Anna Fraczkiewicz

Atomic scale investigation of boron nanosegregation in Fe Al intermetallics

The aim of this work is to investigate the atomic scale distribution of boron in FeAl alloys. Emphasis is put on boron segregation to intragranular lattice defects. The nature of defects was identified by TEM. Then, Field Ion Microscopy (FIM) combined to Three-dimensional Atom Probe (TAP) was used to get the atomic scale distribution of chemical species within the material. This technique is of particular interest as it is able to map out the 3D distribution of chemical species with a depth resolution of a single atomic layer.

A study of thermal vacancies and dislocation structure in Fe–40 at.% Al

The densities of thermal vacancies and residual dislocations in bulk specimens of Fe–40 at.% Al have been investigated using differential dilatometry and X-ray diffraction. A large quenched-in vacancy concentration at temperatures above about 873 K was apparent from the decrease in average lattice parameter. This was correlated to a lowering of the effective enthalpy of vacancy formation from about 91 to 42 kJ mol−1, possibly caused by the presence of different types of point defect in lower- and higher- temperature regimes. The residual dislocations were found to have a major concentration on {100} planes at any given temperature. An increase in the dislocation density and a concurrent fall in the vacancy concentration was observed with a lowering of the quenching temperature from 1223 to 1073 K, indicating the possibility of vacancy annihilation at 1073 K.

Interaction of boron with crystal defects in B2-ordered FeAl alloys

Intermetallic alloys are often doped with boron to suppress their intrinsic room-temperature intergranular brittleness. The commonly admitted mechanism of this effect, i.e. an intergranular segregation of boron, seems not to be the only important feature. In this work, boron interactions with numerous kinds of crystal defects (point defects, dislocations, grain boundaries) are studied in B-doped FeAl (B2) alloys containing 40 at. % Al. The intergranular segregation of boron is first characterized. Both an equilibrium and a non-equilibrium (due to a solute atom / thermal vacancy interaction) segregation mechanisms are identified. Strong tendency of boron to segregate to the dislocations lines is shown by direct measurements by atom probe field ion microscopy (AP FIM). This segregation is shown to induce a local depletion in Al in the vicinity of defects.