schliephake

The development and investigation of novel Mo-Si-Ti alloys has been one of our core competencies in recent years. These alloys are potential candidates for high temperature applications due to their balanced property portfolio of high temperature strength, oxidation resistance and lightweight potential. In our most recent study, we investigated the creep deformation mechanisms of a eutectic Mo-Si-Ti alloy together with our colleagues from Karlsruher Institut für Technologie (KIT), Laboratory for Electron Microscopy #LEM. In contrast to earlier investigations, we found that creep at 1200 °C can be divided into two regimes dominated by (i) diffusion creep below 100 MPa and (ii) dislocation climb controlled creep at and above 100 MPa. An attempt has been made to correlate the microstructural changes occurring at different microstructural length scales with the nature of the creep curve at 1200 °C and 100 MPa. The creep curve shows a transient strain hardening region followed by a distinct minimum and then acceleration of the creep rate. Microstructural investigations using TEM revealed the formation of a high proportion of disperse (Ti,Mo)₅Si₃ precipitates in the solid solution, which led to significant strengthening in the transient creep regime. By simultaneously decreasing the initially high dislocation density in the solid solution, the diffusion creep contribution becomes more dominant to the effective creep behavior. At a minimum, the load and strain are also carried by the silicide phase, which undergoes plastic deformation. Continuous coarsening of precipitates and loss of precipitation strengthening in the solid solution and dynamic recovery in the silicide phase led to creep acceleration at strains above the minimum creep rate.

laube

Compositionally complex alloys based on refractory metals (RCCA) are candidate alloys for future high-temperature applications. If Al is included in these alloys to reduce the density or to achieve oxidation resistance, long-range ordering often occurs. In our latest publication, we were able to show that this contributes significantly to the high yield strength. By combining macroscopic compression testing and nanoindentation measurements by our colleagues of IAM-MMI, we characterized alloys in the Mo-Cr-Ti-Al system over a large temperature range. Once a threshold of Al concentration is surpassed, the system crystallizes in the ordered B2 instead of the disordered A2 crystal structure, and a significant increase in yield strength of about 300 MPa and nanohardness was observed.

Mechanical testing on both length scales as well as chemical and structural investigations precluded the impact of several strengthening mechanisms that could explain this jump. Using state-of-the art modelling of solid solution strengthening in the complex A2 alloys, we were also able to prove that the increase in Al content cannot account for the observed jump. This leaves order strengthening as the only remaining mechanism to explain the increase in strength.

With order strengthening as now-proven relevant strengthening mechanism in RCCA, new alloy design possibilities open up to tailor alloy compositions for specific application profiles. Furthermore, the distinct modeling of strength of compositionally complex B2 alloys is not yet available.

vikram

There is a burgeoning commercial demand for materials that can withstand elevated temperatures without sacrificing strength and ductility. The present work deals with the development of a novel high-temperature Mo-20Si-52.8Ti (at. %) ternary alloy via directional solidification (DS) using modified Bridgeman type apparatus. The microstructure consisted of a body-centered cubic solid solution BCC_ss and a hexagonal silicide (Ti,Mo)₅Si₃ phase with approximate volume fractions of 50%. The phases exhibit a crystallographic orientation relationship as (123)_BCC || (0001)_(Ti,Mo)5Si3  and [111]_BCC || [1120]_(Ti,Mo)5Si3. Mechanical analysis demonstrates that plasticity is primarily accommodated by the BCC_ss phase through dislocation mediation, while the silicide phase acts as a site for crack initiation. Interestingly, crack propagation is impeded and redirected at the interface at the BCC_ss phase. Furthermore, the indentation fracture toughness of the silicide phase is observed to be 3.7 MPa√m, slightly higher than reported values for Nb, Mo, and Cr-based silicides at room temperature. The increased microstructural length scale resulting from directional solidification confers intrinsic advantages, notably enhanced fracture toughness of the alloy. Consequently, this study paves the way for the development of ductile-phase toughened intermetallic, offering new prospects for the design of advanced intermetallics with superior toughness characteristics.