|
|
Compression of ecaped titanium micro-pillars for two principal orientations
Vokoun, David ; Maňák, Jan ; Tesař, Karel ; Habr, Stanislav
The thermomechanical processing by equal-channel angular pressing (ECAP) is used for certain metals and alloys in order to make their structure fine and to increase material strength. In the previous study done at our institute, grade 2 titanium was successfully processed using four consecutive route A passes via a 90° ECAP die with high backpressure at room temperature. Orientation dependence of compressive and tensile loading of ECAPed titanium samples was demonstrated at macro-scale. However, scarce attention has been paid so far to the mechanical behavior of ECAPed titanium samples at micro-scale. In the present study, compression experiments on titanium micro-pillars, fabricated using focused ion beam, are carried out for two main directions in respect to preceding ECAP pressing (insert and extrusion directions). The purpose of this study is to discuss the orientation dependence of mechanical response during compression of the as-ECAPed titanium micro-pillars.
|
|
|
Why monitor acoustic emissions during nanomechanical tests?
Čtvrtlík, Radim ; Václavek, L. ; Tomáštík, J.
Acoustic Emissions (AE) monitoring has been proved as an effective non-destructive technique at the macro scale. Nevertheless, it may also be employed at nano/micro scale during nanomechanical and nanotribological testing. Local mechanical properties of surfaces or micro object are routinely explored using nanoindentation, scratch test or dynamic impact tests that are evaluated based on analysis of depth-load-time records or microscopic observation of residual indents, scratch grooves or impact craters, respectively. Although these approaches have been proven to be sufficient in most cases for a variety of materials, there are many situations where they do not provide sufficient information for a complex understanding of the deformation response. On the other hand, analysis of AE signals generated during these tests may provide valuable complementary information and provide some insight into the dynamics of phenomena like cracking, phase transitions, plastic instabilities, etc.\n
|
|
|
Characterization of titanium laser welds
Chmelíčková, H. ; Hiklová, H. ; Václavek, L. ; Tomáštík, J. ; Čtvrtlík, Radim
Butt welding of commercially pure titanium Grade 1 and Ti6Al4V alloy sheets using a pulsed Nd:YAG laser KLS 246 - 102 LASAG were carried out to determine optimal values of pulse energy and pulse length to create completely penetrated weld. Surface peak power density of about 3.105 W.cm−2 was found as an optimal value. Weld dimensions, both face width and penetration depth, are found to be proportional to increasing energy and decreasing pulse length. Gentle sagging and root penetration were revealed by means of contact surface profilometry. The nanohardness tests on transverse cross-sections detected approximately 50% higher hardness in the fusion zone than in the base material.\n
|
|
|
Laser-induced surface acoustic waves for thin film characterization
Kudělka, R. ; Václavek, L. ; Tomáštík, Jan ; Malecová, S. ; Čtvrtlík, R.
Knowledge of mechanical properties of thin films is essential for most of their applications.However, their determination can be problematic for very thin films. LAW (Laser-induced acousticwaves) is a combined acousto-optic method capable of measuring films with thickness from fewnanometers. It utilizes ultrasound surface waves which are excited via short laser pulses and detectedby a PVDF foil. Properties such as Young’s modulus, Poisson’s ratio and density of both the film andthe substrate as well as film thickness can be explored.Results from the LAW method are successfullycompared with nanoindentation for Young’s modulus evaluation and with optical method for filmthickness evaluation and also with literature data. Application of LAW for anisotropy mapping ofmaterials with cubic crystallographic lattice is also demonstrated
|
|
|
Benefits of use of acoustic emission in scratch testing
Václavek, L. ; Tomáštík, Jan ; Chmelíčková, H. ; Čtvrtlík, R.
Scratch test is regularly used for assessment of cohesive and adhesive strength of thin films and coatings. By default, its evaluation is based on analysis of depth-load-time record and microscopic observation of residual scratch groove. The visual analysis of the residual groove provides the most detailed description of the final damage of the surface (crack patterns, extent of plastic deformation, delamination, etc.), but it may be a time demanding approach. Although the continuous recording of indenter penetration depth and applied load offers instantaneous information about the performance of the tested material, it may not provide sufficient description of the sample’s deformation behaviour. Therefore, other complementary techniques for description of the deformation response to scratch loading are desirable.
|
|
|
Distribution of hydration products in the microstructure of cement pastes
Hlobil, Michal
This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were thresholded based on histogram image analysis combined with microstructural composition obtained from CEMHYD3D hydration model, to separate unhydrated cement grains, crystalline and amorphous hydrates, and capillary pores. The observed spatial distribution of the amorphous hydrate exhibited a strong spatial gradient as the amorphous gel tended to concentrate around dissolving cement grains rather than precipitate uniformly in the available space. A comparative numerical study was carried out to highlight the effect of the spatially (non)uniform hydrate distribution on the compressive strength of the hardened cement paste.
|
host ::
RSS.