Papers by Keyword: TiAl Intermetallics

Abstract: The article presents the first attempt to CVD-aluminise alloys based on an intermetallic phase Ti48Al2Cr2Nb. Moreover, it includes initial results of producing VPA-deposited aluminide coating in industrial environment. Microstructure and phase analyses of the obtained coatings have been conducted. The chemical and phase composition analyses have revealed that the CVD-deposited coating was roughly 8 m thick, and composed of aluminium-rich TiAl phase, whereas the application of VPA method results in a coating which is approximately 18 m thick and consists of three layers made up of TiAl3, TiAl2 i TiAl phases. Both deposition processes were conducted with industrial equipment.

Abstract: The microstructure evolution of TiAl intermetallics with different microstructures loaded under different strain rates and temperatures was investigated. The results showed that the deformation twinning dominated the deformation process under high strain rate, while dislocation slip was another dominating deformation mode under quasi-static loadings. The proportion of twinned grain increased with the increased strain rate. In Duplex TiAl, the plastic deformation was mainly found in equaxied grains and seldom found in lamellar grains.

Abstract: The microstructure evolution of Ti-46.5Al-2Nb-2Cr with different microstructure types loaded under a large range of strain rates and elevated temperatures is investigated by TEM. The results show that deformation twins are the main deformation mode under high strain rate loadings and both ordinary dislocation and super-dislocation are the additional modes under quasi-static loadings. The proportion of twinned grains increases with the increased strain rates.

Abstract: The first results are presented of ESA MAXUS 8 sounding rocket benchmark experiment on γ-TiAl (Nb,B) intermetallics solidification, that was a part of the IMPRESS EU FP6 project. Having the aim to investigate the columnar and equiaxed primary microstructures formation at diffusion controlled melt growth, an experiment was designed applying the power-down directional solidification technique with fixing quench in automatic spaceborne furnace module TEM03-5M. Two related alloys were tested: one inoculated with boron grain refiner Ti-44Al-7.5Nb-2.7B (at.%); and the other Ti-45.5Al-8Nb (at.%) without grain refinement. The series of terrestrial reference processes has been performed accordingly in counter-gravity solidification direction. The numerical studies of heat-mass transfer, melt hydrodynamics and temporal solidification dynamics of these alloys have been implemented under the earth-and zero-gravity approximations using the GIGAN software package (IPPE). The comparison is performed of numerical model findings with the real microstructure and composition of samples, whose analyses were carried out by backscattered scanning electron microscopy (SEM) and X-ray microprobe (EDX) techniques. The convection-induced peritectic macrosegregation effect appearance and development in ground-based reference ingots is analytically observed and numerically studied. The achieved microstructure formation in space benchmarks is discussed.

Abstract: In this paper, TiH₂-47Al-5Nb (at.%) and TiH₂-47Al-7Nb(at.%) alloys were mixed and synthesized using TiH₂, Al and Nb powders. The composition and morphology evolution of the mixed powder were systematically investigated during high energy ball milling. The results show obvious that structure change of the particle during milling, and amorphous, TiAl, Ti₃Al and Ti₂Al phases at nanoscale are formed. The addition of Nb shows an active influence on the decomposition of TiH₂ and formation of TiAl-intermetallics. Compare with Ti-Al system alloy, the forming process of TiAl-intermetallics for TiH₂-Al-Nb system alloy is different and slower. Ti₂Al metastable phase formed after ball milling for 15 h in our experiments.

Abstract: The effect of temperature and strain rate on the mechanical behavior and microstructure evolution of Near Gamma Ti-46.5Al-2Nb-2Cr (NG TiAl) was investigated at temperatures ranging from room temperatures to 840 under strain rates of 0.001, 320, 800 and 1350s-1. The TEM analysis indicated that deformation twinning and stacking fault are the main deformation modes under dynamic loadings and dislocation slip is another important deformation mode under quasi-static loadings. The density of deformation twinning and/or stacking fault increases with the increased temperature and strain rate.

Abstract: The problem of numerical modeling of directional solidification of TiAl refractory intermetallics aboard the MAXUS 8 sounding rocket is considered. The research is of relevance to the FP6 Integrated project IMPRESS (Intermetallic Materials Processing in Relation to Earth and Space Solidification). Attention is paid to columnar-to-equiaxed microstructure transition (CET) phenomenon and mushy zone evolution in Ti-45.9Al-8Nb (at %) alloy being processed in TEM 01-3M high-temperature (up to 17000C) furnace. In this three-zone resistive furnace the “bent” temperature profile is applied with two strongly different axial thermal gradients, presumably allowing the achieving of CET conditions along the sample of 160 mm length. Temperature profile evolution is defined by power-down furnace operation. 2D-numerical study of heat transfer and realtime-scale solidification dynamics of TiAl-Nb under zero gravity approximation is performed. The approaches used for solution of Navier-Stokes equations and phase transition (Stefan) problem are briefly described. The solidification time is shown to be satisfying the 12-minute microgravity limit aboard a MAXUS. The position and the time at which CET may be triggered are predicted and confirmed in line with the Hunt diagram. The comparison is performed of model predictions with the real microstructure of TiAl-Nb reference sample solidified on-ground in TEM 01-3M facility.

Abstract: Mechanical properties of the TiAl alloy produced by centrifugal spray deposition (CSD), compared to that produced by ingot metallurgy (IM), were investigated at different temperatures from 293 to 973K. The result shows that the ultimate strength, yield strength and plasticity of the CSD TiAl alloys, with excellent compression properties and plasticity, are higher than those of as-cast TiAl alloys at room temperature as well as at high temperature. There exists a critical temperature of 873K in the relationship between strength and temperature, in which strength increases with increasing temperature above 873K. The effects of CSD on mechanical properties of the TiAl alloy are discussed, and the higher strength with moderate ductility achieved is because of the finer lamellar structure got in the CSD processing, and this structure is also believed to be beneficial to ductility.

Abstract: The γ-TiAl intermetallic compounds were produced at the temperature ranging from 850°C to 1050°C by the Spark Plasma Sintering (SPS) process. The effects of sintering temperature and holding time on the mechanical properties of γ-TiAl intermetallic compounds were investigated. The γ-TiAl intermetallic compounds sintered at 1050°C for 10 min showed a high relative density more than 98%, and had the best three-point bending strength of 643MPa, fracture toughness of 12 MPa·m1/2 and microhardness of 560MPa. The microstructural observations indicated typical characteristics of intergranular fracture, which meant the poor ductility of γ-TiAl intermetallic compounds.

Abstract: The research involves the development of a powder metallurgical route for producing good quality TiAl targets for making physical vapour deposition (PVD) coatings. Mixtures of elemental titanium and aluminium powders were mechanically milled using a novel discus milling technique under various conditions. Hot isostatic pressing (HIP) was then employed for consolidation of the mechanically alloyed powders. A cathodic arc vapour deposition process was applied to produce a TiAlN coating. A microstructural examination was conducted on the target material and PVD coatings, using X-ray diffractometry (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). It has been found that combining mechanical alloying and HIP enable us to produce a fairly good quality of TiAl based target. The PVD coatings obtained from the TiAl target showed very high microhardness values.