Partners

Abstract:
Research results on both mechanical and acoustic emission (AE) behavior of Mg-Li and Mg-Al alloys matrix composites (AMC) reinforced with ceramic δ-Al2O3 or carbon fibers subjected to the channel-die compression at room and elevated temperatures are presented in this paper. The AE measurements at room temperature showed that, the effect of anisotropy of the fibres distribution (random planar distribution) with respect to the compression axis appeared in the most investigated composites, whereas the AE activity at 140°C revealed a two- range character and the rate of AE events at 140°C was higher than at room temperature. These effects are discussed in terms of both the differences in thermal expansion between the fibres and the matrix as well as the weakening of the coherency between the fibres and the matrix leading to stronger debonding effects at 140°C than at room temperature. The spectral analysis of AE signals was performed with the Windowed Fourier Transform method, which served to plot the spectral density of AE signal as a function of frequency. The alominous and corundum ceramics types were also investigated in order to illustrate the enhanced AE, which was related to the different crack paths in the final stages of the sample degradation. The results were also discussed on the basis of SEM images, including the in-situ observations of microcracking as well as the dislocation strain mechanisms and microcracking ones during the channel-die compression of the Mg-Li-Al AMC.

Keywords:
composites, fibres microcracking, acoustic emission, strain mechanisms, dislocations

Abstract:
The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al and Mg4Li4Zn alloys subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloys and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the relation between plastic flow instabilities and the fracture characteristics. There are discussed the possible influence of the factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries, interaction of solute atoms with mobile dislocations (Cottrell atmospheres) as well as dislocation pile-ups which may lead to the microcracks formation due to the creation of very high stress concentration at grain boundaries. The results show that the plastic flow discontinuities are related to the Portevin-Le Châtelier phenomenon (PL effect) and they are correlated with the generation of characteristic AE pulse trains. The fractography of broken samples was analyzed on the basis of light (optical), TEM and SEM images.

Keywords:
lightweight alloys, Acoustic Emission, fracture, Portevin-Le Châtelier phenomenon, twinning, dislocations, shear bands

Abstract:
The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al alloy subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloy and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the possible influence of factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries or solute atoms along dislocations (Cottrell atmospheres) or dislocation pile-ups at grain boundaries which create very high stress concentration leading to fracture. The results show that the plastic instabilities are related to the Portevin–Le Châtelier phenomenon (PL effect) and they are correlated with the generation of AE peaks. The fractography of breaking samples was analyzed on the basis of light (optical), TEM and SEM images.

Abstract:
Mg and Mg-Li matrix composites were prepared by the melt infiltration of fibrous preform consisting of Saffil alumina fibers and the silica binder. During this process there has occurred decomposition of silica binder and/or surface silica film by displacement redox reactions and the reaction products were characterized using SEM, TEM, SAED, FTIR and XPS techniques. The only reaction products found in Mg infiltrated Saffil preform were MgO and Mg2Si. In Mg-Li melt infiltrated Saffil preform there was found besides MgO and Mg 2 Si also the non- crystalline phase that appears to be the lithium silicate xLi2O ySiO2. Binary lithium silicides and ternary magnesium-lithium silicides were not detected

Keywords:
Mg-Li composites, Saffil fibers, silica binder, melt infiltration, XPS, FTI

Abstract:
The paper deals with the effect of Al content in two-phase Mg-10Li-xAl (x = 0, 1, 5) alloys on their straining behavior before and after severe plastic deformation by high pressure torsion. Both coarse-grained and ultrafine-grained alloys were compression strained in channel-die device with simultaneous acoustic emission monitoring. It has been observed that Hall-Petch strengthening in ultra-fine grained alloys superposes with the strengthening resulting from Al alloying (solution hardening, particulate strengthening). Superposition of these strengthening mechanisms becomes stronger with increase in Al content, presumably due to enhanced grain refining effect.

Keywords:
Mg-Li-Al alloys, grain refinement, high pressure torsion, acoustic emission, Hall-Petch strengthening

Abstract:
The object of investigation was the behaviour of acoustic emission (AE) during channel-die compression of the Mg-Li-Al alloys matrix composites (AMC) reinforced with ceramic δ-Al2O3fibres at room and at elevated temperature. The results of AE measurements at room temperature showed that in the majority of the investigated compositions the effect of anisotropy of the fibres distribution (planar random distribution) appeared with respect to the compression axis, whereas the AE activity at 1400C revealed a two-range character and the level of the rate of AE events was higher than thatat room temperature. These effects are discussed in terms of both, the differences in thermal expansion as well as weakening of the coherency between the fibres and the matrix. The results of AE measurements served to plot the spectral characteristics of the registered AE signals, i.e. the spectral density of AE signal as a function of its frequency. The spectral analysis of AE signals generated during the micro-cracking process of ceramic fibres in channel-die compressed Mg-Li-Al AMC was performed with the Windowed Fourier Transform method. The results are also discussed on the basis of optical and scanning microstructure images including these in-situobservations of microcracking fibres as well as in the context of the dislocation strain mechanisms and microcracking ones during the channel-die compression of the Mg-Li-Al AMC.