Advanced Structural Materials III

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Authors: R. Cuenca-Alvarez, H. Ageorges, Pierre Fauchais
Abstract: The influence of dry particle coating on the properties of coatings produced by d.c. arc plasma spraying is reported. A mechanofusion process is used to coat coarser metallic particles with fine ceramic particles without using either binders or solvents. The key parameters affecting the mechanofusion process and the corresponding plasma spraying method have been varied in order to increase the hardness of the resulting composite coatings. Efforts have been made to disperse homogeneously hard particles (α-Al2O3, SiC) into a metallic matrix (316L stainless steel) and check if it is possible to limit the oxidation of metallic particles during their flight in the plasma jet flowing in air. The hardness of resulting composite coatings depends on the metallic particle size even when the hard ceramic particles are homogenously dispersed into the metallic matrix. Spraying mechanofused powder composed of finer stainless steel particles (64 ,m), results in finer structured deposits that show a higher oxide content. On the contrary, a low oxidation rate of the metallic matrix is observed when coarser metallic particles (120 ,m), covered by a binary layer of α-Al2O3 and SiC are sprayed.
Authors: José Luis Soto, B. Campillo, J.A. Juarez-Islas
Abstract: The microstructure of directional solidified Al-Zn-Mg alloys is characterized and predicted by means of thermal analysis data, multicomponent equations for dendrite growth and data from ternary AlZnMg phase diagram. The resulting microstructure consists of α-Al dendrites with τ precipitates and eutectic in interdendritic regions. Predictions show that at growth velocities up to 9 x 10-4 m/s, the α-Al and τ intermetallic grew simultaneously. Predictions for solute concentration according to the model for dendrite solidification of multicomponent alloys with unequal liquid diffusion coefficients show a good agreement with experimental results.
Authors: Alejandro López-Baltazar, Armando Salinas-Rodríguez, Enrique Nava-Vázquez
Abstract: Interrupted cooling in the run-out table after hot rolling is a processing strategy to produce dual phase ferrite-martensite steel strips. The effects of interrupted cooling temperature and time after austenitizing at 1150 °C are investigated. This is done to establish the conditions for austenite to transform into ferrite-martensite microstructures in a Mn-Cr-Mo strip produced from a CSP (Continuous Strip Processing)-type thin slab. The results show that dual phase microstructures can only be obtained when interrupted cooling is performed at 725
Authors: Enrique Díaz Barriga-Castro, Armando Salinas-Rodríguez, Enrique Nava-Vázquez
Abstract: The aim of the present work is to determine the austenite to ferrite transformation temperatures in a Si-Al non-oriented electrical steel. Critical transformation temperatures on heating and cooling are determined using an in-situ X-ray diffraction technique where the specimen is heated or cooled in a stepwise manner. The transformation temperatures are estimated from changes in the intensities of the (110)α and (111)γ peaks as a function of temperature. The time evolution of the microstructure resulting from isothermal heat treatments at temperatures between 800 and 1000 °C applied after cooling from 1050 °C is followed by quantitative metallography on samples quenched into water. The results show that, on cooling, formation of ferrite starts at about 950 °C and ends at 790 °C, indicating a strong effect of Si and Al on the austenite to ferrite and eutectoid transformations. These results suggest that the low tensile ductility exhibited by this material at temperatures near 1000 °C can be attributed to strain localization in strain-induced ferrite formed at temperatures as high as 1025 °C.
Authors: M. Hinojosa, Edgar I. Morales, N. Mohamed
Abstract: We report the self-affine analysis on fracture surfaces of an A319-type aluminum alloy with different modification and refinement treatments, broken both in Charpy impact and cyclic impulse tests. The in-plane and out-of-plane Hurst exponent as well as the correlation length are obtained using quantitative fractographic techniques. It is found that the Hurst exponent, ζ, has a value of about 0.8 and is not influenced neither by the crack propagation modes nor by the microstructural condition. The self-affine correlation length is found to be related to the grain size resulting from the refining and modifying treatments applied to the alloy.
Authors: Felipe García, Armando Salinas-Rodríguez, Enrique Nava-Vázquez
Abstract: The reaction between solid steel and liquid Al-Zn-Si alloy leads to the formation of a solid intermetallic interfacial layer. In the case of industrial Al-43.5Zn-1.5Si coated steel strips, the thickness of the intermetallic layer is on average 1.35 μm and the kinetics of the reaction is controlled by the effect of Si on Al and Fe diffusivities through the solid intermetallic layer. In this paper it is shown that the thickness of the intermetallic interfacial layer decreases as the Si content in the liquid alloy increases. EDXS microanalysis at the interface of industrial coated steel strips shows that the interfacial intermetallic compounds are chemically similar to those formed in the bulk of Al-43.5Zn-1.5Si liquid baths in continuous coating lines. Differential acid dissolution of the coatings reveals that the intermetallic layer is not planar at the interface with the coating overlay and is formed by grains of different size and chemical composition. Addition of minute quantities of Ti to the Al-Zn-Si liquid alloy causes changes in the morphology of the intermetallic layer and an overall refining of the microstructure.
Authors: Edgar García S., E. Treviño L., Armando Salinas-Rodríguez
Abstract: The hot tensile ductility of solution treated Si-Al electrical steels was investigated at temperatures between 850 and 1150 °C. Samples for mechanical testing were obtained from continuous cast thin slabs and hot rolled strips. A continuous decrease in ductility was observed up to about 1000 °C. After that, the ductility was recovered in strip samples while in slab samples the ductility remained constant at RA<10%. This behavior was associated with the presence of large quantities of undissolved AlN particles formed during slow cooling of the slab. In the case of strip specimens, where the starting slab is not cooled to room temperature, the 1000 °C ductility minimum was attributed to strain localization at grain boundary nucleated ferrite grains. Rapid nucleation and growth of microvoids at AlN particles formed during cooling to test temperatures in the vicinity of Ae3 resulted in intergranular tensile failure by microvoid coalescence.
Authors: Ana Lilia Leal-Cruz, Martin I. Pech-Canul
Abstract: The effect of processing parameters on the weight loss of the silicon solid precursor (Na2SiF6) and the deposition characteristics and morphology of Si3N4 formed onto SiCp/Si porous substrates by CVD has been investigated. The results show that the weight loss of Na2SiF6 is most significantly affected by the processing temperature, followed by the processing time and the type of nitrogen precursor. Formation of Si3N4 is mostly influenced by the substrate temperature, followed by the type of nitrogen precursor and processing time. An increase in processing time and temperature from 60 to 120 min and from 900 to 1300 oC, respectively, favors dissociation of Na2SiF6 and formation of Si3N4. Moreover, N2 enhances Na2SiF6 dissociation and hampers Si3N4 formation, while the N2-NH3 mixture hinders the solid precursor dissociation and favors Si3N4 formation. With regard to microstructure evolution, it is found that in N2 the amount of Si3N4 increases with temperature and the morphology changes from wool-like and light fibers to thicker and compact fibers. When N2-NH3 is used and the processing temperature is increased, the morphology of Si3N4 is modified from deposits with wool-like and compact appearance to whiskers and spheres and finally to thick and compact fibers.
Authors: Miguel Montoya-Dávila, Max A. Pech-Canul, Martin I. Pech-Canul
Abstract: The effect of particle size distribution and particle size ratio of SiCp in SiCp/SiO2 preforms on the microstructure, microhardness of SiCp reinforcements, modulus of rupture, and superficial hardness of Al/SiCp composites produced by pressureless infiltration has been investigated. SiCp/SiO2 preforms in the form of plates (4cm x 3cm x 0.5cm) have been pressureless infiltrated by the alloy Al-15.52 Mg-13.62 Si (wt. %) at 1100 oC for 60 min under inert atmosphere. SiC powders with average particle size of 10, 68 and 140 μm are mixed with SiO2 powders and preforms of 40 % porosity with unimodal, bimodal and trimodal size distributions are prepared by uniaxial compaction. The bimodal (small: large) and trimodal (small: medium: large) preforms are prepared with different particle size ratios in the following levels: 1:1, 3:1, 1:3, 2:2:2, 3:2:1, 3:1:2. Results from characterization by XRD, SEM and energy dispersive X-ray spectrometry show that the typical microstructure of the composites contains the MgAl2O4 (spinel), AlN and MgO phases formed during processing as well as partially reacted silica, SiC, Si and Al. It is found that the density, reinforcement microhardness, modulus of rupture and superficial hardness of the composites increase all with wider particle size distribution. However, whilst the modulus of rupture is mainly affected on going from unimodal and bimodal to trimodal distribution, superficial hardness and microhardness are mostly influenced on going from unimodal to bimodal and trimodal distribution.
Authors: A. Ordóñez, R. González, J.M. Sánchez
Abstract: This work analyses the changes induced on the microstructure and mechanical properties of ultrafine (0.2 and 0.4 μm) hardmetal grade WC-7wt.% Co by HIP after vacuum sintering. A large HIP pressure (155 MPa) is applied to the ultrafine hardmetal system at different temperatures (1000, 1200 and 1400 °C). The well-known correlation between porosity reduction and fracture strength improvement is confirmed. Residual porosity left after liquid phase sintering is removed more effectively at HIP temperatures above the eutectic point of the alloy (1400 °C). In the absence of grain growth inhibitors, hardness decreases continuously, and WC-Co ultrafine microstructure coarsens as HIP temperature rises. However, for specimens containing VC and Cr3C2 additions, hardness increases as HIP temperature rises from 1200 to 1400 °C. It is proposed that this anomalous trend (confirmed by grain size observations) is related to the activation of coalescence mechanisms during solid state HIPing, which are inhibited by the presence of a liquid phase.

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