Formation of nanocrystalline β structure in metastable beta Ti alloy during high pressure torsion

The role played by stress induced martensitic transformation

A. Zafari, X. S. Wei, W. Xu*, K. Xia

*Corresponding author for this work

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

Abstract High pressure torsion was conducted to obtain nano-sized β grains in a metastable Ti-5553 alloy. Much finer grains of <50 nm were achieved, compared to >100 nm in a stable Ti-20 wt.% Mo alloy. The more effective grain refinement was attributed to stress induced martensitic transformation in the former, leading to the formation of thin α" plates which divide β grains into smaller domains. Further deformation resulted in a reverse α" to β transformation with decreasing α" sizes, generating a completely nano β grain structure at very large straining. A detailed description of the β grain refinement mechanism is provided. The reverse transformation is shown to be caused by the significantly increased free energy below a critical α" size of ∼10 nm, consistent with experimental observations. It is also calculated that extremely high energies were required for the formation of α" in nano-sized β grains, making further martensitic transformation impossible. It is concluded that the stress induced martensitic transformation and the subsequent reverse transformation are critical to producing nano-grained metastable β Ti alloys.

Original languageEnglish
Article number12227
Pages (from-to)146-155
Number of pages10
JournalActa Materialia
Volume97
DOIs
Publication statusPublished - 15 Sep 2015
Externally publishedYes

Keywords

  • Grain refinement
  • High-pressure torsion
  • Martensitic phase transformation
  • Severe plastic deformation (SPD)
  • β titanium alloys

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