13th International Conference on Fracture June 16-21, 2013, Beijing, China -1- In-situ SEM/EBSD Study of Deformation and Fracture Behaviour of Flake Cast Iron Mattias Lundberg1,*, Mattias Calmunger1, Ru Lin Peng1 1 Department of Management and Engineering, Linköping University SE-581 83 Linköping *Corresponding author: mattias.lundberg@liu.se Abstract Cast irons’ position as an important engineering material is un-disputed. They are widely used in many important industrial applications such as the automotives and workshop machinery. Nevertheless, fracture mechanisms in cast irons are not fully understood. In this study the fracture path and non-linear elastic behaviour of a fully pearlitic flake cast iron under uniaxial tensile loading have been investigated in a Scanning Electron Microscope (SEM) equipped with an Electron Backscattering Diffraction (EBSD) detector. The tensile load was applied via a specially made sample stage. Under uniaxial tensile loading the graphite flakes act as notches or cracks and therefore the fracture process starts at one or many graphite tips. The crack can propagate in many different ways, at the graphite and matrix interface, through the graphite, at the interface between cementite and ferrite or through the pearlitic grains. At the point where the stress strain curve deviates from its linear path plastic deformation at graphite tips can be noticed. Interface cracking between graphite and the matrix also starts at this point. Keywords in-situ SEM, deformation, fracture, flake cast iron, EBSD 1. Introduction Flake cast iron exhibits brittle failure under tension loading. Due to its many graphite tips acting as notches or small cracks they are considered to be initiation points for cracks in the material. The common knowledge on how cracks propagates in flake cast irons is that the crack initiation point is one or several of the many graphite tips, from the tips the crack propagates along the graphite flakes in the graphite-matrix interface. At strains just before break down, the crack propagates through the matrix and connecting graphite tips. In a work done by Diószegi at al [1] the crack path was found to be through eutectic cells and along the graphite flakes. Depending on cooling condition the main crack path varied so to propagate along dendrites. The mechanisms explaining the non-linear stress-strain curve founded for flake cast irons involves a small plastic zone at graphite tips [2]. A typical stress-strain curve for a flake cast iron with σUTS of 290 MPa can be seen in Figure 3. As can be seen there is a small portion when the curve is straight linear; according to literature the explanation for this linear part is the atomic bonds and the interface forces between the graphite and the matrix [3, 4]. Plastic deformation at graphite tips and separation between graphite and matrix seems to be an excepted description what happens in the material when the stress-strain curve starts to deviate from its linear path during tensile loading [4]. An electron backscatter detector (EBSD) uses kikuchi patterns and a database to identify crystal structure and crystallographic orientation. Especially grain lattice orientation and grain size is commonly determined with this technique [5-10]. Texture analysis of metallic material using EBSD is a commonly usages of this technique [8, 10-11]. Strain variations in each grain near the machined surface give vital information on damages from machining processes [12]. From distribution and amount of low angle grain boundaries (LAGB) in the material, strain variations from external forces as well as from the solidification process can gathered. In a work done by Bjerkaas at al. on an AlMgSi alloy the benefits of this technique (EBSD) shows that grain lattice orientation during
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