On the Influence of the Misorientation of Grains, Grain Size and Boundary Volume on the Strength and Ductility of Ultraf
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On the Influence of the Misorientation of Grains, Grain Size and Boundary Volume on the Strength and Ductility of Ultrafine Grained Cu Florian H. Dalla Torre, Rimma Lapovok , Peter F. Thomson, James D. Sandlin, Chris H.J. Davies and Elena V. Pereloma. School of Physics and Materials Engineering, Monash University, VIC, 3800, Australia ABSTRACT The influence of the microstructure and the misorientation relationship of grains on mechanical properties is investigated in specimens of ultrafine-grained copper processed by equal channel angular extrusion (ECAE) route Bc for 1, 4 and 12 passes. XRD texture analyses have shown that the major texture component is developed during the first pass of ECAE, and remains approximately constant with greater number of passes. EBSD measurements indicate that the majority of grain boundaries are still of low angle (>15°), while after four and twelve passes more than 50 % of all boundaries are high angle ones. TEM analyses have shown that the microstructure evolves from microbands and elongated cells towards a more equiaxed homogenous microstructure. On the microscale observed by TEM the degree of misorientation among subgrains/cells increases and the width of boundaries decreases while the cell/subgrain size remains approximately constant as the number of passes increases. The mechanical properties show a saturation level with a maximum in the yield stress and UTS after 4 passes. The strength of the material decreases between the fourth and the twelfth passes and the uniform elongation increases. It is suggested that the increase in ductility (and decrease in strength) is associated with the decrease in width of boundaries leading to an increase in the mean free path of dislocations. 1. INTRODUCTION Generally, it is well known that nanostructured (NC) and ultrafine grained (UFG) materials exhibit a high strength but only limited ductility. Recently different approaches have been considered to overcome the limitation in ductility [1-4]. One of the approaches involves a change in the deformation mechanism of a microstructure, which is mainly characterized by high angle grain boundaries and a large volume fraction of ‘nonequilibrium’ grain boundaries [1,3]. Such a microstructure can be achieved after a very large deformation via equal channel angular extrusion (ECAE). The deformation mechanism of UFG metals is often considered to be governed not only by lattice dislocation activity, but also by other mechanisms such as grain rotation and grain boundary sliding, especially for an equiaxed randomly textured microstructure [3]. The latter type of deformation mechanisms are often adapted for materials exhibiting even smaller NC grain sizes (below 100 nm), where dislocation activity is strongly reduced and where the deformation is more concentrated in the grain boundaries. In materials processed by ECAE it is, however, not clearly understood, if and how these grain boundary mechanisms act and whether they are beneficial to the mechanical properties. In this respect it is important to
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