Showing papers in "Materials Science and Technology in 2015"
TL;DR: In this paper, columnar to equiaxed transitions during solidification were used to promote the growth of highly misoriented micron scale grains outlining the letters D, O and E, through the thickness of a 25·4 mm tall bulk block comprised of primarily columnar oriented grains made of the nickel base superalloy Inconel 718.
Abstract: Site specific control of the crystallographic orientation of grains within metal components has been unachievable before the advent of metals additive manufacturing (AM) technologies. To demonstrate the capability, the growth of highly misoriented micron scale grains outlining the letters D, O and E, through the thickness of a 25·4 mm tall bulk block comprised of primarily columnar [001] oriented grains made of the nickel base superalloy Inconel 718 was promoted. To accomplish this, electron beam scan strategies were developed based on principles of columnar to equiaxed transitions during solidification. Through changes in scan strategy, the electron beam heat source can rapidly change between point and line heat source modes to promote steady state and/or transient thermal gradients and liquid/solid interface velocity. With this approach, an equiaxed solidification in the regions bounding the letters D, O and E was achieved. The through thickness existence of the equiaxed grain structure outlinin...
424 citations
TL;DR: In this article, a process parameter window is defined, in which the formed melt pool is stable and meets the set requirements, in order to get nearly fully dense parts, and the material properties resulting from this specific material process combination.
Abstract: Owing to their attractive combination of mechanical properties, high heat conductivity and low weight, the Al–Si alloys found a large number of applications in the Additive Manufacturing field for automotive, aerospace and domestic industries. However, due to their high reflectivity and heat conductivity, they are harder to process by Selective Laser Melting. This work elaborates on both the optimisation of process parameters, in order to get nearly fully dense parts, and the material properties resulting from this specific material process combination. A process parameter window is defined, in which the formed melt pool is stable and meets the set requirements. In this process window, the parameter set for optimal density is defined. It is shown that AlSi10Mg parts produced by SLM have mechanical properties higher or at least comparable to the cast material because of the very fine microstructure.
309 citations
TL;DR: In this paper, a review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.
Abstract: Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γR), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γR, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.
300 citations
TL;DR: In this article, a multiscale modeling strategy is described that will serve as the foundation upon which process control and part qualification can be built, including a model at the scale of the powder that simulates single track/single multilayer builds and provides powder bed and melt pool thermal data.
Abstract: The metal laser powder bed fusion additive manufacturing process uses high power lasers to build parts layer upon layer by melting fine metal powders. Qualification of parts produced using this technology is broadly recognised as a significant challenge. Physics based process models have been identified as being foundational to qualification of additively manufactured metal parts. In the present article, a multiscale modelling strategy is described that will serve as the foundation upon which process control and part qualification can be built. This includes a model at the scale of the powder that simulates single track/single multilayer builds and provides powder bed and melt pool thermal data. A second model computationally builds a complete part and predicts manufactured properties (residual stress, dimensional accuracy) in three dimensions. Modelling is tied to experiment through data mining.
238 citations
TL;DR: In this article, a three-dimensional heat transfer and material flow model is developed to numerically simulate the temperature and velocity fields in a laser assisted layer by layer deposition process with coaxially fed powder particles.
Abstract: A three-dimensional heat transfer and material flow model is developed to numerically simulate the temperature and velocity fields in a laser assisted layer by layer deposition process with coaxially fed powder particles. The computed results are tested with independently reported temperature and build geometry for the deposition of multilayered structures of austenitic stainless steel. The results provide detailed insight about the important physical processes and show that the model can be used to understand the effects of process parameters on the thermal cycles, build geometry, cooling rates and solidification parameters in a multilayer additive manufacturing process.
206 citations
TL;DR: In this paper, the authors summarise existing phase selection rules for cast high entropy alloys, which are almost all based on the parametric approach, utilising various descriptors comprising mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch, electronegativity and valence electron concentration.
Abstract: This paper summarises existing phase selection rules for cast high entropy alloys. Essentially, they are almost all based on the parametric approach, utilising various descriptors comprising mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch, electronegativity and valence electron concentration. The overview starts from phase selection rules for solid solutions, intermetallic compounds and the amorphous phase in high entropy alloys. Further discussions are relevant to selection rules for solid solution phases in high entropy alloys, more specifically, for face centred cubic and body centred cubic type solid solutions. Finally, some challenges and future prospects of phase selection rules for high entropy alloys are addressed.
183 citations
TL;DR: In this article, rare earth (RE) elements including Y, Ce, La, Gd and Nd are used to weaken these strong basal textures and significantly improve formability of magnesium alloys.
Abstract: Wrought magnesium alloys are rarely used due to their poor formability which is caused by strong textures created during processing. Addition of rare earth (RE) elements including Y, Ce, La, Gd and Nd weakens these strong basal textures and significantly improves formability. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable magnesium alloys. This fall in texture intensity occurs during recrystallisation and only requires very low solute RE additions, 0·01 at.-% in the magnesium–Ce case. These additions retard dynamic recrystallisation and increase non-basal slip; however, a full understanding of the RE effect has yet to be obtained, with a variety of mechanisms proposed. Recent research in these areas is critically reviewed.
178 citations
TL;DR: In this article, the electrochemical properties of a wide range of high entropy alloys (HEAs) in a 0.6m NaCl solution were reported, with a broad survey of results typifying their electrochemical characteristics, passivity and comparative electrochemistry.
Abstract: The present paper reports on the electrochemical properties of a wide range of high entropy alloys (HEAs) in a 0.6 M NaCl solution. A consolidated treatise of the topic has to date been lacking, and the purpose of the work herein is to present a primitive galvanic series for numerous HEAs, along with a broad survey of results typifying their electrochemical characteristics, passivity and comparative electrochemistry. The results are coupled with microstructural characterisation. The range of potentials for HEAs is comparable to or nobler than austenitic stainless steel, with a number of HEAs displaying higher pitting potential (Epit) values than stainless steels, in spite of possessing heterogeneous microstructures.
139 citations
TL;DR: In this paper, the authors draw on extensive experience in the development of exquisite multiphase microstructures and define new high throughput experiments and computations that must be integrated to respond to this challenge.
Abstract: The field of high entropy alloys has exploded in its first 10 years. Vast opportunities for new compositions and microstructures are offered by this idea, but current efforts have become surprisingly focused on a narrow set of systems and on the search for single phase solid solution alloys. This perspective outlines the challenge to re-engage the full range of compositional and microstructural complexity in the search for new structural metals. We draw on extensive experience in the development of exquisite multiphase microstructures and define new high throughput experiments and computations that must be integrated to respond to this challenge. Broadening the scope is expected to provide dramatic new opportunities in the field of complex concentrated alloys.
131 citations
TL;DR: A critical assessment of the field of quench and partitioning can be found in this paper, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation.
Abstract: Quenching and partitioning is a relatively new heat treatment concept to generate microstructures containing retained austenite stabilised by carbon partitioning from martensite. Research on quench and partitioning has been conducted by numerous groups, and this critical assessment provides some of the authors’ perspectives on progress and understanding in the field, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation. While much progress has been made, the field provides rich opportunity for further understanding and development.
122 citations
TL;DR: In this paper, the electron beam melting (EBM) process is used to control the crystallographic texture of Inconel 718 deposits, which can be used to create complex geometric components with both site-specific microstructures and material properties.
Abstract: Preliminary research has demonstrated the ability to utilise novel scan strategies in the electron beam melting (EBM) process to establish control of crystallographic texture within Inconel 718 deposits. Conventional EBM scan strategies and process parameters yield coarse columnar grains aligned parallel to the build direction. Through varying process parameters such as beam power, beam velocity, beam focus and scan strategy, the behaviour of the electron beam can be manipulated from a line source to a point source. The net effect of these variations is that the resulting crystallographic texture is controlled in a manner to produce either epitaxial deposits or fully equiaxed deposits. This research demonstrates the ability to change the crystallographic texture on the macroscale indicating that EBM technology can be used to create complex geometric components with both site-specific microstructures and material properties.
TL;DR: In this paper, the crystal structure, microstructure, microhardness and compression mechanical properties of AlxNbTiVZr (x = 0, 05, 1, 15) high entropy alloy were examined.
Abstract: The crystal structure, microstructure, microhardness and compression mechanical properties of AlxNbTiVZr (x = 0, 05, 1, 15) high entropy alloy were examined In the as solidified conditions, the alloys consisted from bcc matrix and C14 Laves phase After homogenisation, the NbTiVZr alloy was bcc solid solution, whereas in Al containing alloys, C14 Laves phase and Zr2Al particles were found in the bcc matrix Volume fraction of second phase increased with Al concentration Increase in Al content results in gradual decrease in density of the alloys from 649 g cm− 3 of the NbTiVZr to 555 g cm− 3 of the Al15NbTiVZr alloy The microhardness of the alloys was higher in the alloys with higher Al content and was generally proportional to the volume fraction of second phase particles The compression yield strength of the alloys was of 960–1320 MPa, and NbTiVZr alloy was stronger than Al containing alloys The ductility of the alloys gradually decreased with increase in Al content The factors determi
TL;DR: In this article, a finite element model is developed which uses a dynamic mesh with spatial nonlinear thermal properties to track the point of laser exposure on the powder bed to study thermal evolution during SLM.
Abstract: Selective laser melting (SLM) is an additive manufacturing (AM) process in which parts are fabricated by selectively melting regions of the surface of a metallic powder bed in a layer-by-layer fashion. Various thermal phenomena such as heat conduction, convection, radiation, melting and solidification, dynamic phase changes, and evaporation occur during the SLM process. In addition, laser intensity and powder bed scan speeds during processing complicate understanding of the process due to complex dynamic interactions between the powder bed and laser. In order to study these dynamic interactions, a finite element model has been developed which uses a dynamic mesh with spatial non-linear thermal properties to track the point of laser exposure on the powder bed to study thermal evolution during SLM. The model is able to achieve a refined, localised mesh in the melt zone and heat affected zone (HAZ), surrounded by a relatively coarse mesh outside of the HAZ regions. The dynamic meshing for this implem...
TL;DR: The use of cold spray for repair is often favored because, unlike thermal spray methods, cold spray does not bring about heat related damage to the component under repair as mentioned in this paper, which is often applied to coating applications but can be used for the repair of defects in and damage to metal structures.
Abstract: The deposition of material by cold spray is a relatively new technology. It is often applied to coating applications but can be used for the repair of defects in and damage to metal structures. The use of cold spray for repair is often favoured because, unlike thermal spray methods, cold spray does not bring about heat related damage to the component under repair. The status of the technology and the barriers to widespread commercial application are assessed. The types of repair that have been made with cold spray are discussed, and the unique features of cold spray that permit successful repairs are demonstrated.
TL;DR: The relationship between the filament scale phenomena and the macroscopic properties of parts manufactured by FDM of thermoplastic polymers has been investigated using planar geometry dog bone samples, representing layer by layer lamina in an additively manufactured part as discussed by the authors.
Abstract: The relationship between the filament scale phenomena and the macroscopic properties of parts manufactured by fused deposition modelling (FDM) of thermoplastic polymers has been investigated using planar geometry dog bone samples, representing layer by layer lamina in an additively manufactured part. Finite element simulations of the response of the FDM part(s) at multiple length scales (filament to macro) are compared with full field strain data obtained experimentally for different raster angles and filament gaps. The strain field, strain energy density, and effective Young’s modulus are evaluated. Principal strains resulting from the applied axial loading shifted from the inner rasters to the contours of the FDM planar sample at certain raster angles as the air gap increased, which significantly decreased the effective usage of the material leading to strain localization and premature part failure. The research presented provides a pathway to an effective multiscale approach to optimise the ras...
TL;DR: In this paper, the design approach and validation of a single phase senary refractory high entropy alloy (HEA) MoNbTaTiVW was presented and validated using X-ray diffraction and scanning electron microscopy techniques.
Abstract: The design approach and validation of a single phase senary refractory high entropy alloy (HEA) MoNbTaTiVW was presented in the present study. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified a single phase microstructure in body centred cubic lattice for MoNbTaTiVW. The observed elemental segregation agrees well with the solidification prediction using the Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g cm− 3 respectively. The atomic size difference, the Ω parameter, enthalpy of mixing and entropy of mixing for MoNbTaTiVW HEA are 3.1%, 11.1, − 3.4 kJ mol− 1 and +13.39 J K− 1 mol− 1 respectively.
TL;DR: In this article, the authors employed this process to fine-distribute TiO2 nanoparticles throughout an Al-Mg alloy, aiming to enhance mechanical properties, and the average grain size of the nanocomposite was ∼2 μm.
Abstract: Friction stir processing (FSP) is a solid state route with a capacity of preparing fine grained nanocomposites from metal sheets. In this work, we employed this process to finely distribute TiO2 nanoparticles throughout an Al–Mg alloy, aiming to enhance mechanical properties. Titanium dioxide particles (30 nm) were preplaced into grooves machined in the middle of the aluminium alloy sheet and multipass FSP was afforded. This process refined the grain structure of the aluminium alloy, distributed the hard nanoparticles in the matrix and promoted solid state chemical reactions at the interfaces of the metal/ceramic particles. Detailed optical and electron microscopic studies showed that the microstructural homogeneity was improved with repetition of FSP up to four passes. The average grain size of the nanocomposite was ∼2 μm, while nanometric MgO and Al3Ti particles were formed in situ and homogenously distributed in the metal matrix. Mechanical characterisations showed that the yield strength and e...
TL;DR: In this article, the authors assess the cause of rolling contact fatigue and propose solutions that are backed by evidence that already is available to determine the life of more well behaved bearing applications.
Abstract: One form of damage due to rolling contact fatigue is the formation of localised regions of extremely hard material forming within the body of a bearing. These regions have a relatively homogeneous structure and hence etch mildly with respect to the surrounding unaffected matrix. They, therefore, appear white in a darker background when examined using optical microscopy. We assess here the cause of this damage and propose solutions that are backed by evidence that already is available. The issue is important because of the spate of unexpected failures in large wind turbine bearings and of generic importance in determining the life of more well behaved bearing applications.
TL;DR: In this paper, a simple lattice strain framework is proposed for complex concentrated alloys to address the energetics and kinetics of dislocations and twins, which can be used to design non-equiatomic high entropy alloy matrixes that enhance the properties achieved thus far.
Abstract: Alloys involving multiple solutes where the concentrations are such that it becomes difficult to identify a “solvent”, such as the so called “high entropy alloys”, have the potential for interesting combinations of properties. A key question relates to the fundamental mechanisms of plastic deformation in these alloys. A simple lattice strain framework is proposed for complex concentrated alloys to address the energetics and kinetics of dislocations and twins. It is argued that the lattice strain in highly concentrated alloys raises the base energy of the crystal and thereby reduces the additional energy required to nucleate dislocations and twins. However, the kinetics of dislocation motion are dampened by the lattice strain and local energy variations. This is reflected in lower values of activation volume. This framework can be used to design non-equiatomic high entropy alloy matrixes that enhance the properties achieved thus far.
TL;DR: In this paper, nano-AlN particles were introduced into pure Mg matrix through the powder metallurgy technique incorporating microwave assisted two-directional sintering followed by hot extrusion.
Abstract: In this study, nano-AlN particles were introduced into pure Mg matrix through the powder metallurgy technique incorporating microwave assisted two-directional sintering followed by hot extrusion. The effect of varying volume fraction of nano-AlN addition on the microstructural and mechanical properties of pure Mg was investigated. Microstructural characterisation revealed marginal grain refinement due to the fairly uniform distribution of AlN nano-particulates. X-ray diffraction results indicated basal texture weakening in Mg/0·2AlN composite. Tensile property measurements revealed an overall increase in strength properties and ductility. Among the developed composites, Mg/0·8AlN displayed superior strength (∼30% improvement) and Mg/0·2AlN showed enhanced ductility (∼80% enhancement). Under compressive loading, the developed Mg/AlN nanocomposite formulations exhibited improved strength properties without significant effect on compressibility.
TL;DR: In this paper, the sources of anisotropy in metals are discussed as well as ways of interpreting and modelling this behaviour and a number of case studies relating to mechanical and magnetic properties are presented.
Abstract: Sources of anisotropy in metals are discusssed as well as ways of interpreting and modelling this behaviour. A number of case studies are presented relating to mechanical and magnetic properties.
TL;DR: In this paper, the authors show that beam power and beam velocity combinations yielding constant melt pool cross-sectional areas also yield constant solidification cooling rates for thin walled structures fabricated by wire feed electron beam AM.
Abstract: In additive manufacturing (AM), melt pool dimension control is needed to accurately build a geometry and determine process precision. Microstructure control is needed for its effect on mechanical properties. This research addresses both for Ti–6Al–4V thin walled structures fabricated by wire feed electron beam AM. Model results show that beam power and beam velocity combinations yielding constant melt pool cross-sectional areas also yield constant solidification cooling rates. Experimental measurements back up this finding and show roughly 20 beta grains across the width of a thin wall deposit which is consistent with an earlier study of single bead deposits, suggesting that links between melt pool geometry and beta grain size are independent of deposition geometry, with significant implications for AM process control.
TL;DR: In this article, a magnesium nanocomposites reinforced with copper-graphene nanoplatelet hybrid particles have been prepared through the semipowder metallurgy method, and compared with the monolithic Mg, the Mg-1Cu-xGNPs nan composites exhibited higher tensile and compressive strength.
Abstract: New magnesium nanocomposites reinforced with copper–graphene nanoplatelet hybrid particles have been prepared through the semipowder metallurgy method. Compared with the monolithic Mg, the Mg–1Cu–xGNPs nanocomposites exhibited higher tensile and compressive strength. In tension, nanocomposites revealed substantial enhancement in elastic modulus, 0.2% yield strength, ultimate tensile strength and failure strain (up to +89, +117, +58 and +96% respectively) compared to monolithic Mg. In compression, the nanocomposites showed the greatest improvement in 0.2% yield strength, and the ultimate compressive strength and failure strain (%) (up to +34, +59 and +61% respectively), whilst the compressive elastic modulus first increases and then decreases with an increase in the graphene nanoplatelets (GNPs) contents. The enhanced strength of the composites is likely to result from strengthening mechanisms invoked by the addition of Cu–GNPs hybrids.
TL;DR: In this article, a priori derivation for the extra free energy caused by the passing electric current in metal is presented, and the analytical expression and its discrete format in support of the numerical calculation of thermodynamics in electric current metallurgy are developed.
Abstract: A priori derivation for the extra free energy caused by the passing electric current in metal is presented. The analytical expression and its discrete format in support of the numerical calculation of thermodynamics in electric current metallurgy have been developed. This enables the calculation of electric current distribution, current induced temperature distribution and free energy sequence of various phase transitions in multiphase materials. The work is particularly suitable for the study of magnetic materials that contain various magnetic phases. The latter has not been considered in literature. The method has been validated against the analytical solution of current distribution and experimental observation of microstructure evolution. It provides a basis for the design, prediction and implementation of the electric current metallurgy. The applicability of the theory is discussed in the derivations.
TL;DR: In this paper, a critical assessment of model relations describing the porosity dependence of elastic properties (Young's modulus) and thermal properties (thermal conductivity) is given, and the fact that relative Young's moduli are not equal to relative thermal conductivities except for materials with translational symmetry, proves so-called minimum solid area models to be wrong.
Abstract: A critical assessment of model relations describing the porosity dependence of elastic properties (Young's modulus) and thermal properties (thermal conductivity) is given. It is shown that there are essentially five types of admissible predictive model relations for the relative Young's modulus and thermal conductivity of isotropic porous materials. The cross-property relations resulting from the complete analogy between the model relations for the elastic moduli and thermal conductivity of isotropic porous materials are reviewed and compared. Finally, it is shown that the fact that relative Young's moduli are not equal to relative thermal conductivities except for materials with translational symmetry, i.e. the mere existence and necessity of non-trivial cross-property relations, proves so-called minimum solid area models to be wrong.
TL;DR: In this paper, an assessment of hydrogen-induced cracking in pipeline steel microstructures and crack initiation and propagation processes is presented, where a fracture mechanics based model is used to predict crack propagation.
Abstract: Hydrogen induced cracking (HIC) remains a prominent issue for oil and gas exploration in challenging environments. This assessment discusses HIC in light of hydrogen transport through pipeline steel microstructures and crack initiation and propagation processes. While there has been significant research in hydrogen permeation through steel alloys, additional understanding is necessary in microstructures specific to pipeline steels. Furthermore, a standard model for crack initiation and propagation processes needs to be established; a fracture mechanics based model, which has been used by some researchers, is presented in the present paper to predict crack propagation. Advanced characterisation techniques can help elucidate mechanisms of hydrogen induced crack growth. Ultimately, linking hydrogen transport and cracking processes during HIC will enable optimised alloy and microstructure design.
TL;DR: In this article, a fatigue model based on entropy generation is presented and validated through experiments, which combines statistical mechanics with thermodynamic laws applied at a local scale, and does not require an empirical damage surface or phenomenological constitutive modeling constants.
Abstract: A fatigue model based on entropy generation is presented and validated through experiments. This model is purely physical and combines statistical mechanics with thermodynamic laws applied at a local scale. The model does not require an empirical damage surface or phenomenological constitutive modeling constants. Damage evolution parameter varies from 0 to 1. As it is for the irreversible internal entropy production, this parameter is a non-decreasing quantity that increases with the degradation of a material.
TL;DR: In this paper, the significance of various micro-structural features responsible for the extraordinary mechanical response of nanostructured bainite is elucidated in more detail, using two steels, with different Mn, Ni and V contents.
Abstract: Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19% and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail. Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200–300°C), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.
TL;DR: In this article, a review of the instrumental and methodological approaches responsible for this interest is presented along with some examples where the technique has been successfully applied, as well as some examples of successful applications.
Abstract: Time of flight secondary ion mass spectrometry (ToF-SIMS) has the unique ability to simultaneously obtain chemical information (elemental and molecular) with its spatial distribution on a subcellular scale. Recent progress in instrumentation, in particular the developments of cluster ion beam sources, has resulted in a growing interest in applying ToF-SIMS to a range of biological samples. In this review, the instrumental and methodological approaches responsible for this interest are presented along with some examples where the technique has been successfully applied.
TL;DR: In this paper, the effect of electropulsing on stainless steel is attributed to the electric current induced change of thermodynamic sequences of the phases and electric current accelerated mass diffusion.
Abstract: Precipitation takes place when the austenite stainless steel is heated to a high temperature. This is found significantly different when the electropulsing is implemented during the heat treatment. Considerable less number density and much smaller particle size of precipitates are formed in the sample treated with electropulsing. Electropulsing helps to dissolve precipitates. The effect is not due to Ohm heat. Instead, it is attributed to the electric current induced change of thermodynamic sequences of the phases and the electric current accelerated mass diffusion.