- Email: [email protected]
Phase formation criteria assessment on the microstructure of a new refractory high entropy alloy
Scripta Materialia 131 (2017) 51–54
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Phase formation criteria assessment on the microstructure of a new refractory high entropy alloy A.E. Karantzalis ⁎, A. Poulia, E. Georgatis, D. Petroglou Dept. of Materials Science & Engineering, Univ. of Ioannina, 45110 Ioannina, Greece
a r t i c l e
i n f o
Article history: Received 27 December 2016 Received in revised form 2 January 2017 Accepted 4 January 2017 Available online xxxx Keywords: High entropy alloys X-ray diffraction (XRD) Scanning electron microscopy (SEM) Solidification Segregation
a b s t r a c t This work focuses on the evaluation of various parametric models for the formation of solid solution phases in high entropy alloys. The actual observation of MoWHfZrTi microstructure, along with the theoretical suggestions for single phase selection is discussed. The micro-segregated obtained structure is explained in terms of introducing a possible mechanism based on the estimation of thermodynamic parameters and further analysis of the experimental evidence upon solidification. High values of both micro and macro hardness characterize the synthesized system, indicating a solid solution strengthening mechanism. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction High Entropy Alloys (HEAs) have triggered a new interest in materials design philosophy. Presenting intriguing properties [1–5] and unique microstructural features [6–8] these new systems have gradually opened a new frontier in the metallic materials field [9]. Phase stability is vitally important for HEAs, but understanding this phenomenon is quite complex [10]. However, the ability to predict phase formation/ selection, based on the fundamental thermodynamic properties of the constituent elements, would greatly benefit the capacity for further alloy exploration. Thus, the relationship between physicochemical and thermodynamic parameters of the alloying components in HEAs seems to influence the design concept lately [11]. Towards this direction, several prediction models concerning phase formation criteria in HEAs have gradually been introduced [12–16]. For example, the evaluation of thermodynamic parameters such as atomic size difference, enthalpy of mixing and entropy of mixing [12] seem to be dominant factors in determining the formation of stable solid solution phases. Moreover, a new model based on the use of high-throughput computation of the enthalpies of formation of binary compounds in HEAs systems, predicts the specific combinations of elements most likely to form single-phase high entropy alloys [13]. Additionally, a novel method for rapidly predicting the formation and stability of undiscovered single phase HEAs has been developed, using Miedema's model as a ⁎ Corresponding author. E-mail addresses: [email protected] (A.E. Karantzalis), [email protected] (E. Georgatis).
http://dx.doi.org/10.1016/j.scriptamat.2017.01.004 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
basis. The introduction of parameter, phi, suggests the presence of stable solid solutions at the systems' melting temperature [14]. Perhaps the most critical weakness of the parametric approaches is that most of them are fitted to experimentally obtained data [17]. Indeed, careful assessment of the actual microstructural features is necessary before conclusions are drawn about the characteristics and stability of the corresponding structures [18]. It is undoubtable that the task of HEAs phase selection will add immeasurably to understanding phase formation in high entropy systems, and to our ability to predict complex microstructures in multicomponent alloys. In this present effort, a new – in terms of composition – refractory Mo0.85W0.85Hf1.1Zr1.6Ti0.6 alloy was produced and tested in order to assess whether the predicting models can indeed forecast the observed final microstructures. The system, as a new refractory high entropy alloy, is expected to show improved properties (mechanical, wear, oxidation etc. response) at both ambient and elevated temperatures. It is also important to mention that, irrespectively of the final result (single phase, dual solid solutions, intermetallic phase formation or combinations of them) the HEA approach in designing a new alloy system is of extreme importance as it promises novelty in the designing perception and uniqueness on the microstructural outcome.
2. Results and discussion This paper presents an analysis of this aspect. Specifically, Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy was prepared by melting pure metallic powders and wires in an arc furnace with a non-consumable W-electrode, under a protective argon atmosphere. Two re-melting
52
A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
efforts followed the initial stage of preparation in order to achieve a better distribution of the elements in the final structure. Various phase formation criteria were estimated in an initial effort to correlate theoretical prediction evidence with actual microstructural findings. In particular, estimations of atomic size difference, enthalpy of mixing, entropy of mixing, enthalpy of formation and phi parameter data were evaluated and compared with the presented morphological features of the examined refractory system. A Bruker D8 Advance X-ray diffractometer was used for crystal structure examination, while a JEOL 6510 LV SEM equipped with both backscatter electron (BSE) and energy dispersive spectroscopy (EDS) detectors further characterized the resultant structure of the high entropy alloy. Table 1 presents the calculated parameters of the refractory system, regarding atomic size difference, enthalpy of mixing and entropy of mixing as proposed by Guo et al. [12]. The estimated values for Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy were determined as δ = 7.05, ΔΗmix = − 6.39 kJ/mol and ΔSmix = 12.9 J/K, all ranging within the proposed literature limits. On the other hand, the Troparevsky et al. model [13] suggests the formation of several binary solutions with some ΔΗf values of the binary components escalating outside the recommended boundaries for single phase solid solution formation (−138 b ΔΗf b 37 meV/mol) in a 5 component system. Possible phase segregation phenomena could be spotted in this case. Moreover, according to King et al. [14] the derived values of parameters δ and Φ (using www.alloyASAP.com platform as a calculation tool) were determined as 6.309 and 1.456, respectively, both lying within the proposed ranges for solid solution formation, but also being very close to their marginal limits. As such, some possible deviations from the theoretical response could be expected. To sum up, all the aforementioned proposed prediction models, provide some first clues about the possible phase selection speculations, but do not guarantee a single phase solid solution formation. It is of
high priority to further examine the actual alloy microstructure, in order to receive acceptable data about the real structural mechanisms of the synthesized alloy. Therefore, Fig. 1 presents the Χ-ray spectrum of Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy. The presence of different crystal structures is evident, as two BCC and one HCP based lattices are formed. According on these findings, a first indication about a phase segregation formation is established, verifying the predictions of Troparevsky et al. [20] proposed model and King's et al. [21] uncertainty in forming a single solid solution. Further analysis of the presented microstructure was conducted by SEM examination, as depicted in Fig. 2. The alloy exhibits dark and bright areas (as a sign of possible segregation tendency), while the final structure is significantly refined (b10 μm). A rather equiaxed – especially for the primary W–Mo enriched grains – microstructure was observed, which may raise questions on the establishing for growth driving force conditions (temperature gradients, undercooling rate) ahead of the solidifying fronts, as they may alter the involved segregation mechanisms. A detailed approach to the microstructural findings also led to elemental mapping and line scan analysis (Fig. 2), both verifying the segregation of W and Mo within the bright areas and Zr and Ti within the darker areas. The distribution of Hf seems to be uniformly spread along the final structure. The presence of such segregation phenomena in various refractory high entropy alloys is widely criticized by many researchers [19–23]. For example, Stepanov et al. [21] and Senkov et al. [23] linked the observed micro-segregation in their systems with non-equilibrium solidification within the temperature range between the liquidus-solidus temperatures. As proposed, this tendency is also associated with an increase in the difference in the melting temperatures of the alloying elements. Another approach [20] relates segregation formation with the cooling rate upon solidification. Specifically, depending on the position
Table 1 Calculated values of atomic size difference, enthalpy of mixing, entropy of mixing, enthalpy of formation and phi parameter, according to different proposed prediction models for the initial and actual composition of the examined alloy after specific elemental point analysis. System
Mo0.85W0.85Hf1.1Zr1.6 Ti0.6
Guo et al. [12] δ
ΔΗmix [kJ/mol]
ΔSmix [J/K∙mol]
7.05
−6.39
12.9
Troparevsky et al. [13]
King et al. [14]
ΔΗf [meV/atom]
δ
Φ
6.309
1.456
Estimations based on King et al. model [14]
Actual alloy composition (point EDS analysis)
Dark area Mo = 13.74 at.% W = 0 at.% Hf = 20.65 at.% Zr = 44.16 at.% Ti = 21.45 at.% Light area Mo = 25.39 at.% W = 33.80 at.% Hf = 18.25 at.% Zr = 17.39 at.% Ti = 5.18 at.%
δ
Ηmax
Hss
Φ
Smix
Tm [K]
5.13
−10.88
2.66
1.99
10.72
2271
7.36
−24.26
−3.45
1.62
12.32
2912
A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
53
Fig. 1. X-ray diffraction pattern of Mo0.85W0.85Hf1.1Zr1.6 Ti0.6 HEA.
of the solidified ingot, the presence of micro-segregated zones (lower cooling rates) or homogeneously distributed structures (rapidly solidified zones) are evolved. Keeping these attestations in mind, it is worth to calculate the different parameters of King et al. proposed model [14] with respect to the actual formed phases in Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy (Table 1). As noticed, the light area is characterized by a significantly higher melting temperature point in comparison to the dark phase. This could be strong evidence that phases consisting of higher melting point elements (such as W and Mo) would be the first to form during the solidification process. This assertion is in quite good agreement with previously mentioned references [21,23] which also suggested such possibilities. Further analyzing the proposed parameters of δ and Φ, it can also be observed that they are both close to the values predicted for the initially synthesized system. Relatively high values of δ were estimated in both dark and light phases (5.13 and 7.36, respectively) while in the dark
phase, Φ parameter was calculated as 1.99, with a tension to further reduce to 1.62 upon the light areas of the microstructure. Not being able to end up with a clear statement about the influence of these predictions on the real response of the alloy, it is most likely that the existence of another key factor should be taken into consideration. Indeed, a careful look on the variations of melting point temperatures in dark and light areas seems to give us some valuable hints. In light areas (Tm = 2912 K), the high melting point elements will solidify first, leading clearly to a segregation of the lower melting point elements into the last to solidify liquid (dark area, Tm = 2271 K). This theory, further clarifies the higher concentration of W and Mo in the bright – first to solidify – areas, while keeping together Zr and Ti (with lower melting point temperature values) upon the last stages of solidification. As already mentioned, Hf, possibly presenting a higher effect of lattice distortion, demonstrates a rather uniform distribution in the whole extent of the microstructure.
Fig. 2. SEM backscattered electron micrograph of Mo0.85W0.85Hf1.1Zr1.6 Ti0.6 HEA accompanied by Mapping and Line Scan EDS Analysis.
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A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
At this point, it should also be mentioned that the examined alloy surfaces were derived from the base of the meniscus, near to the higher cooled coper base of the VAM furnace, an area which further promotes segregation phenomena [20]. Lastly, hardness measurements of the examined system revealed values of 7200 ± 70 MPa (measured by HV0.5) and 61 ± 1 HRC. These values are significantly high related to those of the pure constituent elements and as such most likely suggest a solid solution strengthening mechanism, irrespectively of the phase segregation observed in the microstructure. Similar observations have been made by other research efforts, too [23–25]. As a conclusion, the aforementioned criteria in general confirm the skepticism of Pickering et al. [17] and Tsai et al. [18] who claimed that all these parametric studies do give some indications of the likelihood of solid-solution stability, but in some cases they present limited success in predicting the actual phase formation. However, all of them can cleverly be exploited as first-approximation guides in frameworks for alloy selection. In summary, Mo0.85W0.85Hf1.1Zr1.6Ti0.6 refractory high entropy alloy was synthesized by means of vacuum arc melting. A comparison between the suggested predictions of various thermodynamic parameters and the actual microstructural features of the presented system was conducted. The possible mismatches between these approaches were highlighted, while a preliminary effort to evaluate the micro-segregated structure of the examined alloy was attempted. Based on the aforementioned evidence, the importance of melting point temperature differences among the involved constituents during solidification was marked, leading to a putative mechanism underlying the received microstructural characteristics. Finally, the alloy's high values of both micro and macro hardness were presented, indicating a solid solution strengthening mechanism. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
References [1] C.C. Juan, M.H. Tsai, C.W. Tsai, C.M. Lin, W.R. Wang, C.C. Yang, S.K. Chen, S.J. Lin, J.W. Yeh, Intermetallics 62 (2015) 76–83. [2] Z. Tang, L. Huang, W. He, P.K. Liaw, Entropy 16 (2014) 895–911. [3] T.M. Butler, M.L. Weaver, J. Alloys Compd. 674 (2016) 229–244. [4] A. Poulia, E. Georgatis, A. Lekatou, A. Karantzalis, Adv. Eng. Mater. (2016)http://dx. doi.org/10.1002/adem.201600535. [5] V. Braic, M. Balaceanu, M. Braic, A. Vladescu, S. Panseri, A. Russo, J. Mech. Behav. Biomed. Mater. 10 (2012) 197–205. [6] O. Senkov, S. Senkova, C. Woodward, D. Miracle, Acta Mater. 61 (2013) 1545–1557. [7] T.T. Shun, C.H. Hung, C.F. Lee, J. Alloys Compd. 493 (2010) 105–109. [8] L. Lilensten, J.P. Couzinié, L. Perrière, J. Bourgon, N. Emery, I. Guillot, Mater. Lett. 132 (2014) 123–125. [9] Y.F. Ye, Q. Wang, J. Lu, C.T. Liu, Y. Yang, Mater. Today 19 (2016) 349–362. [10] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Mater. Sci. Eng. 375 (2004) 1206–1211. [11] F. Otto, Y. Yang, H. Bei, E.P. George, Acta Mater. 61 (2013) 2628–2638. [12] S. Guo, C.T. Liu, Prog. Nat. Sci-Mater. 21 (2011) 433–446. [13] M.C. Troparevsky, J.R. Morris, P.R.C. Kent, A.R. Lupini, G.M. Stocks, Phys. Rev. X 5 (2015) 011041-1–011041-6. [14] D.J.M. King, S.C. Middleburgh, A.G. McGregor, M.B. Cortie, Acta Mater. 104 (2016) 172–179. [15] O.N. Senkov, D.B. Miracle, J. Alloys Compd. 658 (2016) 603–607. [16] I. Toda-Caraballo, P.E.J. Rivera-Díaz-del-Castillo, Intermetallics 71 (2016) 76–87. [17] E.J. Pickering, N.G. Jones, Int. Mater. Rev. 61 (2016) 183–202. [18] M.H. Tsai, J.H. Li, A.C. Fan, P.H. Tsai, Scr. Mater. 127 (2017) 6–9. [19] O.N. Senkov, J.M. Scott, S.V. Senkova, F. Meisenkothen, D.B. Miracle, C.F. Woodward, J. Mater. Sci. 47 (2012) 4062–4074. [20] J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y. Champion, I. Guillot, Mater. Lett. 126 (2014) 285–287. [21] N.D. Stepanov, D.G. Shaysultanov, G.A. Salishchev, M.A. Tikhonovsky, Mater. Lett. 142 (2015) 153–155. [22] J.P. Couzinié, L. Lilensten, Y. Champion, G. Dirras, L. Perrière, I. Guillot, Mater. Sci. Eng. A 645 (2015) 255–263. [23] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw, Intermetallics 18 (2010) 1758–1765. [24] O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, J. Alloys Compd. 509 (2011) 6043–6048. [25] Y.D. Wu, Y.H. Cai, X.H. Chen, T. Wang, J.J. Si, L. Wang, Y.D. Wang, X.D. Hui, Mater. Design 83 (2015) 651–660.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Phase formation criteria assessment on the microstructure of a new refractory high entropy alloy A.E. Karantzalis ⁎, A. Poulia, E. Georgatis, D. Petroglou Dept. of Materials Science & Engineering, Univ. of Ioannina, 45110 Ioannina, Greece
a r t i c l e
i n f o
Article history: Received 27 December 2016 Received in revised form 2 January 2017 Accepted 4 January 2017 Available online xxxx Keywords: High entropy alloys X-ray diffraction (XRD) Scanning electron microscopy (SEM) Solidification Segregation
a b s t r a c t This work focuses on the evaluation of various parametric models for the formation of solid solution phases in high entropy alloys. The actual observation of MoWHfZrTi microstructure, along with the theoretical suggestions for single phase selection is discussed. The micro-segregated obtained structure is explained in terms of introducing a possible mechanism based on the estimation of thermodynamic parameters and further analysis of the experimental evidence upon solidification. High values of both micro and macro hardness characterize the synthesized system, indicating a solid solution strengthening mechanism. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction High Entropy Alloys (HEAs) have triggered a new interest in materials design philosophy. Presenting intriguing properties [1–5] and unique microstructural features [6–8] these new systems have gradually opened a new frontier in the metallic materials field [9]. Phase stability is vitally important for HEAs, but understanding this phenomenon is quite complex [10]. However, the ability to predict phase formation/ selection, based on the fundamental thermodynamic properties of the constituent elements, would greatly benefit the capacity for further alloy exploration. Thus, the relationship between physicochemical and thermodynamic parameters of the alloying components in HEAs seems to influence the design concept lately [11]. Towards this direction, several prediction models concerning phase formation criteria in HEAs have gradually been introduced [12–16]. For example, the evaluation of thermodynamic parameters such as atomic size difference, enthalpy of mixing and entropy of mixing [12] seem to be dominant factors in determining the formation of stable solid solution phases. Moreover, a new model based on the use of high-throughput computation of the enthalpies of formation of binary compounds in HEAs systems, predicts the specific combinations of elements most likely to form single-phase high entropy alloys [13]. Additionally, a novel method for rapidly predicting the formation and stability of undiscovered single phase HEAs has been developed, using Miedema's model as a ⁎ Corresponding author. E-mail addresses: [email protected] (A.E. Karantzalis), [email protected] (E. Georgatis).
http://dx.doi.org/10.1016/j.scriptamat.2017.01.004 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
basis. The introduction of parameter, phi, suggests the presence of stable solid solutions at the systems' melting temperature [14]. Perhaps the most critical weakness of the parametric approaches is that most of them are fitted to experimentally obtained data [17]. Indeed, careful assessment of the actual microstructural features is necessary before conclusions are drawn about the characteristics and stability of the corresponding structures [18]. It is undoubtable that the task of HEAs phase selection will add immeasurably to understanding phase formation in high entropy systems, and to our ability to predict complex microstructures in multicomponent alloys. In this present effort, a new – in terms of composition – refractory Mo0.85W0.85Hf1.1Zr1.6Ti0.6 alloy was produced and tested in order to assess whether the predicting models can indeed forecast the observed final microstructures. The system, as a new refractory high entropy alloy, is expected to show improved properties (mechanical, wear, oxidation etc. response) at both ambient and elevated temperatures. It is also important to mention that, irrespectively of the final result (single phase, dual solid solutions, intermetallic phase formation or combinations of them) the HEA approach in designing a new alloy system is of extreme importance as it promises novelty in the designing perception and uniqueness on the microstructural outcome.
2. Results and discussion This paper presents an analysis of this aspect. Specifically, Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy was prepared by melting pure metallic powders and wires in an arc furnace with a non-consumable W-electrode, under a protective argon atmosphere. Two re-melting
52
A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
efforts followed the initial stage of preparation in order to achieve a better distribution of the elements in the final structure. Various phase formation criteria were estimated in an initial effort to correlate theoretical prediction evidence with actual microstructural findings. In particular, estimations of atomic size difference, enthalpy of mixing, entropy of mixing, enthalpy of formation and phi parameter data were evaluated and compared with the presented morphological features of the examined refractory system. A Bruker D8 Advance X-ray diffractometer was used for crystal structure examination, while a JEOL 6510 LV SEM equipped with both backscatter electron (BSE) and energy dispersive spectroscopy (EDS) detectors further characterized the resultant structure of the high entropy alloy. Table 1 presents the calculated parameters of the refractory system, regarding atomic size difference, enthalpy of mixing and entropy of mixing as proposed by Guo et al. [12]. The estimated values for Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy were determined as δ = 7.05, ΔΗmix = − 6.39 kJ/mol and ΔSmix = 12.9 J/K, all ranging within the proposed literature limits. On the other hand, the Troparevsky et al. model [13] suggests the formation of several binary solutions with some ΔΗf values of the binary components escalating outside the recommended boundaries for single phase solid solution formation (−138 b ΔΗf b 37 meV/mol) in a 5 component system. Possible phase segregation phenomena could be spotted in this case. Moreover, according to King et al. [14] the derived values of parameters δ and Φ (using www.alloyASAP.com platform as a calculation tool) were determined as 6.309 and 1.456, respectively, both lying within the proposed ranges for solid solution formation, but also being very close to their marginal limits. As such, some possible deviations from the theoretical response could be expected. To sum up, all the aforementioned proposed prediction models, provide some first clues about the possible phase selection speculations, but do not guarantee a single phase solid solution formation. It is of
high priority to further examine the actual alloy microstructure, in order to receive acceptable data about the real structural mechanisms of the synthesized alloy. Therefore, Fig. 1 presents the Χ-ray spectrum of Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy. The presence of different crystal structures is evident, as two BCC and one HCP based lattices are formed. According on these findings, a first indication about a phase segregation formation is established, verifying the predictions of Troparevsky et al. [20] proposed model and King's et al. [21] uncertainty in forming a single solid solution. Further analysis of the presented microstructure was conducted by SEM examination, as depicted in Fig. 2. The alloy exhibits dark and bright areas (as a sign of possible segregation tendency), while the final structure is significantly refined (b10 μm). A rather equiaxed – especially for the primary W–Mo enriched grains – microstructure was observed, which may raise questions on the establishing for growth driving force conditions (temperature gradients, undercooling rate) ahead of the solidifying fronts, as they may alter the involved segregation mechanisms. A detailed approach to the microstructural findings also led to elemental mapping and line scan analysis (Fig. 2), both verifying the segregation of W and Mo within the bright areas and Zr and Ti within the darker areas. The distribution of Hf seems to be uniformly spread along the final structure. The presence of such segregation phenomena in various refractory high entropy alloys is widely criticized by many researchers [19–23]. For example, Stepanov et al. [21] and Senkov et al. [23] linked the observed micro-segregation in their systems with non-equilibrium solidification within the temperature range between the liquidus-solidus temperatures. As proposed, this tendency is also associated with an increase in the difference in the melting temperatures of the alloying elements. Another approach [20] relates segregation formation with the cooling rate upon solidification. Specifically, depending on the position
Table 1 Calculated values of atomic size difference, enthalpy of mixing, entropy of mixing, enthalpy of formation and phi parameter, according to different proposed prediction models for the initial and actual composition of the examined alloy after specific elemental point analysis. System
Mo0.85W0.85Hf1.1Zr1.6 Ti0.6
Guo et al. [12] δ
ΔΗmix [kJ/mol]
ΔSmix [J/K∙mol]
7.05
−6.39
12.9
Troparevsky et al. [13]
King et al. [14]
ΔΗf [meV/atom]
δ
Φ
6.309
1.456
Estimations based on King et al. model [14]
Actual alloy composition (point EDS analysis)
Dark area Mo = 13.74 at.% W = 0 at.% Hf = 20.65 at.% Zr = 44.16 at.% Ti = 21.45 at.% Light area Mo = 25.39 at.% W = 33.80 at.% Hf = 18.25 at.% Zr = 17.39 at.% Ti = 5.18 at.%
δ
Ηmax
Hss
Φ
Smix
Tm [K]
5.13
−10.88
2.66
1.99
10.72
2271
7.36
−24.26
−3.45
1.62
12.32
2912
A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
53
Fig. 1. X-ray diffraction pattern of Mo0.85W0.85Hf1.1Zr1.6 Ti0.6 HEA.
of the solidified ingot, the presence of micro-segregated zones (lower cooling rates) or homogeneously distributed structures (rapidly solidified zones) are evolved. Keeping these attestations in mind, it is worth to calculate the different parameters of King et al. proposed model [14] with respect to the actual formed phases in Mo0.85W0.85Hf1.1Zr1.6Ti0.6 high entropy alloy (Table 1). As noticed, the light area is characterized by a significantly higher melting temperature point in comparison to the dark phase. This could be strong evidence that phases consisting of higher melting point elements (such as W and Mo) would be the first to form during the solidification process. This assertion is in quite good agreement with previously mentioned references [21,23] which also suggested such possibilities. Further analyzing the proposed parameters of δ and Φ, it can also be observed that they are both close to the values predicted for the initially synthesized system. Relatively high values of δ were estimated in both dark and light phases (5.13 and 7.36, respectively) while in the dark
phase, Φ parameter was calculated as 1.99, with a tension to further reduce to 1.62 upon the light areas of the microstructure. Not being able to end up with a clear statement about the influence of these predictions on the real response of the alloy, it is most likely that the existence of another key factor should be taken into consideration. Indeed, a careful look on the variations of melting point temperatures in dark and light areas seems to give us some valuable hints. In light areas (Tm = 2912 K), the high melting point elements will solidify first, leading clearly to a segregation of the lower melting point elements into the last to solidify liquid (dark area, Tm = 2271 K). This theory, further clarifies the higher concentration of W and Mo in the bright – first to solidify – areas, while keeping together Zr and Ti (with lower melting point temperature values) upon the last stages of solidification. As already mentioned, Hf, possibly presenting a higher effect of lattice distortion, demonstrates a rather uniform distribution in the whole extent of the microstructure.
Fig. 2. SEM backscattered electron micrograph of Mo0.85W0.85Hf1.1Zr1.6 Ti0.6 HEA accompanied by Mapping and Line Scan EDS Analysis.
54
A.E. Karantzalis et al. / Scripta Materialia 131 (2017) 51–54
At this point, it should also be mentioned that the examined alloy surfaces were derived from the base of the meniscus, near to the higher cooled coper base of the VAM furnace, an area which further promotes segregation phenomena [20]. Lastly, hardness measurements of the examined system revealed values of 7200 ± 70 MPa (measured by HV0.5) and 61 ± 1 HRC. These values are significantly high related to those of the pure constituent elements and as such most likely suggest a solid solution strengthening mechanism, irrespectively of the phase segregation observed in the microstructure. Similar observations have been made by other research efforts, too [23–25]. As a conclusion, the aforementioned criteria in general confirm the skepticism of Pickering et al. [17] and Tsai et al. [18] who claimed that all these parametric studies do give some indications of the likelihood of solid-solution stability, but in some cases they present limited success in predicting the actual phase formation. However, all of them can cleverly be exploited as first-approximation guides in frameworks for alloy selection. In summary, Mo0.85W0.85Hf1.1Zr1.6Ti0.6 refractory high entropy alloy was synthesized by means of vacuum arc melting. A comparison between the suggested predictions of various thermodynamic parameters and the actual microstructural features of the presented system was conducted. The possible mismatches between these approaches were highlighted, while a preliminary effort to evaluate the micro-segregated structure of the examined alloy was attempted. Based on the aforementioned evidence, the importance of melting point temperature differences among the involved constituents during solidification was marked, leading to a putative mechanism underlying the received microstructural characteristics. Finally, the alloy's high values of both micro and macro hardness were presented, indicating a solid solution strengthening mechanism. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
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