Grain Refinement Mechanism and Evolution of Dislocation Structure of Co-Cr-Mo Alloy Subjected to High-Pressure Torsion


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Isik M., Niinomi M., Liu H., Cho K., Nakai M., Horita Z., ...More

MATERIALS TRANSACTIONS, vol.57, no.7, pp.1109-1118, 2016 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 57 Issue: 7
  • Publication Date: 2016
  • Doi Number: 10.2320/matertrans.m2016052
  • Journal Name: MATERIALS TRANSACTIONS
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.1109-1118
  • Yıldız Technical University Affiliated: Yes

Abstract

Ultrafine-grained materials often possess superior mechanical properties owing to their small grain size. The high-pressure torsion (IIPT) 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, epsilon(eq). Strain-induced gamma ->epsilon transformation in neighboring ultrafine grains is observed in CCMHPT processed at epsilon(eq) = 2.25 and epsilon(eq) = 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 epsilon(eq) = 2.25 and epsilon(eq) = 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8 x 10(16) m(-2) in gamma phase and 3.8 x 10(16) m(-2) in epsilon phase, and maximum subgrain diameters, which are 21.2 nm in gamma phase and 36 nm in epsilon phase, are achieved in CCMHPT processed at epsilon(eq) = 9. IIPT 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 epsilon phase and dislocation density.