Interaction of Slip Bands with Grain Boundary - in situ TEM Observation
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Interaction of Slip Bands with Grain Boundary - in situ TEM Observation Juliana Gemperlová, Alain Jacques1, Antonín Gemperle and Niva Zárubová Institute of Physics AS CR, Na Slovance 2, 182 21 Praha 8, Czech Republic 1 Laboratoire de Physique des Matériaux, Ecole des Mines, parc de Saurupt, 54042 Nancy Cedex, France ABSTRACT Mechanism of the slip transmission across the grain boundaries was studied on Σ3 bicrystals of Fe-4at%Si by transmission electron microscopy. In situ straining experiments as well as post mortem observations were performed. Three distinct events were observed in dependence on the angle α between the slip and grain boundary planes, namely transformation of the tertiary slip system in one grain into the secondary slip system in the other grain for α ≈ 90°, cross slip of dislocations of the primary slip system into the {112} plane parallel to the grain boundary for α ≈ 21°, and an abrupt formation of a sub-grain boundary in one grain for α ≈ 49°. Reasons for these diverse phenomena will be discussed in terms of interactions between the slip dislocations and the grain boundary dislocations. INTRODUCTION The influence of grain boundaries (GBs) on plastic deformation, especially on slip propagation, has been studied extensively (see e.g. reviews [1-4]). A simple geometric and stress criterion were suggested [5] for the transmission of dislocations through a GB. It can be expressed by the factor M = (lA⋅lB)⋅(bA⋅bB)/(bAbB) where lA,B are directions of intersection of the slip planes with the GB and bA,B are the Burgers vectors of dislocations in the two grains A and B. Following it, the easiest propagation (M = 1) would involve dislocations having the Burgers vector common in both grains, and moving in planes which have a common intersection with the interface. This is the case of symmetric Σ3 bicrystals with {112} GB plane. Propagation of slip across the GB has been studied recently [6-8] by X-ray diffraction topography on Fe-4at%Si bicrystals with well-defined GB. Three types of bicrystals, namely Σ3, Σ9 and Σ15, having different M were deformed by compression and tension and investigated both in situ and after deformation using white beam synchrotron radiation. The results led to the following conclusions. The GB form a strong barrier even in the Σ3 bicrystals where the slip geometry is the same in both grains. Wide slip bands of the primary system are formed in one of the grains before propagating to the other grain. An image force [1], stresses exerted by dislocations accumulated at the heads of individual slip bands and the local structure of the GB influence the slip transmission. To elucidate the real reasons why the movement of the slip dislocations is stopped at the GB even in the most favourable geometrical case, in situ TEM experiments were performed. These experiments allow observing and recording in the real time the movement of individual slip dislocations during deformation. Y8.23.1
EXPERIMENTAL DETAILS Symmetric Σ3 [110]/70.53°{112} bicrystals (stereoprojections Figure1) of Fe-4at%Si
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