Johannes May

Strain-rate sensitivity of ultrafine-grained materials

Abstract The strain-rate sensitivity of commercial purity Aluminium (Al 99.5) and of α-iron, with both conventional (CG) and ultrafine (UFG) grain sizes, are investigated by compression and tension tests at different temperatures. Microstructural investigations were performed before and after compression testing in order to investigate the microstructural stability. Pronounced strain rate sensitivity was found for UFG Al at room temperature as well as at elevated temperatures, while for UFG α-iron an enhanced strain-rate sensitivity was found only at elevated temperatures.

Strain Rate Sensitivity of Ultrafine Grained FCC- and BCC-Type Metals

The dependence of the strain rate sensitivity (SRS) of α-Fe and Al 99.5, as typical representatives of fcc- and bcc-type metals, on the testing temperature and with respect to the microstructure is investigated. In particular, the differences between conventional grain size (CG) and ultrafine grain size (UFG) are pointed out. UFG Al 99.5 generally shows an elevated SRS compared to CG Al 99.5. In case of α-Fe the SRS of the UFG state is decreased at room temperature, but increased at 200 °C, compared to the CG state. It is shown that the SRS also influences the ductility of UFG-metals in tensile tests.

Strain Rate Sensitivity of Ultrafine Grained Aluminium Alloy AA6061

The strain rate sensitivity of the aluminium alloy AA6061 has been investigated in a conventional grain sized (CG) state and in two different ultrafine grained (UFG) conditions processed by Equal Channel Angular Pressing (ECAP) for 2 and 6 passes at 100o C. Strain rate jump tests in compression were performed at different temperatures and the strain-rate sensitivity exponent m was determined. The tests were accomplished by microstructural investigations before and after compression testing in CG and UFG conditions. It is shown that all UFG microstructures exhibit strongly increased strain-rate sensitivity (SRS) compared to the CG state. The SRS increases with increasing temperature and is more pronounced for the UFG material processed using 6 ECAP passes. The microstructural investigations show a rather high stability of the grain structure for the UFG conditions up to 250o C. The results are discussed with respect to the relevant deformation mechanisms.

Cyclic Deformation Behaviour and Fatigue Lives of Ultrafine-Grained Aluminium-Magnesium Alloys

The fatigue behaviour of aluminium-magnesium alloys has been investigated in the recrystallized CG state and in an ultrafine-grained (UFG) state after equal channel angular pressing (ECAP). A strong improvement of the fatigue behaviour up to 12 ECAP passes has been found for an AlMg0.5 alloy. The results have been interpreted in consideration of the microstructure of the different states. Additionally, for an investigation of the influence of impurities on the cyclic stability of Aluminium, 3 different AlMg alloys with Magnesium contents of 0.5, 1, and 2 wt.-% have been compared. Total strain controlled fatigue tests have shown an improvement of the cyclic stability with increasing Mg content.

Extended Mechanical Testing of RPV Surveillance Materials Using Reconstitution Technique for Small Sized Specimen to Assist Long Term Operation

For the Ringhals 3 and 4 PWR RPV, results from the irradiation surveillance program are available also for neutron fluences which cover long-term operation (LTO). These standard surveillance result ...