- Email: [email protected]
Competitive formation of intermetallic phases in Y0.7(Nb,Ti)0.3Co2 system: Experiment and thermodynamic modeling
Materials Letters 182 (2016) 90–93
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Competitive formation of intermetallic phases in Y0.7(Nb,Ti)0.3Co2 system: Experiment and thermodynamic modeling Z. Śniadecki Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 23 May 2016 Received in revised form 17 June 2016 Accepted 19 June 2016 Available online 21 June 2016
Process of crystallization of Nb and Ti substituted compounds was analyzed using x-ray diffraction and semi-empirical modeling. Different crystallization products were observed for Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2 despite similar electronegativities and atomic radii of niobium and titanium. Calculations using Miedema's model confirmed the stability of intermetallic compounds in the broad range of compositions at the expense of solid solution and amorphous phase. While in Y0.9(Nb, Ti)0.1Co2 single C15 Laves phase is forming, two phases (YCo2 and NbCo2) are crystallizing in Y0.7Nb0.3Co2. In the case of Ti-substituted sample, three phases with different Ti content were observed due to similar formation enthalpies of intermetallic compounds in analyzed range of compositions ( 33.7 and 32.1 kJ/mol for Y0.9Ti0.1Co2 and Y0.7Ti0.3Co2, respectively). & 2016 Elsevier B.V. All rights reserved.
Keywords: Crystallization Enthalpy of formation Thermodynamics and kinetics of processes in materials Laves phase Glass forming ability Metals and alloys
1. Introduction Depending on the composition, type of dopants, but also on the crystalline structure, compounds from Y-Co series reveal markedly different magnetic properties. Magnetic state of crystalline Y-Co system changes from paramagnetic, through spin glass type to ferri-/ferromagnetic with increasing Co content. Strong influence of structural changes has been denoted mainly for YCo2, which is an exchange-enhanced Pauli paramagnet with itinerant electron metamagnetic transition into ferromagnetic state in high magnetic fields in the crystalline form (cubic Laves phase) [1]. In turn, long range magnetic ordering is observed for amorphous alloy of the same composition and on the surface of crystalline thin film [2,3]. Moreover, nanocrystalline mechanically milled alloy becomes ferromagnetic in the whole grains volume with decreasing grain size [4]. Properties of rapidly quenched YCo2 and Nb or Ti alloyed samples were determined mainly by chemical and topological disorder with clear evidence of magnetic ordering. Additional magnetic contributions have been reported as signs of chemical and topological disorder for small Nb and Ti substitutions [5]. Melt-spun samples possess MgCu2-type structure with lattice constant a changing from 7.223 Å for YCo2, through 7.213 Å for Y0.9Nb0.1Co2 to 7.192 Å for Y0.9Ti0.1Co2. Further studies of Nb- and Ti-substituted systems are provided to give an insight into the formation and stability of Laves phases. While the crystalline E-mail address: [email protected] http://dx.doi.org/10.1016/j.matlet.2016.06.083 0167-577X/& 2016 Elsevier B.V. All rights reserved.
structure determines magnetic properties of mentioned binary and ternary systems, it is of great interest to specify compositional ranges, where different phases (amorphous, solid solution, intermetallics) can be obtained. Substituted Laves phases are the candidates for room temperature refrigeration based on magnetocaloric effect [6].
2. Experiment and calculations The master alloys of Y0.7(Nb,Ti)0.3Co2 were prepared with the use of arc-furnace, by melting of high purity Y (99.9%), Co (99.9%), Ti (99.9%) and Nb (99.9%) in the argon atmosphere. The ingots were rapidly quenched by melt-spinning on a copper wheel rotating with the surface velocity of 40 m/s. The structural characterization was made with the use of x-ray diffractometer (Co-Kα radiation) in Bragg-Brentano geometry. Calculations of formation enthalpy were based on the semiempirical approach known as the Miedema's model [7,8]. Chemical enthalpy was estimated as the only contribution playing a role during formation of intermetallic compounds. Additionally, molar volumes were corrected for volume changes due to alloying. During solidification there is a competition of total free energies of intermetallic compound, solid solution and amorphous phase. Therefore, formation enthalpies of solid solution and amorphous phase were calculated for comparison and to indicate the thermodynamically most stable option. Used approach is described in more details in Ref. [9,10].
Z. Śniadecki / Materials Letters 182 (2016) 90–93
3. Results and discussion X-ray diffraction experiments (patterns shown in Fig. 1.) were carried out to recognize phases crystallized during rapid quenching of Y0.7(Nb,Ti)0.3Co2 ribbons and to investigate their basic parameters. Diffraction data was analyzed utilizing Rietveld refinement. It has been reported that YCo2 and Y0.9(Nb,Ti)0.1Co2 compounds crystallize in single C15 Laves phase with MgCu2-type structure (space group: Fd-3m) [5]. Determined structural parameters are a¼7.223 Å and v¼376.8 Å3 for parent compound, a¼7.213 Å, v¼ 376.8 Å3 for Y0.9Nb0.1Co2 and a¼7.192 Å, v¼372.0 Å3 for Y0.9Ti0.1Co2. Slight decrease of lattice constant suggested that, according to assumed substitution of niobium and titanium for yttrium, Nb and Ti atoms were incorporated into the Y-site. With increasing substitution of Y by Nb (30 at%) the second phase appears (Fig. 1 – upper panel). The new NbCo2 phase has the same structure as YCo2, but its lattice constant equals to a¼6.782 Å, which is comparable with lattice constant obtained in Ref. [11], where it was determined to be ranged from 6.713 Å to 6.802 Å for Nb-rich and Co-rich region of Co-Nb phase diagram. YCo2 lattice constant remains the same as in pure YCo2 compound indicating that whole amount of Nb was built into NbCo2 structure. These two phases grow simultaneously, instead of substituting Y atoms by Nb in YCo2 phase (as it was observed for 10 at% substitution). The volume fraction of YCo2 and NbCo2 phases is equal to 66% and 34%, respectively. The YCo2 alloys with 10 at% substitution of Y by Ti still possess single Laves phase [5]. With higher content of Ti two additional
91
phases appear. The diffraction pattern of Y0.7Ti0.3Co2 is shown in the lower panel of Fig. 1. There are two main phases with the same Fd3m space group, but with different lattice constants: a¼7.205 Å and a¼7.093 Å. Their volume fractions amount to 59% and 24%, respectively. First one is YCo2, slightly substituted with Ti atoms as the lattice constant is decreased by 0.35% in comparison to parent compound. Assuming that lattice constant is changing linearly with Ti substitution and interpolating the lattice constant values for single phase compounds (YCo2 and TiCo2), substitution of Ti should be equal to around 6 at%. Literature based lattice constant of TiCo2 is a¼6.691–6.716 Å [12]. For the second phase, with smaller a value, interpolated Ti content is equal to about 28 at%, while the third Laves phase with the smallest lattice constant (a¼6.751 Å) and the smallest volume fraction (17%) is close to TiCo2 composition, as the Ti content exceeds 93 at%. Concluding structural analysis, presence of binary parent phases, YCo2 and NbCo2 is evident for Nb-containing sample, while for Ti-substituted ribbon mixed phases with different Ti content were observed. Calculations based on the semi-empirical model were utilized to explain the origin of differences in crystallization processes of Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2. Formation enthalpies of different intermetallic compounds were determined and are presented in the upper part of Table 1. The more negative enthalpy values, the higher probability of crystallization of specific phase. YCo2, NbCo2 and Y0.9Nb0.1Co2 exhibit very similar values of formation enthalpies. With increasing content of Nb, the ΔHform tends to be less negative and reaches value around 25.9 kJ/mol for Y0.7Nb0.3Co2. The crystallization of the later and other Ti-substituted phases is hindered in comparison to NbCo2 and YCo2, which in fact crystallize in this sample. Qualitative differences are observed in case of (Y,Ti)Co2 compounds. The most negative heat of formation was calculated for TiCo2. This phase (with expected small Y content) was formed indeed, as shown on the basis of XRD experiment. Starting from YCo2, the composition dependence of ΔHform differ from that observed in Nb-containing system, as the enthalpy of formation values are almost the same, even for the Y0.7Ti0.3Co2. It can be the reason for the presence of different phases with mixed occupation of Y/Ti site, especially the phase with moderate lattice constant and predicted Ti-content of 28 at%. The competition between intermetallic phase, solid solution and amorphous phase takes place during synthesis. The most decisive factor which determines mentioned effect is the difference between enthalpies of formation, which are defined for solid solution and amorphous state in slightly different way than for intermetallic compound. In the case of amorphous phase, the topological contribution is taken into account and is strictly connected with the mismatch of atomic radii of the main elements. More additional terms are contained in solid solution enthalpy of formation and are related to elastic properties and type of
Table 1 Formation enthalpies of Y1-x(Nb,Ti)xCo2 (x¼ 0, 0.1, 0.3) intermetallic compounds (ΔHform), solid solutions (ΔHss) and amorphous phases (ΔHam). All the values are expressed in kJ/mol. YCo2
MCo2
Y0.9M0.1Co2
Y0.7M0.3Co2
31.8 33.7
25.9 32.1
1.9 14.7
6.9 5.3
7.7 2.5
Amorphous phase (ΔHam [kJ/mol]) M ¼ Nb 13.5 15.2 M ¼ Ti 19.4
12.4 13.3
10.6 13.3
Intermetallic compound (ΔHform [kJ/mol]) M ¼ Nb 31.8 33.3 M ¼ Ti 38.3 Solid solution (ΔHss [kJ/mol]) M ¼ Nb 6.1 M ¼ Ti Fig. 1. X-ray diffraction patterns of rapidly quenched Y0.7M0.3Co2 (M – Nb, Ti). For clarity, Miller indices are given for the peaks marked with black circles only, but are valid for all exhibited phases.
92
Z. Śniadecki / Materials Letters 182 (2016) 90–93
Fig. 2. Compositional dependence of formation enthalpies of amorphous phase (ΔHam) and solid solution (ΔHss) for Y-Nb-Co system. Y0.9Nb0.1Co2 and Y0.7Nb0.3Co2 are marked by black circles in both phase diagrams.
crystalline structure. Calculated values of formation enthalpy of solid solution ΔHss and amorphous phase ΔHam are presented in Table 1 (middle and lower part, respectively). In addition, phase diagrams of ΔHam and ΔHss are presented for Nb and Ti containing systems in Figs. 2 and 3, respectively. Comparing the values of ΔHam and ΔHss with ΔHform for intermetallic compounds, one can conclude that in the same composition ranges, there is significant difference in the enthalpies and formation of intermetallic phases is unequivocally preferred. Moreover ΔHam is more negative than ΔHss in almost whole composition range. It would suggest that glassy state can be formed in the composition regions were no intermetallic compounds are observed. One should reject this statement on the basis of experimental results, as the mixture of intermetallic phases is forming in the broad composition range. Additional alloying element should improve glass forming ability, but in the case of Nb and Ti substitutions, Laves phases were synthesized. 4. Conclusions Significant differences in crystallization processes of Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2 were observed by x-ray diffraction and validated by
Fig. 3. Compositional dependence of formation enthalpies of amorphous phase (ΔHam) and solid solution (ΔHss) for Y-Ti-Co system. Y0.9Ti0.1Co2 and Y0.7Ti0.3Co2 are marked by black circles in both phase diagrams.
the calculations based on the semi-empirical Miedema's model. Multiphase systems are forming during solidification, while in Y0.9(Nb,Ti)0.1Co2 single C15 Laves phase is observed [5]. Parent limiting compounds (YCo2 and NbCo2) are crystallizing in case of Nbsubstituted sample. For Ti-substituted ribbon, three phases with different Ti content were observed. It was linked with very similar formation enthalpies of intermetallic compounds in the broad range of Ti content, where none of the phases is prefered. In contrast, formation enthalpy is becoming less negative with increasing Nb fraction in Y-Nb-Co. Intermetallics were confirmed to be more stable than solid solutions and amorphous phases in both cases.
References [1] T. Goto, K. Fukamichi, T. Sakakibara, H. Komatsu, Solid State Commun. 72 (1989) 945. [2] S. Khmelevskyi, P. Mohn, J. Redinger, M. Weinert, Phys. Rev. Lett. 94 (2005) 146403. [3] Yu.S. Dedkov, C. Laubschat, S. Khmelevskyi, J. Redinger, P. Mohn, M. Weinert, Phys. Rev. Lett. 99 (2007) 047204.
Z. Śniadecki / Materials Letters 182 (2016) 90–93 [4] S.N. Jammalamadaka, E.V. Sampathkumaran, V.S.N. Murthy, G. Markandeyulu, Appl. Phys. Lett. 92 (2008) 192506. [5] Z. Śniadecki, M. Werwiński, A. Szajek, U.K. Rößler, B. Idzikowski, J. Appl. Phys. 115 (2014) 17E129. [6] V. Franco, J.S. Blazquez, B. Ingale, A. Conde, Annu. Rev. Mater. Res. 42 (2012) 305. [7] F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, in: F.R. de Boer, D.G. Pettifor (Eds.), Cohesion and Structure, vol. 1, North Holland Physics, Amsterdam, 1988, pp. 1–758.
93
[8] H. Bakker, Enthalpies in Alloys – Miedema's Semi-Empirical Model, Materials Science Foundations 1, Trans Tech Publications Ltd., Switzerland 1998, pp. 1–78. [9] Z. Śniadecki, J.W. Narojczyk, B. Idzikowski, Intermetallics 26 (2012) 72. [10] Z. Śniadecki, J. Alloy. Compd. 615 (2014) S40. [11] F. Stein, D. Jiang, M. Palm, G. Sauthoff, D. Gruener, G. Kreiner, Intermetallics 16 (2008) 785. [12] J.L. Murray, Bull. Alloy Phase Diagr. 3 (1982) 74.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Competitive formation of intermetallic phases in Y0.7(Nb,Ti)0.3Co2 system: Experiment and thermodynamic modeling Z. Śniadecki Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 23 May 2016 Received in revised form 17 June 2016 Accepted 19 June 2016 Available online 21 June 2016
Process of crystallization of Nb and Ti substituted compounds was analyzed using x-ray diffraction and semi-empirical modeling. Different crystallization products were observed for Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2 despite similar electronegativities and atomic radii of niobium and titanium. Calculations using Miedema's model confirmed the stability of intermetallic compounds in the broad range of compositions at the expense of solid solution and amorphous phase. While in Y0.9(Nb, Ti)0.1Co2 single C15 Laves phase is forming, two phases (YCo2 and NbCo2) are crystallizing in Y0.7Nb0.3Co2. In the case of Ti-substituted sample, three phases with different Ti content were observed due to similar formation enthalpies of intermetallic compounds in analyzed range of compositions ( 33.7 and 32.1 kJ/mol for Y0.9Ti0.1Co2 and Y0.7Ti0.3Co2, respectively). & 2016 Elsevier B.V. All rights reserved.
Keywords: Crystallization Enthalpy of formation Thermodynamics and kinetics of processes in materials Laves phase Glass forming ability Metals and alloys
1. Introduction Depending on the composition, type of dopants, but also on the crystalline structure, compounds from Y-Co series reveal markedly different magnetic properties. Magnetic state of crystalline Y-Co system changes from paramagnetic, through spin glass type to ferri-/ferromagnetic with increasing Co content. Strong influence of structural changes has been denoted mainly for YCo2, which is an exchange-enhanced Pauli paramagnet with itinerant electron metamagnetic transition into ferromagnetic state in high magnetic fields in the crystalline form (cubic Laves phase) [1]. In turn, long range magnetic ordering is observed for amorphous alloy of the same composition and on the surface of crystalline thin film [2,3]. Moreover, nanocrystalline mechanically milled alloy becomes ferromagnetic in the whole grains volume with decreasing grain size [4]. Properties of rapidly quenched YCo2 and Nb or Ti alloyed samples were determined mainly by chemical and topological disorder with clear evidence of magnetic ordering. Additional magnetic contributions have been reported as signs of chemical and topological disorder for small Nb and Ti substitutions [5]. Melt-spun samples possess MgCu2-type structure with lattice constant a changing from 7.223 Å for YCo2, through 7.213 Å for Y0.9Nb0.1Co2 to 7.192 Å for Y0.9Ti0.1Co2. Further studies of Nb- and Ti-substituted systems are provided to give an insight into the formation and stability of Laves phases. While the crystalline E-mail address: [email protected] http://dx.doi.org/10.1016/j.matlet.2016.06.083 0167-577X/& 2016 Elsevier B.V. All rights reserved.
structure determines magnetic properties of mentioned binary and ternary systems, it is of great interest to specify compositional ranges, where different phases (amorphous, solid solution, intermetallics) can be obtained. Substituted Laves phases are the candidates for room temperature refrigeration based on magnetocaloric effect [6].
2. Experiment and calculations The master alloys of Y0.7(Nb,Ti)0.3Co2 were prepared with the use of arc-furnace, by melting of high purity Y (99.9%), Co (99.9%), Ti (99.9%) and Nb (99.9%) in the argon atmosphere. The ingots were rapidly quenched by melt-spinning on a copper wheel rotating with the surface velocity of 40 m/s. The structural characterization was made with the use of x-ray diffractometer (Co-Kα radiation) in Bragg-Brentano geometry. Calculations of formation enthalpy were based on the semiempirical approach known as the Miedema's model [7,8]. Chemical enthalpy was estimated as the only contribution playing a role during formation of intermetallic compounds. Additionally, molar volumes were corrected for volume changes due to alloying. During solidification there is a competition of total free energies of intermetallic compound, solid solution and amorphous phase. Therefore, formation enthalpies of solid solution and amorphous phase were calculated for comparison and to indicate the thermodynamically most stable option. Used approach is described in more details in Ref. [9,10].
Z. Śniadecki / Materials Letters 182 (2016) 90–93
3. Results and discussion X-ray diffraction experiments (patterns shown in Fig. 1.) were carried out to recognize phases crystallized during rapid quenching of Y0.7(Nb,Ti)0.3Co2 ribbons and to investigate their basic parameters. Diffraction data was analyzed utilizing Rietveld refinement. It has been reported that YCo2 and Y0.9(Nb,Ti)0.1Co2 compounds crystallize in single C15 Laves phase with MgCu2-type structure (space group: Fd-3m) [5]. Determined structural parameters are a¼7.223 Å and v¼376.8 Å3 for parent compound, a¼7.213 Å, v¼ 376.8 Å3 for Y0.9Nb0.1Co2 and a¼7.192 Å, v¼372.0 Å3 for Y0.9Ti0.1Co2. Slight decrease of lattice constant suggested that, according to assumed substitution of niobium and titanium for yttrium, Nb and Ti atoms were incorporated into the Y-site. With increasing substitution of Y by Nb (30 at%) the second phase appears (Fig. 1 – upper panel). The new NbCo2 phase has the same structure as YCo2, but its lattice constant equals to a¼6.782 Å, which is comparable with lattice constant obtained in Ref. [11], where it was determined to be ranged from 6.713 Å to 6.802 Å for Nb-rich and Co-rich region of Co-Nb phase diagram. YCo2 lattice constant remains the same as in pure YCo2 compound indicating that whole amount of Nb was built into NbCo2 structure. These two phases grow simultaneously, instead of substituting Y atoms by Nb in YCo2 phase (as it was observed for 10 at% substitution). The volume fraction of YCo2 and NbCo2 phases is equal to 66% and 34%, respectively. The YCo2 alloys with 10 at% substitution of Y by Ti still possess single Laves phase [5]. With higher content of Ti two additional
91
phases appear. The diffraction pattern of Y0.7Ti0.3Co2 is shown in the lower panel of Fig. 1. There are two main phases with the same Fd3m space group, but with different lattice constants: a¼7.205 Å and a¼7.093 Å. Their volume fractions amount to 59% and 24%, respectively. First one is YCo2, slightly substituted with Ti atoms as the lattice constant is decreased by 0.35% in comparison to parent compound. Assuming that lattice constant is changing linearly with Ti substitution and interpolating the lattice constant values for single phase compounds (YCo2 and TiCo2), substitution of Ti should be equal to around 6 at%. Literature based lattice constant of TiCo2 is a¼6.691–6.716 Å [12]. For the second phase, with smaller a value, interpolated Ti content is equal to about 28 at%, while the third Laves phase with the smallest lattice constant (a¼6.751 Å) and the smallest volume fraction (17%) is close to TiCo2 composition, as the Ti content exceeds 93 at%. Concluding structural analysis, presence of binary parent phases, YCo2 and NbCo2 is evident for Nb-containing sample, while for Ti-substituted ribbon mixed phases with different Ti content were observed. Calculations based on the semi-empirical model were utilized to explain the origin of differences in crystallization processes of Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2. Formation enthalpies of different intermetallic compounds were determined and are presented in the upper part of Table 1. The more negative enthalpy values, the higher probability of crystallization of specific phase. YCo2, NbCo2 and Y0.9Nb0.1Co2 exhibit very similar values of formation enthalpies. With increasing content of Nb, the ΔHform tends to be less negative and reaches value around 25.9 kJ/mol for Y0.7Nb0.3Co2. The crystallization of the later and other Ti-substituted phases is hindered in comparison to NbCo2 and YCo2, which in fact crystallize in this sample. Qualitative differences are observed in case of (Y,Ti)Co2 compounds. The most negative heat of formation was calculated for TiCo2. This phase (with expected small Y content) was formed indeed, as shown on the basis of XRD experiment. Starting from YCo2, the composition dependence of ΔHform differ from that observed in Nb-containing system, as the enthalpy of formation values are almost the same, even for the Y0.7Ti0.3Co2. It can be the reason for the presence of different phases with mixed occupation of Y/Ti site, especially the phase with moderate lattice constant and predicted Ti-content of 28 at%. The competition between intermetallic phase, solid solution and amorphous phase takes place during synthesis. The most decisive factor which determines mentioned effect is the difference between enthalpies of formation, which are defined for solid solution and amorphous state in slightly different way than for intermetallic compound. In the case of amorphous phase, the topological contribution is taken into account and is strictly connected with the mismatch of atomic radii of the main elements. More additional terms are contained in solid solution enthalpy of formation and are related to elastic properties and type of
Table 1 Formation enthalpies of Y1-x(Nb,Ti)xCo2 (x¼ 0, 0.1, 0.3) intermetallic compounds (ΔHform), solid solutions (ΔHss) and amorphous phases (ΔHam). All the values are expressed in kJ/mol. YCo2
MCo2
Y0.9M0.1Co2
Y0.7M0.3Co2
31.8 33.7
25.9 32.1
1.9 14.7
6.9 5.3
7.7 2.5
Amorphous phase (ΔHam [kJ/mol]) M ¼ Nb 13.5 15.2 M ¼ Ti 19.4
12.4 13.3
10.6 13.3
Intermetallic compound (ΔHform [kJ/mol]) M ¼ Nb 31.8 33.3 M ¼ Ti 38.3 Solid solution (ΔHss [kJ/mol]) M ¼ Nb 6.1 M ¼ Ti Fig. 1. X-ray diffraction patterns of rapidly quenched Y0.7M0.3Co2 (M – Nb, Ti). For clarity, Miller indices are given for the peaks marked with black circles only, but are valid for all exhibited phases.
92
Z. Śniadecki / Materials Letters 182 (2016) 90–93
Fig. 2. Compositional dependence of formation enthalpies of amorphous phase (ΔHam) and solid solution (ΔHss) for Y-Nb-Co system. Y0.9Nb0.1Co2 and Y0.7Nb0.3Co2 are marked by black circles in both phase diagrams.
crystalline structure. Calculated values of formation enthalpy of solid solution ΔHss and amorphous phase ΔHam are presented in Table 1 (middle and lower part, respectively). In addition, phase diagrams of ΔHam and ΔHss are presented for Nb and Ti containing systems in Figs. 2 and 3, respectively. Comparing the values of ΔHam and ΔHss with ΔHform for intermetallic compounds, one can conclude that in the same composition ranges, there is significant difference in the enthalpies and formation of intermetallic phases is unequivocally preferred. Moreover ΔHam is more negative than ΔHss in almost whole composition range. It would suggest that glassy state can be formed in the composition regions were no intermetallic compounds are observed. One should reject this statement on the basis of experimental results, as the mixture of intermetallic phases is forming in the broad composition range. Additional alloying element should improve glass forming ability, but in the case of Nb and Ti substitutions, Laves phases were synthesized. 4. Conclusions Significant differences in crystallization processes of Y0.7Nb0.3Co2 and Y0.7Ti0.3Co2 were observed by x-ray diffraction and validated by
Fig. 3. Compositional dependence of formation enthalpies of amorphous phase (ΔHam) and solid solution (ΔHss) for Y-Ti-Co system. Y0.9Ti0.1Co2 and Y0.7Ti0.3Co2 are marked by black circles in both phase diagrams.
the calculations based on the semi-empirical Miedema's model. Multiphase systems are forming during solidification, while in Y0.9(Nb,Ti)0.1Co2 single C15 Laves phase is observed [5]. Parent limiting compounds (YCo2 and NbCo2) are crystallizing in case of Nbsubstituted sample. For Ti-substituted ribbon, three phases with different Ti content were observed. It was linked with very similar formation enthalpies of intermetallic compounds in the broad range of Ti content, where none of the phases is prefered. In contrast, formation enthalpy is becoming less negative with increasing Nb fraction in Y-Nb-Co. Intermetallics were confirmed to be more stable than solid solutions and amorphous phases in both cases.
References [1] T. Goto, K. Fukamichi, T. Sakakibara, H. Komatsu, Solid State Commun. 72 (1989) 945. [2] S. Khmelevskyi, P. Mohn, J. Redinger, M. Weinert, Phys. Rev. Lett. 94 (2005) 146403. [3] Yu.S. Dedkov, C. Laubschat, S. Khmelevskyi, J. Redinger, P. Mohn, M. Weinert, Phys. Rev. Lett. 99 (2007) 047204.
Z. Śniadecki / Materials Letters 182 (2016) 90–93 [4] S.N. Jammalamadaka, E.V. Sampathkumaran, V.S.N. Murthy, G. Markandeyulu, Appl. Phys. Lett. 92 (2008) 192506. [5] Z. Śniadecki, M. Werwiński, A. Szajek, U.K. Rößler, B. Idzikowski, J. Appl. Phys. 115 (2014) 17E129. [6] V. Franco, J.S. Blazquez, B. Ingale, A. Conde, Annu. Rev. Mater. Res. 42 (2012) 305. [7] F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, in: F.R. de Boer, D.G. Pettifor (Eds.), Cohesion and Structure, vol. 1, North Holland Physics, Amsterdam, 1988, pp. 1–758.
93
[8] H. Bakker, Enthalpies in Alloys – Miedema's Semi-Empirical Model, Materials Science Foundations 1, Trans Tech Publications Ltd., Switzerland 1998, pp. 1–78. [9] Z. Śniadecki, J.W. Narojczyk, B. Idzikowski, Intermetallics 26 (2012) 72. [10] Z. Śniadecki, J. Alloy. Compd. 615 (2014) S40. [11] F. Stein, D. Jiang, M. Palm, G. Sauthoff, D. Gruener, G. Kreiner, Intermetallics 16 (2008) 785. [12] J.L. Murray, Bull. Alloy Phase Diagr. 3 (1982) 74.