Extensive Cold Rolling Research Articles

Hydrogen desorption/absorption properties of the extensively cold rolled β Ti–40Nb alloy

β Ti–Nb BCC alloys are potential materials for hydrogen storage in the solid state. Since these alloys present exceptional formability, they can be processed by extensive cold rolling (ECR), which can improve hydrogen sorption properties. This work investigated the effects of ECR accomplished under an inert atmosphere on H2 sorption properties of the arc melted and rapidly solidified β Ti40Nb alloy. Samples were crushed in a rolling mill producing slightly deformed pieces within the millimeter range size, which were processed by ECR with 40 or 80 passes. Part of undeformed fragments was used for comparison purposes. All samples were characterized by scanning electron microscopy, x-ray diffractometry, energy-dispersive spectroscopy, hydrogen volumetry, and differential scanning calorimetry. After ECR, samples deformed with 40 passes were formed by thick sheets, while several thin layers composed the specimens after 80 passages. Furthermore, deformation of β Ti–40Nb alloys synthesized samples containing a high density of crystalline defects, cracks, and stored strain energy that increased with the deformation amount and proportionally helped to overcome the diffusion's control mechanisms, thus improving kinetic behaviors at low temperature. Such an improvement was also correlated to the synergetic effect of resulting features after deformation and thickness of stacked layers in the different deformation conditions. At the room temperature, samples deformed with 80 passes absorbed ∼2.0 wt% of H2 after 15 min, while samples deformed with 40 passes absorbed ∼1.8 wt% during 2 h, excellent results if compared with undeformed samples hydrogenated at 300 °C that acquired a capacity of ∼1.7 wt% after 2 h. The hydrogen desorption evolved in the same way as for absorption regarding the deformation amount, which also influenced desorption temperatures that were reduced from ∼270 °C, observed for the undeformed and samples deformed with 40 passes, to ∼220 °C, for specimens rolled with 80 passes. No significant loss in hydrogen capacity was observed in the cold rolled samples.

Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries

Summary Zn-Sn interdigitated eutectic alloy (IdEA) foils are highly promising anodes for lithium-ion batteries. Synthesized via severe plastic deformation, a two-phase alloy is converted into a nanostructured foil composed of tin domains distributed throughout an electrically conductive zinc matrix. In addition to providing mechanical integrity, uncycled capacity in the zinc matrix eliminates the lithium-plating risk. These monolithic foil anodes offer an electrode-level volumetric capacity approximately double that of graphite (797 mA h cm−3), with a highly promising formation loss of

Low temperature rolling of AZ91 alloy for hydrogen storage

The hydrogen storage properties of a commercial AZ91 magnesium alloy were investigated after processing by cold rolling in two different conditions: (a) extensive cold rolling at room temperature (CR); (b) rolling with the immersion of the alloy in liquid nitrogen bath after 05 rolling passes. This second condition is named as low temperature rolling (LTR). A full microstructural characterization including scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), X-ray photoelectron and positron annihilation life time spectroscopy (XPS and PALS) were performed in the processed alloys. The hydrogen storage properties were measured using a Sievert-type apparatus. The AZ91-alloy is composed by the β-Mg and Mg17Al12 phases, and after processing by CR, an intense peak broadening and strong [002] texture were observed in the alloys. The LTR processing resulted in a more fragile material containing more microcracks and exposed interfaces than the conventionally cold rolled one. The LTR sample also presented a more refined microstructure in comparison with CR sample. These features resulted in superior hydrogen storage properties for the LTR sample in comparison with the CR sample. In addition, the effect of surface contamination was also studied in detail and it was found to play a significant role during the activation kinetics.

Deformation structures and strengthening mechanisms in an Al-Cu alloy subjected to extensive cold rolling

The deformation structures, mechanical properties and strengthening mechanisms of a 2519 aluminum alloy subjected to cold rolling up to a total reduction of 80% (ε ~ 1.61) in the supersaturated solid solution condition were studied. The formation of cell structure and a one hundred-fold increase in lattice dislocation density up to ∼ 2.1 × 1015m−2 after a 40% reduction leads to increase in yield stress (σYS) and ultimate tensile strength (σUTS) from 135 to 453MPa, from 350 to 503MPa, respectively. The formation of fully lamellar structure and further increase in lattice dislocation up to ρd ~ 5 × 1015m−2 take place at an 80% reduction. As a result, σYS and σUTS increase to 560 and 590MPa, respectively. Yield stress at reductions ≥ 40% are significantly higher than that in an AA2519T87 alloy. Subdivision of initial grains by lamellar boundaries due to deformation banding provide high efficiency of dislocation strengthening due to the accumulation of an extremely high density of lattice dislocations in supersaturated solid solution and retention of sufficient elongation-to-failure of 9% and 5% after reductions of 40% and 80%, respectively.

Processing of MgH2 by extensive cold rolling under protective atmosphere

Cold rolling can significantly increase hydrogen storage properties of Mg and its alloys. Therefore in this paper, two important cold rolling processing parameters were investigated: the amount of passes and the roll rotation frequency, which directly influence particle and crystallite sizes and thus the interfaces for hydrogenation/dehydrogenation. Extensive cold rolling was conducted with MgH2 powder as starting material under an argon atmosphere to avoid contamination. The results indicate that the cold rolled MgH2 was free from oxides even after 50 cold rolling passes. Additionally, considerable improvements in the hydrogen storage properties were achieved with higher amounts of cold rolling passes. A similar effect could also be found for higher roll rotating frequency. This beneficial outcome originated from three important aspects: the decrease of the crystallite size reaching 88% reduction after 50 passes and 82% after 35 passes with rotating frequency of 50 rpm, the refinement of particle size, and a change in the morphology of the initial MgH2 powder from granules to flakes, and ultimately to small particles.

Severely deformed ZK60 + 2.5% Mm alloy for hydrogen storage produced by two different processing routes

In this investigation the commercial ZK60 magnesium alloy modified with 2.5 wt.% of mischmetal was processed by melt spinning (MS) and equal-channel angular pressing (ECAP), both followed by extensive cold rolling (CR). The results indicate that MS was effective to produce small grain sizes in the order of 700 nm, while specimens prepared only by ECAP reached about 50 μm. However, melt spun ribbons showed long activation (first hydrogenation) times of more than 30 h in contrast to ECAP, which took about 10 h. In addition, after the activation process, the hydrogen storage capacity of ECAP was poorer than MS (2.5 and 3.3 wt.% of H2, respectively). Additional processing by CR caused further grain refinement and breakage of intermetallic particles, as well as favoring crystallographic texture in the (002) direction. All these features promoted a significant improvement in the hydrogen storage capacity, and in the absorption kinetics of the specimens processed by MS and ECAP.

Hydrogen storage in heavily deformed ZK60 alloy modified with 2.5 wt.% Mm addition

Severe plastic deformation techniques have been extensively used to improve hydrogen storage properties of magnesium based alloys. In this work, the commercial magnesium alloy ZK60 (Mg-6Zn-1Zr in wt.%) was modified with addition of 2.5 wt.% of mischmetal (55Ce-24La-15Nd-4Pr in wt.%) and subsequently processed by equal-channel angular pressing (ECAP) and extensive cold rolling (CR). Due to the mischmetal addition, a network of intermetallic particles was formed at the grain boundaries. Besides Zn and Zr, the particles also contained the rare-earth elements La and Ce. With CR, a breakage of intermetallics occurred, that could also have played a role in enhancing the hydrogen storage properties. Ultimately, after the combined processing of ECAP followed by CR cold rolling, a significant increase in the storage capacity and activation kinetics was observed.

Superplasticity in a 5024 Aluminium Alloy Subjected to ECAP and Subsequent Cold Rolling

The superplastic behavior of an 5024 alloy, subjected to equal-channel angular pressing (ECAP) followed by extensive cold rolling (CR), was studied in the temperature range 250-500°C and at strain rates ranging from 10–4 to 10–1 s–1. The maximum elongation-to failure of ~1440% with a corresponding strain rate sensitive coefficient m of ~0.42 was attained at 450°C and a strain rate of ~1.4×10–1 s–1. The relationship between superplastic properties and microstructure of the alloy was discussed.

Ultrafine-Grained Structure and Mechanical Properties of a High-Mn Twinning Induced Plasticity Steel

The influence of thermo-mechanical treatment consisting of cold rolling followed by recrystallization annealing on the grain size and mechanical properties of a high-Mn TWIP steel was studied. An Fe-23Mn-0.3C-1.5Al TWIP steel (wt. %) was subjected to extensive cold rolling with a reduction of 80% (true strain of ∼1.6) and then annealed in the temperature interval ranging from 400 to 900 °C during 20 minutes. Recovery processes took place below 500 °C, partial recrystallization was evident at ~550°C and fully recrystallized structure evolved after annealing at 600 °C and higher. The static recovery resulted in a slight decrease in the yield strength from 1400 MPa to 1250 MPa and the ultimate tensile strength from 1540 MPa to 1400 MPa whereas the total elongation of 4% did not changed. The recrystallization development led to a drastic drop of strength and an increase in ductility. The yield strength of 225 MPa, the ultimate tensile strength of 700 MPa and the total elongation of 79% was obtained after annealing at 900 °C. Correspondingly, the grain size increased from 0.2 μm to 6.2 μm with increase in anneal temperature from 550 to 900°C.

Modeling the effect of deformation on strength of a Fe-23Mn-0.3C-1.5Al TWIP steel

Modeling the effect of deformation on mechanical properties of a Fe-23Mn-0.3C- 1.5Al (in wt. %) TWIP steel subjected to extensive cold rolling was performed. The approach including Hall-Petch relation and dislocation strengthening was used for the determination of the strength increment with deformation. Distance between twin boundaries was considered to be the mean free path for dislocation glide in the Hall-Petch relationship. The rate of dislocation accumulation in the TWIP-steel was calculated via the X-ray diffraction (XRD) measurements and compared with direct transmission electron microscopy (TEM) measurements by counting individual dislocations crossing the thin foil surface after small reductions. It was shown that a decrease of the dislocation mean free path by formation of twins with nanoscale thickness is of less importance for the strengthening than an increase of the dislocation density, which was shown to be the major strengthening mechanism. This suggested that twin boundaries were more readily transparent and, therefore, provided less strengthening as compared to ordinary grain boundaries. The developed model has shown a good agreement with the experimental data.

Magnesium-Nickel alloy for hydrogen storage produced by melt spinning followed by cold rolling

Severe plastic deformation routes (SPD) have been shown to be attractive for short time preparation of magnesium alloys for hydrogen storage, generating refined microstructures and interesting hydrogen storage properties when compared to the same materials processed by high-energy ball milling (HEBM), but with the benefit of higher air resistance. In this study, we present results of a new processing route for Mg alloys for hydrogen storage: rapid solidification followed by cold work. A Mg97Ni3 alloy was processed by melt spinning (MS) and by extensive cold rolling (CR). Submitting Mg97Ni3 ribbons between steel plates to cold rolling has shown to be a viable procedure, producing a thin cold welded foil, with little material waste. The as-processed material presents a high level of [002] fiber texture, a sub microcrystalline grain structure with a high density of defects, and also a fine dispersion of Mg2Ni nanoparticles. This refined microstructure allied to the developed texture resulted in enhanced activation and H-sorption kinetics properties.

Hydrogen Activation Behavior of Commercial Magnesium Processed by Different Severe Plastic Deformation Routes

Severe plastic deformation (SPD) techniques are being considered as low cost processing routes for Mg alloys, aiming hydrogen storage applications. The main objective is to develop air-resistant materials, with lower specific surface area in comparison with ball-milled powders, but with still attractive H-sorption kinetics associated to the microstructural refinement. In this study, the effects of different SPD processing routes (high-pressure torsion, extensive cold rolling and cold forging) in the hydrogen activation behavior of Mg was evaluated. The results show that both microstructural and textural aspects should be controlled during SPD processing to obtain Mg alloys with good H-sorption properties and enhanced activation kinetics.

Superplastic behavior of a fine-grained two-phase Mg-9wt.%Li alloy

A two-phase Mg-9Li alloy (where the composition is in weight per cent) was made fine grained (d≈6–35 μm) by extensive cold rolling into foils and press bonding the foils at low homologous temperatures. The material exhibited superplastic properties in the temperature range 423–523 K ((0.49–0.61)T/T m) with elongations to failure as high as 460%. The creep rate was determined to be inversely proportional to approximately the square of grain size and to the second power of stress. The activation energy for superplastic flow was equal to that for lattice diffusion in the major b.c.c. β phase. The Mg-9Li laminate alloy was found to be similar in its flow characteristics to MA-21, a multicomponent Mg-8.1Li fine-grained commercial Soviet alloy.