Unstable Deformation and Fracture Processes in Tensile Tested and Compressed Mg-Li and Cu-Sn Alloys Investigated with Acoustic Emission Method

Research results on both mechanical and acoustic emission (AE) behavior of Mg-Li and Cu-Sn alloys subjected to the tensile and compression deformation at room and elevated temperatures are presented in this paper. AE behavior of Mg-Li-Zn alloys during tensile tests at wide range of elevated temperatures are compared with those of Mg-Li-Al alloys during tensile tests at elevated temperatures as well as during channel-die compression at room temperature. AE behavior is also investigated for Cu-Sn alloys subjected to tensile deformation at elevated temperatures. The main aim of the paper is the documentation and explanation of the observed correlations between AE – generated due to the plastic instabilities induced by Portevin–Le Châtelier effect (PL), twinning or shear band phenomena – and the evolution of fracture of Mg4Li4Zn, Mg4Li5Al and CuSn6 alloys subjected to tensile and compression deformation in a large range of elevated temperatures. The spectral analysis of AE signals, performed with the Windowed Fourier Transform method, the factography of breaking samples, analyzed on the basis of optical, SEM and TEM images, showed that in the case of tensile deformation there occur the transition from the ductile to brittle fracture within a short range of temperatures which is strongly correlated with the AE parameters. Similarly, the results in the case of compression deformation showed that the beginning of cracking was  related with the formation of shear band leading to the strain localization and final fracture. The obtained results are discussed basing on the literature model of transient ductile-brittle fracture, as well as on the conception that the main physical reasons of AE and PL phenomena in metal materials can be explained in terms of the collective behavior of dislocation groups and with the processes of internal and surface synchronized annihilation of many dislocations.