Physics of Microyield of Magnesium Alloys with Titanium

Abstract

Softening effect of grain-boundary sliding (GBS) in h.c.p. α-Mg polycrystals arises in the stress–temperature ranges of interest for automotive applications. Furthermore, decomposition of the oversaturated α-Mg solid solution is accompanied by apparition of a microstructure, which is unstable during a creep. These two main effects deteriorate most likely the microyield resistance and long-term strength of magnesium alloys of Mg–Al–Mn and Mg– Al–Zn systems. According to the data of internal-friction measurements, the introduction of Ca suppresses GBS that gives rise to the GB strengthening of the ternary Mg–Al–Ca system. An introduction of small additions of Ti (about 0.1–0.2%) causes significant solid-solution strengthening due to solute-atmosphere retardation of moving dislocations by the Cottrell mechanism with binding energy of about 0.27 eV. The idea of dynamical self-strengthening of α-Mg matrix during the creep is also supported by precise x-ray diffractometry. As deduced, observed high microyield resistance and excellent long-term strength of the magnesium alloys of Mg–Al–Ca system containing inexpensive additions of Ti are due to minimizing recovery and softening effects at elevated temperatures at the expense of thermal phase-composition and concentration stabilization of α-Mg solid solution. Thermally-activated dislocation relaxation accommodated by the diffusion of alloying elements (Al, Ca, Ti in their best combination) should be considered as dominant (rate-controlled) microyield mechanism, which provides creep strain rate of 10^−9–10^−10 1/s. Its activation increases essentially (by 150–200°C) the heat resistivity of new experimental alloys.

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