Michal Hájek

Effect of Thermomechical Pretreatment on Mechanical Properties of Modified Al-Mn-Fe-Si Based Alloys

Fins in automotive heat exchangers are manufactured from Al-Mn-Fe-Si foils. Since continuous twin-roll casting (TRC) reduces energy and work consumption, it is the preferred manufacturing technology. The precipitation processes resulting from annealing of TRC Al-Mn-Fe- Si based alloys were studied by resistometric measurements during linear heating from room temperature up to 620 °C, hardness measurements and transmission electron microscopy examinations. Primary particles and precipitates of the cubic α-Al15(Mn,Fe)3Si2, hexagonal Al8Fe2Si and orthorhombic Al6(Fe,Mn) phases were identified. Phases of different types prevail in different alloys depending on composition. The increase of cold rolling prestrain prior to annealing induces a significant shift to lower temperatures of the start of precipitation. Prestrain accelerates precipitation kinetics by redistributing solute atoms and favoring their segregation on nucleation sites such as dislocations, subgrain and grain boundaries.

Characterization of Phase Transitions Occurring in Solution Treated Ti-15Mo during Heating by Thermal Expansion and Electrical Resistance Measurements

Metastable β titanium alloy Ti-15Mo was investigated in this study. In-situ electrical resistance and thermal expansion measurements conducted on solution treated material revealed influence of ongoing phase transitions on measured properties. The monotonicity of the dependence of electrical resistance on temperature changes at 225, 365 and 560 °C The thermal expansion deviates from linearity between 305 and 580 °C.

High‐Temperature Processes Occurring during Homogenization of AA6082 Aluminum Alloy

Homogenization conditions of the 6082-type aluminum alloy were tested by means of electrical resistivity measurements and in-situ heating SEM experiments. It was shown that close to 540 °C a partial local melting of dendrite cell boundaries occurs accompanied by a significant Mn, Mg and Si solid solution enrichment. The process is significantly accelerated and the surface damage due to the local melting is more pronounced when the temperature is risen to 560 °C. At higher temperatures a full surface melting and alloy degradation occurs.

Microstructure evolution in ultrafine-grained Ti and Ti-6Al-7Nb alloy processed by severe plastic deformation

Unique in-situ electric resistivity measurement was utilized to identify microstructural changes in ultra-fine grained commercially pure titanium and Ti-6Al-7Nb alloy. Both materials were prepared by equal channel angular pressing. Changes in resistivity evolution during in-situ heating were compared to scanning electron microscopy observations. Both materials are stable up to 440°C. Further heating at rate 5°C/min causes recovery and recrystallization of UFG structure. At 650°C the microstructure is fully recrystallized. High resolution in-situ electric resistivity measurement is capable of detecting recovery and recrystallization in UFG CP Ti and Ti-6Al-7Nb alloy.