Seungwon Lee | University Of Toyama, Japan (original) (raw)

Papers by Seungwon Lee

An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior w... more An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to~150 nm and the Vickers microhardness was significantly increased through the HPT processing. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis including high-resolution transmission electron microscopy revealed that nanosized precipitates having compositions of a metastable Cu 3 Ni 5 Si 2 phase and a stable NiSi phase were formed in the Cu matrix by aging of the HPT-processed samples and these particles are responsible for the additional increase in strength after the HPT processing.

Two body centered cubic (bcc) metals, V and Mo, were processed by high pressure torsion (HPT) at ... more Two body centered cubic (bcc) metals, V and Mo, were processed by high pressure torsion (HPT) at ambient temperature. Hardness variation as well as microstructural evolution was examined with strain under a pressure of 2 to 6 GPa. It was shown that the hardness increases with straining and saturates to a constant level with the grain size of 330-400 nm in V irrespective of the applied pressures. Although the hardness variation with strain is the same for Mo with the grain size of $350 nm at the saturation level when the applied pressure is 6 GPa, the hardness level lowers below the saturation level and the grain size becomes coarser as the pressure is lowered. Tensile tests show that the strength significantly increases with some ductility for V after processing under any pressure and for Mo under lower pressures, but brittle fracture occurs in the Mo specimen processed at 6 GPa. The slower evolution of microstructure as well as the lower hardness levels observed in Mo is due to the applied pressure which is lower than the yield stress and thus due to the insufficient generation of dislocations for grain refinement. [

Two kinds of body centered cubic (bcc) structure refractory metals, pure Cr and Nb, were subjecte... more Two kinds of body centered cubic (bcc) structure refractory metals, pure Cr and Nb, were subjected to severe plastic deformation through high-pressure torsion (HPT) under applied pressures of 2 and 6 GPa for 2, 3, 4 and 5 revolutions at room temperature. Vickers microhardness is plotted as a function of the distance from the disk center and equivalent strain. It is shown that all hardness values fall on a single curve when they are plotted against equivalent strain for both metals. Vickers microhardness increases with increasing equivalent strain at an early stage of straining and then reaches steady state with the grain size of 200250 nm in Cr and 240270 nm in Nb irrespective of the applied pressures. In the steady state, there is no changing in hardness even in applying further straining. Tensile and bending tests show that brittle fracture occurs in Cr but in Nb, the strength significantly increases with some ductility after HPT processing. [

The age-hardenings behavior and precipitate microstructures with high dislocation density and/ or... more The age-hardenings behavior and precipitate microstructures with high dislocation density and/ or ultrafine grains have been studied for 6022Al-Mg-Si and 2091Al-Li-Cu alloys. The highpressure torsion (HPT) specimen of the former alloy exhibited either suppressed age hardenings or even age softening, unlike in the cases of the undeformed and cold-rolled specimens, at room temperature (RT) to 443 K (170°C). On the other hand, the HPT specimen of the latter alloy successfully increased the hardness up to>HV290 at 373 K (100°C), suggesting that concurrent strengthening by ultrafine-grained and precipitation hardenings can be activated if both alloy system and aging temperature are optimally selected. The corresponding transmission electron microscopy (TEM) microstructures attributed such a high level of hardness to the transgranular precipitation of the nanometer-scale particles within ultrafine grains. From the results of in situ small-angle X-ray scattering (SAXS) measurements, methods to maximize the effect of the combined processing of severe plastic deformation (SPD) and the age-hardenings technique are proposed based on the underlying phase transformation mechanisms.

The age hardening behavior of ultrafine-grained Al-Mg-Si alloys with micro-additions of Cu and Ag... more The age hardening behavior of ultrafine-grained Al-Mg-Si alloys with micro-additions of Cu and Ag has been investigated with a special emphasis on the relationship between microstructural features and hardness evolutions. Using transmission electron microscopy, it is shown that the higher hardness and thermal stability induced by a small amount of Cu is due to clustering during grain refinement by severe plastic deformation. It is also demonstrated that the addition of Ag is beneficial for the thermal stability of the ultrafine grain structure as it segregates along grain boundaries and significantly reduces their mobility.

The aging behavior of excess Mg type Al-Mg-Si alloy processed by high pressure torsion (HPT) and ... more The aging behavior of excess Mg type Al-Mg-Si alloy processed by high pressure torsion (HPT) and effect of Cu addition to this alloy have been investigated by hardness test and microstructure observation. The samples were solution heated, quenched, processed by HPT and aged. The hardness of HPT-processed samples increased about 100 HV rather than samples without HPT, and they showed hardening with peak hardness during aging. The level of hardness on alloys was increased by increasing with amount of Cu-addition. The mean grain size in HPT-processed samples was about 200-250 nm for each alloy. All HPT-processed alloys showed peak hardness during aging, and two Cu added alloys show positive value of age-hardening ability for the aging condition in this study. Precipitates in HPT-processed and aged samples existed on and near the grain boundaries, and a few small precipitates are found at some dislocations. According to the analysis of selected area diffraction (SAED) patterns, aluminum, Mg 2 Si and Q-phase are found in the HPT-processed and aged Cu-added alloys. The hardening of HPT-processed samples is probably caused by precipitation at the homogeneous grain boundary networks of 200-250 nm in the sample which acts as heterogeneous nucleation sites for alloys.

Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmis... more Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmission electron microscopy to understand extra diffraction spots that appear in their selected area electron diffraction patterns. These alloys containing Cu exhibit similar extra diffracted spots to each other with diffracted spots or streaks for Al matrix and major precipitates in each alloy. The extra spots cannot be conrmed in Cu-free alloys. The initial cluster, which is based on the β -phase in the Al-Mg-Si alloy, is proposed to be MgSi(/Ge)Mg, CuMgSi(/Ge), AlCuMg, and AlZnMg, while the second clusters, which consist of three initial clusters including anti-phase boundary short-range order, are proposed for Cu-containing alloys. [

Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmis... more Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmission electron microscopy to understand extra diffraction spots that appear in their selected area electron diffraction patterns. These alloys containing Cu exhibit similar extra diffracted spots to each other with diffracted spots or streaks for Al matrix and major precipitates in each alloy. The extra spots were not observed in Cu-free alloys. The initial cluster, which is based on the β -phase in the Al-Mg-Si alloy, is proposed to be MgSi(/Ge)Mg, CuMgSi(/Ge), AlCuMg, and AlZnMg, while the second clusters, which consist of three initial clusters including anti-phase boundary short-range order, are proposed for Cu-containing alloys.

A newly developed severe plastic deformation method, continuous high-pressure torsion (CHPT), was... more A newly developed severe plastic deformation method, continuous high-pressure torsion (CHPT), was modified for continuous processing of metallic wires. In this study, using the CHPT, wires of high-purity aluminum (99.99%) and copper (99.999%) with diameters of 2 mm and total lengths of 100 mm were successfully processed by employing the same features as conventional highpressure torsion (HPT) technique. The results of hardness measurements, 35 Hv for Al and 116 Hv for Cu, after CHPT at an imposed equivalent strain of *13 were consistent with those of conventional HPT using disk and ring specimens, as well as with those of CHPT using sheet specimens. Transmission electron microscopy (TEM) demonstrated that the microstructural elements are elongated in the shear direction after CHPT. The average grain size reaches the steady-state level, *1.3 lm, in Al, but the microstructure is at the non-steady state in Cu with subgrain sizes in the range of 0.3-4 lm.

A powder mixture of Nb-47 wt.% Ti (a well-known composition for superconducting magnets) was subj... more A powder mixture of Nb-47 wt.% Ti (a well-known composition for superconducting magnets) was subjected to severe plastic deformation using high-pressure torsion (HPT) and subsequently annealed at 573 K. Ti gradually dissolved in Nb with increasing shear strain, with a fast kinetics comparable to lattice diffusion at 700-1200 K. At large strains, a complete transition to a nanostructured b phase occurred at room temperature, which is far below the equilibrium temperature of 690 K. Nanoclusters of Ti with a body-centered cubic structure were also detected at large strains. Subsequent annealing led to elemental decomposition, formation of a nanoscale lamellar structure and segregation of Nb at grain boundaries. Superconductivity occurred at temperatures below 9 K, while the transition temperature decreased with increasing shear strain because of supersaturation of Ti in Nb and increased with annealing because of elemental decomposition. The Nb-Ti alloy after HPT exhibited hardness/strength peaks followed by softening at large strains, while hardening occurred after annealing. The maximum hardness, tensile and bending strengths were 4, 1.7 and 2.7 GPa, respectively.

Titanium in the form of bulk and powder was processed by severe plastic deformation using high-pr... more Titanium in the form of bulk and powder was processed by severe plastic deformation using high-pressure torsion (HPT) at cryogenic and room temperatures to investigate the influence of grain size on allotropic phase transformations. Almost a complete a (hexagonal close-packed, hcp) to x (hexagonal) phase transformation occurred under a pressure of 6 GPa at room temperature until the grain size reached the submicrometer level, while the formation of b (body-centered cubic, bcc) phase was not detected. The x-phase fraction and the x ! a transition temperature decreased with processing at cryogenic temperatures and/or with using powders, i.e. with decreasing the grain size to the nanometer scale during the deformation. First-principles calculations found the b phase to be dynamically unstable (neither stable nor metastable), while both a and x phases are dynamically stable at 0 and 6 GPa. This explains why the b phase was not detected in this study using different methods such as X-ray diffraction analysis, high-resolution transmission electron microscopy, automated crystal orientation mapping and electrical resistivity measurements. Mechanical properties of the HPT-processed Ti were also examined.

Several pure metals (magnesium, aluminum, iron, cobalt, nickel, copper, zinc, palladium and silve... more Several pure metals (magnesium, aluminum, iron, cobalt, nickel, copper, zinc, palladium and silver) and single-phase Al-Mg, Al-Ag, Al-Cu, Cu-Al, Cu-Zn, Pd-Ag, Ni-Fe and Ni-Co alloys were processed by severe plastic deformation using high-pressure torsion (HPT). The steady-state grain size was decreased and hardness increased by alloying in all the systems. It was shown that the dominant factor for extra grain refinement by alloying was due to the effect of solute-matrix atomic-size mismatch and modulus interaction on the mobility of edge dislocations. For the selected alloys, unlike pure metals, the grain size was almost insensitive to the melting temperature, and like pure metals, no systematic correlation was established between the grain size and stacking fault energy (chemical interaction) or between the grain size and valence electrons (electrical interaction). The presence of a power-law relation, with n % 0.56, between the hardness normalized by the shear modulus and grain size normalized by the Burgers vector signified the large contribution of grain boundaries to the hardening. The contribution of the solid-solution effect to the total hardening appeared to be <15%.

An age-hardenable Al-4 wt% Cu alloy is severely deformed using high-pressure torsion (HPT) to ref... more An age-hardenable Al-4 wt% Cu alloy is severely deformed using high-pressure torsion (HPT) to refine the microstructure to an average gain size of $ 210 nm. High saturation hardness of 205 Hv and high tensile strength of 820 MPa are achieved after the HPT processing. It is shown that the strength of the HPT-processed alloy is further improved by natural aging at room temperature or by artificial aging at 353 K. A peak hardness followed by softening appears within a few days after natural aging and within a few minutes after aging at 353 K, suggesting the low thermal stability of the alloy. Quantitative evaluation of different strengthening mechanisms shows that the grain boundary hardening through the Hall-Petch relationship and the precipitation hardening through the Orowan relationship are dominant strengthening mechanisms.

A process to achieve strengthening in an Al 6061 alloy by grain refinement to~200 nm using high-p... more A process to achieve strengthening in an Al 6061 alloy by grain refinement to~200 nm using high-pressure torsion (HPT) and fine precipitation using aging treatment is studied. It is shown that although aging of the HPT-processed sample is effective for extra strengthening of the alloy, the imposed shear strain and the aging temperature should be selected carefully. The HPT processing after 5 turns leads high saturation hardness and tensile strength of 163 Hv and 470 MPa, respectively. The hardness at the saturation level remains the same during aging at 373 K (100°C), while the hardness decreases by aging at 423 K (150°C). When the disks are processed for 0.75 turns (lower shear strains) and aged at 373 K (100°C), the hardness increases above the hardness level at the saturation because of the formation of B¢ and b¢ precipitates. Quantitative analyses indicate that three major hardening mechanisms contribute to the total hardening: grain boundary hardening through the Hall-Petch relationship, dislocation hardening through the Bailey-Hirsch relationship and precipitation hardening through the Orowan relationship. This study shows that the contribution of different strengthening mechanisms can be estimated using a linear additive relationship in ultrafine-grained aluminum alloys.

This work presents a study related to the grain refinement of an aluminum A2618 alloy achieved by... more This work presents a study related to the grain refinement of an aluminum A2618 alloy achieved by High-Pressure Torsion (HPT) known as a process of Severe Plastic Deformation (SPD). The HPT is conducted on disks of the alloy under an applied pressure of 6 GPa for 1 and 5 turns with a rotation speed of 1 rpm at room temperature. The HPT processing leads to microstructural refinement with an average grain size of ~250 nm at a saturation level after 5 turns. Gradual increases in hardness are observed from the beginning of straining up to a saturation level. This study thus suggests that hardening due to grain refinement is attained by the HPT processing of the A2618 alloy at room temperature.

Age-hardenable AlMgSi alloys containing additional elements of Ag, Cu, Pt or Pd were processed by... more Age-hardenable AlMgSi alloys containing additional elements of Ag, Cu, Pt or Pd were processed by high-pressure torsion (HPT) and they were subsequently aged at a temperature of 373 K for up to a total period of 5,400 ks. It was found that, in all alloys, the grain sizes after HPT were refined to 300430 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent aging treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying elements. In the Cu-added excess Mg alloy, the hardness increase was large and the higher hardness retained for longer aging time when compared with those of non-added alloys. It was suggested that the precipitation of a few particles within a single grain with the size of a few hundred nanometer contributes to appreciable age hardening.

Age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd were p... more Age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd were processed by high-pressure torsion HPT and they were subsequently aged at a temperature of 373 K for up to a total period of 5400 ks. It was found that, in all alloys, the grain sizes after HPT were refined to 300-400 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent aging treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying compositions. In the Cu-added excess Mg alloy, the hardness increase was large and the higher hardness retained for longer aging time when compared with those of non-added alloys. It was suggested that the precipitation of a few particles within a single grain with the size of a few hundred nanometer contributes to appreciable age hardening.

In this paper we report a quantitative study, using small-angle X-ray scattering, of the precipit... more In this paper we report a quantitative study, using small-angle X-ray scattering, of the precipitation kinetics during ramp heating and isothermal ageing in an AA7075 aluminium alloy processed by high-pressure torsion. The precipitation behaviour has been compared with that of the same material processed in a conventional manner and observations are supplemented by transmission electron microscopy for precipitate and grain size characterization using automated crystal orientation mapping. After severe plastic deformation and natural ageing, the material is shown to contain a high density of GP zones. During ageing, the precipitate size distribution becomes bimodal, with small precipitates behaving similarly to those of the conventionally processed material and large ones associated with the crystalline defects and reaching large sizes at considerably lower temperatures and shorter times as compared to the conventionally processed material.

Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmis... more Age-hardenable Al-Mg-Si, Al-Mg-Ge, and Al-Zn-Mg alloys including Cu were investigated by transmission electron microscopy to understand extra diffraction spots that appear in their selected area electron diffraction patterns. These alloys containing Cu exhibit similar extra diffracted spots to each other with diffracted spots or streaks for Al matrix and major precipitates in each alloy. The extra spots were not observed in Cu-free alloys. The initial cluster, which is based on the β -phase in the Al-Mg-Si alloy, is proposed to be MgSi(/Ge)Mg, CuMgSi(/Ge), AlCuMg, and AlZnMg, while the second clusters, which consist of three initial clusters including anti-phase boundary short-range order, are proposed for Cu-containing alloys.