Α-case Formation Research Articles

Effect of Crystallographic Orientation on the Formation of α-Case in Ti-6Al-4V Titanium Alloy

Effect of Crystallographic Orientation on the Formation of α-Case in Ti-6Al-4V Titanium Alloy

Experimental and theoretical study of the effect of Si on the oxidative behavior of Ti-6Al-4V alloys

This paper reports the first study of the oxidative behavior of Si-containing Ti-6Al-4V alloys in air. The alloys were studied in the temperature range 910–1210 K for a maximum period of 518.4 ks, and their tensile properties at room temperature are reported. The addition of Si decreased the total mass gain during oxidation and enlarged the region in which the mass gain obeyed the parabolic rate law. The oxide scale mainly consisted of rutile and α-Al2O3 phases. The results of the XPS analysis suggest that in rutile, Si exists as both Si4+ ions and SiO2. First-principles calculations indicated that Si4+ ions stably dissolved into the interstitial sites in rutile, which suppresses oxygen migration through rutile by decreasing the number of oxygen vacancies, thereby inhibiting oxide scale formation. The reaction layer consisting of Ti3Al and α″ phases on Ti-1mass% Si alloy suppressed the dissolution of oxygen into the substrate, i.e., α-case formation. The ratio of mass gain in α-case formation to total mass gain during oxidation at 1010 K and 1110 K ranged from 0.077 to 0.42. The addition of Si to the Ti-6Al-4V alloy was effective in improving both its oxidative resistance and tensile strength.

Mechanisms of oxidation of pure and Si-segregated α-Ti surfaces

Using first-principles molecular dynamics, we identified the mechanisms of the oxidation of α-Ti surfaces. Si segregation was found to suppress α-case formation in Ti, which was also confirmed experimentally. Charge transfer from the metal atoms to the gas molecules drives the initial stages of oxidation on the pure and Si-segregated α-Ti (0001) surfaces, while during the later stages, oxidation proceeds via oxygen penetration into the slab. Growth of the oxide network was strongly dependent on the oxidation state of the surface Ti atoms. Oxide growth in the Si-segregated material was retarded with the formation of TiOx (0.5 ≤ x ≤ 1) on the surface, which corresponds to the +1.5 oxidation state of the Ti atoms. The simulations and experiments clearly showed that Si reduces the ingress of oxygen into Ti, even at high temperatures. The primary and critical steps of identifying, understanding, and controlling the mechanisms of oxidation of Ti surfaces at high temperatures, as performed here, are expected to aid the design of new alloys with improved oxidation resistance.

The effect of oxidation on the subsurface microstructure of a Ti-6Al-4V alloy

Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM) are combined for examining α-case formation in Ti-6Al-4V, generated by air exposure at 800 °C. Below the oxide surface, the microstructure separates into a nanoscale mixture of the α-Ti and α2 Ti3Al phases, of compositions Ti70-O30 and Ti65-O10-Al20-V5 respectively. The α2 phase exists either as bands or as nanoscale spherical precipitates. Nitrogen also penetrates the surface, but to a lesser extent, while vanadium partitions to α2 or in distinctly separate phases. The results demonstrate that oxygen stimulates precipitation of α2, helping to explain embrittlement produced in the O-enriched layer beneath the oxide.

Effect of temperature on oxidation behaviour of Ga-containing near-α Ti alloy

Isothermal oxidation testing of a Ga-containing near-α Ti alloy was performed in air at 650, 700, and 750 °C for up to 500 h. Results revealed that the alloy oxidation kinetics followed a parabolic law at 650 °C and a parabolic–cubic law at 700 and 750 °C, while the abundance of Al2O3 in the oxide layers increased with temperature after the dissolution of Ga2O3 species in the Al2O3 phase. The activation energy of the α-case formation was close to the magnitudes obtained for conventional Ti alloys despite its temperature-dependent recrystallization near the oxide/metal interface at 700 and 750 °C.

α-Case formation in Ti-6Al-4V investment casting using ZrSiO4 and Al2O3 moulds

Ti-6Al-4V turbine blade-like geometries were cast in ZrSiO4 and Al2O3 based investment shells at different preheating temperatures to identify the least reactive conditions. The Cold Crucible Induction Melting method was employed to prevent unexpected melt reactions. The resulting sample α-case composition and characteristics of each sample were extensively analysed and the effect of mould stability and preheating temperature were evaluated. This work highlights the diffusion barrier effect that mould-metal reaction products provoke in the detrimental element transport. Interestingly, the results demonstrate that in the less stable preheated ZrSiO4 mould the diffusion layer thickness was 50μm less than that of the more stable Al2O3. This likely occurred because the reaction products present in the outer reaction layer hindered the detrimental element transport and a thinner diffusion layer was obtained.

Evaluation of the α-Case with TiO2 in Mold for Titanium Investment Casting

Molten titanium easily reacts with not only oxygen, carbon, and nitrogen but also the ceramic mold to form an α-case on the casting surface. However, the brittle α-case must be removed by chemical milling because it weakens the mechanical properties of the casting. The aim of this study is to evaluate and suppress the formation of the α-case by including TiO2 in the mold. Titanium powder was added into the alumina mold in order to facilitate the formation of TiO2 by the oxidation of titanium during mold firing. Another way to include TiO2 in the mold was to simply add TiO2 into the alumina mold. Optical microscopy and the micro-Vickers hardness test were used to characterize the α-case. The composition and morphology of the molds were confirmed using X-ray diffraction and field emission scanning electron microscopy. The mold with added titanium (Al2O3 + Ti) was effective in controlling the α-case. However, the interfacial reactions of the mold with the added TiO2 (Al2O3 + TiO2) exhibited distinct differences in α-case formation compared to the Ti-added mold (Al2O3 + Ti), while having almost identical compositions.

The hierarchy of microstructure parameters affecting the tensile ductility in centrifugally cast and forged Ti-834 alloy during high temperature exposure in air

Ductility regression is the main concern in using Ti-834 titanium alloy at temperatures above 500 °C for aerospace applications. The reduction of ductility in titanium alloys at high temperatures is strongly correlated to the exposure time. In the current study the effect of prolonged exposure at 500 °C on the tensile ductility of two differently processed Ti-834 alloys was investigated. In order to simulate actual Ti-834 processing routes, forged and centrifugally cast materials were used. The tensile tests were conducted on various specimens exposed at 500 °C for 100, 200 and 500 h to observe microstructure feature changes. Moreover, the effect of microstructure, microtexture, α-case, α2 and silicide precipitate coarsening during high temperature exposure was studied thoroughly. The cast alloy was found to have a minimum ductility and failed at 1.8% strain after exposure at 500 °C/500 h when the α-case layer was retained during testing, whilst, the ductility of the forged alloy was unaffected. The effects of individual microstructural parameters on the ductility regression in Ti-834 alloy were quantified. The results showed that 7.1% strain differences between the cast and forged alloy are related to microstructural variations including; morphology, lath widths, grain size and shape, grain orientations and microtexture. A total of 9.6% strain loss was observed in centrifugally cast Ti-834 after aging at 500°C/500 h and quantified as follow; 3.6% due to α-case formation during high temperature exposure, 0.2% due to α2-precipitates coarsening, 4.4% due to further silicide formation and coarsening, 1.4% due to additional microstructure changes during high temperature exposure. Furthermore, silicide coarsening on α/β phase boundaries caused large void formation around the precipitates. A theoretical model supported by experimental observations for silicide precipitation in fully colony and duplex microstructures was established. The element partitioning during exposure caused Al and Ti depletion in the vicinity of the β phase in the lamellae, i.e., αs area. This resulted in lowering the strength of the alloy and facilitated the formation of Ti3(SiZr)2 precipitates. The Al depletion and nano-scale partitioning observed at the αs/β boundaries resulted in easy crack initiation and promoted propagation in the centrifugally cast colony microstructure and reduced the basal slip τcrss. Furthermore, silicides were not formed in αp (high Al, Ti and low Zr areas) in the forged duplex microstructure that promoted superior mechanical performance and ductility over the cast alloy.

Catalysts having metal nano-particle catalyst supported on surface-treated natural cellulose fibers and preparation method thereof

Isothermal oxidation testing of a Ga-containing near-α Ti alloy was performed in air at 650, 700, and 750 °C for up to 500 h. Results revealed that the alloy oxidation kinetics followed a parabolic law at 650 °C and a parabolic–cubic law at 700 and 750 °C, while the abundance of Al2O3 in the oxide layers increased with temperature after the dissolution of Ga2O3 species in the Al2O3 phase. The activation energy of the α-case formation was close to the magnitudes obtained for conventional Ti alloys despite its temperature-dependent recrystallization near the oxide/metal interface at 700 and 750 °C.

Interstitial diffusion of O, N, and C in α-Ti from first-principles: Analytical model and kinetic Monte Carlo simulations.

The high affinity of O, N, and C with α-Ti has a serious detrimental influence on the high-temperature properties of these alloys, promoting the formation of α-case. These elements dissolve in interstitial sites and diffuse very fast in α-Ti (10(3)-10(8) times higher than the self-diffusivity of Ti) at high temperature accelerating the growth of α phase surface layer. Understanding the diffusion mechanisms of these elements is crucial to the design of high-temperature Ti alloys. This work aims to determine the stable interstitial sites and migration paths of O, N, and C in α-Ti. Diffusion coefficients were evaluated applying an analytical model, the multi-state diffusion method, and kinetic Monte Carlo simulations informed by first-principles calculations. The results show the reliability of these two methods with respect to the experimental data. In addition to octahedral sites, less traditional interstitial sites are shown to be stable configurations for these elements instead of tetrahedral sites. This requires to update the transition pathway networks through which these elements have been thought to migrate in α-Ti.

Relationship between the volume of the unit cell of hexagonal-close-packed Ti, hardness and oxygen content after α-case formation in Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy

In this study, the influence of oxygen diffusion on the physical properties of Ti–6Al–2Sn–4Zr–2Mo–0.1Si was examined. Measurements were carried out directly on sample cross sections which were preoxidized at high temperature. The lattice parameter evolution was measured using synchrotron X-ray diffraction and was coupled with microhardness and electron probe microanalyzer results with the aim of highlighting their relationships. The results show that the hardness and oxygen gradients along the oxygen diffusion zone in the alloy are similar to the evolution of the α-phase unit-cell volume quantified by X-ray diffraction. Linear relationships were found between these three parameters.

Effect of Oxidation Resistant Al3Ti Coating on Creep Behaviour of a Near α-Ti Alloy Titan 29A

The effect of oxidation resistant Al3Ti coating, applied through pack aluminizing method on creep properties of near α Ti-alloy Titan 29A has been examined at 823 and 873 K and in the stress range of 250–650 MPa. The creep properties of the coated alloy are found to be inferior to those of uncoated alloy. While rupture lives of the coated alloy are 4–25 % lower than those of the uncoated alloy, minimum creep rate (MCR) is only marginally affected. The increase in MCR of the coated alloy is as a consequence of brittle and non-load bearing nature of the Al3Ti coating. The extent of deterioration in creep properties in particular stress rupture life of the coated alloy is more predominant at lower stresses. This can be attributed to the fact that aluminide coating is present prior to creep test in case of the coated alloy, hence surface cracks either pre-exist or form early during creep test, while α-casing forms only during creep test and hence there is delay in formation of brittle α-case at the surface of the uncoated alloy and subsequent development of surface cracks in it.

Interfacial reactions between Ti-1100 alloy and CaO crucible during casting process

Ti-1100 alloys were melted in a controlled atmospheric induction furnace equipped with a CaO crucible. The microstructure, chemical composition, microhardness and metal–crucible interfacial reactions were systematically investigated. The results demonstrate that the primary solidification microstructure in the as-cast alloys was the typical Widmanstätten structure. The interactions between crucible and molten alloys are attributed to slight chemical dissolution and weak physical erosion. According to the line scanning analysis, the interfacial layer (α-case) thicknesses of Ti-1100 samples in the bottom and side wall are about 18 and 17 μm, respectively, which are slightly lower than those presented from microhardness tests (25 and 20 μm). The formation of α-case was caused by interstitial oxygen atoms. The standard Gibbs energy of reaction CaO(s)=Ca+O for Ti-1100 alloy was also determined. The equilibrium constant and the interaction parameter between calcium and oxygen were obtained as lg K=-3.14 and eCaO = 3.54.

Reaction between Ti and boron nitride based investment shell molds used for casting titanium alloys

The aim of this paper was to study the reaction between a Ti-6Al-4V alloy and boron nitride based investment shell molds used for investment casting titanium. In BN based investment shell molds, the face coatings are made of pretreated hexagonal boron nitride (hBN) with a few yttria (Y 2O 3) and colloidal yttria as binder. The Ti-6Al-4V alloy was melted in a controlled atmosphere induction furnace with a segment water-cooled copper crucible. The cross-section of reaction interface between Ti alloys and shell mold was investigated by electron probe micro-analyzer (EPMA) and microhardness tester. The results show that the reaction is not serious, the thickness of the reacting layer is about 30–50 μm, and the thickness of α-case is about 180–200 μm. Moreover the α-case formation mechanism was also discussed.

Modeling of Alpha-Case Formation and Its Effects on the Mechanical Properties of Titanium Alloy Castings

Titanium (Ti) castings generally contain a hard surface layer of alpha-case (α-case) that is enriched of oxygen. For dental Ti prostheses, the hard and brittle α-case can be a significant fraction of the thickness of the entire work piece and must be controlled properly to ensure minimum properties. The objective of this article is to present a theoretical framework for predicting α-case formation in dental Ti castings as a function of cooling rate and the influence of α-case on the tensile ductility and wear resistance. First, a diffusion analysis is presented to describe the oxygen profile and microhardness distribution within the α-case in Ti castings of CP-Ti and Ti-6Al-4V. The reduction in tensile ductility by the α-case is then predicted using an existing tensile ductility model. Furthermore, the improvement in wear resistance by the α-case is predicted using a recent wear model. These results are then used to assess the feasibility of controlling the α-case thickness to optimize both tensile ductility and wear resistance in Ti castings.

Thermodynamic Calculation of Alpha-Case Formation in Titanium Alloys

The alpha-case formation mechanism was elucidated for the economic titanium casting. The α-case formation reaction between Ti and Al2O3 mold was examined in a plasma arc melting furnace. The reaction products were characterized by electron probe micro-analyzer and transmission electron microscopy. The α-case generation between Ti and Al2O3 mold was not able to be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, from our experimental results and thermodynamic calculations and Calphad modeling (Thermo-calc®), it was confirmed that the α-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials. Our newly established α-case formation mechanism will surely lead to a variety of significant applications of the α-case controlled Ti casting.

Titanium and Titanium Alloy Castings for Biomedical Application

The aim of this study is to optimize the economic net-shape forming of titanium and titanium alloys for the biomedical application. The alpha-case formation reaction between titanium, and Al2O3, ZrO2, CaO stabilized ZrO2 and ZrSiO4 mold were examined in a plasma arc melting furnace. Regardless of the thermodynamic approach, α-case formation reactions still remain to be eliminated with the complex chemical milling processes. The reason why the α-case generated cannot be explained by the conventional α-case formation mechanism. However, from the experimental results and thermodynamic consideration, it can be confirmed that the α-case is formed not only by interstitial oxygen atoms but also by substitutional metal atoms dissolved from mold materials. Based on the interstitial and substitutional α-case formation mechanism, α-case controlled net-shape forming of titanium and titanium alloys can be possible for the biomedical application.

Titanium Alloys Casting for Structural Application

The aim of this study is to clarify the metal-mold reaction for the economic net-shape forming of titanium and titanium alloys. The metal-mold reactions between pure titanium, TiAl alloys and Al2O3 mold were examined in a plasma arc melting furnace. The alpha-case generation between pure titanium and Al2O3 mold could not be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, based on the interstitial and substitutional α-case formation mechanism, the α-case formation between pure titanium and Al2O3 mold, and α-case free casting of TiAl alloys against Al2O3 mold can be explained. On the basis of the interstitial and substitutional α-case formation mechanism, α-case controlled net-shape forming of titanium and TiAl alloys can be possible for the structural materials.

Economic net-shape forming of TiAl alloys for automotive parts

Alpha-case formations between investment molds and TiAl alloys were investigated for the economic TiAl alloys net-shape forming. In the case of TiAl alloys, there were no α-case formation reactions. There were neither interstitial nor substitutional α-case formations since TiAl alloys have both negligible solubility of oxygen and low activity in molten states. The fluidity of TiAl alloys is increased with mold preheating temperature since they have a peritectic reaction that appears in the form of envelopment, surrounding each particles of the primary constituent. The results of the investment casting of TiAl alloys confirm that the casting route in our study can be an effective approach for the economic net-shape forming of TiAl alloys.

Net-Shape Forming of TiAl Automotive Parts for Fuel Efficiency

Alpha-case formations between investment molds and TiAl alloys were investigated for economic TiAl alloys net-shape forming. The α-case reactions between mold and TiAl alloys were characterized by using optical microscopy and a hardness tester. In the case of TiAl alloys, there were no α-case formation reactions. The α-case was formed not only by interstitial oxygen elements but also by substitutional metallic elements dissolved from mold materials. There were neither interstitial nor substitutional α-case formations since TiAl alloys have both negligible solubility of oxygen and low activity in molten states. The results of the investment casting of TiAl alloys confirmed that the casting route in our study can be an effective approach for the economic net-shape forming of TiAl alloys automotive parts.