Plastic instability is reported in hexagonal close-packed (HCP) LA41 magnesium alloy during tensile tests. Serration amplitude associated with plastic instability is measured to increase with increasing strain and decrease with increasing strain rate. The model of dynamic strain aging (DSA) is proposed to be the controlling mechanism. Moreover, it is reported that annealing could reduce the flow instability, which has potential practical applications.
Plastic instability associated with the production of Portevin-Le Chatelier (PLC) bands, which is also know as serrated flow or jerky flow, is a topic of current theoretical and engineering interest. Lots of studies in this area are on aluminum alloys; however, due to soaring research on magnesium alloys, similar results have been reported in Mg-Ag [
Microscopically, plastic instability was believed to result from intensive interactions between mobile dislocations and solutes atoms. Cottrell [
Macroscopic features, such as spatiotemporal localization of plastic deformation and propagation of deformation bands, were also investigated systematically. Experimentally, Chmelík et al. [
This paper is intended to report the plastic instability in an HCP magnesium-lithium alloy. The dependence of serration amplitude, which is a major parameter tailoring plastic instability, on strain and strain rate will be discussed in details. Mechanical properties of alloys of the same family will also be compared. It will be shown that plastic instability could be removed through additional annealing treatment, and possible explanations for the disappearance of serrations will also be suggested.
Three types of Mg-Li alloys, LA41, LA40, and LA21, were investigated in this study. The chemical compositions of the three alloys are listed in Table
Chemical compositions of the three alloys under investigation (wt.%).
Li | Al | Mg | |
---|---|---|---|
LA41 | 4.32 | 0.97 | Balance |
LA21 | 2.43 | 0.89 | Balance |
LA40 | 4.06 | — | Balance |
Tensile specimens have the dimension of 5 × 25 mm by diameter and length, respectively. Uniaxial tensile experiments were performed on an MTS-880 machine at 293 K. Extensometer was attached to gauge length in order to gather accurate strain values. Care was taken to minimize the machine effects. Stress-strain curves were recorded and plotted at a speed of 10 points per second.
The reagent used for microstructure revelation was composed of 3 g picric acid, 10 mL acetic acid, 10 mL distilled water, and 50 mL ethyl alcohol. Microstructure was characterized by an environmental scanning electron microscope (ESEM, Philips XL30 FEG) under the acceleration voltage of 10 KV. Transmission electron microscope (TEM, JEM 2000FXII) samples having diameters of 3 mm were thinned in a solution of 30 mL nitric + 70 mL methanol in a twin jet electropolisher to the final thickness of 50 microns. TEM examinations were conducted at the voltage of 200 KV.
Figure
Stress-strain curves of the as-received LA41 under different strain rates at 293 K. The inset shows the locally magnified stress-strain curve at the strain rate of 6.66 × 10−4 s−1.
The inset of Figure
With this definition of serration amplitude, accurate values of
Variations of serration amplitude with strain and strain rate of the as-received LA41.
Current findings are in good agreement with the characteristics of the DSA model [
Figure
Stress-strain curves of three alloys under the strain rate of 6.66 × 10−4 s−1 at 293 K.
It is of great importance to note that serrations are absent in LA21. In a separate paper [
It has been shown that the solute concentration around arrested mobile dislocations is dependent on both strain and strain rate, and the dependence can be described by the following equation [
Another point that needs to be noted is that
Tensile tests were also performed on the as-annealed LA41 alloy, of which the stress-strain curve is shown in Figure
Comparison of stress-strain curves of the as-received and the as-annealed LA41 under the strain rate of 6.66 × 10−4 s−1 at 293 K.
Figure
SEM micrographs of (a) the as-received LA41 and (b) the as-annealed LA41.
Bright field (BF) morphology of the as-annealed LA41.
The existence of annealing-induced twins might exert significant influence on the as-annealed LA41. It is believed that they might act as barriers to mobile dislocations, disabling the DSA process to a large extent. Noting that the annealing temperature is well above 0.4
Plastic instability was observed in HCP LA41 magnesium alloy in the strain rate range from 1.33 × 10−4 s−1 to 6.66 × 10−4 s−1 during tensile tests. Serration amplitude was measured to increase with accumulated strain but decrease with increasing strain rate. The dynamic strain aging model was applied to account for the observed phenomena. It was also demonstrated that additional annealing heat-treatment could remove the plastic instability.
Financial support from National Science Foundation of China (Grant no. 50371089) is greatly acknowledged.