Showing papers on "Microstructure published in 2013"

A Flexible and Highly Pressure‐Sensitive Graphene–Polyurethane Sponge Based on Fractured Microstructure Design

Abstract: A fractured microstructure design: A new type of piezoresistive sensor with ultra-high-pressure sensitivity (0.26 kPa(-1) ) in low pressure range (<2 kPa) and minimum detectable pressure of 9 Pa has been fabricated using a fractured microstructure design in a graphene-nanosheet-wrapped polyurethane (PU) sponge. This low-cost and easily scalable graphene-wrapped PU sponge pressure sensor has potential application in high-spatial-resolution, artificial skin without complex nanostructure design.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

Abstract: After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility

Abstract: The high-temperature stability and mechanical properties of refractory molybdenum alloys are highly desirable for a wide range of critical applications. However, a long-standing problem for these alloys is that they suffer from low ductility and limited formability. Here we report a nanostructuring strategy that achieves Mo alloys with yield strength over 800 MPa and tensile elongation as large as ~ 40% at room temperature. The processing route involves a molecular-level liquid-liquid mixing/doping technique that leads to an optimal microstructure of submicrometre grains with nanometric oxide particles uniformly distributed in the grain interior. Our approach can be readily adapted to large-scale industrial production of ductile Mo alloys that can be extensively processed and shaped at low temperatures. The architecture engineered into such multicomponent alloys offers a general pathway for manufacturing dispersion-strengthened materials with both high strength and ductility.

Microstructures and Mechanical Properties of Ti6Al4V Parts Fabricated by Selective Laser Melting and Electron Beam Melting

Abstract: This work compares two metal additive manufacturing processes, selective laser melting (SLM) and electron beam melting (EBM), based on microstructural and mechanical property evaluation of Ti6Al4V parts produced by these two processes. Tensile and fatigue bars conforming to ASTM standards were fabricated using Ti6Al4V ELI grade material. Microstructural evolution was studied using optical and scanning electron microscopy. Tensile and fatigue tests were carried out to understand mechanical properties and to correlate them with the corresponding microstructure. The results show differences in microstructural evolution between SLM and EBM processed Ti6Al4V and their influence on mechanical properties. The microstructure of SLM processed parts were composed of an α′ martensitic phase, whereas the EBM processed parts contain primarily α and a small amount of β phase. Consequently, there are differences in tensile and fatigue properties between SLM- and EBM-produced Ti6Al4V parts. The differences are related to the cooling rates experienced as a consequence of the processing conditions associated with SLM and EBM processes.

Microstructure and tensile properties of selectively laser-melted and of HIPed laser-melted Ti–6Al–4V

Abstract: Ti–6Al–4V samples have been prepared by selective laser melting (SLM) with varied processing conditions. Some of the samples were stress-relieved or hot isostatically pressed (HIPed). The microstructures of all samples were characterised using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and the tensile properties measured before and after HIPing. It was found that the porosity level generally decreased with increase of laser power and laser scanning speed. Horizontally built samples were found to have a higher level of porosity than vertically built samples. The as-fabricated microstructure was dominated by columnar grains and martensites. HIPing closed the majority of the pores and also fully transformed the martensite into α and β phases. The as-fabricated microstructure exhibits very high tensile strengths but poor ductility with elongation generally smaller than 10%. The horizontally built samples show even lower elongation than vertically built samples. HIPing considerably improved ductility but led to a reduction in strength. With HIPing, the SLMed samples were found to show tensile properties comparable with those thermomechanically processed and annealed samples.

Synthesis and characterization of red mud and rice husk ash-based geopolymer composites

Abstract: A new type of geopolymer composite was synthesized from two industrial wastes, red mud (RM) and rice husk ash (RHA), at varying mixing ratios of raw materials and the resulting products characterized by mechanical compression testing, X-ray diffraction, and scanning electron microscopy to assess their mechanical properties, microstructure, and geopolymerization reactions. Prolonged curing significantly increases the compressive strength and Young’s modulus, but reduces the ductility. Higher RHA/RM ratios generally lead to higher strength, stiffness, and ductility, but excessive RHA may cause the opposite effect. The compressive strength ranges from 3.2 to 20.5 MPa for the synthesized geopolymers with nominal Si/Al ratios of 1.68–3.35. Microstructural and compositional analyses showed that the final products are mainly composed of amorphous geopolymer binder with both inherited and neoformed crystalline phases as fillers, rendering the composites very complex composition and highly variable mechanical properties. Uncertainties in the composition, microstructure, the extent of RHA dissolution, and side reactions may be potential barriers for the practical application of the RM–RHA based geopolymers as a construction material.

Microstructure and Mechanical Properties of Wire and Arc Additive Manufactured Ti-6Al-4V

Abstract: Wire and arc additive manufacturing (WAAM) is a novel manufacturing technique in which large metal components can be fabricated layer by layer. In this study, the macrostructure, microstructure, and mechanical properties of a Ti-6Al-4V alloy after WAAM deposition have been investigated. The macrostructure of the arc-deposited Ti-6Al-4V was characterized by epitaxial growth of large columnar prior-β grains up through the deposited layers, while the microstructure consisted of fine Widmanstatten α in the upper deposited layers and a banded coarsened Widmanstatten lamella α in the lower layers. This structure developed due to the repeated rapid heating and cooling thermal cycling that occurs during the WAAM process. The average yield and ultimate tensile strengths of the as-deposited material were found to be slightly lower than those for a forged Ti-6Al-4V bar (MIL-T 9047); however, the ductility was similar and, importantly, the mean fatigue life was significantly higher. A small number of WAAM specimens exhibited early fatigue failure, which can be attributed to the rare occurrence of gas pores formed during deposition.

Materials Science & Engineering A

Abstract: A two-layer structure composite, in which one layer was B4C/Al compound and the other layer was B4C/ TiB2 compound, was prepared using a two-step method for both hot pressing and infiltration of aluminum in vacuum. The experimental results showed that the two-layer B4C/Al-B4C/TiB2 composite having good interfacial bonding was prepared. Before aluminum infiltration process, the B4C porous layer of the two-layer preform looked like a three-dimensional network of interconnected capillaries that could promote the infiltration of liquid aluminum. For the B4C/TiB2 layer the TiB2 dispersed homogeneously in B4C matrix and the microstructure showed no apparent change before and/or after aluminum infiltrating process. The fracture toughness of this material was improved to about twice that of pure B4C material. A number of twins were observed in the B4C matrix of the two-layer material. & 2012 Elsevier B.V. All rights reserved.

Strain-Induced Ultrahard and Ultrastable Nanolaminated Structure in Nickel

Abstract: Heavy plastic deformation may refine grains of metals and make them very strong. But the strain-induced refinement saturates at large strains, forming three-dimensional ultrafine-grained (3D UFG) structures with random orientations. Further refinement of this microstructure is limited because of the enhanced mobility of grain boundaries. Very-high-rate shear deformation with high strain gradients was applied in the top surface layer of bulk nickel, where a 2D nanometer-scale laminated structure was induced. The strongly textured nanolaminated structure (average lamellar thickness of 20 nanometers) with low-angle boundaries among the lamellae is ultrahard and ultrastable: It exhibits a hardness of 6.4 gigapascal--which is higher than any reported hardness of the UFG nickel--and a coarsening temperature of 40 kelvin above that in UFG nickel.

Mechanical properties of low-density, refractory multi-principal element alloys of the Cr–Nb–Ti–V–Zr system

Abstract: Room temperature and elevated temperature mechanical properties of four multi-principal element alloys, NbTiVZr, NbTiV 2 Zr, CrNbTiZr and CrNbTiVZr, are reported. The alloys were prepared by vacuum arc melting followed by hot isostatic pressing and homogenization. Disordered BCC solid solution phases are the major phases in these alloys. The Cr-containing alloys additionally contain an ordered FCC Laves phase. The NbTiVZr and NbTiV 2 Zr alloys showed good compressive ductility at all studied temperatures while the Cr-containing alloys showed brittle-to-ductile transition occurring somewhere between 298 and 873 K. Strong work hardening was observed in the NbTiVZr and NbTiV 2 Zr alloys during deformation at room temperature. The alloys had yield strengths of 1105 MPa and 918 MPa, respectively, and their strength continuously increased, exceeding 2000 MPa after ∼40% compression strain. The CrNbTiZr and CrNbTiVZr alloys showed high yield strength (1260 MPa and 1298 MPa, respectively) but low ductility (6% and 3% compression strain) at room temperature. Strain softening and steady state flow were typical during compression deformation of these alloys at temperatures above 873 K. In these conditions, the alloys survived 50% compression strain without fracture and their yield strength continuously decreased with an increase in temperature. During deformation at 1273 K, the NbTiVZr, NbTiV 2 Zr, CrNbTIZr, and CrNbTiVZr alloys showed yield strengths of 58 MPa, 72 MPa, 115 MPa and 259 MPa, respectively.

Microstructures and mechanical properties of Co-29Cr-6Mo alloy fabricated by selective laser melting process for dental applications.

Abstract: The selective laser melting (SLM) process was applied to a Co-29Cr-6Mo alloy, and its microstructure, mechanical properties, and metal elution were investigated to determine whether the fabrication process is suitable for dental applications. The microstructure was evaluated using scanning electron microscopy with energy-dispersed X-ray spectroscopy (SEM-EDS), X-ray diffractometry (XRD), and electron back-scattered diffraction pattern analysis. The mechanical properties were evaluated using a tensile test. Dense builds were obtained when the input energy of the laser scan was higher than 400 J mm⁻³, whereas porous builds were formed when the input energy was lower than 150 J mm⁻³. The microstructure obtained was unique with fine cellular dendrites in the elongated grains parallel to the building direction. The γ phase was dominant in the build and its preferential orientation was confirmed along the building direction, which was clearly observed for the builds fabricated at lower input energy. Although the mechanical anisotropy was confirmed in the SLM builds due to the unique microstructure, the yield strength, UTS, and elongation were higher than those of the as-cast alloy and satisfied the type 5 criteria in ISO22764. Metal elution from the SLM build was smaller than that of the as-cast alloy, and thus, the SLM process for the Co-29Cr-6Mo alloy is a promising candidate for fabricating dental devices.

Highly Anisotropic Steel Processed by Selective Laser Melting

Abstract: For additive manufacturing of metals, selective laser melting can be employed. The microstructure evolution is directly influenced by processing parameters. Employing a high energy laser system, samples made from austenitic stainless steel were manufactured. The microstructure obtained is characterized by an extremely high degree of anisotropy featuring coarse elongated grains and a 〈001〉 texture alongside the build direction during processing. Eventually, the anisotropy of the microstructure drastically affects the monotonic properties of the current material.

Enhanced reversibility and unusual microstructure of a phase-transforming material

Abstract: The enhanced reversibility (stable transition temperature even at high strain under a solid-to-solid phase transition), low hysteresis and unusual riverine microstructure (ranging through thermal cycles) of the martensitic material Zn45Au30Cu25 makes it attractive for applications from eco-friendly fridges to medical sensors. Martensitic transformations are diffusionless, solid-to-solid phase transformations characterized by a change of crystal structure that can often be very useful. Applications include medical sensors, eco-friendly refrigerators and energy conversion devices. Repeated transformation cycles, however, can cause thermal hysteresis that modifies the material's properties and can cause permanent damage. Here Richard James and colleagues report the development of a martensitic alloy of zinc, gold and copper that maintains near-reproducible macroscopic properties despite drastic changes in its microstructure during each cycle. As well as providing a system that throws new light on the effects of hysteresis on reversible martensitic phase transformations, this work could help to extend applications for the materials in new areas — towards shape memory alloys for instance. Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators1, eco-friendly refrigerators2,3 and energy conversion devices4. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed ‘reversibility’) is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown2,5,6,7,8,9,10,11,12,13,14 to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling6,9,12,15. Recently, stronger compatibility conditions called the ‘cofactor conditions’5,15 have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles9,16. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

From Powders to Dense Metal Parts: Characterization of a Commercial AlSiMg Alloy Processed through Direct Metal Laser Sintering.

Abstract: In this paper, a characterization of an AlSiMg alloy processed by direct metal laser sintering (DMLS) is presented, from the analysis of the starting powders, in terms of size, morphology and chemical composition, through to the evaluation of mechanical and microstructural properties of specimens built along different orientations parallel and perpendicular to the powder deposition plane. With respect to a similar aluminum alloy as-fabricated, a higher yield strength of about 40% due to the very fine microstructure, closely related to the mechanisms involved in this additive process is observed.

Effects of processing on microstructure and mechanical properties of a titanium alloy (Ti–6Al–4V) fabricated using electron beam melting (EBM), Part 2: Energy input, orientation, and location

Abstract: Selective electron beam melting (EBM) is a layer-by-layer additive manufacturing technique that shows great promise for fabrication of medical devices and aerospace components. Before its potential can be fully realized, however, a comprehensive understanding of processing-microstructure-properties relationships is necessary. Titanium alloy (Ti–6Al–4V) parts were built in a newly developed, unique geometry to allow accurate investigation of the following intra-build processing parameters: energy input, orientation, and location. Microstructure evaluation (qualitative prior-β grain size, quantitative α lath thickness), tensile testing, and Vickers microhardness were performed for each specimen. For a wide range of energy input (speed factor 30–40), small differences in mechanical properties (2% change in ultimate tensile strength (UTS) and 3% change in yield strength (YS)) were measured. Vertically built parts were found to have no difference in UTS or YS compared to horizontally built parts, but the percent elongation at break (% EL) was 30% lower. The difference in % EL was attributed to a different orientation of the tensile axis for horizontal and vertical parts compared to the elongated prior-β grain and microstructural texture direction in EBM Ti–6Al–4V. Orientation within the x – y plane as well as location were found to have less than 3% effect on mechanical properties, and it is possible a second order effect of thermal mass contributed to these results.