Microstructure and creep resistance of Ti-rich Mo + Mo5Si3 + Mo5SiB2 alloys

New materials are necessary to increase efficiency of power generation and aircraft engines by higher combustion temperatures. Recently, it was found that alloying with high amounts of Ti replaces Mo3Si by Mo5Si3 and stabilizes the phase field Mo + Mo5Si3 + Mo5SiB2, which could be beneficial for higher oxidation resistance, since Mo5Si3 is more oxidation resistant than Mo3Si. Additional, Ti-rich Mo-Si-B alloys show an increased creep resistance compared to Ti-free Mo-Si-B alloys and a significant reduction in the alloy density. However, using the compositions reported in literature to stabilizing Mo5Si3 does not lead to reproducible results. This is most likely due to segregation effects and the formation of metastable phases like Ti5Si3. The addition of minor elements can be an option to widen the phase field Mo + Mo5Si3 + Mo5SiB2 as a function of Ti concentration. In this study we want to show the efficiency of such minor alloying additions on the stability of Ti-rich Mo + Mo5Si3 + Mo5SiB2 alloys. This was done by CALPHAD calculations using the commercial Pandat software for several elements such as Al, Cr, Fe, Hf and Zr. Fe seems to be the most promising candidate for stabilizing Mo5Si3. Experimental evaluation was exemplified with Mo-12.5Si-8.5B-xTi-2Fe and Mo-9Si-8B-xTi-2Fe model alloys to determine possible concentration ranges of Ti. Those alloys were produced by repetitive arc-melting of high-purity metals, Si and B in a Zirconium gettered high-purity argon atmosphere, followed by homogenization treatment at different temperatures for various times in a high-purity argon atmosphere. The identification of the resulting phases was done by XRD, SEM and EDS/ESMA analysis. Additionally, the creep resistance of those alloys was determined at temperatures ranging from 1100 to 1300°C and correlated to their microstructural features

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types

Microstructural changes in CoCrFeMnNi under mild tribological load

The lack of a principle element in high-entropy alloys (HEA) leads to unique and unexpected material properties. Tribological loading of metallic materials often results in deformed subsurface layers. As the microstructure feedbacks with friction forces, the microstructural evolution is highly dynamic and complex. The concept of HEAs promises high solid solution strengthening, which might decrease these microstructural changes. Here, we experimentally investigated the deformation behavior of CoCrFeMnNi in a dry, reciprocating tribological contact under a mild normal load. After only a single stroke, a surprisingly thick subsurface deformation layer was observed. This layer is characterized by nanocrystalline grains, twins and bands of localized dislocation motion. Twinning was found to be decisive for the overall thickness of this layer, and twin formation within the stress field of the moving sphere is analyzed. The localization of dislocation activity, caused by planar slip, results in a grain rotation. Fragmentation of twins and dislocation rearrangement lead to a nanocrystalline layer underneath the worn surface. In addition, oxide-rich layers were found after several sliding cycles. These oxides intermix with the nanocrystalline layer due to material transfer to the counter body and re-deposition to the wear track. Having revealed these fundamental mechanisms, the evolution of such deformation layers in CoCrFeMnNi under a tribological load might lead to other HEAs with compositions and properties specifically tailored to tribological applications in the future

Revealing the Role of Cross Slip for Serrated Plastic Deformation in Concentrated Solid Solutions at Cryogenic Temperatures

Serrated plastic deformation is an intense phenomenon in CoCrFeMnNi at and below 35 K with stress amplitudes in excess of 100 MPa. While previous publications have linked serrated deformation to dislocation pile ups at Lomer–Cottrell (LC) locks, there exist two alternate models on how the transition from continuous to serrated deformation occurs. One model correlates the transition to an exponential LC lock density–temperature variation. The second model attributes the transition to a decrease in cross-slip propensity based on temperature and dislocation density. In order to evaluate the validity of the models, a unique tensile deformation procedure with multiple temperature changes was carried out, analyzing stress amplitudes subsequent to temperature changes. The analysis provides evidence that the apparent density of LC locks does not massively change with temperature. Instead, the serrated plastic deformation is likely related to cross-slip propensity