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Microstructure of a near-equimolar refractory high-entropy alloy
Author's Accepted Manuscript
Microstructure of a near-equimolar refractory high-entropy alloy J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y. Champion, I. Guillot
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)00630-2 http://dx.doi.org/10.1016/j.matlet.2014.04.062 MLBLUE16800
To appear in:
Materials Letters
Received date: 28 February 2014 Accepted date: 11 April 2014 Cite this article as: J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y. Champion, I. Guillot, Microstructure of a near-equimolar refractory highentropy alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.04.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure of a near-equimolar refractory high-entropy alloy J.P. Couziniéa*, G. Dirrasb, L. Perrièrea, T. Chauveaub, E. Leroya,Y. Championa, I. Guillota
a
Institut de Chimie et des Matériaux Paris-Est (ICMPE), UMR 7182, CNRS & Université Paris-Est
Créteil, 2-8 rue Henri Dunant, 94320 Thiais, France b
Université Paris 13, Sorbonne Paris Cité, Laboratoire des Sciences, des Procédés et des Matériaux
(LSPM), UPR 3407, CNRS, 99 avenue Jean Baptiste Clément, 93430 Villetaneuse, France *Corresponding author. Tel.: +33 (0) 1 56 70 30 20 ; fax: +33 (0) 1 56 70 30 43. E-mail address: [email protected] (J.P. Couzinié).
Abstract The microstructure of a promising equimolar refractory high-entropy alloy containing Ti, Zr, Hf, Nb and Ta has been investigated after arc melting and induction techniques. The high-entropy alloy displays a fully disordered bcc crystal structure. At the nanoscopic scale, the observed dislocations appear mainly patterned in sub-grain boundaries in the solid solution. At the mesoscopic level and following the cooling rate, the solidified microstructure is different: the alloy exhibits a dendritic structure with presence of Ti, Zr and Hf-rich microsegregations in the interdendritic zones for the slowest cooled parts, whereas it is fully homogenized in the regions for which the cooling rate is intense. Highlights:
• • • •
We investigate the solidification microstructure of a promising refractory high‐ entropy alloy containing Ti, Zr, Hf, Nb and Ta. Dislocations are – for the first time in these refractory alloys – observed and appear mainly arranged in sub‐grain boundaries. We report that the alloy exhibits a dendritic structure with the presence of Ti‐, Zr‐ and Hf‐rich microsegregations in the interdendritic regions for the slowest cooled ingot parts. Fully chemical homogenization is reached in the regions for which the cooling rate is intense.
Keywords Metals and alloys, metallurgy, high-entropy alloys, electron microscopy, solidification, defects
1
1. Introduction The conventional development of new alloys is based on one or two elements as major constituents, and other minor elements for the optimization of final properties. The incorporation of multi-principal elements in alloy preparation has often been pushed away because of the possible (and anticipated) formation of complex microstructures composed of intermetallic compounds, ordered phases, making alloys difficult to process. Contrary to this idea, Yeh et al. recently reported that systems with five or more metallic elements tend to favor possibly multi-element solid solution phases [1]. These systems are termed high-entropy alloys (HEAs) since the key concept seems to be directly linked to their inherent high mixture entropy. Yet, the rules governing the formation of these solid solution phases are unclear, even if empiric parameters derived from the Hume and Rothery rules and depending on size effects and chemical compatibility appear to be fairly effective to predict the formation of the HEAs [2,3]. Since the introduction of the HEA concept, the most commonly used alloying elements are transition metals such as face centered cubic (fcc)-type Cu, Al, Ni; body centered cubic (bcc)-type Fe, Cr, Mo, V and hexagonal close packed (hcp)-type Ti, Co In particular the XCoCrFeNiY (X,Y=Cu, Al, Ti, Mo…) composition has been intensively studied in the past ten years [4-6]. Since 2010, limited efforts have been pursued to explore new HEAs based on refractory constituents [7-9]. Senkov et al. first successfully produced near-equiatomic WNbMoTa and WNbMoTaV HEAs by arc melting [10]. Both alloys exhibited fully disordered bcc solid solution with dendritic grains and tremendous mechanical properties, especially for temperatures above 600°C for which these alloys exhibit clearly superior mechanical performances compared to conventional superalloys. Furthermore, substitution of heavy elements by lighter ones reduces the density and seems to improve particularly the room temperature (RT) ductility of the refractory HEAs. By substituting W and Mo for Ti and Zr, Senkov et al. observed significant increase of the compression strain for TaNbHfZrTi (up to 40% compression strain at RT) [11]. The yield strength is close to 1 GPa and the HEA show a large linear-like strain hardening in the RT–600°C temperature range [12]. Above 600°C, mechanical properties deteriorate and cavitation appears due, seemingly, to grain boundary sliding. Moreover, the mechanical strength at elevated temperatures appears to be extremely sensitive to the strain rate. The TaNbHfZrTi alloy shows promising perspectives for applications as structural material, in particular at high temperatures. However, no detailed analysis on the microstructure of this alloy is reported which is necessary to understand the origin of its performance. In this letter, we
2
report on the preparation of an equimolar TaNbHfZrTi alloy and the preliminary results on the solidification microstructure in particular the formed solid solution phases.
2. Experimental procedures The refractory alloy with equiatomic composition (Ti20Zr20Hf20Nb20Ta20) was prepared using arc melting and homogenized by induction heating. Raw metals were used in the form of slugs and wires with a purity exceeding 99.9 wt.%. Two Ta-Nb and Ti-Zr-Hf master alloys were first prepared by arc melting on a water-cooled copper plate. The alloys were melted three times separately then twice together. High frequency induction melting in a sectorised cooled copper crucible was used next to homogenize the elements, using electromagnetic stirring. Finally, the homogenized refractory alloy was obtained by arc melting in the form of an ingot of 60 mm long and approximately 10 mm of diameter. The crystalline structure was investigated by X-Ray diffraction (XRD), using a diffractometer equipped of a rotary-anode (BRUKER SRA18) using a monochromatic cobalt radiation in the range of 2θ = [35-155°]. The samples were set in a four circles goniometer under a 30° incidence. The determination of phase compositions was performed by electron probe micro-analysis (EPMA) using a CAMECA SX100 micro-analyzer operating at 15 keV. The five elements were simultaneously measured by coupled energy and wavelength dispersive spectrometry (EDS/WDS) using Ti Kα, Zr Lα, Hf Mα, Nb Lα and Ta Mβ. Thin foils were obtained from samples taken in the center of the ingot and finally twin-jet electropolished with a HF / H2SO4 solution in methanol. Observations were performed with a JEOL2000EX transmission electron microscopy (TEM).
3. Results and discussion XRD pattern of the as-cast alloy is given in Fig. 1. The processed HEA possesses a bcc crystalline structure with a lattice parameter determined by Rietveld analysis of 340.10 pm, similar to the 340.40 pm value previously reported by Senkov et al. [Senkov 2011].
3
21 1
32 1
11 0
31 0
0.5
22 2
22 0
20
0
I (norm. units)
1.0
0.0 40
60
80
100
120
2θ (°)
Fig. 1.
The as-cast microstructure has also been investigated by TEM. Figure 2 appears to be the first investigation conducted on this alloy so far. Selected area diffraction shown in Fig. 2 (a) confirms a fully disordered bcc solid solution, exemplified by a [001] zone axis. Figure 2 (b) presents a typical micrograph of the as-cast microstructure and highlights the presence of numerous dislocations, the majority of which are arranged in sub-grain boundaries. Pile-ups are also observed and some dislocation reactions have led to the formation of hexagonal networks.
Fig. 2.
4
Microstructural features of solidification kinetic were analyzed by EPMA. As shown in Fig. 3, and depending on the investigated area and also on the cooling rate, the resulting microstructure is different. A dendritic microstructure is thus observed in the alloy zone 1 (Figure 3 (a)) directly at the contact of the argon atmosphere of the arc-melting chamber and for which the alloy has been slowly cooled.
upper ingot zone 1 central ingot zone 2
Ti a.
b.
Zr
Hf
Nb
Ta
100 m
100 m
lower ingot zone 3
c.
100 m
(a)
(b)
Fig. 3.
EPMA provides evidence that the dendrite arms are Nb and Ta enriched regions (Figure 3 (b)). It suggests that Ta,Nb-rich solid phase first form during solidification, most probably due to the fact that the melting temperature of Ta and Nb are significantly higher than that of the three other elements. Ti, Zr and Hf enriched regions are also visible in the form of micro-segregations in the interdendritic zones. From the EPMA analysis of concentrations (Table 1), the difference in concentration between dendritic and interdendritic zones reaches 6 at.% for Ta, 4 at.% for Zr and 2 at.% for the other elements in the zone where micro-segregations occur (zone 1, Table 1). This difference of composition between the dendrites and the interdendritic regions could be explained by the partition of the alloying elements in the liquid and the solid phases, especially when the solidification process is slow.
5
The dendritic structure tends to disappear in the central and in the lower parts of the ingot (zones 2 and 3 in Figure 3), where the alloy is directly in contact with water-cooled copper plate, i.e. corresponding to a higher solidification rate. Indeed, in the central ingot part (zone 2) some coarse Nb,Ta-rich dendrites are detected, separated by segregated Ti, Zr, Hf-rich zones. In addition, in this zone, neither dendrites nor segregations are observed and all the elements are uniformly distributed in the microstructure, the measured composition being close to the average chemical composition of the alloy (Table 1). The correlation between the chemical elements of the as-cast microstructure has been highlighted by plotting (Ta+Nb) content as function of (Ti+Zr+Hf) content (see the supplementary material): zone 1, (Nb,Ta)rich areas correspond to poor (Ti,Zr,Hf) areas (and vice versa), no correlation can be found in zone 3, indicating that it is a segregation-free zone.
4. Conclusions A refractory high-entropy alloy with near-equimolar TiZrHfNbTa composition has been successfully produced by combined arc melting and induction techniques. The obtained alloys exhibits a bcc solid solution with a 341.10 pm lattice parameter. TEM observations provide evidence that the as-cast alloy is fully disordered and that the nanostructure is composed of dislocations mainly patterned in sub-grain boundaries. Depending on the position in the solidified ingot, the HEA microstructure is evolving. In the lowest cooling rate zones of the ingot Ta- and Nb-rich dendrites and Ti, Zr and Hf micro-segregations in interdendritic regions are emphasized. EPMA results give evidence that the difference of concentrations between the different regions is element-dependent and could reach 6 at.% for Ta, 4 at.% for Zr and 2 at.% for Ti, Hf and Nb. In the rapid solidified zones the microstructure does not display a dendritic structure and the elements are homogeneously distributed in the matrix.
6
References [1] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes. Adv Eng Mat 2004; 6(5): 299303. [2] Guo S, Liu CT. Phase stability in high entropy alloys: Formation of solid-solution phase or amorphous phase. Pro Nat Sci 2011; 21(6): 433-446. [3] Zhang Y, Zhou YJ, Lin JP, Chen GL, Liaw PK. Solid-solution phase formation rules for multicomponent alloys. Adv Eng Mat 2008; 10(6): 534-538. [4] Tong CJ, Chen YL, Chen SK, Yeh JW, Shun TT, Tsau CH, et al. Microstructure characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements. Metall Mater Trans A 2005; 36A(4): 881-893. [5] Zhou YJ, Zhang Y, Wang YL, Chen GL. Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties. App Phys Lett 2007; 90(18): 181904. [6] Otto F, Dlouhy A, Somsen C, Bei H, Eggeler G, George EP. The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy. Acta Mat 2013; 61(15): 5743-5755. [7] Liu CM, Wang HM, Zhang SQ, Tang HB, Zhang AL. Microstructure and oxidation behavior of new refractory high entropy alloys. J Alloy and Compd 2014; 583: 162-169. [8] Senkov ON, Senkova SV, Woodward C, Miracle DB. Low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system: Microstructure and phase analysis. Acta Mat 2013; 61(5): 15451557. [9] Senkov ON, Senkova SV, Miracle DB, Woodward C. Mechanical properties of low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system. Mat Sci Eng A-Struct 2013; 565: 51-62. [10] Senkov ON, Wilks GB, Miracle DB, Chuang CP, Liaw PK. Refractory high-entropy alloys. Intermetallics 2010; 18(9): 1758-1765. [11] Senkov ON, Scott JM, Senkova SV, Miracle DB, Woodward CF. Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy. J Alloy and Compd 2011; 509(20): 60436048.
7
[12] Senkov ON, Scott JM, Senkova SV, Meisenkothen F, Miracle DB, Woodward C. Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy. J Mat Sci 2012; 47(9): 4062-4074.
Figure captions Fig. 1. XRD pattern of the as-cast alloy.
Fig. 2. TEM observations: (a) selected area diffraction indicating the [001] zone axis pattern; (b) brightfield micrograph of the as-cast microstructure. Symbols highlight the presence of sub-grain boundaries and hexagonal dislocation network.
Fig. 3. (a) General view of the as-cast ingot; (b) EPMA element map. Note that the intensity is directly related to the considered element concentration. The black dots in the maps reflect the presence of some porosity in the as-cast HEA.
Table 1. Quantification of the chemical composition for the different ingot zones selected in Figure 3a. D.R.: dendritic regions ; I.R.: interdendritic regions. Ignote Zone 1–1 1
1–2
1–3
2 3 GLOBAL
2–1 3–1 3–2
(at.%) D.R. I.R. AVERAGE D.R. I.R. AVERAGE D.R. I.R. AVERAGE AVERAGE AVERAGE AVERAGE
Ti 20.17 21.21 20.59 19.59 21.75 20.58 19.85 21.94 20.43 21.09 20.86 20.82 20.73
Zr 18.63 20.69 19.45 17.57 21.66 19.45 18.02 22.09 19.14 20.41 20.07 19.95 19.75
Hf 19.82 20.62 20.14 19.44 21.00 20.16 19.67 21.06 20.05 20.54 20.42 20.43 20.29
Nb 18.79 17.83 18.41 19.38 17.28 18.41 19.01 17.21 18.50 17.63 17.76 17.82 18.09
Ta 22.58 19.64 21.40 24.02 18.32 21.40 23.56 17.71 21.88 20.33 20.89 20.97 21.14
8
Microstructure of a near-equimolar refractory high-entropy alloy J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y. Champion, I. Guillot
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)00630-2 http://dx.doi.org/10.1016/j.matlet.2014.04.062 MLBLUE16800
To appear in:
Materials Letters
Received date: 28 February 2014 Accepted date: 11 April 2014 Cite this article as: J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y. Champion, I. Guillot, Microstructure of a near-equimolar refractory highentropy alloy, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.04.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure of a near-equimolar refractory high-entropy alloy J.P. Couziniéa*, G. Dirrasb, L. Perrièrea, T. Chauveaub, E. Leroya,Y. Championa, I. Guillota
a
Institut de Chimie et des Matériaux Paris-Est (ICMPE), UMR 7182, CNRS & Université Paris-Est
Créteil, 2-8 rue Henri Dunant, 94320 Thiais, France b
Université Paris 13, Sorbonne Paris Cité, Laboratoire des Sciences, des Procédés et des Matériaux
(LSPM), UPR 3407, CNRS, 99 avenue Jean Baptiste Clément, 93430 Villetaneuse, France *Corresponding author. Tel.: +33 (0) 1 56 70 30 20 ; fax: +33 (0) 1 56 70 30 43. E-mail address: [email protected] (J.P. Couzinié).
Abstract The microstructure of a promising equimolar refractory high-entropy alloy containing Ti, Zr, Hf, Nb and Ta has been investigated after arc melting and induction techniques. The high-entropy alloy displays a fully disordered bcc crystal structure. At the nanoscopic scale, the observed dislocations appear mainly patterned in sub-grain boundaries in the solid solution. At the mesoscopic level and following the cooling rate, the solidified microstructure is different: the alloy exhibits a dendritic structure with presence of Ti, Zr and Hf-rich microsegregations in the interdendritic zones for the slowest cooled parts, whereas it is fully homogenized in the regions for which the cooling rate is intense. Highlights:
• • • •
We investigate the solidification microstructure of a promising refractory high‐ entropy alloy containing Ti, Zr, Hf, Nb and Ta. Dislocations are – for the first time in these refractory alloys – observed and appear mainly arranged in sub‐grain boundaries. We report that the alloy exhibits a dendritic structure with the presence of Ti‐, Zr‐ and Hf‐rich microsegregations in the interdendritic regions for the slowest cooled ingot parts. Fully chemical homogenization is reached in the regions for which the cooling rate is intense.
Keywords Metals and alloys, metallurgy, high-entropy alloys, electron microscopy, solidification, defects
1
1. Introduction The conventional development of new alloys is based on one or two elements as major constituents, and other minor elements for the optimization of final properties. The incorporation of multi-principal elements in alloy preparation has often been pushed away because of the possible (and anticipated) formation of complex microstructures composed of intermetallic compounds, ordered phases, making alloys difficult to process. Contrary to this idea, Yeh et al. recently reported that systems with five or more metallic elements tend to favor possibly multi-element solid solution phases [1]. These systems are termed high-entropy alloys (HEAs) since the key concept seems to be directly linked to their inherent high mixture entropy. Yet, the rules governing the formation of these solid solution phases are unclear, even if empiric parameters derived from the Hume and Rothery rules and depending on size effects and chemical compatibility appear to be fairly effective to predict the formation of the HEAs [2,3]. Since the introduction of the HEA concept, the most commonly used alloying elements are transition metals such as face centered cubic (fcc)-type Cu, Al, Ni; body centered cubic (bcc)-type Fe, Cr, Mo, V and hexagonal close packed (hcp)-type Ti, Co In particular the XCoCrFeNiY (X,Y=Cu, Al, Ti, Mo…) composition has been intensively studied in the past ten years [4-6]. Since 2010, limited efforts have been pursued to explore new HEAs based on refractory constituents [7-9]. Senkov et al. first successfully produced near-equiatomic WNbMoTa and WNbMoTaV HEAs by arc melting [10]. Both alloys exhibited fully disordered bcc solid solution with dendritic grains and tremendous mechanical properties, especially for temperatures above 600°C for which these alloys exhibit clearly superior mechanical performances compared to conventional superalloys. Furthermore, substitution of heavy elements by lighter ones reduces the density and seems to improve particularly the room temperature (RT) ductility of the refractory HEAs. By substituting W and Mo for Ti and Zr, Senkov et al. observed significant increase of the compression strain for TaNbHfZrTi (up to 40% compression strain at RT) [11]. The yield strength is close to 1 GPa and the HEA show a large linear-like strain hardening in the RT–600°C temperature range [12]. Above 600°C, mechanical properties deteriorate and cavitation appears due, seemingly, to grain boundary sliding. Moreover, the mechanical strength at elevated temperatures appears to be extremely sensitive to the strain rate. The TaNbHfZrTi alloy shows promising perspectives for applications as structural material, in particular at high temperatures. However, no detailed analysis on the microstructure of this alloy is reported which is necessary to understand the origin of its performance. In this letter, we
2
report on the preparation of an equimolar TaNbHfZrTi alloy and the preliminary results on the solidification microstructure in particular the formed solid solution phases.
2. Experimental procedures The refractory alloy with equiatomic composition (Ti20Zr20Hf20Nb20Ta20) was prepared using arc melting and homogenized by induction heating. Raw metals were used in the form of slugs and wires with a purity exceeding 99.9 wt.%. Two Ta-Nb and Ti-Zr-Hf master alloys were first prepared by arc melting on a water-cooled copper plate. The alloys were melted three times separately then twice together. High frequency induction melting in a sectorised cooled copper crucible was used next to homogenize the elements, using electromagnetic stirring. Finally, the homogenized refractory alloy was obtained by arc melting in the form of an ingot of 60 mm long and approximately 10 mm of diameter. The crystalline structure was investigated by X-Ray diffraction (XRD), using a diffractometer equipped of a rotary-anode (BRUKER SRA18) using a monochromatic cobalt radiation in the range of 2θ = [35-155°]. The samples were set in a four circles goniometer under a 30° incidence. The determination of phase compositions was performed by electron probe micro-analysis (EPMA) using a CAMECA SX100 micro-analyzer operating at 15 keV. The five elements were simultaneously measured by coupled energy and wavelength dispersive spectrometry (EDS/WDS) using Ti Kα, Zr Lα, Hf Mα, Nb Lα and Ta Mβ. Thin foils were obtained from samples taken in the center of the ingot and finally twin-jet electropolished with a HF / H2SO4 solution in methanol. Observations were performed with a JEOL2000EX transmission electron microscopy (TEM).
3. Results and discussion XRD pattern of the as-cast alloy is given in Fig. 1. The processed HEA possesses a bcc crystalline structure with a lattice parameter determined by Rietveld analysis of 340.10 pm, similar to the 340.40 pm value previously reported by Senkov et al. [Senkov 2011].
3
21 1
32 1
11 0
31 0
0.5
22 2
22 0
20
0
I (norm. units)
1.0
0.0 40
60
80
100
120
2θ (°)
Fig. 1.
The as-cast microstructure has also been investigated by TEM. Figure 2 appears to be the first investigation conducted on this alloy so far. Selected area diffraction shown in Fig. 2 (a) confirms a fully disordered bcc solid solution, exemplified by a [001] zone axis. Figure 2 (b) presents a typical micrograph of the as-cast microstructure and highlights the presence of numerous dislocations, the majority of which are arranged in sub-grain boundaries. Pile-ups are also observed and some dislocation reactions have led to the formation of hexagonal networks.
Fig. 2.
4
Microstructural features of solidification kinetic were analyzed by EPMA. As shown in Fig. 3, and depending on the investigated area and also on the cooling rate, the resulting microstructure is different. A dendritic microstructure is thus observed in the alloy zone 1 (Figure 3 (a)) directly at the contact of the argon atmosphere of the arc-melting chamber and for which the alloy has been slowly cooled.
upper ingot zone 1 central ingot zone 2
Ti a.
b.
Zr
Hf
Nb
Ta
100 m
100 m
lower ingot zone 3
c.
100 m
(a)
(b)
Fig. 3.
EPMA provides evidence that the dendrite arms are Nb and Ta enriched regions (Figure 3 (b)). It suggests that Ta,Nb-rich solid phase first form during solidification, most probably due to the fact that the melting temperature of Ta and Nb are significantly higher than that of the three other elements. Ti, Zr and Hf enriched regions are also visible in the form of micro-segregations in the interdendritic zones. From the EPMA analysis of concentrations (Table 1), the difference in concentration between dendritic and interdendritic zones reaches 6 at.% for Ta, 4 at.% for Zr and 2 at.% for the other elements in the zone where micro-segregations occur (zone 1, Table 1). This difference of composition between the dendrites and the interdendritic regions could be explained by the partition of the alloying elements in the liquid and the solid phases, especially when the solidification process is slow.
5
The dendritic structure tends to disappear in the central and in the lower parts of the ingot (zones 2 and 3 in Figure 3), where the alloy is directly in contact with water-cooled copper plate, i.e. corresponding to a higher solidification rate. Indeed, in the central ingot part (zone 2) some coarse Nb,Ta-rich dendrites are detected, separated by segregated Ti, Zr, Hf-rich zones. In addition, in this zone, neither dendrites nor segregations are observed and all the elements are uniformly distributed in the microstructure, the measured composition being close to the average chemical composition of the alloy (Table 1). The correlation between the chemical elements of the as-cast microstructure has been highlighted by plotting (Ta+Nb) content as function of (Ti+Zr+Hf) content (see the supplementary material): zone 1, (Nb,Ta)rich areas correspond to poor (Ti,Zr,Hf) areas (and vice versa), no correlation can be found in zone 3, indicating that it is a segregation-free zone.
4. Conclusions A refractory high-entropy alloy with near-equimolar TiZrHfNbTa composition has been successfully produced by combined arc melting and induction techniques. The obtained alloys exhibits a bcc solid solution with a 341.10 pm lattice parameter. TEM observations provide evidence that the as-cast alloy is fully disordered and that the nanostructure is composed of dislocations mainly patterned in sub-grain boundaries. Depending on the position in the solidified ingot, the HEA microstructure is evolving. In the lowest cooling rate zones of the ingot Ta- and Nb-rich dendrites and Ti, Zr and Hf micro-segregations in interdendritic regions are emphasized. EPMA results give evidence that the difference of concentrations between the different regions is element-dependent and could reach 6 at.% for Ta, 4 at.% for Zr and 2 at.% for Ti, Hf and Nb. In the rapid solidified zones the microstructure does not display a dendritic structure and the elements are homogeneously distributed in the matrix.
6
References [1] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes. Adv Eng Mat 2004; 6(5): 299303. [2] Guo S, Liu CT. Phase stability in high entropy alloys: Formation of solid-solution phase or amorphous phase. Pro Nat Sci 2011; 21(6): 433-446. [3] Zhang Y, Zhou YJ, Lin JP, Chen GL, Liaw PK. Solid-solution phase formation rules for multicomponent alloys. Adv Eng Mat 2008; 10(6): 534-538. [4] Tong CJ, Chen YL, Chen SK, Yeh JW, Shun TT, Tsau CH, et al. Microstructure characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements. Metall Mater Trans A 2005; 36A(4): 881-893. [5] Zhou YJ, Zhang Y, Wang YL, Chen GL. Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties. App Phys Lett 2007; 90(18): 181904. [6] Otto F, Dlouhy A, Somsen C, Bei H, Eggeler G, George EP. The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy. Acta Mat 2013; 61(15): 5743-5755. [7] Liu CM, Wang HM, Zhang SQ, Tang HB, Zhang AL. Microstructure and oxidation behavior of new refractory high entropy alloys. J Alloy and Compd 2014; 583: 162-169. [8] Senkov ON, Senkova SV, Woodward C, Miracle DB. Low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system: Microstructure and phase analysis. Acta Mat 2013; 61(5): 15451557. [9] Senkov ON, Senkova SV, Miracle DB, Woodward C. Mechanical properties of low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system. Mat Sci Eng A-Struct 2013; 565: 51-62. [10] Senkov ON, Wilks GB, Miracle DB, Chuang CP, Liaw PK. Refractory high-entropy alloys. Intermetallics 2010; 18(9): 1758-1765. [11] Senkov ON, Scott JM, Senkova SV, Miracle DB, Woodward CF. Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy. J Alloy and Compd 2011; 509(20): 60436048.
7
[12] Senkov ON, Scott JM, Senkova SV, Meisenkothen F, Miracle DB, Woodward C. Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy. J Mat Sci 2012; 47(9): 4062-4074.
Figure captions Fig. 1. XRD pattern of the as-cast alloy.
Fig. 2. TEM observations: (a) selected area diffraction indicating the [001] zone axis pattern; (b) brightfield micrograph of the as-cast microstructure. Symbols highlight the presence of sub-grain boundaries and hexagonal dislocation network.
Fig. 3. (a) General view of the as-cast ingot; (b) EPMA element map. Note that the intensity is directly related to the considered element concentration. The black dots in the maps reflect the presence of some porosity in the as-cast HEA.
Table 1. Quantification of the chemical composition for the different ingot zones selected in Figure 3a. D.R.: dendritic regions ; I.R.: interdendritic regions. Ignote Zone 1–1 1
1–2
1–3
2 3 GLOBAL
2–1 3–1 3–2
(at.%) D.R. I.R. AVERAGE D.R. I.R. AVERAGE D.R. I.R. AVERAGE AVERAGE AVERAGE AVERAGE
Ti 20.17 21.21 20.59 19.59 21.75 20.58 19.85 21.94 20.43 21.09 20.86 20.82 20.73
Zr 18.63 20.69 19.45 17.57 21.66 19.45 18.02 22.09 19.14 20.41 20.07 19.95 19.75
Hf 19.82 20.62 20.14 19.44 21.00 20.16 19.67 21.06 20.05 20.54 20.42 20.43 20.29
Nb 18.79 17.83 18.41 19.38 17.28 18.41 19.01 17.21 18.50 17.63 17.76 17.82 18.09
Ta 22.58 19.64 21.40 24.02 18.32 21.40 23.56 17.71 21.88 20.33 20.89 20.97 21.14
8