Michaela Poková

The influence of dispersoids on the recrystallization of aluminium alloys

Abstract The influence of dispersoids on the softening processes in twin-roll cast and cold-rolled Al – Fe – Mn – Si aluminium alloy was studied. Before rolling, the specimens were thermo-mechanically pre-treated in order to create a different second-phase dispersion. The influence of the initial microstructure was studied by means of electrical resistivity, light and electron microscopy. The evolution of softening was monitored by microhardness measurements. The depletion of the Al matrix from solutes connected with formation of a dense dispersion of precipitates results in the enhancement of (sub)grain-boundary mobility. As a consequence, full recrystallization occurs at lower temperatures in pre-annealed specimens with low solute concentrations. The presence of coarser precipitates formed during the pre-treatment results in the formation of a fine-grained microstructure.

Study of twin-roll cast Aluminium alloys subjected to severe plastic deformation by equal channel angular pressing

Aluminium alloys prepared by twin-roll casting method become widely used in industry applications. Their high solid solution supersaturation and finer grains ensure better mechanical properties when compared with the direct-chill cast ones. One of the possibilities how to enhance their thermal stability is the addition of zirconium. After heat treatment Al3Zr precipitates form and these pin moving grain boundaries when the material is exposed to higher temperatures. In the present work twin-roll cast aluminium alloys based on AA3003 with and without Zr addition were annealed for 8 hours at 450 °C to enable precipitation of Al3Zr phase. Afterwards they were subjected to severe plastic deformation by equal channel angular pressing, which led to the reduction of average grain size under 1 μm. During subsequent isochronal annealing recovery and recrystallization took place. These processes were monitored by microhardness measurements, light optical microscopy and in-situ transmission electron microscopy. The addition of Zr stabilizes the grain size and increases the recrystallization temperature by 100 °C.

The influence of ECAP on microstructure evolution of aluminium alloys during in-situ heating in TEM

Abstract Twin-roll casting of aluminium alloys produces sheets with high solid-solution supersaturation. During high tempe-rature processing the solid solution decomposes and cubic α-Al(Mn, Fe)Si precipitates form. Processing by equal channel angular pressing introduces new grain boundaries into the material and shifts the precipitation to lower temperatures. In-situ annealing in a transmission electron microscope enables dynamic observation of recovery, precipitation, particle dissolution and grain growth. However, most of the processes observed by in-situ transmission electron microscopy occur at temperatures that are lower than temperatures estimated from specimens annealed conventionally in a furnace.

Effect of pre-annealing on microstructure evolution of TRC AA3003 aluminum alloy subjected to ECAP

Abstract An AA3003 aluminum alloy prepared by twin-roll casting was modified by a small amount of zirconium. Annealing at 450 °C led to precipitation of coherent Al 3 Zr phase and a simultaneous co-precipitation of Mn-rich α -Al(Mn, Fe)Si phase. Severe plastic deformation by equal channel angular pressing resulted in the grain refinement and increase of microhardness. Observation by electron back-scatter diffraction and in-situ transmission electron microscopy revealed influence of pre-annealing on microstructure changes during post-deformation heat treatment. Dislocation recovery and precipitation of α -Al(Mn, Fe)Si particles preceded recrystallization at 450 °C in material which was not annealed before deformation. The pre-deformation annealing enabled dynamic recovery during deformation as it depleted the solid solution from Mn atoms. Recrystallization was enhanced by Al 3 Zr precipitates.

Annealing Effects in Twin-Roll Cast AA8006 Aluminium Sheets Processed by Accumulative Roll-Bonding

Ultrafine grained sheets were prepared from a twin-roll cast AA8006 aluminium alloy using accumulative roll-bonding process at room temperature. The evolution of microstructure of sheets after three accumulative roll-bonding passes during isochronal annealing with a constant step of 20 °C/20 min was studied by light and electron microscopy. The influence of the resulting microstructure on mechanical properties was monitored by microhardness measurements. The microhardness increases when the material is annealed up to 160 °C. Above this temperature a fast drop of microhardness occurs followed by a negligible variation at annealing temperatures exceeding 300 °C. In order to map continuously the microstructure changes during annealing, the in situ TEM experiments in the heating stage were performed as a supplement to post-mortem TEM observations.

High Temperature Deformation of Twin-Roll Cast Al-Mn-Based Alloys after Equal Channel Angular Pressing

Twin roll cast Al-Mn- and Al-Mn-Zr-based alloys were subjected to four passes of equal channel angular pressing. The resulting grain size of 400 nm contributes to a significant strengthening at room temperature. This microstructure is not fully stable at elevated temperatures and recrystallization and vast grain growth occur at temperatures between 350 and 450 °C. The onset of these microstructure changes depends on chemical and phase composition. Better stability is observed in the Al-Mn-Zr-based alloy. High temperature tensile tests reveal that equal channel angular pressing results in a softening of all studied materials at high temperatures. This can be explained by an active role of grain boundaries in the deformation process. The maximum values of ductility and strain rate sensitivity parameter m found in the Al-Mn-Zr-based alloy are below the bottom limit of superplasticity (155%, m = 0.25). However, some features typical for superplastic behavior were observed—the strain rate dependence of the parameter m, the strengthening with increasing grain size, and the fracture by diffuse necking. Grain boundary sliding is believed to contribute partially to the overall strain in specimens where the grain size remained in the microcrystalline range.

Microstructure Evolution of AA3003 Aluminum Alloys Enhanced by Zirconium Addition Studied by Electron Microscopy

Zirconium added to aluminium alloys may under suitable conditions form metastable cubic precipitates Al3Zr, which pin moving grain boundaries and thus shift recrystallization to higher temperatures. Twin-roll cast AA3003 aluminium alloy cold-rolled to 5 and 1 mm was subjected to several annealing steps in order to find ideal conditions for precipitation of Al3Zr phase. Mechanical properties were monitored by microhardness measurement and microstructure was observed by light microscope and transmission electron microscope. Annealing to 450 °C with slow heating rate has been used to produce Al3Zr precipitates. This heat treatment also influenced presence of other phases like cubic α-Al(Mn,Fe)Si. The main mechanisms influencing microhardness were hardening by Al3Zr precipitates, softening by recovery and recrystallization and depletion of the solid solution from the major alloying elements.