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.     

Physicomechanical properties and failure of detonation coatings

Conclusions The effect of the complex of external factors such as plastic deformation, temperature, chemical interaction causes structural and phase changes of powder materials in detonation coatings leading to relatively low ductility of the coating. The bend strength of the coatings up to 0.25 mm thick is similar to that of the dense materials. To produce high-quality detonation coatings, it is necessary to avoid hard temperature and kinetic parameters of the spraying process, and the extent of deformation of the particles should be limited to minimize formation, in the coatings, of internal stresses and a defective structure. Coatings made of alloyed steels with a large amount of the hardening phase are especially sensitive to defects.The physicochetnical and mechanical properties of the coatings on the substrates showed the strong mutual effect in the process of formation of the coatings and combined deformation under loading. This fact must be taken into account in selecting coating-substrate pairs for service in the conditions specified in advance.The mechanism of failure of detonation of coatings up to 0.25 mm thick produced by optimum technology in three-point bend loading does not differ from the mechanism of failure of the dense materials. With increasing thickness of the coatings their strength and ductility properties rapidly decrease and the failure mechanism also changes.Translated from Poroshkovaya Metallurgiya, No. 11(299), pp. 88–94, November, 1987.     

Structural and phase characteristics and physicomechanical properties of detonation coatings in the system aluminum oxide-titanium oxide

Conclusions Powdered A1₂O₃—TiO₂ of the highest quality, characterized by a high degree of purity and homogeneity, are obtained by the method of crystallizing melts in a cold crucible. Detonation coatings from such powders have a uniform structure, uniform phase composition, and they have a complex of good physicomechanical properties: density, great separating strength of the substrate, mechanical strength. An addition of TiO₂ to powdered aluminum oxide stabilizes the crystal lattice of the phase -Al₂O₃ which is decisive for the heat and wear resistance of coatings. Moreover, an addition of TiO₂ increases the interparticular bond in the coating and its adhesive strength with the substrate. It is indispensable to point out the selectivity of the properties of detonation coatings type AT-40 in regard to the surfaces of the protected metals. The greatest strength of adheion with the coatings is found in chromium and titanium substrates; in steel, brass, and aluminum ones the strength is somewhat lower. The high selectivity of the properties is most probably due to the difference in sublimation energy and melting points of the materials of the substrate, their hardness and brittleness. It was noted that the adhesive bond between aluminum titanate coatings and the substrates is anomalously strong; this is due to the uniqueness of its properties. Coatings and substrates have a mutual effect on the strength and plastic properties. As a rule, coatings lower the bending strength of the substrate. An exception are chromium alloys which only become embrittled without loss of strength, and substrates of ductile low alloy steels which are somewhat strengthened by coatings.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 59–64, May, 1986.     

钛酸铝对镁阿隆复合材料性能的影响

将钛酸铝作为弥散相加到镁阿隆基材料中，并研究其组成和加入量对复合材料性能的影响。结果表明：钛酸铝能显著的提高复合材料的热态强度和热震稳定性，合理设计钛酸铝的加入量可以获得性能优化的复合材料。     

Superplastic behavior of two-phase titanium aluminides

A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.     

Fracture toughness of gamma-base titanium aluminides

Effects of microstructures, alloying additions, and processing routes on the room- and elevated-temperature fracture toughnesses of gamma-base titanium aluminides were investi-gated. Microstructure was found to have a strong influence on the fracture toughness both at room and elevated temperatures. Lamellar microstructure materials exhibited high fracture toughness compared with duplex microstructure materials, which in turn possessed high fracture toughness compared with the equiaxed microstructure materials. Alloying additions affected the fracture toughness at elevated temperatures significantly. The addition of chromium improved the low-temperature fracture toughness (below 800 °), while the addition of niobium increased the fracture toughness above 800 °. Grain size refining, as a result of isothermal forging after casting and heat treatment, had less influence on the fracture toughness. Formerly Graduate Student, Nagaoka University of Technology     

铝合金表面纳米化--微弧氧化复合涂层摩擦行为

通过表面机械研磨处理在LY12CZ铝合金表面制备表面纳米化(SNC)过渡层,再采用微弧氧化(MAO)技术对纳米晶过渡层进行微结构重构,设计制备出纳米化-微弧氧化(SNC-MAO)复合涂层,并对比研究了铝合金表面微弧氧化涂层及纳米化-微弧氧化复合涂层的摩擦学行为.与微弧氧化涂层相比,纳米化-微弧氧化复合涂层因硬度较高而具有较好的耐磨性.微弧氧化涂层及纳米化-微弧氧化复合涂层与GCr15钢球对磨时具有相同的磨损机理,为对磨钢球向涂层的材料转移和氧化磨损.

     

Field-activated combustion synthesis of titanium aluminides

The feasibility of synthesizing the titanium aluminides Ti₃Al and TiAl through field-activated, self-propagating combustion synthesis is demonstrated. A self-sustaining combustion wave can be initiated only when the imposed field is above a threshold value for each of these two aluminides. At the threshold values, wave propagation resulted in an incomplete reaction between the metals and the products, which contained several phases in addition to the desired one. As the field strength was increased, the reaction approached completion and the amounts of the secondary phases decreased. At a sufficiently high field, a single-phase product was obtained in the case of Ti₃Al, but, in the case of TiAl, the product contained Ti₃Al as a secondary phase even with the highest imposed field. The effect of reactant compact density was investigated for the case of Ti₃Al synthesis. At a fixed value of imposed field, the degree of reaction completion and the conversion to the desired phase increased as the relative density decreased. These observations are discussed in light of the role of the electric field in activating the self-propagating combustion synthesis reactions and the effect of relative density on this activation. The results show that the synthesis by self-propagating high-temperature synthesis (SHS) can be optimized by the combination of field strength and relative density.     

Physicomechanical properties of detonation coatings made from stainless-steel powders

A method has been developed for determining the strength and plasticity characteristics of detonation coatings; the structure and phase factors have been used along with the physicomechanical characteristics and the adhesion to optimize the production conditions. Phase transformations occur during the formation of the coatings that govern the strength, ductility, and tribotechnical properties. In spite of the structural and phase changes that accompany coating formation, layers up to 0.25 mm thick have the properties of the compact materials.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 59–66, July–August, 1994.     

Nanostructured Arc-PVD coatings based on titanium and chromium nitrides

Multicomponent nanostructured coatings based on Ti-Cr-Al-N nitrides with a crystallite size of 10?C100 nm are formed by arc physical vacuum deposition. The dependences of the structure and phase composition of the coatings on the deposition parameters, namely, the bias potential applied to a substrate and the arc current at a chromium cathode, are found. The appearance of chromium nitride phases in the coatings is accompanied by a decrease in the crystallite size. The hardness (up to 32 GPa) and the elastic modulus (up to 700 GPa) of the coatings are determined by both the crystallite size and the microstrains (up to 0.74%) induced by chemical heterogeneity in the coatings. The adhesion strength of the coatings is estimated at 90 N. Cutting hard-alloy tools with the grown coatings are characterized by a high resistance coefficient during continuous (up to 5.1) and discontinuous (up to 5.7) cutting of 38KhNMA steel.     

Auger electron analysis of the initial oxidation of titanium aluminides based on Ti-48Al

The surface of a Ti-48 at. Pct Al alloy was examined by Auger electron microscopy to study oxidation at room temperature. On exposure to air at room temperature, both Al and Ti oxides were observed together with an abundance of C. The amount of C was always larger in the two-phase α₂ + γ region compared to the single-phase γ region. The Ti oxides formed on the surface of they grains were primarily Ti₂O₃ rather than TiO₂. On depth profiling with Ar⁺ ion sputtering, lower oxide states of Ti were found. This was attributed to either the Ar⁺ ion sputtering or the fact that the inner layers of oxide represented oxides of Ti in their lower valence states. The A1₂O₃ was stable and did not exhibit any transient oxidation states. The dominant oxidation product on the surface of sputtered single-phase γ grains after an 84-hour exposure in the ultrahigh vacuum Auger chamber at room temperature is A1₂O₃. A depletion of C and O occurred beneath the oxide surface in some γ grains. The chemical shift between the Al L_2,3MM and A1₂O₃ L_2,3(A1)M_(O)M_(O) peaks in the Auger spectrum of A1₂O₃ formed on the γ phase in TiAl was found to be 11 eV. Y.T. Peng, Graduate Student, Formerly with the Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019,     

Work-hardening and recovery mechanisms in gamma-based titanium aluminides

The work-hardening mechanisms in two-phase γ-titanium aluminide alloys were characterized in terms of the glide obstacles determining the velocity and slip path of dislocations, utilizing transmission electron microscopy (TEM) observations and thermodynamic-glide parameters. There was clear evidence that short-range obstacles in the form of dislocation debris and dipoles were produced during plastic deformation at room temperature. These dislocation obstacles contributed significantly to work hardening. The observed strong strain hardening arose from long-range elastic dislocation interactions and the production of dipole and debris defects. The thermal stability of these deformation-induced defects was assessed by isothermal and isochronal annealing. The results indicated that the dipole and debris defects were relatively unstable upon annealing at moderately high temperatures, which led to significant recovery of work hardening. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Dynamic compaction of titanium aluminides by explosively generated shock waves: Microstructure and mechanical properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[13] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.