Microstructure and Tensile Behavior of Al7075/Al Composites Consolidated from Machining Chips Using HPT: A Way of Solid‑State Recycling

Mohammad Khajouei‑Nezhad,Mohammad Hossein Paydar,Majid Mokarizadeh Haghighi Shirazi,Jenő Gubicza 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.12

In this study the feasibility of using high-pressure torsion (HPT) process as a suitable technique for solid-state recycling ofAl alloy machining chips has been evaluated. One and four turns of HPT were used for consolidation of 7075 Al alloy chips/commercially pure Al powder mixtures at room temperature. The prepared mixtures included different weight fraction ofchips in the range from 20 to 100%. The samples were fabricated from either annealed or non-annealed chips. Additionally,the effect of the fraction of 7075 Al chips on the density, microstructure and tensile behavior of the consolidated sampleswas investigated. It was found that the addition of pure aluminum powder played an important role as a binder in the consolidationprocess and yielded improved mechanical properties due to creating lower porosity and better bonding betweenthe chips. Tensile tests revealed that the highest ultimate tensile strength and fracture strain were obtained for the compositecontaining near 50 wt% annealed 7075 Al chips, and fabricated through 4 turns of HPT. Optical microscopy investigationsdemonstrated that these conditions resulted in a fine microstructure with uniform distribution of chips.

Compressive behavior of Cu-Ni alloy foams: Effects of grain size, porosity, pore directionality, and chemical composition

Gubicza, Jenő,Jenei, Pé,ter,Nam, Kyungju,Ká,dá,r, Csilla,Jo, Hyungyung,Choe, Heeman Elsevier 2018 Materials science & engineering. properties, micro Vol.725 No.-

<P><B>Abstract</B></P> <P>Experiments were conducted to study the compression behavior of Cu-Ni foams prepared using freeze casting. The struts of the foam samples were solid-solutioned with differing Cu/Ni ratios, after which the grain size in the struts was measured using scanning electron microscopy. The compression performance of the samples was studied in both parallel and perpendicular directions to the temperature gradient, and compared with model calculations. It was confirmed that alloying increased the yield strength of the struts. The experimentally determined yield strength and elastic modulus were compared with model calculations, which revealed that the elastic modulus of the foams was lower than the values calculated from the classical compression and Gibson-Ashby models due to variation in the thickness of the struts. It was also found that the alloying of Cu and Ni improved the mechanical performance of the alloy foams because the absorbed energy for the alloys was considerably higher than that for the pure foams.</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>

Study of the compression and wear-resistance properties of freeze-cast Ti and Ti‒5W alloy foams for biomedical applications

Choi, Hyelim,Shil'ko, Serge,Gubicza, Jenő,Choe, Heeman Elsevier 2017 Journal of the mechanical behavior of biomedical m Vol.72 No.-

<P><B>Abstract</B></P> <P>Ti and Ti‒5wt% W alloy foams were produced by freeze-casting process and their mechanical behaviors were compared. The Ti‒5W alloy foam showed a typical acicular Widmanstätten α/β structure with most of the W dissolved in the β phase. An electron-probe microanalysis revealed that approximately 2wt% W was uniformly dissolved in the Ti matrix of Ti‒5W alloy foam with few partially dissolved W particles. The compressive-yield strength of Ti‒5W alloy foam (~323MPa) was approximately 20% higher than that of the Ti foam (~256MPa) owing to the solid-solution-strengthening effect of W in the Ti matrix, which also resulted in a dramatic improvement in the wear resistance of Ti‒5W alloy foam. The compressive behaviors of the Ti and Ti‒5W alloy foams were predicted by analytical models and compared with the experimental values. Compared with the Gibson-Ashby and cellular-lattice-structure-in-square-orientation models of porous materials, the orientation-averaging method provided prediction results that are much more accurate in terms of both the Young's modulus and the yield strength of the Ti and Ti‒5W alloy foams.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Manufacturing of Ti and Ti-W alloy foams by freeze-casting method. </LI> <LI> Compressive test of Ti and Ti-W alloy foams. </LI> <LI> Evaluation of wear resistance of Ti and Ti-W alloy foams. </LI> <LI> Analytical prediction of the Young's moduli and yield strengths of Ti and Ti-5W alloy foams. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Evolution of microstructure and hardness in AZ31 alloy processed by high pressure torsion

Strá,ská,, Jitka,Janeč,ek, Miloš,Gubicza, Jenő,Krajň,á,k, Tomá,š,Yoon, Eun Yoo,Kim, Hyoung Seop Elsevier 2015 Materials science & engineering. properties, micro Vol.625 No.-

<P><B>Abstract</B></P> <P>A commercial MgAlZn alloy (AZ31) was processed by high pressure torsion (HPT) at room temperature, resulting in an extreme microstructure refinement down to the grain size of 150–250nm. The microstructure evolution during HPT was investigated by transmission electron microscopy and X-ray diffraction line profile analysis. The microhardness was measured as a function of the distance from the center of the disk and the number of HPT revolutions. The detailed analysis of dislocation contrast factors in X-ray diffraction line profiles enables to determine the population of the different slip systems as a function of the imposed strain. The influence of microstructure and defect structure evolution on microhardness is discussed in detail.</P>

The effect of hydrogen charging on the evolution of lattice defects and phase composition during tension in 316L stainless steel

El-Tahawy, Moustafa,Um, Teakyung,Nam, Ho-Seok,Choe, Heeman,Gubicza, Jenő Elsevier 2019 Materials science & engineering. properties, micro Vol.739 No.-

<P><B>Abstract</B></P> <P>The effects of hydrogen charging on the evolution of strain-induced lattice defects and phase transformation were investigated in 316L stainless steel. The initial material was obtained by cold rolling to a thickness reduction of 20%. The surfaces of some of the initial samples were cathodically charged with hydrogen. Afterward, both the charged and uncharged samples were subjected to tension until failure. The dislocation density and twin-fault probability in the charged and uncharged specimens during tension were compared. We found that hydrogen charging reduced the degree of increase in the dislocation density and twin-fault probability during the application of tension to the same strain level. Significant martensitic phase transformation was observed in the uncharged samples tested to the strain of 10% or higher. In the hydrogen-charged samples, only a slight increase in the martensite phase fraction was detected. A correlation between the α′-martensite fraction and the dislocation density was found for the studied samples, suggesting that the lower degree of martensitic phase transformation in the charged 316L steel was caused by the smaller amount of stress developed due to the lower dislocation density. In accordance with the differences observed in the phase composition and defect densities, the hydrogen-charged material exhibited lower surface hardness.</P>

Exceptionally high strength and good ductility in an ultrafine-grained 316L steel processed by severe plastic deformation and subsequent annealing

El-Tahawy, Moustafa,Pereira, Pedro Henrique R.,Huang, Yi,Park, Hyeji,Choe, Heeman,Langdon, Terence G.,Gubicza, Jenő Elsevier 2018 Materials letters Vol.214 No.-

<P><B>Abstract</B></P> <P>An investigation was conducted to evaluate the effect of annealing at different temperatures on the tensile properties of ultrafine-grained 316L stainless steel processed by high-pressure torsion (HPT). A “moderate-temperature” annealing at 740 K resulted in reduced strength and elongation due to the annihilation of mobile dislocations. A “high-temperature” annealing at 1000 K yielded a remarkably good combination of yield strength (∼1330 MPa) and elongation to failure (∼43%) which can be attributed to the almost full reversion of α′-martensite formed during HPT into γ-austenite while the grain size remained very fine with a value of about 200 nm.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 316L steel with a nanocrystalline microstructure was processed by HPT. </LI> <LI> The effect of annealing on the tensile properties was studied. </LI> <LI> Annealing to 740 K yielded a pronounced embrittlement and a strength reduction. </LI> <LI> Annealing to 1000 K led to a good combination of strength and elongation to failure. </LI> <LI> The excellent tensile behavior was caused by the very fine austenitic microstructure. </LI> </UL> </P>

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>

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>