Microstructure of ultrafine-grained fcc metals produced by severe plastic deformation

J. Gubicza,N.Q. Chinh,Gy. Kr?llics,I. Schiller,T. Ung?r 한국물리학회 2006 Current Applied Physics Vol.6 No.2

Ion implantations with energies less than 100 keV and current densities of 10151018 ions/cm2 were conducted to polymers, met-als, and diamond gemstone. Physical and chemical phenomena and mechanical properties of the implanted materials wereinvestigated.Single or mixed ions of N, He, C were implanted onto polyethylene terephtalate (PET) to see the surface hardening eects. Multi-ple ion-implantation resulted in more increase in the surface hardness than single ion implantation at the same ion energy and dose.XPS analysis showed that CN compounds were formed when both N and C ions are implanted into PET, implying that hard par-ticle formation by reactions between the implanted ions and/or between the implanted N ions and carbon in PET in addition to thecross linking may be the mechanism of this signicant increase in hardness.Ion implantation with 70 keV N ions of >5 · 1016/cm2 ness (Ra) from 0.04l m to 0.02l m. The implanted nitrogen was detected up to at least 300 nm from the surface of the stainless steelas measured with Auger electron spectroscopy. X-ray photo-electron spectroscopy analysis showed that the implanted N formedmostly Cr2N without post irradiation annealing. Hardness depth proles obtained with nano-indentation technique showed thatthe peak hardness of 14 Gpa formed at. 50 nm depth from the N ion implanted surface was about at least 2 times higher thannon-irradiated specimen.N ions were implanted into the diamond in order to change the optical band gap and then to change the emitted color. In spite ofthe restricted ion penetrated depth, uniform and vividly changed color was observed after heat treatment of the nitrogen-implanteddiamond in the vacuum or inert gas atmosphere. The changed color appeared to be black. Chemical states of the implanted nitrogen25% after N doping.. 2005 Elsevier B.V. All rights reserved.PACS:81.65.. b

Structural characterization of ultrafine-grained interstitial-free steel prepared by severe plastic deformation

Č,í,ž,ek, J.,Janeč,ek, M.,Krajň,á,k, T.,Strá,ská,, J.,Hruš,ka, P.,Gubicza, J.,Kim, H.S. Elsevier 2016 Acta materialia Vol.105 No.-

<P>Interstitial free steel with ultrafine-grained (UFG) structure was prepared by high-pressure torsion (HPT). The development of the microstructure as a function of the number of HPT turns was studied at the centre, half-radius and periphery of the HPT-processed disks by X-ray line profile analysis (XLPA), positron annihilation spectroscopy (PAS) and electron microscopy. The dislocation densities and the dislocation cell sizes determined by XLPA were found to be in good agreement with those obtained by PAS. The evolution of the dislocation density, the dislocation cell and grain sizes, the vacancy cluster size, as well as the high-angle grain boundary (HAGB) fraction was determined as a function of the equivalent strain. It was found that first the dislocation density saturated, then the dislocation cell size reached its minimum value and finally the grain size got saturated. For very high strains after the saturation of grain size the HAGB fraction further increased. The PAS investigations revealed that vacancies introduced by severe plastic deformation agglomerated into small clusters consisting of 9-14 vacancies. The evolution of the yield strength calculated from the microhardness as a function of strain was explained by the development of the defect structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

Microstructure and Thermal Stability of Copper - Carbon Nanotube Composites Consolidated by High Pressure Torsion

Jenei, P.,Yoon, E.Y.,Gubicza, Jenő,Kim, Hyoung Seop,Lá,bá,r, J.L.,Ungá,r, Tamá,s Trans Tech Publications, Ltd. 2012 Materials science forum Vol.729 No.-

<P>Blends of Cu powders and 3 vol. % carbon nanotubes (CNTs), and an additional sample from pure Cu powder were consolidated by High Pressure Torsion (HPT) at room temperature (RT) and 373 K. The grain size, the lattice defect densities as well as the hardness of the pure and composite materials were determined. Due to the pinning effect of CNTs, the dislocation density is about three times larger, while the grain size is about half of that obtained in the sample consolidated from the pure Cu powder. The increase of the HPT-processing temperature from RT to 373 K resulted in only a slight increase of the grain size in the Cu-CNT composite while the dislocation density and the twin boundary frequency were reduced significantly. The flow stress obtained experimentally agrees well with the value calculated by the Taylor-formula indicating that the strength in both pure Cu and Cu-CNT composites is determined mainly by the interaction between dislocations. The addition of CNTs to Cu yields a significantly better thermal stability of the UFG matrix processed by HPT.</P>

Microstructure and hardness of copper-carbon nanotube composites consolidated by High Pressure Torsion

Jenei, P.,Yoon, E.Y.,Gubicza, J.,Kim, H.S.,Labar, J.L.,Ungar, T. Elsevier Sequoia 2011 Materials science & engineering. properties, micro Vol.528 No.13

Blends of Cu powders and 3vol.% carbon nanotubes (CNTs) were consolidated by High Pressure Torsion (HPT) at room temperature (RT) and 373K. The grain size, the lattice defect densities as well as the hardness of the composite samples were determined. It was found that the Cu-CNT composite processed at RT exhibited a half as large mean grain size and a three times higher dislocation density than those observed in the specimens either consolidated from pure Cu powder or processed from bulk Cu by HPT. The small grain size and the pinning effect of CNT fragments on dislocations led to significant twin boundary formation during HPT. The increase of the temperature of HPT-processing to 373K resulted in a slight increase of the grain size, and a strong decrease of the dislocation density and the twin boundary frequency in the composite. The correlation between the microstructural parameters and the flow stress calculated from the hardness was discussed.

Defect structure and hardness in nanocrystalline CoCrFeMnNi High-Entropy Alloy processed by High-Pressure Torsion

Heczel, Anita,Kawasaki, Megumi,Lá,bá,r, Já,nos L.,Jang, Jae-il,Langdon, Terence G.,Gubicza, Jenő Elsevier 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.711 No.-

<P><B>Abstract</B></P> <P>An equiatomic CoCrFeMnNi High-Entropy Alloy (HEA) produced by arc melting was processed by High-Pressure Torsion (HPT). The evolution of the microstructure during HPT was investigated after ¼, ½, 1 and 2 turns using electron backscatter diffraction and transmission electron microscopy. The spatial distribution of constituents was studied by energy-dispersive X-ray spectroscopy. The dislocation density and the twin-fault probability in the HPT-processed samples were determined by X-ray line profiles analysis. It was found that the grain size was gradually refined from ∼60 μm to ∼30 nm while the dislocation density and the twin-fault probability increased to very high values of about 194 × 10<SUP>14</SUP> m<SUP>−2</SUP> and 2.7%, respectively, at the periphery of the disk processed for 2 turns. The hardness evolution was measured as a function of the distance from the center of the HPT-processed disks. After 2 turns of HPT, the microhardness increased from ∼1440 MPa to ∼5380 MPa at the disk periphery where the highest straining is achieved. The yield strength was estimated as one-third of the hardness and correlated to the microstructure.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The microstructure and the hardness in HPT processed CoCrFeMnNi HEA were studied. </LI> <LI> The dislocation density increased up to 194 × 10<SUP>14</SUP> m<SUP>−2</SUP> after 2 turns of HPT. </LI> <LI> The grain size decreased to 27 nm at the periphery of the disk processed by 2 turns. </LI> <LI> After 2 turns the twin fault probability increased to 2.7% at the disk periphery. </LI> <LI> The hardness increased from 1440 MPa to 5380 MPa due to 2 turns of HPT. </LI> </UL> </P>

Mechanical behavior and microstructure of compressed Ti foams synthesized via freeze casting

Jenei, P.,Choi, H.,Toth, A.,Choe, H.,Gubicza, J. Elsevier 2016 Journal of the mechanical behavior of biomedical m Vol.63 No.-

Pure Ti and Ti-5%W foams were prepared via freeze casting. The porosity and grain size of both the materials were 32-33% and 15-17@?m, respectively. The mechanical behavior of the foams was investigated by uniaxial compression up to a plastic strain of ~0.26. The Young@?s moduli of both foams were ~23GPa, which was in good agreement with the value expected from their porosity. The Young@?s moduli of the foams were similar to the elastic modulus of cortical bones, thereby eliminating the osteoporosis-causing stress-shielding effect. The addition of W increased the yield strength from ~196MPa to ~235MPa. The microstructure evolution in the grains during compression was studied using electron backscatter diffraction (EBSD) and X-ray line profile analysis (XLPA). After compression up to a plastic strain of ~0.26, the average dislocation densities increased to ~3.4x10<SUP>14</SUP>m<SUP>-2</SUP> and ~5.9x10<SUP>14</SUP>m<SUP>-2</SUP> in the Ti and Ti-W foams, respectively. The higher dislocation density in the Ti-W foam can be attributed to the pinning effect of the solute tungsten atoms on dislocations. The experimentally measured yield strength was in good agreement with the strength calculated from the dislocation density and porosity. This study demonstrated that the addition of W to Ti foam is beneficial for biomedical applications, because the compressive yield strength increased while its Young@?s modulus remained similar to that of cortical bones.

Effect of Addition of Rare Earth Elements on the Microstructure, Texture, and Mechanical Properties of Extruded ZK60 Alloy

S. Najafi,M. Sabbaghian,A. Sheikhani,P. Nagy,K. Fekete,J. Gubicza 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.6

Microstructure, texture and mechanical properties of extruded ZK60 alloys containing different rare earth (RE) elementswere studied. Two alloys containing 1 and 2 wt% Ce-rich lanthanides as well as two other alloys with 1 and 2 wt% Y wereprepared and denoted as ZK60–1RE, ZK60–2RE, ZK60–1Y and ZK60–2Y, respectively. The results showed that the additionof RE elements refined the grain size of the ZK60 alloy, and Y had a more significant effect on the grain size reduction. The finer grain size in the ZK60–1Y and ZK60–2Y alloys (2.3 and 2.1 μm, respectively) compared to ZK60–1RE andZK60–2RE alloys (6.2 and 3.7 μm, respectively) was attributed to the pinning effect of very fine secondary phases on grainboundaries. While the ZK60–1Y alloy exhibited a fiber texture with a slight rotation of < 1010 > poles around the extrusiondirection, the ZK60–1RE alloy showed a new texture component known as RE texture. Moreover, there were no changes inthe fiber-like texture component of ZK60–2RE and ZK60–2Y alloys. The mechanical properties of the alloys were assessedby shear punch test. Accordingly, the addition of RE and Y elements enhanced the shear strength, and the maximum ultimateshear strength of 176 MPa was obtained for the ZK60–2RE alloy. The higher dislocation density, the lower volume fractionof dynamically recrystallized regions compared to other alloys, and the large fraction of secondary phase particles wereresponsible for this high strength value.

High temperature thermal stability of nanocrystalline 316L stainless steel processed by high-pressure torsion

El-Tahawy, Moustafa,Huang, Yi,Choi, Hyelim,Choe, Heeman,Lá,bá,r, Já,nos L.,Langdon, Terence G.,Gubicza, Jenő Elsevier 2017 Materials science & engineering. properties, micro Vol.682 No.-

<P><B>Abstract</B></P> <P>Differential scanning calorimetry (DSC) was used to study the thermal stability of the microstructure and the phase composition in nanocrystalline 316L stainless steel processed by high-pressure torsion (HPT) for ¼ and 10 turns. The DSC thermograms showed two characteristic peaks which were investigated by examining the dislocation densities, grain sizes and phase compositions after annealing at different temperatures. The first DSC peak was exothermic and was related to recovery of the dislocation structure without changing the phase composition and grain size. The activation energies for recovery after processing by ¼ and 10 turns were ~163 and ~106kJ/mol., respectively, suggesting control by diffusion along grain boundaries and dislocations. The second DSC peak was endothermic and was caused by a reverse transformation of α’-martensite to γ-austenite. The hardness of annealed samples was determined primarily by the grain size and followed the Hall–Petch relationship. Nanocrystalline 316L steel processed by HPT exhibited good thermal stability with a grain size of ~200nm after annealing at 1000K and a very high hardness of ~4900MPa.</P>