Isothermal section of the Ti-Si-B system at 1250 ° C in the Ti-TiSi2-TiB2 region

Isothermal section of the Ti-Si-B system at 1250 ° C in the Ti-TiSi₂-TiB₂ region

Alfeu Saraiva Ramos^I; Renato BaldanII,^*; Carlos Angelo Nunes^II; Gilberto Carvalho Coelho^II,IV; Paulo Atsushi Suzuki^II; Geovani Rodrigues^III

^IInstituto de Ciência e Tecnologia, Universidade Federal de Alfenas – UNIFAL-MG, Rod. José Aurélio Vilela, 11.999, Cidade Universitária, CEP 37715-400, Poços de Caldas, MG, Brasil

^IIEscola de Engenharia de Lorena – EEL, Universidade de São Paulo – USP, Est. Municipal do Campinho, s/n, CEP 12602-810, Lorena, SP, Brasil

^IIIUniversidade Federal de Itajubá – UNIFEI, Av. BPS, 1303, Pinheirinho, CEP 37500-903, Itajubá, MG, Brasil

^IVNúcleo de Pesquisa, Centro Universitário de Volta Redonda – UniFoa, Campus Três Poços, Av. Paulo Erlei Alves Abrantes, 1325, Três Poços, CEP 27240-560, Volta Redonda, RJ, Brasil

ABSTRACT

A partial isothermal section (Ti-TiSi₂-TiB₂ region) of the ternary Ti-Si-B system at 1250 ° C was determined from heat-treated alloys prepared via arc melting. Microstructural characterization has been carried out through scanning electron microscopy (SEM), x-ray diffraction (xRD) and wavelength dispersive spectrometry (WDS). The results have shown the stability of the near stoichiometric Ti₆Si₂B phase and a negligible solubility of boron in the Ti-silicides as well as of Si in the Ti-borides. The following three-phase equilibria have been observed in the Ti- TiSi₂-TiB₂ region: Ti_ss+TiB+Ti₆Si₂B, Ti_ss+Ti₆Si₂B+Ti₅Si₃, Ti₅Si₃+Ti₆Si₂B+TiB, Ti₅Si₃+TiB+Ti₃B₄, Ti₅Si₃+Ti₃B₄+TiB₂, Ti₅Si₃+TiB₂+Ti₅Si₄, Ti₅Si₄+TiB₂+TiSi and TiSi+TiB₂+TiSi₂.

Keywords: Ti-Si-B system, titanium alloys, phase diagram, isothermal section, phase equilibria.

1. Introduction

Initial information on phase equilibria in the Ti-Si-B system was provided by Meschter¹ and Maex et al.² from thermodynamic calculations, using data from the binary phases of the Ti-B and Ti-Si systems. Figure 1a,b shows the isothermal sections calculated by these authors.

From experimental results, Ramos et al.^3,4 showed the existence of a stable ternary phase with Ti₆Si₂B stoichiometry in this system. Additionally, the phase relations involving this phase at 1250 ° C and its formation reaction from the liquid state were also showed. Yang et al.⁵ carried out a thermodynamic calculation of the Ti-Si-B system, proposing a liquidus projection and an isothermal section at 1250 ° C from selected experiments and the results from Ramos et al.^3,4. The experiments carried out by Yang et al.⁵ involved the production by arc-melting, annealing at 1250 ° C for 100h and microstructural characterization of four samples of composition Ti-21Si-10B, Ti-19Si-6B, Ti-16.5Si-3.5B and Ti-14Si-1B. All these compositions are located in the Ti-rich region of the Ti-Si-B system, specifically in the βTi+Ti₅Si₃+Ti₆Si₂B three-phase field of this system at 1250 ° C.

In order to extend the phase stability knowledge of this system, this work evaluated the phase relations in the Ti-TiSi₂-TiB₂ region of the Ti-Si-B system at 1250 ° C and the results were compared with those from Yang et al.⁵.

2. Experimental Procedure

Several ingots (~ 8 g) of different compositions were produced from high-purity commercially available materials: titanium (99.8 % min.), silicon (99.999 % min.) and boron (99.5 % min.). Table 1 shows the nominal compositions of the alloys used in this work. The samples were prepared via arc-melting with a non-consumable tungsten electrode on a water-cooled hearth under high purity argon atmosphere gettered by titanium. The ingots were melted five times in an effort to produce homogeneous alloys. Then, parts of the as-cast ingots were heat-treated at 1250 ° C for up to 240 h under argon in quartz tubes. In order to confirm the stability of the Ti₃B₄ and support some proposed phase relations which could not be experimentally proved, a Ti_42.89B_57.11 (Ti₃B₄ stoichiometry) alloy was arc-melted and heat-treated at 1250 ° C for 30 days and at 1500 ° C for 55 h.

All the samples were characterized by scanning electron microscopy (SEM) and selected alloys were evaluated via microanalysis wavelength dispersive spectrometry (WDS).

The SEM/BSE images were acquired in a LEO 1450VP SEM equipment. The WDS measurements were carried out in a model 440 Stereoscan/Leica Microscope at 15 kV/10 nA using PET and LSM 200 crystals to quantify the contents of Ti/Si and B, respectively. Pure elements were used as standards, and the results were obtained using a Φρz correction procedure. At least three measurements for each phase were carried out.

3. Results and Discussion

Figure 2 shows the partial isothermal section at 1250 ° C of the Ti-Si-B system determined in this work. Data for the binaries Ti-Si and Ti-B systems were taken from⁶ and includes the following phases: Ti_ss, Ti₅Si₃, Ti₅Si₄, TiSi, TiSi₂, TiB, Ti₃B₄ and TiB₂. There exists only one ternary phase, Ti₆Si₂B, which was discovered by Ramos et al.⁴ and is isotypic with Ni₆Si₂B.

The WDS measurements data can be roughly evaluated via the measured values of B and Si in the borides and silicides present in Table 1. These borides and silicides are near stoichiometric phases in the correspondent binaries phase diagrams. For TiB, the concentration of B should be near 50 at.% and the measured values were in the range of 52.6 to 54.3 at.% B. For TiB₂, it is expected 66.7 at.% B and the measured values were in the range of 65.6 to 69.1 at.% B. For Ti₅Si₃, the concentration of Si should be near 37.5 at.%, the measured values are in the range of 34 to 37.9 at.% Si, however, in this case, some solubility range might exist. For Ti₅Si₄, the concentration of Si should be near 40 at.%, the measured values are in the range of 40.8 to 43 at.% Si. For TiSi, the concentration of Si should be near 50 at.%, the measured values varied from 47 to 50.2 at.% Si. For TiSi₂, the concentration of Si should be 66.7 at.%, the measured value is 66.3 at.% Si.

The microstructural characteristics of the alloys which allowed the establishment of the isothermal section shown in Figure 2 are presented below.

Alloy #40 (Ti₆₅Si_32.5B_2.5) presented Ti_ss, Ti₅Si₃ and Ti₆Si₂B phases in the as-cast as well as in the heat-treated samples. Figure 3a shows a SEM/BSE micrograph of this alloy in the heat-treated condition where Ti_ss and Ti₆Si₂B are minor phases in a Ti₅Si₃ matrix. The WDS results shown in Table 2 indicate a low solubility of Si and B in Ti_ss as well as of B in Ti₅Si₃. The cracks present in the Ti₅Si₃ phase (Figure 3a) are formed during cooling in the solid state, associated with the high anisotropy of thermal expansion of this phase⁷.

 

Alloy # 55 (Ti₇₀Si_7.5B_22.5) presented Ti_ss, TiB and Ti₆Si₂B phases in the as-cast as well as in the heat-treated samples. Figure 3b shows a SEM/BSE micrograph of the heat-treated alloy where all the phases are present in significant amounts. The WDS data (Table 2) shows once again the low solubility of Si and B in Ti_ss as well as of Si in TiB. In addition, because the compositions of the Ti_ss and Ti₆Si₂B phases in the alloys #40 and #55 are approximately the same (Table 2), the Ti_ss+Ti₆Si₂B two-phase region should be quite narrow at 1250 ° C.

The calculated results of Yang et al.⁵ have shown some solubility of Si in the Ti_ss at 1250 ° C. However, considering the low solubility of Si and B in Ti_ss, the low solubility of B and Si in the silicides and borides respectively, as well as the difficult to accurately determine the concentration of boron, we have not included any solubility data in Figure 2.

Alloy #14 (Ti_66.7Si_22.2B_11.1) presents the nominal composition of the stoichiometric Ti₆Si₂B phase. Five phases were observed in the as-cast sample: TiB₂, TiB, Ti₆Si₂B, Ti₅Si₃ and Ti_ss. After heat-treatment at 1250 ° C, the microstructure formed was essentially constituted of the Ti₆Si₂B, with minor amount of Ti_ss and TiB, as shown in Figure 3c. These results suggested that the Ti₆Si₂B should be near stoichiometry at 1250 ° C. Contrasting to the behavior observed for the samples with high volume fraction of Ti₅Si₃ phase, no crack was noticed in the Ti₆Si₂B phase, likely due to its lower thermal expansion anisotropy⁷.

Alloy # 69 (Ti₆₀Si₂₅B₁₅) presented Ti_ss, Ti₅Si₃, TiB, TiB₂ and Ti₆Si₂B in the as-cast and TiB, Ti₅Si₃ and Ti₆Si₂B in the heat-treated sample, showing the complete dissolution of TiB₂ and Ti_ss during heat-treatment. Figure 3d shows a SEM/BSE micrograph of the heat-treated alloy. The WDS analysis shows very low solubility of B in the Ti₅Si₃ phase as well as of Si in the TiB phase.

As can be concluded from the previous results, at 1250 ° C the ternary Ti₆Si₂B phase equilibrates with Ti_ss, TiB, and Ti₅Si₃ phases through narrow two-phase fields.

Alloy # 46 (Ti₅₅Si₄₀B₅) presented Ti₅Si₃, Ti₅Si₄, TiSi, TiSi₂ and TiB₂ in the as-cast and Ti₅Si₃ Ti₅Si₄ e TiB₂ in the heat-treated sample, indicating the dissolution of TiSi and TiSi₂ during the heat-treatment. Figure 4a shows a SEM/BSE micrograph of this alloy after heat-treatment. The WDS results show the low solubility of B in both Ti₅Si₃ and Ti₅Si₄ as well as of Si in TiB₂.

 

Alloy #77 (Ti₅₀Si₄₅B₅) presented Ti₅Si₄, TiSi, TiSi₂ and TiB₂ in the as-cast and Ti₅Si₄, TiSi and TiB₂ in the heat-treated sample, showing the dissolution of the TiSi₂ phase during the heat-treatment. Figure 4b shows a SEM/BSE micrograph of this alloy after heat-treatment where all the phases are present in significant amount. The WDS analysis from the Ti₅Si₄ and TiSi phases has shown a very low B solubility in these phases as well as of Si in TiB₂.

Alloy # 79 (Ti₄₀Si₅₀B₁₀) revealed the presence of Ti₅Si₄, TiSi, TiSi₂ and TiB₂ in the as-cast and TiSi, TiSi₂ and TiB₂ after heat-treatment, indicating the dissolution of the Ti₅Si₄ phase. Figure 4c shows a SEM/BSE micrograph of the heat-treated sample where all the phases are present in significant amounts. The WDS data shown in Table 2 indicates the low solubility of B in both TiSi and TiSi₂ phases.

Alloys # 91, 92 and 93 presented TiB, Ti₅Si₃ and TiB₂ in both as-cast and heat-treated microstructures. These results indicate the difficulty to form the Ti₃B₄ from the as-cast microstructures and therefore to equilibrate these alloys in the phase fields involving this phase. Another possibility was that the Ti₃B₄ phase is not stable, in spite of its indication in the currently accepted Ti-B phase diagram. In order to check the stability of the Ti₃B₄ phase in the Ti-B system, an alloy with composition Ti_42.89B_57.11 (Ti₃B₄ stoichiometry) was arc-melted and heat-treated at 1250 ° C for 30 days. The as-cast microstructure was formed by the phases TiB₂, Ti₃B₄, TiB and Ti_ss, the volume fraction of Ti₃B₄ being near 13%. The heat-treatment did not change the phases nor modify significantly amounts in the as-cast microstructure. A second as-cast sample of same composition was heat-treated at 1500 ° C for 55 h and now a significant increase of Ti₃B₄ volume fraction (13% => 43%) was observed, even though thermodynamic equilibrium conditions were not reached. Spear et al.⁸ have evaluated the stability of the Ti₃B₄ from annealing (1690 ° C to 2070 ° C for ½ to 2h) of as-cast samples with composition in the 50 to 67 at.% B range. Both xRD and SEM results showed an increase in the quantity of Ti₃B₄ phase due to annealing, indicating the stability of the Ti₃B₄ phase. Based on the results of Spear et al.⁸ and our own results presented above, it is assumed that the Ti₃B₄ is also stable at 1250 ° C. Thus, the existence of the TiB+Ti₅Si₃+Ti₃B₄ and Ti₃B₄+Ti₅Si₃+TiB₂ three-phase regions is proposed to be consistent with the results of alloys #69 and #46 shown previously. Furthermore, the proposed isothermal section is in agreement with those calculated by Yang et al.⁵.

4. Conclusions

Phase equilibria in the Ti-TiSi₂-TiB₂ region of the Ti-Si-B system have been experimentally evaluated at 1250 ° C from heat-treated alloys prepared via arc melting. In general, it was found to be difficult to equilibrate alloys located in the TiB-Ti₅Si₃-TiB₂ region. The ternary phase (Ti₆Si₂B) previously reported in the literature has been confirmed in this study. Very low solubility of Si in the borides as well as of B in the silicides has been noticed, thus, all the two-phase fields should be very narrow. The following three-phase equilibria have been observed: Ti_ss+TiB+Ti₆Si₂B; Ti_ss+Ti₆Si₂B+Ti₅Si₃; Ti₅Si₃+Ti₆Si₂B+TiB; Ti₅Si₃+TiB+Ti₃B₄; Ti₅Si₃+Ti₃B₄+TiB₂; Ti₅Si₃+TiB₂+Ti₅Si₄; Ti₅Si₄+TiB₂+TiSi; TiSi+TiB₂+TiSi₂.

Received: April 21, 2013

Revised: November 9, 2013

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Publication Dates

• Publication in this collection
  22 Oct 2013
• Date of issue
  Apr 2014

History

• Reviewed
  09 Nov 2013
• Received
  21 Apr 2013