Improving the oxidation resistance of refractory metals via aluminum diffusion coatings and halogen effect

Recently, refractory metals and their alloys have received increasing attention for the purpose of substituting Ni-based single crystal superalloys. Compared to commonly used materials, refractory metals have significantly higher melting points while their mechanical properties seem adequate at high temperatures. The main challenge for their application is, that refractory metals show low oxidation resistances at elevated temperatures. Aluminum diffusion layers are promising coatings to suppress harmful oxidation by forming a protective oxide layer. This study deals with the application of such aluminum reservoir layers and with the investigation of their protective properties. Four different unalloyed refractory metals, molybdenum, tantalum, tungsten and niobium, were used as substrate materials. The coatings were manufactured via a pack cementation process carried out for 8 h at 1000°C. Homogeneous intermetallic layers with thicknesses up to 49 µm and high aluminum contents were characterized using optical microscope, EPMA, and XRD analysis. The oxidation resistance of the samples was investigated using thermogravimetric analysis. The experiments were carried out at 1300°C for up to 100 h in synthetic air. Via mass change curves the oxidation kinetics were analyzed as well as the formed oxide layers using, again, optical microscopy, EPMA, and XRD analysis. It was found that an additional application of a halogen treatment can significantly reduce the oxidative attack of the substrate and support the formation of a continuous protective Al2O3 layer. Furthermore, the effect of varying the amount of halogen on oxide layer formation is shown. The Al2O3 growth mechanism and aluminum depletion of the underlying reservoir layer in the different refractory metals were investigated by comparing uncoated, coated, and additional halogen-treated samples

Coatings for Superalloys

High-temperature coatings for superalloys can be divided into three categories: Two of them, diffusion and overlay coatings, are both used to protect a system from oxidation and corrosion. The third type, thermal barrier coatings, protects the substrate from thermal degradation

Simulated Role-Play to Improve Attitudes and Empathy Towards Older Adults in Accelerated Nursing Students

Nearly 18% of the United States population will be at least 65 years or older by 2025. Studies suggest student nurses have a negative attitude toward working with this population. There is a need to increase the number of graduate nurses who have positive attitudes towards older adults and an interest in working with this population upon graduation. This pre- and post-test intervention study aimed to: 1) examine the effectiveness of two educational modalities, a case study and simulated role-playing scenarios, on improving attitudes and empathy towards older adults; 2) explore factors affecting attitudes and empathy in accelerated baccalaureate of science in nursing (ABSN) students towards older adults. A convenience sample of 45 ABSN students completed pre- and post-test questionnaires. The results showed role-playing simulation scenarios had no impact on improving attitudes and empathy towards older adults, however, within group, the case study intervention was found to improve students’ attitudes. No significant predictor was found for attitudes; one significant predictor was found for empathy. Empathy is highly correlated with attitude. Further study is needed to explore educational modalities that improve attitudes and empathy in the ABSN student population

Assessment of Mechanical TBC Failure in Complex Geometries

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Nitridation during oxidation as a challenge for Cr-based alloys and its mitigation by alloying

The high temperature behavior of pure chromium above 1000°C shows very interesting features such as vaporization, scale spallation and worst of all extreme nitridation when exposed to air. A detailed investigation of the different mechanisms can explain the wide range and controversial results given in the literature when the oxidation and its oxidation rates are reported for pure Chromium. Especially, the attack by nitridation deteriorates the mechanical properties by severe embrittlement. It also has a huge impact on the mass change during oxidation. To mitigate this embrittlement an alloying strategy is developed by the addition of silicon and germanium. The oxidation behavior of binary Cr-Si and ternary Cr-Ge-Si alloys at ultra-high temperatures (T=1200°C-1350°C) is reported (see also Figure 1) and the morphological evolution of the oxide scale and the metal subsurface zone was investigated using scanning electron microscopy, electron probe micro-analysis, and X-ray diffraction techniques. The A15-phase Cr3Si is shown to have a crucial influence on prevention of nitridation. During oxidation of a two phase Crss-Cr3Si system an A15 barrier develops in form of a continuous intermetallic layer underneath the surface. The in-situ formed barrier layers shown to be able to successfully prevent nitridation and the same time to improve the oxidation kinetics. Beyond that, ternary additions of Ge to the Cr-Si system strengthens this effect even more and significantly improves the oxidation kinetics of the chromium alloys at ultra-high temperatures to a level comparable to alumina formers (Figure 2). Please click Additional Files below to see the full abstract

The Importance of the Fluorine Effect on the Oxidation of Intermetallic Titanium Aluminides

Due to the low Al activity within technical titanium aluminides and the similar thermodynamic stabilities of Al- and Ti-oxide these alloys always form a mixed oxide scale at elevated temperatures consisting of TiO2, Al2O3 and also nitrides if the exposure takes place in air. This mixed scale does not provide any oxidation protection especially under thermocyclic load or in water vapor containing environments. Thus accelerated oxidation occurs. Alloying of additional elements such as Nb improves the oxidation behavior if the additions stay within a certain concentration range but such additions cannot suppress non-protective mixed scale formation. Coatings are another way to protect these materials but several obstacles and new degradation mechanisms exist such as delamination e.g. due to CTE mismatch or development of brittle intermetallic phases due to interdiffusion. Therefore, other suitable protective measures have to be undertaken to make sure a protective oxide scale will develop. The so called halogen effect is a very promising way to change the oxidation mechanism from mixed scale formation to alumina formation. After optimized halogen treatment the alumina layer is very protective up to several thousand hours even under thermocyclic load and in atmospheres containing water vapor or SO2

Hydrogen permeation and embrittlement of ferritic SOEC/SOFC interconnect materials

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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