Marie Kvapilová

Effect of Equal-Channel Angular Pressing (ECAP) on creep in aluminium alloys

This paper examines the effect of equal-channel angular pressing (ECAP) on creep behaviour of pure aluminium, binary Al-0.2wt.%Sc alloy and ternary Al-3wt.%Mg-0.2wt.%Sc alloy. The ECAP was conducted at room temperature with a die that had a 90° angle between the channels and 8 repetitive ECAP passes followed route BC. Constant stress compression creep tests were performed at 473 K and stresses ranging between 16 to 80 MPa on ECAP materials and, for comparison purposes, on the initial coarse-grained materials. The results showed that the creep resistance of the ECAP processed Al-Sc and Al-Mg-Sc alloys was markedly deteriorated with respect to unpressed coarse-grained materials.

Equal-Channel Angular Pressing and Creep in Ultrafine-Grained Aluminium and Its Alloys

Creep strength and ductility are the key creep properties of creep-resistant materials but these properties typically have opposing characteristics. Thus, materials with conventional grain sizes may be strong or ductile but there are rarely both. In this connection, recent findings of high strength and good ductility in several submicrometer metals and alloys are of special in‐ terest [1]. Reduction of the grain size of a polycrystalline material can be successfully produced through advanced synthesis processes such as the electrodeposition technique [2] and severe plastic deformation SPD [1,3-6]. Although creep is an exceptionally old area of research, above mentioned processing techniques have become available over the last two decades which pro‐ vide an opportunity to expand the creep behaviour into new areas that were not feasible in ear‐ lier experiments. Creep testing of nanocrystalline (grain size d < 100 nm) and ultrafine-grained (d < 1 μm) materials is characterized by features that may be different from those documented for coarse-grained materials and thus cannot easily be compared.

An Evaluation of Creep Mechanisms in Ultrafine-Grained Metals

Our earlier published creep data are analyzed for ultrafine-grained pure aluminium and copper processed by equal-channel angular pressing (ECAP). The analysis demonstrates conclusively that creep occurs in the investigated materials after ECAP by the same mechanism as in conventional coarse-grained materials with intergranular dislocation glide and climb as the dominant rate-controlling flow process. Under creep conditions examined in this work diffusion creep is not important in pure aluminium and copper because the ultrafine grains are unstable at elevated and/or high temperature creep and the grains grow sufficiently to preclude any significant contribution from Nabarro-Herring or Coble 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.

The effect of ultrafine-grained microstructure on creep behaviour of 9% Cr steel

The effect of ultrafine-grained size on creep behaviour was investigated in P92 steel. Ultrafine-grained steel was prepared by one revolution of high-pressure torsion at room temperature. Creep tensile tests were performed at 873 K under the initially-applied stress range between 50 and 160 MPa. The microstructure was investigated using transmission electron microscopy and scanning electron microscopy equipped with an electron-back scatter detector. It was found that ultrafine-grained steel exhibits significantly faster minimum creep rates, and there was a decrease in the value of the stress exponent in comparison with coarse-grained P92 steel. Creep results also showed an abrupt decrease in the creep rate over time during the primary stage. The abrupt deceleration of the creep rate during the primary stage was shifted, with decreasing applied stress with longer creep times. The change in the decline of the creep rate during the primary stage was probably related to the enhanced precipitation of the Laves phase in the ultrafine-grained microstructure.

Combination of Experiments and Continuum Damage Mechanics Approach for Prediction of Creep Fracture in an Advanced 9%Cr Steel

As candidate materials for high-temperature components, most attention has been paid to improving tempered martensitic creep-resistant 9-12%Cr steels. In this work, creep damage and fracture behaviour of an advanced W-modified P92 steel (ASTM Grade P92) was investigated at 600 and 650°C. Tensile creep tests were followed by fractographic analysis of crept and broken specimens. Besides experimental investigations, the creep damage tolerance parameter λ has been used to assess the creep fracture mode. In accordance with experiments the values of λ indicate variety in the fracture mode and provide some evidence on accelerated degradation of the creep strength. The SEM investigations of creep fracture surface revealed substantial changes in microfractographic features of creep fracture. At high applied stress level, the fracture was frequently transgranular due to local loss of a stability of plastic deformation. The fracture ductility drops with decreasing applied stress, demonstrating ductile dimple (transgranular) to brittle (intergranular cavitation) transition of the fracture mode. It was suggested that both the creep deformation and fracture processes are controlled by the same processes and the rate controlling mechanism is most probably climb of intergranular mobile dislocations.

Creep damage and fracture in advanced tungsten modified 9%cr ferritic steel

Advanced tungsten modified 9%Cr ferritic steel (ASTM Grade P92) is a promising material for the next generation of fossil and nuclear power plants. Unfortunately, there are rather few published reports on damage processes in P92 steel during high temperature creep and the effect of damage evolution on the creep strength is not fully understood. In this work, the creep behaviour of P92 steel in as-received condition and after long-term isothermal ageing was investigated at 600 and 650°C using uniaxial tension creep tests. To quantify the effect of each damage process on the loss of creep strength, most of creep tests were followed by microstructural and fractographic investigations. It was found that the large Laves phase particles, which coarsened during creep exposure, served as preferential sites for creep cavity nucleation.

Quantitative characterization of microstructure in copper processed by equal-channel angular pressing

Experiments were conducted on extremely coarse-grained pure copper to evaluate the effect of equal-channel angular pressing (ECAP) on microstructure evolution in the as-pressed state and after creep exposure using various stereological methods. The microstructure formed by severe plastic deformation is an unusual structure which can be hardly characterized only by the mean grain size especially after low number of ECAP passes. The purpose of this paper is a detailed examination of (sub)boundaries and grain boundaries in the microstructures of the pressed material. The inhomogeneity of deformed microstructures is also evaluated. The detailed description of ECAP microstructures should contribute to the better understanding of mechanical properties of the pressed materials.

Effect of Equal-Channel Angular Pressing on the Creep Resistance of Precipitation-Strengthened Alloys

Despite intensive research efforts in precipitation-strengthened alloys processed by equal-channel angular pressing (ECAP), so far only a few investigations have been conducted on their creep behaviour and the results from these studies are controversial. This paper examines these differences and evaluates some factors influencing the creep resistance of the binary Al-0.2%Sc and Cu-0.2% Zr alloys processed by ECAP.

Microstructure Stability and Creep Behaviour of a Cu-0.2wt.%Zr Alloy Processed by Equal-Channel Angular Pressing

A dispersion-strengthened Cu-0.2 wt.% Zr alloy was subjected to equal-channel angular pressing (ECAP) at room temperature for up to 12 passes through route BC using a die having a channel angle of 90°. The microstructural investigations were performed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Tensile creep tests were conducted at temperature 673 K and at the applied stress in the range from 80 to 180 MPa. The migration of boundaries and subsequent grain growth were restricted by Cu9Zr2 precipitates. The study was performed in order to evaluate the effects of severe plastic deformation and precipitation on creep behaviour and microstructure of the pressed alloy. It was found that creep behaviour is strongly dependent on number of ECAP passes. The pressed alloy after up to 4 ECAP passes exhibited a considerable improvement in creep properties in comparison with the unpressed alloy.