Sintering and creep properties of MIM titanium aluminides (Ti(43-47)Al5Nb0.2B-0.2C)

February 14, 2022

Titanium aluminides show outstanding high temperature mechanical properties with low density, making them attractive light materials for aerospace and automotive applications. However, conventional processing methods that involve solidification (e.g., casting), can result in segregation which can be eliminated only through thermomechanical treatments. Powder metallurgical processes, including Metal Injection Moulding, can offer both near-net shape fabrication to substantially reduce manufacturing costs, as well as providing a fine and isotropic microstructure in small to medium complex shaped parts.

Some research has already indicated that MIM produced titanium aluminides have good mechanical properties at room temperature achieving a tensile strength of >600 MPa. There is, however, only limited information available on the creep properties of these sintered materials, and how the different microstructural features of sintered TiAl (such as colony size, lamellar spacing, presence of precipitates, porosity, etc.) influence creep properties. Understanding these unique microstructural characteristics is important to allow wider application of MIM titanium aluminides.

Recent collaborative research undertaken at Universidade Estadual de Campinas, Brazil, and Helmholtz-Zentrum Hereon, Germany, has been investigating the effect of Al variations on the sintering behaviour and creep resistance in MIM processed titanium aluminides based on the alloy TNB-V5 (Ti45Al5N0.2B-0.2 C at%). The results of this work have recently been published in a paper by J Soyama, W Limberg, T Ebel, and F Pyczak in the journal Advanced Engineering Materials (Vol.22, 2020, 2000377, 7pp).

The authors reported that an argon gas atomised prealloyed Ti45Al5Nb0.2B-0.2C (TNB-V5) powder (<45 µm) was used as the starting material, and to achieve the aluminium variations high-purity elemental titanium and aluminium powders with particle size also of <45 μm were added to the prealloyed TNB-V5 powder. The titanium addition was used to decrease the aluminium content to 43 at% – designated 43Al, whereas addition of elemental aluminium increased it to 47 at% and was designated 47Al. In addition, high-purity niobium and boron powders were added in small amounts to keep the relative composition constant at the TNB-V5 value.

The binder system used to prepare MIM feedstocks from these powder mixtures was composed of paraffin waxes, polyethylene-vinyl-acetate, and stearic acid. The paraffin waxes and stearic acid were removed chemically in a bath of hexane for 15 h at 45°C. The remaining backbone polymer was removed thermally inside the sintering furnace. The feedstock was injection moulded to produce tensile test bars from which were cut cylindrical specimens of about 8 mm in length and 4.1 mm diameter.

The sintering temperatures investigated were 1450°C and 1500°C with a dwell time of 2 h in a high vacuum of ≈10-4 mbar. After sintering, the cooling rates were in the order of 10°C min-1 from the sintering temperature. The authors stated that the process of sintering is very sensitive and based on the dilatometry results, it was possible to consider the two sintering temperatures of 1450°C and 1500°C. In the case of 43Al and 47Al, a lower temperature could suffice for proper sintering as densification took place below 1500°C. However, the higher sintering temperature of 1500°C could improve densification due to the presence of liquid phase, as in the case of TNB-V5 (45Al).

Fig.1 Microstructures of sintered specimens at different temperatures. a) 43Al sintered at 1450°C. b) 43Al sintered at 1500°C. c) 45Al sintered at 1450°C. d) 45Al sintered at 1500°C. e) 47Al sintered at 1450°C. f ) 47Al sintered at 1500°C. (From paper: ‘Sintering and creep resistance of Powder Metallurgy processed Ti-(43-47)Al-5Nb-0.2B-0.2C’ by J Soyama, et al., Journal of Advanced Engineering Materials (Vol. 22, 2020, 2000377)

Fig. 1 shows SEM images of specimens with the three aluminium variations sintered at 1450 and 1500°C for 2 h. In all cases, a fully lamellar microstructure was achieved with different colony sizes. From the SEM images it was possible to identify pores, represented by the dark (mostly circular features) elongated particles that correspond to borides and areas of increased Nb content, as shown in Fig.1b. It was also established that colony size and porosity were influenced by the sintering temperatures. Sintering at 1450°C led to higher porosity of ≈10% in the case of the most porous 47Al, whereas sintering at 1500°C could significantly decrease the porosity to values <1% for the alloys with 43Al and 45Al, whereas for 47Al, a value of about 7% was achieved.

Fig. 2 Compression creep curves of different aluminium variations measured at 800°C and 350 MPa loading: (a) Creep strain (b) Creep rate. (From paper: ‘Sintering and creep resistance of Powder Metallurgy processed Ti-(43-47)Al-5Nb-0.2B-0.2C’ by J Soyama, et al., Journal of Advanced Engineering Materials (Vol. 22, 2020, 2000377)

The researchers also investigated the mechanical properties of the MIM TiAl alloys. They established that, at room temperature, high tensile strength of around 500–600 MPa was achieved for Ti45Al5Nb0.2B-0.2C and Ti43Al5Nb0.2B-0.2C, whereas Ti47Al5Nb0.2B-0.2C resulted in reduced tensile strength due to higher porosity (Table 1). The most creep-resistant alloy was Ti45Al5Nb0.2B-0.2C, which could be sintered to very low porosity and showed serrated colony boundaries. For the same sintering parameters, Ti47Al5Nb0.2B-0.2C showed higher creep resistance in comparison to Ti43Al5Nb0.2B-0.2C (Fig. 2), which was attributed to the larger colony size achieved after sintering induced by the higher Al content. This, they state, indicates that the effect of porosity was secondary to colony size in compressive creep at 800°C and 350 MPa loading.

Table 1 Room temperature tensile properties of specimens sintered at 1500°C for 2 h. (From paper: ‘Sintering and creep resistance of Powder Metallurgy processed Ti-(43-47)Al-5Nb-0.2B-0.2C’ by J Soyama, et al., Journal of Advanced Engineering Materials (Vol. 22, 2020, 2000377)

www.onlinelibrary.wiley.com/journal/15272648

In the latest issue of PIM International…

Download PDF

Extensive MIM, CIM industry and sinter-based AM industry news, plus the following exclusive deep-dive articles and reports:

  • Calling all product designers: Discover what Metal Injection Moulding could do for you through these award-winning parts
  • Binder Jetting of a dual-phase steel: Process insight and optimisation using the Master Sintering Curve
  • The rise of filament-based metal AM: New materials and machines present opportunities for MIM producers
  • Sinter-based Additive Manufacturing at the 20th Plansee Seminar on Refractory Metals and Hard Materials
  • Ceramitec 2022: Opportunities abound for producers of technical ceramics by CIM and AM

The latest news from the MIM, CIM and sinter-based AM industries

Don't miss any new issue of PIM International, and stay up to date with the latest industry news. Sign up to our fortnightly newsletter.

Sign up

News from the industry…

    Discover our magazine archive…

    The free-to-access PIM International magazine archive offers unparalleled insight into the world of MIM, CIM and sinter-based AM from a commercial and technological perspective through:

    • Reports on visits to leading part manufacturers and industry suppliers
    • Articles on technology and application trends
    • Information on materials developments
    • Reviews of key technical presentations from the international conference circuit
    • International industry news

    All past issues are available to download as free PDFs or view in your browser.

     

    Browse the archive

     

    Looking for suppliers of materials, production equipment and finished MIM, CIM or sinter-based AM parts?

    Discover suppliers of these and more in our advertisers’ index and buyer’s guide, available in the back of PIM International.

    • Metal powders
    • MIM, CIM & AM parts producers
    • Binders & feedstocks
    • Feedstock mixers
    • Furnaces & furnace supplies
    • Atmospheres & gas generation
    • HIP systems & services
    • Injection moulding machines
    • AM technology
    • Debinding systems
    Download PDF
    Share via
    Copy link
    Powered by Social Snap