Showing papers on "Superplasticity published in 2018"

Liquid-Like, Self-Healing Aluminum Oxide during Deformation at Room Temperature.

Abstract: Effective protection from environmental degradation relies on the integrity of oxide as diffusion barriers. Ideally, the passivation layer can repair its own breaches quickly under deformation. While studies suggest that the native aluminum oxide may manifest such properties, it has yet to be experimentally proven because direct observations of the air-environmental deformation of aluminum oxide and its initial formation at room temperature are challenging. Here, we report in situ experiments to stretch pure aluminum nanotips under O2 gas environments in a transmission electron microscope (TEM). We discovered that aluminum oxide indeed deforms like liquid and can match the deformation of Al without any cracks/spallation at moderate strain rate. At higher strain rate, we exposed fresh metal surface, and visualized the self-healing process of aluminum oxide at atomic resolution. Unlike traditional thin-film growth or nanoglass consolidation processes, we observe seamless coalescence of new oxide islands wit...

Superplasticity in an organic crystal

Abstract: Superplasticity, which enables processing on hard-to-work solids, has been recognized only in metallic solids. While metallic materials and plastics (polymer solids) essentially possess high plastic workability, functional crystalline solids present difficulties in molding. Organic crystals especially are fragile, in the common view, and they are far from the stage of materials development. From the viewpoint of practical application; however, organic crystals are especially attractive because they are composed of ubiquitous elements and often exhibit higher performance than metallic materials. Thus, finding superplastic deformation of organic crystals, especially in a single-crystal-to-single-crystal manner, will pave the way for their material applications. This study confirmed superplasticity in a crystal of a simple organic compound: N,N-dimethyl-4-nitroaniline. The crystal exhibits single-crystal-to-single-crystal superplastic deformation without heating. This finding of “organosuperplasticity” will contribute to the future design of functional solids that do not lose their crystalline quality in molding. Superplasticity enables processing on hard-to-work solids but superelastic deformation, especially in a single-crystal-to-single-crystal manner, was not demonstrated for organic crystals so far. Here the authors demonstrate a single-crystal-to-single-crystal superplasticity in a crystal of N,N-dimethyl-4-nitroaniline.

Transition from poor ductility to room-temperature superplasticity in a nanostructured aluminum alloy

Abstract: Recent developments of nanostructured materials with grain sizes in the nanometer to submicrometer range have provided ground for numerous functional properties and new applications. However, in terms of mechanical properties, bulk nanostructured materials typically show poor ductility despite their high strength, which limits their use for structural applications. The present article shows that the poor ductility of nanostructured alloys can be changed to room-temperature superplastisity by a transition in the deformation mechanism from dislocation activity to grain-boundary sliding. We report the first observation of room-temperature superplasticity (over 400% tensile elongations) in a nanostructured Al alloy by enhanced grain-boundary sliding. The room-temperature grain-boundary sliding and superplasticity was realized by engineering the Zn segregation along the Al/Al boundaries through severe plastic deformation. This work introduces a new boundary-based strategy to improve the mechanical properties of nanostructured materials for structural applications, where high deformability is a requirement.

Achieving room temperature superplasticity in the Zn-0.5Cu alloy processed via equal channel angular pressing

Abstract: The Zn-0.5Cu alloy was investigated in as-cast, homogenized state and after carrying out equal channel angular pressing four times at room temperature via route BC. Microstructure analysis using light microscopy and SEM/EBSD was performed, as well as tensile tests, misorientation angle distribution and texture analysis. The initial microstructure with the average grain size of 560 µm was refined to a grain size of approx. 1 µm. The obtained microstructure consists of uniaxial grains separated mostly by high angle grain boundaries. Mechanical characterization was performed at strain rates from 2 × 10−6 s−1 to 1 s−1 at room temperature. Ultrafine-grained zinc-copper alloy exhibited 510% elongation with the strain rate sensitivity equal 0.31, which was the first observation of room temperature superplasticity in Zn-Cu alloys. Based on the microstructure, misorientation angle and texture analysis, the main operating deformation mechanisms were distinguished for samples deformed at strain rates from 2 × 10−5 s−1 to 10−1 s−1.

Ultra-grain refinement and enhanced low-temperature superplasticity in a friction stir-processed Ti-6Al-4V alloy

Abstract: An ultrafine microstructure consisting of α grains (~ 0.51 µm) and a small amount of β phase was successfully achieved in a friction stir-processed (FSPed) Ti-6Al-4V alloy. The fraction of high angle grain boundaries (HAGBs) with random crystallographic orientations reached 89.3% revealed that dynamic recrystallization was responsible for the ultra-grain refinement mechanism during friction stir processing (FSP). Low-temperature superplasticity (LTSP) of such an ultrafine microstructure was demonstrated in the temperature range of 550–650 °C and strain rates of 1 × 10−4–3 × 10−3 s−1. Specifically, an extremely superior LTSP of 1130% was achieved at 600 °C and 3 × 10−4 s−1, which was explained by means of the ultrafine equiaxed grains, a large proportion of HAGBs with random orientations as well as the presence of β phase. The predominant superplastic deformation mechanism was considered as grain boundary sliding associated with grain boundary diffusion.

Effect of a minor titanium addition on the superplastic properties of a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

Abstract: A CoCrFeNiMn high-entropy alloy (HEA) with an addition of 2 at% Ti was processed by high-pressure torsion to produce a grain size of ~30 nm and then tested in tension at elevated temperatures from 873 to 1073 K using strain rates from 1.0 × 10−3 to 1.0 × 10−1 s−1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of 830% at a temperature of 973 K. It is shown that the Ti addition contributes to the formation of precipitates and, combined with the sluggish diffusion in the HEA, grain growth is inhibited to provide a reasonable stability in the fine-grained structure at elevated temperatures. By comparison with the conventional CoCrFeNiMn HEA, the results demonstrate that the addition of a minor amount of Ti produces a smaller grain size, a higher volume fraction of precipitates and a significant improvement in the superplastic properties.

Aluminium scandium alloys

Abstract: Scandium is the first of the transition elements and occupies an intermediate position between the typical rare-earth elements and the light metallic elements. The most significant technological potential for scandium is as an alloying element in aluminium. Indeed, extensive research has demonstrated that a minor alloying addition of scandium can result in a major increase in strength, with reports of a 50–100 MPa increment per 0.1 wt.%Sc added. This enhanced strength can be attributed to the fine spherical L12 Al3Sc dispersoids, typically with a diameter under 10 nm. Sc acts as a solid solution hardener and Al3Sc as a precipitation hardener and recrystallization inhibiter and has also been reported to act as a favourable nucleation site for strengthening phases. This increase in strength is generally accompanied by an improved or stable ductility and the strong antirecrystallization of Al3Sc enables the creation of superplastic alloys, formable at strain rates up to 10− 2 s− 1. Finally, scandium has been reported to improve various corrosion-related properties. In this chapter, a brief history and details of the chemical and physical metallurgy of the scandium element will be presented. The impact of scandium on phase transformations, microstructure, and bulk properties in the industrially relevant Al alloys (casting and wrought, heat-treatable, and nonheat-treatable) is reviewed. A particular emphasis on the interaction between scandium and other alloying elements is also outlined.

A Review on Superplastic Formation Behavior of Al Alloys

Abstract: Superplastic formation technology is considered to be an effective and promising method to conquer formation difficulties of Al alloys, especially thin-walled or complex structure. In this paper, fundamentals of superplastic deformation of Al alloys are summarized and the superplastic deformation behaviors of main kinds of Al alloys are summarized and compared, including Al-Mg alloys, Al-Li alloys, and Al-Zn-Mg alloys as well as aluminum matrix composites. Then, the superplastic deformation mechanism for different Al alloys is thoroughly discussed. Last but not the least, the factors affecting the superplastic deformation process are fully analyzed.

Comparison between superplastic deformation mechanisms at primary and steady stages of the fine grain AA7475 aluminium alloy

Abstract: Evolution of the deformation behaviour, surface and bulk structures at superplastic flow of the АА7475 aluminium-based alloy were studied by scanning and transmission electron microscopes and a focused ion beam technique. Differences between deformation behaviour at a primary stage with strains below 0.69 and a steady stage with strains above 0.69 were discussed. The research showed the grain growth and grain elongation to the stress direction at a primary stage of deformation. Stabilisation of both mean grain size and grain aspect ratio was found at a steady stage of deformation. Grain neighbour switching, grain rotation, dispersoid free zones and some insignificant intragranular strain were observed at both stages. Appearing and disappearing of the grains on the sample surface, with increased dislocation activity occurred at the steady stage of deformation. The current results highlighted the importance of diffusion creep as a dominant mechanism at the beginning of superplastic deformation and as an accommodation mechanism of the grain boundary sliding at the steady stage of deformation. The dislocation creep as an additional accommodation mechanism of the grain boundary sliding at the steady stage of superplastic deformation is suggested.

Superplasticity in a commercially extruded ZK30 magnesium alloy

Abstract: The high temperature mechanical behavior of a commercial ZK30 magnesium alloy was studied by means of tensile tests at various temperatures and strain rates. This behavior was related to the complex as-received microstructure that evolves during testing to a microstructure formed by few large grains (≈ 30 µm) combined with a large amount of small grains (1–5 µm). Large elongations to failure up to 360% and low apparent stress exponents between 2.6 and 2.9 at low strain rates are a hint of the activation of grain boundary sliding as the controlling deformation mechanism. This is corroborated by the equiaxed microstructure after testing. The stress exponents higher than 2 are attributed to the accelerated grain growth of the dual grain size microstructure.

Superplasticity in Ultrafine-Grained Materials.

Abstract: Abstract Superplasticity refers to the ability of a polycrystalline solid to exhibit a high elongation, of at least 400% or more, when testing in tension. The basic characteristics of superplastic flow are now understood and a theoretical model is available to describe the flow process both in conventional superplastic materials where the grain sizes are a few micrometers and in ultrafinegrained materials processed by severe plastic deformation where the grain sizes are in the submicrometer range. This report describes the basic characteristics of superplastic metals, gives examples of flow in ultrafine-grained materials, demonstrates the use of deformation mechanism mapping for providing a visual display of the flow processes and provides a direct comparison with the conventional model for superplastic flow. The report also describes the potential for using nanoindentation to obtain detailed information on the flow properties using only exceptionally small samples.

Consecutive crystallographic reorientations and superplasticity in body-centered cubic niobium nanowires

Abstract: Plasticity of metallic nanowires is often controlled by the activities of single deformation mode. It remains largely unclear whether multiple deformation modes can be activated in an individual metallic nanowire and how much plasticity they can contribute. In situ nanomechanical testing reveals a superior plastic deformation ability of body-centered cubic (BCC) niobium nanowires, in which a remarkable elongation of more than 269% is achieved before fracture. This superplastic deformation originates from a synergy of consecutively nucleated multiple reorientation processes that occur for more than five times via three distinct mechanisms, that is, stress-activated phase transformation, deformation twinning, and slip-induced crystal rotation. These three coupled mechanisms work concurrently, resulting in sequential reorientations and therefore superplastic deformation of Nb nanowires. Our findings reveal a superior mechanical property of BCC Nb nanowires through the close coordination of multiple deformation modes, which may have some implications in other metallic nanowire systems.

Effect of layered microstructure on the superplasticity of friction stir processed AZ91 magnesium alloy

Abstract: Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity. The FSP tools with different tool pin length were used to develop three kinds of layered microstructured materials. A full thickness fine grained microstructure (FFG), a half thickness fine grained with remaining half in as-cast condition (HFG). The last variation was one third thickness modified into fine grain from both the surfaces and the middle section having as-cast microstructure (SFG). FSP was performed at a tool rotational speed of 720 rpm and at a transverse speed of 150 mm/min. The coarse α-Mg dendrites with large plate like interconnected β-Mg17Al12 interdendrites were the characteristics microstructure of as-cast AZ91 alloy. Fine grains and uniformly distributed precipitates were the characteristics of FSPed microstructure. High temperature tensile tests were carried out at 350 °C using three different initial strain rates i.e. 5 × 10−3 s−1, 1 × 10−3 s−1 and 5 × 10−4 s−1. The FFG material showed superplasticity at all strain rates and highest ductility of 680% was achieved at the strain rate of 5 × 10−4 s−1. The AC and HFG material displayed very low elongation while SFG material exhibited superplasticity of 388%. The superplastic behaviour in SFG was due to increase in the fraction of fine grained microstructure and modification of as-cast microstructure on both the surfaces. Microstructure and texture studies revealed that grain boundary sliding accommodated by grain boundary migration and grain rotation was responsible for superplasticity in FSPed region.

Low temperature superplasticity of a dual-phase Mg-Li-Zn alloy processed by a multi-mode deformation process

Abstract: A dual-phase Mg-Li-Zn alloy was processed by a severe plastic deformation method which is a method of combination of extrusion and rolling processes and enables production of a very fine grain structure. After this processing, the Mg-Li-Zn alloy exhibited a significantly large fracture elongation of 1400% at 473 K at 0.001 s−1. Moreover, an elongation of more than 600% was observed at 473 K even at high strain rate of 0.01 s−1. Also, at a lower temperature of 423 K, the alloy exhibited a large fracture elongation of 720% at 0.001 s−1. The values of the strain rate sensitivity were approximated to 0.5, which suggested that the superplastic deformation is based on grain boundary sliding. Dislocation glide is identified to be an accommodation mechanism according to the texture evolution during the superplastic deformation. The different trends of the changes of the textures in the α and β phases indicated an inhomogeneity of grain boundary sliding between phases.

Factors influencing superplasticity in the Ti-6Al-4V alloy processed by high-pressure torsion

Abstract: Two different initial microstructures, martensitic and lamellar, were developed in a Ti-6Al-4V alloy to examine their effect on the high temperature mechanical properties and superplasticity after high-pressure torsion (HPT). Significant grain refinement was achieved in both conditions with grain sizes after HPT processing of ~30 and ~40 nm, respectively. The nanocrystalline alloy in both conditions was subjected to mechanical testing at 923–1073 K using strain rates in the range from 10−3 to 10−1 s−1. The martensitic and lamellar alloys exhibited excellent ductility at these high temperatures including superplastic elongations at 973 K with maximum elongations of 815% and 690%, respectively. The fcc phase was stable at elevated temperatures in the martensitic alloy and the results suggests the fcc phase may contribute to the superior superplastic properties of the martensitic alloy.

Superplastic forming of shells from sheet blanks with thermally unstable coatings

Abstract: The article is devoted to superplastic forming of non-uniformly heated sheet blanks from aluminum alloy AMg6M . To create an uneven temperature field over the surface of the blank, coatings made of sublimated substances were used from at temperatures 50…150°C below the superplastic forming temperature (450°C for AMg6M alloy). An aqueous solute of chloride and ammonium iodide having a sublimation temperature at normal pressure and a latent heat of conversion equal to 338 and 404°C, 330 and 355kJ/kg, respectively, was sprayed onto the central zones of the blanks. Superplastic forming of shells was carried out in two modes: 1) with simultaneous sublimation of the coating; 2) with the beginning of the sublimation of the coating upon reaching the height, which is formed by the workpiece, equal to 20-30% of the final shell height. The experiments showed a decrease in the thickness of the shells to be formed up to 4-10% on the working surface of the shells (without taking into account their flange zones) and the savings of sublimate during superplastic forming in the second mode. The optimal subliming coating for AMg6M alloy was ammonium chloride.

Grain refinement and superplastic flow in a fully lamellar Ti-6Al-4V alloy processed by high-pressure torsion

Abstract: A cold-rolled Ti-6Al-4V alloy was subjected to consecutive heat treatments at 1283 K for 1 h and at 823 K for 3 h in order to produce a fully lamellar microstructure. Thereafter, the material was processed by high-pressure torsion (HPT) through various numbers of turns up to a maximum of 30. It is shown that the HPT processing leads to exceptional grain refinement with average grain sizes of ~ 70 and ~ 50 nm after 20 and 30 turns, respectively. Tensile testing was conducted at 873 and 923 K with different initial strain rates using the material processed through 20 turns of HPT and this gave a maximum superplastic elongation of 820% at the relatively low temperature of 923 K when testing with an initial strain rate of 5.0 × 10−4 s−1. The associated strain rate sensitivity for this low temperature superplasticity was estimated as m ≈ 0.5 which is consistent with flow by grain boundary sliding.

Studies on titanium alloys for aerospace application

Abstract: Since the development of the Ti54M titanium alloy in 2003, its application within the aerospace sector has gradually increased due to the combination of properties such as improved forgeability and machinability, low flow stress at elevated temperatures, and superplastic characteristics. However, for the successful exploitation of Ti54M a comprehensive understanding of its mechanical characteristics, microstructure stability, and superplastic behaviour is required. The superplastic forming of titanium alloys is characterised by high deformation at slow strain rates and high temperatures which influence the material microstructure, and in turn, determine the forming parameters. These mechanisms make the prediction of the material behaviour very challenging, limiting its application within the aerospace industry. Even though Ti54M has been commercially available for over 10 years, further studies of its mechanical and superplastic properties are still required with the aim of assessing its applicability within the aerospace industry as a replacement for other commercial titanium alloys. Therefore, in this work a study of the mechanical and superplastic properties of Ti54M, in comparison with other commercial titanium alloys used in the aerospace industry - i.e. Ti-6AL-4V, and Ti-6-2-4-2 - is presented. The final objective of this study, carried out at the Advanced Forming Research Centre (AFRC, University of Strathclyde, UK), is to obtain material data to calibrate and validate a model capable of estimating the behaviour and grain size evolution of titanium alloys at superplastic conditions.

Investigation on the thickness distribution of highly customized titanium biomedical implants manufactured by superplastic forming

Abstract: Mechanical performances of titanium biomedical implants manufactured by superplastic forming are strongly related to the process parameters: the thickness distribution along the formed sheet has a key role in the evaluation of post-forming characteristics of the prosthesis. In this work, a finite element model able to reliably predict the thickness distribution after the superplastic forming operation was developed and validated in a case study. The material model was built for the investigated titanium alloy (Ti6Al4V-ELI) upon results achieved through free inflation tests in different pressure regimes. Thus, a strain and strain rate dependent material behaviour was implemented in the numerical model. It was found that, especially for relatively low strain rates, the strain rate sensitivity index of the investigated titanium alloy significantly decreases during the deformation process. Results on the case study highlighted that the strain rate has a strong influence on the thickness profile, both on its minimum value and on the position in which such a minimum is found.

Deformation mode transitions in Cu 50 Zr 50 amorphous/Cu crystalline nanomultilayer: A molecular dynamics study

Abstract: The amorphous/crystalline (A/C) nanomultilayers have been aroused great interest in people due to its first-class mechanical properties. The effects of the thickness of crystalline and amorphous on the deformation mechanism of A/C CuZr/Cu nanomultilayers under tension loading here are investigated by molecular dynamics method. The results indicate that the mechanical behavior of nanomultilayer strongly depends on the bidirectional synergistic deformation mechanism between crystal phase and amorphous phase. The deformation behavior of nanomultilayers can be controlled by integrating the thickness of different phases to achieve high strength and superplastic multilayer materials. For the nanomultilayers with constant crystal layer, the plastic deformation changes from shear band propagation to a pronounced interface slip-accommodation mechanism, and ultimately to crack propagation mode with decreasing amorphous thickness. For the nanomultilayers with fixed amorphous layer thickness, the mechanical behavior changes from localization to plastic co-deformation mode with the crystalline thickness decreases. This study proposes an approach for achieving a good balance between strength and ductility, which is useful for the synthesis of A/C nanomultilayer with high strength and predominant ductility.

Roll-to-Roll Nanoforming of Metals Using Laser-Induced Superplasticity

Abstract: This Letter describes a low-cost, scalable nanomanufacturing process that enables the continuous forming of thin metallic layers with nanoscale accuracy using roll-to-roll, laser-induced superplasticity (R2RLIS). R2RLIS uses a laser shock to induce the ultrahigh-strain-rate deformation of metallic films at room temperature into low-cost polymeric nanomolds, independently of the original grain size of the metal. This simple and inexpensive nanoforming method does not require access to cleanrooms and associated facilities, and can be easily implemented on conventional CO2 lasers, enabling laser systems commonly used for rapid prototyping or industrial cutting and engraving to fabricate uniform and three-dimensional crystalline metallic nanostructures over large areas. Tuning the laser power during the R2RLIS process enables the control of the aspect ratio and the mechanical and optical properties of the fabricated nanostructures. This roll-to-roll technique successfully fabricates mechanically strengthened ...

High-strength Mg-6Zn-1Y-1Ca (wt%) alloy containing quasicrystalline I-phase processed by a powder metallurgy route

Abstract: A high-strength Mg-6Zn-1Y-1Ca (wt%) alloy has been processed by a powder metallurgy route. Rapidly solidified powders with a particle size below 100 µm were used as a way for preventing formation of ternary MgZnCa compounds during subsequent extrusion at 250 °C. The microstructure of the extruded alloy consists of an ultrafine-grain magnesium matrix, with an average grain size of 444 nm, embedding a high volume fraction of fine I-phase particles aligned along the extrusion direction. The alloy combines an excellent ductility (14% of elongation to failure) with a high strength (ultimate strength of 469 MPa and yield stress of 461 MPa) at room temperature, mainly due to grain size refinement (around 70% of the yield stress). The strength is kept high up to 150 °C (yield stress of 279 MPa). Above this temperature, the mechanical strength falls to very low values but the ability to deform plastically is considerably enhanced, exhibiting superplastic behaviour from 200 to 350 °C, with a maximum elongation of 477% at 350 °C.