Michal Knapek

The Effect of Matrix Composition on the Deformation and Failure Mechanisms in Metal Matrix Syntactic Foams during Compression

The influence of the matrix material on the deformation and failure mechanisms in metal matrix syntactic foams was investigated in this study. Samples with commercially pure Al (Al) and Al-12 wt % Si (AlSi12) eutectic aluminum matrix, reinforced by hollow ceramic spheres, were compressed at room temperature. Concurrently, the acoustic emission response and the strain field development on the surface were monitored in-situ. The results indicate that the plastic deformation of the cell walls is the governing mechanism in the early stage of straining for both types of foams. At large stresses, deformation bands form both in the Al and AlSi12 foam. In Al foam, cell walls collapse in a large volume. In contrast, the AlSi12 foam is more brittle; therefore, the fracture of precipitates and the crushing of the matrix take place within a distinctive deformation band, along with an occurrence of a significant stress drop. The onset stress of ceramic sphere failure was shown to be not influenced by the matrix material. The in-situ methods provided complementary data which further support these results.

Anisotropy of mechanical and thermal properties of AZ31 sheets prepared using the ARB technique

In the accumulative roll bonding (ARB) technique, repeated stacking of material followed by conventional roll-bonding is carried out. For this process the surfaces are cleaned with ethanol and then joined together by rolling. The rolled material is then cut into two halves, again surface treated and roll-bonded. This process may be repeated several times. For the magnesium alloy AZ31 (Mg-3Al-1Zn) rolling at an elevated temperature of 400 °C is necessary for ARB because of the low plasticity of hexagonal magnesium alloys at lower temperatures. Samples for this study were prepared using 1 to 3 ARB passes through the rolling mill. It was found that the ARB substantially refined the grain size of sheets to the micrometer scale. The microstructure and texture of the deformed samples were studied by light and electron microscopy. The mechanical properties of the ARB samples were explored using tensile test-pieces cut from the sheets with the tensile axis taken either parallel or perpendicular to the rolling direction, where a significant anisotropy in both mechanical properties and Youngs modulus was found. Anisotropy is explained on the basis of the specific microstructure and texture formed during the ARB process.

Micron-scale deformation: a coupled in-situ study of strain bursts and acoustic emission

Plastic deformation of micron-scale crystalline materials differs considerably from bulk samples as it is characterized by stochastic strain bursts. To obtain a detailed picture of the intermittent deformation phenomena, numerous micron-sized specimens must be fabricated and tested. An improved focused ion beam fabrication method is proposed to prepare non-tapered micropillars with excellent control over their shape. Moreover, the fabrication time is less compared with other methods. The in situ compression device developed in our laboratory allows high-accuracy sample positioning and force/displacement measurements with high data sampling rates. The collective avalanche-like motion of the dislocations is observed as stress decreases on the stress-strain curves. An acoustic emission (AE) technique was employed for the first time to study the deformation behavior of micropillars. The AE technique provides important additional in situ information about the underlying processes during plastic deformation and is especially sensitive to the collective avalanche-like motion of the dislocations observed as the stress decreases on the deformation curves.

Assessing the frost resistance of illite-based ceramics through the resonant frequency of free vibration and internal damping

Experimental samples with porosity ranging from 6 % to 50 % were prepared by sacrificing template method from illite-rich clay. The frost resistance was assessed by measuring the shift in resonant frequency of free flexural vibration of prismatic samples and by the change of internal damping due to freeze-thaw cycles. Resonant frequency was determined by the impulse excitation technique and internal damping of the material was evaluated from the width of resonant peak. Samples were saturated with water and subjected to freeze-thaw cycles in the temperature range from –22 to 20 °C. Resonant frequency and internal damping were measured after 50, 100, 200, and 300 freeze-thaw cycles. Resonant frequency drops with increasing number of freeze-thaw cycles, and the effect is more pronounced in the case of samples with higher porosity. Internal damping of material was not found to be a suitable quantity for assessing the frost damage.

Acoustic Emission Analysis of Plane Strain-Compressed Mg Single Crystals

Mg single crystals with three different crystallographic orientations have been channel die (plane strain) compressed at applied rate of 10−3s−1 and at room temperature. The concurrent acoustic emission (AE) measurement was used as a method to analyze the dislocation dynamic during plastic deformation of Mg single crystals. The AE count rate and the maximum amplitudes of the AE event were correlated with stress-strain curves to determine the activity of various deformation mechanisms. The presence of large AE signals in the first stage of plastic deformation in single crystals with favorable orientation for (10.2)-twinning indicates twin nucleation and subsequent decrease of the AE activity, which can be connected with twin growth and collective dislocation processes. Compression along the c-axis (hard deformation mode) produces almost no AE activity.

Deformation of Open‐Cell Microcellular Pure Aluminum Investigated by the Acoustic Emission Technique

Acoustic Emission (AE) is used to characterize the plastic deformation of open-cell pure aluminum foams produced by salt replication. Measurements were performed on samples with cells 25, 75 or 400 μm in average diameter, all with a relative density near 24%. AE signals were measured during compression tests conducted with a constant cross-head speed. Deformation is uniform along the porous metal samples. Recorded AE accompanying plastic deformation of the cellular structure exhibits intermittent behaviour, with the probabilities of AE event energies distributed according to a power-law similar to those previously found in many different materials. The present study thus confirms the universal scale-free character of global plastic deformation dynamics and extends the observation to pure aluminum.