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 ∘C, 100 ∘C and 200 ∘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.
Highlights
Dislocation accumulation and distribution around crack tips play a great role in fracture mechanics
Ar ion milling was applied on two perpendicular surfaces by a Leica EM TIC 3X polisher at 10 kV to create a sharp edge. 9 identical cantilevers were prepared by a Tescan Lyra3 Ga+ focused ion beam (FIB) with beam currents 30 kV, 10 nA decreasing to 30 kV, 0.5 nA to minimize the FIB-affected surface layer
All specimen showed non-negligible crack tip plasticity, while elastic-plastic fracture mechanics (EPFM) were used to determine the fracture toughness characterized by the onset of stable crack growth, which is denominated as ”fracture initiation toughness”
Summary
Dislocation accumulation and distribution around crack tips play a great role in fracture mechanics. The characterization of the plastic zone around crack during fracture tests remains a difficult task. The introduction of the J-integral technique at the microscale by Wurster et al [4] has opened the possibility to analyse semi-brittle fracture processes at small scales and calculate the fracture toughness of materials showing nonnegligible plastic deformation during fracture. Such behaviour is typical for body centered cubic (BCC) metals, which are usually found in numerous industrial applications, where their mechanisms of deformation need to be accurately defined for ambient and non-ambient conditions [5].
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