Ultrafine-grained Deformation Research Articles

On the Microstructural Characteristics of UFG Microalloyed Steel Influencing the Mechanical Behavior under Dynamic Loading

This paper presents a summary of a preliminary research aimed at producing ultrafine-grained (UFG) and heterogeneous microstructure in microalloyed steel and testing these materials under dynamic loading conditions (strain rates 800 s-1 and 1800s-1). The UFG and bimodal-structures, due to grain size, structural composition or morphology of structural components, were produced by an advanced thermomechanical processing, namely rolling in: hot, two-phase and cold-hot combined conditions. The advantage of bimodal microstructures is their maximization of mechanical behavior under extreme loading conditions due to promoted accumulation and interactions of geometrically necessary dislocations. The dynamic work-hardening behavior has been studied as a function of solute atoms and fine-scale, second-phase particles in the UFG and bimodal-structures. The substantial complexity of the phenomena, which occur through the evolution of microstructure and texture in response to dynamic loading, presents formidable challenges to theoretical model development of plastic deformation of UFG and bimodal-structures. Such an extraordinary work hardening provides an attractive strategy to develop optimal combination of mechanical properties i.e. strength/ductility ratio. A multi-scale analysis capable of including material behavior in different scales should be applied to discuss mechanical response of mentioned above microstructures and to help to analyze their influence on mechanical behavior under dynamic loading. The investigation was performed for a material of common application: high strength microalloyed steel X70. The experimental results show that strain rate sensitivity of the heterogeneous microstructures obtained by various thermomechanical rolling routes are significant, but not by a similar magnitude with the microstructure compositions and increasing strain rate.

Strain driven grain boundary sliding and/or rotation during tensile deformation of ultrafine grained C–Mn steel

The microstructure evolution of ultrafine grained C–Mn steel during tensile deformation was investigated using scanning electron microscopy. The surface morphologies and orientation imaging micrographs at different locations near the fractures were discussed. No obvious evident work hardening was identified and partially attributed to the strain driven grain boundary motion of grain rotation and/or grain boundary sliding, especially at the initial stage, while the dislocation activities gradually participate in as deformation proceeds.

Viscous nature of deformation of ultra-fine grain aluminum processed by equal-channel angular pressing

Abstract Deformation and microstructural evolution of ultra-fine grained (UFG) aluminum produced by equal-channel angular pressing has been examined over a wide range of strain-rates at room temperature and liquid nitrogen temperature (77 K). Elevated strain-rate sensitivity of UFG structures at both temperatures was revealed and compared with the coarse-grain counterparts. Results indicated that this enhanced viscous nature may be attributed in part to an increase in dislocation density as well as to some sort of reaction related to grain boundary diffusivity.

STRUCTURES AND MICROSTRUCTURES OF MANTLE ROCKS: KEYS TO THE RHEOLOGY OF THE LITHOSPHERE

Upper mantle peridotites from ophiolitic and alpine-type massifs, as well as those brought up as basalt-borne xenoliths, show a spectrum of structures and microstructures that potentially reflect the mechanical behaviour of the upper mantle during flow. It is uncertain, however, if microstructures are unambiguous in this respect. Based on experimental studies and materials science know-how, we use microstructure in its broadest sense to infer the nature of the microphysical processes once operating in the upper mantle rocks in question. Aside the many uncertainties involved in such extrapolation of experimentally determined mechanical data via the microstructure, there is the even more difficult question if the flow and fine structure of a natural rock, deformed at depth and high temperature, really reflects the flow of interest. Inherent to the fact that these rocks are now found at the Earth’s surface, the microstructures seen in naturally deformed mantle rocks may to a certain extent represent the waning stages of deformation and recrystallization in the mantle, i.e., the microstructure frozen in the rock may not appropriately reflect the main flow. The question thus arises in how far the deformation microstructures in mantle rocks really represent a significant part of the ductile flow history. In this context it seems appropriate to use the concept of a steady-state microstructure, i.e., a microstructure that continuously evolves during flow without changing its principal characteristics such as average grain size, grain shape and grain orientation, and that likely reflects the microphysical processes during flow. These concepts are particularly important at high homologous temperatures where deformation-induced rotation recrystallization (development of new grains from subgrains during dislocation creep) may or may not be balanced by migration recrystallization (recrystallization due to the migration of grain boundaries). It may be envisaged that high-temperature flow in the asthenosphere or lower lithosphere thus leads to seemingly undeformed granular rocks, even protogranular in the sense of commonly used peridotite microstructure classifications, or that the samples recovered preserve more clearly deformed, porphyroclastic microstructures including coarse to fine-grained tectonites. A distinct class of microstructures in peridotites concerns very fine to ultrafine-grained deformation microstructures in sometimes spectacular mylonitic shear zones. The mechanical aspects of the initiation and growth of these structures is surrounded with uncertainties, but there is growing consensus regarding their mechanical significance once these structures have developed: they are mechanically extremely weak due to the fact that flow in such ultrafine-grained rocks will be dominated by grainsize-sensitive diffusion creep mechanisms. Mylonites in upper mantle rocks often overprint earlier flow structures, probably because they tend develop at lower temperatures and higher stresses inherent in the later stages of a mantle fragment’s history in the Earth’s upper mantle, e.g., related to emplacement at crustal levels in rifts, or near the ocean floor in ophiolites.