Abstract
Creep behaviour of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was analysed over a wide temperature range from 343 to 673 K at stresses from 2 to 200 MPa in order to investigate the mechanisms governing the creep of this alloy. The stress dependences of the creep rate ε ranging from 10^-9 to 10^-3 s^-1 were evaluated by means of compressive creep experiments with stepwise loading. The stress dependence cannot be described by a simple power-law or exponential relationship. The best description is found with the modified Garofalo’s sinh relationship ε = A_M(sinh[B_M(σ-σ_th)])ⁿ with a natural exponent n = 5. This description well reflects the apparent threshold creep behaviour of the alloy observed at high temperatures and low stresses. The parameters of this relationship support the assumption that lattice diffusion plays a decisive role in creep mechanisms under all experimental conditions. Most likely, a climb controlled creep mechanism governs at lower stresses and higher temperatures, and a glide-controlled mechanism dominates at higher stresses and lower temperatures. The threshold stress σ_th is connected with precipitation of a very stable (Mg,Al)₂Ca phase distributed in a discontinuous skeleton at grain boundaries as well as within the grain interior. The level of the threshold stress is determined by a mechanism with a relatively low activation energy Q ≅ 20 kJ/mol.

Key words
magnesium alloys, creep, threshold behaviour, threshold stress concept, mechanisms of creep.

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