Showing papers on "Microstructure published in 2001"

A process model for friction stir welding of age hardening aluminum alloys

Abstract: In the present investigation, a numerical three-dimensional (3-D) heat flow model for friction stir welding (FSW) has been developed, based on the method of finite differences. The algorithm, which is implemented in MATLAB 5.2, is provided with a separate module for calculation of the microstructure evolution and the resulting hardness distribution. The process model is validated by comparison with in-situ thermocouple measurements and experimental hardness profiles measured at specific time intervals after welding to unravel the strength recovery during natural aging. Furthermore, the grain structure within the plastically deformed region of the as-welded materials has been characterized by means of the electron backscattered diffraction (EBSD) technique in the scanning electron microscope (SEM). Some practical applications of the process model are described toward the end of the article.

Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study

Abstract: Three aluminium metal matrix composites containing reinforcing particles of B4C, SiC and Al2O3 (0-20 vol. %) were processed. The stir-casting manufacturing route followed by hot extrusion was utilized, being one of the cost-effective industrial methods. In this study, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites. The microstructural distribution of reinforcing particles in all three composites was studied by means of optical and scanning electron microscopy (SEM). The distribution and chemical composition of the phases formed at matrix/particulate interface of the processed composites were also investigated by SEM and energy dispersive x-ray spectroscopy (EDX). A clear interfacial reaction product/layer was found at Al/SiC interface for composites held for a relatively long processing time (> 30 minutes). No reaction product was observed at Al/B4C and Al/Al2O3 interfaces at the resolution limit of the SEM used. On the other hand, two secondary phases (alumina and another phase containing aluminium, boron and carbon) were found in the aluminum matrix away from the interface in Al-B4C composites. From the fracture surface analysis, B4C reinforced Al composite seemed to exhibit a better interfacial bonding compared to the other two composites.

Materials characterization of ZrO2–SiO2 and HfO2–SiO2 binary oxides deposited by chemical solution deposition

Abstract: The thermal stability, microstructure, and electrical properties of xZrO2⋅(100−x)SiO2 (ZSO) and xHfO2⋅(100−x)SiO2 (HSO) (x=15%, 25%, 50%, and 75%) binary oxides were evaluated to help assess their suitability as a replacement for silicon dioxide gate dielectrics in complementary metal–oxide–semiconductor transistors. The films were prepared by chemical solution deposition using a solution prepared from a mixture of zirconium, hafnium, and silicon butoxyethoxides dissolved in butoxyethanol. The films were spun onto SiOxNy coated Si wafers and furnace annealed at temperatures from 500 to 1200 °C in oxygen for 30–60 min. The microstructure and electrical properties of ZSO and HSO films were examined as a function of the Zr/Si and Hf/Si ratio and annealing temperature. The films were characterized by x-ray diffraction, mid- and far-Fourier transform infrared (FTIR), Rutherford backscattering spectroscopy, and Auger electron spectroscopy. At ZrO2 or HfO2 concentrations ⩾50%, phase separation and crystallizatio...

Magnesium strengthened by SiC nanoparticles

Abstract: Mg was reinforced with SiC nanoparticles by powder metallurgical technique. The SiC nanoparticles were generated by laser-induced gas phase reaction in a flow reactor and had a median particle diameter of 30 nm. In order to distribute the nanoparticles in the Mg matrix, Mg micropowder with a median particle diameter of 40 μm was mixed or ball milled with the nanoscaled ceramic powder followed by hot extrusion. The mechanical properties of the new material were investigated by tensile tests and creep measurements and the microstructure of the composites were examined by light and transmission electron microscopy (TEM).

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

Abstract: A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.

Distribution of tensile property and microstructure in friction stir weld of 6063 aluminum

Abstract: Dominant microstructural factors governing the global tensile properties of a friction-stir-welded joint of 6063 aluminum were examined by estimating distribution of local tensile properties corresponding to local microstructure and hardness. Yield and ultimate tensile strengths of the as-welded weld were significantly lower than those of the base material. Postweld aging and postweld solution heat-treatment and aging (SHTA) restored the strengths of the weld to the levels of the base material. Elongation was found to increase with increasing strength. Hardness tests showed that the as-welded weld was soft around the weld center and that the aged weld and the SHTA weld had relatively homogeneous distributions of high hardness. Hardness profiles of the welds were explained by precipitate distributions and precipitation sequences during the postweld heat treatments. The strengths of the welds were related to each minimum hardness value. In a weld having a heterogeneous hardness profile, the fracture occurred in the region with minimum hardness. When a weld had a homogeneous hardness profile, its fracture site depended on both crystallographic-orientation distribution of the matrix grains and strain tensor of the imposed deformation, i.e., it fractured in the region with a minimum average Taylor factor.

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys

Abstract: Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al6(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.

Optical, electronic, and transport properties of nanocrystalline titanium nitride thin films

Abstract: Spectroscopic ellipsometry (SE) was employed to get insights on the optical, electronic, and transport properties of nanocrystalline titanium nitride (TiNx) films with respect to their microstructure and stoichiometry. The films’ properties can be tailored by varying the energy of bombarding ions during sputter deposition and the substrate temperature (Td). The best metallic behavior of TiNx (resistivity 40 μΩ cm and conduction density 5.5×1022 electrons/cm3) has been observed in films developed with energy above 100 eV and Td⩾400 °C. A redshift of the optical gaps has been observed for overstoichiometric films, suggesting it as a sensitive probe to investigate the TiNx stoichiometry. The energy, strength, and broadening of the interband transitions were studied with respect to the energy of ions and Td and they were explicitly correlated with the TiNx crystal cell size and grain orientation. On the other hand, the study of intraband absorption has provided the conduction electron density with respect to ...

Friction Stir Processing: A New Grain Refinement Technique to Achieve High Strain Rate Superplasticity in Commercial Alloys

Abstract: Friction stir processing is a new thermo-mechanical processing technique that leads to a microstructure amenable for high strain rate superplasticity in commercial aluminum alloys. Friction stirring produces a combination of very fine grain size and high grain boundary misorientation angles. Preliminary results on a 7075 Al demonstrate high strain rate superplasticity in the temperature range of 430-510 °C. For example, an elongation of >1000 % was observed at 490 °C and 1 × 10 -2 s -1 . This demonstrates a new possibility to economically obtain a superplastic microstructure in commercial aluminum alloys. Based on these results, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.

AC Impedance Spectroscopy in Characterizing Time-Dependent Corrosion of AZ91 and AM50 Magnesium Alloys Characterization with Respect to Their Microstructures

Abstract: The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E corr ) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the α-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al 2 O 3 in the magnesium oxide (or hydroxide) layer a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance, This was attributed to an increase of the number of Mg 2 Si particles, accelerating the dissolution n of eutectic areas.

Nanostructure formation on the surface of railway tracks

Abstract: The microstructure of the surface layer of railway tracks is investigated. It is shown that the surface layer of the rail transforms during exploitation into a nanocrystalline Fe–C alloy. The mechanism of the nanostructure formation is discussed. It is shown that the transformation of pearlite to the nanostructured Fe–C alloy layer is caused by the heavy plastic deformation at the wheel–rail contact zone. The transformation of the microstructure of the surface may take place at rail–wheel contact temperatures less than 230°C and its mechanism is similar to that taking place during mechanical alloying.

Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis

Abstract: Binary Cu/ZnO catalysts with varying molar ratios (90/10 through 10/90) were studied under methanol synthesis conditions at 493 K and at atmospheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N2O reactive frontal chromatography, N2O RFC) after reduction in 2 vol% H2 at 513 K. Activity data were supplemented with a detailed analysis of the microstructure, i.e., crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N2O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn K edge show that about 5% ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e.g., size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.

Self-assembling three-dimensional colloidal photonic crystal structure with high crystalline quality

Abstract: High-quality colloidal crystal multilayers were fabricated from aqueous solutions by the vertical deposition method. The effect of the evaporation temperature on the crystalline quality of colloidal crystals was carried out. It is found that with the increase of the evaporation temperature, the colloidal crystal shows an increasing tendency towards equilibrium face-centered-cubic phase, and the resulted sample also shows few dislocations and vacancies when the balance in the processes of nucleus formation, particle transport, and crystallization can be kept. However, with the further increase of the evaporation temperature (above 55 °C), a vast amount of defects appear in the crystal because the fast water evaporation rate, which results in a fast crystal growth rate, will spoil the balance. Optical measurements correspond well to the microstructure results.

Precipitation behavior and its effect on strengthening of an HSLA-Nb/ti steel

Abstract: The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 µm, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needle-shaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650°C to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.

Fabrication and evaluation of plasma sprayed nanostructured alumina-titania coatings with superior properties

Abstract: Reconstituted nanostructured powders were plasma sprayed using various processing conditions to produce nanostructured alumina‐titania coatings. Properties of the nanostructured coatings were related to processing conditions through a critical plasma spray parameter (CPSP) that in turn, can be related to the amount of unmelted powder incorporated into the final coating. Those coatings that retain a significant amount of unmelted powder show optimum microstructure and properties. Selected physical and mechanical properties were evaluated by X-ray diffraction (XRD), optical and electron microscopy, quantitative image analysis and mechanical testing. Constituent phases and the microstructure of the reconstituted particles and plasma sprayed coatings were examined with the aid of quantitative image analysis as a function of processing conditions. Mechanical properties including hardness, indentation crack growth resistance, adhesion strength, spallation resistance during bend- and cup-tests, abrasive wear resistance and sliding wear resistance were also evaluated. These properties were compared with a commercial plasma sprayed alumina‐titania coating with similar composition. Superior properties were demonstrated for nanostructured alumina‐titania coatings plasma sprayed at optimum processing conditions. © 2001 Elsevier Science B.V. All rights reserved.

Ferroelectric ceramics: defects and dielectric relaxations

Abstract: Dielectric relaxations in ferroelectric ceramics occur at various frequencies, depending on the type of chemical or physical defects These defects depend on either intrinsic or extrinsic heterogeneities due to special heat treatments (quenching, annealing,…), ionic substitutions, grain size additives, and grain boundary nature The value of the relaxation frequency fr increases from about 102 to 1012 Hz as the scale of the defect phenomenon decreases from microstructure, to nanostructure, to unit-cell, to atomic vibrations This type of study requires a pluridisciplinary approach involving solid state chemistry, materials science, solid state physics and various industrial aspects The applications in the area of electronic ceramics are related to the value of fr In absorbants, the usable frequency is close to fr, whereas it is far from fr in good insulators In view of numerous experimental examples, a classification is proposed to predict which materials may be suitable for a given application

Additive effects on electrical properties of (Bi1/2Na1/2)TiO3 ferroelectric ceramics

Abstract: Microstructure, dielectric, ferroelectric and piezoelectric properties of bismuth sodium titanate (Bi1/2Na1/2)TiO3 (BNT) were studied for a candidate as lead-free piezoelectric ceramics. In the case of Mn addition, the Curie temperature, Tc, decreases rapidly with increasing amount of doped MnCO3. The resistivity, ρ, is enhanced to 3×1014 (Ωcm) (at 40°C) for BNT+MnCO3 0.2 (wt.%) and electromechanical coupling factor, k33, was obtained the relatively high value under the condition of the low poling field, Ep for the Mn-doped BNT ceramics. It seems that Mn ions exist in the grain, and substitute for the A- or B-site of the perovskite structure, and eject Bi ions to the air.

Microstructure and mechanical/thermal properties of Cr–N coatings deposited by reactive unbalanced magnetron sputtering

Abstract: Chromium nitride (CrN) is a hard material and a well-established coating for applications where severe corrosion and friction conditions are present. In this work, we report on the influence of nitrogen/argon flow rate ratio, ion energy and ion/atom flux ratio on the microstructure, hardness, residual stresses and thermal stability of magnetron sputtered chromium nitride coatings. The coatings were characterized with respect to thickness, morphology, chemical composition, microstructure and hardness. Hardness values up to 38.4 GPa could be obtained for stoichiometric CrN, which strongly depend on the grain size and residual stress. Thermal coating properties were evaluated using stress measurements during thermal cycling and XRD analyses after annealing at 500 and 700°C. Film stresses up to 700°C were measured from the bending of coated silicon specimens using the Stoney formula. Stress relaxation occurring during this temperature treatment strongly depends on the biaxial stresses in the as-deposited state. The interrelationships between growth conditions, microstructure, mechanical and thermal properties will be presented and discussed.

Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability

Abstract: This study included a comparison of the baseline Sn-3.5Ag eutectic to one near-eutectic ternary alloy, Sn-3.6 Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0 Cu-0.45 Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1–3 C/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu6Sn5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150 C. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.

Corrosion resistance of multi-layered plasma-assisted physical vapour deposition TiN and CrN coatings

Abstract: The corrosion resistance of mild steel coated with single layered and multi-layered TiN and CrN coatings has been studied. The base material was coated with TiN and CrN by an electron-beam plasma-assisted physical vapour deposition (PAPVD) technique. ‘Single’ layers of TiN or CrN, which normally include an interlayer (approx. 100–200 nm) of Ti or Cr under the main TiN or CrN film, were prepared; four layers of TiN (or CrN) were produced by four sequential repetitions of the single-layer process. The microstructural features and corrosion performance were then compared. It is shown that applying four layers of coating can enhance the corrosion performance of PVD TiN and CrN coated mild steel. The corrosion resistance improvement is not only attributed to the increase in thickness, but also to the internal microstructure and phase composition. CrN coatings produced in this work proved to be particularly promising in terms of corrosion resistance, owing to their dense non-columnar structure which contained a mixture of three phases: Cr (b.c.c.), Cr2N (hexagonal) and CrN (f.c.c.). Mild corrosion reactions were observed in CrN coated steel during various electrochemical tests in aqueous salt solution, indicating that inter-phase corrosion had caused a redistribution of the current flow, so as to eliminate current concentration at small through-coating pinholes. This prevented rapid galvanic attack at the coating/substrate interface. More importantly however, in comparison with the columnar structure of TiN coatings, the dense structure with fine equiaxed crystallites made the CrN coatings less permeable to the corrosive medium.