Showing papers on "Superplasticity published in 2006"

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Abstract: Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of ∼0.6 and ∼1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10 −4 s −1 . This result compares with an elongation of ∼70% achieved in the extruded condition without ECAP under similar testing conditions.

Mechanical behaviour and microstructural evolution of magnesium alloy AZ31 in tension at different temperatures

Abstract: The mechanical behaviour and microstructure changes of the magnesium alloy AZ31 have been studied in tension at various temperatures, from RT to 250 °C. Continuous dynamic recrystallization (DRX) under tension was observed for temperatures higher than 150 °C. Texture components corresponding to a high activation of 〈c + a〉 slip systems were observed from 200 °C. DRX occurs by rotation recrystallisation. At 200 °C a strong texture is conserved despite the occurrence of DRX, which is typical for rotation recrystallisation and concurrent grain deformation by slip. At 250 °C, where superplastic behaviour is observed, the texture of small grains weakens due to grain boundary sliding as an additional deformation mechanism. The dramatic changes in the mechanical behaviour at elevated temperature can thus be explained by the occurrence of DRX, the high activities of non-basal slip systems and the occurrence of grain boundary sliding.

The development of superplastic ductilities and microstructural homogeneity in a magnesium ZK60 alloy processed by ECAP

Abstract: An extruded ZK60 magnesium alloy was processed by equal-channel angular pressing (ECAP) and then tested in tension at elevated temperatures. The results show the alloy is superplastic at a testing temperature of 473 K with an optimum ductility of ~1310% when using an initial strain rate of 2.0 × 10?4 s?1. The results demonstrate that optimum superplasticity is achieved at intermediate strain rates and there are decreases in the elongations to failure at both faster and slower strain rates. Microhardness measurements were taken both on the cross-sectional plane and along the axial direction after processing by ECAP. These measurements show the alloy is essentially homogeneous in the as-pressed condition.

Deformation Induced Vacancies with Severe Plastic Deformation: Measurements and Modelling

Abstract: In discussing hardening characteristics in terms of crystalline lattice defects, in most cases the properties and kinetics of dislocations and their arrangement have been considered. However, during plastic deformation also vacancies and/or vacancy type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point. The effect of high vacancy concentrations on the hardening characteristics is twofold: (i) direct effects by impeding the movement of dislocations (ii) indirect one by inducing climbing and annihilation of edge dislocations leading to softening or even absolute decreases in strength. This paper presents first measurements of deformation induced vacancies in SPD materials which have been achieved by combined evaluation of resistometry, calorimetry and X-ray diffraction. The density of vacancies during and after SPD deformation is found to be markedly higher than in cases of conventional deformation and/or coarse grained material which may be partly attributed to the particular conditions of SPD namely the enhanced hydrostatic pressure as well as the changes in deformation path. It is suggested to make this high vacancy concentration responsible for both dynamic and static recovery and/or recrystallisation processes recently found during and after SPD, being potential reasons for enhanced ductility and superplasticity which only occur with nanomaterials originating from SPD. Recent publications show that in alloys, SPD induced vacancies can also enable the existence of phases which do not appear in the equilibrium diagram.

Low-Temperature Superplasticity of Ultra-Fine-Grained Ti-6Al-4V Processed by Equal-Channel Angular Pressing

Abstract: The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 µm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10−5 to 10−2 s−1. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10−4 s−1. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.

Failure mechanisms in superplastic AA5083 materials

Abstract: The mechanisms of tensile failure in four 5083 aluminum sheet materials are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to control failure of the AA5083 materials under uniaxial tension at elevated temperatures: cavitation and flow localization (i.e., necking). Conditions for which failure is controlled by cavitation correspond to those under which deformation is primarily by grain-boundary-sliding creep. Conditions for which failure is controlled by flow localization correspond to those under which deformation is primarily by solutedrag creep. A geometric parameter, Q, is used to determine whether final failure is controlled by cavitation or by flow localization. Differences in elongations to failure between the different AA5083 materials at high temperatures and slow strain rates are the result of differences in cavitation behaviors. The rate of cavitation growth with strain is nearly constant between the AA5083 materials for identical testing conditions, but materials with less tensile ductility evidence initial cavitation development at lower strain levels. The rate of cavitation growth with strain is shown to depend on the governing deformation mechanism; grain-boundary-sliding creep produces a faster cavitation growth rate than does solute-drag creep. A correlation is found between the early development of cavitation and the intermetallic particle-size population densities of the AA5083 materials. Fine filaments, oriented along the tensile axis, are observed on fracture surfaces and within surface cavities of specimens deformed primarily under grain-boundary-sliding creep. As deformation transitions to control by solute-drag creep, the density of these filaments dramatically decreases.

Friction-Stir Processing

Abstract: : Friction-stir processing (FSP) is an emerging surface-engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving strength and ductility, increase resistance to corrosion and fatigue, enhance formability, and improve other properties. FSP can also produce fine-grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non-consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. Large surface areas can be traversed rapidly by using the appropriate tool design accompanied by rastering. Frictional heating and extreme deformation occurs causing plasticised material (constrained by the shoulder) to flow around the tool and consolidate in the tool's wake. FSP zones can be produced to depths of 0.5 to 50mm, with a gradual transition from a fine-grained, thermodynamically worked microstructure to the underlying original microstructure. FSP has been applied to Al, Cu, Fe, and Ni-based alloys with resulting property improvements. Details of the benefits and limitations of FSP, along with examples of current and potential applications are presented below. Some examples of benefits include (1) a doubling of strength of cast nickel-aluminium-bronze, (2) a five-fold increase in ductility of Al alloy A356, (3) increased fatigue life by friction stir processing the surface of fusion welds, (4) depending on the microstructure, a 3 to 20 times increase in the corrosion resistance of a Cu-Mn alloy, and (5) bending of 25 mm thick 2519 Al plate to 85 at room temperature without surface cracking. The additional advantages of low-plasticity burnishing (LPB) following FSP, to change any residual near-surface tensile stresses introduced by FSP to compressive residual stresses, are also discussed.

Severe plastic deformation : toward bulk production of nanostructured materials

Abstract: Preface Processing mechanics and structure formation during SPD Physical Fundamentals Of Formation And Stabilisation of Nanostructures In Metals And Multiphase Alloys Under Severe Plastic Deformation The Limit of Grain Refinement during ECAP Deformation Modelling of Grain Boundary Structure in Nanostructured Materials Severe Plastic Deformation of Single Crystals Structure and Properties of Near-nanostructured Iron Ausforming and Marforming of NiTi Shape Memory Alloys Using Severe Plastic Deformation Anomalous Phase Transformations During Severe Cold Deformation Leading to Formation of Nanocrystalline Alloys The structure and properties of iron-based nanocrystalline alloys produced using cyclic martensitic transformations Phase Transformation in Intermetallics Induced by Shock-wave Loading Influence of Structure Features on Physical Properties of Nanocrystalline Metals from Severe Plastic Deformation Structural Sensitivity of Mechanical Properties SPD Materials Microstucture/Mechanical Properties Relationship in SePD Materials Room Temperature Mechanical Properties of Submicrocrystalline Commercial Aluminium Alloys Processed by Severe Plastic Deformation Internal Long-range Stress Fields in Ultrafine Grained Materials Mechanisms of Creep in Bulk Nanostructured Metallic Materials Structure and Resistance to Deformation of UFG Metals and Alloys Improving the Mechanical Properties of Severely Deformed Zinc-based Za Alloys through Equal-channel Angular Extrusion Enhanced Superplasticity of SPD-produced Nanostructured Metallic Materials Low-temperature Superplasticity in Commercial Magnesium Alloys Processed by ECAP Superplasticity and Solid-state Bonding of Materials Magnetic and Electronic Properties Characteristics of Electronic Structure for Nanocrystalline Metal Magnetic Properties and Defects in Palladium, Titanium and Copper after Severe Plastic Deformation Severe Plastic Deformation of the Hard Magnetic Materials Based on R2fe14b (R-Nd, Pr) Intermetallics Deformation Analysis in SPD Finite Element Simulation of Equal Channel Angular Pressing Thermo-Mechanical Finite Element Analysis of Equal Channel Angular Pressing State of the Art in SPD methods A novel method for the production of bulk nanostructured materials: Equal-channel angular extrusion (ECAE) Nanograined Structure In Steels Prepared by Severe Plastic Deformation Microstructure of Bimetallic Joint Of Titanium and Orthorhombic Titanium Aluminide (Explosion Welding) Production of Bulk Nanostructured Metals by Accumulative Roll Bonding (ARB) Process Scale Up and Commercialisation of ECAE Sputtering Products with Submicrocrystalline Structures.

Flow and cavitation in a quasi-superplastic two-phase magnesium–lithium alloy

Abstract: Tensile tests were conducted at temperatures from 473 to 623 K on a two-phase Mg–9.5 wt.% Li–1.0 wt.% Zn alloy with an initial lamellar structure. It is shown that the alloy exhibits a quasi-superplastic behavior with a maximum recorded elongation of 290% at 523 K when testing with an initial strain rate of 1.0 × 10?4 s?1. The experiments give a strain rate sensitivity of 0.33 and an activation energy of 92 kJ mol?1. Metallographic inspection after failure revealed the occurrence of extensive internal cavitation and quantitative measurements were undertaken to determine the shapes and sizes of these internal cavities at two different testing temperatures. It is shown that the results suggest a transition with increasing strain from cavity growth by diffusion to growth controlled by plastic flow in the surrounding crystalline matrix.

Ceramic matrix composites : microstructure, properties and applications

Abstract: Part 1 Fibre, whisker and particulate-reinforced ceramic composites: Fibrous monolithic ceramics Whisker reinforced silicon nitride ceramics Fibre reinforced glass/glass-ceramic matrix composites Particulate composites. Part 2 Graded and layered composites: Functionally-graded ceramic composites SiAlON based functionally graded materials Design of tough ceramic laminates by residual stresses control Hardness of multilayered ceramics. Part 3 Nanostructured ceramic composites: Nanophase ceramic composites Nanostructured coatings on advanced carbon materials Processing and microstructural control of metal reinforced ceramic matrix nanocomposites Carbon nanotubes-ceramic composites Machinable nanocomposite ceramics. Part 4 Refractory and speciality ceramic composites: Magnesia-spinel (MgAl2O4) composite refractory materials Thermal shock of ceramic matrix composites Superplastic ceramic composites. Part 5 Non-oxide ceramic composites: Sialons Carbon-ceramic alloys Silicon nitride and silicon carbide-based ceramics Oxynitride glasses-glass ceramics Functionally graded ceramics.

Pinning of grain boundaries by second phase particles in equal-channel angularly pressed Cu–Fe–P alloy

Abstract: Isothermal annealing was applied to an equal-channel angularly pressed (ECAP) Cu-Fe-P alloy to investigate the stability of the fine-grained microstructure. The starting as-cast microstructure of the alloy contained two sets of second phase particles, fine gamma-Fe particles, and coarse Fe3P intermetallic particles. Following ECAP, while the fine Fe particles were well dispersed in the interior of the grains, many Fe3P particles were found outside the grains, along or at the junctions of grain boundaries. The presence of Fe3P particles at the grain boundaries enhanced the stability of the fine grain structure by delaying grain growth until 873 K when a coarse-grained structure developed. The different roles of the two sets of particles are discussed in terms of the difference in their deformation behavior. (c) 2006 Elsevier B.V. All rights reserved.

Cooperative grain boundary sliding in nanocrystalline materials

Abstract: Superplastic behaviour of microcrystalline materials is now believed to be controlled by cooperative grain boundary sliding (CGBS). An increasing role of grain boundary mediated plasticity with decreasing grain size down to the nanoscale was predicted leading to the prospect of enhanced superplasticity in nanocrystalline materials. Nevertheless, materials with nanosized grains have revealed a significant decrease in plasticity contrary to theoretical prediction. Direct evidence of CGBS in nanocrystalline Ni3Al alloy from SEM surface analysis and in-situ TEM tensile testing was detected, confirming one similarity in the rheology of deformation processes between micro- and nanomaterials. Thus, differences in deformation behaviour of materials at these two length scales are related to the probability of sliding surface formation, sliding distance and related accommodation mechanisms.