MECHANICAL-BEHAVIOR OF A BULK NANOSTRUCTURED IRON ALLOY

Bulk, fully dense materials were prepared from Fe-10Cu with grain diam eters between 45 nm and 1.7 mu m. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by therm al treatment of the consolidated powders. The bulk materials were rela tively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pet. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanis m of plastic deformation was intense shear banding, which was accompan ied by a perfectly plastic stress-strain response (absence of strain h ardening). There was a large tension-compression asymmetry in the stre ngth, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual fo r metals, has been observed in amorphous polymers and metallic glasses . On the other hand, the fine-grained Fe-10Cu materials behaved like c oarse-grained iron in some respects, particularly by obeying the Hall- Fetch equation with constants reasonably close to those of pure iron a nd by exhibiting low-temperature mechanical behavior which was very si milar to that of steels. Transmission electron microscopy (TEM) studie s found highly elongated grains within shear bands, indicating that sh ear banding occurred by a dislocation-based mechanism, at least at gra in sizes above 100 nm. Similarities and differences between the fine-g rained Fe-10Cu and metals, polymers, metallic glasses, radiation-damag ed metals, and quench-damaged metals are discussed.