Composition,structure, and properties of SHS-compacted cathodes of the Ti-C-Al-Si system and vacuum-arc coatings obtained from them

The phase composition, structure, and properties of self-propagating high-temperature synthesis (SHS)-compacted materials of the Ti-C-Al-Si system are investigated. It is shown that metal-like titanium compounds can be used as cathodes for vacuum-arc evaporators. The coatings fabricated from SHS cathodes are single-phase and represent cubic titanium nitride of the (Ti, Al, Si)N composition. The volume of a microdrop phase in them is smaller by a factor of 2.5–3.0 as compared with TiN, while their microstructure is not fragmented into low-strength columnar elements; the CSR size is twice smaller than that of titanium nitride. Being approximately identically hard, the (Ti, Al, Si)N coatings substantially surpass TiN coatings in elastic and plastic failure resistance due to the lower elasticity modulus. When milling the tungsten-copper alloy, the hardness of hard-alloy milling cutters with the (Ti, Al, Si)N coating is higher by a factor of 2.4 than with the TiN coating fabricated from the titanium cathode with the magnetic plasma flow separation.     

Synthesis of MoSi2-TiSi2 pseudobinary alloys by reactive sintering

MoSi₂-TiSi₂ pseudobinary alloys are synthesized from mixtures of elemental powders of molybdenum, silicon, and titanium by reactive sintering under a pseudoisostatic pressure of 150 MPa. When the titanium content in the alloy increases from 0 to 33 at. pct, the density of the alloy decreases from approximately 6 to 4 g/cm³, while the relative density is more than 95 pct independent of the titanium content. Vickers hardness of the alloy is approximately 800 when the alloy consists of a monophase structure of α-MoSi₂ or β-MoSi₂. However, the hardness increases to approximately 950 when the alloy consists of a dual-phase structure of (α-MoSi₂+β-MoSi₂) or (β-MoSi₂+γ-TiSi₂). The oxidation resistance of the alloy at 773 K is approximately tripled when the titanium content increases from 0 to 1.7 at. pct, but the effect of the titanium content on the oxidation resistance becomes less remarkable as the titanium content increases.     

Influence of annealing on depth distributions and microstructure of ion-implanted Ti6Al4V

Ti6Al4V alloy was ion implanted with carbon, nitrogen, platinum, or gold. The effect of heat treatment at 500°C on the depth distributions of oxygen and implanted atoms was investigated using backscattering spectrometry. The phases in the near-surface region were determined using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Platinum and gold are enriched in both α- and β-titanium solid solutions. Implanted carbon forms titanium carbide at lower concentration than nitrogen forms titanium nitride. The depth profiles of Au, Pt, and N are not altered by annealing. Implanted carbon as well as oxygen diffuse to larger depth upon heat treatment. Noble metal-implanted layers are permeable to oxygen diffusion. At concentrations exceeding 35 at. pct, implanted carbon and nitrogen act as a diffusion barrier for oxygen.     

Corrosion behavior of zirconium diboride-based ceramics and electrospark coatings in 3% NaCl solution

The paper examines the corrosion behavior of AlN-ZrB₂, AlN-TiN, and ZrB₂-LaB₆ composite materials and associated electrospark coatings based on aluminum alloy AL9 in 3% NaCl solution. It is shown that the electrochemical corrosion resistance of composites is close to that of titanium nitride. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 54–57, 2007.     

Deformation kinetics of the Ti-6Al-4V alloy at low temperatures

The deformation kinetics of theα + β titanium alloy Ti-6 A1-4V were investigated over the temperature range of 4.2 to 760 K. It was found that the Gibbs free energy of activation AG at 0 K and zero effective stress is ∼ 1.3 eV (∼ 1.25 × 10⁵ J/mole) (∼0.17 μ_ob³), the maximum force for the dislocation-obstacle interaction is ∼ 80 × 10^-6 dyne (∼ 80 × 10^-11N) (∼ 0.19 μ_ob²) and the activation distance x₀ ^* at which the force first rises rapidly ∼ 1.5b. These quantities, and others, are the same as those for unalloyed titanium, where it was established that interstitial solutes are the obstacles controlling the dislocation motion. The results for the Ti-6A1-4V alloy are in slightly better accord with ρG being independent of temperature than proportional to the shear modulus, but the evidence is not conclusive. Former Graduate Student, Metallurgical Engineering and Materials Science Department, University of Kentucky. Formerly Research Associate, Metallurgical Engineering and Materials Science Department, University of Kentucky     

Erosion-resistant coatings for gas turbine compressor blades

The effect of ion-plasma coatings made from high-hardness metal compounds on the erosion and corrosion resistance and the mechanical properties of alloy (substrate) + coating compositions is comprehensively studied. The effects of the thickness, composition, deposition conditions, and design of coatings based on metal nitrides and carbides on the relative gas-abrasive wear of alloy + coating compositions in a gas-abrasive flux are analyzed. The flux contains quartz sand with an average fraction of 300–350 μm; the abrasive feed rate is 200 g/min; and the angles of flux incidence are 20° (tangential flow) and 70° (near-head-on attack flow). Alloy + coating compositions based on VN, VC, Cr₃C₂, ZrN, and TiN coatings 15–30 μ m thick or more are shown to have high erosion resistance. A detailed examination of the coatings with high erosion resistance demonstrates that a zirconium nitride coating is most appropriate for protecting gas turbine compressor blades made of titanium alloys; this coating does not decrease the fatigue strength of these alloys. A chromium carbide coating is the best coating for protecting compressor steel blades.     

The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl₃, TiAl, Ti₃Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti₃Al coating deposited in nitrogen possesses high wear resistance and heat resistance.     

Surface hardening of Ti alloys by gas-phase nitridation: Kinetic control of the nitrogen surface activity

A new concept for surface hardening of titanium alloys has been developed and successfully applied to Ti-6Al-4V alloys: gas-phase nitridation under kinetic control of the nitrogen activity. This method avoids the formation of detrimental second-phase nitrides by nitriding under a very low nitrogen activity, combined with rapid diffusion of nitrogen into the specimen. The surface hardness of the Ti-6Al-4V alloy was increased by a factor of 2 to ≈ 12 GPa, with only modest attenuation of ductility. We have realized conditions for generating case depths of ≈ 25 μm in reasonable nitridation times. The nitrogen activity in the gas phase is generated by heating a powder pack of Cr and Cr₂N. A closed two-zone system allows the powder pack and the specimen to be at different temperatures, optimizing both the nitrogen partial pressure and the nitrogen diffusion into the specimen. This low-cost, conformal nitridation process generates a smoothly graded nitrogen concentration profile and can be applied to finished Ti alloy components.     

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings for hard-alloy cutting tools

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings with various contents of chromium and nitrogen are obtained by the magnetron sputtering of multiphase composite targets. Their structure and phase composition are investigated by X-ray phase analysis, transmission and scanning electron microscopy, X-ray photoelectron spectroscopy, and optical emission glow-discharge spectroscopy. The Ti-Cr-B-N and Ti-Cr-Si-C-N coatings are based on the fcc phase with texture (100) and crystallite size <25 nm. The Si₃N₄-based hexagonal phase was also revealed in the Ti-Cr-Si-C-N coatings. An investigation into the properties of coatings with the use of methods of nanoindentation, scratch testing, and by performing tribological tests showed that they have a hardness of up to 30 GPa, an adhesion strength no lower than 35 N, and their friction coefficient falls in the range of 0.35–0.57. Coatings also possess high thermal stability, resistance to oxidation, and corrosion stability in a 1N H₂SO₄ solution. The data obtained in tests of hard-alloy cutting tools indicate that the deposition of nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings increases its resistance by a factor of 11–17.     

Thermodynamic activities in the alloys of the Ti-Al-Nb system

The vaporization of the solid alloys of the Ti-Al-Nb system has been extensively studied, with emphasis on the α ₂-Ti₃Al, (α ₂-Ti₃Al+γ-TiAl), and γ-TiAl phase fields, by using Knudsen effusion mass spectrometry at temperatures between 1170 and 1635 K. Twenty-four alloy samples of different compositions, were investigated and their Ti and Al partial pressures were determined. The thermodynamic activities of Ti and Al, as well as partial enthalpies and entropies of mixing, were evaluated for the mean temperature of 1473 K. The data obtained are discussed with respect to their significance for high-temperature corrosion of titanium aluminides and the solubility behavior of Nb in the γ-TiAl phase.     

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U as a function of λ₂ is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.     

Tribological Properties of Finely-Dispersed Coatings with Electric-Spark Alloying of Materials of the System Ti ― Al ― N

Mass transfer, composition, structure, and also the tribological and corrosion properties of coatings prepared with electric-spark alloying (ESA) of titanium alloy VT6 and steels 45 and 40Kh with electrode materials of the systems Ti ― Al and TiN ― AlN with different component ratios are studied. In both cases a finely-dispersed island structure forms with high wear and corrosion resistance. The best results are obtained for ESA with intermetallics. The main phase of ESA-coatings prepared with intermetallics is Al₂O₃, and from the nitrides it is TiN. It is suggested that the difference in tribological behavior of “intermetallic” and “nitride” coatings is due to the difference in their roughness and phase composition.     

Formation and stability of a nitride with the structure of beta manganese in Ni-Cr-N ternary system

A nitride with the metal-atom arrangement of β manganese, which is designated as π phase, was confirmed to exist as an equilibrium phase in the ternary Cr-Ni-N system. A Cr-40 mass pct Ni binary alloy was nitrided under a pure nitrogen atmosphere at 1523 K to prepare a Cr-40Ni-5N alloy consisting of a nickel-rich face-centered cubic (fcc) γ phase and a dichromium nitride, Cr₂N, designated as ε phase. Upon aging the alloy at 1273 K, the π phase was formed through a peritectoid reaction between the γ and ε phases. A volume fraction of the π phase reached 95 pct after 3.6 × 10⁵ s, and no more change of the volume fraction was observed, even after 3.6 × 10⁷ s aging. The chemical composition of the π phase was determined to be Cr₁₃Ni₇N₄, whose lattice parameter wasa = 0.6323 nm. The π phase became unstable above 1473 K, which explains a previous unsuccessful attempt to produce the Cr-Ni-N ternary π phase by the replacement of molybdenum in Mo₁₂Ni₈N₄ by chromium at 1473 K.     

Combined hardening of alloy Ti-6Al sheet workpieces during reversible hydrogen alloying

The effect of the initial hydrogen concentration, warm rolling, and vacuum annealing conditions on the formation of the phase composition, structure, and mechanical properties of rolled sheet workpieces made of a Ti-6Al α alloy is studied. When the initial hydrogen concentration increases to C _Hⁱⁿⁱ = 0.3–0.9%, the grain size decreases and the phase composition of the alloy is complicated. In the grain size range 27–5 μm, the yield strength of the alloy obeys the Hall-Petch relation with the lattice friction stress σ _i = 662 MPa. When the initial hydrogen concentration increases, the grain-boundary hardening intensity and the yield strength increase. At an average α grain size of 5 μm, the yield strength increases from 770 MPa in the alloy with C _Hⁱⁿⁱ = 0.004% to 970 MPa in the alloy with C _Hⁱⁿⁱ = 0.7%. The maximum yield strength (σ_y = 1064 MPa) is obtained for the alloy with C _Hⁱⁿⁱ= 0.5% after vacuum annealing at 650°C. The conditions and contributions of solid-solution hardening, grain-boundary hardening, precipitation hardening induced by the formation of the α₂ phase, and strain hardening to the total hardening of the alloy are considered.     

Properties of nanostructured TiN-Ni ceramic-metal coatings obtained by ion-plasma vacuum-arc method

Physicomechanical and tribological properties of TiN-Ni ceramic-metal coatings prepared by ion-plasma vacuum-arc deposition are investigated. It is established that the hardness (H) increases from 23 to 54 GPa with the Ni content from 0 to 12 at %, which is determined by the influence of the nanostructured nitride component of coatings. Coefficients HE ^?1 and H ³ E ^?2, which characterize the material resistance against the elastic and plastic failure deformation, reach 0.104 and 0.567 GPa, respectively. The further increase in the nickel concentration in coatings to 26 at % leads to a decrease in H to 23–25 GPa, which is associated with the influence of the increasing amount of soft plastic metal and the formation of noticeable porosity in the bulk of coatings. The friction coefficient of studied coatings is 0.45, against 0.58 (for the TiN coating) and 0.72 (for the hard-alloy base). The cohesion failure mechanism of TiN-Ni nanostructured coatings (C _Ni = 2.8–12 at %) is established, and critical loads which characterize the appearance of the first crack (13.5–14.2 N) and the local coating attrition up to the substrate (61.9–64.4 N) are determined. The complete attrition of coatings does not occur up to a load of 90 N, which points to their high adhesion strength. The developed nanostructured ceramic-metal coatings are characterized by high heat resistance up to 800°C.     

High-Pressure Equation of the State of a Zirconium-Based Bulk Metallic Glass

The high stress U _s -U _p Hugoniot equation of state (EOS) of a zirconium-based bulk metallic glass (BMG, Zr₅₇Nb₅Cu_15.4Ni_12.6Al₁₀) was determined using plate impact experiments on disk-shaped samples of 10-mm diameter and 2-mm thickness. The National Institute for Materials Science (NIMS) two-stage light-gas gun was used for the high stress measurements (∼26 to 123 GPa), and the Georgia Institute of Technology (GT) single-stage gas gun was used for the lower stress measurements (∼5 to 26 GPa). The NIMS experiments were instrumented with streak photography and used the inclined mirror (IM) method to simultaneously measure shock velocity and free surface velocity. The GT experiments used polyvinylidene fluoride (PVDF) stress gages and velocity interferometry (VISAR) to simultaneously measure the shock velocity, free surface velocity, and stress. Results from the streak camera records and PVDF gages + VISAR traces, as well as impedance matching calculations, were used to generate the U _s -U _p Hugoniot EOS for the BMG over a wide range of stresses. The U _s -U _p data show evidence of a low pressure phase, a transition to a mixed phase region at ∼26 GPa, followed by transition at ∼67 GPa to a high-pressure phase of bulk modulus of 288 GPa. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Ceramic-metallic (TiN-Cu) nanostructured ion-plasma vacuum-arc coatings for cutting hard-alloy tools

The structure and properties of TiN-Cu coatings with a broad range of copper concentrations (C _Cu = 0.6–20 at %), which were fabricated by the ion-plasma vacuum-arc deposition on a TT10K8B hard-alloy tool, including its cutting resistant tests, were investigated. The introduction of copper into the coating composition diminishes crystallites of the nitride phase from 100 to 20 nm. The hardness of coatings increases from 20 to 40 GPa, with an increase in C _Cu to 7–8%. The further increase in the copper content, which is accompanied by diminishing crystallites of the nitride phase, is characterized by a decrease in hardness to 14–15 GPa, which is associated with the influence of soft plastic metal. Resistant cutting tests of steel 35KhGSA of removable multifaceted plates (RMP) with the TiN-Cu coatings indicate that the optimally selected composition (TiN-7-8 at % Cu) increases RMP resistance more than by a factor of 6 and 2.5 as compared with tools without the coating and with the TiN coating deposited according to the basic technology, respectively.     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Deformation kinetics of α-titanium at low temperatures using stress relaxation

Stress relaxation tests were carried out on titanium wire (0.2 at. pct O_eq) as a function of grain size (5 to 42 μm) at 77 to 623 K to study the rate-controlling mechanism. Values for the activation volume were derived from the slope of the stress relaxation curves. The Gibbs free energy of activation ΔG at σ* = 0 and OK was ∼1.4 eV (∼0.2 μo₂b²), the maximum forcef*i of the dislocation-obstacle interaction ∼80 x 10^-6 dyne (~0.2 μ₀b²) and the activation distancex* at which the force first increases rapidly ∼2b. These values are in good agreement with those obtained in the more conventional strain rate cycling tests and support the earlier conclusion that the rate controlling mechanism during the low temperature deformation of titanium is thermally activated overcoming of interstitial solute obstacles on the first order prism planes.