Age Hardening in Ultrafine-Grained Al-2 Pct Fe Alloy Processed by High-Pressure Torsion

Abstract

A cast Al-2 wt pct Fe alloy was processed by high-pressure torsion (HPT) at room temperature and then subjected to artificial aging at temperatures of 373 K and 473 K (100°C and 200°C). The aging behavior was studied by Vickers microhardness measurements and by microstructural analyses using transmission electron microscopy and X-ray diffraction. The initial intermetallic structures, composed of a mixture of Al + Al 6 Fe and Al + Al 3 Fe eutectics phases, were partially dissolved in the matrix up to a supersaturation of~1 wt pct Fe. The microstructure was refined by HPT to an ultrafine-grained level with a minimum grain size of~120 nm in the matrix and a dispersion of particles less than 400 nm. Age hardening was achieved within 0.25 hours at 473 K (200°C), to a maximum UTS of~700 MPa as a result of nano-sized precipitation within the ultrafine grains. The uniform elongation exceeded~12 pct even at intermediate levels of imposed strain by HPT, while it decreased to~6 pct with the subsequent aging treatment. The thermal stability of the ultrafine-grained structure was verified to exceed 16 days at 373 K (100°C) and 12 hours at 473 K (200°C).

Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy stabilized by nanoscaled intermetallic particles

“…4). Similar hardening by annealing has been reported by several authors in some metallic alloys processed by SPD [5,27,29,84,85]. This could be attributed to precipitation of second phase particle, but this is very unlikely in our case, especially in the material processed by N=10 revolutions where original intermetallic particles are still relatively coarse and where the amount of Fe driven in solid solution is very low.…”

“…2(a) shows the typical as-cast dendritic structure with pro-eutectic zones in dark (zone A) and Fe-rich eutectic zones in bright (zone B). Similar as-cast structures have been described in previous works [5][6][7]29,[43][44][45][46]. The HAADF STEM image ( Fig. 2(b)) shows similar zones at a higher magnification and clearly exhibits in bright the fine-scaled intermetallic particles within the eutectic areas (labeled B and B').…”

“…2). In similar Al-Fe alloys, the stable Al13Fe4 phase is often reported in as-cast conditions [5][6][7]29,[44][45][46] but it has also been shown that fast cooling conditions usually promote the formation of Al6Fe [43]. This metastable phase exhibits a higher Al / Fe ratio and thus a larger volume fraction is formed.…”

“…It clearly indicates that a huge strain (1000 revolutions, corresponding to a shear strain of 55000) is necessary to achieve a near complete fragmentation of the original coarse intermetallics to the STEM Dark Field (a, b, c) and STEM HAADF (d, e, f, g, h, i) images after HPT with N = 10 revolutions (a, d, g), N = 100 revolutions (b, e, h) and N = 1000 revolutions (c, f, i) nanoscale with a nearly uniform distribution of final particles. Electron diffraction in the TEM and TEM-EDS was applied on a large number of nanoscaled particles resulting from the fragmentation process (see supplementary material), and they all exhibited the crystallographic structure of the Al6Fe phase, indicating that no significant strain-induced phase transformation occurred during the HPT process.It has been shown in the literature that intermetallic particles could be partly dissolved during SPD[5,6,[26][27][28][29][30]53]. Measuring the volume fraction of intermetallic particles from STEM-HAADF images was only possible after 1000 revolutions where particles are homogeneously distributed and where it is possible to obtain representative data from a limited number of images.…”

Optimization of Strength‐Electrical Conductivity Properties in Al–2Fe Alloy by Severe Plastic Deformation and Heat Treatment

“…This strengthening results from the combined effect of grain refinement, non‐stable supersaturated solid solution (SSSS) of Fe in Al, and refinement of intermetallic particles during deformation . Recent works have also shown that artificial aging after high‐pressure torsion (HPT) increases both conductivity and tensile strength to the level of 49–51% IACS and above 300 MPa, respectively . Decomposition of supersaturated solid solution, that takes place during aging, leaves the clean aluminum matrix with fine intermetallic particles, which simultaneously enhance alloy conductivity and strength.…”