ISSN:
1662-9752
Quelle:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Thema:
Maschinenbau
Notizen:
Thermomechanical processing involving severe plastic deformation (SPD) is a popularapproach to ultrafine grain formation in bulk samples. In the present study, two grades of highpurityα-iron were deformed within the ferritic domain in cold and warm torsion to large strains (〉〉1). Examination of the deformed samples using orientation imaging microscopy revealed a highlyfragmented, lamellar structure aligned almost parallel to the direction of shear. Between 37 and54 % of boundaries detected are high angle ones (HAB). Some of these HAB are associated withthe original grain boundaries. However, a good number are believed to originate from dislocationaccumulation processes, during which the misorientation angle across certain strain-induced lowangle boundaries (LAB) rises with increasing strain. The resultant microstructure is composed ofultrafine crystallites on the order of 1 – 2 1m. In fact, localised regions of equiaxed grains on themicron scale were observed within samples deformed between room temperature and 300 °C.Nonetheless, other areas remain relatively unfragmented despite persistent straining until failure. Athigher temperatures, the microstructure is more homogeneous, but the average grain size iscoarsened. Optimal grain refinement thus appears to be a compromise between several competingfactors: large strains at relatively low temperatures for high dislocation density, higher temperaturesto enable sufficient dynamic recovery, and low grain boundary mobility that is aided by lowtemperatures and/or pinning by solute atoms or second phase particles. Furthermore, thedevelopment of a torsion texture composed of a single ideal orientation at large strains isunfavourable towards the generation of HAB
Materialart:
Digitale Medien
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/02/15/transtech_doi~10.4028%252Fwww.scientific.net%252FMSF.539-543.2898.pdf
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