE in the present system (Fig. 4c). Interestingly, AE for VF for Cr melt, was reported to be unusually high (185 kJ/mol). Recently, Ding et al. [12] reported the ab-initio molecular dynamics (AIMD) simulation on melts of CrCoNi, CrFe-
Fig. 3. SEM images of: (a) sample quenched from 1000 °C, (b) Sample quenched from 1250 °C, (c) and (d) are the EDS pattern at Point 1 and Point 2 in (b).
S. Rohila et al. / Materials Letters 256 (2019) 126668
3
observed AE, endothermic peak during DSC analysis and microstructural evidences strongly suggest, the densification at higher temperature is controlled by viscous flow mechanism and diffusion of constituent atoms in melt phase. 4. Conclusions Sintering behavior of the mechanically alloyed AlCoCrFeNi HEA powder was studied through the dilatometric experiments. At higher temperature, sample exhibited a rapid shrinkage during non-isothermal heating and yield a sintered density of about 87%. Sintering was found to be characterized with an activation energy of 166 ± 13 kJ/mol and attributed to the viscous flow mechanism and atomic diffusion in liquid state. Viscous phase was found to be a Cr-rich phase, which had formed during mechanical alloying stage. Utilization of presence of small fraction of low melting phase, could be one of the alternate route to sinter high entropy alloy powders. Declaration of Competing Interest 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. Acknowledgement Fig. 4. (a) Arrhenius plots for different mechanisms, (b) comparison of present data with the range of activation energies reported on literature for GBD & VD [9,10] and VF [11,12], and (c) comparison of present VF activation energy with literature data [11,12].
DST-SERB (Govt. of India) is gratefully acknowledged for FIST-I funding for SEM to MSME, IITH. References
CoNi and CrMnFeCoNi to understand the characteristic of the melt phase. Though the composition of the present alloy (Al containing) is slightly different than the above alloys; it appears that the AE obtained for VF mechanism (166 ± 13 kJ/mol) is nearly the same (Fig. 4c). In the additive sintering or liquid phase sintering; two factors are very important: a) grain boundary wetting, and b) solubility of base alloy in to additive phase or liquid phase. A good wetting of the grain boundaries, enables grain sliding and rearrangements, i.e. densification. In addition, if the liquid phase has a good solubility for the base alloy, it accelerates the mass transport process. In the current HEA, all elements have capability of forming solid solution to a great extent; that means, a good solubility would be there in the viscous phase too. It appears that Cr seems to be segregated at the grain boundaries and diffused more to the viscous phase, hence, the fraction of secondary phase increased during sintering (Fig. 3d). During cooling, the solidified phase also has a good wetting, hence, it formed a network (bright phase) around the solid grains (Fig. 3b). Segregation and wetting phenomena was reported earlier [13] on a low carbon ferritic steel also, where a network of cementite (Fe3C) was formed due to grain boundary wetting around a-Fe (ferrite) grains. In another study [14], on the ultrafine grained Fe-C alloy produced through the high pressure torsion, segregation of carbon at the grain boundaries yielded the network of cementite around the ferrite grains. In the present HEA powder,
[1] B.S. Murty, J.W. Yeh, S. Ranganathan, High-Entropy Alloys, 1st ed., Butterworth-Heinemann, London, 2014, doi.org/10.1016/C2013-0-14235-3. [2] D.B. Miracle, O.N. Senkov, Acta Mater. 122 (2017) 448–511, https://doi.org/ 10.1016/j.actamat.2016.08.081. [3] M. Vaidya, G.M. Muralikrishna, B.S. Murty, J. Mater. Res. 34 (2019) 664–686, https://doi.org/10.1557/jmr.2019.37. [4] S.V. Divinski, A.V. Pokoev, N. Esakkiraja, A. Paul, Diffusion Foundations 17 (2018) 69–104, https://doi.org/10.4028/www.scientific.net/df.17.69. [5] R.B. Mane, B.B. Panigrahi, Mater. Lett. 217 (2018) 131–134, https://doi.org/ 10.1016/j.matlet.2018.01.092. [6] S. Rohila, R.B. Mane, G. Ummethala, B.B. Panigrahi, J. Mater. Res. 34 (2019) 777–786, https://doi.org/10.1557/jmr.2019.9. [7] A. Munitz, S. Salhov, S. Hayun, N. Frage, J. Alloys Compd. 683 (2016) 221–230, https://doi.org/10.1016/j.jallcom.2016.05.034. [8] J. Han, A.M. Senos, P. Mantas, J. Eur. Ceram. Soc. 19 (1999) 1003–1006, https:// doi.org/10.1016/s0955-2219(98)00362-8. [9] R.B. Mane, Y. Rajkumar, B.B. Panigrahi, Powder Metall. 61 (2018) 131–138, https://doi.org/10.1080/00325899.2018.1433268. [10] R.B. Mane, B.B. Panigrahi, J. Mater. Res. 33 (2018) 3321–3329, https://doi.org/ 10.1557/jmr.2018.225. [11] L. Battezzati, A.L. Greer, Acta Metallurgica 37 (1989) 1791–1802, https://doi. org/10.1016/0001-6160(89)90064-3. [12] J. Ding, M. Asta, R.O. Ritchie, Appl. Phys. Lett. 113 (2018), https://doi.org/ 10.1063/1.5045216. [13] B.B. Straumal, Y.O. Kucheev, L.I. Efron, A.L. Petelin, J.D. Majumdar, I. Manna, J. Mater. Eng. Perform. 21 (2012) 667–670, https://doi.org/10.1007/s11665-0120130-6. [14] B.B. Straumal, S.V. Dobatkin, A.O. Rodin, S.G. Prostasova, A.A. Mazilin, D. Goll, B. Baretzky, Adv. Eng. Mater. 13 (2011) 463–469, https://doi.org/10.1002/ adem.201000312.