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Precipitation of W2B5 and β-WB from Ti0.3W0.4Cr0.3B2 solid solutions
Materials Letters 62 (2008) 3836–3838
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Precipitation of W2B5 and β-WB from Ti0.3W0.4Cr0.3B2 solid solutions S. Bartels a, W. Gruber a, B. Cappi b, R. Telle b, H. Schmidt a,⁎ a b
Institut für Metallurgie, AG Materialphysik, Technische Universität Clausthal, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany Institut für Gesteinshüttenkunde, RWTH Aachen, Mauerstr. 5, 52064 Aachen, Germany
A R T I C L E
I N F O
Article history: Received 31 March 2008 Accepted 25 April 2008 Available online 6 May 2008 Keywords: Precipitation Kinetics Transition metal borides Ceramics
A B S T R A C T We investigated the formation of W2B5 and β-WB precipitates in supersaturated Ti0.3W0.4Cr0.3B2 solid solutions during isothermal annealing at temperatures between 1400 and 1600 °C. X-ray diffractometry measurements showed that the precipitation process is very complex including metastable phase formation and mutual transformation between the two precipitation phases. The rate constants of precipitation are determined as a function of reciprocal temperature and discussed in framework to literature data. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Considerable interest in transition metal diboride ceramics is fostered by attractive materials properties such as high melting points, high hardness, and good electrical and thermal conductivity [1]. Besides these qualities, many of these diborides are distinguished by large temperature dependent homogeneity ranges which are bordered by miscibility gaps as demonstrated for the system TiB2–WB2–CrB2 [2,3]. Annealing of homogeneous solid solutions at temperature below the homogenization temperature leads to the precipitation of plateletshaped particles with a high aspect ratio. These platelets are expected to improve both fracture toughness and creep resistance and are the basis of the development of in-situ reinforced boride ceramics [2,4,10]. The main precipitates which are formed in the supersaturated WxTiyCrzB2 matrix during annealing at temperatures between 1400 and 1700 °C are diborides of structure type W2B5 with a stoichiometry close to 1:2 in accordance with the phase diagram [2,5]. The precipitation kinetics of W2B5 can be described by a model based on the modified theory of Johnson, Mehl, Avrami, and Kolmogorov (JMAK) including grain boundary nucleation [6]. In addition to W2B5 precipitates, for certain WxTiyCrzB2 compositions also monoboride precipitates of β-WB type are found in conflict to the TiB2–WB2 quasi-binary phase diagram as known up to date [2,4]. At the moment it is still unclear what are the prerequisites (temperature, composition, annealing time) for the formation of this phase. For a closer inspection of this phenomenon, we investigated the precipitation of W2B5 and β-WB as a function of temperature and annealing time by X-ray diffractometry (XRD) and
⁎ Corresponding author. E-mail address: [email protected] (H. Schmidt). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.04.077
secondary electron microscopy (SEM). As a model system, ceramics with a chemical composition of Ti0.3W0.4Cr0.3B2 were used, where both types of precipitates are formed. 2. Experimental The samples were produced by reaction sintering of TiB2, WB2, and CrB2 powders (H. C. Starck, Germany) at the Institut für Gesteinshüttenkunde (Mineral Engineering), RWTH Aachen. The powders contain less than 1 wt.% of impurities (C, Fe, N, O). The reaction sintering process was carried out in a uni-axial graphite hot press at a pressure of 60 MPa at 1800 °C for 30 min in argon using boron nitride coated graphite crucibles. Afterwards, the bulk specimens were homogenized at 2000 °C for 8 h at ambient pressure in argon. Samples with a chemical composition of Ti0.3W0.4Cr0.3B2 were investigated in this study as determined by EDX and XRD. For the precipitations experiments slabs of 5 × 5 × 2 mm3 were cut from the interior of the hot-pressed samples, polished with diamond paste down to 1 μm and cleaned with ethanol. These samples were isothermally annealed at temperatures between 1400 °C and 1600 °C in an argon atmosphere at ambient pressure and quenched afterwards. XRD investigations were carried out by a SIEMENS D5000 diffractometer in θ/2θ mode using CoKα radiation (40 kV, 40 mA). The X-ray diffractograms were analyzed with the program Powder Cell for Windows [7] using structural data of Ref. [8]. SEM studies were made using a CamScan 44 apparatus with a tungsten cathode. 3. Results and discussion Investigations with XRD, SEM, and EDX of samples with composition Ti0.3W0.4Cr0.3B2 after homogenisation at 2000 °C proved that single phase polycrystalline solid solutions with a grain size of 20–50 μm are present, all crystallized in
S. Bartels et al. / Materials Letters 62 (2008) 3836–3838
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Fig. 1. X-ray diffractograms of a Ti0.3W0.4Cr0.3B2 ceramic annealed at 1500 °C for various time steps.
Fig. 3. Lattice constants for the TiB2 type matrix phase as a function of annealing time at 1500 °C.
the AlB2 structure type (space group: P6/mmm) [8]. No significant amount of precipitates was detected. Annealing at temperatures between 1400 and 1600 °C leads to precipitation of tungsten rich secondary phases with W2B5 structure (space group: P63/mmc) and/or β-WB structure (space group: Cmcm) [8]. As known from literature [2,4] precipitates of W2B5 structure contain preferentially W and only small amounts of Cr and Ti (b 5 at.%). In contrast, for precipitates with β-WB structure an high amount of Ti and Cr can be incorporated into the structure, leading to stabilization. For a detailed understanding of the precipitation process, X-ray diffraction studies and some additional SEM studies were carried out as a function of annealing time at constant temperature. The following procedure was applied: first, the homogeneous material was isothermally annealed at a distinct temperature for a given time. Then the material was characterized and afterwards annealed again. This procedure was repeated until the X-ray diffractograms did not change any more during further annealing, meaning that the precipitated phase remains constant. The (004) reflection of the W2B5 phase and the (110) reflection of the β-WB phase are used for monitoring the precipitation process as a function of annealing time. From the integrated intensities of the Bragg peaks, the relative amount of crystallized phase was determined. Details of this procedure can be found in Refs. [5,6]. The results obtained at a temperature of 1500 °C are exemplarily illustrated in Figs. 1 and 2. As shown in Fig. 2, in a first step, on a time scale of about 1 h a significant amount of the W2B5 type phase precipitates (region A). Afterwards, after about 3 h of annealing the W2B5 phase vanishes and simultaneously the β-WB phase is formed reaching its maximum value at about 10 h of annealing (region B). Further annealing leads to a reduction of the β-WB phase. Simultaneously the W2B5 phase is formed again (region C) and is present up to annealing times of over 100 h. The maximum amount of each phase precipitated is about 25–40% for β-WB at an annealing time of 10 h and 25–30% for W2B5 for times larger than 100 h, as determined by XRD. These results show that the observation of the β-WB phase in precipitated transition metal diboride samples [2] is the consequence of a cyclic variation of the type and amount of precipitated phase
present as a function annealing time. Investigations at a temperature of 1600 °C exhibited qualitatively the same results as for 1500 °C, however, the phase formation processes are faster (see below). At 1400 °C due to a slower phase formation kinetics only the first precipitation step (region A) can be observed within 30 h of annealing. For further analysis, from the Bragg peaks in the angle range between 5 and 80° the lattice parameters of the distinct phases were calculated using an appropriate software package [7]. For the hexagonal matrix phase with TiB2 structure, the a parameter remains constant during the whole precipitation process and is calculated to be a=(3.021±0.002) Å. In contrast, the c parameter increases significantly during the first 6 h of annealing (see Fig. 3), corresponding roughly to the time where the maximum amount of β-WB phase is formed. Concerning the W2B5 phase the parameters remain approximately constant during the whole precipitation process with values of a=(2.978±0.002) Å and c=(13.85±0.01) Å. The same is true for the β-WB phase with values of a=(3.139±0.002) Å, b=(8.254±0.005) Å, and c= (3.035±0.002). The present results can be explained under the assumption that three processes govern phase formation in Ti0.3W0.4Cr0.3B2 solid solutions: the precipitation of a W2B5 type phase, the precipitation of a β-WB type phase, and the transformation of the two phases into each other according to a reaction with free boron. The increase of the c lattice constant of the TiB2 matrix phase during annealing reflects an ongoing precipitation process, while the matrix phase is depleted predominantly in tungsten, and tungsten is transferred into the precipitates. However, in the range where the β-WB phase vanishes and W2B5 phase is formed again (10–120 h) the c lattice constant remains constant. This is a strong hint that here predominantly no new phase is precipitated from the matrix, but more likely, a transformation of β-WB into W2B5 takes place. Due to the high mobility of the boron atoms [9] it is possible that free boron,
Fig. 2. Relative fraction of precipitated phase (W2B5 and β-WB) as a function of annealing time at 1500 °C for a ceramic with composition Ti0.3W0.4Cr0.3B2. The meaning of the different regions marked with A, B, and C are explained in the text.
Fig. 4. Rate constants of precipitation as a function of reciprocal temperature for selected samples in the system (TixWyCrz)B2: (Ti0.4W0.5Cr0.1)B2 (dot), (Ti0.3W0.5Cr0.2)B2 (circle) [5] and Ti0.3W0.4Cr0.3B2 [this work]. The regions corresponding to the rate constants are indicated (see Fig. 2).
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S. Bartels et al. / Materials Letters 62 (2008) 3836–3838
presumably located at grain boundaries, reacts with β-WB to form W2B5. The detailed explanation for the cyclic occurrence of the two precipitation phases is still unclear at the moment and needs further clarification. Especially, an exact determination of the phase diagram, improved micro-structural investigations e. g. by TEM, and measurements of the diffusivities might be helpful for a comprehensive picture. For further analysis the rate constants of precipitation (or better phase formation) are extracted from the experimental data (see Fig. 2) for the three regions by determining a characteristic time t⁎ = 1/kp where the transformed fraction has a value of 0.632 (in analogy to a JMAK type of transformation [5,6]). The rate constants of precipitation are shown as a function of reciprocal temperature in comparison to literature data in Fig. 4. As shown in a previous study [5], the rate constants for (Ti0.5 − xW0.5Crx)B2 (x = 0.1, 0,2) are identical within error limits and exhibit a non-monotonic behaviour where log kp increases linearly with temperature between 1400 and 1600 °C, forms a maximum around Tmax = 1650 °C and progressively decreases for higher temperatures. In contrast, the samples of the present study show a different behaviour. The rate constants for the formation of the W2B5 type phases are about one order of magnitude higher (region A) or lower (region C) compared to the literature data, indicating a completely different phase formation mechanism for the chemical composition investigated in this study. Since the first precipitation step is only metastable, the final formation of this phase has a smaller rate constant as given in the literature for the other compositions. We assume that the higher Cr concentration and the corresponding lower W concentration of the present sample is the reason for a considerable decrease of the precipitation rate. This might have an influence on the diffusivities or/and on the driving force of nuclei formation. In which way the transient precipitation of the β-WB triggers this kinetic behaviour is unclear at present and needs further clarification.
4. Conclusions The present experiments on the precipitation of W2B5 and β-WB type phases from supersaturated Ti0.3W0.4Cr0.3B2 solid solutions exhibited that the two phases are formed in a complex, time-dependent behaviour. With increasing annealing time, first W2B5 is formed,
afterwards W2B5 vanishes, while additional β-WB precipitates and finally β-WB is transformed into W2B5. These results clearly show that the unexpected report of a β-WB precipitation phase in literature is the consequence of metastable phase formation process. Acknowledgements We thank E. Ebeling for ceramographic preparation of the samples and S. Lenk for their assistance in SEM/EDX analyses. This work has been funded by the German Research Foundation (DFG), grant No. SCHM 1569/1. References [1] Telle R. Boride and carbide Ceramics. In: Swain M, editor. Materials Science and Technology. Structure and Properties of CeramicsWeinheim: VCH-Wiley; 1993. p.173. [2] Mitra I, Telle R. J Solid State Chem 1997;133:25. [3] Telle R, Fendler E, Petzow E. J Hard Mater 1992;3:211. [4] Schmalzried C, Telle R, Freitag B. Mader W Z Metallkd 2001;92:1197. [5] Schmidt H, Fotsing ER, Borchardt G, Schmalzried C, Telle R. Int J Mater Res 2006;97:821. [6] Fotsing ER, Schmidt H, Borchardt G, Schmalzried C, Telle R. Philos Mag 2005;85:4409. [7] Kraus W. Nolze G J Appl Cryst 1996;29:301. [8] Villars P, Calvert LD. Pearson's Handbook of Crystallographic Data for Intermetallic Phases. Ohio: ASM International; 1991. [9] Schmidt H, Borchardt G, Schmalzried C, Telle R, Weber S, Scherrer H. J Appl Phys 2003;93:907. [10] Telle R, Momozawa A, Music D, Schneider. J Solid State Chem 2006;179::2850.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Precipitation of W2B5 and β-WB from Ti0.3W0.4Cr0.3B2 solid solutions S. Bartels a, W. Gruber a, B. Cappi b, R. Telle b, H. Schmidt a,⁎ a b
Institut für Metallurgie, AG Materialphysik, Technische Universität Clausthal, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany Institut für Gesteinshüttenkunde, RWTH Aachen, Mauerstr. 5, 52064 Aachen, Germany
A R T I C L E
I N F O
Article history: Received 31 March 2008 Accepted 25 April 2008 Available online 6 May 2008 Keywords: Precipitation Kinetics Transition metal borides Ceramics
A B S T R A C T We investigated the formation of W2B5 and β-WB precipitates in supersaturated Ti0.3W0.4Cr0.3B2 solid solutions during isothermal annealing at temperatures between 1400 and 1600 °C. X-ray diffractometry measurements showed that the precipitation process is very complex including metastable phase formation and mutual transformation between the two precipitation phases. The rate constants of precipitation are determined as a function of reciprocal temperature and discussed in framework to literature data. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Considerable interest in transition metal diboride ceramics is fostered by attractive materials properties such as high melting points, high hardness, and good electrical and thermal conductivity [1]. Besides these qualities, many of these diborides are distinguished by large temperature dependent homogeneity ranges which are bordered by miscibility gaps as demonstrated for the system TiB2–WB2–CrB2 [2,3]. Annealing of homogeneous solid solutions at temperature below the homogenization temperature leads to the precipitation of plateletshaped particles with a high aspect ratio. These platelets are expected to improve both fracture toughness and creep resistance and are the basis of the development of in-situ reinforced boride ceramics [2,4,10]. The main precipitates which are formed in the supersaturated WxTiyCrzB2 matrix during annealing at temperatures between 1400 and 1700 °C are diborides of structure type W2B5 with a stoichiometry close to 1:2 in accordance with the phase diagram [2,5]. The precipitation kinetics of W2B5 can be described by a model based on the modified theory of Johnson, Mehl, Avrami, and Kolmogorov (JMAK) including grain boundary nucleation [6]. In addition to W2B5 precipitates, for certain WxTiyCrzB2 compositions also monoboride precipitates of β-WB type are found in conflict to the TiB2–WB2 quasi-binary phase diagram as known up to date [2,4]. At the moment it is still unclear what are the prerequisites (temperature, composition, annealing time) for the formation of this phase. For a closer inspection of this phenomenon, we investigated the precipitation of W2B5 and β-WB as a function of temperature and annealing time by X-ray diffractometry (XRD) and
⁎ Corresponding author. E-mail address: [email protected] (H. Schmidt). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.04.077
secondary electron microscopy (SEM). As a model system, ceramics with a chemical composition of Ti0.3W0.4Cr0.3B2 were used, where both types of precipitates are formed. 2. Experimental The samples were produced by reaction sintering of TiB2, WB2, and CrB2 powders (H. C. Starck, Germany) at the Institut für Gesteinshüttenkunde (Mineral Engineering), RWTH Aachen. The powders contain less than 1 wt.% of impurities (C, Fe, N, O). The reaction sintering process was carried out in a uni-axial graphite hot press at a pressure of 60 MPa at 1800 °C for 30 min in argon using boron nitride coated graphite crucibles. Afterwards, the bulk specimens were homogenized at 2000 °C for 8 h at ambient pressure in argon. Samples with a chemical composition of Ti0.3W0.4Cr0.3B2 were investigated in this study as determined by EDX and XRD. For the precipitations experiments slabs of 5 × 5 × 2 mm3 were cut from the interior of the hot-pressed samples, polished with diamond paste down to 1 μm and cleaned with ethanol. These samples were isothermally annealed at temperatures between 1400 °C and 1600 °C in an argon atmosphere at ambient pressure and quenched afterwards. XRD investigations were carried out by a SIEMENS D5000 diffractometer in θ/2θ mode using CoKα radiation (40 kV, 40 mA). The X-ray diffractograms were analyzed with the program Powder Cell for Windows [7] using structural data of Ref. [8]. SEM studies were made using a CamScan 44 apparatus with a tungsten cathode. 3. Results and discussion Investigations with XRD, SEM, and EDX of samples with composition Ti0.3W0.4Cr0.3B2 after homogenisation at 2000 °C proved that single phase polycrystalline solid solutions with a grain size of 20–50 μm are present, all crystallized in
S. Bartels et al. / Materials Letters 62 (2008) 3836–3838
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Fig. 1. X-ray diffractograms of a Ti0.3W0.4Cr0.3B2 ceramic annealed at 1500 °C for various time steps.
Fig. 3. Lattice constants for the TiB2 type matrix phase as a function of annealing time at 1500 °C.
the AlB2 structure type (space group: P6/mmm) [8]. No significant amount of precipitates was detected. Annealing at temperatures between 1400 and 1600 °C leads to precipitation of tungsten rich secondary phases with W2B5 structure (space group: P63/mmc) and/or β-WB structure (space group: Cmcm) [8]. As known from literature [2,4] precipitates of W2B5 structure contain preferentially W and only small amounts of Cr and Ti (b 5 at.%). In contrast, for precipitates with β-WB structure an high amount of Ti and Cr can be incorporated into the structure, leading to stabilization. For a detailed understanding of the precipitation process, X-ray diffraction studies and some additional SEM studies were carried out as a function of annealing time at constant temperature. The following procedure was applied: first, the homogeneous material was isothermally annealed at a distinct temperature for a given time. Then the material was characterized and afterwards annealed again. This procedure was repeated until the X-ray diffractograms did not change any more during further annealing, meaning that the precipitated phase remains constant. The (004) reflection of the W2B5 phase and the (110) reflection of the β-WB phase are used for monitoring the precipitation process as a function of annealing time. From the integrated intensities of the Bragg peaks, the relative amount of crystallized phase was determined. Details of this procedure can be found in Refs. [5,6]. The results obtained at a temperature of 1500 °C are exemplarily illustrated in Figs. 1 and 2. As shown in Fig. 2, in a first step, on a time scale of about 1 h a significant amount of the W2B5 type phase precipitates (region A). Afterwards, after about 3 h of annealing the W2B5 phase vanishes and simultaneously the β-WB phase is formed reaching its maximum value at about 10 h of annealing (region B). Further annealing leads to a reduction of the β-WB phase. Simultaneously the W2B5 phase is formed again (region C) and is present up to annealing times of over 100 h. The maximum amount of each phase precipitated is about 25–40% for β-WB at an annealing time of 10 h and 25–30% for W2B5 for times larger than 100 h, as determined by XRD. These results show that the observation of the β-WB phase in precipitated transition metal diboride samples [2] is the consequence of a cyclic variation of the type and amount of precipitated phase
present as a function annealing time. Investigations at a temperature of 1600 °C exhibited qualitatively the same results as for 1500 °C, however, the phase formation processes are faster (see below). At 1400 °C due to a slower phase formation kinetics only the first precipitation step (region A) can be observed within 30 h of annealing. For further analysis, from the Bragg peaks in the angle range between 5 and 80° the lattice parameters of the distinct phases were calculated using an appropriate software package [7]. For the hexagonal matrix phase with TiB2 structure, the a parameter remains constant during the whole precipitation process and is calculated to be a=(3.021±0.002) Å. In contrast, the c parameter increases significantly during the first 6 h of annealing (see Fig. 3), corresponding roughly to the time where the maximum amount of β-WB phase is formed. Concerning the W2B5 phase the parameters remain approximately constant during the whole precipitation process with values of a=(2.978±0.002) Å and c=(13.85±0.01) Å. The same is true for the β-WB phase with values of a=(3.139±0.002) Å, b=(8.254±0.005) Å, and c= (3.035±0.002). The present results can be explained under the assumption that three processes govern phase formation in Ti0.3W0.4Cr0.3B2 solid solutions: the precipitation of a W2B5 type phase, the precipitation of a β-WB type phase, and the transformation of the two phases into each other according to a reaction with free boron. The increase of the c lattice constant of the TiB2 matrix phase during annealing reflects an ongoing precipitation process, while the matrix phase is depleted predominantly in tungsten, and tungsten is transferred into the precipitates. However, in the range where the β-WB phase vanishes and W2B5 phase is formed again (10–120 h) the c lattice constant remains constant. This is a strong hint that here predominantly no new phase is precipitated from the matrix, but more likely, a transformation of β-WB into W2B5 takes place. Due to the high mobility of the boron atoms [9] it is possible that free boron,
Fig. 2. Relative fraction of precipitated phase (W2B5 and β-WB) as a function of annealing time at 1500 °C for a ceramic with composition Ti0.3W0.4Cr0.3B2. The meaning of the different regions marked with A, B, and C are explained in the text.
Fig. 4. Rate constants of precipitation as a function of reciprocal temperature for selected samples in the system (TixWyCrz)B2: (Ti0.4W0.5Cr0.1)B2 (dot), (Ti0.3W0.5Cr0.2)B2 (circle) [5] and Ti0.3W0.4Cr0.3B2 [this work]. The regions corresponding to the rate constants are indicated (see Fig. 2).
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presumably located at grain boundaries, reacts with β-WB to form W2B5. The detailed explanation for the cyclic occurrence of the two precipitation phases is still unclear at the moment and needs further clarification. Especially, an exact determination of the phase diagram, improved micro-structural investigations e. g. by TEM, and measurements of the diffusivities might be helpful for a comprehensive picture. For further analysis the rate constants of precipitation (or better phase formation) are extracted from the experimental data (see Fig. 2) for the three regions by determining a characteristic time t⁎ = 1/kp where the transformed fraction has a value of 0.632 (in analogy to a JMAK type of transformation [5,6]). The rate constants of precipitation are shown as a function of reciprocal temperature in comparison to literature data in Fig. 4. As shown in a previous study [5], the rate constants for (Ti0.5 − xW0.5Crx)B2 (x = 0.1, 0,2) are identical within error limits and exhibit a non-monotonic behaviour where log kp increases linearly with temperature between 1400 and 1600 °C, forms a maximum around Tmax = 1650 °C and progressively decreases for higher temperatures. In contrast, the samples of the present study show a different behaviour. The rate constants for the formation of the W2B5 type phases are about one order of magnitude higher (region A) or lower (region C) compared to the literature data, indicating a completely different phase formation mechanism for the chemical composition investigated in this study. Since the first precipitation step is only metastable, the final formation of this phase has a smaller rate constant as given in the literature for the other compositions. We assume that the higher Cr concentration and the corresponding lower W concentration of the present sample is the reason for a considerable decrease of the precipitation rate. This might have an influence on the diffusivities or/and on the driving force of nuclei formation. In which way the transient precipitation of the β-WB triggers this kinetic behaviour is unclear at present and needs further clarification.
4. Conclusions The present experiments on the precipitation of W2B5 and β-WB type phases from supersaturated Ti0.3W0.4Cr0.3B2 solid solutions exhibited that the two phases are formed in a complex, time-dependent behaviour. With increasing annealing time, first W2B5 is formed,
afterwards W2B5 vanishes, while additional β-WB precipitates and finally β-WB is transformed into W2B5. These results clearly show that the unexpected report of a β-WB precipitation phase in literature is the consequence of metastable phase formation process. Acknowledgements We thank E. Ebeling for ceramographic preparation of the samples and S. Lenk for their assistance in SEM/EDX analyses. This work has been funded by the German Research Foundation (DFG), grant No. SCHM 1569/1. References [1] Telle R. Boride and carbide Ceramics. In: Swain M, editor. Materials Science and Technology. Structure and Properties of CeramicsWeinheim: VCH-Wiley; 1993. p.173. [2] Mitra I, Telle R. J Solid State Chem 1997;133:25. [3] Telle R, Fendler E, Petzow E. J Hard Mater 1992;3:211. [4] Schmalzried C, Telle R, Freitag B. Mader W Z Metallkd 2001;92:1197. [5] Schmidt H, Fotsing ER, Borchardt G, Schmalzried C, Telle R. Int J Mater Res 2006;97:821. [6] Fotsing ER, Schmidt H, Borchardt G, Schmalzried C, Telle R. Philos Mag 2005;85:4409. [7] Kraus W. Nolze G J Appl Cryst 1996;29:301. [8] Villars P, Calvert LD. Pearson's Handbook of Crystallographic Data for Intermetallic Phases. Ohio: ASM International; 1991. [9] Schmidt H, Borchardt G, Schmalzried C, Telle R, Weber S, Scherrer H. J Appl Phys 2003;93:907. [10] Telle R, Momozawa A, Music D, Schneider. J Solid State Chem 2006;179::2850.