Nokeun Park

Novel thermomechanical processing methods for achieving ultragrain refinement of low-carbon steel without heavy plastic deformation

ABSTRACT Two novel two-step thermomechanical routes were developed to produce ultrafine-grained ferrite microstructures in a 10Ni–0.1C steel without high-strain deformation. Homogeneous ultrafine ferrite (UFF) structures having mean grain sizes down to 460 nm were fabricated by a total equivalent strain of 0.92 and exhibited high yield strength of 820 MPa with large uniform elongation of 10% and total elongation of 29%. The formation of UFF was attributed to two different phenomena, i.e. dynamic transformation from austenite to ferrite and dynamic recrystallization of ferrite. GRAPHICAL ABSTRACT IMPACT STATEMENT Ultragrain refinement and remarkable strength-ductility properties can be achieved by controlling dynamic transformation and recrystallization in a thermomechanically controlled process without high-strain deformation.

Combination of dynamic transformation and dynamic recrystallization for realizing ultrafine-grained steels with superior mechanical properties

Dynamic recrystallization (DRX) is an important grain refinement mechanism to fabricate steels with high strength and high ductility (toughness). The conventional DRX mechanism has reached the limitation of refining grains to several microns even though employing high-strain deformation. Here we show a DRX phenomenon occurring in the dynamically transformed (DT) ferrite, by which the required strain for the operation of DRX and the formation of ultrafine grains is significantly reduced. The DRX of DT ferrite shows an unconventional temperature dependence, which suggests an optimal condition for grain refinement. We further show that new strategies for ultra grain refinement can be evoked by combining DT and DRX mechanisms, based on which fully ultrafine microstructures having a mean grain size down to 0.35 microns can be obtained without high-strain deformation and exhibit superior mechanical properties. This study will open the door to achieving optimal grain refinement to nanoscale in a variety of steels requiring no high-strain deformation in practical industrial application.

Fabrication of fine recrystallized grains and their mechanical property in HPT processed pure magnesium

To clarify the grain size effect on mechanical property in fine grained pure Mg, mechanical properties of fully recrystallized Mg with fine grain sizes were investigated. The specimens having fully recrystallized microstructures with mean grain sizes of 2.8 μm and 7.8 μm were fabricated by HPT processing and subsequent annealing. The 2.8 μm specimen showed discontinuous yielding with a high yield stress and low strain hardening while the 7.8 μm specimen showed continuous yielding with a low yield stress and high strain hardening. The as-HPT processed specimen showed a larger ductility than that in the subsequently annealed specimens.

Systematic Approach to Clarify the Mechanism of Dynamic Transformation in Fe-6Ni-0.1C Alloy

In order to study about dynamic transformation phenomenon, Fe-6Ni-0.1C alloy was hot-deformed in uniaxial compression using thermo-mechanical simulator at various temperatures ranging from 600 to 1000 °C at various strain rates from 0.001 to 1 s-1 after austenitization. As the value of Zener-Hollomon (Z) parameter increased, softening of the stress from the empirically expected value, which was extrapolated from stresses deformed at low Z value, was observed through systematical analysis of peak stresses. It suggested that this softening phenomenon was attributed to the dynamic transformation, since ferrite is softer than austenite at elevated temperature. The microstructural observation also supported that ferritic transformation occurred during compressive deformation. Even above Ae3 temperature the softening of the peak stress of austenite was still observed, which implied that dynamic ferritic transformation might occur above Ae3 temperature.

Erratum to: Recrystallization Behavior of CoCrCuFeNi High-Entropy Alloy

Nokeun Park’s affiliation has been updated as shown in this erratum. The third sentence in the following text has been added to the Experimental Procedures section on page 1482 as clarification regarding the chemical composition of the whole sample: The material used in the current study was an equimolar CoCrCuFeNi alloy (20 mol pct for each element) that was cast with the pseudo float melting process that brings almost homogeneous distribution of each element. An as-cast HEA rod 10 mm in diameter and 60 mm in length was homogenized at 1373 K (1100 C) for 12 hours, followed by water cooling. The chemical composition of homogenized specimen was measured by wavelength-dispersive X-ray spectroscopy, and the atomic percent was Co21Cr22Cu22Fe21Ni14.

Microstructural Evolution of Ferrite Grains during Dynamic Transformation in 10Ni-0.1C Steel

The effect of strain on the microstructural evolution of ferrite grains during dynamic transformation was investigated using a 10Ni-0.lC steel uniaxially compressed at a strain rate of 10−2 s−1 and temperature of 520 ℃. The deformation of the ferrite formed at relatively early stages of dynamic transformation led to the formation of subgrains inside the ferrite grains. The misorientation between subgrains changed from low angles to high angles with increasing strain. The formation of equiaxed grains surrounded by high angle boundaries was confirmed, of which volume fraction increased with increasing compression strain. The results indicated that the grain refinement in the process was not only due to dynamic transformation but also due to the deformation and dynamic recovery/recrystallization of ferrite grains transformed at relatively early stages of dynamic transformation.

Deformation-assisted diffusion for the enhanced kinetics of dynamic phase transformation

ABSTRACT Comparison on the kinetics of two different phase transformations, including phase transformation after deformation and phase transformation during deformation (i.e. dynamic transformation, DT), reveals a new discovery that the transformation kinetics can be significantly enhanced in DT even under low driving forces. DT enables continuous generation of defects (e.g. dislocations) near the phase boundary, which can act as short-circuiting diffusion paths for atoms. The diffusivity of atoms is enhanced and the activation energy for the atom jump across the phase boundary is lowered under stress during DT, resulting in more pronounced grain growth as well as accelerated transformation kinetics. GRAPHICAL ABSTRACT Impact Statement Deformation-enhanced grain growth is revealed in dynamic phase transformation, which will promote microstructure and property design of structural materials where phase transformations occur.

Effect of Precipitate on Microstructure Evolution and Hardness of Al-Cu Alloy during Torsion Deformation

The effect of precipitate on microstructure evolution and hardness of Al-Cu alloy during torsion deformation has been investigated, by comparing the evolution of microstructure in aged Al-2wt.% Cu alloy with commercial purity aluminum (1100Al). The microstructure evolution is studied by Transmission Electron Microscopy and Electron Backscatter Diffraction, and hardness is measured using Vickers hardness measuring instrument. It is found that the presence of precipitate enhance the grain refinement and hardness of Al-Cu alloy. By applied equivalent strain of 3.26, the (sub) grain size of 86 nm is achieved. In contrast, the presence of precipitate is found to be inhibiting the development of high angle grain boundary.

Dynamic Softening of Flow Stress during Dynamic Ferrite Transformation

This study using a 6Ni-0.1C steel confirmed the relationship between the change in a fraction of dynamically transformed ferrite and the dynamic softening in stress-strain curve during dynamic transformation above ortho-equilibrium austenite-to-ferrite transformation temperature. Dynamic softening in stress-strain curve was well-fitted with a form of Avrami equation as a function of strain, and it corresponded with the change in fraction of ferrite. The slope of work-hardening rate was increased due to the additional softening phenomenon, i.e. dynamic transformation, to dynamic recovery of austenite. Dynamic softening of austenite, which has been considered as a typical evidence of dynamic recrystallization, could be interpreted as a response of dynamic transformation to ferrite.