Effects of Ni Additions on the High Temperature Expansion, Melting and Oxidation Behaviors of Cobalt-Based Superalloys

Nickel is often added to cobalt-based superalloys to stabilize their austenitic structure. In this work the effects of Ni on several high temperature properties of a chromium-rich cobalt-based alloy reinforced by high fraction of TaC carbides are investigated. Different thermal analysis techniques are used: differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA) and thermogravimetry (TG). Results show that the progressive addition of nickel did not induce great modifications of microstructure, refractoriness or thermal expansion. However, minor beneficial effects were noted, including reduction of the melting temperature range and slight decrease in thermal expansion coefficient. The most important improvement induced by Ni addition concerns the hot oxidation behavior. In this way, introducing several tens wt % Ni in this type of cobalt-based alloy may be recommended.

The high-temperature expansion of the thermal sunset

We give a prescription for calculating the high-temperature expansion of the thermal sunset integral to arbitrary order. We derive all terms odd in \bm{T}𝐓, and rederive previous results up to \bm{\mathcal{O}(T^0)}𝒪(𝐓0) for both bosonic and fermionic thermal sunsets in dimensional regularisation. We perform analytical and numerical cross-checks. Intermediate steps involve integrals over three Bessel functions.

Structural state and thermodynamic stability of Al–Cu alloys

It is known that processes occurring in binary system melts affect the crystallization process and the phase composition of alloys. To predict these processes, we should determine the region of thermodynamic stability of the melt. In this paper, the structural properties of hypoeutectic and hypereutectic alloys in Al–Cu system are studied depending on the heating temperature above the liquidus line and aftercooling rate. It is shown that overheating of Al–Cu melts to 150 K above the liquidus line and further cooling leads to complete suppression of the process of formation of primary aluminum crystals in hypoeutectic alloys and [Formula: see text] phase in hypereutectic alloys. For the first time, by accounting in Gibbs energy of binary Al–Cu alloy for the first degree approximation of high-temperature expansion of thermodynamic potential, the dependence of temperature of line of the melt thermodynamic stability on copper content in alloy is obtained.

Determination of thermodynamic stability of FeB monoboride

In this paper we investigate the phase composition and phase transformations in the Fe-B system alloys with boron content in the range of 9.0–15.0 wt.%. We use microstructural, X-ray diffraction, differential thermal and durometric analyzes to determine the physical properties of the alloys. The experimental findings show that in the as-cast alloy structure there is Fe5B3 phase in small quantities along with FeB monoboride and Fe2B boride. The Fe5B3 phase is formed as a result of the peritectic reaction L+FeB→Fe5B3 at the temperature of 1680 K. The eutectic transformation L→Fe5B3 +Fe2B occurs in the boron concentration range of 8.8–10.5 wt.%. After annealing of the Fe-B alloys at the temperature of 1473 K and cooling with the rate of 102 K/s we observe the occurring of the Fe5B3 phase. To spot an opportunity of the secondary monoboride formation in the alloys under consideration, we calculate the thermodynamic characteristics of stability of the system. Accounting for the contribution of the first degree approximation of high-temperature expansion of thermodynamic potential of FeB iron monoboride in a Fe-B binary alloy enables us to study its thermodynamic stability. It is shown that stability decrease of FeB at 1423 K allows suggesting that at this temperature the phase transformation occurs and this fact correlates to the differential thermal analysis results.

Structural State and Thermodynamic Stability of Fe-B-C Alloys

The studies were performed for the specimens of Fe-B-C alloys with boron content of 0.005–7.0 wt.% and carbon content of 0.4–5.5 wt.%, the rest was iron. As a result of the experiment carried out in this work, the phase composition and phase transformations occurring in the alloys are investigated and the liquidus surface is constructed; it is shown that the point with minimum temperature of 1375 K at the liquidus surface is observed at boron content of 2.9 wt.% and carbon content of 1.3 wt. %. For the first time, considering the contribution of the first degree approximation of high-temperature expansion of thermodynamic potential into the Gibbs energy of Fe-B-C melt, we obtain the surface of thermodynamic stability of Fe-B-C melt, depending on temperature and content of boron and carbon in the alloy. The findings show that in order to obtain the homogeneous Fe-B-C melt, which does not contain micro-inhomogeneous structures in the form of short-range microregions, it is necessary to perform overheating more than to 150 K.

The peculiarities of formation and thermodynamic functions of τ-phase

It is shown that for alloys with boron content of 0.1–6.5% (wt.) and carbon content of 0.3–4.0% (wt.) without pretreatment no formation of cubic boron carbide takes place under crystallization. The cubic boron carbide can be obtained by pre-annealing at a temperature of 1173 K for an hour and further heating to a temperature of 30 K above the liquidus and cooling of alloys with boron content of 2.5–4.0% (wt.) and carbon content of 0.8–3.0% (wt.). Formation of crystals of cubic boron carbide is possible as a constituent of multiphase inclusions for alloys with boron content of 0.1–0.3% (wt.) and carbon content up to 0.4–0.5% (wt). It should be noted that for alloys with boron content of 4.2–6.0% (wt.) and carbon content of more than 3.0% (wt.) the pretreatment does not result in formation of cubic boron carbide. The increase in boron content in the alloy to 0.3–0.5% (wt.) and carbon content to 0.5–0.7% (wt.) leads to formation of the eutectic α-Fe+Fe23(CB)6, which is arranged on the boundaries of pearlite grains. The thermodynamic functions of Fe23(CB)6 cubic boron carbide are derived for the first time using the Hillert and Staffonsson model and accounting for the first degree approximation of high-temperature expansion of the thermodynamic potential for binary alloys. We obtain temperature dependences of such thermodynamic functions for Fе23(CB)6 phase as Gibbs energy, entropy, enthalpy and heat capacity CP , as well as calculate their values at the formation temperature of the phase. The approach used in this paper enables to give the most complete from the thermodynamic point of view description of cubic boron carbide formed from a liquid.