Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures

Abstract

Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.

Micro-mechanical and tribological properties of aluminum-magnesium nanocomposites processed by high-pressure torsion

“…As reviewed recently [47], this approach produces an ordering of the defect structures within the grain boundaries leading to an equilibrium state without any significant grain growth. In addition, short-term annealing reduces the dislocation density in the grain interior of the UFG material after SPD so that the dislocation storage capability may increase and thus the strain hardening capability is enhanced wand this leads to the possibility of high ductility in the SPD-processed material.…”

“…It is now well established that there is generally a significant loss in ductility in bulk materials having UFG microstructures where a marked increase in hardness and strength is achieved by grain refinement through SPD [44][45][46][47]. Specifically, the lower overall ductility in the UFG materials is generally attributed to an interrelationship between the lower strain hardening and an increasing strain rate sensitivity [48,49].…”

The Strength–Grain Size Relationship in Ultrafine-Grained Metals

“…[115] In Figs This procedure may be illustrated using results from a thorough investigation of a cast Al -7% Si alloy which is a material used in automotive applications where an increase in ductility will be profitable. [121] This alloy was processed by HPT at 6.0 GPa up to 10 turns at temperatures of 298 K (25°C) or 445 K (172°C), the processed grain size was ~0.4 μm, the strain rate sensitivity increased with increasing numbers of HPT turns from ~0.03 to ~0.14 and the average Si particle size was reduced from ~2.8 to ~0.12 μm after 5 cycles of HPT. [122] annealing after SPD increased the elongation but not sufficiently to move the datum into the HSHD region in materials such as the Al-1% Mg alloy.…”

“…[122] annealing after SPD increased the elongation but not sufficiently to move the datum into the HSHD region in materials such as the Al-1% Mg alloy. [121] Thus, the quantitative diagram in Fig. 7 is a useful tool for assessing different materials and processing methods when attempting to achieve HSHD.…”

“…Elongation to failure correlated with the fraction of HAGBs measured in an Al-7% Si alloy processed by HPT. [121] Figure 11. Stress-strain curves for Al-7034 showing the weakening through increasing numbers of ECAP passes.…”