Strengthening of concrete damaged by mechanical loading and elevated temperature

Ahmad, Hammad,Hameed, Rashid,Riaz, Muhammad Rizwan,Gillani, Asad Ali Techno-Press 2018 Advances in concrete construction Vol.6 No.6

Despite being one of the most abundantly used construction materials because of its exceptional properties, concrete is susceptible to deterioration and damage due to various factors particularly corrosion, improper loading, poor workmanship and design discrepancies, and as a result concrete structures require retrofitting and strengthening. In recent times, Fiber Reinforced Polymer (FRP) composites have substituted the conventional techniques of retrofitting and strengthening of damaged concrete. Most of the research studies related to concrete strengthening using FRP have been performed on undamaged test specimens. This contribution presents the results of an experimental study in which concrete specimens were damaged by mechanical loading and elevated temperature in laboratory prior to application of Carbon Fiber Reinforced Polymer (CFRP) sheets for strengthening. The test specimens prepared using concrete of target compressive strength of 28 MPa at 28 days were subjected to compressive and splitting tensile testing up to failure and the intact pieces of the failed specimens were collected for the purpose of repair. In order to induce damage as a result of elevated temperature, the concrete cylinders were subjected to $400^{\circ}C$ and $800^{\circ}C$ temperature for two hours duration. Concrete cylinders damaged under compressive and split tensile loads were re-cast using concrete and rich cement-sand mortar, respectively and then strengthened using CFRP wrap. Concrete cylinders damaged due to elevated temperature were also strengthened using CFRP wrap. Re-cast and strengthened concrete cylinders were tested in compression and splitting tension. The obtained results revealed that re-casting of specimens damaged by mechanical loadings using concrete & mortar, and then strengthened by single layer CFRP wrap exhibited strength even higher than their original values. In case of specimens damaged by elevated temperature, the results indicated that concrete strength is significantly dropped and strengthening using CFRP wrap made it possible to not only recover the lost strength but also resulted in concrete strength greater than the original value.

Microstructural development and mechanical properties of nanostructured copper reinforced with SiC nanoparticles

Akbarpour, M.R.,Salahi, E.,Hesari, F.A.,Yoon, E.Y.,Kim, H.S.,Simchi, A. Elsevier Sequoia 2013 Materials science & engineering. properties, micro Vol.568 No.-

Nanostructured Cu and Cu-2vol% SiC nanocomposite were produced by high energy mechanical milling and hot pressing technique. Microstructure development during fabrication process was investigated by X-ray diffraction, scanning electron microscope, scanning transmission electron microscope, and electron backscatter diffraction techniques. The results showed that the microstructure of copper and copper-based nanocomposite composed of a mixture of equiaxed nanograins with bimodal and non-random misorientation distribution. The presence of SiC nanoparticles refined the grain structure of the copper matrix while the fraction of low angle grain boundaries was increased. Evaluation of mechanical properties by compression test showed enhanced yield strength from 505+/-17MPa for the nanostructured copper to 630+/-12MPa for the reinforced metal with 2vol% SiC. We correlated the strength of the nanostructured materials to their microstructural features based on the strengthening mechanisms. The contribution of different mechanisms including Orowan strengthening, high angle grain boundaries, and density of dislocations were analyzed. It is shown that the high angle grain boundaries in nanostructured materials play a significant role in the strengthening mechanism. The effect of nanoparticles is presented and discussed.

In-Situ Precipitated Needle Like Nanocrystalline β-Ti Reinforced Porous Titanium Alloy via Molten Salt Electrolysis

Hao Wang,Jiahao Zhao,Honglin Li,Zhifeng Huang,Yangwei Wang,Qiang Shen,Fei Chen 대한금속·재료학회 2024 METALS AND MATERIALS International Vol.30 No.1

Fabricating porous active metals through chemical dealloying poses challenges due to their reactivity and vulnerability tooxidation in aqueous solutions. The objective of this study was to create micron-sized porous Ti alloy by utilizing the Ti–Mo system as a precursor alloy for chemical dealloying. The impact of phase composition and initial microstructure of theprecursor alloys (Tix at% Mo100 − x at%, x = 60 ~ 70) on the morphology of the resulting porous Ti alloy was systematicallyinvestigated. To improve the mechanical strength and minimize oxidized phases during the dealloying process, a molten saltelectrolysis (MSE) method was employed. The strengthening mechanism of MSE on porous Ti alloys encompassed three keyaspects. Firstly, it effectively reduced the presence of oxidized phases, thereby eliminating surface defects. Secondly, MSEfacilitated grain growth and eliminated voids and cracks at the grain boundaries, leading to enhanced mechanical properties. Thirdly, the involvement of a secondary phase contributed to the overall strengthening mechanism. Following MSE treatment,the oxygen content in the porous Ti alloy decreased from over 13 to 5 at%, and needle-like nanocrystalline β-Ti precipitatesformed within the ligament structure. The accumulation and aggregation of compression-induced dislocations at the grainboundaries of the precipitated phase further improved the mechanical properties. In summary, this work presents an innovativeapproach to fabricating porous Ti alloy with low oxygen content, high strength, and adjustable microstructure. It elucidatesthe strength enhancement mechanism by MSE, providing insights for future materials development and applications.

Importance of Individual Evaluation of Crystallographic Texture and Microstructure Effects on Biocompatibility and Corrosion Performance of Ti6Al4V Alloy

Hongchi Zhou,Qiang Shen,Fei Chen,Yaojun Lin 대한금속ᆞ재료학회 2023 METALS AND MATERIALS International Vol.29 No.2

This paper reports systematic studies on precipitation behavior of a solid-solution treated single-phase Nb-containingCoCrNi-based face-centered-cubic high-entropy alloy, CoCrNi1.5Nb0.2,at a series of aging temperatures of 660–960 °C fordifferent aging times of 1–48 h, and on the effect of precipitation behavior on tensile strength and ductility. Aging at 660 °Cfor 1–48 h produced disc-shaped coherent γ"-(Ni,Co,Cr)3(Nb,Cr) precipitates of nanometric diameter and thickness withthe D022superlattice, offering precipitation strengthening via dislocation shear mechanism. The diameter monotonicallyincreases with increasing aging time, and the thickness increases with increasing aging time from 1 to 18 h but essentiallyremains unchanged with aging time of 18–48 h; the volume fraction of γ" precipitates constantly increases with increasingaging time. Consequently, yield strength (YS) and ultimate tensile strength (UTS) of CoCrNi1.5Nb0.2increase with increasingaging time due to increased volume fraction and dimensions of γ" precipitates. Uniform elongation (UE) of CoCrNi1.5Nb0.2decreases with increasing aging time due to increased strength of CoCrNi1.5Nb0.2and dimensions of γ" precipitates. Aging at760 °C for 1 h yielded dimensions of γ" precipitates similar to, but volume fraction of γ" precipitates lower than, those duringaging at 660 °C for 48 h, leading to lower YS and UTS and higher UE than those in CoCrNi1.5Nb0.2aged at 660 °C for 48 h. Unlike aging at 660 °C for 1–48 h and at 760 °C for 1 h, aging at 860 and 960 °C for 1 h generated semi-coherent lath-shapedε-(Ni,Co,Cr)3(Nb,Cr) precipitates of submicrometric to micrometric length and submicrometric width with the D019structure,changing precipitation strengthening mechanism to Orowan dislocation bypass mechanism. Submicrometric to micrometricε precipitates provide limited precipitation strengthening, only attaining modest YS and UTS. Despite submicrometric tomicrometric length of ε precipitates, CoCrNi1.5Nb0.2still exhibits a relatively high UE due to strength-ductility tradeoff.

Effect of Precipitation Behavior on Mechanical Properties of a Nb-Containing CoCrNi-Based High-Entropy Alloy

Hongchi Zhou,Yaojun Lin,Fei Chen,Qiang Shen 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.3

This paper reports systematic studies on precipitation behavior of a solid-solution treated single-phase Nb-containingCoCrNi-based face-centered-cubic high-entropy alloy, CoCrNi1.5Nb0.2,at a series of aging temperatures of 660–960 °C fordifferent aging times of 1–48 h, and on the effect of precipitation behavior on tensile strength and ductility. Aging at 660 °Cfor 1–48 h produced disc-shaped coherent γ"-(Ni,Co,Cr)3(Nb,Cr) precipitates of nanometric diameter and thickness withthe D022superlattice, offering precipitation strengthening via dislocation shear mechanism. The diameter monotonicallyincreases with increasing aging time, and the thickness increases with increasing aging time from 1 to 18 h but essentiallyremains unchanged with aging time of 18–48 h; the volume fraction of γ" precipitates constantly increases with increasingaging time. Consequently, yield strength (YS) and ultimate tensile strength (UTS) of CoCrNi1.5Nb0.2increase with increasingaging time due to increased volume fraction and dimensions of γ" precipitates. Uniform elongation (UE) of CoCrNi1.5Nb0.2decreases with increasing aging time due to increased strength of CoCrNi1.5Nb0.2and dimensions of γ" precipitates. Aging at760 °C for 1 h yielded dimensions of γ" precipitates similar to, but volume fraction of γ" precipitates lower than, those duringaging at 660 °C for 48 h, leading to lower YS and UTS and higher UE than those in CoCrNi1.5Nb0.2aged at 660 °C for 48 h. Unlike aging at 660 °C for 1–48 h and at 760 °C for 1 h, aging at 860 and 960 °C for 1 h generated semi-coherent lath-shapedε-(Ni,Co,Cr)3(Nb,Cr) precipitates of submicrometric to micrometric length and submicrometric width with the D019structure,changing precipitation strengthening mechanism to Orowan dislocation bypass mechanism. Submicrometric to micrometricε precipitates provide limited precipitation strengthening, only attaining modest YS and UTS. Despite submicrometric tomicrometric length of ε precipitates, CoCrNi1.5Nb0.2still exhibits a relatively high UE due to strength-ductility tradeoff.

Development and Characterization of a Novel Fine-Grained W–Ni–Mn–Mo Alloys with La2O3 Addition Prepared Via Low-Temperature Sintering

Zhi‑Bo Li,Xin Lan,Guo‑Hua Zhang,Kuo‑Chih Chou 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.7

W–Ni–Mn heavy alloys are a kind of promising metal-matrix composites. A novel fully-compacted and fine-grained W–Ni–Mn–Mo composites with La2O3addition were fabricated in this work by the low-temperature sintering and vacuumheat-treatment techniques. W nanopowder synthesized by a two-stage reduction approach was employed as raw material. The heat-treated alloys contained equiaxed W grains (only 7.43 μm) surrounded by a Ni–Mn-based matrix phase, whileLa2O3particles were dispersed at grain boundaries. The 91 W–4.2Ni–2.8Mn–2Mo–1La2O3 alloy sintered at 1200 °C for2 h exhibited superior comprehensive properties, with a maximum tensile strength of ~ 904 MPa, elongation of ~ 19.8% andhardness of ~ 452 HV1. Additions of Mo and La2O3contribute to triggering the fine-grained strengthening, oxide dispersionstrengthening and solid solution strengthening mechanisms.

Enhancement of Tensile Properties of AZ91–Ca–Sb Magnesium Alloy with SiC Nanoparticles Additions

Sourav Ganguly,Smarajit Sarkar,A. K. Mondal 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.10

The present work investigates the effect of SiC nanoparticles (SiCnp) additions on the tensile properties of the squeeze-castAZ91 + 2.0Ca + 0.3Sb (wt%) alloy at ambient, 423 K and 473 K temperatures. All the AZ91 + 2.0Ca + 0.3Sb + xSiCnp [x = 0.5wt% (0.29 vol%), 1.0 wt% (0.58 vol%), and 2.0 wt% (1.16 vol%)] nanocomposites illustrate greater yield strength and ultimatetensile strength in contrast to the AZ91 + 2.0Ca + 0.3Sb alloy at all the temperatures employed. The yield strength, ultimatetensile strength and elastic modulus of both the alloy and the nanocomposites decline as the temperature increases, whereasthe work to fracture increases with a rise in temperature. Among the nanocomposites, the one comprising of 2.0SiCnp demonstratesthe best tensile properties. All the nanocomposites display superior strain hardening response than the alloy, andthe maximum strain hardening is perceived in the AZ91 + 2.0Ca + 0.3Sb + 2.0SiCnp nanocomposite. The improved tensileproperties of the nanocomposites are ascribed to the reduced grain size, the increase in dislocation density owing to Coefficientof Thermal Expansion (CTE) mismatch between the alloy and the SiC nanoparticles, the Orowan strengthening aswell as the presence of a relatively higher amount of Al2Caphase in the nanocomposites. The contribution to the improvementof strength of the nanocomposites in decreasing order of their influence are the strengthening due to CTE mismatch,the Hall–Petch strengthening, and Orowan strengthening. The fracture surfaces of the tensile specimens tested at 298 Kconfirmed the presence of transgranular cleavage fracture which remained unchanged at 473 K as well.

Ultra-fine Grained and Dispersion-strengthened Titanium Materials Manufactured by Spark Plasma Sintering

Handtrack Dirk,Sauer Christa,Kieback Bernd 한국분말야금학회 2006 한국분말야금학회 학술대회논문집 Vol.2006 No.1

Ultra-fine grained and dispersion-strengthened titanium materials (Ti-Si, Ti-C, Ti-Si-C) have been produced by high energy ball milling and spark plasma sintering (SPS). Silicon or/and carbon were milled together with the titanium powder to form nanometer-sized and homogeneously distributed titanium silicides or/and carbides as dispersoids, that should prevent grain coarsening during the SPS compaction and contribute to strengthening of the material. The microstructures and the mechanical properties showed that strength, hardness and wear resistance of the sintered materials have been significantly improved by the mechanisms of grain refinement and dispersion strengthening. The use of an organic fluid as carrier of the dispersoid forming elements caused a significant increase in ductility.

Creep in Grain Coarsened ODS MA NiAl

Ur, Soon-Chul,Suh, Sungjae,Nash, Philip 대한금속학회 2002 METALS AND MATERIALS International Vol.8 No.4

NiAl based oxide dispersion strengthened (ODS) intermetallic alloys have been produced by mechanical alloying (MA) and consolidated by hot extrusion. Subsequent isothermal annealing was carried out to induce normal grain growth (NGG), and a thermomechanical treatment was performed to induce secondary recrystallization (SRx). SRx resulted in pronounced elongated grain growth without dispersoid coarsening, whereas concurrent equiaxed coarsening of grains and dispersoids occurred in NGG specimens. Creep properties of grain coarsened ODS MA NiAl were investigated, and the associated creep mechanisms were evaluated. The creep properties of SRxed MA NiAl are compared with those of as-consolidated MA NiAl and other counterparts. It has been shown that SRx results in improved creep resistance compared to NGG mechanism. The apparent activation energy and the stress exponent for creep indicate that SRx MA NiAl exhibits intermediate creep behavior of the two limiting dislocation creep modes; climb controlled and viscous glide controlled. However, a grain size dependent creep has been shown, indicating that grain boundary sliding mechanism contributes to the overall deformation in MA NiAl.

Development of Fe-12%Cr Mechanical-Alloyed Nano-Sized ODS Heat-Resistant Ferritic Alloys

Kim, Ick-Soo,Choi, Byung-Young 대한금속학회 2002 METALS AND MATERIALS International Vol.8 No.3

The development of mechanical alloying (MA)-oxide dispersion strengthened (ODS) heat-resistant ferritic alloys of Fe-12%Cr with W, Ti and Y_2O_3 additions were carried out. Fe-12%Cr alloys with 3%W, 0.4%Ti and 0.25% Y_2O_3 additions showed a much finer and more uniform dispersion of oxide particles among the alloy system studied. Nano-sized oxides dispersed in the alloys suppress the grain growth during annealing at a high temperature and resulted in the remarkable improvement of creep strength. The oxide phase was identified as a complex oxide type of Y-Ti-O.