Superplastic behaviour of AZ91 magnesium alloy processed by high– pressure torsion

An investigation has been conducted on the tensile properties of a fine–grained AZ91 magnesium alloy processed at room temperature by high pressure torsion (HPT). Tensile testing was carried out at 423 K, 473 K and 573 K using strain rates from 1×10–1 s–1 to 1×10–4 s–1 for samples processed in HPT for N = 1, 3, 5 and 10 turns. After testing was completed, the microstructures were investigated by scanning electron microscopy and energy dispersive spectroscopy. The alloy processed at room temperature in HPT exhibited excellent superplastic behaviour with elongations higher than elongations reported previously for fine–grained AZ91 alloy produced by other severe plastic deformation processes, e.g. HPT, ECAP and EX–ECAP. A maximum elongation of 1308 % was achieved at a testing temperature of 573 K using a strain rate of 1×10–4 s–1, which is the highest value of elongation reported to date in this alloy. Excellent high–strain rate superplasticity (HSRSP) was achieved with maximum elongations of 590 % and 860 % at temperatures of 473 K and 573 K, respectively, using a strain rate of 1×10–2 s–1. The alloy exhibited low–temperature superplasticity (LTSP) with maximum elongations of 660 % and 760 % at a temperature of 423 K and using strain rates of 1×10–3 s–1 and 1×10–4 s–1, respectively. Grain–boundary sliding (GBS) was identified as the deformation mechanism during HSRSP, and the glide–dislocation creep accommodated by GBS dominated during LTSP. Grain–boundary sliding accommodated with diffusion creep was the deformation mechanism at high test temperature and slow strain rates. An enhanced thermal stability of the microstructure consisting of fine equiaxed grains during deformation at elevated temperature was attributed to the extremely fine grains produced in HPT at room temperature, a high volume fraction of nano ?–particles, and the formation of ?–phase filaments

Microstructural and hardness evolution of additively manufactured Al–Si–Cu alloy processed by high-pressure torsion

Nanostructured Al-9%Si-3%Cu alloy was achieved by direct metal laser sintering (DMLS) and then processed using high-pressure torsion (HPT) processing, which resulted in considerable grain refinement down to 60 nm associated with a substantial dislocation density up 6.2×1014m-2 and a significant reduction in the porosity. Hardness measurements across the horizontal and vertical cross-sections showed an improvement in the strength homogeneity for processed samples after 10 turns of HPT processing. These results indicate that a controllable ultrafinegrained microstructure can be achieved by employing additive manufacturing, followed by effective severe plastic deformation processing