Class Of High-temperature Materials Research Articles

Ti-Fe Phase Evolution and Equilibria Toward β-Ti Superalloys

Recent design and development of precipitate reinforced refractory metal alloys demonstrate the possibility of A2 + B2 bcc superalloys as a new class of high temperature materials. Existing β-Ti alloys do not typically employ reinforcement with intermetallics, as in other high temperature alloys; to this effect sufficient additions of Fe, a low cost β-Ti stabiliser, can promote formation of an ordered-bcc intermetallic phase, β′-TiFe (B2), offering scope to develop a β + β′ dual-phase field. However, key uncertainties exist in the base Ti-Fe binary. The current research evaluates the formation of ordered-bcc TiFe precipitates within a disordered-bcc β-Ti matrix through variable heat treatment strategies. The microstructure optimisation has revealed new insight into the Ti-Fe phase equilibria at near eutectoid temperatures in the purported dual-phase field, where a complex interplay between β-Ti, β′-TiFe and α-Ti exists.

Chromium-based bcc-superalloys strengthened by iron supplements

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials.

Tungsten-based bcc-superalloys

Applications from nuclear energy to rockets and jet engines are underpinned by advanced high temperature materials. Whilst state of the art, the performance of current nickel-based superalloys is fundamentally limited to Ni’s melting point, Tm=1455∘C. Here, we develop an analogous superalloy concept but with superior high temperature capability by transitioning to a bcc tungsten base, Tm=3422∘C. This strategy involves reinforcing bcc β-W by β′ TiFe intermetallic compound, which results in impressive high temperature compressive strengths of 500 MPa at 1000∘C. This bcc-superalloy design approach has wider applicability to other bcc alloy bases, including Mo, Ta, and Nb, as well as to refractory-metal high entropy alloys (RHEAs). By investigation of the underlying phase equilibria, thermodynamic modelling, characterisation and mechanical properties, we demonstrate the capability of ternary W-Ti-Fe tungsten-based bcc-superalloys as a new class of high temperature materials.

Transport mechanisms during the high-temperature oxidation of ternary \u03b3/\u03b3\u2032 Co-base model alloys

Over a decade ago, γ′-strengthened Co-base alloys were introduced as potential replacement for conventional Ni-base Superalloys. Insufficient resistance against high-temperature oxidation restricts the number of possible applications. The present study contributes to the understanding of elementary mechanisms such as material transport during extensive oxide scale formation on γ/γ′ Co-base alloys to explain their inferior oxidation behaviour. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections. By means of electron backscattered diffraction (EBSD), the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16O2- and 18O2-containing atmospheres proved counter-current material transport. The combination of focused ion beam (FIB) and secondary ion mass spectroscopy (SIMS) visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale. An elaborate review of available transport paths for oxygen is given during the discussion of results. All experimental findings were combined to a coherent explanation of the inferior oxidation resistance of this relatively new class of high-temperature materials at temperatures above 800 °C.

Hot deformation behavior and DRX mechanism in a γ-γ/ cobalt-based superalloy

γ-γ/cobalt-based superalloys have been the main focus of attention as a new class of high temperature materials. The present research studies the hot deformation, microstructure evolution, and dynamic recrystallization (DRX) mechanisms of a cast Co–Al–W superalloy during the hot compression process. The microstructure of the starting material is constituted of large MC carbide particles within a coarse grain structure. Two hot deformation regimes, i.e., low temperature (950°C–1050 °C) and high temperature (1100°C–1200 °C) were designed and performed. Flow curve and constitutive analysis using a hyperbolic sine equation evidenced two distinct behaviors among the regimes. Microstructural evaluation revealed a large volume fraction of fine γ/particles with radius less than 100 nm, precipitated during heating and isothermal holding, which precluded DRX in the low temperature regime. In the high temperature regime, new grains were formed through a special discontinuous DRX at three different regions in the microstructure: original grain boundaries, vicinity of MC carbides and the interior of the relatively coarse grains; these may be attributed to the mechanisms of SIBM, PSN and sub-grain growth, respectively. A conceptual descriptive model for the especial DRX phenomenon was proposed.

Phase equilibrium and mechanical properties of Cr-Mo-Nb-Si-B alloys Composed of BCC and T2-silicide phase

A new class of high-temperature materials based on refractory elements was investigated with an aim to improve the energy efficiency of thermal power plants. Alloys based on Nb and Mo composed of BCC solid solution (BCCss) (Nb-Mo) and T2-silicide (Nb,Mo)5(Si,B)3 are promising candidates as high-temperature materials. Further investigation on the alloy phase equilibria of this system is required to improve the mechanical properties and oxidation resistance through optimization of the phase compositions. Cr is one candidate to modify the properties of the alloy because Cr is expected to stabilize the T2 compound phase along with B. Here, the phase equilibria among BCCss and the T2 compound are widely investigated in the Cr-Mo-Nb-Si-B system, and a BCCss-T2 two-phase microstructure is found in Mo-rich alloys. The B/Si ratio in the T2 phase increases with the Cr content, while almost no B solubility was found in BCCss. As the Si content increases in alloys, the A15 silicide phase ((Cr, Mo, Nb)3Si) and/or Laves phase appear. Nanoindentation tests were conducted to investigate the mechanical properties of the BCCss phase of the alloys in the Cr-Mo-Nb-Si-B system. The nanohardness and reduced elastic modulus of these alloys tended to be higher with an increase in Cr.

γ′ Precipitation Study of a Co-Ni-Based Alloy

A Co-Ni-based alloy strengthened by γ′-(L12) precipitates was utilized to investigate the precipitation evolution after various cooling rates and several aging conditions. In this study, the precipitate size and volume fraction have been studied via scanning electron microscopy and transmission electron microscopy. The influence of the precipitation evolution was measured via microhardness tests. The cooling rate study shows a more sluggish γ′ precipitation reaction compared to that observed in a Ni-based superalloy. Following a rapid cooling rate, the application of appropriate double aging treatments allows for the increase of the γ′ volume fraction as well as the control of the size and distribution of the precipitates. The highest hardness values reach those measured on supersolvus cast and wrought Ni-based superalloys. The observed γ′ precipitation behavior should have implications for the production, the heat treatment, the welding, or the additive manufacturing of this new class of high-temperature materials.

Co‐Re‐based Alloys: A New Class of High Temperature Materials?

The potential of Co-Re-based alloys as a new high temperature material class for service beyond 1200°C is explored. Complete miscibility of Co and Re and high melting point of Re provides good opportunity to design high temperature alloys. The rational for focusing on Co-Re-based alloys for gas turbines application beyond the temperature capabilities of Ni- superalloys is presented. Four alloys are evaluated to study microstructure evolution and explore various hardening concepts.

Stochastic analysis of the fracture of solids with microcracks

The present study is concerned with the stochastic formulation of the failure of solids with microcracks. The analysis is directed towards the development of microcracks in a given material structure due to the application of an external constant load. The formulation is based on the theory of stochastic population processes and that of point processes. The consideration of certain aspects of the “extreme value” theory is however necessary in characterizing the evolution of the microcrack population. It is shown to be convenient to first consider a collection of microcracks in a specific material domain (microdomain) which is regarded as a “family of objects” and represented by its state so that: {z1, z2 ..., z n }=z ⊂ Z ⊂\(\mathfrak{X}\), where Z is the state space and\(\mathfrak{X}\) a general probabilistic function space. Governing equations for the development of microcracks are derived and the results of the analysis are then compared with experimental observations obtained from short-time creep tests on a class of high temperature materials.

Platinum-Enriched Superalloys

The initial phase in the development of a new class of high temperature materials based upon the incorporation of platinum group metals as alloy constituents in nickel based superalloys has been reported here previously. It was shown that the new alloys provide enhanced resistance to environmental corrosion when compared with their conventional base metal counterparts, without detriment to high temperature strength. This paper describes the results of more detailed studies of the constitutional and structural aspects of these complex alloys which led to the design of a developmental alloy, RJM2012, specifically tailored for operation under the aggressive environmental conditions found in the industrial and marine gas turbine.