Ultrafine-grained Bulk Research Articles

BULK NANOSTRUCTURED MATERIALS OBTAINED BY SHOCK WAVES COMPACTION OF ULTRAFINE TITANIUM AND ALUMINUM

Theoretical and experimental Investigations of shock wave consolidation processes of Ti - Al nano sized and ultra-disperse powder compositions are discussed. For theoretical calculations of the shock wave loaded materials were used the hydrodynamic theory and experimental adiabatic of Ti and Al . The normal and tangential stresses in the cylindrical steel tube (containers of Ti - Al reaction mixtures) were estimated using the partial solutions of elasticity theory. The mixtures of ultra-disperse Ti and nano sized (≤ 50nm) Al powder compositions were consolidated to full or near-full density by explosive-compaction technology. The ammonium nitride based industrial explosives were used for generation of shock waves. To form ultra-fine grained bulk TiAl intermetallics with different compositions, ultra-disperse Ti particles were mixed with nano-crystalline Al . Each reaction mixture was placed in a sealed container and explosively compacted using a normal and cylindrical detonation set-up. Explosive compaction experiments were performed in range of pressure impulse (5-20) GPa. X-ray diffraction (XRD), structural investigations (SEM) and micro-hardness measurements were used to characterize the intermetallics phase composition and mechanical properties. The results of analysis revealing the effects of the compacting conditions and precursor particles sizes, affecting the consolidation and the properties of this new ultra high performance alloys are discussed.

Strengthening Behavior of Bulk Ultra Fine Grained Aluminum Alloys

Billets of aluminum and aluminum alloys have been deformed at room temperature using a die having equal channels of 10 mm diameter intersecting at an inner angle of 120° and outer arc of 60° by equal channel angular pressing (ECAP) to ultra fine grain (UFG) size level, adopting route Bc. Mechanical properties were evaluated by tensile testing and microhardness measurement. Effects of alloying elements on strengthening were explored. The strengths increase rapidly at first few passes and then reach to a saturation level. The improvement in strength at initial passes of ECAP is due to work hardening and subgrain or dislocation cell formation. However, strengthening at large number of passes is due to the grain refinement alone. The rate of strengthening as a function equivalent strain decreases to a minimum. The strengthening level of bulk UFG alloys is about 3.5 to 4.5 times to that of starting materials. The major cause of strengthening is grain refinement apart from solute strengthening. Among Mg, Zn and Ag alloying elements, the strengthening effect is highest for Mg and lowest for Ag. Ductility is regained without affecting the strength after sufficient number of passes when microstructure becomes equiaxed and ultra-fine in size. However, ductility of UFG Al alloys is lower than that of their coarse grained counterpart.

Texture Evolution of Armco Iron during Accumulative Roll Bonding Process

The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size <1µm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding (ARB). The important parameter of ARB is the number of cycles and then the von Mises equivalent strain. By means of the Electron BackScattered Diffraction (EBSD) technique, the texture evolution with the number of cycles was studied. The microhardness was also measured in function of the equivalent strain. Finally, the mean grain size and the fraction of high angle grain boundaries were determined as a function of the number of cycles.

Low Temperature Plasticity of an Ultrafine‐Grained Al–3Mg Alloy Prepared by Accumulative Roll Bonding

AbstractMany efforts have been made recently to produce ultrafine grained (UFG) structural metallic materials having submicrometer grain sizes, since UFG materials are expected to perform superior mechanical properties, i.e., a high strength, good elongation and, mainly superplastic formability at lower temperatures and higher strain rates. One of the promising ways to produce bulk UFG materials is the severe plastic deformation (SPD).

Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part I: Microstructure Evolution

Aluminum alloys are widely used because they are lightweight and exhibit high strength. In recent years, spark plasma sintering (SPS) technology has emerged as a viable approach to sinter materials due to its application of rapid heating and high pressure. In this study, SPS was chosen to consolidate dense ultrafine-grained (UFG) bulk samples using cryomilled nanostructured Al 5083 alloy (Al-4.5Mg-0.57Mn-0.25Fe, wt pct) powder. Both bimodal microstructure and banded structure were observed through transmission electron microscopy (TEM) investigation. The evolution of such microstructures can be attributed to the starting powder and the process conditions, which are associated with the thermal, electrical, and pressure fields present during SPS. A finite element method (FEM) was also applied to investigate distributions in temperature, current, and stress between metallic powder particles. The FEM results reveal that the localized heating, deformation, and thermal activation occurring at interparticle regions are associated with the formation of the special microstructure.

Preparation of bulk ultrafine-grained Mg-3Al-Zn alloys by consolidation of ball milling nanocrystalline powders

Powder metallurgy was used to fabricate a bulk ultrafine grain size (UFG) Mg-3Al-Zn alloys. Nanocrystalline alloy powders with an average grain size of 45 nm were synthesized via ball milling of elemental powders of Mg, Al and Zn. The milled powders canned in Al containers were subjected to cold (at room temperature) or hot press (for 40 min at 633K), respectively, in a vacuum furnace. The sintered samples were extruded at 423 K to further solidify. The results show that the average grain size is 180 nm for the cold samples, and that is 600 nm for the hot samples. The cold-press alloys show a yield stress of 426 MPa. The high strength of UFG Mg is attributed to the fine grain strengthening mechanism resulting from the strong dependence of strength on the grain size for HCP metals. The consolidated samples of the cold-press and hot-press alloys have a final density of (1.777±0.006) and (1.800±0.006) g/cm 3, respectively.

Processing, microstructure and corrosion behaviour of multi-axially forged low carbon steels

Increasing attention is being paid to processing bulk ultrafine grained (UFG) materials for structural applications. Along with mechanical properties, study of corrosion behaviour of these materials is important for them to find wider applications. In this work, low carbon steels that are extensively used for general purpose structural applications are studied. Low carbon steels were processed by using multi-axial forging, a technique of severe plastic deformation. Steel specimens were processed using six passes of warm multi-axial forging operations, to produce UFG structures. The present work further investigated the corrosion behaviour of these severely deformed UFG steels. Potentiodynamic tests, immersion tests, and salt spray tests were performed in chloride environments. In most environments employed in this study, UFG steels showed improved uniform corrosion behaviour compared to their coarse grained counterparts. The improvement in corrosion properties is explained based on microstructural evolution with respect to grain size, dislocation population and substructure development.

Microstructure inhomogeneity in ultra-fine grained bulk AZ91 produced by accumulative back extrusion (ABE)

This paper focuses on the effects of the accumulative back extrusion (ABE) process on microstructure inhomogeneity and room temperature mechanical properties of AZ91 magnesium alloys. The results show that ABE processing leads to significant grain refinement with an average grain size of ∼2 μm in the shear zone from initial grain size of ∼150 μm at the central region of a specimen subjected to high shear through 1 pass at 300 °C. In the outer region of the sample which is in contact with die surface and central region which is in contact with inner punch land, there is no significant change in the microstructure, but some Mg17Al12 precipitation occurs around the massive Mg17Al12 phase after ABE. In the other regions subjected to deformation channel of the specimen, there is more grain refinement along with increased Mg17Al12 dissolution. It is also shown that the ABE process affects the morphology of the Mg17Al12 phase. Microhardness measurements verified the microstructure inhomogeneity results.

The mechanical behaviour of an ultrafine grained Ti–47Al–2Cr (at%) alloy in tension and compression and at different temperatures

A bulk ultrafine grained (UFG) Ti–47Al–2Cr (at%) alloy has been produced using a powder metallurgy process that combines high energy mechanical milling (HEMM) of a mixture of Ti, Al and Cr powders to produce a Ti/Al/Cr composite powder and hot isostatic pressing (HIP) of the composite powder compact. The purpose of the present study is to determine the mechanical behaviour of the alloy in tension and compression at room temperature (RT) and elevated temperatures, and also to compare the compression behaviour of the material with its tensile behaviour. It has been found that due to the residual pores, lack of full level interparticle bonding and high oxygen content (0.87 wt%) in the consolidated samples, the UFG TiAl based alloy has a very low room temperature tensile fracture strength of ∼100 MPa and shows no tensile ductility. However these microstructural defects and high oxygen content have much less significant effect on the room temperature compressive mechanical properties, and the alloy shows a high compressive yield strength of ∼1410 MPa, and some ductility (plastic strain to fracture ∼4%). At elevated temperatures of 800 °C and above, the alloy shows high tensile and compressive ductility as demonstrated by 75% tensile elongation to fracture and no cracking in upset forging with a height reduction of 50% at 900 °C. The yield strength of the alloy at 900 °C is 55 MPa in tension and ∼33 MPa in compression, both of which are lower than those of coarse grained TiAl based alloys with similar compositions at 900 °C. This is due to a higher creep rate of the UFG alloy caused by the small grains. The good formability of the UFG TiAl based alloy as reflected by the lower critical temperature above which the alloy becomes highly formable indicates that the material can be used as a suitable precursor for secondary thermomechanical processing and super-plastic forming.

Influence of Crystallite Size on Consolidation of Carbon Nanotube Reinforced AA 4032 Composite Powders by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) is the one of the promising methods of severe plastic deformation to obtain bulk ultrafine grain structures. However, ECAP can also be used for powder consolidation. In the present study, fully dense bulk AA 4032 alloy was consolidated from nanocrystalline and microcrystalline powders. These materials were processed by ECAP until four passes at ambient temperature. It is observed that hardness and densification increased significantly with increase in number of ECAP passes. Transmission electron microscopic and scanning electron microscopic examinations evidenced that crystallite size of the nanopowders are unaltered, however a significant crystallite size reduction from around 50 µm down to submicron size is observed. Moreover, higher densification is achieved in microcrystalline powders than nano powders, whereas higher hardness in the case of nanopowders compared to microcrystalline powders.

Role of structural parameters of ultra-fine grained Cu for its fatigue and crack growth behaviour

High cycle fatigue (HCF) life time curves and fatigue crack growth rates of bulk ultra-fine grained (UFG) copper deformed by high pressure torsion (HPT) were determined. Cu of two different purities as well as a bimodally structured HPT Cu were investigated. The results show increased HCF properties of the UFG materials compared to coarse grained (CG) Cu. Especially HPT Cu with lower purity shows enhanced fatigue resistance due to higher microstructural stability. Contrary, crack growth rates in HPT Cu are increased. In case of the high purity Cu, cyclic deformation induced coarsening of the UFG microstructure nearby the crack is found at threshold crack growth rates leading to a retardation of the fatigue crack propagation. Within these coarse grains typical fatigue surface slip marks as observed in CG Cu are found.

Microstructure and Texture of Magnesium Single Crystals Processed by ECAP

Equal channel angular pressing (ECAP) is a promising severe plastic deformation technique for production of ultrafine-grained bulk metals with face centered cubic (fcc) structure. However, the process is often complicated in hexagonal close packed (hcp) metals such as magnesium due to its low forming capability. In this contribution, magnesium single crystals were processed by ECAP through a single pass in order to reveal processes taking place in hcp lattice during severe plastic deformation. The microstructure and texture were investigated by SEM-EBSD. The deformed microstructure contains shear bands, recrystallized regions and mechanical twins. Activity of twinning systems and texture formation are discussed regarding different initial orientation of the single crystals.

Scratch-induced deformation in fine- and ultrafine-grained bulk alumina

The nanoscratch behavior of two bulk α-alumina samples with 1.3 μm and 290 nm average grain sizes, respectively, was investigated using a nanoindenter in scratch mode, in combination with atomic force and scanning electron microscopy. A ductile to brittle transition was observed in the fine-grained sample, while the ultrafine-grained sample exhibited predominantly ductile deformation with a fish-bone feature indicative of a stick–slip mechanism. These findings suggest that grain refinement can increase the potential for plastic deformation in ceramics.

Tensile and compressive test in nanocrystalline and ultrafine carbon steel

Plastic deformation behavior was investigated in near fully dense nanostructured and ultrafine-grained bulk samples of carbon steel (0.55 wt% C) under compression and tension tests. The specimens were obtained by hot pressure from mechanically milled powder at 400 and 500 °C. Subsequent heat treatments at temperatures going from 600 to 900 °C produced samples with ferrite grain sizes from 30 nm to 17 μm. Nanocrystalline grained steel samples presented very high strength with low ductility. Once, in the ultrafine range, as the ferritic grain size was increased, the strength was decreased and the ductility was improved. The porosity and carbon atoms within the structure were analyzed in order to explain the results of strength and strain obtained.

Prototyping of Ultrafine-Grained Steel Fabrication

Ultrafine-grained (UFG) steel is a promising idea for future demands based on merits like less alloying, high strength, and excellent toughness. However, heavy deformation is concerned as a requisite to obtain the ultrafine-grained microstructure. Hence, nobody has ever attempted to make bulk UFG steels. In the UFG steel fabrication, major concerns are how much force is necessary, how the UFG microstructure is homogeneously inherited, and how the plate geometries can be precisely controlled. In our attempt, finite element model (FEM) computer simulation was utilized to answer these questions. Furthermore, collaborating with private companies, we have fabricated various types of bulk UFG steels. The FEM computer simulation has successfully bridged the gap between National Institute for Materials Science (NIMS) fundamental knowledge and practical fabrication by private companies.

Corrosion and ion release behavior of ultra-fine grained bulk pure copper fabricated by ECAP in Hanks solution as potential biomaterial for contraception

Ultra-fine grained (UFG) bulk pure copper has been successfully fabricated by equal-channel angular pressing (ECAP), with the grain size about 380 nm after 8 passes. The potentiodynamic polarization results of the ECAP copper specimens tested in Hanks solution revealed that the corrosion current of UFG copper is higher than that of the coarse grained copper. The cupric ion release behaviors of UFG copper immersed in Hanks solution for 30 days only displayed a burst release during the first 3 days (in comparison to the 1–2 months for the conventional Cu) from 115 μg/day to 12.5 μg/day, after which the ion release remained constant and slow. During the immersion experiments, Cu 2O was the only corrosion product found on the surface and it took 10 days or so to form a uniform Cu 2O layer. Uniform corrosive damage on the surface and few localized corrosion is observed. The above results indicate that UFG copper could have high potential as biomedical materials for contraception.

Ductility of Bulk Nanocrystalline and Ultrafine Grain Iron and Steel

This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, in addition to new approaches such as spark plasma sintering and high pressure torsion, have increased the total plastic strain of nanostructured iron. The development of bimodal structures enables the existence of strain hardening and more uniform deformation. The paper also includes a steel study, which finds that the hardness of milled powder and the role of carbon atoms inside ferrite grains make it more difficult to improve the ductility of nanostructured samples.

Cyclic deformation and fatigue properties of very fine-grained metals and alloys

Bulk ultrafine-grained (UFG) metals and alloys prepared by techniques of severe plastic deformation (SPD), in particular by equal channel angular pressing (ECAP), exhibit exceptional mechanical properties. With respect to potential applications of this new class of very fine-grained bulk materials, the cyclic deformation and fatigue behaviour, relative to that of conventional grain size (CG) materials, is of crucial importance. In the present review, the research performed on the fatigue properties of very fine-grained bulk materials during the last decade will be discussed. Mainly bulk UFG materials prepared by ECAP will be considered and, to a smaller extent, also bulk truly nanocrystalline materials. The discussion will focus on simple materials and on more complex alloys and structural materials. Presentation of fatigue performance in total strain fatigue life diagrams has been found to be particularly suitable to compare the strong UFG and the more ductile CG materials in both the high cycle fatigue (HCF) and the low cycle fatigue (LCF) ranges. In general, the fatigue strength of UFG materials is enhanced considerably by grain refinement, in particular in the HCF regime. However, at the same time, the LCF performance is impaired by microstructural instabilities of the strongly hardened but less ductile UFG materials, as manifested in cyclic softening, fatigue-induced grain coarsening (by dynamic recrystallization at rather low homologous temperature!) and massive shear banding. These effects are discussed critically, also with respect to the effects of the route of ECAP-processing employed, the purity of the material – and the mode of fatigue testing. Remedies by mild annealing treatments which have been partially successful in improving the LCF strength by enhancing the ductility at the expense of a moderate loss of strength will be discussed. Examples of modelling of the cyclic deformation and fatigue properties will be presented, and some directions of future research will be outlined.

Characterization of ultra-fine grained steel samples produced by high pressure torsion via magnetic Barkhausen noise analysis

High pressure torsion (HPT) is one of the most widely used severe plastic deformation methods which enable to obtain a crack-free ultra-fine grained bulk sample. In the process, a disk-shaped sample is pressed between two anvils and deformed via surface friction forces by rotating one of the anvils. The aim of this study is to nondestructively characterize the variations in the deformation uniformity of the severely deformed disks. Various sets of low carbon steel samples were obtained by applying the constrained HPT process up to 6 turns. Magnetic Barkhausen noise (MBN) method was used in order to evaluate the samples in a nondestructive manner via a commercial system. The results were compared with those of metallographic examination, hardness measurements and X-ray diffraction (XRD). The initial stages of HPT revealed the effects of conventional plastic deformation on MBN; however with further straining, grain refinement caused increase in MBN signals.

Preparation and magnetic characterization of ultrafine-grained ζ-Mn 2N 0.86 compound bulk

The ultrafine-grained ζ-Mn 2N 0.86 compound bulk was prepared by a novel method that combined the solid-gas reaction, ball milling and spark plasma sintering. Characterizations on the magnetic properties showed that the Néel temperatures of the ball-milled powder and the ultrafine-grained bulk of Mn 2N 0.86 compound are drastically reduced as compared with the coarse-grained polycrystalline ζ-phase Mn-N compounds. Furthermore, it was found that the antiferromagnetism and the weak ferromagnetism coexist in the ball-milled powder and the ultrafine-grained bulk of Mn 2N 0.86 compound.