Deformation Mechanisms at Grain Boundaries in Polycrystal Plasticity

114 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.Strengthening by grain boundaries is commonly attributed to the dislocation pile-up at the boundary describing the effect of grain size, i.e. the Hall-Petch relation. In this work, the importance of the boundary structure is emphasized, along with grain size, in predicting the flow stress using alternative deformation mechanisms. The effect of boundary structure on the plastic deformation of metals is modeled by computing the aggregate response of composite grains in the visco-plastic self-consistent scheme. Assuming a planar boundary, the composite grain model accounts for the interaction between neighboring grains by satisfying the compatibility and equilibrium constraints across the boundary. For silver with 2 mum grains, in-situ Transmission Electron Microscopy studies suggest that annealing twin boundaries are sources for lattice dislocations. These sources contribute to an extended yield point, modeled using a formulation for slip system hardening that accounts for the evolution of mobile and forest dislocation densities---depicting boundary-dislocation and dislocation-dislocation interactions, respectively. In addition, the composite grain model is applied to predict the unique rolling texture of Cu/Nb nanostructured multilayers occurs due to the confined layer slip of single Orowan loops. The model regards each grain as a composite composed of Cu and Nb crystals. A hardening effect is introduced to account for the interaction between glide and interface dislocations. This unconventional hardening promotes symmetry of slip activity and consequently slows evolution of the rolling texture for Cu/Nb nanolayers.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

A New Approach to Direct Friction Stir Processing for Fabricating Surface Composites

Friction stir processing (FSP) is a green fabrication technique that has been effectively adopted in various engineering applications. One of the promising advantages of FSP is its applicability in the development of surface composites. In the current work, a new approach for direct friction stir processing is considered for the surface fabrication of aluminum-based composites reinforced with micro-sized silicon carbide particles (SiC), eliminating the prolonged preprocessing stages of preparing the sample and filling the holes of grooves. The proposed design of the FSP tool consists of two parts: an inner-threaded hollow cylindrical body; and a pin-less hollow shoulder. The design is examined with respect to three important tool processing parameters: the tilt angle of the tool, the tool’s dispersing hole, and the tool’s plunge depth. The current study shows that the use of a dispersing hole with a diameter of 6 mm of and a plunge depth of 0.6 mm, in combination with a tilting angle of 7°, results in sufficient mixing of the enforcement particles in the aluminum matrix, while still maintaining uniformity in the thickness of the composite layer. Metallographic examination of the Al/SiC surface composite demonstrates a uniform distribution of the Si particles and excellent adherence to the aluminum substrate. Microhardness measurements also show a remarkable increase, from 38.5 Hv at the base metal to a maximum value of 78 Hv in the processed matrix in the surface composites layer. The effect of the processing parameters was also studied, and its consequences with respect to the surface composites are discussed

Microstructure Refinement of 301 Stainless Steel via Thermomechanical Processing

The current study applied thermomechanical processing (TMP) on 301 austenitic stainless steel to produce an ultrafine-grained austenitic structure, examining the dual effects of deformation at subzero temperature and TMP cycles on the strain-induced α′-martensitic transformation and austenite reversion occurring upon subsequent annealing. Three TMP schemes were adopted: (1) one cycle using a strain of 0.30, (2) two cycles using a strain of 0.20, and (3) three cycles using a strain of 0.15. Each cycle consisted of tensile deformation at −50 °C followed by annealing at 850 °C for 5 min. Compared to other schemes, the use of three cycles of the 0.15 strain scheme resulted in a significant formation of the martensitic phase to about 99 vol.%. Consequently, the austenite reversion occurred strongly, providing a mixture of the austenitic structure of reverted ultra-fine grains and retained coarse grains with an average grain size of 1.9 µm. The development of a mixed austenitic structure was found to lower the austenite stability and thus enhance the α′-martensitic transformation upon deformation in subsequent cycles. Moderate growth of high-angle grain boundaries occurred in the austenitic phase for all schemes, reaching a maximum of 64% in cycle 3 of the 0.15 strain scheme. The tensile behavior during subzero deformation was generally characterized by an initial strain hardening by slip (stage I), followed by a remarkable increase in strain hardening rate due to the strain-induced α′-martensitic transformation (stage II). Further straining promoted breakage of the α′-martensite banded lath structure for forming dislocation cell-type martensite, which was marked by a decline in strain hardening rate (stage III). Accordingly, the latter hardening stage had a lesser hardness enhancement of deformed samples with an increasing number of cycles. Nevertheless, the yield strength for samples processed by the 0.15 strain scheme improved from 450 MPa in cycle 1 to 515 MPa in cycle 3

Microstructure and microhardness of OFHC copper processed by high-pressure torsion

An ultra-high purity oxygen free high conductivity (OFHC) Cu was investigated to determine the evolution of microstructure and microhardness during processing by high-pressure torsion (HPT). Disks were processed at ambient temperature, the microstructures were observed at the center, mid-radius and near-edge positions and the Vickers microhardness was recorded along radial directions. At low strains, ?3 twin boundaries are formed due to dynamic recrystallization before microstructural refinement and ultimately a stabilized ultrafine grain structure is formed in the near-edge position with an average grain size of ~280 nm after 10 turns. Measurements show the microhardness initially increases to ~150 Hv at an equivalent strain of ~2, then falls to about ~80 Hv during dynamic recrystallization up to a strain of ~8 and thereafter increases again to a saturated value of ~150 Hv at strains above ~22. The delay in microstructure and microhardness homogeneity by dynamic recrystallization is attributed to the high purity of Cu that enhances dislocation mobility and causes dynamic softening during the early stages of straining

Data from: Evidence that metallic proxies are unsuitable for assessing the mechanics of microwear formation and a new theory of the meaning of microwear

Mammalian tooth wear research reveals contrasting patterns seemingly linked to diet: irregularly-pitted enamel surfaces, possibly from consuming hard seeds, vs. roughly-aligned linearly-grooved surfaces, associated with eating tough leaves. These patterns are important for assigning diet to fossils, including hominins. However, experiments establishing conditions necessary for such damage challenge this paradigm. Lucas et al. (2013) slid natural objects against enamel, concluding anything less hard than enamel would rub, not abrade, its surface (producing no immediate wear). This category includes all organic plant matter. Particles harder than enamel, with sufficiently angular surfaces, could abrade it immediately, prerequisites that silica/silicate particles alone possess. Xia et al. (2015) countered with experiments using brass and aluminium balls. Their bulk hardness was lower than enamel, but the latter was abraded. We examined the ball exteriors to address this discrepancy. The aluminium was surfaced by a thin rough oxide layer harder than enamel. Brass surfaces were smoother, but work-hardening during manufacture gave them comparable or higher hardness than enamel. We conclude that Xia et al.’s results are actually predicted by the mechanical model of Lucas et al. To explain wear patterns, we present a new model of textural formation, based on particle properties and presence/absence of silica(tes)