Microstructure of Ti-4Fe-7Al alloy quenched in a salt bath after solution treatment

Masataka Ijiri, Akira Okumura, Takafumi Ishikawa, Kenji Kadowaki, Yoshito Takemoto

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

The Ti-4Fe-7Al alloy exhibits some interesting phenomena upon tempering. The noteworthy features are rapid hardening and shape change in a U-shaped specimen with heating, which are caused by the β(bcc)→α? orthorhombic) transformation at elevated temperature. The transformation behavior on the TTT (time-temperature-transformation) diagram estimated from the age hardening at various temperatures indicated a C-curve with a nose at 450°C, which means that the transformation occurs through a thermal activation process. However, STEM-EDS analysis of α? revealed that the transformation had occurred without atom diffusion. In this study, every solution treatment was carried out at 1050°C for 30 min under a vacuum, then the specimen was quenched into a salt bath of 200-550°C and held for a certain period prior to water quenching. These treated specimens were compared with the quenched and tempered specimens with respect to the microstructure and the hardening behavior. The hardness of the specimens held at 200-500°C (30 s) increased with increasing holding temperature. The maximum hardness, 610Hv, which was almost the same as that of a tempered specimen, was obtained at around 450°C. The microstructure of the specimen held at 450°C (30 s) showed a common morphology composed of acicular martensite variants, except for the size. Because the several variants would cancel the transformation strain of each other, no severe unevenness of the surface was generated, unlike the tempered specimen. On the other hand, the hardness of the specimen held at 550°C (30 s) decreased drastically owing to the phase separation to α + β, and the TiFe precipitates were generated from β upon the extension of the holding time. By the TEM observation of the electropolished thin foil of the specimen held at 450°C (30 s), unreasonable microstructure that was different from the XRD result was obtained. Just after the electropolishing, the SAD patterns showed that the α?-struc-ture has a commensurate relation with the β-lattice. However, α? turned spontaneously into an incommensurate relation after one year. On the other hand, TEM observations of the foil prepared by FIB (focused ion beam) were consistent with the XRD result. It was suggested that the hydrogen absorption during electropolishing caused a structural transition of α? martensite.

Original languageEnglish
Pages (from-to)691-696
Number of pages6
JournalNippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals
Volume80
Issue number11
DOIs
Publication statusPublished - 2016

Keywords

  • Electropolishing
  • Hydrogen absorption
  • Variant
  • α″-Martensite transformation

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Metals and Alloys
  • Materials Chemistry

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