Microstructure Of Transition Zone Research Articles

Effect of nanosilica and fiber on mechanical properties and microstructure of recycled coarse aggregates road concrete

Recycled coarse aggregate has its own characteristics of large porosity, high water absorption and high crushing index, which adversely affects the mechanical strength and durability of recycled aggregate concrete. In this study, the mechanical properties, durability and microstructure of recycled aggregate concrete (RAC) modified with nano-SiO2 (NS) and fibres were investigated to prepare road RAC, using NS polypropylene fibres (PPF) and basalt fibres (BF) as test variables. Compared with RAC only incorporating NS, NS and fibre synergism can significantly enhance the mechanical properties of recycled concrete, compressive strength increased by 27–51 %, flexural strength increased by 12 %, and abrasion resistance increased by 25–72 %. The synergistic effect of NS and PPF significantly improves the frost resistance of recycled concrete. NS has a volcanic ash effect which can react with calcium hydroxide to produce a large number of C-S-H in the secondary hydration reaction, making the internal structure of the cement matrix denser. Interfacial transition zone microstructure is more compact, thus interfacial transition zone porosity is effectively reduced. The hydration products are attached to the fibre surface, which enhances the bonding ability between the fibers and matrix. Therefore, the fibres effectively inhibit the generation and development of internal micro-cracks. NS optimises the internal pore structure of recycled aggregate concrete and enhances the interfacial transition zone, and NS synergistically with PPF can improve the strength and durability of various mechanical properties of recycled concrete. The research results provide the scientific basis for the application of road RAC.

The interfacial transition zone microstructure of ground ferronickel slag incorporated self-compacting concrete investigated by nanoindentation

The effect of using ground ferronickel slag (GFNS) as a supplementary binder on the interfacial transition zone (ITZ) and strength properties of self-compacting concrete (SCC) were investigated. Cement was replaced by 20–50% GFNS, and mechanical properties of the SCC were assessed. The ITZ was characterized by EDS line scanning. The elastic modulus and hardness properties of ITZ were determined by grid nanoindentation tests. Microstructural changes were followed up by XRD, SEM and EDS analysis, while the macro-level characterization was carried out by compressive and splitting tensile strengths. The strengths showed a decreasing trend with increasing GFNS content. The 28-day compressive strength of the mix with 35% GFNS was 66 MPa as compared to 79 MPa for the control mix. Moreover, up to 35% replacement level, this study finds a compatible ITZ, with a thickness of around 55 μm. The elastic modulus and the hardness values from the nanoindentation test suggest a decrease of the calcium silicate hydrate with the increase of GFNS that decreased strength. XRD data confirmed this finding. The line scanning data from EDS reveals that the presence of Al, Fe and S in ITZ was not significant. Hence, the Ca/Si ratio plays a pivotal role in determining the ITZ characteristic, which increases with the increase of GFNS content in the binder. Strength diminishes gradually with increasing Ca/Si ratio, possibly due to the damaged silicate skeleton and the hydrolytic weakening effect. Therefore, microstructural observations and nanoindentation results well elucidate the strength results, and these experimental outcomes will provide a strong base for utilizing GFNS in SCC.

Microstructure evolution and phase transformation behavior of Ti17 and γ-TiAl bimetallic structure fabricated by directed energy deposition

A bimetallic structure prepared by directed energy deposition (DED) can simultaneously meet multiple property demands in service. However, in existing studies, formation mechanism and controlling method of transition zone (TZ) remain elusive due to narrow TZ in Functionally graded Materials (FGMs). In this article, a bimetallic structure of Ti17 and γ-TiAl alloy with a wide transition zone prepared by DED was obtained with special powder feeding method, and microstructure of TZ was studied in more details. According to change of element content and microstructure, the wide transition zone (TZ) is distinguished as TZ1 and TZ2. With the increase of Al content from TZ1 to TZ2, α phase grows up and (α2+γ)lamellar precipitates according to a certain burgers orientation relation. Meanwhile, massive γ phase nucleates at α grain boundary, and β phase transforms into ordered phase B2. Microhardness of TZ1 is much higher than that of Ti17 because the effect of solid-solution strengthening. The microhardness of TZ2 is reduced by precipitation of softer phases (α2+γ)lamellar and massive γ. In TiAl, the increase of Al content leads to coarsening of (α2+γ)lamellar and massive γ, so microhardness declines. The sequence of phase transitions in different zones is simulated and the results are consistent with the observed microstructure. Due to low strength microstructure with coarsening α phase and fine (α2+γ)lamellar, the average tensile strength of gradient sample at room temperature is only 167 MPa, and fracture shows typical cleavage fracture characteristic.

The bond properties between ultra-high-performance concrete and normal strength concrete substrate: Bond macro-performance and overlay transition zone microstructure

This study investigates the bond performance between ultra-high-performance concrete (UHPC) and normal strength concrete (NSC) substrate. A hoop constraint method is designed to assess the shrinkage effect of repair materials on bond macro-performance. The bond strength and permeability of the same specimen are determined using the hoop constraint method. Additionally, the properties of repair materials, such as drying shrinkage, thermal gravity analysis, and mercury intrusion porosimetry, were investigated. The overlay transition zone (OTZ) microstructure is analysed to reveal the bonding mechanism. The results indicate that the hoop constraint method effectively reflects the ability of UHPC to overcome the significant autogenous shrinkage effect and achieve good bond macro-performance. Moreover, the UHPC-combined substrate exhibits better bond strength than that of the NSC because the repair interface of the UHPC is more compact than that of the NSC. Furthermore, the OTZ width of the UHPC-combined substrate is narrower than that of the NSC, and in the OTZ, the modulus of the UHPC is higher than that of the NSC.

Structure and Properties of Ti/Ti64 Graded Material Manufactured by Laser Powder Bed Fusion.

Multimaterial additive manufacturing is an attractive way of producing parts with improved functional properties by combining materials with different properties within a single part. Pure Ti provides a high ductility and an improved corrosion resistance, while the Ti64 alloy has a higher strength. The combination of these alloys within a single part using additive manufacturing can be used to produce advanced multimaterial components. This work explores the multimaterial Laser Powder Bed Fusion (L-PBF) of Ti/Ti64 graded material. The microstructure and mechanical properties of Ti/Ti64-graded samples fabricated by L-PBF with different geometries of the graded zones, as well as different effects of heat treatment and hot isostatic pressing on the microstructure of the bimetallic Ti/Ti64 samples, were investigated. The transition zone microstructure has a distinct character and does not undergo significant changes during heat treatment and hot isostatic pressing. The tensile tests of Ti/Ti64 samples showed that when the Ti64 zones were located along the sample, the ratio of cross-sections has a greater influence on the mechanical properties than their shape and location. The presented results of the investigation of the graded Ti/Ti64 samples allow tailoring properties for the possible applications of multimaterial parts.

Micro-mechanism and mechanical properties of solid-powder hot isostatic pressing diffusion-bonded Ti2AlNb alloy

Achieving the near-net shaping of brittle and difficult-to-machine materials is still challenging. Thus, we explore a method to prepare Ti-22Al-25Nb alloy by solid-powder hot isostatic pressing (HIP) diffusion bonding. The grain size, microstructure, interface features and mechanical properties of the fabricated alloy were systematically investigated. The results show that the solid-powder interface realizes a complete metallurgical bonding, and the grain size, composition and microstructure in transition zone is formed on the side of preform through recrystallization. There is a huge difference for the grain size between the powder forming zone and the preforming zone. As a result, the fabricated sample for solid-powder transition zone exhibits an excellent mechanical properties, with a tensile strength of 940 MPa, elongation of 2.9% and torsional strength of 815 MPa, respectively. In response to the torsional force, the crack starts from the preforming zone, and the crack deflection and branching occurs in the transition zone, thereby preventing the crack from propagating to the powder forming zone. The torsional strength of the solid powder HIP diffusion bonding zone is basically the same as that of the preformed zone. This study proposes a new solution for fabricating brittle and difficult-to-process materials and is of great significance in the development of the overall near-net shaping technology for complex components of such material.

Microstructure and mechanical properties of TA15-Ti2AlNb bimetallic structures by laser additive manufacturing

The TA15-Ti2AlNb bimetallic samples with no defects were prepared by laser additive manufacturing. The evolution of the composition and microstructure in transition zone, and the mechanical properties were investigated. Composition in the transition region shows a step-and-platform distribution due to the rapid solidification and diffusion during the depositing process. The phase transformation from TA15 alloy side to Ti2AlNb-based alloy side is α (lath) +β→α (coarse acicular) +β→β+α2→α2+O + B2. The TA15-Ti2AlNb bimetallic structures present tensile properties with average tensile strength of 1025 MPa and elongation of 7.3%. The fracture position of samples is located in diffusion zone, and the fracture mode is quasi-cleavage fracture. The TA15-Ti2AlNb bimetallic circular structure with bell mouth was fabricated and this work shows great potential for fabricating bimetallic structures by laser additive manufacturing technology.

Ideal supplementary cementing material – Metakaolin: A review

AbstractThough being an ancient trend, usage of the homogeneous material cement in the construction industry is steadily getting eradicated with the springing up of supplementary cementing materials (SCM). Metakaolin is an imminent mineral admixture extracted from the mineral ore kaolinite, which enhances the interfacial zone by more efficient packing at the cement paste-aggregate particle interface, thus reducing the bleeding and producing a denser, more homogeneous transition zone microstructure. This paper depicts the various repercussions of the pozzolanic material metakaolin in the fresh and hardened properties of concrete when replaced with cement in finite amount. Also, it states the behavior of high-performance concrete and self-compacting concrete with metakaolin.

Wear-resistant boride reinforced steel coatings produced by non-vacuum electron beam cladding

In this work, we present a wear-resistant coating fabricated by non-vacuum electron beam cladding of Fe62Cr10Nb12B16 at.% powder on a mild steel substrate. The protective coating was 1.3 mm thick, dense, and exhibited an α-(Fe,Cr) matrix reinforced by a significant fraction of hard borides formed upon solidification. Micrometric and nanometric borides homogeneously dispersed within the matrix were formed due to the homogeneous melting and the relatively fast cooling to suppress the excessive phase growth. An intimate metallurgically bonded interface between the coating and substrate was characterized by low compositional dilution and a fine eutectic-like transition zone microstructure anchoring the dissimilar materials. The coatings displayed a higher wear resistance compared to the mild steel substrate, showing specific wear rates, κ, about one order of magnitude lower (10−5 against 10−4 mm3/N.m, respectively). The abrasive wear mechanism was dominant for the coating sample when tested at low sliding velocity, 10 cm/s, due to the detachment of hard borides from the surface and their incorporation into the tribosystem. The adhesive wear mechanism was found to be dominant at higher sliding velocities of 20 and 40 cm/s. Dry sand/rubber wheel testing revealed the higher resistance of the coating against abrasive wear compared to the mild steel substrate. Regardless of the wear mechanism, the Fe62Cr10Nb12B16 at.% coatings showed a superior sliding and abrasive wear resistance and represented an interesting protective measure to extend the service of inexpensive mild steel components.

Formation of Magnesium Silicide in bulk diffusion couples

Formation of Magnesium Silicide in binary Mg/Si diffusion couples and in reactions of Mg with Silicon Dioxide was investigated. The position of the Kirkendall marker plane inside the reaction zone of the bulk diffusion couples revealed that at 500 °C, Mg is virtually the only mobile species in the Mg2Si intermetallic compound. Such diffusion behaviour was rationalized using crystallographic information about anti-fluorite structure of the Mg2Si phase. Using general relations between integrated diffusion coefficient for a stoichiometric compound, tracer diffusivities of the components and data on thermodynamic stability of the phases involved in the interaction, the tracer diffusion coefficient DMg⁎ in the Mg2Si was derived.During diffusion-controlled interaction of Mg with Silicon Oxide, the Mg2Si intermetallic is formed within the transition zone microstructure as a continuous phase with the periodically arranged bands of MgO-particles embedded in the matrix phase. In the case of bulk Mg/quartz-SiO2 couples annealed at 500 °C, MgSiO3 and Mg2SiO4 silicates are also found in the reaction zone next to the SiO2-substrate. This can be explained with the help of the equilibrium Mg–Si–O phase diagram. However, no formation of Magnesium Silicates was observed in diffusion-controlled reactions between Mg and fused (amorphous) Silica.At the early stages of the interaction in bulk Mg/SiO2 couples, the situation was found to be different. No quasi-binary compounds MgSiO3 and Mg2SiO4 were detected in the diffusion zone after reaction of Mg with monocrystalline SiO2 as well as fused Silica at 550 °C. In both cases, the layer adjoining the SiO2-substrate is Magnesium Oxide.The formation of periodic layered morphology is considered to be a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state. The suggested mechanism is operative when the components have distinctly different mobilities in adjacent reaction product layers.

Mechanical properties and interfacial transition zone microstructure of concrete with waste basalt powder addition

This study evaluates the use of waste basalt powder as a replacement of the fine aggregate to enhance concrete properties. Experiments were carried out to determine the compressive and flexural strengths of concrete with basalt powder and the influence of the basalt powder on the microstructure of the ITZ. Results indicate that addition of basalt powder as a replacement of sand, improves the compressive and flexural strength of concrete. It is mainly due to the filler effect of the basalt powder. The ITZ in concrete with basalt powder addition is denser and stronger than in concrete without basalt.

Study on microstructure of transition zone and its strong contrast of single T700 carbon fibers

The transition zone (TZ) between the skin and core of Toray T700 carbon fiber was investigated by transmission electron microscopy. The higher basal-plane orientation was identified in the TZ compared with the skin and core, but it disappeared after heat treatment at 2800°C. Plasmon peak energy in the TZ was higher than that in the skin and core about 0.7–0.8eV, indicating the TZ with higher density. No element concentration existed in the TZ. The TZ with strong contrast manifests itself before and after heat treatment, and formation mechanism of its strong contrast was proposed.

SHS joining by thermal explosion in (Ni + Al)/Nb/(Ni + Al + Nb) sandwiches: Microstructure of transition zone

Thermal explosion in (Ni + Al)/Nb/(Ni + Al + Nb) sandwiches was found to results in formation of (a) NiAlNb composite containing NiAl, NiAlNb, and trace amounts of Nb5Ni and (b) good joining with a transition zone.

Properties, microstructure and hydration products of lightweight aggregate concrete with metakaolin and slag addition

The effects of metakaolin (MK) and slag on the strength, shrinkage, chloride resistance and interfacial transition zone microstructure of lightweight aggregate concrete were characterized by a range of analytical techniques. Slag decreased early age strength while MK increased late age strength, which was further increased by the combination of MK and slag. MK was more effective on reducing chloride diffusion coefficient and shrinkage than slag. These properties were further improved by the combination of MK and slag. Linear correlation was observed between strength and chloride diffusion coefficient with higher strength presenting lower diffusion coefficient. Portlandite, ettringite, hemicarboaluminate and monocarboaluminate were identified as hydration products. MK promoted the formation of hemicarboaluminate and monocarboaluminate. Portlandite content decreased at 28days in concrete with MK and slag, resulting in an interfacial transition zone with denser microstructure and lower Ca/Si ratio, due to the pozzolanic reaction.

STUDY OF MICROSTRUCTURE AND STRESS CORROSION CRACKING BEHAVIOR IN WELDING TRANSITION ZONE OF Ni–BASED ALLOYS

Welding technique is generally used in nuclear power plant for manufacturing and machining important components such as steam generator. The similar metal weld and dissimilar metal weld were commonly involved to connect and fixup tubings in steam generators. However, in recent years large amounts of cracking accidents have been observed in the welded joints. A concern has been raised about the integrity and reliability in the joint transition zone due to the high susceptibility of heat affected zone (HAZ) and fusion zone (FZ) to stress corrosion cracking (SCC). In this study, the similar metal and dissimilar metal joints were investigated and compared, focusing on the correlation between microstructure, residual strain and SCC behavior. The microstructures of transition zone in Ni-based Alloys 690 and 52 similar metal joint and Alloys 182 and A533B low alloy steel dissimilar metal joint were investigated comprehensively by SEM, EBSD, TEM. The residual strain distribution in the HAZ of 690-52 similar metal joint was quantitatively measured. The SCC behavior of 182-A533B dissimilar metal joint in high temperature oxygenated water were simulated by creviced bent beam specimen. The HAZ in the similar joint exhibits higher residual strain, sensitive microstructure and high susceptibility to SCC, therefore, the HAZ region deserve more attention during the inspection and examination of components. The FZ of the dissimilar metal joint exhibits complicated microstructure and chemical

Influence of Mineral Admixtures on Crack Resistance of High Strength Concrete

As high strength concrete (HSC) is widely used in construction, more and more attention has been paid to crack resistance of it. In order to improve crack resistance of HSC, we study the influence of mineral admixtures (ground slag, silica fume and fly ash) on brittleness and characteristic length as crack resistance’ indexes. Testing researches shows, without admixture, crack resistance of HSC is the lowest; when one of the three mineral admixtures added, crack resistance increases dramatically; when two of the three mineral admixtures are added in the concrete, crack resistance increases a little more and it does not change very much no matter which two are mixed; crack resistance comes out the highest when the three mineral admixtures are added in concrete. Mechanism analysis shows, Adding fine and high active ground slag, silica fume and fly ash into concrete can greatly improve microstructure of transition zone, decrease Ca(OH)2, ettringite and porosity in concrete, increase C-S-H gel and greatly reduce the original micro-cracks in the transition zone.

Effect of heat input on precipitation characteristic and order degree of Ni 3Al in light beam surfacing layer

With the use of SEM, EDS and X-ray diffraction, the microstructure and phase composition of Ni 3Al intermetallic coating obtained by light beam surfacing were investigated. Moreover, the effect of heat input on the precipitation characteristic and order degree of Ni 3Al was also analyzed. The results show that with the Ni and Al mixed powder of 3:1 atomic ratio as surfacing material, the light beam surfacing layer cannot entirely consist of Ni 3Al owing to the oxidation burning loss of aluminum and the dilution of the molten pool by base metal. The surfacing layer with lower heat input (3000 J/mm) is composed of the eutectic of Ni 3Al and γ-(Fe,Ni). The increase of heat input (4200 J/mm) leads to intensified dilution of molten pool and increased oxidation burning loss of aluminum, which results in the reduced aluminum content in molten pool. The microstructure of the surfacing layer with higher heat input is the hypoeutectic of Ni 3Al and γ-(Fe,Ni), a lot of particle α-Fe precipitates within γ-(Fe,Ni) phases, and the order degree of Ni 3Al increases. With the increase of heat input, the microstructure of transition zone between surfacing layer and base metal changes from γ-(Fe,Ni) to lath martensite with hardness of HV740.

Effects of cement particle size distribution on performance properties of Portland cement-based materials

The original size, spatial distribution, and composition of Portland cement particles have a large influence on hydration kinetics, microstructure development, and ultimate properties of cement-based materials. In this paper, the effects of cement particle size distribution on a variety of performance properties are explored via computer simulation and a few experimental studies. Properties examined include setting time, heat release, capillary porosity percolation, diffusivity, chemical shrinkage, autogenous shrinkage, internal relative humidity evolution, and interfacial transition zone microstructure. The effects of flocculation and dispersion of the cement particles in the starting microstructures on resultant properties are also briefly evaluated. The computer simulations are conducted using two cement particle size distributions that bound those commonly in use today and three different water-to-cement ratios: 0.5, 0.3, and 0.246. For lower water-to-cement ratio systems, the use of coarser cements may offer equivalent or superior performance, as well as reducing production costs for the manufacturer.

99/02540 Coal combustion fly ash as construction material in a sanitary landfill project

The effects of metakaolin (MK) and slag on the strength, shrinkage, chloride resistance and interfacial transition zone microstructure of lightweight aggregate concrete were characterized by a range of analytical techniques. Slag decreased early age strength while MK increased late age strength, which was further increased by the combination of MK and slag. MK was more effective on reducing chloride diffusion coefficient and shrinkage than slag. These properties were further improved by the combination of MK and slag. Linear correlation was observed between strength and chloride diffusion coefficient with higher strength presenting lower diffusion coefficient. Portlandite, ettringite, hemicarboaluminate and monocarboaluminate were identified as hydration products. MK promoted the formation of hemicarboaluminate and monocarboaluminate. Portlandite content decreased at 28 days in concrete with MK and slag, resulting in an interfacial transition zone with denser microstructure and lower Ca/Si ratio, due to the pozzolanic reaction.

Transition zone in high performance concrete during hydration

A number of transition zone studies are available in the literature for normal strength concrete. High performance concrete behaves differently from normal strength concrete and very few transition zone studies are available for high performance concrete. The weakest unique transition zone between cement paste and coarse aggregate particle controls many important properties of concrete such as compressive strength. In this paper the relationship between transition zone microstructure and compressive strength property of high performance concrete using Australian industrial silica fume and Australian aggregates is investigated. Based on the scanning electron microscopic studies of transition zone, it has been found that high performance concrete transition zone thickness increases while compressive strength increases. Further, high performance concrete transition zone thickness is much less than the thickness of transition zone of normal strength concrete. This thin transition zone also enhances compressive strength. It has been observed that silica fume presence in the transition zone significantly enhances compressive strength. Silica fume particles consume calcium hydroxide, which is present in transition zone, and make the zone dense and uniform.