| Summary: | Structural and kinetic aspects of precipitation in the systems Mg-Th and Mg-Nd , with various ternary additions, have been studied, using electron microscopy and electrical resistivity techniques.
The sequence of precipitation in the Mg-Th alloy is solid solution → β" → β.
Small additions of manganese and silver have little effect on the precipitation process but additions of zirconium and zinc alter the process significantly. The β” reaction is completely suppressed by an addition of 0.3% zinc and a phase not present in the binary system, β’ Mg2Th, is formed as two polymorphs in the zirconium containing alloy.
The sequence of precipitation in the Mg-Nd system is completely changed by the zinc addition. Alloys containing Mg 2.8% Nd 1.3% Zn have the following precipitation sequence
Solid solution → low temperature reaction → plates γ”││ (0001) → rods on γ (0001) in <1120> & <1010>
The low temperature reaction has not been elucidated but it occurs with an activation energy approximating to that of vacancy migration in magnesium, and may possibly be associated with short range order.
The structures of these various alloys have been correlated with their creep and mechanical properties. In particular, the creep properties of the Mg-Nd-Zn alloy have been shown to be superior to those of the binary Mg-Nd alloy and the improvement has been attributed to the γ" phase restricting dislocation motion on the {1011} planes.
|
|---|