| The uppermost layer of a type 304 stainless steel for use in an ultrahigh vacuum system was characterized by an atom-probe. Specimens were prepared from a cold-worked wire heated at 1000°C for 5 min in vacuum. After heating at 450°C, 600°C and 800°C for 5 min in the atom-probe, surface compositions were determined layer by layer. The surface Cr content was increased to 30 at.% in the 450°C heating and to 50 at.% in the 600°C and 800°C heating. The Cr segregated zone was limited to one or two atomic layers in each case. This was followed by a few atomic layers of a Cr depleted zone with a Cr content of nearly 0 at.% in the 450°C heating. By raising the heating temperature to 600°C and 800°C, this Cr depleted zone was disappeared. Beneath the Cr depleted layers, the Cr content was nearly the same value as in the bulk. In the grain boundary, after heating at 600°C, (Cr+Fe)23C6 and (Cr+Fe)7C3 precipitates were observed. |
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