Bronislava Gorr

Microstructure Evolution in a New Refractory High-Entropy Alloy W-Mo-Cr-Ti-Al

The microstructure of a body-centered cubic 20W-20Mo-20Cr-20Ti-20Al alloy in the as-cast condition as well as its microstructural evolution during heat treatment was investigated. Different characterization techniques, such as focused ion beam-scanning electron microscope, X-ray diffraction, and transmission electron microscope, were applied. Experimental observations were supported by thermodynamic calculations. The alloy exhibits a pronounced dendritic microstructure in the as-cast condition with the respective dendritic and interdendritic regions showing significant fluctuations of the element concentrations. Using thermodynamic calculations, it was possible to rationalize the measured element distribution in the dendritic and the interdendritic regions. Observations of the microstructure evolution reveal that during heat treatment, substantial homogenization takes place leading to the formation of a single-phase microstructure. Driving forces for the microstructural evolution were discussed from a thermodynamic point of view.

High temperature oxidation of mechanically alloyed Mo–Si–B alloys

Abstract Nowadays, even fourth generation nickel base superalloys are approaching their fundamental limitation, the melting point. Hence, a further increase in efficiency, i.e. of jet engines, can only be realised by developing new materials for the use at temperatures beyond 1200°C. A new alloy concept using the Mo–Si–B system for ultrahigh temperature applications is discussed. Those alloys have melting points ∼2000°C, while retaining good mechanical properties and oxidation resistance in the desired temperature range. A three phase Mo–9Si–8B alloy (composition in at.-%) consisting of α-Mo, Mo3Si and Mo5SiB2 (T2) was produced by powder metallurgical processing route. At temperatures higher than 1000°C in laboratory air, a protective SiO2/B2O3 glass layer develops on the alloy surface giving excellent oxidation resistance. However, in the temperature range between 700 and 900°C, non-protective and highly volatile molybdenum oxide cause the disintegration of the material (the so called pesting phenomenon). Additions of Zr and La2O3 to the Mo–Si–B alloy systems were investigated to improve the performance of the alloys in the pesting temperature range. The oxidation kinetics was determined by means of thermogravimetric analysis and discontinuous oxidation experiments. Microstructural examinations were performed by means of optical and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. The microstructural observations were compared with the theoretical prediction of phase stability using computational thermodynamic calculations. A significant improvement of the alloys during oxidation in the pesting temperature range was found. The rate of formation of molybdenum oxides could be drastically reduced at intermediate temperature range. At high temperatures (>1000°C), a homogeneous and protective SiO2 oxide layer was formed on the alloy surface leading to a slow growing oxide scale.

Oxidation behaviour of experimental Co–Re-base alloys in laboratory air at 1000°C

Abstract The oxidation behaviour of experimental Co – Re-based alloy at 1000 °C was studied. A set of binary, ternary and quaternary alloys from the Co – Re – Cr – C system was used as model alloys to understand the role each alloying element plays on oxidation. The morphology and composition of the oxide scale that formed was analysed by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. It was found that the present Co – Re alloys with 23 at.% and 30 at.% Cr additions behaved very similarly to Co – Cr binary alloys with equivalent Cr content. The oxide scale was multilayered, consisting of a dense CoO outer layer, a porous mixed oxide layer containing Co-oxide and Co – Cr spinel, and a discontinuous and non-protective Cr3O2 layer. The binary Co – Re alloy behaved differently in oxidation, and it formed only a monolithic CoO scale. However, Re in combination with Cr promotes Cr – Re-rich phase formation, which oxidises preferentially compared to the Co matrix. Carbon ties up part of the Cr to form Cr23C6 type carbides. However, these carbides are not stable at 1000 °C and dissolved with time, therefore C had only a minor role in the oxidation behaviour. In general, increasing Cr content in the alloy improved oxidation resistance.

Effect of Si addition on the oxidation resistance of Co–Re–Cr-alloys: Recent attainments in the development of novel alloys

Abstract The influence of silicon on the oxidation behaviour of Co—Re—Cr-alloys has been studied at 1 000°C and 1 100°C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr2O3 layer. The Si addition to the Co—Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr2O3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr2O3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate.

Effect of in-situ gas changes on thermally grown chromia scales formed on Ni–25Cr alloy at 1000°C in atmospheres with and without water vapour

Abstract Regarding oxidation resistance, most high temperature alloys rely on the formation of a protective chromia surface scale during service. In the present study, the oxidation behaviour of a Ni–25%Cr model alloy was investigated during single- and two-stage oxidation in Ar–O2 and Ar–H2O gas mixtures at 1000°C. In the two-stage experiments, the test gas was changed after a predefined oxidation time from dry to wet gas, and vice versa, without intermediate specimen cooling. Oxidation kinetics were measured using thermogravimetry and post exposure characterisation was accomplished using scanning and transmission electron microscopy, focused ion beam and energy dispersive X-ray spectroscopy techniques. The single stage exposure to Ar–O2 resulted in the formation of a voided, poorly adhering chromia scale exhibiting a coarse, equi-axed morphology. In Ar–7%H2O a fine, columnar grained oxide scale was formed which was free of microvoidage within the scale and at the scale/metal interface and therefore exhibited excellent adherence to the metallic substrate. During two-stage exposure to Ar–O2 and subsequently to Ar–H2O, a fine grained, columnar shaped oxide developed on top of the coarse oxide scale formed in the first oxidation stage. The opposite effect occurred when the exposure started in the wet environment. The mechanisms which are responsible for the observed oxidation features are discussed on the basis of classical oxidation theory in combination with previous results on oxidation of chromia forming alloys in oxygen rich gases and water vapour containing, low p(O2) environments.

Contribution of Lattice Distortion to Solid Solution Strengthening in a Series of Refractory High Entropy Alloys

We present an experimental approach for revealing the impact of lattice distortion on solid solution strengthening in a series of body-centered-cubic (bcc) Al-containing, refractory high entropy alloys (HEAs) from the Nb-Mo-Cr-Ti-Al system. By systematically varying the Nb and Cr content, a wide range of atomic size difference as a common measure for the lattice distortion was obtained. Single-phase, bcc solid solutions were achieved by arc melting and homogenization as well as verified by means of scanning electron microscopy and X-ray diffraction. The atomic radii of the alloying elements for determination of atomic size difference were recalculated on the basis of the mean atomic radii in and the chemical compositions of the solid solutions. Microhardness (μH) at room temperature correlates well with the deduced atomic size difference. Nevertheless, the mechanisms of microscopic slip lead to pronounced temperature dependence of mechanical strength. In order to account for this particular feature, we present a combined approach, using μH, nanoindentation, and compression tests. The athermal proportion to the yield stress of the investigated equimolar alloys is revealed. These parameters support the universality of this aforementioned correlation. Hence, the pertinence of lattice distortion for solid solution strengthening in bcc HEAs is proven.

Effect of specimen thickness on chromia scaling of Ni25Cr in N2–O2–H2O test gases at 1000°C

Abstract The isothermal oxidation behaviour of a Ni25Cr model alloy was studied using specimens of different thicknesses at 1000°C in dry and wet N2–1%O2. The oxidation mechanisms were evaluated using thermogravimetry and SEM/electron backscatter diffraction analyses of oxide scale cross-sections. The oxidation rates decreased with increasing specimen thickness and increasing water vapour additions in the gas. The findings can be explained by considering the effect of H-defects and in-scale stress state on point defect concentrations in the chromia lattice.

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO2, intermediate Al2O3, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.

High-Temperature Oxidation Behaviour of Co-Re-Cr-based Alloys: Limitations and Ways to Improve

Ni-based alloys are the most widely used alloy system in high-temperature applications. However, the use of Ni-based alloys is limited to temperatures below 1100°C. The experimental Co-Re-Cr-based alloys are promising for high-temperature applications for service temperatures beyond 1200°C. A complete miscibility in the Co-Re system allows to steadily elevate the melting point of the system with the rhenium content. In addition, rhenium takes the role as solid solution strengthening element. In the case of Co-based alloys, the oxidation resistance at high temperature is mainly based on the formation of a protective Cr2O3 scale. The purpose of the present investigations is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. Earlier investigations of the authors showed a rather poor oxidation resistance during exposure to laboratory air. Oxidation at 1000°C in air yielded an oxide scale that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. As a consequence of the lacking protectiveness of the oxide layer the vaporization of rhenium oxide takes place and hence leads to a rapid loss of Re. The aim of recent investigations is to study the effect of Si on the high-temperature oxidation behaviour of Co-Re-Cr alloys by means of kinetic and microstructural examinations. It was found that Si stabilizes the Cr2O3 scale, enhancing the oxidation resistance significantly. Hence, the synergetic effect of chromium with silicon could be considered as an encouraging perspective to improve the oxidation resistance of Co-Re-Cr alloys. Apart from that, other concepts to enhance the oxidation resistance of this class of materials are discussed, such as the formation of a borosilicate layer or protective Al2O3 scale on the substrate surface.

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co3O4, Co(Ni)O and Ni-doped CoCr2O4, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr2O3 layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.