The contribution of grain boundary sliding in tensile deformation of an ultrafine-grained aluminum alloy having high strength and high ductility

Abstract

An as-cast Al-7 % Si alloy was processed by high-pressure torsion (HPT) for up to 10 turns at temperatures of 298 or 445 K. The HPT-processed samples had ultrafine-grained structures and they were tested in tension at room temperature at various strain rates in the range from 1.0 9 10 -4 to 1.0 9 10 -2 s -1 . The contributions of grain boundary sliding (GBS) to the total strain were measured directly using atomic force microscopy. Samples simultaneously showing both high strength and high ductility contained the highest fractions of high-angle grain boundaries (HAGB) and exhibited the highest contributions from GBS, whereas samples showing high strength but low ductility gave negligible values for the sliding contributions. It is concluded that high strength and high ductility require both an ultrafine grain size and a high fraction of HAGB.

The Strength–Grain Size Relationship in Ultrafine-Grained Metals

“…In the as-cast Al-7% Si alloy, grain boundary sliding contributes only 1% to the total strain but in the SPD-processed alloy the contribution is ~14%. [123] This shows the increasing importance of grain boundary sliding as a flow process under these experimental conditions. Thirdly, high resolution transmission electron microscopy has provided direct evidence for the presence of non-equilibrium grain boundaries containing an excess of extrinsic dislocations in alloys processed by SPD [124] and it is reasonable to anticipate that the movement of these extrinsic dislocations permits relatively easy sliding.…”

“…[129] For the Al-7% Si alloy discussed earlier, the contribution of GBS to the total strain was as high as ~14% after 5 turns of HPT and ~17% after 10 turns. [123] Further confirmation for the possibility of low temperature GBS in UFG metals comes from diffusion data on pure Ni processed by ECAP which show ultra-fast diffusion at lower temperatures [<400 K (127°C)] with a low activation energy but relaxation of the nonequilibrium grain boundaries at high temperatures [>400 K (127°C)]. [130,131] …”

“…10 where the elongation to failure increases with the fraction of HAGBs. [123] Secondly, the strain rate sensitivity increases with the imposed strain. The strain rate sensitivity of Cu, for example, increases from ~0.06 to ~0.14 after 16 passes of ECAP [109] and for the Al-7% Si alloy there is an increase from ~0.03 to ~0.14 after 10 turns of HTP.…”

Producing Bulk Ultrafine-Grained Materials by Severe Plastic Deformation: Ten Years Later

“…[27][28][29] Lastly, it has been found recently that a heterogeneous lamellar (HL) Ti with $25 vol.% of recrystallized grains ($4 lm) embedded in a UFG hard matrix has a strain-hardening rate and ductility higher than that of CG Ti with a grain size of 43 lm while maintaining the strength of UFG Ti (Fig. 1).…”

Shear fracture mechanism in micro-tension of an ultrafine-grained pure copper using synchrotron radiation X-ray tomography

“…4. For the UFG pure copper with partial recrystallized microstructure produced by ECAP processing, the main deformation mechanisms during micro-tension are dislocation slip for the coarse grains and grain boundary sliding for the ultrafine grains [19,20]. It appears that the ideal A/ A and B/ B shear textures make the geometrical condition of <110> closely aligned for the activation of the relatively easy (111)<110> slip system in f.c.c.…”