Abstract Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental ...research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions.
This study presents oscillatory and rotational viscosity measurement results on Al-Cu alloys with the goal of achieving effective viscosity data of partially solidified alloys from fully liquid to ...nearly fully solid state. Rotational viscosity measurements indicate fragmentation of dendrites and a concomitant sudden drop in viscosity of the solid–liquid two-phase mixture upon cooling and increase in solid fraction. This measurement artifact is avoided with oscillatory measurements. The viscosity of partially solid Al-Cu over the hypoeutectic range is nearly composition independent when evaluated as a function of solid fraction. The oscillatory measurements yield a complex viscosity with the real part reflecting the elastic behavior and the imaginary part indicating the viscous part of the viscosity behavior. The measurements of the phase angle in this study reveal that the rheological behavior of the solid phase in the liquid–solid two-phase region resembles that of a liquid more than that of a solid phase.
Highlights
Oscillatory rheology is shown to be better suited to characterize partially solidified metal alloys than rotational rheology.
The viscosity of the liquid phase does not change measurably during solidification despite an increase in the solute content.
The solid phase in the liquid–solid two-phase region behaves rheologically more like a liquid than a solid.
The sliding wear behavior was investigated for Fe50−xCr15Mo14C15B6Erx (x=0, 1, 2at%) bulk metallic glasses (BMG). Minor alloying with 1–2at% erbium resulted in an increase in hardness and a ...deterioration of the indentation fracture toughness, but the wear resistance increased with increasing Er content. The Fe48Cr15Mo14C15B6Er2 bulk metallic glass was then annealed and the wear resistance increased in the following order: crystallized, as-cast, and structurally relaxed condition. Transmission electron microscopy studies did not reveal sliding wear-induced crystallization, but instead shear band formation. The results demonstrate that micro-alloying is a useful approach for improving the wear resistance of bulk metallic glasses and that the combination of micro-alloying and annealing can substantially improve the wear resistance.
► Wear resistance of Fe50Cr15Mo14C15B6 can be improved with Er micro-alloying. ► Best wear behavior if achieved with 2at% Er addition and annealing. ► Continuous hardness increase and fracture toughness decrease with Er addition. ► Dry-sliding at room temperature induces multiple shear bands but no crystallites.
A coupled thermomechanical finite element analysis is performed in order to simulate orthogonal cutting of normalized AISI 9310. Damage parameters are optimized to define the behavior of the material ...subjected to orthogonal cutting. AISI 1045, AISI 4140, and A2024-T351 are selected as precursors to validating the present finite element approach for orthogonal cutting of normalized AISI 9310. The numerical results obtained in this study include the average cutting force, residual stresses and strains, chip morphology, and tool temperature. These results are validated for each material with experimental results found in literature. The current study optimizes the Johnson–Cook damage parameters for steel materials in order to capture physical chip morphology. A correlation analysis is then performed using the validated finite element model for the AISI 9310 material to better understand the effect of specific input parameters such as the damage parameters, coefficient of friction, fracture energy, heat generation fractions and tool velocity on output results such as stresses and strains in the workpiece, chip thickness ratio and tool temperature. This analysis provides input-output relations for a physically reasonable range of input parameters and supports that the damage parameters, coefficient of friction, and fracture energy have a very strong influence on the residual stresses and strains, and the chip morphology. The coefficient of friction has a strong influence of tool temperature. Correlation analysis results can help manufacturers in understanding the nature of residual stresses and distortion, and in choosing optimized process parameters suitable for and applicable to the specific workpiece material. Tool wear that is observed in actual cutting of normalized AISI 9310 is also discussed. This study will benefit the manufacturing industry with the understanding of how specific cutting processing parameters will impact the distortions and residual stresses in the machined AISI 9310 parts.
An ASTM B213 standard Hall Flowmeter Funnel experiment was conducted for Ti 6–4 powder particles and simulated utilizing a discrete element method approach implemented in the LIGGGHTS package. ...Particle interactions were described with a modified simplified Johnson–Kendall–Roberts theory that includes adhesion as a function of the particle surface free energy. Experimental data for the powder particle size distribution were used as input for the simulations. Adjustable parameters, such as cohesion energy density, coefficient of restitution and dynamic friction, were tuned to match the general shape of the experimentally obtained particle pile. Geometrical properties of the simulated powder pile, including its diameter, height and inside/outside slope angles, were computed and compared with the experimental results where available. Local particle size distributions for different areas within the pile (top vs. bottom) were obtained, indicating the dominance of larger particles at the top of the pile, akin to the Brazil nut effect.
The character, morphology and distribution of inclusions has been studied in additively manufactured 17-4PH stainless steel parts produced from gas-atomized powder by selective laser melting. A ...combination of advanced electron microscopy techniques has been used to show that such parts contain oxide inclusions ranging from a few nm to tens of μm across. The coarser inclusions have morphologies that mimic the oxides which accumulate between melt tracks at the build surface, suggesting that these inclusions are incorporated into the build during deposition of subsequent layers. Such features could have deleterious anisotropic effects upon the fatigue and fracture resistance of the built parts. The finer inclusions are equi-axed, reside preferentially at grain boundaries, and could inhibit grain growth during processing via Zener pinning effects. The chemistries of the inclusions include elements such as Al that are not part of the alloy specification. This indicates that the inclusions are exogenous defects that are entrained in the melt during gas atomization due to interactions with the crucible or nozzle materials. Examples of encapsulated oxide material in the powder feedstock support this explanation. These observations highlight the need for careful control of powder pedigree when using additive manufacturing for critical structural components.
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•Inclusions consisting of amorphous oxides of silicon and aluminum are observed in additively manufactured 17-4 PH samples.•The largest inclusions are many microns across with characteristic shapes, and contain crystalline alloy nanoparticles.•Oxide phases with similar morphologies are found between the tracks at the build surface of the samples.•The origin of the oxides is shown to be entrained refractory material within the powder feedstock.•These observations highlight the importance of powder pedigree in metal additive manufacturing.
Ti-6Al-4V powder has been recycled 30 times in an electron beam melting system. A combination of electron microscopy techniques has been used to show that the recycled powder has a 35% higher oxygen ...content, and that the particles have a more irregular morphology, a narrower particle size distribution, and a much more variable microstructure than the virgin powder. The microstructures in the recycled powder particles vary from a martensitic α′ structure, which is identical to that in the virgin powder, to a two-phase α + β structure. This variability is related to the complex thermal history of the unmelted metal powder in the system. Despite these differences, all of the particles exhibit essentially the same surface oxide thickness; the excess oxygen in the recycled powders is instead located in the β phase. The possible consequences for the structure and properties of the resultant additively manufactured parts are discussed.
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•17-4PH stainless steel thin-walled samples were additively manufactured by SLM.•Samples parallel and at 45˚ to the scan axes gave beam path lengths of 19−0.8 mm.•Longer beam paths ...gave microstructures comprising mostly coarse-grained ferrite.•The shortest beam paths gave structures with mostly austenite and some martensite.•Re-heating of ferrite leads to austenite, with martensite forming during cooling.
Components with varying dimensions are found in numerous applications. The current work examines how microstructures and phases change for additively manufactured 17-4PH thin walls as a function of laser path length, path direction, and wall thickness. Two sample sets were designed, each consisting of four walls with thicknesses of 6.4 mm to 0.8 mm. In the first set, the wall axes were parallel to the scan axes, such that the laser path length varied from layer to layer with the laser path either being parallel or perpendicular to the wall. In the second set, the walls lay at 45° to the scan axes, such that the laser path had the same length in all layers and gradually decreased with wall thickness. Substantial changes in phase stability and microstructure are observed as the wall thickness decreases, with ferritic phases and coarse grains changing to fine grains and an increasing volume fraction of austenite. These changes are attributed to changes in the local temperature-time profile as the length of the laser paths change from 19 mm to 0.8 mm. These observations demonstrate the range of microstructure and phase control options available in additive manufacturing with judicious selections of part layouts on build plates and of laser beam directions.
Nanocrystals develop in amorphous alloys usually during annealing treatments with growth- or nucleation-controlled mechanisms. An alternative processing route is intense deformation and nanocrystals ...have been shown to develop in shear bands during the deformation process. Some controversy surrounded the idea of adiabatic heating in shear bands during their genesis, but specific experiments have revealed that the formation of nanocrystals in shear bands has to be related to localized deformation rather than thermal effects. A much less debated issue has been the spatial distribution of deformation in the amorphous alloys during intense deformation. The current work examines the hypothesis that intense deformation affects the regions outside shear bands and even promotes nanocrystal formation in those regions upon annealing. Melt-spun amorphous Al
Y
Fe
alloy was intensely cold rolled. Microcalorimeter measurements at 60 °C indicated a slight but observable growth of nanocrystals in shear bands over the annealing time of 10 days. When the cold-rolled samples were annealed at 210 °C for one hour, transmission electron images did not show any nanocrystals for as-spun ribbons, but nanocrystals developed outside shear bands for the cold rolled samples. X-ray analysis indicated an increase in intensity of the Al peaks following the 210 °C annealing while the as-spun sample remained "X-ray amorphous". These experimental observations strongly suggest that cold rolling affects regions (i.e., spatial heterogeneities) outside shear bands and stimulates the formation of nanocrystals during annealing treatments at temperatures well below the crystallization temperature of undeformed ribbons.
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