Evaluation of texture weakening and microstructural evolution in an Fe–10Ni–7Mn martensitic steel severely deformed by six turns of high-pressure torsion

Abstract

The effect of high-pressure torsion (HPT) on textural components and microstructural evolution of an Fe–10Ni–7Mn steel was investigated. Six turns of HPT, with a rotation rate of 1 rpm and with a pressure of 6.0 GPa, were applied to a solution annealed steel. Microstructural and textural evolution, including dislocation characteristics and grain size/misorientation measurements, were accomplished using electron backscatter diffraction (EBSD). The results showed that a fully martensitic structure, containing a high density of dislocations, was developed during the solution annealing stage and subsequently the application of six turns of HPT process converted the low-angle grain boundaries (LAGBs) of the solution annealed condition into high-angle grain boundaries (HAGBs). Thus, the sub-grain boundaries established in the solution annealed condition were altered into very sharp HAGBs. The grain size was also decreased from ∼ 25 μm in the solution annealed situation to ∼210 nm in the severely deformed condition of six-turn high-pressure torsion state. Yield strength, and tensile strength were increased from 765 MPa, and 840 MPa in the solution annealed case to 1890 MPa, and 2230 MPa in the six-turn HPT-processed sample, respectively. Under such a condition, ductility was decreased from 15.2% to 6.1% in the severely deformed state. Applying six turns of HPT developed a predominantly {111}//ND texture. However, the overall texture intensity of the HPT-processed sample was weakened to a value of about one-quarter of the as-quenched solution treated sample. This was related to dynamic recrystallization and grain subdivision as well as a dislocation density reduction in the HPT-processed sample. The texture components of strong Copper ({112}<111>), S ({123}<634>) and some average intensity Cube {100}<0001> were observed in the {100} pole figure in the solution annealed condition. As well, γ-fibre (<111>//ND) and β-fibre were seen in this condition. However, {110}<uvw> fibres were mostly developed in the six-turn sample with the texture components of E1{011‾}<111>, E2{01‾1}<111>, J1{01‾1}<2‾11>, J2+{11‾0}<1‾1‾2>, D2{1‾1‾2}<111> , D1{112‾}<111> and F {110}<001>. In addition, the Cu{112}⟨111⟩ component was significantly diminished. This weakened Cu{112}⟨111⟩ component would have caused enhancement of the mechanical properties, particularly the formability and toughness, as previously reported by the current authors. Finally, it was found that applying severe plastic deformation (SPD) in the form of HPT reduces the effect of martensitic texture in the solution treated steel.