G.M. La Vecchia

Co-continuous Al/Al2O3 composite produced by liquid displacement reaction: Relationship between microstructure and mechanical behavior

Co-continuous 63%Al2O3/37%Al(Si) composite, known as C4 composite, was produced by submersion of silica glass specimens in a molten metal bath. The effect of temperature and composition of the metal bath on the reactive penetration rate was investigated. An infiltration speed exceeding 2 mm/h, increasing with temperature, and suitable for practical applications, was observed above 1100°C. Mechanical properties of C4 specimens were measured, at room temperature, and related to composite microstructure. This latter was investigated by optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction and porosimetry.The strong interfacial bonds between metal and ceramic resulted in low thermal expansion, high stiffness, and good compression and bending strength. On the contrary, the composite showed a rather poor tensile strength, due to a high ceramic percentage and, partially, to porosity. In spite of the low ductility shown by the investigated composite, the metal network provides a toughening effect, which resulted in a ductile micro-mechanism of fracture, localized in the zones characterized by the presence of the metal matrix.

Effect of microblasting on cathodic arc evaporation CrN coatings

Abstract Cathodic arc evaporation CrN double layer coatings were deposited onto tool steels in two steps by industrial apparatus. Microblasting was performed on CrN first layer in order to remove metallic droplets and surface defects and to promote the adhesion of the second layer. Surface morphology effects were assessed by a scanning electron microscope coupled with Image analyser software. Nanoindentation and scratch tests were performed to evaluate the changes of coating adhesion and mechanical properties. Structure, microstructure and residual stress were evaluated by X-ray diffraction. In particular, by means of Cu and Cr wavelengths, the residual stresse of both steel substrates and coatings was calculated. The results show that microblasting has no effect on coating hardness and slightly affects the adhesion of the coating to the substrate. On the other hand, microblasting increases coating resistance to cohesive failure and the substrate compressive stress at the coating interface.

Effect of Different Corrosion Levels on the Mechanical Behavior and Failure of Threaded Elements

This research concerns the study of corrosion effects on different threaded elements to analyse bolted joints working in a saline environment. In particular, this article examines the mechanical behavior of a M8 junction realized by different steels and subjected to different degrees of corrosion. Four materials were investigated: a low-alloyed structural steel in the original condition and the same after zinc-plating; a quenched and tempered steel; and an austenitic stainless steel. For each steel were tested a certain number of threaded rods exposed to a saline water solution, simulating the atmospheric exposure to an aggressive environment. The samples were characterized by different intervals of exposure. During these periods, the threaded elements (threaded rods) were kept in tension applying a tightening torque proportional to the yield strength of the tested steel, to evaluate their susceptibility to stress corrosion cracking. Before and after the different corrosion steps, some metallographic analyses, static tensile and fatigue tests, were carried out to determine, for each sample, the degree of corrosion and its influence on the reduction of the junction resistance. In order to evaluate the stress-intensifying factor and its variation with the imposed degree of corrosion was carried out a FEM analysis.

Corrosion and Wear Behavior of CAE Deposited CrN-PVD Coatings

Corrosion and wear resistance of Cathodic Arc Evaporation (CAE) CrN coatings deposited on a tool steel were investigated considering the effect of the number of layers (mono or double) and the influence of defects. The CrN coatings were characterized for mechanical (scratch, nanoindentation test), corrosion behavior (polarizarion tests) and tribological properties (pin-on-disk tests). Scanning Electron Microscope analyses on samples before and after the tests were also made in order to identify the type of the defects and the role they play in the coating damage. Monolayer coating shows a good wear behavior with a stable and low friction coefficient, but its corrosion performance is poor. On the contrary, the presence of an interface into the double layer improves the corrosion resistance, but it is a weak point for the coating delamination during wear tests.

J-R curve dependence on specimen geometry and microstructure in two austenitic-ferritic stainless steels

Abstract The geometry dependence of J-R curves is a still-debated matter as it is of relevance for their transferability from laboratory test pieces to structural components. In the present research work J-R curves have been obtained for SENT and SENB specimens machined in the longitudinal and transverse direction from two duplex stainless-steel plates. The role of crack tip constraint, contained or uncontained yield, and microstructural directionality, has been considered to explain the observed geometry dependence of the J-R curves.

Effect of Titanium on the Mechanical Properties and Microstructure of Gray Cast Iron for Automotive Applications

Lamellar gray cast iron, with a mainly pearlitic microstructure, is widely used in the automotive industry, mostly in the manufacturing of brake disks. This work analyzes in depth the effects of small variations of titanium content on the microstructure and mechanical properties of cast iron brake disks. For this purpose, eight different heats of EN-GJL-250 cast iron were selected, with a similar chemical composition but with different titanium contents, varying from 0.013 to 0.031%. The drops in mechanical strength and hardness values measured on the high-Ti samples were correlated to microstructural variations quantitatively observed by means of optical and scanning electron microscope. It was found that titanium combines to form titanium nitrides, suppressing the beneficial microstructural effects of nitrogen at solidification. Residual nitrogen, if present in sufficient quantity, promotes the nucleation of primary austenite from the liquid and the formation of a fine microstructure, with small eutectic cells and lower graphite content. Such a microstructure provides brake disks with better mechanical properties. The interpretation of results was further supported by thermal analysis and thermodynamic calculations.

Analysis of fatigue crack propagation in nickel-plated components

This paper presents a procedure for the prediction of the fatigue life of thin-coated components. The procedure requires residual stress and hardness measurements in the surface layers of material of a coated component (coating and bulk) to develop numerical axisymmetric and three-dimensional finite element models capable of predicting the fatigue life. The application of the procedure to a thin-coated steel component (electroless nickel-plated) enabled confirmation of the proposed model. Besides this particular application, the method can be extended to the evaluation of the fatigue resistance of any other kind of thin-coated component.

Gravity casting processing of brake discs: use of partially recycled (Al–Si–Mg)/SiCp composite

Abstract Brake discs of (Al–Si–Mg)/SiC composite were produced by gravity casting. To this purpose ingots, containing 50 vol.-%of SiC particles, produced by pressureless metal infiltration at Lanxide Corp., were melted, diluted with an unreinforced alloy (thus achieving a SiC content of 30 vol.-%), and cast in permanents moulds. The possibility of repeating the casting process using recycled material was investigated. Brake discs were fabricated by using 50 or 100% of recycled composite. The influence of material recycling on both microstructure and mechanical strength of composite items was studied by X-ray diffractometry, optical and scanning electron microscopy, and X-ray energy dispersive analysis. Tensile tests were carried out on round bars obtained contemporaneously to the discs by casting. Only a small decrease in strength and an increase in inclusion concentration resulted from material recycling. As brake discs have frequently to experience temperature changes, due to the friction occurring during braking, their sensitivity to thermal fatigue was investigated. Thermal cycling tests were carried out in the temperature range 25–220°C. In these conditions, only negligible material weakening was observed after 2000 cycles.