K. Kuchařová

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.

Microstructural processes in creep of an AZ 91 magnesium-based composite and its matrix alloy

Abstract Constant stress tensile creep tests were conducted on an AZ 91–20 vol.% Al2O3 short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by TEM revealed good fiber–matrix interface bonding during creep exposure. The microstructures of the AZ 91 alloy and its composite were similar with regard to two types of β-phase precipitates; the enhanced precipitation of the Mg17 (Al, Zn)12 phase on the fibers is promoted by heterogeneous nucleation due to the Al enrichment of the matrix near to the alumina fibers.

High temperature creep behaviour of an Al-8.5Fe-1.3V-1.7Si alloy reinforced with silicon carbide particulates

Abstract The creep behaviour of an Al-8.5Fe-1.3V-1.7Si (the 8009Al type, all numbers indicate wt.%) alloy reinforced with 15 vol.% silicon carbide particulates — the Al-8.5Fe-1.3V-1.7Si-15SiCp composite — is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behaviour is observed to be associated with the true threshold stress that decreases more strongly with increasing temperature than the shear modulus of aluminium. The minimum creep strain rate is controlled by the lattice diffusion in the composite matrix, and the true stress exponent is close to 5. The results are compared with those obtained investigating the creep behaviour of an unreinforced Al-8.5Fe-1.3V-1.7Si alloy in the same temperature range. The creep strength of the composite as characterised by the minimum creep strain rate is found to be up to six orders of magnitude higher in the composite than in the alloy. This creep strengthening is attributed to a much higher true threshold stress in the composite than in the alloy, which is primarily due to finely dispersed alumina particles appearing in the composite matrix as a result of composite fabrication. The creep behaviour is interpreted in terms of athermal detachment of dislocations from interacting particles admitting a temperature dependence of the relaxation factor that characterises the strength of dislocation/particle interaction.

Creep behaviour of an oxide dispersion strengthened Al-5mg alloy reinforced by silicon carbide particulates : an oxide dispersion strengthened Al-5Mg-30SiCp composite

Abstract The creep behaviour of oxide dispersion strengthened (ODS) Al–5%Mg alloy reinforced by 30vol.% silicon carbide particulates – an ODS Al–5Mg–30SiC p composite — is investigated at three temperatures ranging from 623 to 723 K. The creep is associated with a true threshold stress depending on temperature more strongly than the shear modulus of the matrix solid solution. At applied stresses only slightly higher than the true threshold stresses, the apparent stress exponent m c reaches values as high as ∼85 and the apparent activation energy Q c values as high as ∼2000 kJ mol −1 at 673 K. However, the true stress exponent is close to five and the minimum creep strain rate is controlled by the lattice diffusion in the composite matrix. The difference between the apparent and the true stress exponents as well as between the apparent and the true activation energies of creep is attributed to the strong temperature dependence of the threshold stress. The origin of the true threshold stress and its temperature dependence is discussed. Alloying the aluminium matrix with 5% Mg is shown to have only a slight effect on the creep strength of the ODS Al–30SiC p composite.

The role of matrix microstructure in the creep behaviour of discontinuous fiber-reinforced AZ 91 magnesium alloy

Abstract Constant stress tensile creep tests were conducted to failure at temperatures of 423 and 473 K on an AZ 91 (Mg–9wt.%Al–1wt.%Zn) alloy reinforced with 20 vol.% Al 2 O 3 short fibres and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material was considerably improved by comparison with the matrix alloy. A microstructural investigation revealed that the most frequent morphology of the β-phase precipitates in the composite is continuous Mg 17 Al 12 platelets. Detailed investigations using transmission electron microscopy indicate that the matrix microstructure has no significant influence on the creep properties of these two materials. This result confirms the proposal that the creep strengthening of the composite is controlled by an effective load transfer between the matrix and the fibres.

Disappearance of the true threshold creep behaviour of an ODS Al–30SiCp composite at high temperatures

Abstract The creep behaviour of an oxide dispersion strengthened (ODS) Al–30SiC p composite has been investigated in a broad temperature interval ranging from 623 to 798 K. At temperatures up to 723 K true threshold creep behaviour occurs, while at temperatures above 723 K no true threshold stress has been found. Accepting the creep model of Rosler and Arzt (Acta Met. Mater. A150 (1992) 21), the creep behaviour of this composite at temperatures ranging from 748 to 798 K is interpreted in terms of the thermally activated detachment of dislocations from small alumina particles in the composite matrix. The disappearance of the true threshold stress at temperatures above 723 K is then due to a transition from athermal to thermally activated detachment of dislocations from alumina particles.

Creep in copper dispersion strengthened with alumina particles (ODS copper)

Abstract The creep behaviour of ODS copper is investigated in two distinctly different temperature intervals. In both intervals creep exhibits threshold behaviour; the true threshold stress decreases with increasing temperature. In the lower temperature interval (673–773 K) the creep strain rate is dislocation core diffusion controlled and the true stress exponent is close to 7. In the higher temperature interval (923–1023 K) creep is controlled by the lattice self-diffusion and the true stress exponent is close to 5. In both temperature intervals the apparent activation energies are much higher than the respective activation enthalpies of diffusion, i.e. the true activation energies of creep, and the apparent stress exponents are much higher than the respective true stress exponents. The differences of apparent and true activation energies are fully explained by the observed temperature dependences of the true threshold stresses. The differences between the apparent and the true stress exponents are shown to be inherent to the true threshold creep behaviour.

Dislocation slip and deformation twinning interplay during high temperature deformation in γ-TiAl base intermetallics

Abstract Interrupted compression creep tests were performed at 973 K and 350 MPa to study strain dependent microstructural changes in near-γ equiaxed Ti–48Al–2Cr–2Nb–B alloy. Dislocation densities and volume fraction of deformation twins were measured in the range of strain where sharp creep rate minima occur. The steep decrease of creep rate during initial 1% of strain is associated with a considerable increase in density of ordinary dislocations and superdislocations. Twinning processes are rather limited during this primary stage. On the other hand, in the strain range of minimum creep rate the volume fraction of deformation twins grows due to the increasing number of twinned grains and decreasing spacing between individual twins. Based on these results it is suggested that, in the pure- and near-γ TiAl equiaxed grain microstructures, the increase of creep rate after minimum and its new gradual saturation reflects a contribution of deformation twins to the creep strain accumulation kinetics. The contribution is twofold: (i) deformation twins relax incompatibility stresses set in during primary creep; and (ii) twinning supplies up to 30% of the overall strain in advanced stages of creep.

Metallographic investigation of reinforcement damage in creep of an AZ 91 matrix composite

Specimens of the AZ 91 composite were creep tested to failure and then examined internally to evaluate the significance of fibre cracking and/or breaking. It is demonstrated that the occurrence of fibre damage is influenced significantly by the nature of the procedures used to prepare samples for subsequent metallographic observations. Using an improved procedure, it is shown that the cracking or breaking of fibres is a relatively unimportant process in the creep failure of the AZ 91 composite.

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.