Microstructural Formation and Creep Resistance of Sintered Titanium Aluminides with Co and Ni Addition

Abstract

One of the most effective approaches for the densification of titanium aluminides processed by powder metallurgy involves the addition of elements that induce the formation of a liquid phase during sintering. This work investigated the sintering behavior and creep resistance of Ti-45Al (in at.%) with ternary Co or Ni additions. Conventional sintering under a high vacuum atmosphere led to densifications of approximately 90% of the theoretical density in the Ti-45Al-2Co and Ti-45Al-2Ni alloys, while binary Ti-45Al was limited to a maximum of 57%. Microstructural analysis revealed the formation of equilibrium intermetallics γ-TiAl and α₂-Ti₃Al, along with Ni- and Co-rich regions concentrated at the boundaries of primary particles. Compression creep tests at 800°C under a 120 MPa load demonstrated a significant increase in mechanical strength, especially with the addition of Co, which showed 1% of plastic deformation in approximately 18 hours.

Keywords:
titanium aluminides; powder metallurgy; sintering; creep

1. Introduction

Titanium aluminides are intermetallic compounds with low density, high mechanical strength, and excellent creep behavior¹. Therefore, these materials are typically used in high-temperature applications². However, processing titanium aluminides using conventional techniques is not straightforward, as the material is brittle and has a complex solidification path that can cause segregation. Consequently, conventional techniques such as casting and machining result in high manufacturing costs. Thus, powder metallurgy presents itself as a great alternative, as it is a near-net-shape route that provides a refined microstructure with isotropic mechanical properties³. However, processing via powder metallurgy usually requires sintering for densification, which in the case of titanium aluminides requires high temperatures⁴. Additionally, mixtures of elemental powders can result in high porosity due to the formation of intermediate intermetallics such as TiAl₂ and TiAl₃⁵.

Alves Nogueira da Silva et al.⁶ sintered titanium aluminides with different Al contents ranging from Ti-10Al to Ti-45Al (at.%) using the blended elemental powders approach. The authors reported a high porosity of approximately 40% with the binary alloy Ti-45Al sintered at 1300 °C and 1400 °C. The low densification resulted from the exothermic reaction for the formation of the intermetallics that created a swelling effect. Although the equilibrium intermetallic phases γ (TiAl) and α₂ (Ti₃Al) were identified, low mechanical properties resulted due to the high porosity.

Soyama et al.⁷ investigated the effect of elemental Ti and Al additions on pre-alloyed TNB-V5 (Ti-45Al-5Nb-0.2B-0.2C). The authors reported that high sintering temperatures >1480 °C were necessary with elemental Al additions due to the formation of intermediate intermetallics. The exothermic reaction for the intermetallic formation took place after the melting of elemental Al and required high temperatures to homogenize the composition and achieve the equilibrium intermetallic phases. Despite the use of pre-alloyed powders in the greater part for specimen preparation, the small amounts of Al resulted in higher porosities at the same sintering temperatures.

Therefore, to ensure adequate densification in titanium aluminides, different sintering strategies were considered, including the aid of liquid phase⁸. Xia et al.⁹ investigated small Co additions to pre-alloyed Ti-48Al-2Cr-2Nb. The authors reported that a ternary compound CoAl₂Ti formed at temperatures between 1220 and 1260 ºC, and a liquid phase was created by the reactions of CoAl₂Ti + α → Liquid and CoAl₂Ti + α → γ + Liquid. A densification of nearly 100% was achieved with 1.5 at.% Co addition to the pre-alloyed Ti-48Al-2Cr-2Nb powder with sintering at 1300 °C for 2 hours. In another work, Xia et al.¹⁰ studied the effect of small Ni additions on pre-alloyed TNB-V5 (Ti-45Al-5Nb-0.2B-0.2C). The authors reported a densification of about 100% with 1.25% Ni content sintering at 1375 °C for 1 hour. The significant improvement in sintering behavior was explained by the formation of an intermediate intermetallic compound, Al₃NiTi₂, that induced the formation of a liquid phase by three reactions between 1250 °C and 1350 °C: Al₃NiTi₂ + γ → Liquid, Al₃NiTi₂ + γ → Liquid + α, and Al₃NiTi₂ + α → Liquid.

Titanium aluminides typically show high creep resistance; however, since powder metallurgy processing using blended elemental powders often results in residual porosity and heterogeneities at the microscale, the mechanical properties can be impaired. Up to now, only a few studies have investigated the creep resistance of conventionally sintered titanium aluminides¹¹-¹³. Moreover, considering that sintering titanium aluminides requires alloying elements to enhance densification, the characterization of mechanical properties is essential to reveal the potential of powder metallurgy techniques. Therefore, the objective of this work was to investigate the effect of Ni and Co addition to Ti-45Al (at.% %) to improve the sintering behavior and assess the creep resistance at 800 °C, which corresponds to a harsh condition for high temperature materials.

2. Experimental Procedure

The raw materials were Ti, Al, Co, and Ni powders provided by Stanford Advanced Materials, USA. The mean particle size was <45 μm. Specimens with the compositions Ti-45Al, Ti-45Al-2Co, and Ti-45Al-2Ni (all compositions mentioned are in at.%) were prepared by manually mixing elemental powders using a mortar and pestle for approximately 3 minutes. Compaction was conducted uniaxially with the powder mixtures placed in a cylindrical closed die, producing specimens with 8.1 mm in diameter and 5.5 mm in height. A compaction pressure of 390 MPa was applied. Sintering was carried out under vacuum at different temperatures ranging from 1100 to 1300 °C for 2 hours using an XVAC XERION Ofentechnik.

The sintered specimens were mounted in bakelite, ground, and polished to 1 µm with diamond paste. Microstructural characterization was conducted by Light Optical Microscopy with a Leica DM500 and Scanning Electron Microscopy with a Zeiss EVO MA15 in backscattered electron mode. Image analysis for porosity measurements was conducted using the software ImageJ. At least 5 images with the same magnification were used for the porosity measurements. The relative density was calculated based on the porosity, according to the relation: $RelativeDensity=1-Porosity$.

The SEM was equipped with an energy-dispersive X-Ray Spectroscopy system (EDS) from Oxford. X-Ray Diffraction (XRD) patterns were recorded using an X’Pert3 PRO Powder from Malvern Panalytical. The XRD used Cu-Kα radiation with an acceleration voltage of 40 kV and 30 mA of tube current. The sintering behavior was investigated by dilatometry using an L78RITA from LINSEIS under a vacuum atmosphere and a heating rate of 10 C/min. The temperature profile was similar to the experiments conducted with the sintering furnace, applying a dwell time of 2 hours at the sintering temperature.

The creep strength was measured by compression creep conducted at 800 °C with 120 MPa loading. The creep experiments were conducted using an Instron-Satec creep testing machine model SF-16 2230 with a lever arm ratio of 1/16. The rough sintered surface was ground at both extremes of the cylindrical specimen before the creep test.

3. Results and Discussion

The sintering behavior was investigated by dilatometry experiments, according to Fig. 1. The change in linear dimension (specific strain) and the strain rate as a function of temperature for the binary Ti-45Al is shown in Fig. 1 (a), (b) and (c) for the sintering temperatures of 1100, 1200, and 1300 °C, respectively. In all cases, a strong dilatation of about 10-14% was observed around the melting point of Al at approximately 660 °C. This sudden change in length took place due to a strong exothermic reaction for the formation of Al-rich intermetallics that are out of equilibrium⁶,⁷. With the formation of liquid Al, the wetting of Ti particles occurred, thus enhancing the reaction rate.

Figure 1
Sintering behavior of binary Ti-45Al (a) at 1100 °C, (b) 1200 °C, (c) 1300 °C of sintering temperature, and (d) ternary Ti-45Al-2Co and Ti-45Al-2Ni (b).

The formation of the Al-rich intermetallics might severely impair the sintering process⁶. As could be observed in Fig. 1 (a) and (b), only a small shrinkage (<1%) was observed at the sintering temperatures of 1100 and 1200 °C. The change in strain rate marked the start of solid state sintering, however, only the first stage of sintering was reached, considering the small shrinkage and the high porosity. This indicates that higher sintering temperatures are necessary for the binary Ti-45Al. Further changes in dimension observed were caused by cooling (thermal shrinkage), as indicated by the strain rate after the isotherm.

On the other hand, with sintering at 1300 °C, a pronounced change in strain rate was observed slightly before the isotherm at approximately 1240 °C. The strain rate peak (highest shrinkage) was observed at the beginning of the isotherm at 1300 °C, which decreased until the end of the sintering dwell time of 2 hours. The most intense shrinkage was achieved with sintering at 1300 °C, reaching almost 10% of linear dimension variation during the 2 hours of sintering. This indicates that the sintering process was more effective for densification at 1300 °C for the binary Ti-45Al.

Fig. 1 (d) presents the dilatometry curves and strain rate for the ternary alloys Ti-45Al-2Ni and Ti-45Al-2Co. Both curves also presented a swelling effect in the range of the melting point of Al due to the formation of Al-rich intermetallics. A sudden change in the strain rate was observed at higher temperatures, which took place at slightly different temperatures for Co and Ni additions. The discontinuous change in the specimen length (shrinkage) indicated the formation of a liquid phase that caused dimensional instability. Considering that the experiment is conducted horizontally, and a small load is applied to hold the specimens in place, the formation of the liquid phase could be clearly identified. Therefore, the sintering temperatures for the ternary Ti-45Al-2Ni and Ti-45Al-2Co should be above 1200 °C.

Sintering experiments were performed using the vacuum furnace, and the relative densities after sintering are shown in Fig. 2. For the binary Ti-45Al, independent of the sintering temperature, the relative density was limited to approximately 57%. The high porosity was the result of swelling due to intermetallic formation that could not be compensated by the diffusion mechanisms present during the sintering process. In the course of sintering, the powder particles are drawn to each other, which macroscopically is observed as shrinkage¹⁴. The absence of shrinkage could indicate hindrance in the sintering process. The intermetallics require a formation reaction during heating (reactive sintering) that causes the powder particles to come apart due to the volume difference between the elemental metallic powders and the intermetallic phases⁵. Therefore, when the sintering mechanisms are not able to counteract the swelling effect, high porosity results⁶. Consequently, additional densification mechanisms are required, such as liquid phase sintering.

Figure 2
Relative densities after sintering for 2 hours at different temperatures. The sintering experiments were conducted in a vacuum furnace.

According to Fig. 2, with Co or Ni additions, the relative densities achieved with sintering at 1100 °C were about 57%, similar to the binary Ti-45Al, because at this temperature only solid state sintering mechanisms were active. However, at 1300 °C, a significant improvement in densification was observed, which led to approximately 90% of the theoretical density. The dilatometry measurements indicated a liquid phase formation for Ti-45Al-2Ni and Ti-45Al-2Co at approximately 1188 °C and 1130 °C, respectively. Therefore, with sintering temperatures of 1100 °C and 1300 °C, it was possible to identify the effect of liquid phase sintering in the densification of the investigated alloys.

Moreover, according to the study of Mihalcea et al.¹⁵ that investigated liquid phase sintering of a composite Ti-Al-4V with CoCrMo, a eutectic reaction between Ti and Co takes place at 1130 °C, leading to the formation of Ti₂Co. In titanium aluminides, the ternary TiAl₂Co has also been previously reported to enhance densification⁹,¹⁶. A similar effect is assumed to take place with Ni addition. As reported by Gandova¹⁷, Ni and Ti should also form liquid phase starting from 986 °C with the reaction NiTi → L+Ni₂Ti, and at 1120 °C Ni₃Ti → L+NiTi, finally at 1310 °C NiTi → L. Therefore, the literature data support the fact that liquid phase formation is feasible in Ni or Co-containing TiAl, and it could help densification.

The formation of liquid phase during sintering is a well-known mechanism for densification and is widely applied to hard metals¹⁸. The mechanism consists of the formation of a transient or permanent liquid phase at the sintering temperature. The transient liquid phase is consumed during the sintering process, whereas the permanent liquid phase remains and is solidified during cooling from the sintering temperature¹⁹. The liquid phase is able to fill the empty spaces between the powder particles, and thus, an important requirement is the wettability of the liquid with the solid phases. With high wettability, the liquid is homogeneously spread and acts as a binding agent, drawing the powder particles together, therefore, improving the densification¹⁴.

The sintered microstructures of the different alloys are shown in Fig. 3. Ti-45Al, Fig. 3 (a), presented large and irregular pores due to the low densification. However, it was possible to identify sintering necks and lamellar colonies. Moreover, γ (TiAl) and α₂ (Ti₃Al) grains were also observed, according to Fig. 3 (b), as well as some remaining α-Ti cores that were not homogenized during sintering.

Figure 3
SEM images from Ti-45Al (a) and (b), Ti-45Al-2Ni (c) and (d), and Ti-45Al-2Co (e) and (f) sintered at 1300 °C for 2 hours. (a) Overview of Ti-45Al. Large and irregular pores could be identified. (b) Detail of Ti-45Al showing lamellar colonies, γ phase, and pores. (c) Overview of Ti-45Al-2Ni. (d) Detail of Ti-45Al-2Ni showing a lamellar colony and a Ni-rich region at the interface. (e) Overview of Ti-45Al-2Co. (f) Detail of Ti-45Al-2Co showing the lamellar colony surrounded by γ phase and Co-enriched areas.

The microstructure of the ternary Ti-45Al-2Ni and Ti-45Al-2Co, Fig. 3 (c) and Fig. 3 (d), contained lamellar colonies, pores, and γ grains, in addition to some regions with enrichments in Ni and Co. The α₂ grains were not distinguishable from the Ni- and Co-rich areas. According to the EDS measurements, the γ phase could dissolve small amounts of Ni and Co of about 1%. However, the Ni- and Co-rich areas contained approximately 15-20% Co or Ni, which indicates that they might correspond to ternary aluminides such as CoAl₂Ti and Al₃NiTi₂. The Ni- and Co-rich areas were found mainly at grain boundaries, suggesting that they were related to the mechanism of liquid phase sintering. Considering that a mixture of elemental powders was used to prepare the specimens, with the formation of the liquid phase, the ternary aluminides formed through the reaction with the liquid phase remained between the primary particles, resulting in enhanced densification.

The X-Ray diffraction patterns of the investigated alloys are shown in Fig. 4. The peaks corresponding to the equilibrium phases γ (TiAl) and α₂ (Ti₃Al) were identified in all cases, indicating that sintering at 1300 °C for 2 hours was able to promote the formation of the desired intermetallics. However, a small peak corresponding to α-Ti could also be identified. This peak was more intense in the case of Ti-45Al, which presented undiffused Ti cores in the microstructure, according to Fig. 3 (a). Peaks of the ternary aluminides could not be observed, probably due to the small phase fraction.

Figure 4
X-Ray diffraction patterns of the investigated alloys.

Fig. 5 presents the compression creep resistance of the investigated alloys. The binary Ti-45Al showed the least resistance to creep, reaching 1% of strain in about 2 hours. The ternary Ti-45Al-2Ni and Ti-45Al-2Co displayed a significant improvement in creep resistance. Ti-45A-2Co reached an almost 10-fold increase in comparison to the binary Ti-45Al. The creep measurements were focused on the primary creep regime, which is typically related to the initial plastic deformation of less than 2%. Primary creep is extremely important in the case of titanium aluminides since the applications usually do not tolerate considerable creep deformation. Therefore, developing powder metallurgy processing routes that show adequate primary creep resistance is essential. Ti-45Al was very porous, therefore, its performance in creep was poor. However, with the ternary additions of Ni and Co, the densification was improved, which naturally induced higher mechanical properties. Additionally, the solid solution strengthening of the γ phase by small contents of Ni and Co (about 1%) also contributed to the enhanced creep resistance.

Figure 5
Compression creep curves (a) and time to 0.5 and 1% of strain (b).

Soyama et al.¹¹,¹² investigated the compression creep resistance of TNB-V5 (Ti-45Al-5Nb-0.2B-0.2C) processed by powder metallurgy. TNB-V5 is a well-known titanium aluminide developed for high creep resistance²⁰. The results indicated approximately 10 hours to reach 1% of creep strain. The creep tests, however, were conducted at a higher load (350 MPa) at 800 °C, and the porosity was low, <3%. Singh et al.²¹ reported 20 hours to reach 1% of creep deformation using a cast Ti–45Al–8Nb–2Cr–0.2B tested in compression (800 °C/125 MPa). Liang et al.²² investigated the alloy Ti-43Al-6Nb-1Cr-1.5V produced by induction skull melting and measured about 40 hours to 1% of creep strain at 800 °C with 200 MPa loading. Therefore, the reported results are in the same order of magnitude as the ternary Ti-45Al-2Ni and Ti-45Al-2Co. This indicates the great potential for liquid phase sintering in titanium aluminides.

Conventional sintering of titanium aluminides using the blended elemental powder approach leads to high porosity and thus low mechanical properties. However, by choosing proper alloying elements that induce the formation of a transient liquid phase during sintering, the densification could be improved. The addition of 2% Ni or Co increased the densification to about 90% of the theoretical density, transforming a nearly unsinterable Ti-45Al to a reasonably densified Ti-45Al-2Ni and Ti-45Al-2Co. Additionally, the primary creep resistance of the sintered alloys was considerably improved, indicating the great potential for powder-metallurgy-processed titanium aluminides.

4. Conclusions

Processing titanium aluminides by powder metallurgy is not trivial and sintering is the most critical step. The sintering behavior of Ti-45Al prepared with the blended elemental powder approach, as well as ternary additions of 2 at.% Ni or Co were investigated. The following conclusions could be drawn:

• Binary Ti-45Al achieved a maximum densification of about 60% of the theoretical density with sintering at 1300 °C for 2 hours.

• Ternary additions of 2 at.% Co or Ni forming the alloys Ti-45Al-2Ni and Ti-45Al-2Co significantly improved the densification, reaching about 90% of the theoretical density, which was the result of a transient liquid phase during sintering.

• The equilibrium intermetallics γ (TiAl) and α₂ (Ti₃Al) and undiffused α-Ti were observed in the sintered alloys. Additionally, Ni- and Co-rich areas were also identified.

• The compression creep resistance measured at 800 °C and 120 MPa loading indicated a beneficial effect of ternary additions due to the reduced porosity and solid solution strengthening. Ti-45Al-2Co was the most creep resistant composition, achieving 1% of deformation in about 18 hours.

5. Acknowledgments

The authors would like to thank the financial support from FAPESP – São Paulo Research Foundation, grant numbers 20/04177-6, 18/18293-8, and 18/04564-0. CAPES – Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, finance code 001. FAEPEX – Fundo de apoio ao ensino, pesquisa e extensão da Unicamp, grant number 3348/23. We are also grateful for the help with vacuum sintering, dilatometry, and compression creep measurements conducted at Helmholtz-Zentrum Hereon, Dr. Thomas Ebel, Prof. Dr. Florian Pyczak, Dr. Jonathan Paul, and Wolfgang Limberg.

• Data Availability
  Data will be made available upon request from the corresponding author.

6. References

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Edited by

• Associate Editor:
  Igor Vasconcelos.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

Data will be made available upon request from the corresponding author.

Publication Dates

• Publication in this collection
  18 July 2025
• Date of issue
  2025

History

• Received
  10 Jan 2025
• Reviewed
  19 May 2025
• Accepted
  08 June 2025