A deformation-processed Al-matrix/Ca-nanofilamentary composite with low density, high strength, and high conductivity

Abstract

Light, strong materials with high conductivity are desired for many applications such as power transmission conductors, fly-by-wire systems, and downhole power feeds. However, it is difficult to obtain both high strength and high conductivity simultaneously in a material. In this study, an Al/Ca (20 vol.%) nanofilamentary metal-metal composite was produced by powder metallurgy and severe plastic deformation. Fine Ca metal powders (~ 200 μm) were produced by centrifugal atomization, mixed with pure Al powder, and deformed by warm extrusion, swaging, and wire drawing to a true strain of 12.9. The Ca powder particles became fine Ca nanofilaments that reinforce the composite substantially by interface strengthening. The conductivity of the composite is slightly lower than the rule-of-mixtures prediction due to minor quantities of impurity inclusions. The elevated temperature performance of this composite was also evaluated by differential scanning calorimetry and resistivity measurements.

Deformation processed Al/Ca nano-filamentary composite conductors for HVDC applications

“…The micrographs in figure 2 clearly show a change in the filament size, with a measured increase in thickness from 0.9µm to 1.5µm for the selected strain level. This difference is very close to that expected, indicating that the transformation to Al 2 Ca is complete, as was also confirmed by separate differential scanning calorimetry (DSC) and X-ray ray diffraction studies [4,5]. The filaments are able to deform with a consistent thickness as Al and Ca have nearly equal flow stresses, avoiding problems with a softer phase flowing around a harder second phase [6,12].…”

“…The resultant morphology is in contrast to the expectations of classic composite morphology theory, which predicts cylindrical filaments due to axi-symmetrical deformation of a fcc secondary phase in a fcc matrix. A possible explanation is a temporary crystal structure transformation in the Ca from fcc to bcc during extrusion leading to plane-strain deformation giving rise to a ribbon-like shape [4,5,6], but this has not been further explored in this study. Following the conversion of the Ca reinforcement to Al 2 Ca, the filament size increases, as shown in figure 2.…”

“…The ultimate tensile strength (UTS) was used to characterize the samples as it is more reproducible and reliable than yield strength for small specimens and is the strength value most often cited for transmission conductor materials [9]. [3,6,10] and in recent years for the Al/Ca composite system [4,5,11]. Figure 1 shows that the ribbon-shaped morphology seen in the prior generation of Al/Ca composites exists at lower strain levels, but is altered by extensive mechanical working.…”

“…Both Al and Ca are ductile fcc metals with high conductivities, similar mechanical properties, and low densities. One key finding from previous study of Al/Ca composites was the identification of a transformation of the reinforcing Ca phase to the Al 2 Ca intermetallic phase at temperatures as low as 175˚C, a temperature sometimes reached by transmission conductors during periods of heavy electrical demand [4,5]. This study examines the effect of intentionally converting the reinforcement phase to an intermetallic species to produce a material with high-temperature stability while retaining the superior performance properties that have been shown in the prior Al/Ca composite materials.…”

Effect of friction stir welding on mechanical properties and fracture mechanism of 7B52 Al laminated composite

“…It is produced by rolling, casting and other manufacturing processes to achieve metallurgical bonding on the interface [5]. A novel Al matrix Ca reinforced nanocomposites was developed by Tian et al [6,7] using powder metallurgy and severe deformation processing (i.e. extrusion, swaging and wire drawing) to achieve metallurgical bonding at metallic interfaces with high strength, high electrical conductivity.…”