Preageing of magnesium alloys

Certain magnesium alloy systems can obtain superior mechanical strength through age hardening. However, conventional ageing heat treatments for these alloys are rarely more complex than a single isothermal hold - especially when compared to those of aluminium alloys - so age hardenable magnesium alloys may not be achieving their full strengthening potential. The use of preageing, where a lower temperature heat treatment is utilised before a hotter secondary ageing step, has proven successful in boosting the strength of magnesium alloys previously, but these trials are few in number, and the testing conditions are limited. In this work, a wide range of preageing temperatures and times were trialled on commercial and experimental magnesium alloys to determine the effectiveness of the strengthening technique. The results showed that preageing can produce a significant boost in hardness, can reduce total ageing times, and can provide a degree of mechanical property customisation through control of precipitate habit plane and morphology. However, the effectiveness of the technique is alloy-system dependent, where the fundamental precipitate nucleation and phase evolution is a key contributor to the success of preageing; as is the energy barrier to nucleation in an alloy system, whether that be controlled by alloy chemistry or thermomechanical processing. The results are discussed in the context of alloy design and industrial processing.