Plastic Instabilities Induced by the Portevin - Le Châtelier Effect and Fracture Character of Deformed Mg-Li Alloys Investigated Using the Acoustic Emission Method

Abstract

IntroductionUnsatisfying ability to plastic deformation at elevated temperatures, mostly leading to the reduction of deformability and/or hot plasticity is characteristic for many metals and alloys [1][2][3][4]. The occurrence of hot-shortness phenomenon, followed by the fracture of intercrystalline character is an important reason for technological difficulties. Previous investigations of the authors of the paper [5][6][7][8][9] of mechanical properties of alloys with the application of AE method were devoted to the Mg-Li-Al alloys and related composites generally in the context of the method of intensive deformation processes leading to their excellent mechanical properties, such as great strength and plasticity or even superplasticity. On the other hand, the investigations of metal and alloy plastic instability using the AE technique were carried out [10][11][12] mainly in the context of basic aspects of PL effect, twinning or shear band in both poly-and single metal and alloy crystals. The fracture and strengthening properties of Mg-Li based alloys (and composites) were investigated, for example in [13][14][15][16], but without the use of AE method, which proved to be a very useful technique of material examination. In this work the results of the investigations of the correlation between the AE phenomenon, the plastic instability, induced by PL effect, twining or shear bands, and the both, intergranular and transcrystalline fracture of Mg4Li5Al and Mg4Li4Zn alloys subjected to tensile and compression tests at wide range of elevated temperatures are presented.Alloys based on magnesium with lithium, as the lightest ones from among the known metallic construction materials, are very attractive from the point of view of their application as the materials for light, yet durable constructions to be used in the automotive industry (e.g. car engine housings), light housings of computers and aerospace technology. The basic Mg-Li alloys exist in three phase areas. The hexagonal α phase appears in the concentration range of Li up to 4 wt.%. If the content of Li is more than 12 wt.% -the β phase of cubic lattice occurs. The alloys of Li content from 4% up to 12 wt.% form the α+β two-phase mixture. The mechanical properties of α phase are worse from that of the β phase, which is more plastic and thus reveals good machinability and weldability. Alloying additions, e.g. Al (or Zn) from 3% to 5%, slightly increase the density of the alloy, but lead to the precipitation of coherent particles of transition phase, θ-MgLi 2 Al/θ-MgLi 2 Zn, which additionally strengthens the matrix and leads to the improvement of mechanical properties [14]. The present paper addresses the optical microscopic, as well as TEM and SEM observations of the failure of samples after tensile and compression tests.

Archives of Metallurgy and Materials

“…The role of precipitations and the comparison of mechanical properties of some Mg-Li-Al and Mg-Li-Zn alloys were discussed in [2]. Ultra-fine-grained Mg alloys and composites were objects of investigation in [3] and [4].…”

Behaviour of Acoustic Emission in Deformation and Microcracking Processes of Mg Alloys Matrix Composites Subjected to Compression Tests

“…Moreover, some recent our publications [17][18][19][20] are related also to above mentioned questions, though especially they are referred to the problems of the influence of plastic instabilities on the fracture mechanisms in deformed metallic materials.…”