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.
Highlights
Academic Editor: Alexander VorozhtsovReceived: 29 January 2021Accepted: 11 March 2021Published: 13 March 2021Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.In the stress-relaxation (SR) test, the samples are deformed up to a certain stress, σ(0), the testing machine is abruptly stopped and the subsequent decrease in stress, σ, is recorded depending on time, t [1,2,3,4]
Where ε is the strain rate; ε0 is a model constant depending on the dislocation density, vibration frequency, and the area swept by a dislocation segment after the successful activation; and G* is the Gibbs free energy of activation, which depends on the effective stress, σ∗, necessary to overcome local obstacles for dislocation motion: σ* = σa − σi Licensee MDPI, Basel, Switzerland
The preform consisting of short commercial Saffil fibers
Summary
Academic Editor: Alexander VorozhtsovReceived: 29 January 2021Accepted: 11 March 2021Published: 13 March 2021Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.In the stress-relaxation (SR) test, the samples are deformed up to a certain stress, σ(0), the testing machine is abruptly stopped and the subsequent decrease in stress, σ, is recorded depending on time, t [1,2,3,4]. Successive SR tests may be used to study different aspects of plastic deformation [5,6,7,8]. The strain rate of plastic flow, ε , controlled by thermal fluctuations can be written as: G ∗ (σ∗ ) ε = ε0 exp − (1). KT where ε is the strain rate; ε0 is a model constant depending on the dislocation density, vibration frequency, and the area swept by a dislocation segment after the successful activation; and G* is the Gibbs free energy of activation, which depends on the effective stress, σ∗ , necessary to overcome local obstacles for dislocation motion: σ* = σa − σi (2)
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