Orientation imaging microscopy of ultrafine-grained nickel
Introduction
There are two important techniques for refining the microstructure of metals and alloys without contamination and without creating any internal porosity: these techniques are high-pressure torsion (HPT) and equal channel angular pressing (ECAP) [1], [2]. Both of these procedures introduce severe plastic deformation (SPD) and they lead to a refinement of the microstructure to the nanometer or submicrometer range. These processing techniques have become of interest recently because they have the potential for producing substantial improvements in the mechanical and physical properties of metallic alloys including combinations of high strength, enhanced ductility and superplasticity at relatively low temperatures and rapid strain rates [1], [2], [3], [4]. Many of these enhanced properties are attributed to the introduction during processing of arrays of predominantly high-angle random grain boundaries. In the early reports of processing by ECAP, the presence of high-angle grain boundaries was inferred indirectly from transmission electron microscopy (TEM) because of the presence of rings in selected area electron diffraction (SAED) patterns [5], [6]. However, it is recognized that inspection of the SAED patterns provides only qualitative information on the nature of the grain boundaries and alternative procedures are now available, such as orientation imaging microscopy (OIM) [7], [8] and Kikuchi pattern analysis in TEM [9], which permit quantitative measurements of the grain boundary misorientation distributions. In several recent investigations, these more sophisticated techniques have been applied after ECAP processing of samples of pure Al [10], [11], [12], [13], several Al alloys [14], [15], [16] and pure Cu [17], [18], [19]. However, no similar observations have been reported to date on any samples processed using HPT. Furthermore, there are no reports describing grain boundary character distributions in pure nickel after processing by either ECAP or HPT despite the fact that nickel is an ideal model material for grain refinement by SPD processing because the stacking fault energy, which is intermediate between that of pure aluminum and pure copper, leads to a much smaller grain size than in pure Al but a more homogeneous microstructure than in pure Cu [20]. Accordingly, the present investigation reports the application of OIM to determine grain boundary misorientation distributions in samples of pure Ni processed by HPT and ECAP.
Section snippets
Experimental material and procedure
The experiments were conducted using high purity (99.99%) nickel. Samples were processed by HPT or ECAP: details of these two methods are given elsewhere [1], [2], [5], [6], [21]. The HPT samples were in the form of disks with diameters of ∼10 mm and thicknesses of ∼0.3 mm. These disks were processed by HPT at room temperature under an applied pressure of 6 GPa to a total of five complete revolutions, equivalent to a true logarithmic strain of ∼6 [22]. The ECAP samples were in the form of
Results and discussion
Fig. 1 shows examples of the TEM images and the associated SAED patterns for ultrafine-grained nickel processed using (a) HPT and (b) ECAP. The SAED patterns were recorded with an aperture size of 1.8 μm. It is apparent that the microstructure after HPT is very homogeneous and the SAED pattern consists of rings with many diffracted beams so that there are many small grains with multiple orientations within the selected field of view. The average grain size after HPT was measured as ∼0.17 μm [22]
Concluding remarks
Experiments were conducted to determine the microstructures and grain boundary character distributions in ultrafine-grained nickel after processing by HPT and ECAP. Both procedures lead to large fractions of high-angle boundaries but with higher fractions of low-angle boundaries than anticipated from a random distribution. The fractions of twin boundaries and special grain boundaries after HPT and ECAP are also higher than in a random distribution. It is assumed that three deformation processes
Acknowledgements
This work was partially supported by INTAS under Grant no. 99-1216 and by the US Army Research Office under Grant no. DAAD19-00-1-0488. One of the authors (APZ) thanks the DGR of Generalitat of Catalonia for financial support. The authors also thank Slavek Poplawski (McGill University) for experimental assistance.
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