Alena Orlová

Microstructural development during high temperature creep of 9% Cr steel

The modified 9% Cr steel Type P91 is one of the materials presently employed in power plant pipework components. The detailed microstructural analysis of a trial melt produced by Vitkovice Steel, Ostrava is reported in the present work. The microstructural evolution during creep at 873 K was investigated by means of transmission electron microscopy and computer image analysis. Two main microstructural elements of tempered martensite ferritic steels, namely subgrains and secondary phase particles, are studied quantitatively. The influence of stress free ageing and the influence of stress under creep conditions on particle coarsening and subgrain growth is determined.

On the microstructural development of the tempered martensitic Cr-steel P 91 during long-term creep—a comparison of data

Abstract Microstructural data for creep of the tempered martensitic 9 wt.%CrMoV steel P 91 at 873 K were analyzed. It was found that the variation of the subgrain size with strain conforms to an exponential change from the initial to the stress dependent steady-state value. Analysis of the size distributions of precipitated particles confirmed the previous result that in P 91 there is a superposition of growth of particles with the dynamic precipitation of new particles mostly of type MX. From the distributions the numerical fractions of two types of particles, a predominating population of M 23 C 6 carbides and a population of fine particles representing mostly carbonitrides of type MX, and their sizes could be deduced. The variation of the creep rate with strain is in qualitative agreement with these microstructural changes.

Analogy between creep cycles and stress relaxation series for activation volume measurement

Abstract Two techniques that are commonly used to measure the activation volume in the course of constant strain rate deformation experiments consists of performing either strain rate jump or stress relaxation tests. In this paper, a new method based on repeated creep tests is presented. It is demonstrated that, as for the technique of stress relaxation series, this new procedure allows the measurement of the apparent activation volume and the corresponding strain hardening correction term, which yield the effective activation volume. Only this last parameter is relevant for characterizing the thermally activated processes that control plastic flow.

Choice of evolution equation for internal stress in creep

Abstract The internal stress values in the course of the primary and the steady-state stages of creep in a Fe3wt.%Si alloy were measured by a technique analogical to the strain transient dip test technique. Two types of evolution equation, i.e. phenomenological and derived from dislocation kinetics, were examined by correlating them with the experimental data. In the application of the second group of equations, characteristics of the real dislocation structure were taken into account. The results of structure investigations are discussed in relation to the concept of internal stress in both homogeneous and heterogeneous dislocation structures, in the latter case using the composite model of the dislocation structure.

Interpretation of steady-state creef rate in mechanically alloyed AlCO alloys

Abstract The steady-state creep rate of pure aluminium and of four mechanically alloyed aluminium alloys was obtained within more extensive study of the creep behaviour of these materials. Creep data can be rationalized by introducing a threshold stress below which the creep rate is negligible. The estimated threshold stress grows with the increasing content of both carbon and oxygen. Within the threshold stress concept it can be shown that the data are in accord with the natural creep law. he steady-state creep rate can alternatively be described by the equation of thermally activated detachment of dislocations from dispersed particles. A new procedure for evaluation of the relaxation parameter characterizing this process is presented.

Constitutive description of creep curves based on internal stress evolution

A constitutive description of creep curves is derived from the time evolution of the internal stress in creep determined by means of strain transient dip test technique and from the kinetic equation between this stress and the instantaneous creep rate. Several constitutive equations have been tested on creep curves of aluminium. It is shown that fitting of the curves and its statistical judgement cannot be used as the only criterion for an investigation of the evolution and role of the internal stress in creep. Such procedure must be completed by another independent procedure. As an appropriate procedure, the measurement of the internal stress in the steady state creep has been successfully applied.

Relation between the internal stress measured in creep and the stress generated by the dislocation structure in the fcc metals

From the steady-state creep rate data treated as a function of the applied and the measured effective stress and temperature, a phenomenological dependence of the internal stress on the applied stress and temperature was derived. The result determined the expected character of the applied stress- and temperature dependences of the dislocation density, which was considered the microstructure parameter of the internal stress σ i = α MGbρ 1/2 (σi is the internal stress, α the dislocation interaction factor, M the Taylor factor, G the shear modulus, b the Burgers vector length and ρ the dislocation density). A scaling of the expected dislocation density by fitting it to measured dislocation density data yielded reasonable values of the parameter α in the Taylor formula, but the experimental data indicated a weaker applied stress dependence of the measured dislocation density than that of the expected dependence. An admission of an empirical formula fitting the dependence of dislocation density on the applied ...From the steady-state creep rate data treated as a function of the applied and the measured effective stress and temperature, a phenomenological dependence of the internal stress on the applied stress and temperature was derived. The result determined the expected character of the applied stress- and temperature dependences of the dislocation density, which was considered the microstructure parameter of the internal stress σ i = α MGbρ 1/2 (σi is the internal stress, α the dislocation interaction factor, M the Taylor factor, G the shear modulus, b the Burgers vector length and ρ the dislocation density). A scaling of the expected dislocation density by fitting it to measured dislocation density data yielded reasonable values of the parameter α in the Taylor formula, but the experimental data indicated a weaker applied stress dependence of the measured dislocation density than that of the expected dependence. An admission of an empirical formula fitting the dependence of dislocation density on the applied stress and temperature leads to a suggestion that the parameter α might be dependent on applied stress and temperature.

Internal stress and heterogeneous dislocation structure in creep

The present work deals with the problem of microstructural interpretation of internal stress measured in high-temperature creep by the strain-transient dip-test technique. The development of microstructure in the course of creep is considered a transition from uniformly distributed dislocations to a well-developed substructure. The substructure is supposed to have a composite character that consists of cell or subgrain boundaries with few dislocations in cell or subgrain interiors. Model equations for the relation of internal stress to the parameters of the dislocation structure are discussed and examined with reference to experimental data. The evolution of internal stress in creep, evaluated using different formulae, is compared with the evolution of macroscopically measured internal stress. The use of applied-stress dependences of microstructure parameters permits quite realistic estimates of the values of internal stress in steady-state creep.

Participation of non-compact dislocation glide in creep of copper at high temperatures

Abstract A possible participation of dislocation glide in non-compact crystal planes of copper in the high temperature creep process is assessed by means of values of activation energy. The activation energy of creep was determined alternatively from conventional data analysis, from the concept of effective and internal stresses applied to steady-state creep, and from constant-structure creep data. The results are discussed and compared with activation enthalpies of the {001} and {110} glide calculated from the kink-pair model. The calculated values are higher than the activation enthalpy of lattice diffusion. However, the activation energy of creep, which is lower than the value for lattice self-diffusion indicates rather a participation of a process with a lower activation energy than of the relatively difficult non-compact glide.

On the applied stress dependence of the subgrain size

The present work studies the collection of experimental data from which Raj and Pharr (Mater. Sci. Eng., 81 (1986) 217) deduced a universal empirical dependence of the subgrain size on the applied stress. In accord with their result and some theoretical predictions the normalized subgrain size ds/b was ssumed to be proportional to G/σ (G is the shear modulus, b the Burgers vector length, σ the applied stress). The evaluated factor of proportionality K1, having the value within the interval from 0.76 to 180 in the inspected data sets, was discussed from the point of view of various factors which can influence the experimental data.