Studying the Thermally Activated Processes Operating during Deformation of hcp and bcc Mg–Li Metal-Matrix Composites

Abstract

Stress-relaxation tests were performed during plastic deformation at room temperature of three magnesium Mg–Li alloys reinforced with 10 vol% of short Saffil fibers. For comparison, the composite with the Mg matrix was studied. The time dependencies of the stress decrease were analyzed with the aim to determine the activation volume and the main types of thermally activated processes occurring during plastic flow. The Mg4Li matrix alloy exhibited the hcp structure, while the composite with the Mg12Li matrix alloy had the bcc structure. The third alloy, Mg8Li, combined both phases, hcp and bcc. The stress acting in the matrix was divided into two components: the internal stress and the effective stress. Activation volume and stress-sensitivity parameters were determined as a function of effective stress and strain. While the values of the activation volume depending on the effective stress lay on one “master” curve, the strain dependence was different for all materials. The main thermally activated process in the hcp structure was the dislocation motion in the noncompact planes, while in the bcc structure, massive recovery processes connected with an increase in dislocations were identified.

Thermoactivated Dislocation Motion in Rolled and Extruded Magnesium: Data of the Low-Temperature Acoustic Experiment

“…In contrast to materials with a cubic crystal structure, Mg has several slip systems with very different critical resolved shear stresses. It can lead to competition between different elementary mechanisms of plastic deformation at low temperatures, most of which are thermally activated at moderately low temperatures [14]. Present paper reports dynamic dislocation effects observed in amplitude-independent and amplitude-dependent regions of ultrasonic vibrations in commercially pure magnesium polycrystals subjected to different types of plastic deformation.…”

“…In Figure 6a, the dependences of the activation energy U(ε 0 ) for five sections δ H (T,ε 0 ) = const are shown. Solid lines in Figure 8a shows the phenomenological stress dependences of the activation energy [14,21]:…”

“…This situation is caused by higher dislocation density in the rolled sample. The presence of dislocations in crystals leads to a decrease in the Young's modulus, i.e., a quasi-static modulus defect may be observed, which in the low-frequency limit is determined by the dislocation density Λ and the average length Lc of dislocation segments [14]:…”

“…Consequently, these two types of treatment result in different level of plastic deformation, different dislocation density as well as in difference in microstructures and textures. Elastic and inelastic properties of magnesium and magnesium alloys have been previously studied mostly in the temperature range near and above room temperature [5][6][7][8][9][14][15][16][17][18]. To fill to some extent this gap, in the present work linear and nonlinear dislocation acoustic effects were studied in the temperature range 51-300 K.…”

Effect of Temperature on the Complex Modulus of Mg-Based Unidirectionally Aligned Carbon Fiber Composites

“…The strength and elastic modulus of magnesium alloys are low. The properties of magnesium alloys can be significantly enhanced via the addition of particles, fibers, nanofibers, and nanotubes to form metal matrix composites (MMC) [ 5 , 6 , 7 , 8 , 9 , 10 ]. The Young’s modulus of magnesium alloys can be increased via the particles, whiskers, and fibers.…”