Lightweight single-phase Al-based complex concentrated alloy with high specific strength

Abstract

Developing light yet strong aluminum (Al)-based alloys has been attracting unremitting efforts due to the soaring demand for energy-efficient structural materials. However, this endeavor is impeded by the limited solubility of other lighter components in Al. Here, we propose to surmount this challenge by converting multiple brittle phases into a ductile solid solution in Al-based complex concentrated alloys (CCA) by applying high pressure and temperature. We successfully develop a face-centered cubic single-phase Al-based CCA, Al₅₅Mg₃₅Li₅Zn₅, with a low density of 2.40 g/cm³ and a high specific yield strength of 344×10³ N·m/kg (typically ~ 200×10³ N·m/kg in conventional Al-based alloys). Our analysis reveals that formation of the single-phase CCA can be attributed to the decreased difference in atomic size and electronegativity between the solute elements and Al under high pressure, as well as the synergistic high entropy effect caused by high temperature and high pressure. The increase in strength originates mainly from high solid solution and nanoscale chemical fluctuations. Our findings could offer a viable route to explore lightweight single-phase CCAs in a vast composition-temperature-pressure space with enhanced mechanical properties.

Fig. 1. Characterization of the as-cast and HPHT-synthesized Al₅₅Mg₃₅Li₅Zn₅ samples.

a Synchrotron XRD patterns of the as-cast sample (black) and the sample synthesized at 15 GPa and 1500 K by LVP (red). b, c HRTEM and SAED images of the HPHT-synthesized sample, revealing an SP-FCC structure. The SAED pattern is obtained along the [011] direction. d Bright-field TEM image, showing a fairly uniform distribution of numerous dislocations in the HPHT-synthesized sample. e EBSD image of the HPHT-synthesized sample, showing the grains with different orientations. The inset shows the orientations corresponding to different colors. f APT reconstruction of the HPHT-synthesized sample. Maps of 5.8 at.% Li, 8 at.% Zn iso-concentration and their overlap are displayed to depict local chemical fluctuations. g Composition analysis along a line crossing the interface between the matrix and nanoscale clusters. The error bars are standard deviations of the mean.

Fig. 2. Mechanical properties.

a Stress-strain curves of the as-cast and SP-FCC Al₅₅Mg₃₅Li₅Zn₅ CCAs at room temperature under compression. The insets a1 and a2 are the optical images of the SP-FCC sample before and after compression, respectively. b Specific fracture strength and density of the SP-FCC Al₅₅Mg₃₅Li₅Zn₅ CCA, in comparison with those of various lightweight CCAs. The data of other alloys are taken from literature^,–. Different colored shadows and symbols represent the performance of different CCAs.

Fig. 3. Deformation behavior of the SP-FCC Al₅₅Mg₃₅Li₅Zn₅ CCA.

a, b Fracture surfaces of the as-cast and SP-FCC Al₅₅Mg₃₅Li₅Zn₅ CCAs, respectively. c Representative bright-field TEM image of dislocation morphologies in a fractured SP-FCC sample, imaged under (−112) type diffraction conditions. d The corresponding SAED pattern along the [−112] direction of the fractured SP-FCC sample.

Fig. 4. Phase evolution at high pressures.

a Synchrotron XRD patterns of the as-cast Al₅₅Mg₃₅Li₅Zn₅ alloy during compression up to 22 GPa at room temperature. No phase transition was observed. b Synchrotron XRD patterns of the Al₅₅Mg₃₅Li₅Zn₅ alloy as a function of temperature when subjected to a compression pressure of 10 GPa. The intermetallic compounds (IMCs) in the as-cast alloy remained stable up to ~ 950 K at 10 GPa, then a phase transition started at ~ 950 K. NaCl served as the pressure-transmitting medium and thermal insulator here. The red and black tick marks in Fig. 4b represent the peaks of IMCs and NaCl.

Fig. 5. Change of radius, local atomic strain (λ), electronegativity, and excess configurational entropy (S_E) as a function of applied pressure.

a Pressure-Radius relationship for four components up to 50 GPa. The radius under high pressure was determined by high-pressure volume data of four components from literature^–. b The local atomic strain (λ) as a function of pressure in Al₅₅Mg₃₅Li₅Zn₅. The left y-axis shows the λ parameter as a function of pressure (blue solid curve) while the right y-axis presents the separate contribution from each solute atom (Mg, Li, and Zn) to the λ parameter as a function of pressure (dashed lines). The dotted horizontal line marks the position where λ = 0.1. c The electronegativity of four components as a function of pressure. d The change of excess configurational entropy (S_E) with pressure in Al₅₅Mg₃₅Li₅Zn₅.