G. Welsch

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.

Silicon nitride coating on titanium to enable titanium-ceramic bonding

Failures that occur in titanium-ceramic restorations are of concern to clinicians. The formation of poorly adhering oxide on titanium at dental porcelain sintering temperatures causes adherence problems between titanium and porcelain, which is the main limiting factor in the fabrication of titanium-ceramic restorations. To overcome this problem a 1-microm thick Si3N4 coating was applied to a titanium surface using a plasma-immersion implantation and deposition method. Such a coating serves as an oxygen diffusion barrier on titanium during the porcelain firings. The protective coating was characterized in the as-deposited condition and after thermal cycling. Cross sections of Ti/Si3N4-porcelain interface regions were examined by various electron microscopy methods and by energy dispersive analysis of X-rays to study the Si3N4 films effectiveness in preventing titanium oxidation and in forming a bond with porcelain. The experiments have shown that this Si3N4 coating enables significant improvement in Ti-ceramic bonding.

Deformation modes of the α-phase of ti-6al-4v as a function of oxygen concentration and aging temperature

The tensile properties of lamellar Ti-6A1-4V and the deformation modes of the α-phase of this alloy were investigated as a function of oxygen concentration and as a function of aging heat treatment. Oxygen affects the mechanical properties through microstructural modifications which depend on the choice of aging parameters. The variations in Youngs modulus, yield strength, ultimate tensile strength, and ductility, are correlated with α/β volume ratio and with α-deformation characteristics. Homogeneityvs inhomogeneity of slip, change of the predominant slip modes from prismatic slip to fine planar slip on pyramidal planes, and the occurrence of Ti3Al precipitates influence the deformation behavior of the α-phase and thus influence the mechanical properties of the alloy. The deformation behaviors of the lesser β-phase regions were not investigated, and only speculations can be made on the extent of their influence on the alloys mechanical properties.

Literature Survey on Diffusivities of Oxygen, Aluminum, and Vanadium in Alpha Titanium, Beta Titanium, and in Rutile

A survey of diffusion data of interstitial oxygen and of the substitutional elements aluminum and vanadium is presented for alpha and beta titanium. It is based on a survey of literature. Oxygen is an important interstitial element in titanium alloys. Oxygen’s large chemical affinity to titanium is indicated by Ti—O bond energy of 2.12 eV,1 comparable to the Ti—Ti bond energy of 2.56 eV.2 Oxygen is difficult to eliminate completely from titanium, and commercial titanium alloys usually contain from 0.10 to 0.20 wt pct oxygen. Oxygen significantly affects the mechanical properties of titanium alloys1,3 and is sometimes used as an alloying element. The effects of oxygen on phase transformation ,4,5,6 Youngs modulus,7,8 hardness,9,10 fracture toughness,11 and other mechanical properties12 have been amply documented. Aluminum and vanadium are the most frequently used substitutional alloying elements. Aluminum is an alpha stabilizer and vanadium is a beta stabilizer.

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Youngs modulus and yield strength. Finely dispersed α2(Ti3Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti3Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Youngs modulus and yield strength.

Oxide-formation characteristics in the early stages of oxidation of Fe and Fe-Cr alloys

Electron microscopy investigations have been conducted on the oxides formed on Fe and Fe-Cr alloys at elevated temperatures (700–800‡C) and at low oxygen partial pressures (∼10−3Pa). Oxide nucleation and growth on chromium-rich iron alloys are significantly different from that for pure iron. On pure Fe and on Fe-3%Cr, wustite and magnetite particles nucleate and grow out of the surface, while on the higher Cr-containing alloys (≥9 wt. % Cr) the spinel oxide (Fe, Cr)3O4 nucleates and grows into the metal. The differences in oxide formation in the early stages of oxidation are explained in terms of the diffusion of different species being rate-controlling and in terms of rapid diffusion, for example, at the metal-oxide interface.

Influence of ion implantation on the microstructure of oxide scales formed on a 20Cr/25Ni/Nb-stabilized stainless steel in carbon dioxide at 825°C

Specimens of a stainless steel (20%Cr, 25%Ni stabilized with niobium and also containing 0.9% Mn and 0.6% Si) implanted with lanthanum to a dose of 1017 ion cm−2 , together with unimplanted specimens, have been oxidized in carbon dioxide at 825° C for times up to 9735 h. Transverse sections through the oxide scales formed on the respective specimens have been studied by analytical electron microscopy. After this exposure the scale on the unimplanted 20/25/Nb stainless steel consists of an outer, large-grained, spinel layer, a middle fine-grained Cr2O3 layer and an inner, discontinuous silicon rich, niobium and chromium bearing, amorphous layer. The main effects of the lanthanum implantation are to improve oxidation resistance and maintain scale adherence during thermal cycling. The microstructural changes in the scale formed on the lanthanum implanted 20/25/Nb steel include finer Cr2O3 oxide grains and an intermediate region between the outer spinel and inner Cr2O3 layers comprised of both oxides. The lanthanum concentrates in this region and appears to act as a marker due to its low diffusivity. Mechanisms of scale development on the 20/25/Nb stainless Red and the influence of lanthanum implantation are discussed.

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)3O4 spinel, an intermediate layer of (Fe,Cr)2O3 sesquioxide, and two outer layers of Fe2O3 hematite, each with different morphologies. Wustite (Fe1−xO) and distorted cubic oxide (γ-(Fe,Cr)2O3) were observed during the iniital parabolic oxidation only.

Microstructural investigation of the oxidation of an Fe-3 pct Cr alloy

The microstructure and microchemistry of the oxide scale on an Fe-3 wt pct Cr alloy have been investigated after oxidation in the temperature range 700 to 800 °C. Transmission and scanning electron microscopy along with energy dispersive X-ray analysis and Auger electron spectroscopy techniques were used for the investigation. Multilayered scales are observed which vary in composition and structure; the innermost oxide is an Fe-Cr spinel of the type Fe(Fe2•xCrx)O4. The intermediate layer and the outer oxide layer are both α-Fe2O3 hematite. The outer hematite layer is nonadherent and wrinkling is observed. Spallation occurs readily at the inner hematiteJspinel interface and at the spinel oxideJalloy interface. The poor oxidation resistance of the alloy is discussed in terms of these observations.

Analysis of closed cell metal composites

Composites of closed metal cells, in which the cells have uniform wall thickness and which confine a fluid in their interior, are modeled using cylindrical cells with square, hexagonal and circular cross sections. The analysis of their predicted mechanical behaviors in compression includes the effects of fluids and pressure in the cell interiors. A specialized finite element program is developed to account for the internal pressure. The finite element calculations predict the compressive stiffness of filled cell composites. Results for elastic and plastic cell wall deformation are also presented. Compared to the ambient air cell structures, the composites of internally pressurized or fluid-filled cells show increased compressive stiffness and delayed onset of plastic yielding.