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

Abstract

Ultra ne-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 ultra ne grains. In this study, the grain re nement 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 pro le analysis. The microstructure of the CCM alloy subjected to HPT processing (CCM HPT ) was evaluated as a function of torsional rotation number, N and equivalent strain, ε eq . Strain-induced γ→ε transformation in neighboring ultra ne grains is observed in CCM HPT processed at ε eq = 2.25 and ε eq = 4.5. Low-angle crystal rotation around the [110] fcc direction occurs in different locations in the same elongated grain neighboring ultra ne grains, which suggests the formation of low-angle grain boundaries in CCM HPT processed at ε eq = 2.25 and ε eq = 4.5. Two possible grain re nement mechanisms are proposed. The maximum dislocation densities, which are 2.8 × 10 16 m −2 in γ phase and 3.8 × 10 16 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 CCM HPT processed at ε eq = 9. HPT processing causes a substantial increase in the tensile strength and hardness owing to the grain re nement and a signi cant increase in the volume fraction of ε phase and dislocation density.

High‐Pressure Torsion: Experiments and Modeling

“…In addition to the dislocation mechanisms for ultrafine-grained microstructure formation, some other mechanisms of the grain refinement have been described in the literature. For example, Isik et al [41] reported that, in the case of Co-Cr-Mo alloy, the deformation-induced phase transformation (γ ! ε) contributes to grain refinement via the formation of ε platelets which subdivide the γ grains.…”

Optimization of Microstructure and Mechanical Properties of Co–Cr–Mo Alloys by High-Pressure Torsion and Subsequent Short Annealing

“…The elongation of CCM HPT at each rotation number shows a similar value of 1-2%. Therefore, even though the rotation of 90 degrees (N = 0.25) might seem to be rather small, the mechanical properties of CCM HPT at N = 0.25 is optimized compared to those of CCM HPT with higher rotation number 14,18) . In this study, the mechanical properties of short annealed CCM CR were not evaluated.…”

Diffusive and Displacive Phase Transformations Under High Pressure Torsion

“…Together with observation of grain refinement, a huge amount of experimental data is collected on the Vickers microhardness during and after SPD of copper, aluminium, titanium, magnesium alloys, steels etc. [10,17,29,[55][56][57][58][59][60][61][62][63][64][65][66][67]. Usually microhardness increases during SPD [10,17,29,[55][56][57][58][59][60][61][62][63][64][65][66][67].…”

“…Usually microhardness increases during SPD [10,17,29,[55][56][57][58][59][60][61][62][63][64][65][66][67]. It correlates also with tensile strength [13,24,25,57,60,[68][69][70]. It is true not only for rotation angle for HPT but also for number of passes during equal channel angular pressing (ECAP) [62].…”

“…The most prominent examples are iron, cobalt, titanium. For example, the Co-Cu alloys before HPT contain the (metastable at room temperature) fcc α-Co phase, but after HPT the ε-Co phase appears in the samples [1,59,60,122]. Even more interesting is the situation in Ti, Zr and Hf where the high-pressure ω-phase exists [123].…”