Magnesium creep (elongation under load) is the time-dependent strain that takes place under a given load. There are generally three distinct stages of magnesium creep:
- Primary—the creep strain that occurs at a diminishing rate.
- Secondary—the creep strain that shows a minimum and almost constant rate.
- Tertiary—the creep strain that exhibits an accelerated rate, usually leading to rupture.
Creep in AZ91
The creep behavior of the most commonly used magnesium die cast alloy, AZ91, at room temperature and low stresses is 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 cast alloys at temperatures exceeding 120°C has resulted in the introduction of alloys containing silicon or rare-earth metals.
The following graphs illustrate the properties of magnesium alloys at varying temperatures and pressures:
- Magnesium Alloys Creep Curves 100C and 100MPa
- Magnesium Alloys Creep Curves 100C and 50MPa
- Magnesium Alloys Creep Curves 150C and 30MPa
- Magnesium Alloys Creep Curves 150C and 50MPa
- Magnesium Alloys Creep Curves 200C and 30MPa
- Magnesium Alloys Creep Curves 50C and 100MPa
- Magnesium Alloys Creep Curves 50C and 50MPa