Abstract
High-temperature creep, cyclic deformation in saturation, and a number of technologically important processes are typical examples of the so-called “steady-state” deformation. These cases are usually defined in terms of the constancy of the mechanical parameters. Moreover, it is usually assumed that the deformation-induced microstructure undergoes no further changes. However, clear evidence shows that non-negligible microstructural changes continue to occur in the so-defined steady-state high-temperature creep and in cyclic saturation. It can be shown that the so-called “steady-state” deformation is actually a quasi-stationary deformation which is characterized by the initial development of a “mechanical steady state”, which is followed with a delay by a “microstructural steady state.” Only the latter can then be considered as a true steady state. A deeper analysis reveals a persistent slight increase of the dislocation density, with geometrically necessary dislocations in the cell walls/subgrain boundaries, causing the latter to transform gradually into sharper boundaries with higher misorientations. These findings, based on a detailed analysis of a wide range of experimental studies, are found to be almost identical for both high-temperature creep and cyclic deformation in saturation and are hence considered as characteristic of quasi-stationary deformation. The analysis clarifies, as a by-product, specific effects which arise from the increasing heterogeneity of the dislocation pattern (patterning). Thus, a marked decrease of the arrangement factor “alpha” in the Taylor flow stress is noted, as patterning proceeds, in agreement with predictions of the so-called composite model. Since this effect is compensated partially by the increase of the dislocation density, the flow stress remains rather insensitive to subtle microstructural changes. Based on these facts, the need for revision of current flow-stress formulations in future dislocation modeling is emphasized.
Highlights
THE so-called ‘‘steady-state’’ deformation is frequently encountered under a variety of circumstances in different areas of science and engineering
These findings show that the dislocation density continues to increase in steady state and contradicts the current general opinion that the dislocation density is constant in steady state
One finds in the literature statements like ‘‘paradoxically, realistic strain hardening properties in uniaxial deformation are obtained without accounting for dislocation patterning’’[21] or that there exists ‘‘a relative insensitivity of the dislocation strengthening relation to the arrangement of the microstructure.’’[23] in the present work, evidence will be provided, showing that as the dislocation density continues to increase mildly during mechanical steady-state deformation, the a-factor varies and can decrease by about 20 pct
Summary
THE so-called ‘‘steady-state’’ deformation is frequently encountered under a variety of circumstances in different areas of science and engineering. – High-temperature creep, and in – Cyclic deformation in saturation In these two cases, ‘‘steady-state’’ deformation is generally defined in terms of the constancy of the mechanical parameters which define the deformation, e.g., the stress and the strain rate. A strong motivation for this approach lies in the increasing evidence that, in the so-called ‘‘steady-state’’ deformation, small but non-negligible microstructural changes occur, as was documented earlier.[8,9] This important aspect and its implications will be discussed in detail subsequently It follows that, strictly speaking, there is no true steady state, defined in terms of constancy of both stress (deformation strength) and microstructure.
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