Different types of interatomic potentials for bcc metals were compared to DFT calculations (see Fig.1) which showed that none of the initially available potentials was able to correctly reproduce crack tip processes. This led to the development of a new MEAM potential and the optimization of a machine-learned ACE potential. Furthermore, the potentials’ ability to simulate the dislocation motion was assessed.
Based on these studies, The crack initiation toughness was determined for W and in stoichiometric and off-stoichiometric B2 NiAl (Fig.2) for different fracture systems, crack tip configurations and radii.
A large part of the project was devoted to large-scale, fully 3D atomistic studies of crack-microstructure interactions. In particular, the interaction of pre-existing dislocations (Fig. 4) with cracks was studied in detail, which proved common, textbook-level conceptions wrong. Further simulations were performed to study crack-GB interactions, and GB fracture (Fig.5) as well as crack-void interactions (Fig.3). Also here, new mechanisms of crack-defect interactions were identified.
The atomistic simulations were complemented with tailored micromechanical fracture tests. These included randomly oriented crack systems, cracks with different crack tip qualities, cracks at or near GBs, cracks interacting with precipitates (Fig.8) and fracture of irradiated, W as well as single crystalline, pre-deformed W to study the effect of populating dedicated slip systems. To elucidate the role of thermal activation, fracture experiments were performed at different temperatures and with different strain rates. These experiments were supplemented with finite element simulations (Fig.7).
The CNRS group in Grenoble managed to combine their 3D discrete dislocation dynamics code with an XFEM implementation of fracture and included many of the atomistically observed crack-dislocation interaction mechanisms. This allows now for the first time to perform detailed parameter studies on a realistic model of crack tip plasticity at the micron scale, including crack tip shielding and blunting effects, crack propagation, and changes in crack configuration and dislocation nucleation at the crack tip (Fig.6). The CraDis code is published as open-source.
Taken together, the experimental and simulation results show clearly, that the usual tenet, that increasing the yield stress through the introduction of microstructural obstacles to dislocation motion leads to an increased brittleness does not generally hold, even on the crystallite level. In particular, at low temperatures, dislocation source terms can become important, and fracture toughness is determined by a complex, 3D competition between dislocation nucleation, glide and multiplication, and crack advance, reorientation, deflection and blunting, which can all be influenced by microstructure design.
The results of the project were disseminated in journal articles and conference presentations. We organized a well-attended international symposium on the fundamentals of fracture at Europe’s largest Physics conference the DPG Spring Meeting at the beginning and end of the project (2018 and 2023). A media campaign furthermore resulted in a TV report and a whole-page feature in the regional newspaper.
Our insights in 3D fracture processes will be exploited in a continuum model for fracture in semi-brittle materials.
