uh, baz seems to have an important point (just did a quick search and posting what i found... didn't read the whole article
http://www.keytometals.com/Article45.htm):
Depending upon the composition, magnesium alloys begin to melt at a temperature in the range of about 685 to 1200F (360 to 650C). The common alloys begin to soften and weaken appreciably on exposure to temperatures as low as 200F (95C). However, some special compositions have been recently developed which maintain yield and tensile strength quite well at temperatures up to 400F (205C) or higher.
The usual commercial magnesium alloys of the aluminum-zinc (manganese) type are relatively stable up to about 300F (150C) and may be used for some applications below that temperature. Solution heat-treated castings and hard-rolled sheet in the usual alloys are unstable above 300F (150C) and are not suitable for use at elevated temperatures.