Atomic Pillar-Based Nanoprecipitates Strengthen AlMgSi Alloys

Abstract

of these junctions by chemical functionalization (23) or physical processing can markedly increase the TCR as shown in the annealing experiment ( fig. S3). The presence of intertube junctions dramatically decreases the efficiency of heat transport along the SWNT film, thereby thermally insulating the sensitive element from the supporting substrate, which enhances the temperature response (18). Optimization of the room-temperature performance of the SWNT-based bolometer may provide a cost-efficient alternative to pyroelectric detectors, vanadium dioxide, and amorphous silicon-based bolometer arrays (18).The SWNT networks used here are a mixture of semiconducting and metallic SWNTs; metallic pathways present within such networks reduce the temperature dependence of the resistance (23). The ultimate enhancement of the TCR would be achieved by the exclusive use of semiconducting SWNTs, but even with current preparations, the implementation outlined above in conjunction with recent advances in carbon nanotube thin film preparation technology (3, 7, 22,27,28) allows the manufacture of high-density 2D arrays of SWNT bolometers that are suitable for applications in thermal imaging, spectroscopy, and infrared astronomy (18).References and Notes 1. J. A. Misewich et al., Science 300, 783 (2003). 2. M. Freitag et al., Phys. Rev. Lett. 93, 076803 (2004 Atomic-resolution electron microscopy reveals that pillarlike silicon double columns exist in the hardening nanoprecipitates of AlMgSi alloys, which vary in structure and composition. Upon annealing, the Si 2 pillars provide the skeleton for the nanoparticles to evolve in composition, structure, and morphology. We show that they begin as tiny nuclei with a composition close to Mg 2 Si 2 Al 7 and a minimal mismatch with the aluminum matrix. They subsequently undergo a onedimensional growth in association with compositional change, becoming elongated particles. During the evolution toward the final Mg 5 Si 6 particles, the compositional change is accompanied by a characteristic structural change. Our study explains the nanoscopic reasons that the alloys make excellent automotive materials.A luminum is essential to modern civilizations because of its light weight, strength, and workability. Its many applications include fuel-efficient transportation vehicles (e.g., it comprises about 80% of a commercial aircraft_s unloaded weight), building construction, and food packaging. Pure aluminum is soft and has little strength or resistance to plastic deformation. However, alloyed with small amounts of other elements, it can provide the strength of steel at only half the weight. With thermal treatments, the added alloying elements can form nanometer-sized precipitates, which act as obstacles to dislocation movement in the crystal (atomic matrix), strengthening the aluminum. This phenomenon is known as precipitation hardening, and the hardening nanoprecipitates are named GP zones after the pioneer work by Guinier and Preston on AlCu alloys (1, 2).AlMgSi accounts for a large percentage of...

Atomic-resolution chemical mapping of ordered precipitates in Al alloys using energy-dispersive X-ray spectroscopy

“…Other structures are still disputed after aberration-corrected ADF imaging due to a content of elements with similar atomic numbers: The β'' phase in Al-Mg-Si alloys was first well established as Mg 5 Si 6 (Zandbergen et al, 1997;Chen et al, 2006), then proposed as both Al 2 Mg 5 Si 4 (Hasting et al, 2009) and Al 3 Mg 4 Si 4 (Ninive et al, 2014). The Q' phase in Al-Mg-Si-Cu alloys has been suggested as Al 3 Mg 9 Si 7 Cu 2 on the basis of density functional theory calculations (Wolverton, 2001), with no decisive experimental evidence.…”

Concurrent substitutional and displacive phase transformations in Al-Mg-Si nanoclusters

“…AlMg-Si alloys are a typical example: they have important applications in the automotive industry and the precipitation process is here of critical importance for obtaining the required mechanical properties, yet many of the precipitate structures are not superstructures of the fcc matrix and the early stage precipitates undergo a series of structural and compositional changes, leading to the formation of the ␤Љ Mg 5 Si 6 phase. [4][5][6] Figure 1 shows an early needle-shaped Mg x Si y Al z nanocluster as embedded in aluminum. The typical dimensions of these needles are 4 ϫ 4 ϫ 50 nm 3 , or even smaller.…”

Effect of Minor Cu Addition on the Precipitation Sequence of an As-Cast Al-Mg-Si 6005 Alloy

“…4a. According to the work of Chen et al [29], the transformation from G.P. zones to β precipitates is accompanied with the evolution of both structure and composition.…”