Hans Chen

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.

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 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.

Development of Oxidation Resistant Refractory High Entropy Alloys for High Temperature Applications: Recent Results and Development Strategy

Refractory High Entropy Alloys (HEAs) can be considered as promising materials for high temperature applications because of their high melting point and outstanding high temperature strength. The microstructure of the equimolar alloy Nb-Mo-Cr-Ti-Al consists of a disordered body centered cubic (BCC) phase and a small amount of the Laves phase, while the equimolar alloy Ta-Mo-Cr-Ti-Al exhibits the ordered B2 and several Laves phases in addition to the BCC phase. The experimental studies reveal that these alloys possess a beneficial combination of high temperature strength and corrosion protectiveness. The compressive yield stress of the alloy Nb-Mo-Cr-Ti-Al and Ta-Mo-Cr-Ti-Al at 1200 °C is determined to 100 MPa and 200 MPa, respectively. The oxidation resistance of the alloy Ta-Mo-Cr-Ti-Al in the temperature range between 900 and 1100 °C is comparable to that of multi-phase Ni-based alloys. The main drawback of both alloys is their low ductility at room temperature. Strategies for the future alloy development are discussed.