AJ alloys (AJ52 and AJ62) are die castable alloys of magnesium that have good creep resistance at high temperature. They contains magnesium, aluminum, and strontium.^[1]

In the names the 'J' refers to Strontium.

Alloy   Mg     Al%  Sr%  Mn%  AJ62  Balance  6    2    0.34   [2] AJ52  Balance  5    2    0.4    [3] 

AJ52 has higher creep resistance, and AJ62 has better castability. Both are used in the BMW magnesium aluminum composite engine block.^[4]

References

1. ↑
2. ↑ http://www.archivesmse.org/vol28_6/2865.pdf. MSE AJ62
3. ↑ http://iweb.tms.org/Mg/magtech2002/018.pdf
4. ↑