Research Projects

Research Projects

Running Projects

Hydrogen-associated decohesion and localized plasticity in a high-Mn -two phase- lightweight steel
Hydrogen embrittlement (HE) is one of the most dangerous embrittlement problems in metallic materials and  advanced high-strength steels (AHSS) are particularly prone to HE with the presence of only a few parts-per-million of H. However, the HE mechanisms in these materials remain elusive, especially for the lightweight steels where the composition and microstructure significantly differ from the traditional plain-carbon steels. Here we focus on a high-Mn and high-Al lightweight steel and unravel the effects of H-associated decohesion and localized plasticity on its H-induced catastrophic failure.

The dual role of martensitic transformation in fatigue crack growth
About 90% of all mechanical service failures are caused by fatigue. Avoiding fatigue failure requires addressing the wide knowledge gap regarding the micromechanical processes governing damage under cyclic loading, which may be fundamentally different from that under static loading. This is particularly true for deformation-induced martensitic transformation (DIMT), one of the most common strengthening mechanisms for alloys. Here, we identify two antagonistic mechanisms mediated by martensitic transformation during the fatigue process through in situ observations and demonstrate the dual role of DIMT in fatigue crack growth and its strong crack-size dependence. Our findings open up avenues for designing fatigue-resistant alloys through optimal use of DIMT. They also enable the development of physically based lifetime prediction models with higher fidelity.
Hydrogen embrittlement in high manganese lightweight steel
In this project we study the degradation of hydrogen embrittlement resistivity of austenitic high-Mn and high-Al lightweight steels upon age hardening and discover ways to mitigate this deterioration. more
In-situ investigation of H interaction with stacking faults (SFs) at the stress concentrated crack tip 
The main aspect of this project is to understand how hydrogen interacts with dislocations/ stacking faults at the stress concentrated crack tip. A three-point bending test has been employed for this work. more
Chemical boundary engineering
In order to solve key challenges in lightweight transportation and safe infrastructures stronger steels with high ductility are urgently needed. In this work we introduce a new unique chemical boundary engineering (CBE) approach, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. more
Macroscopic to nanoscopic in situ investigation on yielding mechanisms in ultrafine grained medium Mn steels
Within this project we show that medium Mn steels can develop a pronounced discontinuous yielding when the austenite matrix fraction lies about 65 vol%. This phenomenon is investigated by a combination of multiple in situ characterization techniques covering the macroscopic down to the nanoscopic scale. more
Dual-nanoprecipitation enables ultrastrong lightweight compositionally complex nanosteels
We introduce here a new approach in which we strengthen a low-density solid solution matrix simultaneously by a dual-nanoprecipitation system containing both kappa-carbides and B2 particles. Since the conventional thermodynamic working point is not accessible to realize this dual-precipitation strategy, we designed a low-density (6.6 g/cm3) steel-type alloy, which uses merits of the recently introduced multi–principal element approach referred to as compositionally complex alloys (CCAs) or high-entropy alloys (HEAs).

Overview of Advanced High-Strength Steels
Steels are backbone materials of civilization since more than 3000 years. They retrieve their properties not from expensive chemical compositions but rather from complex nano- and microstructures. They cover a wider spectrum of properties than any other material. more
Interplay of chemistry and faceting at grain boundaries in a multicomponent engineering Al-alloy
 In this project, we reveal the subtle yet important interplay between the faceting of grain boundaries and their chemical decoration with solutes in an engineering Al-Zn-Mg-Cu alloy. Previously, the interplay of chemistry and faceting was revealed for specific grain boundaries in well-defined bicrystals, which are realistically not encountered in engineering alloys. more
Damage and fracture mechanisms in multiphase medium Mn steels
In this project we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. more
Macroscopic to nanoscopic in situ investigation on yielding mechanisms in ultrafine grained medium Mn steels: Role of the austenite-ferrite interface
In this project we show that medium Mn steels with an austenite matrix (austenite fraction ~65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. more
Confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys
In this ongoing project, we investigate spinodal fluctuations at crystal defects such as grain boundaries and dislocations in Fe-Mn alloys using atom probe tomography, electron microscopy and thermodynamic modeling [1,2]. more

Finished Projects

Integrated experimental–simulation analysis of stress and strain partitioning in multiphase alloys
Limitations in the characterization of the partitioning of multiphase alloys, which takes place at the submicron scale, lead to a microstructure optimization of these alloys typically based on the evaluation of the averaged response referred to the macroscopic stress-strain-curves. We introduce a novel experimental-numerical methodology to strengthen the integrated understanding of the microstructure and mechanical properties of these alloys. more
Advances in the Optimization of Thin Strip Cast Stainless Steels
This project is about the understanding and optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in-line forming operations, and heat treatments for optimizing microstructure and properties have been studied. more
Deformation and fracture in fine- and ultrafine-grained ferrite-martensite dual-phase steels
For this project three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 um) but with the same martensite content (30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. more
Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Dual-Phase Steels
For this project two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 um by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). more
Advanced design of a ductile 2 GPa Fe-Cr-C martensitic-austenitic steel by nanoscale austenite reversion through partitioning, segregation, and kinetic freezing
Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultra-high strength ferritic stainless steel with excellent ductility. more
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