Stress Relaxation in AX41 Magnesium Alloy Studied at Elevated Temperatures

Abstract

For recent years, research and development of magnesium alloys have shown that the Mg-based alloys have great potential for applications as the lightweight materials. Among them, the Mg-Al-Ca alloys exhibit good resistance due to the presence of a heat-stable phase. During plastic deformation in a certain range of temperature and strain rate, different micromechanisms may play important role. The analysis of deformation microstructures has shown that one should consider dislocation-based mechanisms in order to explain the deformation behaviour. It is widely accepted that the resolved shear stress s necessary for the dislocation motion in the slip plane can be divided into two components:where s i is the (internal) athermal contribution to the stress, resulting from long-range internal stresses impeding the plastic flow.where G is the shear modulus, a 1 is a constant describing interaction between dislocations, b is the Burgers vector of dislocations and q t is the total dislocation density. The effective shear stress s* acts on dislocations during their thermally activated motion when they overcome short range obstacles. The mean velocity of dislocations v is connected with the plastic shear strain rate by the Orowan equation:where q m is the density of mobile dislocations. The dislocation velocity (the plastic shear strain rate) is controlled by obstacles (their strength, density) and it depends on temperature and the effective shear stress. In polycrystalline materials, the resolved shear stress s and its components are related to the applied stress r and its corresponding components by the Taylor orientation factor W: r = Ws. A simple relation between the resolved shear strain rate and strain rate is c=We.The most common equation used in describing the average dislocation velocity as a function of the effective stress is an Arrhenius type. The plastic strain rate _ e for a single thermally activated process can be expressed as:where _ e 0 is a pre-exponential factor containing the mobile dislocation density, the average area covered by the dislocations in every activation act, the dislocation Burgers vector, the vibration frequency of the dislocation line, and the geometric factor. T is the absolute temperature and k is the Boltzmann constant. DG(r * ) is the change in the Gibbs free enthalpy depending on the effective stress r * = r ap Àr i and its simple form isHere DG 0 is the Gibbs free enthalpy necessary for overcoming a short range obstacle without the stress and V = bdL is the activation volume where d is the obstacle wide and L is the mean length of dislocation segments between obstacles. It should be mentioned that L may depend on the stress acting on dislocation segments. [**] This work is a part of the research plan MSM 1M2560471601that is financed by the Ministry of Education, Youth and Sports of the Czech Republic. 100ºC ε 0.00 0.02 0.04 0.06 0.08 0.10 σ (MPa) 50 100 150 200 250 σ ap σ R σ i Fig. 1. A part of the true stress-true strain curve at 100°C. The points on the curve indicate the stresses...

Influence of strain rate on deformation behaviour of an AX52 alloy processed by equal channel angular pressing (ECAP)

“…Theory of the thermally activated processes occurring during plastic deformation predicts that the activation volume is inversely proportional to the effective stress component σ*. (This component may be estimated for example in the stress relaxation experiments [34]). Kocks et al formulated an empirical relation for the dependence of the Gibbs enthalpy ∆G on the effective stress σ* in the following form [35]:…”

Effect of aging treatment on the microstructure, creep resistance and high-temperature mechanical properties of Mg–6Zn–3Cu alloy with La- and Ce-rich rare earth additions

“…However, poor mechanical properties at elevated temperatures, especially creep resistance, have restricted their uses [1][2][3]. This has been attributed to the low thermal stability of their microstructural constituents, which cannot withstand the imposed high temperatures [4][5][6]. To address these disadvantages, attempts have been made to develop new Mg base alloys having structural stability at high temperatures.…”

Comparison of the effects of La- and Ce-rich rare earth additions on the microstructure, creep resistance, and high-temperature mechanical properties of Mg–6Zn–3Cu cast alloy

“…Mg-Al alloys are the most common types of magnesium alloys, which are widely used for their moderate strength at room temperature [1][2][3]. This has been attributed to the low thermal stability of their microstructure containing the net shape Mg 17 Al 12 precipitates along grain boundaries [4][5][6]. Accordingly, attempts have been made to develop new Mg base alloys having structural stability at high temperatures.…”