NUCLEATION MECHANISM OF DISCONTINUOUS PRECIPITATION IN MG-AL ALLOYS AND RELATION WITH THE MORPHOLOGY

The nucleation rate of discontinuous precipitation in Mg-Al has been measured as a function of temperature, initial grain size and solute content. From these measurements, it appears that at high temperatures (T greater-than-or-equal-to 220-degrees-C) all precipitation nodules nucleate via Fournelle and Clark's mechanism, whereas at lower temperatures (T approximately 140-degrees-C), at least one of the mechanisms identified by Tu and Turnbull or Purdy and Lange is also active. The proportion of double seam nodules determined by optical microscopy decreases from more than 1/2 to 0 when the temperature increases. In the low temperature domain, this behaviour is in agreement with that predicted by Baumann, Williams and Michael.