Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Improvement of the biomedical properties of titanium using SMAT and thermal oxidation

Titanium and its alloys are excellent candidates for biomedical implant. However, they exhibit relatively poor tribological properties. In this study, a two-step treatment including surface mechanical attrition treatment (SMAT) combined with thermal oxidation process has been developed to improve the tribological properties and biocompatibility of Ti. Ti after two-step treatment shows excellent wear-resistance and biocompatibility among all Ti samples, which can be ascribed to the highest surface energy, well crystallinity of rutile layer on its surface. Overall, the two-step treatment is a prospective method to produce excellent biomedical Ti materials.

Effect of heat treatment on diffusion, internal friction, microstructure and mechanical properties of ultra-fine-grained nickel severely deformed by equal-channel angular pressing

Severe plastic deformation via equal-channel angular pressing was shown to induce characteristic ultra-fast diffusion paths in Ni (Divinski et al., 2011). The effect of heat treatment on these paths, which were found to be represented by deformation-modified general high-angle grain boundaries (GBs), is investigated by accurate radiotracer self-diffusion measurements applying the 63Ni isotope. Redistribution of free volume and segregation of residual impurities caused by the heat treatment triggers relaxation of the diffusion paths. A correlation between the GB diffusion kinetics, internal friction, microstructure evolution and microhardness changes is established and analyzed in detail. A phenomenological model of diffusion enhancement in deformation-modified GBs is proposed.

Enhanced mechanical response of an ultrafine grained Ti-6Al-4V alloy produced through warm symmetric and asymmetric rolling

An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti-6Al-4V alloy with an average grain size of 110-230. nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365. MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

Microstructural Evolution of Ti-6Al-7Nb Alloy During High Pressure Torsion

Ti-6Al-7Nb alloys are being evaluated for biomedical applications, in substitution of the more conventional Ti-6Al-7V. Both types of alloys present a microstructure containing the alpha and the beta phases, which result in good compromise for mechanical applications. In the present work Ti-6Al-7Nb alloys were processed by High Pressure Torsion (HPT), varying the number of revolutions and thus the total imposed strain. X-Ray Diffraction (XRD) results revealed the formation of different crystallographic textures in samples subjected to HPT. Microhardness distribution, across the diameters of the disks, is rather homogeneous for all samples, with higher values for those subjected to 03 and 05 turns. Transmission electron microscopy (TEM) micrographs have showed that an ultra-fine grained microstructure was obtained in all the samples.

Efeito no processo ECAP (EQUAL CHANNEL ANGULAR) na microestrutura e nas propriedades mecânicas de uma liga comercial de alumínio AA3104

Pós-graduação em Engenharia Mecânica - FEG

Effect of cold-working process on corrosion behavior of copper

The microstructure, microhardness, texture and corrosion resistance of cold-swaged and cold-wiredrawn copper rods were evaluated. Elongated grains along the deformation direction were observed for both materials and the width of these grains decreased with the increase of reduction in area. Wiredrawn copper rods have higher microhardness than the swaged rods for the same reduction in area. The copper grains in both cold-worked rods presented a preferential orientation in the [1 1 0] crystallographic direction but this trend was more pronounced for swaged rods. The corrosion resistance of wiredrawn copper rods investigated in H(2)SO(4) solutions was lower than that of swaged rods, and for both deformed materials the corrosion resistance decreased with the deformation degree. (C) 2011 Elsevier Ltd. All rights reserved.

Prediction of ductile failure in CP-Titanium as criterion of SPD process design

Application of damage model in combination with finite element analysis to design and optimization of equal channel angular pressing - conform of commercially pure titanium against ductile failure is demonstrated. The properties required for precise simulation of the process and prediction of damage accumulation (equivalent stress as function of equivalent strain and temperature and low bound ductility function) are obtained in the temperature interval 20-400 °C and described in details.

Machining of coarse grained and ultra fine grained titanium

Machining of titanium is quite difficult and expensive. Heat generated in the process of cutting does not dissipate quickly, which affects tool life. In the last decade ultra fine grained (UFG) titanium has emerged as an option for substitution for more expensive titanium alloys. Extreme grain refinement can be readily performed by severe plastic deformation techniques. Grain refinement of a material achieved in this way was shown to change its mechanical and physical properties. In the present study, the microstructure evolution and the shear band formation in chips of coarse grained and UFG titanium machined to three different depths and three different feeding rates was investigated. A change in thermal characteristics of commercial purity Ti with grain refinement was studied by comparing heating/cooling measurements with an analytical solution of the heat transfer boundary problem. It was demonstrated that an improvement in the machinability can be expected for UFG titanium. © 2012 Springer Science+Business Media, LLC.

Production of Ti-6Al-4V billet through compaction of blended elemental powders by equal-channel angular pressing

In this work, compaction by warm equal-channel angular pressing (ECAP) with back pressure was used to produce Ti-6Al-4V billets from both commercially pure (CP) titanium and titanium hydride (TiH 2) powders, which were mixed with pulverised binary Al-V master alloys of two distinct Al/V ratios and with elemental aluminium powder to arrive at the nominal alloy composition. It was demonstrated that the right combination of temperature, high hydrostatic pressure and plastic shear deformation permits consolidation of the powder mixture to maximum green densities of 99.26%. Moreover, after direct compaction of blended elemental powders by equal-channel angular pressing (ECAP) with back pressure, the sintering temperature required for chemical and microstructural homogenisation of the compacts could be reduced by 150-250°C. This was possible due to high green density, increased contact area between powder particles and the formation of fast diffusion paths associated with grain refinement by severe plastic deformation. The sintered Ti-6Al-4V billets exhibited a maximum density of 99.88%, Vickers hardness of 409-445 HV1 and ultimate tensile strength in the range of 1000-1080MPa. In contrast to findings of other authors, the use of TiH 2 powders in conjunction with ECAP processing did not bring any benefits with regard to the production of the Ti-6Al-4V alloy.

Precipitation and solute clustering in aluminium: advanced characterisation techniques

Recent development of characterisation techniques and computer simulation has extended our ability to access atomic scale information regarding materials microstructure evolution. New results from such techniques have significantly progressed our knowledge about solute behaviour during the earliest stages of decomposition of the solid solution. This chapter updates current understanding about solute clustering and discusses the effect of solute clustering and micro-alloying on precipitate microstructure evolution in aluminium alloys. In addition, a brief review is given on the effect of severe plastic deformation on precipitate evolution in Al alloys.

Bimodal grain size distributions in UFG materials produced by SPD - their evolution and effect on the fatigue and monotonic strength properties

In ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) techniques such as ECAP (equal channel angular pressing), bimodal grain size distributions have been observed under different circumstances, for example shortly after ECAP, after rest or anneal and/or after mild cyclic deformation at rather low homologous temperature. It has been shown that the mechanical monotonic and fatigue properties of some UFG materials can be modified (sometimes enhanced) by introducing a bimodal grain size distribution by a mild annealing treatment which leads, in some cases, to a good combination of strength and ductility. Here, the conditions under which bimodal grain size distributions evolve by (adiabatic) heating during ECAP and during subsequent annealing or cyclic deformation will be explored, and the effects on the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed. In particular, the role of temperature rise during ECAP will be considered in some detail.

A model of grain fragmentation based on lattice curvature

A new model is proposed that aims to capture within a single modelling frame all the main microstructural features of a severe plastic deformation process. These are: evolution of the grain size distribution, misorientation distribution, crystallographic texture and the strain-hardening of the material. The model is based on the lattice curvature that develops in all deformed grains. The basic assumption is that lattice rotation within an individual grain is impeded near the grain boundaries by the constraining effects of the neighbouring grains, which gives rise to lattice curvature. On that basis, a fragmentation scheme is developed which is integrated in the Taylor viscoplastic polycrystal model. Dislocation density evolution is traced for each grain, which includes the contribution of geometrically necessary dislocations associated with lattice curvature. The model is applied to equal-channel angular pressing. The role of texture development is shown to be an important element in the grain fragmentation process. Results of this modelling give fairly precise predictions of grain size and grain misorientation distribution. The crystallographic textures are well reproduced and the strength of the material is also reliably predicted based on the modelling of dislocation density evolution coupled with texture development.

A neutron diffraction investigation of residual stresses in rail ends after severe deformation of rail surfaces

Insulated rail joints (IRJs) are a primary component of the rail track safety and signalling systems. Rails are supported by two fishplates which are fastened by bolts and nuts and, with the support of sleepers and track ballast, form an integrated assembly. IRJ failure can result from progressive defects, the propagation of which is influenced by residual stresses in the rail. Residual stresses change significantly during service due to the complex deformation and damage effects associated with wheel rolling, sliding and impact. IRJ failures can occur when metal flows over the insulated rail gap (typically 6-8 mm width), breaks the electrically isolated section of track and results in malfunction of the track signalling system. In this investigation, residual stress measurements were obtained from rail-ends which had undergone controlled amounts of surface plastic deformation using a full scale wheel-on-track simulation test rig. Results were compared with those obtained from similar investigations performed on rail ends associated with ex-service IRJs. Residual stresses were measured by neutron diffraction at the Australian Nuclear Science and Technology Organisation (ANSTO). Measurements with constant gauge volume 3x3x3 mm3 were carried in the central vertical plane on 5mm thick sliced rail samples cut by an electric discharge machine (EDM). Stress evolution at the rail ends was found to exhibit characteristics similar to those of the ex-service rails, with a compressive zone of 5mm deep that is counterbalanced by a tension zone beneath, extending to a depth of around 15mm. However, in contrast to the ex-service rails, the type of stress distribution in the test-rig deformed samples was apparently different due to the localization of load under the particular test conditions. In the latter, in contrast with clear stress evolution, there was no obvious evolution of d0. Since d0 reflects rather long-term accumulation of crystal lattice damage and microstructural changes due to service load, the loading history of the test rig samples has not reached the same level as the ex-service rails. It is concluded that the wheel-on-rail simulation rig provides the potential capability for testing the wheel-rail rolling contact conditions in rails, rail ends and insulated rail joints.

Deformation behaviours of coarse-grained and nanocrystalline Mg-5wt% Al alloys

Magnesium alloys have been of growing interest to various engineering applications, such as the automobile, aerospace, communication and computer industries due to their low density, high specific strength, good machineability and availability as compared with other structural materials. However, most Mg alloys suffer from poor plasticity due to their Hexagonal Close Packed structure. Grain refinement has been proved to be an effective method to enhance the strength and alter the ductility of the materials. Several methods have been proposed to produce materials with nanocrystalline grain structures. So far, most of the research work on nanocrystalline materials has been carried out on Face-Centered Cubic and Body-Centered Cubic metals. However, there has been little investigation of nanocrystalline Mg alloys. In this study, bulk coarse-grained and nanocrystalline Mg alloys were fabricated by a mechanical alloying method. The mixed powder of Mg chips and Al powder was mechanically milled under argon atmosphere for different durations of 0 hours (MA0), 10 hours (MA10), 20 hours (MA20), 30 hours (MA30) and 40 hours (MA40), followed by compaction and sintering. Then the sintered billets were hot-extruded into metallic rods with a 7 mm diameter. The obtained Mg alloys have a nominal composition of Mg–5wt% Al, with grain sizes ranging from 13 μm down to 50 nm, depending on the milling durations. The microstructure characterization and evolution after deformation were carried out by means of Optical microscopy, X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Scanning Probe Microscopy and Neutron Diffraction techniques. Nanoindentaion, compression and micro-compression tests on micro-pillars were used to study the size effects on the mechanical behaviour of the Mg alloys. Two kinds of size effects on the mechanical behaviours and deformation mechanisms were investigated: grain size effect and sample size effect. The nanoindentation tests were composed of constant strain rate, constant loading rate and indentation creep tests. The normally reported indentation size effect in single crystal and coarse-grained crystals was observed in both the coarse-grained and nanocrystalline Mg alloys. Since the indentation size effect is correlated to the Geometrically Necessary Dislocations under the indenter to accommodate the plastic deformation, the good agreement between the experimental results and the Indentation Size Effect model indicated that, in the current nanocrystalline MA20 and MA30, the dislocation plasticity was still the dominant deformation mechanism. Significant hardness enhancement with decreasing grain size, down to 58 nm, was found in the nanocrystalline Mg alloys. Further reduction of grain size would lead to a drop in the hardness values. The failure of grain refinement strengthening with the relatively high strain rate sensitivity of nanocrystalline Mg alloys suggested a change in the deformation mechanism. Indentation creep tests showed that the stress exponent was dependent on the loading rate during the loading section of the indentation, which was related to the dislocation structures before the creep starts. The influence of grain size on the mechanical behaviour and strength of extruded coarse-grained and nanocrystalline Mg alloys were investigated using uniaxial compression tests. The macroscopic response of the Mg alloys transited from strain hardening to strain softening behaviour, with grain size reduced from 13 ìm to 50 nm. The strain hardening was related to the twinning induced hardening and dislocation hardening effect, while the strain softening was attributed to the localized deformation in the nanocrystalline grains. The tension–compression yield asymmetry was noticed in the nanocrystalline region, demonstrating the twinning effect in the ultra-fine-grained and nanocrystalline region. The relationship k tensions < k compression failed in the nanocrystalline Mg alloys; this was attributed to the twofold effect of grain size on twinning. The nanocrystalline Mg alloys were found to exhibit increased strain rate sensitivity with decreasing grain size, with strain rate ranging from 0.0001/s to 0.01/s. Strain rate sensitivity of coarse-grained MA0 was increased by more than 10 times in MA40. The Hall-Petch relationship broke down at a critical grain size in the nanocrystalline region. The breakdown of the Hall-Petch relationship and the increased strain rate sensitivity were due to the localized dislocation activities (generalization and annihilation at grain boundaries) and the more significant contribution from grain boundary mediated mechanisms. In the micro-compression tests, the sample size effects on the mechanical behaviours were studied on MA0, MA20 and MA40 micro-pillars. In contrast to the bulk samples under compression, the stress-strain curves of MA0 and MA20 micro-pillars were characterized with a number of discrete strain burst events separated by nearly elastic strain segments. Unlike MA0 and MA20, the stress-strain curves of MA40 micro-pillars were smooth, without obvious strain bursts. The deformation mechanisms of the MA0 and MA20 micro-pillars under micro-compression tests were considered to be initially dominated by deformation twinning, followed by dislocation mechanisms. For MA40 pillars, the deformation mechanisms were believed to be localized dislocation activities and grain boundary related mechanisms. The strain hardening behaviours of the micro-pillars suggested that the grain boundaries in the nanocrystalline micro-pillars would reduce the source (nucleation sources for twins/dislocations) starvation hardening effect. The power law relationship of the yield strength on pillar dimensions in MA0, MA20 supported the fact that the twinning mechanism was correlated to the pre-existing defects, which can promote the nucleation of the twins. Then, we provided a latitudinal comparison of the results and conclusions derived from the different techniques used for testing the coarse-grained and nanocrystalline Mg alloy; this helps to better understand the deformation mechanisms of the Mg alloys as a whole. At the end, we summarized the thesis and highlighted the conclusions, contributions, innovations and outcomes of the research. Finally, it outlined recommendations for future work.