Normal and anormal microstructure of plasma nitrided Fe-Cr alloys

Plasma nitriding behavior of Fe-Cr alloys has been studied at temperatures in the range of 773–873 K in order to provide basic knowledge for microstructure design of nitrided layers and to improve the wear resistance. In the nitriding temperature of 773 K, typical microstructure of nitrided layers was observed as reported elsewhere. However, anormal microstructure of nitrided layers was observed under a nitriding condition, at 873 K for 176.4 ks (49 h). In Fe-13Cr alloy, nitrided layer showed stripe-pattern, each sub-layer of which has different chromium content. Nitrided layer hardness increased gradually from the specimen surface to the nitriding front before dropping drastically to the same level as matrix hardness. The stripe-pattern was also observed for Fe-3Cr alloy at the vicinity of nitriding front for the same nitriding conditions. On the other hand, nitrided layers in Fe-8Cr and Fe-19Cr alloys are composed from different sub-layers, containing different concentration of chromium. These phenomena cannot be explained only by nitrogen diffusion process during the nitriding.     

Fatigue fracture of nitrided layers

We describe the mechanism of fatigue fracture of a nitrided layer. The fatigue limits of the nitrided layer and steel are related to the residual stresses and microhardness. The distribution of stresses in nitrided bending specimens is determined and the site of fatigue fracture on the boundary of the nitrided layer is detected. The results of our analytic investigations coincide with the experimental data. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 113–116, March–April, 2006.     

TEM and EBSD investigation of continuous and discontinuous precipitation of CrN in nitrided pure Fe-Cr alloys

Pure Fe-Cr alloys (1 and 3 wt% Cr) were gas nitrided (NH₃, N₂, N₂O mixture at 823 K). Two modes of CrN precipitation: continuous (fine disc-shaped precipitates) and discontinuous (lamellae-like precipitates) were identified and investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both types of precipitates presented a cubic NaCl-type structure and Baker-Nutting orientation relationship with respect to the ferritic matrix. A quantification procedure based on TEM images exploitation revealed that the size and the number of the fine precipitates vary inversely with nitriding depth. This result was compared to the profile of micro-hardness. SEM observations showed that only superficial regions in the Fe-3 wt% Cr were transformed by the discontinuous precipitation of CrN. Electron back-scattered diffraction (EBSD) studies of lamella revealed singular initiation and growth features. A qualitative mechanism of lamella initiation and growth is discussed.     

Optical and microstructural investigations of porous silicon

Raman scattering and photoluminescence (PL) measurements on (100) oriented n-type crystalline silicon (c-Si) and porous silicon (PS) samples were carried out. PS samples were prepared by anodic etching of c-Si under the illumination of light for different etching times of 30, 60 and 90 min. Raman scattering from the optical phonon in PS showed the redshift of the phonon frequency, broadening and increased asymmetry of the Raman mode on increasing the etching time. Using the phonon confinement model, the average diameter of Si nanocrystallites has been estimated as 2.9, 2.6 and 2.3 nm for 30, 60 and 90 min samples, respectively. Similar size of Si crystallites has been confirmed from the high resolution transmission electron microscopy (HRTEM). Using 2TO phonon mode intensity, we conjectured that the disordered Si region around the pores present in 30 min PS dissolved on etching for 90 min. The photoluminescence (PL) from PS increased in intensity and blue shifted with etching time from 2.1–2.3 eV. Blue shifting of PL is consistent with quantum confinement of electron in Si nanocrystallites and their sizes are estimated as 2.4, 2.3 and 2.1 nm for 30, 60 and 90 min PS, respectively which are smaller than the Raman estimated sizes due to temperature effect. Unambiguous dominance of quantum confinement effect is reported in these PS samples.     

Electrochemical and microstructural studies of sintered and sintered-plasma nitrided steel containing different alloying elements

The corrosion of sintered and sintered-plasma nitrided steels containing different alloying elements was evaluated through analysis of the potentiodynamic polarization curves of the samples at pH = 7, with 1.25 M KNO₃ as electrolyte. The Fe-3.0% Ni and Fe-0.1% C sintered alloys show better performance in relation to electrochemical corrosion, than sintered pure iron samples. In addition, Fe-4.0% Mn, Fe-1.5% Mo and Fe-1.5% Si alloys exhibited increased anodic current densities in relation to the pure iron sample. After the nitriding process the anodic current densities of all samples containing an alloying element were considerably diminished. The sintered-nitrided pure iron sample was the only nitrided part whose current density was higher than the current density of the non-nitrided sample.     

Mechanical and microstructural properties of SnAgCu solder joints

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85 °C for 1000 h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag₃Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.     

Phase, structural and microstructural investigations of plasma sprayed hydroxyapatite coatings

The properties and performance of plasma sprayed hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, i.e. HA) coatings are closely related to their manufacturing process. The objective of the current study is to investigate the phase, structure and microstructure of the coatings and their formation mechanism due to different processing parameters. Hydroxyapatite powders were atmospherically plasma sprayed (APS) using various process parameters. The phase, structure and microstructure of the coatings were investigated and their microhardness measured. Both crystallinity and hydroxyl contents decreased with increasing spray power and stand-off distance (SOD), and increased from the coating interface to the surface. Crystallinity alone cannot reflect coating quality due to the existence of various forms of HA, i.e. unmelted, recrystallized and dehydroxylated, as well as the gradient structures consisting of these forms. Coating microstructure varied from a porous structure to a smooth glassy structure or a typical lamellar structure, and some newly formed nanocrystalline regions were revealed. These effects were associated with the temperature–time experiences of particles, their cooling rates and the heat and hydroxyl accumulation during coating buildup. The coating with highest recrystallization displayed the highest microhardness.     

Chemical and microstructural investigations of high-temperature interactions between AlN and TiO2

Chemical and microstructural changes occurring when AIN and TiO₂ are put in contact at 1400 to 1600 C in nitrogen have been studied. Experiments were carried out with single-crystal TiO₂ embedded in AIN powder compacts, and with AIN + TiO₂ powder mixtures of varying relative proportions. A displacement type of reaction is shown to take place yielding Al₂O₃ + TiN, with an intermediate step when a reduced titanium pseudobrookite (Al, Ti)₂TiO₅ phase is generated and subsequently tends to disappear. For appropriate AIN/TiO₂ initial ratios, a fine homogeneous two-phase composite microstructure may be obtained between Al₂O₃ and TiN reaction products.Based on a communication to Journées d'Etudes sur les nitrures (JENI VI), Saint-Etienne, France, 17–18 September 1984.     

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel

Tests were performed on a 2¼  Cr–1  Mo steel to measure the fracture toughness at initiation, K _Ic and at arrest, K _Ia . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K _Ia : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500  m  s⁻¹ ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine K _Ia values. These tests reveal a greater temperature shift (∼100 °C) between K _Ic and K _Ia in 2¼  Cr–1  Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼  Cr–1  Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism     

Mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling

Nitriding is usually applied to alloyed steels with the scope of increasing their surface hardness and wear resistance. Warm working has been found to produce a fine-grained microstructure, which makes possible further treatment of low carbon steels. In combination with a low temperature thermochemical treatment, such as nitriding, warm working can be used to produce machine parts with a though core and with a hard, wear resistant surface layer. This paper presents a study of mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling. The rolling was performed in the following conditions: temperature 670–550°C, rolling speed 1.39 s^-1 and deformation ratio 36.4%. After rolling, the samples were reheated to 550°C for a duration varying from a few minutes to 10 hours. The microstructural changes were assessed by light microscopy and quantitative microscopy analysis. Warm rolled samples were ion nitrided at 510–520°C in dissociated ammonia. The microstructure was analyzed by scanning electron microscopy and the mechanical properties were evaluated by tensile testing, surface hardness and friction coefficient measurements. Prior application of warm rolling makes possible (in the sense that is a viable solution) the ion nitriding of low carbon steels in order to produce machine parts with improved mechanical properties in the core (due to warm rolling) and longer service life (due to ion nitriding).     

Structural investigations of RTA boron-doped thin a-Si layers

The structural changes in as- sputtered thin a-Si layer, and after boron doping with rapid thermal annealing are investigated by transmission electron microscopy. Stable hexagonal amorphous/crystalline series of SiO₂ structures, as signed as SiO₂ (SnO₂-V), not revealed in high temperature SiO₂ layers, are observed in all films investigated. Different types of crystalline and high ordered SiO₂ structures are obtained in the BSG film, used for doping. Boron penetration in the a-Si layer starts the crystallization at B/Si ratios lower than 10⁻³. RTA process leads to inhomogeneous disordered polycrystalline silicon layer, with large areas of poly-and monocrystalline silicon, coexisting with various crystalline SiO₂ structures. Faster crystallization and larger monocrystalline silicon regions are observed at higher temperatures and longest durations of the annealing process.     

Mechanical and microstructural properties of waste andesite dust-based geopolymer mortars

Waste andesite dust (WAD) occurs during sawing and other stone dressing processes of andesite stone. The disposal of WAD may cause storage and environmental pollution problems. The use of WAD in geopolymer production may be a solution to these problems. The mechanical and microstructural properties of geopolymer mortars synthesized from WAD were investigated in this work. To investigate these properties, the geopolymer mortars were manufactured using WAD with different molarity variations of NaOH as alkali-activator. The produced fresh WAD-based geopolymer mortars were cured at 100 °C in three different periods (12, 24, and 48 h). The results revealed that the ultrasonic pulse velocity (U_pv), flexural strength (f_fs), compressive strength (f_cs) of WAD-based geopolymer mortars increased as increasing NaOH molarity until reaching an optimum concentration (12 M for this work). Curing periods also significantly affected the mechanical and microstructural properties of WAD-based geopolymer mortars. The results showed that the higher f_fs and f_cs values can be obtained with a longer curing periods. Moreover, considering overall performance analysis, geopolymer synthesis with WAD promises a solution for sustainable mortar production and waste elimination.     

Mechanical and microstructural behavior of nanocomposites produced via cold spray

Cold spraying is an innovative coating technology mainly based on the high speed impact of metals and ceramic particles on different substrates. Through the employment of low temperature gases (Air, He, N₂) spray particles (usually 1–50 μm in diameter) are accelerated to a high velocity (typically 300–1200 m/s) that is generated through a convergent–divergent de Laval type nozzle. Severe plastic deformation of particles impacting on the substrate occurs at temperature well below the melting point leading to the unique mechanical properties experienced by such kinds of coatings. In the present paper the main processing parameters affecting the microstructural and mechanical behavior of metal–metal cold spray deposits are described. The effect of processing parameters on grain refinement and mechanical properties were analyzed for different particles (Ti–TiAl₃, Al–Al₂O₃, Ni–Cr₃C₂, Ni–BN, Cu–Al₂O₃, Co–SiC). The results belonging to the properties of the formed nanocomposites were compared with those of the pure parent materials sprayed in the same conditions. Many experimental conditions have been analyzed in terms of particle dimensions and composition, substrate temperature and composition, gas temperature and pressure, nozzle properties. In particular, those conditions leading to a strong grain refinement with an acceptable level of the mechanical deposit properties such as porosity, adhesion strength and hardness were underlined.     

Mechanical properties of diffused chromized layers

The mechanical properties of thermo-diffused layers obtained by vacuum chromizing of steel were studied. It was established that to produce strong and ductile chromized layers by this method it is necessary to decarburize the substrate material before or after the treatment.     

金尾矿蒸压加气混凝土水化机理和微观结构分析

为明确金尾矿蒸压加气混凝土制品的水化机理和微观结构的内在关系,通过对比硬化坯体(NAC)、蒸压恒温养护0 h(AAC-0)和蒸压恒温养护8 h(AAC-8)的3组样品的XRD谱、IR谱图分析其水化产物种类的变化,并对比SEM照片分析了其微观结构变化.研究结果表明,随着蒸压养护过程的进行,坯体内的水化产物出现阶段性变化,最终由富钙型水化硅酸钙向托贝莫来石转化,由于托贝莫来石生成的局限性和同步性,导致微观孔壁结构出现明显的分层现象,后生成的托贝莫来石层使孔结构成为中空的刚性球,作为"骨料",起到骨架和支撑的作用.     

RBS-C and ellipsometric investigations of radiation damage in hot-implanted GaAs layers

Semi-insulating (1 0 0) GaAs single crystalline substrates have been doubly Al⁺-implanted using ion beams of the 250 keV energy and the fluence F = 3.5 × 10¹⁶ cm⁻², and 100 keV with F = 9.6 × 10¹⁵ cm⁻² at six target temperatures ranging from 250 to 500 °C. The radiation damage introduced by such “hot implantation” was subsequently investigated by Rutherford Backscattering Spectrometry with Channeling (RBS-C) and Variable Angle Spectroscopic Ellipsometry (VASE) techniques. Using these experimental methods we determined a degree of lattice disorder. With the increasing implantation temperature the degree of disorder substantially decreases. No evidence of full amorphization of the implanted GaAs layers has been found in the present studies. The results of non-destructive ellipsometric characterization are in good agreement with the RBS-C investigations.