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ABSTRACT: Microstructure and mechanical properties of laser deposited complex quaternary Ti-34Nb-7Zr-7Ta (all wt%), an orthopedic load-bearing implant alloy, has been investigated in detail in both as-deposited as well as heat-treated (β-solutionized and quenched) conditions. The difference in stress-strain behavior of this alloy in the above conditions has been characterized using scanning electron microscopy (SEM), orientation imaging microscopy (OIM™) and transmission electron microscopy (TEM). Compared to the sample in heat-treated condition, the as-deposited sample showed evidence of strong growth related texture. Again in the as-deposited post tensile-tested condition formation of a high density of shear bands, possibly arising from slip localization due to shearing of ω precipitates in the β matrix is observed. TEM investigations also show the presence of lenticular shaped deformation induced ω phase within the shear bands. In contrast, in case of the β-solutionized sample, twinning and the formation of stress-induced plates appears to be the primary mode of deformation. The change in deformation mechanism and thus the tensile property of this alloy could be attributed to the crystallographic texture along the growth direction as well as diffusion mediated isothermal ω precipitates, that cause an enrichment of Nb and Ta in the β matrix, during the laser-deposition process. This is no longer present after the solutionizing treatment.
Journal of the mechanical behavior of biomedical materials. 08/2012; 16C:21-28.
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ABSTRACT: This article discusses the competing mechanisms of martensite formation vs eutectoid decomposition via pearlitic or bainitic mechanisms during continuous cooling of a Ti-5wtpct Cu hypoeutectoid alloy, which falls under the
category of active eutectoid systems. Faster cooling rates result in a mixed microstructure of nanoscale bainite consisting
of a far-from-equilibrium Ti2Cu phase and martensitic alpha plates, as determined from three-dimensional atom probe (3DAP) coupled with energy-filtered
transmission electron microscopy (EFTEM). Slower cooling resulted in near-equilibrium eutectoid-based microstructures.
Metallurgical and Materials Transactions A 05/2012; 42(5):1139-1143. · 1.54 Impact Factor
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ABSTRACT: The atomic-scale study of solid-solid interfaces in complex multi-phase multicomponent systems is a challenging but important
endeavor. This article highlights the coupling of recently developed advanced characterization techniques, such as high resolution
scanning transmission electron microscopy, carried out in an aberration-corrected microscope, and atom probe tomography, to
address the structural and compositional transition at the atomic scale across solid-solid interfaces, such as the γ/γ′interface
in nickel-base superalloys and the α/β interface in titanium alloys. Possible implications of such investigations of the interface
on the understanding of physical and mechanical properties are discussed.
JOM: the journal of the Minerals, Metals & Materials Society 04/2012; 62(12):64-69. · 1.42 Impact Factor
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ABSTRACT: The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has
been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy
(EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24°C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale
secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr
while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary
precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760°C leads to coarsening of the primary
γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.
Metallurgical and Materials Transactions A 04/2012; 40(13):3059-3068. · 1.54 Impact Factor
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ABSTRACT: The mechanical properties of Ni-base superalloys are strongly influenced by ordered gamma'-Ni3Al precipitates, whose growth and coarsening are controlled by the site occupancy of elements within the structures. The site occupancy behavior of Cr in gamma'-Ni3Al is investigated by ab initio based Density Functional Theory based computational studies and 3D atom probe tomography. Three formalisms related to site occupancy are discussed, including standard defect formation formalism, antisite based formalism, and vacancy based formalism. The calculated substitutional energies of the Cr atom at the Ni and Al sublattice sites indicate that Cr has a strong preference for the Al site. Comparisons between the formalisms indicate that standard defect formation formalism gives an inconsistent result and vacancy based mechanism will dominate in the substitution process. The effect of additional Cr atoms calculated by the interaction energies of two Cr atoms suggests that Cr atoms prefer to be on nearest neighbor lattice sites: either in the Al, Ni, or mixed sublattice sites.
06/2011;
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ABSTRACT: The atomic-scale study of solid-solid interfaces in complex multi-phase multi-component systems is a challenging but important endeavor. This article highlights the coupling of recently developed advanced characterization techniques, such as high resolution scanning transmission electron microscopy (HRSTEM), carried out in an aberration-corrected microscope, and 3D atom probe (3DAP) tomography, to address the structural and compositional transition at the atomic scale across solid-solid interfaces, such as the gamma/gamma interface in Ni-base superalloys and the alpha/beta interface in titanium alloys. Possible implications of such investigations of the interface on the understanding of physical and mechanical properties are discussed.
06/2011;
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S Nag,
A Devaraj,
R Srinivasan,
R E A Williams,
N Gupta,
G B Viswanathan,
J S Tiley,
S Banerjee,
S G Srinivasan,
H L Fraser,
R Banerjee
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ABSTRACT: Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.
Physical Review Letters 06/2011; 106(24):245701. · 7.37 Impact Factor
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ABSTRACT: The transition in structure and composition across the titanium carbide/nickel hybrid interface has been determined at near atomic resolution by coupling high-resolution transmission electron microscopy with three-dimensional atom probe tomography. The titanium carbide phase adopts a rocksalt-type structure, is highly non-stoichiometric with a composition of (Ti+Ni)2C, exhibits an orientation relationship with the nickel matrix with a structurally well-defined semi-coherent interface, and a compositionally distinct interfacial layer (~ 2 nm), that is marginally enriched in titanium and nickel and depleted in carbon.
12/2009;
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ABSTRACT: The compositional and microstructural evolution of different generations of precipitates of the ordered gamma-prime phase during the continuous cooling, followed by isothermal aging, of a commercial nickel base superalloy, Rene 88DT, has been characterized by three dimensional atom probe tomography (3DAP) coupled with energy-filtered transmission electron microscopy studies. After solutionizing in the single gamma phase field, during continuous cooling at a relatively slow rate (approx. 24 degrees C/min), the first generation primary gamma-prime precipitates, forming at relatively higher temperatures, exhibit nearequilibrium compositions, while the smaller scale secondary gamma-prime precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often consisting of excess Co and Cr, while being depleted in Al and Ti content. The compositions of the gamma matrix near these precipitates also exhibit similar trends with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates.
02/2009;
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ABSTRACT: Using atom probe tomography, the partitioning of alloying elements between α and β in the alloy Ti metal-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe) has been investigated as a function of heat-treatment. It has been shown that β-solutionizing followed by step-quenching to a higher temperature (700°C) or slow-cooling leads to substantial partitioning of the alloying elements, including an enrichment of slow-diffusing Mo at the α/β interfaces. In contrast, it was found that the combination of β-solutionizing, quenching to room temperature and aging at 400°C leads to rather limited partitioning of these alloying elements.
Philosophical Magazine. 02/2009; 89(6):535-552.
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ABSTRACT: This article investigates the intra-granular precipitation of nanometer-scale α platelets in the β matrix of a complex quaternary
β titanium alloy, Ti-35Nb-7Zr-5Ta (all in wt%), for orthopedic implant applications, during β-solutionizing/quenching/aging
type heat-treatments. The role of metastable ω precipitates on the nucleation and growth of these α precipitates, has been
specifically addressed by coupling transmission electron microscopy (TEM) and 3D atom probe (3DAP) tomography studies on this
alloy. Athermal ω precipitates form in this alloy on quenching from above the β-transus temperature. On isothermal annealing
at low temperatures (~400°C), these ω precipitates coarsen, rejecting Zr into the adjacent β matrix as determined by 3DAP
studies. Concurrently, the nucleation and growth of α precipitates is initiated at or near the ω/β interfaces, as determined
by TEM studies. In addition to coherency strains induced by the ω precipitates, the local enrichment of Zr adjacent to these
precipitates appears to play an important role in aiding the nucleation and growth of Zr-rich α precipitates in this alloy.
Journal of Materials Science 01/2009; 44(3):808-815. · 2.02 Impact Factor
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ABSTRACT: The complex quaternary Ti-35Nb-7Zr-5Ta orthopedic alloy has been successfully deposited from a powder feedstock consisting of a blend of elemental titanium, niobium, zirconium, and tantalum powders, using the laser engineered net-shaping (LENStrade mark) process. In the as laser-deposited form, these alloys exhibit a substantially higher tensile strength as compared with more conventionally processed counterparts of similar composition, while maintaining excellent ductility and a low modulus. Furthermore, the as-deposited alloys appear to exhibit a <001> texture, with a substantially large number of grains of the beta phase aligning one of their <001> axes nearly normal to the substrate or parallel to the growth direction. The microstructure of the as-deposited as well as tensile-tested alloys have been characterized in detail using scanning electron microscopy (SEM), orientation microscopy (OM), and transmission electron microscopy (TEM). Formation of a high density of shear bands, possibly arising from slip localization due to precipitates of the omega phase in the beta matrix, is clearly evident in the tensile-tested sample. The enhanced tensile strength and low modulus in these laser-deposited alloys coupled with the ability to form near-net shape components makes LENS an attractive processing technology for orthopedic implants.
Journal of Biomedical Materials Research Part A 08/2006; 78(2):298-305. · 2.63 Impact Factor
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ABSTRACT: This paper discusses the structural and compositional changes at the nanometer scale associated with the nucleation and growth of α precipitates in the β titanium alloy Ti-5553 (Ti–5Al–5Mo–5 V–3Cr–0.5Fe) with ω precipitates acting as heterogeneous nucleation sites. The microstructural evolution in this alloy, during β-solutionizing, quenching and aging type heat-treatments, has been investigated by combining results from scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy, high-resolution TEM and three-dimensional atom probe (3DAP) tomography. Athermal ω precipitates form in this alloy on quenching from above the β transus temperature. On isothermal annealing at low temperatures, these ω precipitates coarsen to form chemically ordered ω precipitates, accompanied by the nucleation of the stable α phase. Annealing at higher temperatures leads to dissolution of ω and further growth of α precipitates accompanied by clustering of different α variants in self-accommodating morphologies. 3DAP results indicate that annealing at lower temperatures (∼350 °C) leads to initial nucleation of α precipitates with a non-equilibrium composition, nearly identical to that of the β matrix. Subsequent aging at higher temperatures (∼600 °C) leads to more pronounced partitioning of alloying elements between the two phases. These results indicate that the structural body-centered cubic to hexagonal close-packed transformation and the compositional partitioning of alloying elements occur in sequential steps, resulting in a mixed-mode displacive-diffusional transformation, similar to the bainite transformation in steels.
Acta Materialia. 57(7):2136-2147.
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ABSTRACT: Using atom probe tomography, the three-dimensional morphology and composition of nanoscale omega precipitates in a simple binary titanium–molybdenum alloy have been uniquely determined. The results include the average composition of the omega precipitates and the partitioning of Mo across the ω/β interfaces. These investigations also revealed compositional inhomogeneity within the nanoscale precipitates, with the presence of a Mo-rich core, presumably arising from the relatively slow rate of Mo rejection during isothermal growth of these omega precipitates.
Scripta Materialia. 61(7):701-704.
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ABSTRACT: The compositional and microstructural evolution of different generations of γ′ precipitates during the continuous cooling of a commercial nickel base superalloy, Rene88DT, has been characterized by three dimensional atom probe tomography coupled with energy-filtered transmission electron microscopy studies. After solutionizing in the single γ phase field, continuous cooling at a very high rate results in a monomodal size distribution of γ′ precipitates with a high nucleation density and non-equilibrium compositions. In contrast, a relatively slower cooling rate (~ 24 °C/min) results in a multi-modal size distribution of γ′ precipitates with the larger first generation primary precipitates exhibiting close to equilibrium composition, along with the smaller scale secondary γ′ precipitates, exhibiting non-equilibrium composition (excess of Co and Cr, depleted in Al and Ti). The composition of the γ matrix near these precipitates also exhibits similar trends with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates.Highlights► Effect of cooling rate on the precipitation of γ′ particles in commercial nickel base superalloy. ► Couples EFTEM and 3DAP studies to determine the composition and morphology of γ′ precipitates. ► Determination of near and far field compositional variations within the gamma matrix leading to subsequent precipitation.
Materials Characterization. 62(9):878-886.
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ABSTRACT: In recent years there has been a significant thrust directed towards the development of novel implant alloys based on β-Ti. Two recently developed and promising biocompatible β-Ti alloys are Ti–35Nb–7Zr–5Ta and Ti–29Nb–4.6Zr–13Ta. While both these alloy compositions, based on the quaternary Ti–Nb–Zr–Ta system, are promising, there is still a tremendous scope for improvement in terms of alloy design in this and other systems via optimization of alloy composition and thermo-mechanical treatments. Here a novel combinatorial approach has been used for the development of implant alloys with optimized compositions and microstructures. Using directed laser deposition, compositionally graded alloy samples based on the Ti–Nb–Zr–Ta system have been fabricated. These samples have been heat-treated to affect different microstructures in terms of the volume fraction and distribution of the α phase in the β matrix as a function of composition. Subsequently, composition-specific indentation-based hardness and modulus information has been obtained from these samples to construct a database relating the composition and microstructure to the mechanical properties. These databases have been used to train and test fuzzy-logic based neural-network models for predicting the mechanical properties. The trained models have also been used to predict the influence of different alloying additions on the hardness and modulus. These predictions have subsequently been verified by detailed experimental characterization, shedding light on the factors influencing the strength and modulus in these alloys. Such modeling approaches for the development of novel implant alloys can be highly beneficial since they offer the possibility of identifying promising compositions without the necessity for extensive experimental test cycles.
Materials Science and Engineering: C.
