M. C. Cristina

Influence of extrusion temperature on the microstructure and the texture of 6061Al–15 vol.% SiCw PM composites

Abstract A systematic study of the microstructure and the texture of powder metallurgy, PM, 6061Al metal matrix composites, MMCs, as influenced by the extrusion temperature, has been conducted. For this purpose, a containerless PM route has been developed to controlling the parameters involved during processing of materials. A strong fiber texture with two components: and , (with the fiber axis parallel to the long extrusion direction) is developed in the matrix of all materials. A limited particle stimulated nucleation process, PSN, occurs during extrusion of the composites when the extrusion temperature, Textr, is low. The SiC in the composites is divided in two populations; whiskers oriented with the extrusion axis (oriented whiskers) and randomly oriented whiskers (random whiskers) plus particles. Whereas the average whisker length does not change significantly with Textr, the “degree” of alignment of oriented whiskers (which follows a Gaussian distribution function) is clearly accentuated in the composites extruded at high Textr with respect the composites extruded at low Textr.

Texture of magnesium alloy films growth by physical vapour deposition (PVD)

Abstract The texture developed by four physical vapor deposited Mg alloys grown at two collector temperatures and their correlation with the microstructure have been studied. The alloys Mg–2.3Zr, Mg–10.6Zr and Mg–12Ti (wt%) deposited at around 150°C exhibited elongated grains growing in the normal direction to the deposit surface, but tilted with respect to the Z axis, with pores and cracks at the boundaries. A strong fibre texture of the ( 0001 ) basal plane component forming the fibre axis and the normal to the substrate plane an angle of 24, 18 or 12° depending on alloying element concentration has been found. The Mg–14Ti–1Al–0.9Mn (wt%) alloy, deposited at higher temperature, consisted of a region of columnar grains growing from the collector side followed by a thin region of equiaxed grains. Furthermore, a fibre texture with two components, the ( 0002 ) basal plane and the ( 1120 ) and the ( 1010 ) components in the collector and free surfaces, respectively, were present. Differences found in this alloy with respect to the alloys deposited at lower temperature were related to the surface diffusion phenomenon of adatoms caused by a higher collector temperature. The change in the second texture component from the collector to the free surface can explained by the decrease in the deposit strain energy.

Microstructural evolution of annealed Al–5 wt% Ca–5 wt% Zn sheet alloy

Microstructural evolution on annealing of Al–5 wt% Ca–5 wt% Zn sheet alloy has been systematically investigated by means of texture analysis and optical and transmission electron microscopy. Heat treatments in the temperature range of 100 to 540 °C for times between 0.11 (7 min) and 90.5 h have been performed. After annealing, the main texture component of the as- received material, {1 1 3} <3 3 2>, was retained, whereas the minor components, {0 1 3} <3 1 0> and {0 1 1} <1 0 0>, showed clear changes depending on the annealing conditions. Additional minor components appeared upon heat treatments. Continuous recrystallization, subgrain accommodation by means of rotation and subsequent extensive grain growth are the processes which account for all the microstructural changes observed.

Lüders bands formation in a rapidly solidified Ni3al alloy ribbon

The phenomenology of Luders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Luders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Luders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures ( 470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σeff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Luders front motion and responsible for serration appearance.

Characterization of rapidly solidified ultrahigh boron steels

Abstract Rapidly solidified powders of two binary FeB alloys and two boron-containing tool steels exhibiting a fine lamellar eutectic microstructure were investigated in both rapidly solidified and consolidation conditions. Both binary alloys contained ferrite and the metastable boride Fe 3 B, the latter of which was transformed into the stable boride Fe 2 B upon annealing at 610°C. The hardness of the as-quenched powders was very high (up to 1050 HV) because of high volume fractions of both phases. A loss of hardness with increasing annealing temperature was observed and may be attributed to the transformation of the metastable boride. Both tool steel powders showed the presence of austenite and stable borides. Fine, uniform microstructures consisting of 2–3 μm grains were developed in all materials after consolidation at temperatures ranging from 800 to 1150°C. When compression tested at 900°C, all materials exhibited a low stress exponent of 2–3, which may be explained in terms of grain boundary sliding. It appears that the borides containing chromium, nickel and/or molybdenum in both tool steels are harder than iron-borides at high temperatures, making the tool steels superior to the binary alloys over the entire temperature range up to 1100°C.

On the magnetic behavior of the MA 956 superalloy

MA 956 superalloy is a ferritic stainless material which develops a fine, dense, and well-adhered alpha-alumina layer upon heat treatment at elevated temperatures. This unique capability makes MA 956 attractive for surgical implants. In this work, the magnetic behavior of the material before and after thermal oxidation treatment required to develop the alumina layer is investigated. The thermal oxidation treatment yields a microstructure of elongated grains and a significant change in the texture. Despite these strong microstructural differences between the as-received and heat-treated materials, the hysteretic behavior is not greatly affected by them. MA 956 is a soft magnetic material irrespective of the material condition. The coercive force and residual magnetization of the material are somewhat lower under heat-treated conditions than in the as-received condition.