A. Orlová

Dislocation structure and applied, effective and internal stress in high-temperature creep of alpha iron

Abstract The dislocation structure of alpha iron formed in high-temperature creep mas investigated in the transmission electron microscope and its quantitative characteristics—dislocation density, subgrain diameter and subgrain misorientation—were measured in the primary as well as in the steady-state creep. The steady state substructure characteristics are correlated with the steady state values of the applied, effective and internal stress and the development of the substructure in the primary creep is discussed. The results of the structure observation correspond well to the macroscopic creep data (Pahutora, Orlova, Kuchařova, Cadek, to be published).

Internal stress and dislocation structure of aluminum in high-temperature creep

Abstract The dislocation structure formed in high-temperature creep of aluminum has been investigated in the transmission electron microscope. The effect of applied stress and temperature on the quantitative characteristics—dislocation density in sub-grains, subgrain size and misorientation—has been discussed. The correlation of the steady-state substructure characteristics with the macroscopic mean internal stress shows the main contribution of the internal stress field of dislocations in subgrains.

Some substructural aspects of high-temperature creep in metals

Abstract Some recent data obtained by investigating dislocation substructure formed in high-temperature creep of several metals are summarized, and some substructural aspects of both steady-state and primary creep are discussed on the basis of the data analysis. It is found that similar densities of dislocations unbound in sub-boundaries (free dislocations), ρ, exist in various metals in steady-state creep at the same value of the ratio of applied stress to elastic modulus. In steady state the mean subgrain diameter is proportional to the reciprocal of the square root of the total dislocation density ρc=ρ+ρSB ρSB being the density of dislocations forming sub-boundaries. Recovery in steady-state creep most probably takes place almost exclusively by annihilation of free dislocations and not by the entry of these dislocations into the sub-boundaries. The total dislocation density was found to increase in the course of primary creep. Most of the dislocations generated go to build the sub-boundaries as the den...

Dislocation structure in the high temperature creep of metals and solid solution alloys: a review

Abstract The present knowledge and ideas on the dislocation substructures in high temperature creep of metals and solid solution alloys are reviewed with special reference to contributions that appeared in the last decade. The microstructure development during creep is briefly described first. Further, substructural phenomena are discussed in relation to the applied stress and creep strain, and contemporary ideas of the relation of substructure to long-range internal stresses are outlined. Knowledge of the dislocation structure under dynamic conditions is summarized; the structure and motion of subboundaries, dislocation-subboundary interaction, dislocation generation and motion, and the structural steady state are discussed. Finally, some data on the statistical distribution of lengths of the dislocation links forming the three-dimensional networks as well as data on the mobile dislocation density and long-range internal stresses are reviewed. Also, a statistical treatment of the three-dimensional network is outlined. In conclusion, several problems that require further attention are indicated.

Instability origin of dislocation substructure

Abstract The formation of a dislocation substructure in high-temperature deformation is treated as a stability problem. A crystal in which dislocations can glide and climb is modelled as an anisotropic viscous medium. The subgrain structure development is interpreted as a deformation-induced instability of internal bending (folding) type first described by Biot. The suggested continuum mechanics model seems to provide a rational explanation of the main observed features of the subgrain structure: misorientation, tendency to regularity and relatively uniform subgrain size.

Creep in an aluminium alloy strengthened by Al4C3 particles

Abstract Creep in an Al-4.0mass% Cu-1.5mass%Mg alloy strengthened by about 4.5 vol.%Al 4 C 3 particles (Novamet IN-9021 alloy) of mean grain diameter about 0.9 μm was investigated in the temperature range 623–723 K and with applied stresses ranging from 7.5 × 10 −4 G to 3.1 × 10 −3 G (where G is the shear modulus). The creep is characterized by an apparent activation energy about five times higher than the activation enthalpy of lattice self-diffusion in pure aluminium; the steady state creep rate varies as approximately the sixteenth power of applied stress. Two different interpretations of experimental data are considered: 1. (i) The creep results from the motion of lattice dislocations associated with a strongly temperature- and applied-stress-dependent back stress σ B . 2. (ii) The creep is due to diffusional transport of matter controlled by motion of grain boundary dislocations with which a back stress σ 0 , again strongly temperature and applied stress dependent, is associated. Both interpretations are critically discussed; because of small grain size of the alloy investigated and, in particular, because of strong applied stress and temperature dependences of the back stress, the latter interpretation, although qualitative, is believed to be more realistic.

Steady-state creep in alpha iron as described in terms of effective stress and dislocation dynamics

Abstract Using the strain transient dip test technique the mean effective stress [sgrave]∗ was measured in steady-state creep of alpha iron in a temperature interval 400 to 700°C. At temperatures 400 to 450°C the mean effective stress was found to increase with increasing temperature, while at temperatures 550 to 700°C it is temperature-independent. At intermediate temperatures a more complicated behaviour of mean effective stress was observed. The parameter m∗ = (∂ ln v g/∂ ln [sgrave]∗) describing the effective stress sensitivity of dislocation glide velocity v g was estimated, assuming the moving dislocation density ρ m to be a constant fraction of the density ρ of dislocations unbound in sub-boundaries. The contribution to the effective stress dependence of steady-state creep rate of the effective stress dependenoe of ρm was found to be considerably smaller—at least at high effective stresses—than that of v g. In the temperature interval 550 to 700°C the apparent activation energy of dislocation glide...

Estimation of the volume fraction of hard and soft regions formed during high temperature creep

Abstract Several new methods for estimating the volume fraction of hard and soft regions formed during high temperature creep of metallic materials are presented. These methods start from available microscopic (dislocation density, subgrain size, subgrain misorientation) and macroscopic (applied stress, steady state creep rate) data. The application of the proposed methods is illustrated by means of data obtained in creep testing of aluminium at 573 K.

On the origin of the dislocation substructure during high-temperature creep

Abstract On the surface of a single-slip oriented copper crystal, slip and deformation bands, along with the thermally etched deformation substructure, could be observed after high-temperature creep. The substructure was further investigated in the transmission electron microscope. It has been proved that under given creep conditions the subgrain boundaries are formed predominantly in the plane normal to the slip direction and in the primary slip plane. The suggested conception of substructure formation has been based on the activity and interaction of several slip systems.

The effect of stress changes during creep on the dislocation structure of Al-5.5 at.% Mg

Abstract The effect of stress changes during high-temperature creep on the dislocation structure of Al-5.5 at.% Mg has been investigated by means of transmission electron microscopy. The behaviour of the dislocation loops during unloading at the test temperature after a creep test agrees well with a dislocation model which is based on the assumption that edge and screw dislocations in the alloy glide with different velocities which qualitatively correspond to a viscous glide due to gragging of solute atmospheres and a relatively free glide respectively. Reloading to about 10% of the initial creep stress does not introduce a sudden change in the dislocation structure.