  High angular resolution electron backscatter diffraction (HR-EBSD) was
coupled with focused ion beam (FIB) slicing to characterize the shape of the
plastic zone in terms of geometrically necessary dislocations (GNDs) in W
single crystal in 3 dimensions. Cantilevers of similar size with a notch were
fabricated by FIB and were deformed inside a scanning electron microscope at
different temperatures (21$^{\circ}$C, 100$^{\circ}$C and 200$^{\circ}$C) just
above the micro-scale brittle-to-ductile transition (BDT). J-integral testing
was performed to analyse crack growth and determine the fracture toughness. At
all three temperatures the plastic zone was found to be larger close to the
free surface than inside the specimen, similar to macro-scale tension tests.
However, at higher temperature, the 3D shape of the plastic zone changes from
being localized in front of the crack tip to a butterfly-like distribution,
shielding more efficiently the crack tip and inhibiting crack propagation. A
comparison was made between two identically deformed samples, which were
FIB-sliced from two different directions, to evaluate the reliability of the
GND density estimation by HR-EBSD. The analysis of the distribution of the Nye
tensor components was used to differentiate between the types of GNDs nucleated
in the sample. The role of different types of dislocations in the plastic zone
is discussed and we confirm earlier findings that the micro-scale BDT of W is
mainly controlled by the nucleation of screw dislocations in front of the crack
tip.
