Creep (elongation under load) is defined as the time-dependent strain that takes place under a given load. There are generally, three distinct stages of creep:

• Primary - the creep strain that occurs at a diminishing rate
• Secondary - the creep strain that show a minimum and almost constant rate
• Tertiary - the creep strain that exhibits an accelerated rate, usually leading to rupture.

The creep behavior of the most commonly used magnesium die casting alloy, AZ91, at room temperature and low stresses is well described by the relationship:

where e is the steady-state creep rate, A is a constant, and the stress exponent n equals 4.6. AZ91 is seldom used at high temperatures because it loses much of its strength above approximately 120°C (250°F). Efforts to improve the creep strengths of magnesium die casting alloys at temperatures exceeding 120°C have resulted in the introduction of alloys containing silicon or rare-earth metals.

The following graphs illustrate the properties of magnesium alloys at temperature and pressure: