Phases magnesium alloys

Mg yA1 2> or P-(MgAl). Thus aluminum occurs in magnesium alloys both in soHd solution and as the intermediary intermetaUic phase. The latter is clear white and in slight rehef in poHshed and etched samples. In as-cast alloys, the hard phase occurs in massive form, but when precipitated from sohd solution a lamellar stmcture is formed similar to peadite in steel. When produced by aging at low temperatures, it appears as fine particles. 

Aluminum-copper-lithium alloys, 2 321 equilibrium and metastable phases, 2 322t S-Aluminum-copper-magnesium alloy, 2 318-320... 

Figure 4.17. The binary phase diagrams of the magnesium alloy systems with the divalent metals ytterbium and calcium (Ca is a typical alkaline earth metal and Yb one of the divalent lanthanides). Notice, for this pair of metals, the close similarity of their alloy systems with Mg. The compounds YbMg2 and CaMg2 are isostructural, hexagonal hP12-MgZn2 type.

Intergranular S.C.C. of magnesium alloys has also been reported, attributed to a grain-boundary phase of Mgi7Ali2 that causes galvanic attack of the adjacent matrix [8]. 

Table 3.1-2 Solubility data and intermetaUic phases in binary magnesium alloys [1.4]...

There are two main reasons for the poor corrosion resistance of many magnesium alloys (Makar and Kruger, 1990) - firstly, internal galvanic corrosion caused by second phases or impurities (Chapter XX in Emley, 1966) and, secondly, the quasi-passive hydroxide film on magnesium is much less stable than the passive films which form on metals such as aluminum and stainless steels. This quasi-passivity results in only poor pitting resistance for magnesium and magnesium alloys. 

Magnesium alloys are highly susceptible to galvanic corrosion. Galvanic corrosion is usually observed as heavy localized corrosion of the magnesium adjacent to the cathode (Froats et al., 1987). Cathodes can be external, e.g. other metals in contact with the magnesium, or internal, e.g. second or impurity phases. These two kinds of galvan-... 

The corrosivity of a magnesium alloy is governed by the corrosion reactions of the individual constituent phases of the alloy. If an alloy contains constituents that are very reactive with a particular environment, the alloy will often have low corrosion resistance in that environment. The reactions of pure magnesium are of particular interest. These reactions provide the basis for understanding the corrosion of magnesium alloys. 

Second phases have a pronounced influence on the corrosion of magnesium, because most elements affect the corrosion resistance of magnesium alloys only after they form second phases. 

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