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This article studies the surface characteristics of quench- and temper-treated AISI 440A martensitic stainless steels, which were rough cut using wire electrical discharge machining (WEDM). The microstructure of the recast layer on the cut surface was investigated using scanning and transmission electron microscopes, and the phase compositions were analyzed with an energy-dispersive X-ray (EDX) spectrometer. Experimental results showed that the thickness of the recast layer varied with the heat-treatment condition of the workpiece, the largest thickness was obtained with a quenched specimen, and the thickness decreased with increasing tempering temperature. Intergranular surface cracks were observed only from the as-quenched specimen, whereas surface cracks were not found in the rough-cut specimens after tempering above 200 °C. It is reckoned that reliefing of the thermal residual stress in the quenched workpiece induced the surface intergranular cracks. Microstructures of the recast layer on the rough-cut surfaces of the 600 °C tempered specimen were examined using cross-sectional transmission electron microscopy (TEM) specimens. An amorphous layer exists at some parts of the outermost cut surface. A high density of wire electrode droplets of spherical shape, approximately 10 to 60 nm in size, was found throughout the porous recast layer. Besides, many high-chromium containing sigma spheres with sizes of approximately 120 to 200 nm were precipitated at the bottom part of the recast layer, and its formation mechanism was proposed. Adjacent to the recast layer was a heat-affected zone (HAZ) with a thickness of about 4 µm, in which temper-induced carbides were fully dissolved. The HAZ comprised basically two distinct regions: the first region adjacent to the recast layer was composed of a lath martensite structure, while the other region was an annealed ferrite structure.

Copper/silicon carbide composites (Cu/SiC p ) were made by the powder metallurgy method. Electroless plating was employed to deposit a copper film on SiC p powder before mixing with Cu powder in order to improve the bonding status between Cu and SiC particles during sintering. Thermal expansion property of as-formed product was measured in the temperature range from 50 to 550 °C. The results showed that copper coating on silicon carbide particles could render uniform distribution of SiC p in the copper matrix. The composites exhibited positive thermal hysteresis behavior when cooled down from the peak temperature to room temperature, which can be explained in terms of the residual stresses and the interfacial bonding between copper and silicon carbide. The magnitude of this strain was a function of the SiC p volume fraction and the number of thermal cycles. The thermal expansion property of composites was measured and compared with those predicted from various theoretical models.

Cu-SiCp composites made by the powder metallurgy method were investigated. To avoid the adverse effect of Cu-SiCp reaction, sintering was controlled at a reaction temperature less than 1032 K. Electroless plating was employed to deposit a copper film on SiCp powder before mixing with Cu powder in order to improve the bonding status between Cu and SiC particles during sintering. It was found that a continuous copper film could be deposited on SiCp by electroless copper plating, and a uniform distribution of SiCp in Cu matrix could be achieved after the sintering and extrusion process. The mechanical properties of Cu-SiCp composites with SiCp contents from 0.6 to 10 wt% were improved evidently, whereas electrical properties remained almost unchanged as compared with that of the pure copper counterpart. In the electrical discharge machining (EDM) test, the as-formed composite electrodes exhibited a character of lower electrode wear ratio, justifying its usage. The optimum conditions for EDM were Cu–2 wt% SiC...

The influence of niobium on the L12 structure phase and thus on the thermal stability of the alloys was investigated for four melt-spun Al-Cr-Zr-Nb alloys; three alloys contain about 4vol% of L12-Al3 (ZrxNb1-x) precipitate with different Zr/Nb ratios and one alloy contains about 8vol% of L12 precipitate. The microstructures of as-spun ribbons were characterized using an optical microscope and scanning transmission electron microscope. DSC and microhardness tests indicated that the alloys aged at 723 and 773K showed excellent thermal stability after annealing at 648 and 673K for up to 1000h. The Al-3.14Cr-4.03Zr-1.95Nb alloy, for example, attains microhardness of 204kgf/mm2 in agehardened condition and retains 93% of that value, i. e. 190kgf/mm2, after annealing at 648K for 1000h. Microstructural changes during annealing were identified and correlated with the microhardness data. The coarsening rate of the L12 structure precipitate is very slow at the test temperatures. The coarsening kinetics of the L12 structure Al3(Zr2/3 Nb1/3) precipitate in the Al-3.14Cr-4.03Zr-1.95Nb alloy were studied at 648, 673 and 723K. Antiphase boundary evolved inside the L12 particles during annealing at 723K.

The corrosion fatigue (CF) behaviour, under constant deflection bending conditions with a pulsating tension stress form, of 2024-T3 aluminium alloy, unanodized and anodized to form a thick porous film, in 3.5% NaCl solution has been investigated. It was found that E corr varies very little until specimen fracture under low frequency CF conditions, whereas E corr drops rapidly when approaching the later fracturing stage of the CF process under high-frequency conditions for unanodized specimens. However, a slow drop in E corr was detected from the commencement of the CF process, and lasted up to a much more rapid drop at a later fracturing stage for the anodized specimen. This behaviour presumably can be explained by the cracking of the anodic film and the theory of imperfect recovery of the surface film. It is suggested that the E corr monitoring technique may be useful for determining the remnant CF life for existing structural parts of this alloy or other aluminium alloys regardless of whether or not they are anodized. Furthermore, the T3 temper provides a microstructure which may retard main-crack formation and penetration in the CF process of the anodized alloy, thus mitigating partly the negative effect of the readily crackable anodic film.

The temperature rise in the substrate as well as in the bulk solution and the development of a local brown-coloured 'burnt' region on the anodic film are the characteristics of high rate anodizing of aluminum in oxalic acid. In this study, measurement of the barrier layer thickness in both the predicted burning region and the normal region indicates that they increase simultaneously as the voltage builds up to form the secondary voltage peak on the voltage/time transient; a depletion of oxalate anions allied with the temperature rise at the pore base is thought to govern this behaviour. Characteristic strong dotted anodic glows occurred simultaneously with a slight drop of voltage, about 1V, from the secondary peak value were observed just prior to the initiation of the local burning sector; also, the local substrate temperature always rises when burning is approaching. These studies suggest that a thermal modified electronic breakdown process is responsible for the burning phenomenon.

Anodisation dure dans l'acide sulfurique. Les resultats experimentaux des courbes transitoires courant/temps, de microdurete, de rugosite, de resistance a l'abrasion, d'etude metallographique et d'analyse de composition, montrent que dans les conditions utilisees le courant pulse alternatif donne de mauvais resultats et le courant pulse continu de meilleurs resultats