Michael Huppmann

Microstructure and mechanical properties of differently extruded AZ31 magnesium alloy

Abstract Profiles of the magnesium alloy AZ31 were produced by conventional indirect extrusion and by indirect extrusion into counterpressure with variations of the billet temperature TB from 200 °C up to 350 °C. Microstructural examination by optical microscopy showed that for extrusion trials carried out with billet temperatures TB ≤ 250 °C fine dynamically recrystallized grains are found while elongated grains of the former cast structure remain. Above TB ≥ 300 °C most grains show higher dynamic recrystallization kinetics and additional static recrystallization and increase to grain sizes of d = 30 – 50 μm. At TB ≤ 250 °C the typical extrusion fiber texture is found, at TB ≥ 300 °C a double fiber texture is formed due to static recrystallization. The results of the quasistatic mechanical tests are due to these differences in the microstructure.

In-situ observation of creep damage evolution in Al–Al2O3 MMCs by synchrotron X-ray microtomography

Abstract Creep damage by pore nucleation and growth limits the lifetime of components subjected to loading at high temperature. In this work, the creep damage evolution in the aluminium matrix composite AA6061 + 22 % (Al2O3)P is observed in-situ by the fast-tomography technique using a miniature creep device. Reconstructions of the gauge volumes of the creep samples show particle delamination, pore formation and growth at 300 °C and additional particle fracture at 200 °C during the creep process. The results reveal that both the number of pores and the total pore volume show exponential growth with creep time, whereas the average volume of individual pores remains almost constant. The total pore volume increases linearly with the creep strain with a stress-dependent factor of proportionality.

Development of Twin Fractions and Microstresses of the Hot Extruded Magnesium Alloy AZ31 under Cyclic Loading Conditions

For investigating the twinning and detwinning behaviour under cyclic loading conditions in-situ stress measurements with different applied strain amplitudes were performed at the beamline 7T-MPW at BESSY II. Intensity measurements of the (10-10), (11-20) and (0002) reflections served for the determination of the twinned volume fraction. The measurements of the (hkil) dependent strains and stresses gave information on the load partitioning between different grain orientations. The intensity measurements performed in the compression regime showed that the evolution of the (0002) reflection intensity depends on the strain amplitude. For strain amplitudes ≥ 0.75 % an increase in intensity is observed whereas for smaller strain amplitudes the (0002) intensity decreases with increasing cycle numbers. In the tensile regime an increase in the (0002) reflection intensity is found for the whole range of applied strain amplitudes.

Microstructural development of the hot extruded magnesium alloy AZ31 under cyclic testing conditions

Abstract A study of the internal strain (stress) evolution during uni-axial cyclic deformation along the prior extrusion axis with fully reversed total constant strain amplitudes εA (0.5% < εA < 5%) was investigated by using in-situ high energy synchrotron X-ray diffraction. The deformation is dominated by {1012} 〈1011〉 twinning and detwinning mechanisms within the textured hot extruded magnesium alloy AZ31. The results show a strong load partitioning between the internal stresses of (1010) and (1120) parent grains and of the (0002) twinned daughter grains that are relaxed. Following the evolution of the twinning/detwinning behavior and the hkil dependent microstresses as a function of cycling numbers, two different regimes were observed whereby it was found that the transition between these regimes is marked by an applied strain amplitude of eA = 0.625%.

The effect of heat treatments on the microstructure, texture and mechanical properties of the extruded magnesium alloy ME21: Dedicated to Prof. Dr.-Ing. Heinrich Wollenberger on the occasion of his 80thbirthday

Abstract Heat treatments of the hot extruded magnesium alloy ME21 were performed at 400 °C – 550 °C and various annealing times. The evolution of the microstructure, the texture and the resulting mechanical properties were investigated. The heat treatments result in grain growth and in an intensification of the so-called rare earth texture. The grain coarsening results in a decrease in the yield strength according to the Hall – Petch relationship. The rare earth texture also contributes to a decrease in the yield strength due to the easy activation of basal slip systems. However, this texture improves the ductility at room temperature. The combination of large grain sizes, dissolved Mg12Ce precipitates and the rare earth texture delivers fracture strains up to  – 55 % under compression. The elongation under tension is less affected.