Composition and Performance of an Improved Magnesium AS41 Alloy

Calculated phase diagrams, iron tolerance limits, and corrosion of Mg-Al alloys

“…Hillis and coworkers using AZ91, 6 AM60, 12 and AS41. 10 These experiments demonstrated that high-purity alloys could be produced from low-purity alloys by control of the casting temperature. For the AZ91 experiments, they used ~40 kg heats of high-purity AZ91-base alloys containing ~9%Al, 0.5%Zn, ~390 ppm Fe, <10 ppm Ni and <100 ppm Cu.…”

“…Some useful insights are gained regarding the mechanism of the iron tolerance limit for the die-cast (DC) and gravity-cast (GC) magnesium alloys AZ91-DC and AZ91-GC, 6 AM60-DC, 12 AS41-DC, 10 and AE42-DC. is the alloy-dependent iron tolerance limit which depends on the manganese concentration and on the alloy (i.e., the weighting factor X is alloy-dependent, particularly dependent on alloy aluminum content, as shown in Table I).…”

“…Figure 4 presents a pseudo-binary section calculated through the Mg-AlMn-Fe phase diagram at 0.4%Mn and 0.02%Fe; this section was calculated to try to understand the controlled casting experiments (described in the prior section on high-purity castings) carried out by Hillis and co-workers with AZ91, 6 AM60, 13 and AS4. 10 Figure 4 indicates that cooling of an alloy containing 6% aluminum causes initially the precipitation of an iron-rich BCC phase labeled BCC_B2. Between ~675 and ~670ºC there is a three-phase region of liquid + BCC_B2 + Al 8 Mn 5 .…”

“…Between ~675 and ~670ºC there is a three-phase region of liquid + BCC_B2 + Al 8 Mn 5 . Following the experiments of Hillis and co-workers 6,10,13 it is assumed that the BCC_B2 phase settles out as the cast is slowly cooled and that the alloy as-cast at 670ºC contains no BCC_B2 phase but only magnesium liquid + Al 8 Mn 5 . If it is assumed that the phase Al 8 Mn 5 is passive (i.e., it is no more effective as a cathode than pure magnesium) then a 6% aluminum alloy cast at 670ºC has an iron content below the iron tolerance limit and would be expected to show a low corrosion rate.…”

Magnesium and Magnesium Alloys

“…Zinc makes the Mg alloy electrochemically more noble, thereby minimizing the corrosion rate [59]. Silicon is intentionally added to only the AS alloys, to combine with Mg, forming Mg 2 Si, which precipitation strengthens the alloy and is relatively innocuous to the corrosion behavior.…”