High-strain-rate superplasticity of nanocrystalline aluminum alloy 1570

The structure and mechanical properties of nanocrystalline aluminum alloy 1570 obtained by means of severe plastic deformation have been studied. Being tested in a temperature range from 300 to 400°C, the alloy exhibits high-strain-rate superplasticity. At 400°C, the superplasticity is manifested in a very broad range of strain rates, extending from 5 × 10^?3 to 1 s^?1.     

The use of severe plastic deformation techniques in grain refinement

Severe plastic deformation (SPD) has emerged as a promising method to produce ultrafine-grained materials with attractive properties. Today, SPD techniques are rapidly developing and are on the verge of moving from lab-scale research into commercial production. This paper discusses new trends in the development of SPD techniques suchas high-pressure torsion and equal-channel angle pressing, as well as new alternative techniques for introducing SPD. The paper also contains a comparative analysis of SPD techniques in terms of their relative capabilities for grain refinement, enhancement of properties, and potential to economically produce ultrafine-grained metals and alloys. For more information, contact Terry C. Lowe, Science and Technology Base Programs, Los Alamos National Laboratory, Los Alamos, NM 87545; (505) 667-7824; fax (505) 665-3199; e-mail tlowe@lanl.gov.     

Microhardness measurements and the Hall-Petch relationship in an AlMg alloy with submicrometer grain size

An Al-3% Mg solid solution alloy was subjected to intense plastic deformation, using either equal-channel angular (ECA) pressing or torsion straining, to produce grain sizes in the submicrometer range. Static annealing at elevated temperatures led to grain growth and average grain sizes of up to > 100 μm. As-fabricated and statically annealed specimens were used to determine the variation in microhardness with grain size, and results confirm that the Hall-Petch relationship persists down to at least the finest grain size examined experimentally (∼90 nm). The results provide no evidence to support the claims of a negative Hall-Petch slope when the average grain size is very small, but there is evidence of a decrease in the slope of the Hall-Petch plot at the very finest grain sizes (< 150 nm); this is attributed to the increased participation of mobile extrinsic dislocations in the boundary regions when taking the hardness measurements.     

Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ? B19 transformation

The results of the comparative analysis of the Ti₅₀Ni₂₅Cu₂₅-alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.     

Investigation into the corrosion rate and features of the samples made of nanostructured aluminum alloy in the H<Subscript>2</Subscript>S-containing medium

The corrosion rate was determined and corrosion damage features of the samples made of AK4-1 aluminum alloy were investigated in the NACE solution containing hydrogen sulfide (H₂S). The alloy was investigated in the ultra-fine-grain state compared with the coarse-grain state formed after standard treatment T6 (quenching + ageing). The alloy was nanostructured by equichannel angular pressing (ECAP). It is shown that the alloy corrosion rate after ECAP is higher than after T6 treatment by a factor of 1.9. Herewith, the total corrosion takes place in alloy after ECAP, while pitting corrosion is also observed after T6 treatment in addition to the total corrosion. The corrosion effect strongly affects surface-roughness parameters of the samples made of AK4-1 alloy after ECAP compared with the samples after T6 treatment.     

Determining the shear plasticity of metals on the basis of torsion-tension tests

The ratio of the axial and angular deformation rates affects the stress state of samples in torsion with tension (compression). A method is proposed for determining the plasticity of metals on the basis of tests involving torsion with tension (compression).     

Automated Control System for the Assembly of a Composite Punching Tool

An automated system for simulation and control of the assembly of a composite hole-punching tool is developed. It consists of an information system used in preparations for assembly, with decision-making support on the basis of precedents; and modules for simulation and correction of the assembly process.     

Refinement of tungsten microstructure upon severe plastic deformation

The effect of equal-channel angular pressing and high-pressure torsion on the formation of an ultrafine-grained structure in commercially pure tungsten has been studied. The structure was tested by the methods of transmission and scanning electron microscopy using electron backscatter diffraction analysis. The material microhardness depending on the stored deformation has been evaluated. In the samples subjected to severe plastic deformation, the quantitative structure characteristics (the average size of grains/subgrains and misorientation-angle distribution) have been determined.