Volume Fractions of Tantalum Carbides Deduced from the Ta Contents in the Matrix of Three 1250°C–Aged Cobalt–Based Alloys; Comparison with Thermodynamic Calculations

Some superalloys for service at high temperature under stresses are strengthened by tantalum carbides. Their creep resistance depends on the quantity of TaC and this is the reason why it is often important to control the volume fraction of these carbides in the microstructure. Metallographic preparation followed by electron imaging and surface fraction measurements by image analysis is a frequent way for that. Another possibility is to deduce the mass fraction of TaC, and after their volume fraction, from the chemical composition of the matrix when the alloys are only double–phased, on the {matrix + TaC} type. In this work three alloys – chemically designed to be made exclusively of matrix and TaC – were elaborated and isothermally exposed to an elevated temperature for a duration long enough to allow the alloys being at their thermodynamic equilibria. The chemical compositions of the alloy and of its matrix were measured and the results allowed evaluating their TaC mass fractions which were converted in volume fractions. The obtained TaC fractions were compared to results issued from thermodynamic calculations. Good agreement was found for the three alloys, and this allowed to exploit the used software and thermodynamic database to explore further the microstructures at the same high temperature, notably to know the conditions on the Co, Ni, Cr, Ta and C contents to keep the {matrix + TaC} structure and to avoid any possible partial melting.

Thermal Expansion and Microstructure Behavior at Elevated Temperature of various {Ni, Co}-based Cast Superalloys

This paper aims to investigate the thermal expansion behavior, up to an elevated temperature, of superalloys based on nickel and cobalt with various proportions and designed to be strengthened by tantalum carbides. The as-cast microstructures of these superalloys and their evolutions at two very high temperatures were also of interest. All results are discussed by considering the Ni/Co repartition in the base element position. It appears that when the Ni content is higher than the Co one: 1/the thermal expansion is slower, 2/the as-cast microstructures as well as the ones stabilized at high temperature contain not only TaC but also chromium carbides, and 3/the hardness in as-cast or aged state is lower.

EFFECT OF SUBSTRATE TEMPERATURE ON TANTALUM CARBIDES INTERLAYERS SYNTHESIZED ONTO WC-Co SUBSTRATES FOR ADHERENT DIAMOND DEPOSITION

Tantalum carbides (TaXC) interlayers have been synthesized by double glow plasma surface alloying (DG-PSA) method at different temperature for subsequent deposition of diamond coatings. The evolution of the microstructures, phase composition and adhesion of the interlayers dependent on substrate temperature has been discussed. The results show that the layers are composed of TaXC (i.e. Ta2C, TaC) with nanocrystalline microstructure and small amounts of CoTa2. The layer produced at 700∘C is formed of specific flower-shaped rings embedded in smooth structures. As the temperature increases to 800∘C, interacted rings are covered the full surface, and the surface roughness is increased. As the temperature increases further, the rings are replaced by irregular-shaped pits, caused a decreasing surface roughness. Besides the special microstructure with interactional rings and relatively high roughness, the layer prepared at 800∘C possesses higher adhesion, better wear performance and higher hardness than those of other layers. The coating obtained on the interlayer pretreated at 800∘C exhibits the best adhesion. Thus, the TaXC interlayers synthesized at 800∘C are demonstrated as a suitable option for adherent diamond coatings deposited onto WC-Co substrates.