Grain refinement mechanism and evolution of dislocation structure of Co-Cr-Mo alloy subjected to high-pressure torsion

Murat Isik, Mitsuo Niinomi, Huihong Liu, Ken Cho, Masaaki Nakai, Zenji Horita, Shigeo Sato, Takayuki Narushima, Hakan Yilmazer, Makoto Nagasako

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Abstract

Ultrafine-grained materials often possess superior mechanical properties owing to their small grain size. The high-pressure torsion (HPT) process is a severe plastic deformation method used to induce ultra-large strain and produce ultrafine grains. In this study, the grain refinement mechanisms in the Co-28Cr-6Mo (CCM) alloy, evolution of dislocation density as a result of HPT and its effects on mechanical properties were investigated. The dislocation density and subgrain diameter were also calculated by X-ray line profile analysis. The microstructure of the CCM alloy subjected to HPT processing (CCMHPT) was evaluated as a function of torsional rotation number, N and equivalent strain, ϵeq. Strain-induced γ→ϵ transformation in neighboring ultrafine grains is observed in CCMHPT processed at ϵeq= 2.25 and ϵeq= 4.5. Low-angle crystal rotation around the [110] fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCMHPT processed at ϵeq= 2.25 and ϵeq= 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8 × 1016 m-2 in γ phase and 3.8 × 1016 m-2 in ϵ phase, and maximum subgrain diameters, which are 21.2 nm in γ phase and 36 nm in ϵ phase, are achieved in CCMHPT processed at ϵeq= 9. HPT processing causes a substantial increase in the tensile strength and hardness owing to the grain refinement and a significant increase in the volume fraction of ϵ phase and dislocation density.

Original languageEnglish
Pages (from-to)1109-1118
Number of pages10
JournalMaterials Transactions
Volume57
Issue number7
DOIs
Publication statusPublished - Jan 1 2016

All Science Journal Classification (ASJC) codes

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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    Isik, M., Niinomi, M., Liu, H., Cho, K., Nakai, M., Horita, Z., Sato, S., Narushima, T., Yilmazer, H., & Nagasako, M. (2016). Grain refinement mechanism and evolution of dislocation structure of Co-Cr-Mo alloy subjected to high-pressure torsion. Materials Transactions, 57(7), 1109-1118. https://doi.org/10.2320/matertrans.M2016052