Microstructural study in graphitised hypereutectoid cast and commercial steels

In the present paper, past research work with new and/or improved processes for willing graphitisation in steels is reviewed. Experiments were carried out to study the carbide dissociation in two different hypereutectoid steels (cast and commercial steels) during graphitisation process by annealing primary martensitic structures at 670°C. Graphite phase evolution during graphitisation treatment was investigated by light optical and scanning electron microscopy and energy dispersive X-ray analyses. It has been reported with promising results that a uniform distribution of alloying elements is found around the graphite particles, which resulted in cast steel. Furthermore, graphite particles in the cast steel were observed to be larger and more spherical than that in commercial steel, which seems to be due to lower Mn/S ratio in cast steel composition.     

Crystalline – Amorphous AA4048 – Al60Nb40 composites processed by mechanical alloying and powder compaction

Commercial crystalline AA4048 powders and mechanically alloyed amorphous Al₆₀Nb₄₀ (at.%) powders were used for fabrication of crystalline-amorphous composites containing 10, 20 and 30?vol% of amorphous phase. High pressure high temperature technique was used for powders compaction. The applied pressure was 7.7?GPa and temperature was in the range 600–1000?°C. The powders and bulk samples were characterized by structural investigations (X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, X-ray fluorescence spectroscopy), hardness and microhardness tests and measurements of density. The obtained sinters revealed relative density above 98.7%. The amorphous component was observed as agglomerates or as single particles surrounding the grains of crystalline AA4048 phase. The crystallization of amorphous phase was not observed. Simultaneously, up to 2.5?wt% of Fe was detected as impurity from milling media. Significant increase of hardness was observed, from 226 to 288HB, resulting from the presence of amorphous component which prevent from cracks propagation during deformation.     

Effect of heat treatment on microstructure and interface of SiC particle reinforced 2124 Al matrix composite

The microstructure and interface between metal matrix and ceramic reinforcement of a composite play an important role in improving its properties. In the present investigation, the interface and intermetallic compound present in the samples were characterized to understand structural stability at an elevated temperature. Aluminum based 2124 alloy with 10 wt.% silicon carbide (SiC) particle reinforced composite was prepared through vortex method and the solid ingot was deformed by hot rolling for better particle distribution. Heat treatment of the composite was carried out at 575 °C with varying holding time from 1 to 48 h followed by water quenching. In this study, the microstructure and interface of the SiC particle reinforced Al based composites have been studied using optical microscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electron probe micro-analyzer (EPMA) associated with wavelength dispersive spectroscopy (WDS) and transmission electron microscopy (TEM) to identify the precipitate and intermetallic phases that are formed during heat treatment. The SiC particles are uniformly distributed in the aluminum matrix. The microstructure analyses of Al–SiC composite after heat treatment reveal that a wide range of dispersed phases are formed at grain boundary and surrounding the SiC particles. The energy dispersive X-ray spectroscopy and wavelength dispersive spectroscopy analyses confirm that finely dispersed phases are CuAl₂ and CuMgAl₂ intermetallic and large spherical phases are Fe₂SiAl₈ or Al₁₅(Fe,Mn)₃Si. It is also observed that a continuous layer enriched with Cu and Mg of thickness 50–80 nm is formed at the interface in between Al and SiC particles. EDS analysis also confirms that Cu and Mg are segregated at the interface of the composite while no carbide is identified at the interface.     

Effect of titanium and titania on chemical characteristics of hydroxyapatite plasma-sprayed into water

HA and its composite particles (HA/Ti, HA/TiO₂) were plasma-sprayed into water as well as on the Ti substrate, respectively. The microstructure and phase compositions of the sprayed HA and its composite particles before and after impinging on the substrate were studied by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The results showed that the HA in the composite particles sprayed into water had a higher crystallinity than that in the composite coating. The addition of Ti or TiO₂ could both influence the decomposition of HA, but no chemical reacting product between them was formed before and after impinging on the substrate. However, EDS analyses showed the occurrence of interdiffusion of elements between HA and TiO₂, which was favorable to enhance the cohesive strength of particles in the composite coating. The post heat treatment at 650 °C for 2 h can effectively improve the crystallinity of coating by transforming amorphous phases into HA.     

Developing bearing steels combining hydrogen resistance and improved hardness

Thermodynamic and kinetic computational modelling are combined to conceive a hydrogen resistant bearing steel. Existing hydrogen resistant steels are not appropriate for bearings due to their low hardness. The proposed microstructure combines a martensitic matrix in which fine cementite precipitates impart strength, and V₄C₃ nano-scaled particles acting as hydrogen traps. It is demonstrated that the conflicting objectives of ultra-hardness and hydrogen resistance can be concealed by: (1) Adding 0.5 wt.% V to 100Cr6, which allows to preserve existing steel production technology. (2) Following a novel heat treatment procedure consisting of austenitisation (and a subsequent temperature spike to dissolve coarse V₄C₃), followed by tempering at 600 °C where V₄C₃ particles form (and a subsequent temperature spike to dissolve coarse cementite), followed by quench and tempering at 215 °C, where fine cementite strengthening particles form. The enhanced trapping capacity of the new steel is demonstrated via thermal desorption; the presence of the desired microstructure after heat treatment is proved via transmission electron microscopy. Concomitant with the trapping ability, a significant hardness increase was observed; this was ascribed to the controlled V₄C₃ precipitation.     

Evaluation and characterization of nanostructure hydroxyapatite powder prepared by simple sol-gel method

Many attempts have been focused on preparing of synthetic hydroxyapatite (HA), which closely resembles bone apatite and exhibits excellent osteoconductivity. Low temperature formation and fusion of the apatite crystals have been the main contributions of the sol-gel process in comparison with conventional methods for HA powder synthesis. This paper describes the synthesis of nano-HA particles via a sol-gel method. Nanocrystalline powder of hydroxyapatite (HA) was prepared using Ca(NO₃)₂·4H₂O and P₂O₅ by a simple sol-gel approach. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for characterization and evaluation of the phase composition, morphology and particle size of products. The presence of amorphous and crystalline phases in the as-dried gel precursor was confirmed by the evaluating technique. Single phase of HA was also identified in the heat treated powder by XRD patterns. SEM and TEM evaluations showed that the obtained powder after heat treatment at 600 °C was agglomerated and composed of nanocrystalline (25-28 nm) HA particles. Increasing the sintering temperature and time could cause decomposition of HA into β-tricalcium phosphate and calcium oxide. The prepared nanocrystalline HA is able to improve the contact reaction and the stability at the artificial/natural bone interface for medical applications.     

Synthesis of V2O3/C composites with different morphologies by a facile route and phase transition properties of the compounds

V₂O₃ and amorphous carbon composites (V₂O₃/C composites) with different morphologies (e.g. nanospheres, nanorods and nanosheets) were, for the first time, successfully synthesized by a facile hydrothermal route and subsequent calcination. The as-obtained samples were characterized by X-ray powder diffraction (XRD), energy dispersive spectrometery (EDS), elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The morphology of V₂O₃/C composites could be easily controlled by varying the reaction time, and, as a result, V₂O₃/C composites with nanospheres, nanorods and nanosheets were selectively synthesized. Furthermore, the phase transition property of V₂O₃/C composites was measured by differential scanning calorimetry (DSC), suggesting that V₂O₃/C composites exhibit the phase transition similar to V₂O₃, which could expand the potential applications of materials related to V₂O₃ in the future.     

The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150°C and 850°C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850°C for 96 hours. In the lower Al or Al-free steels, a -Fe₂Nb₃ phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe₂Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.     

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si₃N₄ material which was produced by the nitridation pressureless sintering (NPS) method using Y₂O₃ and Al₂O₃ as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.     

Crystallization and thermally induced surface relief effects in a Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Y<Subscript>10</Subscript> bulk metallic glass

The crystallization behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) under different reheating conditions was investigated. X-ray diffraction spectrometery (XRD), differential scanning calorimetery (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to examine the crystallization of different samples and the surface relief generated on as-polished surfaces during heat treatment. Different phase constituents were found in samples that experienced different reheating stages. It is proposed that both the reheating temperature and holding time have a significant effect on the phase constituents. The BMG was found to generate surface corrugations of amplitude 1–2 μm during annealing above its crystallization temperature. Such thermally induced surface relief effects are probably a result of the development of surface stresses generated by volumetric changes associated with crystallization of the residual amorphous phase.     

Precipitation behaviors of carbides and Cu during continuous heating for tempering in Cu-bearing medium C martensitic steel

The precipitation behaviors of carbides and Cu during continuous heating for tempering were investigated in Cu-bearing medium C martensitic steel by means of dilatometry, electrical resistivity, and transmission electron microscopy. The addition of 1.5 wt% Cu suppressed carbide precipitation during quenching from 900 °C, resulting in a large amount of solute C atoms in virgin martensite. The addition of Cu increased both the finish temperature of ε-carbide precipitation and the amount of ε-carbide precipitates during continuous heating. The precipitation of cementite was retarded and the amount of cementite precipitates increased by the addition of Cu. Retarded cementite precipitation in the Cu-bearing steel was attributed to sluggish Cu partitioning from cementite particles to the martensite matrix, the hindrance to the migration of cementite interfaces by Cu particles, and the slowed diffusions of C and Fe atoms. Cu precipitation was accelerated by cementite precipitation because cementite interfaces and the high Cu concentration near cementite particles provided nucleation sites for Cu precipitation. The hardness of the tempered Cu-bearing steel was higher than that of the tempered Cu-free steel at the temperatures of over 300 °C due to both Cu precipitation hardening and retarded cementite precipitation.     

Mechanochemical synthesis of amorphous and crystalline magnesium diboride

We have studied the phase composition of materials obtained by mechanochemical processing and subsequent heat treatment of mixtures of magnesium and boron powders in the atomic ratio 1: 2. Differential dissolution, differential scanning calorimetry, and X-ray diffraction data indicate that, during mechanical processing, some of the magnesium reacts with boron to form amorphous magnesium diboride. During annealing of the activated powder mixture, X-ray amorphous magnesium diboride forms at 340°C and crystallizes at 480°C. As shown by high-resolution transmission electron microscopy, the unreacted crystalline magnesium is covered with an amorphous layer consisting of magnesium diboride and boron. The amorphous material obtained by milling contains nuclei of MgB₂ crystallites 3–5 nm in size. During subsequent heating of the activated mixture, magnesium and boron react further to form amorphous magnesium diboride and the amorphous phase crystallizes. Heating of mechanically activated mixtures to just below the crystallization temperature allow MgB₂ nanoparticles to be produced. The formation of nanocrystalline magnesium diboride nuclei along with the amorphous phase during mechanochemical processing facilitates mechanochemical synthesis compared to thermal synthesis.     

Accelerated spheroidization of hypoeutectoid steel by the decomposition of supercooled austenite

A thermal cycling treatment was used to produce a spheroidized structure of hypoeutectoid steel from direct decomposition of supercooled austenite. Scanning electron microscopy and quantitative metallography were employed to study the changing microstructure during the thermal cycling treatment. A conventional spheroidizing annealing was also investigated for comparison. It has been shown that the thermal cycling treatment results in a structure of cementite spheroidites homogeneously distributed in a ferrite matrix within a very short processing time, and the coarsening of cementite particles is controlled by the coupled diffusion of both carbon and iron atoms.     

AEM characterization of interfacial reactions between Ti-coating and 95% Al2O3-5% silicate ceramics

Characterization of the interface reaction region of Ti-95% Al₂O₃ upon exposure to heat treatment at 980 °C has been carried out to investigate changes in microstructure and chemistry of interfacial phases by using energy dispersive X-ray spectroscopy and convergent beam electron diffraction methods. Reactions of titanium-coating with an amorphous grain boundary phase and Al₂O₃ substrate form suicide layers and columnar Ti₃Al grains at the interface respectively.     

Synthesis of cobalt-orthotitanate inverse spinel nano particles via a novel low temperature solvothermal method: structural,opto-electronical,morphological, surface characterization and photo-catalytical application in mineralization of Remazol Red RB 133

Synthesis of single phase cobalt-orthotitanate inverse spinel nano particles is reported for the first time via a novel solvothermal method at low calcination temperature at 400?°C. The pure phase cobalt-orthotitanate spinel nano particles were prepared using cobalt nitrate and titanium tetraisopropoxide (1:1 molar ratio) as cobalt and titanium sources. The synthesised Co₂TiO₄ nanoparticles were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, surface area analyses (Brauner–Emmett–Teller) (BET) and UV–Vis diffuse reflectance spectroscopy. The XRD results proved the formation of a single phase cobalt-orthotitanate (Co₂TiO₄) nanoparticles by calcination temperature of 400?°C. The FE-SEM results showed that nano particles possess a uniform spherical morphology with an average size of 51 nm. Porosity and specific surface area of cobalt-orthotitanate nano particles was measured by nitrogen adsorption using BET and the results showed surface area of 44.47 m²/g. DRS results showed an optical band gap value of 1.802 eV for cobalt-orthotitanate nano powder. An excellent performance as a nano photo-catalyst toward the degradation of Remazol Red RB 133 (RR133) as a single azo textile dye with excellent efficiency. Mineralization of RR133 by highly active cobalt-orthotitanate nano-catalyst coated on glass surface was applied and 97% TOC removal was observed. This is due to high electron–hole charge separation and high surface area of nano-catalyst.     

Effect of alloying elements on the segregation and dissolution of CuAl2 phase in Al-Si-Cu 319 alloys

The hypoeutectic 319 aluminum alloy (Al-7%Si-3.5%Cu) was used in the present study to investigate the effect of diverse alloying elements on the dissolution of the copper phase (CuAl₂) during solution heat treatment. Elements such as Sr, Fe and P were added to the base alloy individually and in various combinations. The cooling curves of these alloys were obtained by solidifying the alloy melts in a preheated graphite mold (600°C, cooling rate 0.8°C/s). From these the first derivate curves were plotted and used to determine the effect of the additives on the precipitation temperature of the Al-CuAl₂ eutectic reaction. Microstructural examination was carried out using optical microscopy, image analysis, and electron probe microanalysis (EPMA), with energy dispersive X-ray (EDX) and wavelength dispersive spectroscopic (WDS) analysis facilities. Samples from different alloys were solution heat treated at 505°C for various times up to 100 hours. The results explicitly reveal that solution heat treatment plays a critical role on the dissolution of the CuAl₂ phase. It is found that Sr leads to segregation of the CuAl₂ phase away from the Al-Si eutectic regions, which slows down its dissolution during solution heat treatment. The -Al₅FeSi phase platelets act as preferred precipitation sites for the copper phase and hence lessen the degree of segregation. Thus, addition of Fe can accelerate the copper phase dissolution. However, phosphorus addition has a negative effect on CuAl₂ dissolution due to (i) its solubility in the CuAl₂ phase particles, and (ii) the formation of (Al,P)O₂ oxide particles which act as nucleation sites for the precipitation of the block-like CuAl₂ phase. It retards the complete dissolution of this copper phase even after 100 hr solution treatment. In the case when phosphorus and iron are added together, the negative effect of phosphorus can be neutralized to some extent.     

Anomalous formation of micrometer-thick amorphous oxide surficial layers during high-temperature oxidation of ZrAl2

The thermal oxidation of ZrAl₂ in the temperature range of 550–750 °C in pure oxygen has been investigated by a combinational experimental approach using X-ray diffraction, scanning electron microscopy/energy dispersive spectrometer, Auger electron spectroscopy and cross-sectional transmission electron microscopy. The thermal oxidation leads to the growth of anomalously thick (up to 4.5 μm) amorphous (Zr_0.33Al_0.67)O_1.66 surficial layers at temperatures as high as 750 °C. The oxidation kinetics obeys a parabolic law with an activation energy of 143 kJ/mol. The underlying mechanism for the formation of such micrometer-thick amorphous oxide surficial layers has been discussed on the basis of interface thermodynamics and the occurrence of high interface stability associated with a synchronous oxidation of Al and Zr elements.     

铝合金表面微弧氧化纳米SiO2复合涂层的元素及物相分析

为提高微弧氧化层性能,通过向微弧氧化电解液中添加纳米SiO2颗粒,在7A52铝合金表面制备了纳米SiO2复合微弧氧化层.利用扫描电镜、能谱仪和X射线衍射仪表征了纳米SiO2复合微弧氧化层的微观组织、元素分布特征及物相组成.研究表明:纳米SiO2颗粒与微弧氧化层复合生长到一起,在微弧氧化层中大致均匀分布;纳米SiO2在微弧氧化层中主要以无定形态存在,同时SiO2与微弧氧化层主体成分Al2O3发生相变反应,生成新物相-莫来石.     

Effect of volume fraction of undissolved cementite on the high cycle fatigue properties of high carbon steels

A present study was performed to investigate the effect of volume fraction of undissolved cementite on the high-cycle fatigue (HCF) properties of high carbon steels. High carbon steels were heat treated to have three distinctive volume fractions of undissolved cementite phase in the martensite matrix. With increasing holding time at the austenitizing temperature of 800 °C, the volume fraction of undissolved cementite gradually decreased from 9% to 1%. A series of tensile and micro-hardness tests showed that the specimen with 1% volume fraction of undissolved cementite exhibited the highest strength, which was mainly attributed to the increased carbon content in the martensite matrix. Though of its highest tensile strength, however, the fatigue strength was the lowest in the specimen with 1% volume fraction of undissolved cementite, which was the reverse trend as compared to the general relationship between fatigue strength and tensile strength. This was attributed to the higher propensity of crack initiation in the specimen containing fewer amounts of undissolved cementite, where cracks were observed to nucleate at the interfaces between nonmetallic inclusions (Al₂O₃ and SiO₂) and martensite matrix during HCF tests. A probabilistic model was developed to predict fatigue strength of heat treated high carbon steel and compared with the experimental data.     

Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900 °C for 1 h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni₃Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10 nm to 50 nm were precipitated and uniformly distributed in the Ni₃Ti phase to form a thin granular layer after heat treatment at 700 °C. The granular layer spread from the edge toward the center of the Ni₃Ti phase with increasing temperature. A large number of fine equiaxed Cr₂₃C₆ particles with 0.2–0.5 μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900 °C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.