Microstructural stability and flow properties of an Al-Mg-Sc alloy processed at different temperatures using severe plastic deformation

Abstract

Experiments were systematically conducted to evaluate the thermal stability and flow properties of an Al-3Mg-0.2Sc alloy both without and with processing at different temperatures using equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). 
 The average grain size of the solution treated material was ~300 µm and this was reduced to ~250 nm after 8 passes of ECAP at room temperature (RT). Tests were conducted in both the coarse and ultrafine-grained (UFG) Al alloy to determine the mechanical properties and deformation mechanisms over seven orders of magnitude of strain rate from ~10-4 to ~103 s-1 at 298, 523 and 673 K. The results confirm that there is no apparent breakdown in the Hall-Petch relationship in Al-Mg-Sc alloys with average grain sizes down to ~0.1 µm. Profuse shear banding and grain refinement were observed in the coarse-grained metal during dynamic testing at 4  103 s-1 but in the ECAP-processed alloy there was minor grain coarsening. Dynamic strain ageing occurred in both the coarse and UFG Al-Mg-Sc alloy tested at RT for strain rates below ~10-1 s-1 with a transition in flow mechanism from dislocation climb in the coarse-grained material to superplasticity in the ECAP-processed alloy at 673 K with strain rates from ~10-4 to ~10-2 s-1. 
After processing through severe plastic deformation (SPD) procedures, samples of the Al alloy were annealed for 1 h at temperatures up to 773 K or mechanical tested at high temperatures using strain rates from 3.3  10-4 to 1 s-1. The microstructural evolution was investigated using advanced techniques such as transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). The mechanical properties were assessed through microhardness measurements and tensile testing using miniature specimens having the same dimensions. 
 ECAP processing at 300 K led to higher hardness values and finer grain structures than ECAP at 600 K (~0.60 µm). The alloy processed by ECAP at 600 K exhibited lower driving pressures for grain boundary migration and therefore an improved thermal stability compared with the metal processed at RT. As a result, it displayed larger grain sizes than the alloy processed by ECAP at RT after annealing at T ≥ 623 K. After annealing at 723 and 773 K, the metal processed by ECAP at 300 K displayed a bimodal distribution of grains, whereas samples processed by ECAP at 600 K exhibited uniform microstructures. High superplastic elongations were attained in miniature specimens of the ECAP-processed material when tested at ~523-773 K. However, they were notably lower than after tensile testing using regular ECAP samples. For tests performed at T ≥ 673 K, the alloy processed at 600 K displayed superior superplasticity and it achieved a maximum elongation of ~1490 % after testing at 723 K. Conversely, low temperature superplasticity was obtained at faster deformation rates in samples processed by ECAP at 300 K. 
HPT processing promotes further hardening and grain refinement in the Al alloy by comparison with ECAP. The HPT-processed samples displayed an average boundary spacing of ~0.15 µm and hardness values of > 180 Hv. The metal processed by HPT at 450 K also exhibited a lower dislocation density of ~7  1012 m-2 and a more uniform microstructure. The microstructural stability was enhanced by conducting HPT processing at 450 K. Although abnormal coarsening was observed in the HPT discs after annealing at 623 and 673 K, the metal processed at 450 K exhibited slower coarsening kinetics and it had grain sizes below 2 µm after annealing at 673 K. After HPT at RT, the Al alloy displayed excellent superplasticity at low homologous temperatures and it achieved a maximum elongation of ~850 % for tests performed at 523 K. However, the overall elongations decreased at T ≥ 623 K and superplasticity was only attained at 673 K using a strain rate of 4.5  10-3 s-1. The Al-3Mg-0.2Sc alloy processed through 10 turns of HPT at 450 K displayed superior superplastic ductilities among all SPD processing conditions. Elongations of > 1100 % were achieved after testing at 673 K using strain rates from 3.3  10-4 to 1.0  10-1 s-1. A record elongation of ~1880 % for HPT-processed metals was attained at 1.5  10-2 s-1 at 673 K. High strain rate superplasticity was also obtained for an extended range of strain rates at temperatures down to 473 K. Analysis of the data confirms a stress exponent of n = 2 for samples of the SPD-processed alloy having elongations of ≥ 400 %. This indicates that superplastic flow by grain boundary sliding is accommodated by dislocation climb in Al-Mg-Sc alloys. The calculated activation energies for superplasticity lie within the range of ~99-125 kJ mol-1 for all processing temperatures and they are higher than the activation energy for grain boundary diffusion in pure Al (~86 kJ mol-1).

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ORCID for Yi Huang: orcid.org/0000-0001-9259-8123

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