Processing and mechanical behavior of ultrafine grain materials

Citation

Jain, Mohit Kumar (1995) Processing and mechanical behavior of ultrafine grain materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8fty-t229. https://resolver.caltech.edu/CaltechETD:etd-10112007-090033

Abstract

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The mechanical behavior of ultrafine grain Fe-28A1-2Cr and 304 stainless steel was examined by conducting conventional mechanical testing and pre- and post-deformation microstructural characterization on bulk samples.

Shock wave consolidation was used to produce a fully-dense nanophase Fe-28A1-2Cr (grain size = 80 nm) intermetallic compound. In tension, the nanophase intermetallic failed in a brittle fashion with failure strength comparable to the coarse grain intermetallic of similar composition. However, the nanophase intermetallic yielded at 2.1 GPa during quasi-static compressive deformation and deformed to true strains greater than 1.4 without work hardening. The elastic-perfectly plastic behavior of nanophase Fe-28A1-2Cr is significantly different from that of coarse-grained intermetallic of the same composition, which yielded at 0.25 GPa and works hardened to 1.5 GPa before failure (at true strain of about 0.37). Microstructural examination before and after compressive deformation revealed that a significant portion of the microstructure refined to 10 nm grains surrounded by amorphous material. A similar grain refinement process was observed in 80 urn Fe- 28A1-2Cr produced by ingot metallurgical technique.

A novel thermo-mechanical processing technique was developed for the production of a ultrafine grain 304 stainless steel (grain size = 200 nm). The key steps to this processing technique involved (1) formation of ultrafine dislocation cell structure, and (2) the conversion of dislocation cells into grains with medium to high misorientation by initiating grain boundary sliding in the microstructure. The ultrafine grain steel (grain size = 200 nm) was about six times stronger ([...] = 1700 MPa) than coarse-grained steel of the same composition. Grain size hardening behavior of 304 stainless steel was also investigated over a broad range of grain sizes (200 nm to 200[...]).

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