Project 10: Small scale deformation and failure of Mo silicides
Doctoral researchers
Dr.-Ing. Georg Winkens (1st cohort)
M. Sc. Sri Rathinamani Ramdoss (2nd cohort)
M. Sc. Sarah Bohn (3rd cohort)
Supervisors
Prof. Ruth Schwaiger,
Prof. Martin Heilmaier
Project
Mo-Si-Ti alloys, notably the eutectic composition Mo-20Si-52.8Ti (at%), have garnered significant attention due to their remarkable resistance to oxidation and creep [1]. However, their practical use is currently constrained by their limited ductility at room temperature and a brittle-to-ductile transition occurring at around 1100 °C. This composition consists of a body-centered cubic (BCC) disordered Mo-rich solid solution and an intermetallic hexagonal (Ti,Mo)5Si3 phase. The goal of this project is to understand fundamental mechanisms behind the deformation and failure in this alloy system and its individual phases.
During the previous funding period, the properties of the BCC Mo-Ti system were extensively investigated from the nano- to the millimeter scale by chemical and mechanical analysis methods to isolate substitutional solid solution strengthening. Model comparisons revealed competitive yield strength modelled by the motion of edge dislocations compared to screw dislocations for Ti-rich Mo-Ti solid solutions, driven by the non-linear change in lattice parameter. Similar results in Mo-Nb solid solutions then confirmed these findings in a second binary system, indicating that edge dislocation-controlled strength might be more widespread in BCC solid solutions than expected.
In parallel, Cr–Mo–Si alloys, particularly Cr–36.1Mo–3Si (at.%), have emerged as promising alternatives because they combine excellent oxidation resistance with appreciable room-temperature plastic deformability. Research during the second funding period therefore focused on Cr–Mo–Si alloys with enhanced solid-solution-strengthening potential while maintaining excellent resistance against pesting. The alloy showed unexpected observation of deformation twinning at room temperature, a phenomenon that is highly unusual in BCC materials. Consequently, the research concentrated on understanding how solid-solution strengthening influences the competition between deformation twinning and dislocation slip. By combining nanoindentation, microhardness measurements, and macroscopic compression testing, the individual contributions of the different plastic deformation mechanisms were deconvoluted, revealing pronounced size-dependent plasticity. The concurrent activation of deformation twinning and dislocation slip significantly enhanced the work-hardening capability of the alloy.
Despite these promising findings, compression testing and post-deformation microstructural analyses revealed the formation of intergranular cracks. These observations raise concerns regarding the tensile performance of the alloy and identify grain-boundary cohesion as a critical limitation.
The proposed project therefore aims to improve grain-boundary strength. To improve this, using DFT, the project will combine detailed microstructural characterization of with small-scale mechanical testing of individual grain boundaries using FIB-fabricated cantilever bending experiments. These investigations will be complemented by macroscopic compression testing to assess the influence of grain-boundary strengthening on the overall deformation behavior and mechanical performance of the alloy.
[1] F. Hinrichs et al., Nature 646 (2025), 331-337, doi:10.1038/s41586-025-09516-8.