Effects of microalloying and ceramic particulates on mechanical properties of TiAl-based alloys

Micro-alloyed titanium aluminides and TiAl-based composite with a fully dense structure have been synthesized by a powder metallurgical method. X-ray diffraction and EDX studies show that the monolithic TiAl-based alloy and its composite consist of TiAl and Ti₃Al compounds with the former as the major phase. Additions of Nb and/or (Mo + Cr) exert remarkable effects on the resultant phase compositions and mechanical properties of TiAl-based alloys. Both the added Mo and Cr elements are dissolved into the TiAl-based compounds to exhibit solid solution strengthening effect whereas Nb is only partially dissolved and the rest appears in the Nb-rich compound, Nb₂Al. In general, the micro-alloyed TiAl and the composite exhibit superior flexural strength and fracture toughness to TiAl-based monolithic compound.     

Titanium Aluminides: Science, Technology, Applications and Synthesis by Mechanical Alloying

The production. characteristics. and commercialization of monolithic and composite titanium aluminides are summanzed with emphasis on use in the demanding aerospace industry. The attractive elevated temperature properties combined with alow density make these materials of great interest, but inherently low "forgiveness", and environmental concerns, must be overcome before widespread use will occur One synthesis method for the production of monolithic titanium aluminides-mechanical alloying- will be discussed in detail     

“Bioceramic-titanium nickelide” functional composites for medicine

“Bioceramic-titanium nickelide” functional composites are proposed and investigated. It is shown that the incorporation of an alloy (titanium nickelide) possessing superelasticity and shape memory enhances the strength properties of the composite while the porosity through the material needed for the ingrowth of living tissue is maintained. These composites are biocompatible and exhibit a property similar to the superelasticity characteristic of living tissue. Pis’ma Zh. Tekh. Fiz. 23, 1–6 (April 26, 1997)     

On laws governing the splitting failure of flat metallic specimens subjected to thermal shock

Computed and experimental data on the splitting failure of copper, nickel, titanium, molybdenum, brass, and bronze metallic foils from 0.005 to 1 mm thick under thermal shock initiated by the x-radiation of a nuclear explosion are presented. It is proposed that the concepts “average energy liberated over the thickness (mass) of the specimen” ε, “specific absorbed energy” W, and “splitting strength of the material” σ be used as criterial characteristics of failure thresholds of optically thin flat metallic specimens (foils). It is demonstrated that the critical average energy liberation ε_*, which results in splitting, decreases logarithmically (ε_* =A _*-B _*log Δ) with increasing thickness Δ of the irradiated specimens in the interval Δ≈0.001–1 mm, and the critical specific potential energy W_* reguired to effect splitting increases with increasing optical mass m of the specimen under the law W_*=−αmlog (βm), where A_*, B_*, α, and β are certain parameters. It is shown that the longevity of the copper, nickel, titanium, molybdenum, brass, and bronze under radiation-induced thermal shock decreases exponentially with increasing amplitude of the failing stress (splitting strength) and can be described on the basis of the kinematic concept of strength. Deceased. Translated from Problemy Prochnosti, No. 1, pp. 37–47, January–February, 1997.     

The Composite Eshelby Tensors and their applications to homogenization

Summary In recent studies, the exact solutions of the Eshelby tensors for a spherical inclusion in a finite, spherical domain have been obtained for both the Dirichlet- and Neumann boundary value problems, and they have been further applied to the homogenization of composite materials [15], [16]. The present work is an extension to a more general boundary condition, which allows for the continuity of both the displacement and traction field across the interface between RVE (representative volume element) and surrounding composite. A new class of Eshelby tensors is obtained, which depend explicitly on the material properties of the composite, and are therefore termed “the Composite Eshelby Tensors”. These include the Dirichlet- and the Neumann-Eshelby tensors as special cases. We apply the new Eshelby tensors to the homogenization of composite materials, and it is shown that several classical homogenization methods can be unified under a novel method termed the “Dual Eigenstrain Method”. We further propose a modified Hashin-Shtrikman variational principle, and show that the corresponding modified Hashin-Shtrikman bounds, like the Composite Eshelby Tensors, depend explicitly on the composite properties.     

Experimental studies on process-induced morphological characteristics of macro- and microstructures in laser consolidated alloys

Laser consolidation (LC) developed by National Research Council’s Industrial Materials Institute (NRC-IMI-London) since mid-1990s, is a laser cladding based rapid manufacturing and material additive process that could fabricate a “net-shape” functional metallic shape through a “layer-upon-layer” deposition directly from a computer aided design model without using molds or dies. In order to evaluate the LC processability of different materials, some representative nickel-based superalloys (IN-625, IN-718, IN-738, and Waspaloy), stainless steels (austenitic SS316L and martensitic SS420), and lightweight alloys (Ti–6Al–4V titanium alloy and Al-4047 aluminum alloy) have been investigated. Like other laser cladding based processes, due to process-induced rapid directional solidification, the LC alloys have demonstrated certain unique morphological characteristics. Moreover, the “as-consolidated” LC alloys, in nature, are in the “as-quenched” state, and some precipitation processes from their matrices, which are sometimes critical to the development of mechanical performance of the materials, could be effectively suppressed or retarded. Post-heat treatments, therefore, could necessarily facilitate the process of achieving their required operational microstructures. In this article, a comprehensive investigation was performed including metallurgical soundness and process-induced morphological characteristics of the LC materials, and microstructure development brought by post-LC heat treatments using optical microscope, scanning electron microscope, and X-ray diffraction. The implications on the mechanical performance of the LC materials were discussed as well in order to provide essential information for potential industrial applications of the LC materials.     

Long-term assessment of the implant titanium material—artificial saliva interface

This paper studies the long-term (20,000 exposure hours) behavior of titanium and Ti–5Al–4V alloy—Carter–Brugirard saliva interface and the short-term (500 exposure hours) resistance of titanium and Ti–5Al–4V alloy—Tani&Zucchi saliva interface. Potentiodynamic polarization method was applied for the determination of the main electrochemical parameters. Linear polarization measurements for to obtain the corrosion rates were used. Monitoring of the open circuit potentials (E _oc) for long-term have permitted to calculate the potential gradients due to the pH, ΔE _oc(pH) and to the saliva composition ΔE _oc(c) changes which can appear “in vivo” conditions and can generate local corrosion. Atomic force microscopy (AFM) has analyzed the surface roughness. Ion release was studied by atomic absorption spectroscopy (AAS). In Carter–Brugirard saliva both titanium and Ti–5Al–4V alloy present very stable passive films, long-term stability, “very good” resistance, low values of the open circuit potential gradients, which cannot generate local corrosion. In Tani&Zucchi artificial saliva, pitting corrosion and noble pitting protection potentials (which cannot be reached in oral cavity) were registered; titanium ion release is very low; surface roughness increase in time and in the presence of the fluoride ions, denoting some increase in the anodic activity.     

Surface behaviour of biomaterials: The theta surface for biocompatibility

“Biomaterials” are non-living substances selected to have predictable interactions with contacting biological phases, in applications ranging from medical/dental implants to food processing to control of biofouling in the sea. More than 30 years of empirical observations of the surface behaviours of various materials in biological settings, when correlated with the contact-angle-determined Critical Surface Tensions (CST) for these same materials, support the definition of the “theta surface”. The “theta surface” is that characteristic expression of outermost atomic features least retentive of depositing proteins, and identified by the bioengineering criterion of having measured CST between 20 and 30 mN/m. Biomaterials applications requiring strong bioadhesion must avoid this range, while those requiring easy release of accumulating biomass should have “theta surface” qualities. Selection of blood-compatible materials is a main example. It is forecast that future biomaterials will be safely and effectively translated directly to clinical use, without requiring animal testing, based on laboratory data for CST, protein denaturation, and cell spreading alone.     

Work of adhesion influence on the rheological properties of silica filled polymer composites

As the work of adhesion, W _a, increases between a silica filler surface and a polymer matrix, the dynamic viscosity and the shear modulus of the composite material increase. The logarithms of these properties decrease linearly as W _a decreases. At lower dynamic test frequencies, a change in W _a has a more dramatic impact on these properties than at higher frequencies. An “effective silica particle size” model can be used to explain why W _a affects the viscosity and the shear modulus of a composite. According to that model, the thickness of the interphase layer increases as the W _a increases. An increase in effective particle size decreases the “free” polymer volume, and the decrease free volume polymer causes both the viscosity and the shear modulus to increase. Increasing the dynamic test frequency releases some of the immobilized polymer from the filler surface which causes the effective particle size to decrease. As the effective particle size decreases because of the increased testing frequency and approaches the mean size of the original filler, the impact of the W _a value on viscosity and shear modulus should decrease. However, the friction experienced between the filler interphase and the polymer, the so called “skin friction”, depends on the magnitude of W _a and the more general term, bond energy density (BED). The skin friction determines the viscosity of the composite, particularly at lower frequencies. Higher W _a values induce higher skin friction and thereby higher flow resistance (viscosity) as polymer chains move along the filler surface.     

Structural model of a microinhomogeneous cracked material

We develop a mathematical model of microscopically inhomogencous but macroscopically isotropic materials with statistically distributed components of tensors of stiffness and strength. In this model, the material is represented as the continuous set of “characteristic” (i.e., typical of a given material) disjoint microscopic domains (microvolumes). The microinhomogeneous material is identified with an “effectively homogeneous” material in such a way that, at the same points, the components of the displacement vector determined for these materials are equal. It is assumed that, for each “characteristic” microvolume the parameters of stiffness and strength of the material are constant and can be obtained as values of an arbitrary random variable distributed according to the Weibull law and averaged over a certain random interval of any length. The components of the tensor averaged as indicated above are also regarded as random variables distributed according to the normal law with the same probability of hitting any arbitrarily located “characteristic” microvolume. The model is based on the assumption that the material is isotropic both macroscopically and in any “characteristic” microvolume. The stress-strain state of the microinhomogeneous material is described by the “effective” (averaged over its volume) components of the stress tensor. The model takes into account cracks in the material if their length exceeds the size of the relevant “characteristic” volume. The model is justified for the case of an infinite microinhomogeneous cracked plane under uniaxial tension. It is shown that the parameters determining the stressed state of this plane are not independent in the vicinity of the crack tip. The relevant constraints are given by equations of the model. The choice of these parameters which ignores the indicated constraints leads to results contradicting well-known physical facts. By using the symmetry properties of the system under consideration and physical reasoning, we obtain equations for the determination of the size of “characteristic” domains and physically reasonable dependences of the maximal “effective” tensile stresses and their direction on the parameter of inhomogeneity of the material and average volume of defects. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Pidstryhach Institute of Applied Problems in Mechanics and Mathematics, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 5–16, July–August, 1996.     

Novel basalt fibre reinforced glass matrix composites

A novel hot-pressing technique for the manufacturing of basalt fibre reinforced glass matrix composites was investigated. Two-dimensional (2D) fibre mats were sandwiched between borosilicate glass powder layers, thus configuring a much simpler processing route than that commonly employed for the production of fibre-reinforced glasses. Besides economic benefits, the use of fibre mats may lead to technologic advantages due to the possibility of readily coating the fibres with a suitable material (e.g. titanium oxide) by means of the sol-gel method. The coating of basalt fibre mats with TiO₂ is proposed for preventing the fibres from an excessive adhesion to the glass matrix. The developed composites containing 15 vol% of 2D-fibre reinforcement exhibited promising bending strength (∼90 MPa) and desirable “graceful” fracture behaviour without catastrophic failure. Thus the present study represents a convenient approach for production of advanced low-cost fibre reinforced glass matrix composites for structural applications.     

Time dependent behavior of elements combining ultra-high performance fiber reinforced concretes (UHPFRC) and reinforced concrete

Structural elements combining Ultra-High Performance Fiber Reinforced Concretes (UHPFRC) and concrete offer a high potential in view of rehabilitation and modification of existing structures. The investigation of the time-dependent behavior of composite “UHPFRC-concrete” elements is a fundamental step in the determination of durability and serviceability. For this, an experimental program was conducted on large composite “UHPFRC-concrete” beams and a numerical model was validated with the test results. The experimental results and a parametric study performed with the numerical model showed that UHPFRC and normal strength reinforced concrete are compatible in the long-term and that the critical period of composite “UHPFRC-concrete” elements are the first 90 days after the casting of the UHPFRC layer. Thus, the high potential of such composite elements can be exploited also in the long term.     

Uncertainty of measurement results in various scales

The relationship between the concepts of “error” and “uncertainty” of measurement results is examined. The concepts of “standard uncertainty,” “combined standard uncertainty,” and “expanded uncertainty” are shown to be inapplicable in nonmetric scales of quantities and properties, in which the general concept of “uncertainty” in the broad sense is recommended. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 29–30, May, 2000.     

Novel geopolymer based composites with enhanced ductility

Geopolymer materials have been proposed for various applications due to their excellent fire resistance and low processing cost. One requirement that geopolymer composites need to meet for structural applications is graceful failure, as catastrophic failure during service can result in significant loss of life. In this paper, the properties of novel low cost composites made by infiltrating stainless steel mesh with geopolymer resin are reported. Four point flexure tests on these composites showed metal-like yielding behavior instead of catastrophic failure and the “yield strength” was significantly higher than the flexure strength of the geopolymer matrix. Exposure to high temperatures, 800 and 1,050°C, resulted in the “yield strength” decreasing to ∼59 and ∼44% level respectively compared to the strength of as-processed samples. However, the good ductility was still retained after the high temperature treatment, which makes this novel composite a strong candidate for the applications where catastrophic failure upon fire/high temperature exposure needs to be avoided.     

Comparative viability of processing routes for intermetallic based materials

Abstract

Processing routes for intermetallic based materials are briefly surveyed and compared. For monolithic intermetallics the two main factors determining process routes both derive from the low room temperature ductility of most intermetallics. They are the need to maintain material cleanness, thus reducing fracture initiation sites, and the desire to achieve fine grain size to seek to improve ductility. For the titanium based aluminides there is also a need to minimise interstitial, particularly oxygen, pick-up during processing. For intermetallic based composites, a broad range of processes is already being investigated. In many of these, issues of cleanness may be more difficult to control than for the monolithic composites. With continuous fibre reinforced composites a further process impetus is the need to control interfacial interactions between the fibre and the matrix.

MST/1560     

Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.     

Influence of oxynitriding on the wear resistance of VT14 titanium alloy

We investigate the wear resistance of VT14 titanium alloy after oxynitriding realized by modifying nonstoichiometric titanium nitride by oxygen. It is shown that the oxide component in an oxynitride coating improves the wear resistance of the alloy in a friction pair with BrAZh9-4l brass. It is established that the wear rate of a “VT14 titanium alloy with oxynitride coating–U8 steel” tribopair is an order smaller than that of a “VT14 titanium alloy with oxynitride coating–BrAZh9-4l brass” tribopair. It is also established that the tribologic behavior of the former tribopair improves after replacing AMH-10 lubricant by I-40A lubricant.     

Evaporation of NH3 in NH3/H2 atmosphere by 25 bar for neutral gas absorption refrigeration units

The present work considers the evaporation process ofNH ₃ inNH ₃/H₂ atmosphere, that takes place in neutral gas absorption refrigeration units. The data obtained are used to calculate the “psychrometric problem” and the evaporation process within an insulated tube. The “psychrometric problem” deals with the determination of the mass fraction of theNH ₃/H ₂ gas mixture from known “dry” and “wet bulb” temperatures for Lewis numbers not equal to one. The results are in good agreement with some existing experimental data.     

Scientometric study of laser patent literature

An analysis of the patents filed and scientific papers published and abstracted in theJournal of Current Laser Abstracts (JCLA) for the period 1967–95 indicates that innovative activity in laser science and technology was at its peak in the early 70s. However, scientific activity surpassed the innovative activity in the early 80s. There was a continuous shift in emphasis from “applications of lasers” to “experimental laser research” and to “theoretical laser research”. Further analysis of the 1840 patents field in 1970–71, 1975–76, and 1980–85 indicates that most of the firms filing patents were situated in USA and thus USA is the leading country filing patents in this area followed by Japan. “Spectroscopy of laser output” followed by “Communication applications of laser” got the maximum emphasis.     

The Kramers-Kronig relations for permittivity, “True” screening radius,and critical point of a coulomb system

A correlation between the peculiarities of the electric permittivity (EP) ɛ(q, ω) of a homogenous and isotropic Coulomb system (CS) in the limit of ω → 0, q → 0 and q → 0, ω → 0 is established on the basis of exact limiting relations; general expression for static EP ɛ(q, 0) of a CS is obtained in the region of small wave vectors q. The concepts of a “true” dielectric and “ideal” conductor are introduced. The definition of a “true” screening radius is considered. A critical point of two-component CS is shown to correspond to either the “true” dielectric state or the “ideal” conductor one.