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

Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys

The oxidation and creep behaviour of novel eutectic-eutectoid Mo-Si-Ti alloys were studied and compared to previously investigated entirely eutectic Mo-20Si-52.8Ti (at%) and eutectoid Mo-21Si-34Ti reference alloys [Schliephake et al. in Intermetallics 104 (2019) 133-142]. While the latter reference alloys showed either outstanding oxidation behaviour in the temperature range of 800 to 1200 °C (eutectic alloy) or reasonable creep resistance (eutectoid alloy), a combination of both was successfully achieved in a Ti-rich alloy variant (Mo-21Si-43.4Ti). The ubiquitous catastrophic oxidation (“pesting”) of Mo-based alloys at 800 °C is suppressed in this alloy and reasonable oxidation resistance at higher temperatures is observed. For the first time, the unexpected oxidation resistance of the alloys exhibiting eutectic volume fractions of more than 50 vol% is rationalised by a systematic deconvolution of mass gain by scale formation and mass loss by evaporation of volatile species. Furthermore, creep is revealed to be based on similar creep mechanisms throughout the alloy series. Therefore, the observed improvement in creep resistance of the pesting-resistant Ti-rich alloy variant over the eutectic alloy is attributed to the decreasing homologous temperature when testing both at 1200 °C

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

Enhanced oxidation resistance of Ti-rich Mo-Si-B alloys by pack-cementation process

To increase efficiency by higher combustion temperatures of aircraft engines and energy generation, new high temperature materials are inevitable. Mo-Si-B alloys for example satisfy several requirements such as good oxidation and creep resistance. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to Ti-free Mo-Si-B alloys by the formation of Ti-silicide precipitates during processing. However, due to the formation of a duplex SiO2 – TiO2 oxide layer, where fast inwards diffusion of oxygen takes place, the oxidation resistance is poor. In this study we show that oxidation resistance of Mo-Si-B-Ti alloys can be enhanced drastically at temperatures ranging from 800 to 1200°C for several hundreds of hours by pack-cementation application of a borosilica based coating. The Mo-12.5Si-8.5B-27.5Ti (in at.%) substrate was produced by repetitive arc-melting of high-purity metals, Si and B in a high-purity argon atmosphere. After homogenization treatment at 1600°C for 100h slices of this alloy were prepared for pack-cementation. The pack-cementation was done in an atmosphere of high-purity argon at 1000°C for 40h, followed by a conditioning step at 1400°C for 10h in air. The resulting layer consists of an outer borosilica layer followed by an inner MoSi2 and Mo5Si3 layer. To study the oxidation behavior, both isothermal and cyclic oxidation tests were carried out. After an initial mass loss during the first hours of oxidation, a steady state is reached for tests up to 1000 hours. To demonstrate the high stability of the outer borosilicate layer SEM cross-sections were prepared after different times of oxidation

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