I have noticed when examining performance of various heating elements in different atmospheres that in a vacuum maximum temperatures are lower than the in air temperatures. Why is this?
(Typical heating element alloys are Nickel-Chromium and Iron-Chromium-Aluminum)
As Olin Lathrop says below the evaporation of the element becomes significantly increased in a vacuum. I found this interesting quote in an old book:
This idea of working in a vacuum brought with it new difficulties as regards the construction of a furnace. The ordinary platinum resistance furnace proved of little value, as, owing to diminution of pressure, the volatility of the platinum foil, at high temperatures, was increased to such an extent that it was destroyed in a short time.