bluuebunny/crossref_metadata_2025_split huggingface dataset:
Experimental Investigation and Thermodynamic Modeling of Phase Equilibria in the Hf-Ti-Si System
Authors: Yang Y. , Bewlay B. P. , Jackson M. R. , Chang Y. A. 
Published: April 2011

ABSTRACTPhase equilibria in ternary Hf-Ti-Si alloys were studied in the as-solidified and heat treated conditions using scanning electron microscopy, x-ray diffraction, and electron beam microprobe analysis. Selected solid-solid phase equilibria at 1350°C and a partial liquidus projection of the Hf-Ti-Si system at the metal rich end of the phase diagram were established. These data were then used to develop a thermodynamic description of the Hf-Ti-Si system using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated isothermal section at 1350°C and the liquidus projection can satisfactorily account for the available experimental phase equilibria data and solidification paths. Both the calculations and the experimental data suggested that the metal-rich end of the ternary phase diagram possesses one transition reaction: L + (Hf, Ti)5Si3 → Hf(Ti)2Si + β(Hf, Ti, Si).
Sputtered Coatings Based on the Al2Au Phase
Authors: Mitterer Christian , Lenhart Helmut , Mayrhofer Paul H. , Kathrein Martin 
Published: April 2011

ABSTRACTTransition metal nitride-based, wear-resistant hard coatings on cutting tools and other substrates often lack distinct colorations allowing product differentiation and self-lubricating properties. In the present work, the possibility of achieving these objectives for sputtered coatings based on the purple-red Al2Au phase within the Al-Au system was investigated. Coatings were characterized with respect to morphology, chemical and phase composition, hardness, optical, oxidation and tribological properties. Al2Au-containing coatings were deposited with dense, fine-grained structures yielding a hardness of 4 GPa and pink coloration. The coatings were stable up to about 850°C, where the onset of oxidation occurs. Low friction coefficients against alumina were achieved between 500 and 700°C. The concept of applying Al2Au-containing coatings as a colored self-lubricating layer on top of a hard coated cemented carbide tool warrants further investigations.
Micro Fracture Toughness Testing of TiAl Based Alloys with a Fully Lamellar Structure
Authors: Takashima K. , Halford T. P. , Rudinal D. , Higo Y. , Takeyama M. 
Published: April 2011

ABSTRACTA micro-sized testing technique has been applied to investigate the fracture properties of lamellar colonies in a fully lamellar Ti-46Al-5Nb-1W alloy. Micro-sized cantilever specimens with a size ≈ 10 × 10 × 50 μm3 were prepared by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the micro-sized specimens by focused ion beam machining. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values obtained were in the range 1.4–7 MPam1/2. Fracture surface observations indicate that these variations are attributable to differences in local lamellar orientations ahead of the notch. These fracture toughness values are also lower than those having been previously reported in conventional samples. This may be due the absence of significant extrinsic toughening mechanisms in these micro-sized specimens. Fracture mechanisms of these alloys are also considered on the micrometer scale. The results obtained in this investigation give important and fundamental information on the development of TiAl based alloys with high fracture toughness.
Experimental Studies and Thermodynamic Simulation of Phase Transformations in γ-TiAl Based Alloys
Authors: Chladil Harald F. , Clemens Helmut , Leitner Harald , Bartels Arno , Gerling Rainer , Marketz Wilfried T. , Kozeschnik Ernst 
Published: April 2011

ABSTRACTPhase transformations and phase transition temperatures in several Nb-rich γ-TiAl based alloys were investigated experimentally and compared to thermodynamic simulations. The present study combines light-optical and scanning electron microscopy, X-ray diffraction and differential-scanning-calorimetry for the characterization of the prevailing phases and phase transformations. Thermodynamic simulation based on the CALPHAD method was used for predict phase stabilities. The results from experiments on a variety of γ-TiAl based alloys are compared to thermodynamic calculations. Finally, the influence of carbon on the transition temperatures is presented.
Phase Transformation in Orthorhombic Ti2AlNb Alloys Under Severe Deformation
Authors: Greenberg B. A. , Kazantseva N. V. , Pilugin V. P. 
Published: April 2011

ABSTRACTIt was found that severe plastic deformation of orthorhombic alloys caused phase transformations of the displacement type and those associated with a change in the degree of long-range order, namely B2→ω(B82 ), B2→B19 and B2→β (BCC) (in the case of alloy with initial B2-phase structure) and O→B19→A20 (initial O-phase structure, Ti2AlNb). Unlike to ordinary metals, severe plastic deformation of the titanium aluminum intermetallics leads to decreasing of the strength of the material. The B19 and A20 phases are metastable. They are absent in the equilibrium phase diagrams of the compounds under investigation. The formation of the disordered phase states with extensive sliding and having great plasticity under severe deformation makes possible to consider severe deformation as the way for increasing of plasticity of the titanium aluminides.
Texture development during hot extrusion of a gamma titanium aluminide alloy
Authors: Oehring M. , Appel F. , Brokmeier H.-G. , Lorenz U. 
Published: April 2011

ABSTRACTThe evolution of preferred orientations during processing appears to be of significant importance for the use of γ titanium aluminide alloys, since the desired lamellar microstructures exhibit a strong anisotropy of mechanical properties. As texture evolution certainly is dependent on several factors, involving deformation properties, recrystallization kinetics and particularly the phase constitution, different processing temperatures were investigated. By comparing the results it is indicated that the determined textures can be understood by the deformation modes of the dominating phase at hot-working temperature and the subsequent phase transformations.
A Study of the Deformation Behavior of Lamellar γ-TiAl by Numeric Modeling
Authors: Schaden T. , Fischer F. D. , Clemens H. , Appel F. , Bartels A. 
Published: April 2011

ABSTRACTIn this paper the deformation behavior of a fully lamellar microstructure, which is usually present in cast γ-TiAl based alloys, is studied by numerical modeling. After large compressive deformation at elevated temperatures the lamellar colonies are often bent or buckled depending on their orientation, which is representative for a deformation instability characterized by a large wave length. Such a deformation behavior is triggered by both, structural defects of the lamellae and their somewhat irregular arrangement. In addition, a shear band-type deformation mode occurs according to an instability mode exhibiting a short wave length. These two deformation modes interact in a rather subtle way, which leads to a very inhomogeneous deformation pattern.
Effect of Long-Term Aging and Creep Exposure on the Microstructure of TiAl-Based Alloy for Industrial Applications
Authors: Lapin Juraj , Nazmy Mohamed , Staubli Marc 
Published: April 2011

ABSTRACTThe effect of long-term aging and creep exposure on the microstructure of a cast TiAl-based alloy with nominal chemical composition Ti-46Al-2W-0.5Si (at.%) was studied. The aging experiments were performed at temperatures between 973 and 1073 K for various times ranging from 10 to 14000 h in air. Constant load tensile creep tests were performed at applied stresses ranging from 150 to 400 MPa and at temperatures between 973 and 1123 K up to 25677 h. During aging and creep testing the α2(Ti3Al)-phase in the lamellar and feathery regions transforms to the γ(TiAl)-phase and fine needle-like B2 precipitates. Microstructural instabilities lead to a softening of the alloy. The effect of this softening on long-term creep resistance is negligible at temperatures of 973 and 1023 K.
Creep Behavior and Microstructural Stability of Ti-46Al-9Nb with Different Microstructures
Authors: Bystrzanowski S. , Bartels A. , Clemens H. , Gerling R. , Schimansky F.-P. , Dehm G. 
Published: April 2011

ABSTRACTIn this paper the creep behavior and the microstructural stability of Ti-46Al-9Nb (in at.%) sheet material were investigated in the temperature range of 700°C to 815°C. The study involves three different types of microstructure, namely fully lamellar with narrow lamellar spacing, duplex and massively transformed. Short-term creep experiments conducted at 700°C and 225 MPa confirmed that the lamellar microstructure with narrow lamellar spacing exhibits a much higher creep resistance when compared to the massively transformed and duplex ones. During longterm creep tests up to 1500 hours stress exponents (in the range of 4.4 to 5.8) and apparent activation energies (of about 4 eV) have been estimated by means of load and temperature changes, respectively. Both, stress exponents and activation energies suggest that under the applied conditions diffusion-assisted climb of dislocations is the dominant creep mechanism. The thermal stability of the different microstructures under various creep conditions has been analyzed by electron microscopy and X-ray diffraction. Our investigations revealed considerable stress and temperature induced microstructural changes which are reflected in the dissolution of the α2 phase accompanied by precipitation of a Ti/Nb - rich phase situated at grain boundaries. This phase was identified as a ω-related phase with B82-type structure. It was shown, that in particular the duplex microstructure is prone to such microstructural instabilities.
TEM characterisation of fatigue tested lamellar Ti-44Al-8Nb-1B
Authors: Jiang H , Hu D , Jones I P 
Published: April 2011

ABSTRACTDetailed microstructural examination by TEM of fine-grained polycrystalline lamellar Ti-44Al-8Nb-1B after fatigue testing (R=0.1) at room temperature has been carried out. The operative slip systems were identified as 1/2<110> ordinary dislocations and 1/6<112> twinning. The results showed no strong relation between operative slip systems and macroscopic Schmid factor and it is believed the local stress conditions control the operation of the slip systems. Translamellar cracking was observed to be associated with fine transverse twins in the gamma lamellae.
Creep of TiAl alloys at 750°C under moderate stress
Authors: Couret Alain , Malaplate Joel 
Published: April 2011

ABSTRACTThe creep properties at 750°C of TiAl alloys produced by cast and powder metallurgy processes are investigated by creep tests and subsequent TEM investigations. It is demonstrated that the same dislocation mechanism is controlling the primary and secondary creep stages in these two alloys. This mechanism is the mixed climb of ordinary dislocations. Explanations to the creep behaviour during primary and secondary stages are then proposed.
Microstructure-Property Relationships of Two Ti2AlNb-based Intermetallic Alloys: Ti-15Al-33Nb(at.%) and Ti-21Al-29Nb(at.%)
Authors: Cowen Christopher J. , Li Dingqiang , Boehlert Carl J. 
Published: April 2011

ABSTRACTTwo Ti2AlNb intermetallic orthorhombic (O) alloys, Ti-15Al-33Nb and Ti-21Al-29Nb(at.%), were subtransus processed into sheets, using pancake forging and hot-pack rolling, and evaluated in tension (25 and 650°C) and creep (650–710°C) and the properties and deformation behavior were related to microstructure. Some of the microstructural features evaluated were grain boundary character, grain size, phase volume fraction, and morphology. The alloy Al content was important to strength and elongation-to-failure (εf), where higher Al contents lead to greater tensile strengths and lower εf values and a corresponding brittle fracture response. However, the room temperature (RT) strengths of Ti-15Al-33Nb, which exhibited greater BCC phase volume fractions than Ti-21Al-29Nb and ductile failure (εf >2%), were always greater than 775 MPa. The creep stress exponents (n) and activation energies (Qapp) suggested that a transition in the dominant creep deformation mechanism exists and is dependent on stress and microstructure. Supertransus heat treatment, which increased the prior-BCC grain size and resulted in a lath-type O+BCC microstructure, resulted in reduced creep strains and strain rates. In fact, the supertransus heat-treated Ti-15Al-33Nb microstructures exhibited greater creep resistance than subtransus heat-treated Ti-21Al-29Nb microstructures. Combining the creep observations with the tensile response, the supertransus heat treated Ti-15Al-33Nb lath O+BCC microstructures exhibited the most attractive combination of tensile strength, εf values, and creep resistance.
Role of Biomechanics in Functional Tissue Engineering
Authors: An Kai-Nan 
Published: April 2011

ABSTRACTFunctional tissue engineering establishes functional criteria for design and manufacture of the scaffold matrix for repair and replacement. It also provides useful and strategic information in mechanical stimulation of the cells seeded in the matrix before and after surgical placement to enhance the success of tissue engineering. Biomechanics plays an important role in accomplishing these requirements by assessing the in vivo environment and the material properties.
Atomistic And Continuum Studies Of Flaw Tolerant Nanostructuresin Biological Systems
Authors: Buehler Markus J. , Yao Haimin , Ji Baohua , Gao Huajian 
Published: April 2011

AbstractBone-like biological materials have achieved superior mechanical properties through hierarchical composite structures of mineral and protein. Geckos and many insects have evolved hierarchical surface structures to achieve superior adhesion capabilities. What is the underlying principle of achieving superior mechanical properties of materials? Using joint atomistic-continuum modeling, we show that the nanometer scale plays a key role in allowing these biological systems to achieve such properties, and suggest that the principle of flaw tolerance and design for robustness may have had an overarching influence on the evolution of the bulk nanostructure of bone-like materials and the surface nanostructure of gecko-like animal species. We illustrate that if the characteristic dimension of materials is below a critical length scale on the order of several nanometers, Griffith theory of fracture no longer holds. An important consequence of this finding is that materials with such nano-substructures become flaw-tolerant, as the stress concentration at crack tips disappears and failure always occurs at the theoretical strength of materials, regardless of defects. The atomistic simulations complement continuum analysis and reveal a smooth transition between Griffith modes of failure via crack propagation to uniform bond rupture at theoretical strength below a nanometer critical length. This modeling resolves a long-standing paradox of fracture theories, and these results have important consequences for understanding failure of small-scale materials. Additional investigations focus on shape optimization of adhesion systems. We illustrate that optimal adhesion can be achieved when the surface of contact elements assumes an optimal shape. The results suggest that optimal adhesion can be achieved either by length scale reduction, or by optimization of the contact shape. Whereas change in shape does not lead to robustness, reducing the dimension results in robust adhesion devices.
Contribution of Collagen, Mineral and Water Phases to the Nanomechanical Properties of Bone
Authors: Bembey Amanpreet K. , Koonjul Vanessa , Bushby Andrew J. , Ferguson Virginia L. , Boyde Alan 
Published: April 2011

ABSTRACTCortical bone is an anisotropic material, and its mechanical properties are determined by its composition as well as its microstructure. Mechanical properties of bone are a consequence of the proportions of, and the interactions between, mineral, collagen and water. Mid-shaft palmar cortical tissue from the equine third metacarpal bone is relatively dense and uniform with low porosity. The mainly primary osteons are aligned to within a few degrees of the long axis of the bone. Beams of compact cortical bone were prepared to examine effects of dehydration and embedding and to study contribution of collagen and mineral to nano-scale material properties. Five beams were tested: untreated (hydrated); 100% ethanol (dehydrated); or embedded in poly-methylmethacrylate (PMMA) for one normal, one decalcified, and one deproteinated bone sample. Elastic modulus was obtained by nanoindentation using spherical indenters, with the loading direction transverse [1] and longitudinal to the bone axis. By selectively removing water, mineral and organic components from the composite, insights into the ultrastructure of the tissue can be gained from the corresponding changes in the experimentally determined elastic moduli.
Effects of Osteopontin Deficiency and Aging on Nanomechanics of Mouse Bone
Authors: Kavukcuoglu B. , West C. , Denhardt D.T. , Mann A. B. 
Published: April 2011

ABSTRACTOsteopontin (OPN), a phosphorylated glycoprotein, is among the most abundant non-collageneous bone matrix proteins produced by osteoblasts and osteoclasts. OPN has been implicated in bone formation, resorption and remodeling. However, previous studies have presented contradictory results regarding the effect of OPN on the mechanics and microstructure of bone. This study has used nanoindentation to identify local variations in elastic modulus and hardness of OPN deficient (OPN -/-) and wild-type control (OPN+/+) mouse bones. Specifically, the study has looked at changes in the mechanical properties of OPN-/- and OPN+/+ mouse bones with the mouse's age. Cortical sections of femurs from different age groups ranging from 3 weeks to 58 weeks were tested and compared. The results suggest that there are large, abrupt variations in mechanical properties across the femur's radial section for 3-week-old mouse bone. The hardness (H) drops significantly towards the inner and outer sections so the cortical bone has a mean H=3.66 GPa with a standard deviation of 2.44 GPa. In contrast, the hardness of the 58-week-old mouse bone had a standard deviation of 0.35 GPa and a mean H=1.45 GPa. The hardness across the radial axis of the 58-week-old bone was found to be quite uniform. The elastic modulus showed similar variations to the hardness with respect to age and position on the bone. We conclude that the mechanical properties of the mouse bones decrease substantially with maturity, and statistically the hardness and elastic modulus are more uniform in mature bones than young ones. Surprisingly we found a similar variation in both OPN-/- and OPN+/+ bones, with no statistically significant difference in the mechanical properties of the OPN -/- and OPN+/+ bones. The results for OPN-/- and OPN+/+ mouse bones are particularly important as control of OPN activity has been postulated as a potential treatment for bone pathologies that exhibit a change in the bone mineralization, such as osteoporosis, osteopetrosis and Paget's disease. Understanding the effects of OPN on bone mechanics is a vital step in the development of these new treatments.
Spatially Dependent Mechanical Properties Of Rat Whiskers For Tactile Sensing
Authors: Herzog E. K. , Bahr D. F. , Richards C. D. , Richards R. F. , Rector D. M. 
Published: April 2011

ABSTRACTA new generation of sensors based on biologically inspired whisking action will help determine the presence and location of solid objects and fluid vortices similar to mechanisms used by whisker bearing animals such as rats and seals. By using nanoindentation, we demonstrate that mechanical properties are essentially uniform by cross section, but vary longitudinally from the whisker base (a 3.9 GPa elastic modulus) to the tip (a 3.1 GPa elastic modulus). Several recent studies show propagation of high frequency information through whiskers that are tuned by their physical properties. In order to fully understand and model these properties, this study demonstrates a more complex whisker structure than previously assumed.
Nano-Mechanical Investigation of the Byssal Cuticle, a Protective Coating of a BioElastomer
Authors: Holten-Andersen Niels , Slack Nelle , Zok Frank , Herbert Waite J. 
Published: April 2011

ABSTRACTThe mechanical properties of the mussel byssal thread have been investigated via nano-indentation, with the emphasis on the differences between the cuticle and the fibrous interior. The cuticle hardness was found to be 30–40% higher than that of the underlying fibrous interior. In contrast, the Young's moduli in the two regions were virtually identical to one another. Elemental analysis via energy dispersive spectroscopy indicated surprisingly high levels of Al and Br in the cuticle considering the low amounts found in seawater. A potential role of Al in byssal thread mechanics is discussed in light of the unique capability of the cuticle to accommodate strains of 70% by the underlying fibrils in the core without delamination.
Effects of Wetting and Desiccation on the Creep Properties of Spider Dragline Silk
Authors: Ritchie Joanne , Smith Christopher , Bell Fraser I. , McEwen Iain J. , Viney Christopher 
Published: April 2011

ABSTRACTWe have previously devised a simple method for quantifying the creep resistance of silk fibres – i.e. the ability of silk to maintain dimensional stability while supporting a constant load. We demonstrated (C. Smith et al., J. Arachnol. 31:421–424, 2003) that creep of spider dragline is significant at stresses that are small compared to the conventional yield strength. In addition, the existence of a limiting creep stress was revealed: if samples are loaded smoothly and quickly to a constant stress lying above the limiting creep stress, they break within a few seconds of the stress being applied. The magnitude of the limiting creep stress is equal to approximately one fifth of the fracture stress recorded in conventional constant strain rate tests (in which the stress is applied more gradually).Here we develop the method and broaden the study of creep to investigate the effects of immersion in water and in ethanol. Experiments are conducted by attaching a small weight (an appropriate number of office staples) to an approximately 20 cm length of dragline from Nephila clavipes spiders, and monitoring extension as a function of time while the samples are suspended vertically in the liquid. The silk is previously conditioned (immersed without imposing any geometrical constraint) in the liquid, to allow it to supercontract or relax if the liquid promotes such a response. Allowance for buoyancy is made when calculating the net force applied to samples by the weight of the staples. Water is found to have a plasticizing effect; it exacerbates the rate of creep in response to a given small load, and it decreases the limiting creep stress. Ethanol, a desiccant, delays the transition from creep dominated by changes in chain conformation to creep dominated by chain slip, and increases the limiting creep stress. These results draw attention to an important limitation that must be overcome if biomimetic silk is to be used in applications where dimensional stability is required while loads are being supported for long times, especially in a wet environment. The results also point to the molecular origins of the creep sensitivity, and thus to ways of making silk and silk analogues less susceptible to creep.