V. Sklenička

An assessment of time and strain to creep fracture in a low alloy heat-resistant CrMoV steel based on a physical-metallurgical approach

Abstract An analysis of the intergranular damage process taking place during creep in a low alloy heat-resistant 1.2wt.%Cr-0.7wt.%Mo-0.3wt.%V steel at 873 K was performed. The experimentally determined stress dependences of time and strain to creep fracture were compared with those estimated from the following mechanisms of cavity growth (the time necessary for cavity nucleation was assumed to be insignificant compared with the time to fracture): (a) coupled diffusion and plastic growth, (b) diffusion growth and (c) grain boundary sliding growth. It was found that mechanism (c) does not play any role at stresses below about 250 MPa. Direct evidence supporting the mechanism of coupled diffusion and plastic growth at stresses from 250 to 200 MPa obtained by analysing experimental data is presented. At even lower stresses (σ ⩽ 150 MPa) the diffusion-constrained growth mechanism may be expected. At stresses above 250 MPa, transgranular fracture takes place.

Strain components in high temperature creep of a Cu-30% Zn alloy

Abstract The surface measurements of strain components in high temperature creep of a coarse grained Cu - 30% Zn alloy were carried out in order to determine the contributions to creep strain of participating processes—grain interior deformation, grain boundary sliding and intercrystalline void formation. An equation describing the contribution of intercrystalline void formation was developed on the basis of detailed metallographic observations. Grain boundary sliding and grain interior deformation were found to be mutually independent processes, whereas intercrystalline void formation, consisting of intercrystalline cracking preceded by cavitation, represents a process accommodating stress concentrations created by grain boundary sliding and grain interior deformation. The relative importance of these three processes strongly depends on testing conditions. Thus, at applied stresses not exceeding 1.0 kg/mm2 the contribution to creep strain of grain interior deformation is almost negligible at temperatures lower than about 500°C while it predominates at temperatures higher than about 700°C. The contribution to creep strain of intercrystalline void formation is proportional to the sum of the contributions of grain interior deformation and grain boundary sliding.

High-temperature creep and grain-boundary behaviour in a Cu—10% Zn alloy

Abstract High-temperature creep and grain-boundary behaviour in a Cu — 10% Zn alloy was investigated in a temperature interval of 773 to 973 K. It was found that grain-boundary sliding (GBS) and grain-interior deformation (GID) are two independent mechanisms of the creep deformation. The ratio of the activation energies of these processes is close to about 0.6 and, consequently, GID controlled by volume diffusion dominates at higher while GBS controlled by grain-boundary diffusion dominates at lower temperatures. The differences of creep behaviour as to GIC, GBS and intercrystalline void formation (IVF) are discussed. It is shown that grain-boundary migration (GBM) in creep is more frequent in a Cu — 10% Zn alloy than in a Cu — 30% Zn alloy and that the migration distance is proportional to the total GBS vector. The main part of GBM can be explained as a structurally necessary process accompanying GBS.

On stress dependence of strain rates due to grain boundary sliding and grain interior deformation in high-temperature creep of a Cu-30% Zn alloy

Abstract Stress dependence of strain components ∊g and ∊gb due to grain interior deformation (GID) and grain boundary sliding (GBS), respectively, as well as of the corresponding component strain rates ∊g and ∊gb in high-temperature creep of a Cu-30% Zn alloy were investigated. The stress dependences of ∊g and ∊gb were found to be distinctly different, which, with other results, serves as a proof that GID and GBS are mutually independent processes in creep of the Cu-30% Zn alloy. The nomogram describing both the stress and temperature dependences of ∊g and ∊gb was obtained using the present results and the results of an investigation of temperature dependences of these component strain rates.

Localization and Development of Damage Under High Temperature Loading Conditions

The standard metallographic methods of the estimation of the degree of intergranular damage (cavitation) based on the quantitative microscopy are rather unsatisfactory since the typical feature of cavitation is its pronounced inhomogeneity. This work attempts to highlight the problem areas just in this respect. The attention is focused on the semiquantitative and quantitative investigation of inhomogeneities in the spatial distribution of cavities and the proposed approaches are illustrated by recent experimental results. The total of acquired information gives the detailed quantitative picture of cavitation in different stages of loading and can be used in constitutive models of creep fracture process. The critical amount of creep damage accumulation is defined and demonstrated experimentally. Finally, the application of the percolation model of cavitation to explain theoretically the critical amount of cavitation has been used.