Atomic-Scale Quantification of Grain Boundary Segregation in Nanocrystalline Material

Abstract

Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.

Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys

“…As recently reviewed by Herbig,8 full electron microscopy characterization of specific features can be carried out on needle-shaped specimens prior to APT analysis by utilizing specially designed holders. This approach has, for example, led to a better understanding of diffusional mechanisms resulting in local solute segregation at crystalline imperfections [9][10][11][12][13][14] that critically impact the macroscopic material behavior. APT had previously revealed details of the composition of structural imperfections, 15 the presence of which were confirmed by field ion microscopy 16 or TEM.…”

Deformation‐Induced Martensite: A New Paradigm for Exceptional Steels

“…[ 35 ] This results in a carbon-supersaturated Fe matrix and segregation of carbon atoms to grain boundaries stabilizing the nanosized Fe grain structure. [ 13,14,32,36 ] For the same samples, Li el al. reported a transition from a lamellar pearlite to a nanocrystalline microstructure during wire drawing at very high drawing strains.…”

“…The high strength is associated with the refi nement of the originally lamellar eutectoid body-centeredcubic (bcc) α-Fe (ferrite) + Fe 3 C (cementite) structure of the pearlite, which leads to a nanocomposite that is stabilized by carbon segregation to the α-Fe grain boundaries. [ 13,14 ] With ongoing structure refi nement, more and more carbon must be accommodated inside the ferrite due to the dissolution of the cementite. [ 15,16 ] As a consequence, the concentration of carbon in the α-Fe exceeds the equilibrium solubility limit by far.…”

Atom probe tomography reveals options for microstructural design of steels and titanium alloys by segregation engineering

“…Besides these confined types of chemical and structural changes of lattice defects also conventional phase transformation and phase growth can occur at segregation decorated internal defects when the system is (in some cases locally) rendered into the two-phase regime. Here, we provide some examples how these different types of segregation phenomena and confined structural states at lattice defects can be used to design complex microstructures in metallic alloys by using simple heat treatments [12][13][14]. We give examples of corresponding segregation effects, complexions and phase transformation phenomena in Fe-Mn steels taken from earlier work [12][13][14][15][16][17][18][19].…”