Kenong Xia

Mechanical Properties of Discontinuous SiC Reinforced Aluminum Composites at Elevated Temperatures

High temperature mechanical properties of discontinuous, whisker and particulate, SiC reinforced aluminum composites, including 2124 and 6061 alloy matrices, are reviewed. It is shown that the behavior of these composites is similar to conventional oxide dispersion strengthened alloys. Namely, they exhibit a low strain rate senstivity and a high apparent activation energy for creep deformation. Despite the fact that the addition of SiC significantly improves the mechanical properties of aluminum at room temperature, the mechanical strength of the composite at elevated temperatures is dominated by the strength of the aluminum matrix This is because the SiC dispersoids are, in general, too coarse and they are not effective barriers for dislocation motion. It is also demonstrated that SiC particulate composites are less creep resistant than SiC whisker composites.

Liquidus casting of a wrought aluminum alloy 2618 for thixoforming

Abstract A wrought aluminium alloy 2618 was cast from its liquidus temperature to obtain a fine structure of equiaxed, nondendritic primary grains. In contrast, the structures obtained from casting the alloy from above its liquidus temperature displayed the usual dendritic grains. The primary grains were characterized by image analysis which showed that the average grain size in two dimensions was ∼44 μm and the average circularity factor was ∼18, indicating that the grains were both fine and globular. Some particles were also viewed in three dimensions and this showed mostly round and smooth surfaces although some protrusions could be seen to emerge on some particles. The misorientation angles between adjacent particles were measured by the electron back-scattered diffraction (EBSD) and most were >15°, indicating genuinely separate grains. The grains in the as-cast material remained fine for up to 30 min at 610°C during reheating, although particle coalescence led to eventual coarsening. The liquidus cast billets were thixoformed at 610°C and the microstructures after forming were examined. It is concluded that the liquidus casting is capable of producing thixotropic materials without resorting to stirring, grain refining or severe deformation and reheating.

The toughening and strengthening of ceramic materials through discontinuous reinforcement

The incorporation of a discontinuous reinforcement, in the form of fibres or particulates, into a ceramic matrix provides the possibility of introducing toughness and strength. This review summarizes the various toughening and strengthening mechanisms and then examines and analyses the reported experimental observations on various ceramic composite systems.

3D study of the structure of primary crystals in a rheocast Al–Cu alloy

An Al-10.25 wt.% Cu alloy was mechanically stirred at 1000 rpm while being cooled to and isothermally held at 619°C for various times before casting. The microstructure of as-cast materials consists of pseudo-particles and pseudo-clusters in two-dimensional (2D) views. Serial sectioning was performed and it was found that pseudo-particles belonging to a pseudo-cluster were connected in three dimensions and a pseudo-cluster was actually a single primary particle. A three dimensional (3-D) model of such a primary particle was constructed from 2D sections which showed the complexity of its structure. It is suggested that the theories based on agglomeration and disagglomeration of particles during rheocasting be re-examined in light of the experimental results.

Ultrafine-grained titanium of high interstitial contents with a good combination of strength and ductility

A dehydrided Ti powder of very high oxygen content was successfully consolidated using back pressure equal channel angular processing into a fully dense bulk ultrafine-grained Ti showing apparent compressive ductility as well as high true yield and ultimate strengths of 1350 and 1780MPa, respectively. Interstitial solid solution strengthening contributed to the majority of the increase in strength with additional contribution from ultrafine grains. Significantly, the material also exhibited much improved ductility for such a high interstitial content, thanks probably to the nonequilibrium grain boundaries and bimodal grain structure introduced during severe plastic deformation.

Effects of addition of rare earth element Gd on the lamellar grain sizes of a binary Ti-44Al alloy

Rare earth (RE) elements have been known to refine the grains in materials such as steels, cast irons, aluminum alloys, titanium alloys and magnesium alloys. Additions of RE elements to FeAl, Fe{sub 3}Al, NiAl and TiAl were shown to increase the ductility and strength of the alloys at room temperature. However, the microstructures of the fully lamellar TiAl alloys with additions of RE elements have not been investigated in detail. In the present work the effects of adding the RE element Gd on the refinement of lamellar grains in a binary Ti-44Al alloy were studied. The microstructures after casting and/or hot pressing followed by various heat treatments are presented.

High strength ultrafine/nanostructured aluminum produced by back pressure equal channel angular processing

High strength ultrafine/nanograined aluminum materials with ultimate strength up to 740MPa and Vickers microhardness up to 2285MPa were produced using back pressure equal channel angular processing of ultrafine-sized aluminum powder at 400°C. Microstructure analyses revealed that the attained high strength and microhardness were derived from the presence of nanosized aluminum and γ-alumina grains (5–10nm) as well as residual amorphous alumina. The interaction between the severe shear deformation and the preexisting amorphous alumina, concurrent oxidation, and amorphous to γ-alumina transition was considered to be responsible for the formation of such a refined and complex nanostructure.

The Influence of Grain Size and Grain Orientation on Sensitization in AA5083

Aluminum alloy AA5083 specimens (with a nominal composition of Al-4.4Mg-0.5Mn) from the same original plate were prepared with a variation in grain size imparted by cold rolling, cryo-rolling, equal channel angular pressing and high-pressure torsion. Electron backscatter diffraction was used to determine the grain size and misorientation. The effect of grain size on the degree of sensitization for AA5083 was studied via the ASTM G67 nitric acid mass lost test, upon specimens artificially sensitized at 150°C for 7 d. It was revealed that the intergranular corrosion caused by sensitization in AA5083 was significantly influenced by the grain size and processing methods used for grain refinement.

Microstructure observations in rare earth element Gd-modified Ti-44 at% Al

Abstract Rare earth element Gd (0.15 at%) was added to Ti-44 at% Al to refine the microstructure. Almost all of the Gd atoms were found to form an oxide phase which was tentatively identified as Gd2TiO5. In the as-cast ingot the oxide phase possessed a complicated “finger” shaped morphology and formed a networked structure between the primary crystals. This morphology was broken down in the subsequent heat treatment at 1350°C to become discrete particles in the form of particulate, platelets or rods, often with a faceted appearance, although clusters of the particles reminiscent of their as-cast morphology were common. The distribution of the particles was improved by hot pressing which appeared to reduce the particle size and de-cluster them. Although the particles were not actually pinning the grain boundaries, their distribution seemed to have a significant effect on lamellar grain sizes after heat treatment. The addition of Gd appeared also to promote the formation of lamellae during cooling.

Kinetics of the α grain growth in a binary Ti–44Al alloy and a ternary Ti–44Al-0.15Gd alloy

Abstract Grain growth in the single α region was studied both in a binary Ti–44Al alloy and in a ternary Ti–44Al–0.15Gd alloy. Cast ingots were solution treated at 1623 K for various times from 2 min to 20 h and the average grain sizes were measured. The α grain growth was much slower in the Gd-containing alloy. The kinetics of grain growth can be described by the traditional power–law relationship between the grain size (μm) and time (s) with a rate constant of 308 μm/s 0.17 and a time exponent of 0.17 for the binary alloy, and 316 μm/s 0.045 and 0.045 for the Gd-containing alloy, respectively. Some ingots were solution treated at temperatures from 1543 to 1673 K for 1 h and the logarithmic grain size was plotted against reciprocal temperature. The activation energy obtained was 81 kJ/mol for the binary alloy and 50 kJ/mol for the ternary alloy. The implications of these values were discussed.