Pedro Henrique R. Pereira

Examining the mechanical properties and superplastic behaviour in an Al-Mg-Sc alloy after processing by HPT

Experiments were conducted using the solution treated Al-3 % Mg-0.2 % Sc alloy in order to examine the mechanical properties and superplastic behaviour after processing by high-pressure torsion. Hardness values of ~200 Hv were detected in the edge of the samples after 10 turns of HPT, although the microhardness distribution was inhomogeneous along the diameter of the discs. Excellent superplastic properties were achieved in this Al alloy after HPT processing and further tensile testing at temperatures over the range from 523 to 673 K, demonstrating good agreement with the theoretical model for grain boundary sliding. High strain rate superplasticity was consistently observed at temperatures from 573 to 623 K, with elongations to failure up to ~800 % at 573 K and at a strain rate of 1.4×10-2 s-1. Further analysis of the experimental data revealed that the stress exponent at the testing conditions displaying elongations >400 % was ~2 thereby demonstrating good agreement with the theoretical model for grain boundary sliding. The calculated activation energies lie in the range from ~98 to ~118 kJ mol-1 and are similar to the value obtained for the same alloy after ECAP which is within the range for grain boundary diffusion in pure Al and interdiffusion in Al-Mg alloys

Grain refining of a Ti-6Al-4V alloy by high-pressure torsion and low temperature superplasticity

A cold-rolled Ti-6Al-4V sheet was subjected to heat treatmentprior to processing by high-pressure torsion (HPT). Quantitative measurements revealed that the volume fractions were 25% equiaxed ? phase and 75% lamellar (?+?). The grain size of the ? phase was 9.5±1.5 ?m. The processing by HPT was performed at room temperature with a pressure of 6.0 GPa and a rotation speed of 1 rpm. Processing of the material was conducted through a total number of revolutions, N, of 1/4, 1, 5, 10 and 20. After 1 turn in HPT, the traces of shear deformation appeared. When the turns increased to 5, shear deformation was more severe. After 10 turns, the fibrous structure basically disappeared and ultrafine equiaxed grains were formed. When the deformation proceeded to 20 turns, a series ofhigh-angle boundaries were formed in the sample and the average size of equiaxed grains was 77±15 nm.After HPT processing, the microhardness increased as the numbers of HPT turns increased and the distribution of hardness tended to be relatively homogeneous when the HPT continued to 20 turns.Superplastic deformation tests were performed at 873K at different strain rates in the material processed by HPT and an elongation of 790% was achieved when deforming at 873K and 10-4 s-1

Using finite element modelling to examine the flow process and temperature evolution in HPT under different constraining conditions

High-pressure torsion (HPT) is a metal-working technique used to impose severe plastic deformation into disc-shaped samples under high hydrostatic pressures. Different HPT facilities have been developed and they may be divided into three distinct categories depending upon the configuration of the anvils and the restriction imposed on the lateral flow of the samples. In the present paper, finite element simulations were performed to compare the flow process, temperature, strain and hydrostatic stress distributions under unconstrained, quasi-constrained and constrained conditions. It is shown there are distinct strain distributions in the samples depending on the facility configurations and a similar trend in the temperature rise of the HPT workpieces.

Influence of Initial Heat Treatment on the Microhardness Evolution of an Al-Mg-Sc Alloy Processed by High-Pressure Torsion

An Al-3% Mg-0.2% Sc alloy was subjected to annealing or solution treatment and further processed by HPT at room temperature. Microhardness measurements were taken along the middle-sections of the discs and they demonstrated that a very substantial hardening is achieved during HPT processing regardless of the initial heat treatment. Hardness values of ~200 Hv were recorded at the edge of the samples although the microhardness distribution remained inhomogeneous along the diameters of the discs after 20 turns of high-pressure torsion. In addition, the microhardness of the solution treated Al-Mg-Sc samples continued to increase with the equivalent strain imposed by the anvils even after 30 turns of HPT processing whereas the hardness at the edges of the annealed discs saturated after 10 turns. These differences in the hardness evolution are attributed to the higher Mg content in solid solution in the case of the solution treated samples and its influence on delaying the recovery rate of this aluminium alloy.

Thermal stability and mechanical properties of HPT-processed CP-Ti

A grade 2 commercial pure titanium was processed by high-pressure torsion (HPT) at room temperature to 20 turns. X-ray analysis showed that an ω phase formed during HPT processing but disappeared immediately during 10 minutes post-HPT short-term annealing even at a very low temperature of 473 K. The thermal stability of HPT-processed microstructural evolution was studied by electron backscatter diffraction (EBSD). After short-term annealing at lower temperatures (473 and 673 K), the disc centre had relative higher hardness value than the edge area, and it was found that the centre retained the feature of deformed microstructure whereas the edge showed recovery / recrystallization of the microstructure. Short-term annealing at higher temperatures (873 and 973 K) led to almost uniformly distributed hardness and microstructures in the disc centre and edge areas. Grain structures and hardness measurements indicate complete recrystallization occurred at 873 K.

Thermal stability and superplastic behaviour of an Al-Mg-Sc alloy processed by ECAP and HPT at different temperatures

An Al-3%Mg-0.2%Sc alloy was processed by ECAP and HPT at different temperatures. Afterwards, samples subjected to 10 turns of HPT at 300 and 450 K, 8 passes of ECAP at 300 K and 10 passes of ECAP at 600 K were annealed for 1 hour at 523 K and their mechanical properties and microstructure were examined using microhardness measurements and EBSD analysis. In addition, tensile specimens with similar dimensions were machined from the HPT and ECAP processed materials and further tensile tested at 523 K. The results demonstrate that the Al alloy processed by HPT at 450 K exhibits higher microhardness values (~138 Hv) and a smaller average grain size (~0.28 µm) after annealing at 523 K among all SPD processing conditions. Accordingly, the material subjected to HPT at an elevated temperature displays superior superplastic properties such that an elongation of ~1020 % was attained after testing at 523 K at 1.0 × 10-3 s-1. Furthermore, detailed EBSD analysis revealed a significant fraction of low-angle grain boundaries (LAGBs > 33 %) in the ECAP-processed material after annealing which may be responsible for the inferior superplastic behaviour by comparison with the HPT-processed samples (LAGBs < 13 %) tested at 523 K.

Recovery or non-recovery in Al-0.1% Mg and Al-1% Mg alloy during high-pressure torsion processing

The Al-1% Mg and Al-0.1% Mg alloys were both processed by high-pressure torsion (HPT) at room temperature. In the Al-1% Mg alloy, the hardness values in the disc centre area are lower than in the disc edge area after 1/2 and 1 turn, and the area of lower hardness values in the disc centre decreases as the number of turns increases from 1/2 to 1 turn. Finally, the hardness values are reasonably homogenous along the disc diameter as the number of turns increases to 5 and 10 turns. The Al-0.1% Mg alloy displays a different hardness evolution behavior: the hardness values in the disc centre are higher than at the disc edge 1/2 and 1 turn, and the area of higher hardness values decreases as the numbers of turn increases from 1/2 to 1 turn. The hardness values evolve towards homogeneity along the disc diameter after 5 and 10 turns. EBSD microstructure investigations in the Al-0.1% Mg alloy reveal that a few low-angle boundaries exist at the disc edge after 1/2 turn. It is suggested that the higher hardness values in the disc centre in the Al-0.1% Mg alloy are related to rapid recovery at the disc edge where the material is subjected to heavy straining.

An evaluation of high temperature tensile properties for a magnesium AZ31 alloy processed by high-pressure torsion

A magnesium alloy AZ31 was processed by high-pressure torsion (HPT) at room temperature. Microstructure investigations show the material has a grain size as fine as ~450 nm after 5 turns of HPT processing. X-ray texture measurements show most grains have the {0001} fibre, with their c-axis parallel to the HPT torsion axis. Tensile specimens were cut from HPT disc and pulled to failure over a range of strain rates (4.5×10-5, 1.3×10-4, 1.3×10-3 and 1.3×10-2 s-1) at temperatures of 623 and 673 K. The tensile elongations from HPT specimens are lower than for published results using equal-channel angular processing (ECAP) specimens although AZ31 has a finer grain size after HPT than after ECAP. The reasons for the lower elongations in HPT specimens are related to the thermal stability of the processed microstructure, the texture components and the tensile specimen size. Earlier investigations confirmed there was significant grain growth at 623 and 673 K in HPT samples, which would contribute to the low ductility of AZ31 in tensile testing. The main {0001} fibre in HPT samples means in tensile specimens most grains have their basal plane parallel to the surface of the tensile specimen, leading to the low ductility because the critical resolved shear stress does not operate on the basal plane due to the small Schmid factor that is nearly zero. The tensile specimen thickness in HPT is thinner than in ECAP and it is known that the ductility decreases when reducing the specimen thickness