Analysis of surface structure of titanium alloy nitride by slow positron beam

Takeda, Yusuke; Iida, Kiyoshi*; Sato, Shinji*; Matsuo, Tadatoshi*; Nagashima, Yasuyuki*; Okubo, Nariaki   ; Kondo, Keietsu  ; Hirade, Tetsuya  

The impact stress exfoliates a Nitride surface layer on Titanium alloy prepared in some conditions. So we prepared specimens in two different conditions, (1) 810C, 600min and (2) 850C, 720min, and performed depth profile analysis of their surfaces with Doppler broadening measurements of positron annihilation -rays (DB) that is sensitive for defects by Slow Positron beams. Although the specimens (2) is harder than (1) according to the Vickers Hardness test, DB results indicated that defect layer of specimen (1) is thicker than (2). It means that the defect layer thickness was not the cause of surface hardening. Furthermore, chemical elements at the positron annihilation sites could be analyzed qualitatively by DB and the change of the composition appeared in deeper region than the defect layer. The defect layer depth does not match the depth where chemical composition changes by DB appeared. TEM and EDS observation showed different tendency from the results by DB. It is because positrons probe specific sites, and the combination of these methods will be a strong tool to investigate surface structures.