3D HR-EBSD characterization of the plastic zone around crack tips in tungsten single crystals at the micron scale

TL;DR

This study uses 3D HR-EBSD combined with FIB slicing to analyze the shape and dislocation structure of the plastic zone around crack tips in tungsten single crystals at different temperatures, revealing temperature-dependent plastic zone morphology and dislocation behavior.

Contribution

It introduces a novel 3D characterization method for plastic zones in tungsten single crystals, providing new insights into dislocation types and crack tip shielding effects at the micron scale.

Findings

Plastic zone size is larger near the surface at all temperatures.

Higher temperatures cause the plastic zone to change shape to a butterfly-like distribution.

Nucleation of screw dislocations mainly controls the micro-scale brittle-to-ductile transition.

Abstract

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.