Study on Rare Earth-Containing Phases in TiAl Based Alloys Prepared by Non-Equilibrium Solidification Processing

Microstructure evolution of rare earth rich phase of rapidly-solidified (RS) TiAl based alloys was investigated. The two rapid-solidification techniques employed are melt-spinning technique (MS) and Hammer-and-Anvil technique (HB). MS ribbons and HA foils were obtained in the experiment. The results demonstrate that with the increasing of cooling rates of TiAl based alloys great changes are taken place in the microstructures of rare earth rich phase, from scattering mainly on grain boundaries of as-cast ingot to distributing homogeneously as very fine fibers or powders (nanometer grade) on the matrix. The fine paralleling second phase fibers in the HA foils are considered to be connected with gamma/alpha (2) lamellar colonies. Selected area electronic diffraction (SAED) patterns of the rare earth rich phase is in accordance with that of intermetallic AlCe.

Prediction and improvement of shrinkage porosity in TiAl based alloy

The present research has developed a novel investment casting process for ingot production of TiAl alloys through forming a small vertical temperature gradient on the mold.The advantage of this process is to guarantee that the castings solidify sequentially from bottom to top.The analysis of numerical simulation and experimental results showed that the shrinkage porosity of Ti-47Al-2Cr-2Nb alloy was significantly improved by forming a vertical temperature gradient of 3 oC/mm on the mold,while the increase of pouring temperature and pressure on the molten alloys had no apparent effect on the reduction of shrinkage porosity.The critical value of the Niyama criterion that can reliably predict the shrinkage porosity in Ti-47Al-2Cr-2Nb alloy was identified by the comparison of experimental and simulated results.

A NEW HEAT TREATMENT PROCESSING FOR TiAl BASED ALLOY

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Physical simulation of hot deformation of TiAl based alloy

In order to establish a model between the grain size and the process parameters, the hot deformation behaviors of Ti 49.5Al alloy was investigated by isothermal compressive tests at temperatures ranging from 800 to 1?100 ℃ with strain rates of 10 -3 10 -1 s -1 . Within this range, the deformation behavior obeys the power law relationship, which can be described using the kinetic rate equation. The stress exponent, n , has a value of about 5.0, and the apparent activation energy is about 320 J/mol, which fits well with the value estimated in previous investigations. The results show that, the dependence of flow stress on the recrystallized grain size can be expressed by the equation: σ=K 1d rex -0 56 . The relationship between the deformed microstructure and the process control parameter can be expressed by the formula: lg d rex =-0 281?1gZ +3 908?1.

Superplastic behavior of TiAl based alloy at relatively low temperatures ranging from 800℃ to 1075℃

Superplastic behaviors of TiAl based alloy with initial grain size of about 2?μm obtained by multistep forging were investigated at 800 ～ 1?075?℃ with strain rates of 8×10 -5 s -1 ～2×10 -3 s -1 . The results show that the material exhibits excellent low temperature superplasticity. Flow softing resulting from dynamic recrystallization is observed at relatively low temperatures (≤ 1?000?℃) or at higher strain rates (≥2×10 -4 s -1 ). Continuous strain hardening resulting from strain enhanced grain growth occurs at higher temperatures or at lower strain rates. A maximum elongation of 533% is obtained at 800?℃ with strain rate of 2×10 -5 s -1 , and at 1?050?℃, a maximum elongation of 570% is obtained at strain rate of 8×10 -5 s -1 . At a fixed strain rate of 2×10 -4 s -1 , when the alloy is deformed at 850?℃, the microstructure is refined, however at 1?050?℃, is coarsened. The as deformed microstructure shows relatively high strain rate sensitivity value and it keeps nearly stable during deformation. The activation energy is calculated to be 290?kJ/mol at 950 ～ 1?075?℃, with the grain size exponent, p =2, and 224?kJ/mol at 800 ～ 900?℃ with p =3. Therefore, it is suggested that the dominant mechanism during superplastic deformation at 800 ～ 900?℃ is grain boundary sliding controlled by grain boundary diffusion; however at 950 ～ 1?075?℃ is grain boundary sliding controlled by lattice diffusion.

Microstructural evolution of a forged TiAl based alloy during heat treatment at subtransus temperature

Microstructural evolution of a forged TiAl based alloy during heat treatment from 1 180 ℃ to 1 300 ℃ was investigated. The grain sizes of the alpha phases as well as the sizes and the volume fractions of the gamma phases were evaluated as a function of heat treatment temperature and time. When the alloys are isothermally heat treated at subtransus temperatures, the sizes of gamma phases(D γ) increase slightly with heat treatment temperature and time and those of alpha phases(D α) and the volume fractions of gamma phases(φ γ) vary significantly with holding time in the early stages of heat treatments, but after heat treatments for 2 h, φ γ reveal little variations with holding times and D α approach limits, which can be described by D α0 =0.65D γ/φ γ. Besides, it has been found that the alpha phases in the specimens heat treated at 1 260 ℃ and 1 300 ℃ contain lamellar structures, at low temperatures, however, appear featureless.

Synthesis of TiAl based alloy by mechanical milling, warm pressing and reactive sintering

The synthesis process TiAl based alloy by mechanical milling, warm pressing and reactive sintering was investigated. The microstructure of Ti-Al mixture powders milled was analyzed by backscattering electron image （BSE）. Then, the powders were warmly pressed in a vacuum hot-pressing device and reactively sintered at different temperatures for different times, the microstructures of TiAl based alloys after sintering were analyzed by scanning electron microscopy（SEM）, energy dispersive spectroscopy（EDS） and optical microscopy（OM）. The compositions of all stages were also analyzed by X-ray diffraction（XRD）. The results show that warm pressing can increase the relative densities of Ti-Al powder billets obviously. The billets warmly pressed with high relative density can be transformed into near fully-densified TiAl based alloy through pre-sintering at low temperature and sintering at high temperature for appropriate time, and the microstructure of the TiAl based alloy is the typical lamellar structure composed of γ-TiAl and α2-Ti3Al.

Electron Structure of γ/α2 Phase Boundaries in TiAl Based Alloys

The valence electron structure of γ/α2 phase boundaries in lamellar colonies in Ti-47Al-2M (M=Nb, Cr, V) (at. pct) was investigated by empirical electron theory of solid and molecules (EET) and its bond-length-difference (BLD) method. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac (TFD) theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that of γ/α2 interface is continuous. Furthermore, it is found that adding alloying elements (including Nb, Cr, and V) can improve the electron density (ρ) of γ/α2 interface, decrease Aρ of γ/α2 interface. With the electron structure analysis together with properties analysis,the effect mechanism of alloying elements (Nb, Cr, V) improving mechanical properties was explained.

SURFACE CARBURIZATION OF TiAl BASED ALLOY

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Effect of carbon on tribological property of plasma carburized TiAl based alloy

Plasma carburization at two different methane-to-argon gas ratios (5:5 and 6:5) was carried out on the cast TiAl based alloy of Ti-46.5Al-2.5V-1Cr (mole fraction, %) in order to enhance its wear resistance. The results show that after carburization under both carburizing atmospheres, Ti2AlC and TiC are the main carbides in the carburized layer and the value of surface hardness reaches more than HK 822, but for the carburized TiAl treated at CH4:Ar of 5:5, the surface carbon concentration is higher and the carburized depth is slightly thicker than that of alloy carburized at CH4:Ar of 6:5. The result of the ball-on-disk test against hardening-steel counter bodies shows that the wear resistance of the TiAl based alloy carburized under two different carburizing atmospheres is improved compared with non-carburized TiAl. The tribological property is related to the carbon content, and the carburized layer obtained at CH4:Ar of 5:5 possesses a stable friction coefficient, lower volume loss or wear rate and narrow wear scar. The characteristic of the carburized layer was examined by using optical microscopy, glow discharge spectrum and micro-hardness tester.

Effect of Sr on forming properties of Al-Mg-Si based alloy sheets

The effects of Sr element on the forming properties of the Al-Mg-Si based alloy sheets were studied by tensile test, metallograph, DSC, XRD, SEM and TEM. The results show that the tensile strength of aluminum alloy sheet added 0.033%(mass fraction)Sr increases comparing with that of free Sr. Simultaneously, the forming properties of sheets evidently increase, the elongation hardenability (n) and plastic strain ratio (r) and Erichsen number increase 27.8%, 11.1%, 10.8% and 12%, respectively, and the forming limit diagram increases evidently, too. The analysis shows that Sr is surface active element, which can refine grains of alloys, promote precipitation, reduce activation energy ofβ' phase, and lead the formation ofα-(Al8Fe2Si) phase instead ofβ-(Al5FeSi) phase. As a result, the forming properties of the alloy sheet increase.

Vacuum Brazing of TiAl Based Alloy with 40Cr Steel

The vacuum brazing of TiAl based alloy with 40Cr steel was investigated using Ag-Cu-Ti filler metal. The experimental results show that the Ag, Cu, Ti atoms in the filler metal and the base metal inter-diffuse toward each other during brazing and react at the interface to form an inter-metallic AlCu 2Ti compound which joins two parts to produce a brazing joint with higher strength.

X-ray elastic constant determination and residual stress of two phase TiAl-based intermetallic alloy

To evaluate the residual stress in TiAl based alloys by X ray diffraction, X ray elastic constants (REC) of a γ TiAl alloy were determined. From these results, the stress state of a given phase in a duplex TiAl based alloy under a uniaxial tensile loading has been characterized by X ray diffraction. The results show that the X ray elastic constants and the microscopic stresses of the given phase are different from the apparent elastic constants and the macroscopic stresses of the alloy. The reason of the different distribution of the alloy was also discussed. [

Effect of Lamellar Orientation on Crack Paths in PST Crystals of γ-TiAl BasedAlloys

The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.

MICROSTRUCTURES OF TiAl BASED ALLOYS PREPARED BY HOT PRESSING ELEMENTAL POWDERS

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Interface structure and formation mechanism of diffusion-bonded joints of TiAl-based alloy to titanium alloy

Vacuum diffusion bonding of a TiAl based alloy (TAD) to a titanium alloy (TC2) was carried out at 1 273 K for 15～120 min under a pressure of 25 MPa . The kinds of the reaction products and the interface structures of the joints were investigated by SEM, EPMA and XRD. Based on this, a formation mechanism of the interface structure was elucidated. Experimental and analytical results show that two reaction layers have formed during the diffusion bonding of TAD to TC2. One is Al rich α(Ti)layer adjacent to TC2,and the other is (Ti 3Al+TiAl)layer adjacent to TAD,thus the interface structure of the TAD/TC2 joints is TAD/(Ti 3Al+TiAl)/α(Ti)/TC2.This interface structure forms according to a three stage mechanism,namely(a)the occurrence of a single phase α(Ti)layer;(b)the occurrence of a duplex phase(Ti 3Al+TiAl)layer;and(c)the growth of the α(Ti)and (Ti 3Al+TiAl)layers.

JOINING OF TiAl BASEDALLOYS BY HOT PRESSING

By hot pressing in an inert atmosphere, the joining interface between coarse lamellar structures and that between a coarse lamellar structure and a fine duplex structure of a Ti 33Al 3Cr(%, mass fraction) alloy, and the effect of heat treatment on the microstructure at the interfaces have been studied. The results showed that the microstructure at the interface between two coarse lamellar structures depends on the orientations of lamellae. And microcavities are found at such interfaces. After heat treatment at 1 250 ℃, the recrystallization structure widened. As to the interface between a coarse lamellar structure and a fine duplex structure, a new duplex structure formed within the original duplex structure at the interface and further widened after subsequent heat treatment at 1 250 ℃ for 4 h. Microcavities are rare at such interfaces, and the joining appears better than that between two coarse lamellar structures.

STRENGTHENING MECHANISMSIN TiAl BASED ALLOYS

The influence of heat treatment and of thermomechanical processing on the structure and properties of a range of TiAl based alloys has been assessed and in agreement with other reports it has been found that increased refinement of the microstructure leads to improved mechanical strength at room temperature, both for the lamellar and the duplex structures. In the case of alloys cooled rapidly from the alpha phase field the increased refinement in lamellar spacing leads to significant increases in room temperature strength but thermomechanical processing can lead to far greater increases. The origin of this increase in strength in samples with a lamellar structure has been assessed in terms of the ability of dislocations to cross gamma/gamma and gamma/alpha 2 lamellar interfaces. It was concluded that the alpha 2 gamma interfaces and the alpha itself are important factors in strengthening the lamellar alloys. The stability of the various structures developed either by appropriate heat treatments or by thermomechanical processing has been investigated by exposing samples for a range of times at temperatures between 700 and 1 000 ℃. It has been found that the yield strength and the ultimate tensile strength generally decreased by about 20% during high temperature exposure at 700 ℃ for 3 000 h. The detailed behaviour on exposure at 700 ℃ has been found to be a function of alloy composition, with complex precipitates being formed in some alloys, but in all cases the amount of alpha 2 decreased with increased heat treatment time. It has been found that during exposure the alpha 2 lamellae decomposed to gamma phase by a mechanism that can involve the formation of thin gamma lamellae within the original alpha 2 lamellae.

INVESTIGATION OF MULTI-STEP THERMO-MECHANICALTREATMENT OF CAN-FORGED TiAl-BASED ALLOY

The effects of a new technology, multi-step thermo-mechanical treatment, on the microstructure, deformation substructure and mechanical properties at ambient and elevated tempemture of can-forged Ti-33 Al-3 Cr-0.5Mo (wt%) Alloy were investigated.The results show that, after multi-step thermo-mechanical treatment of can-forged specimens, homogeneous, fine microstructures can be obtained in the whole TiAl-based alloy specimens. Furthermore, the specimen with homogeneous and fine full lamellae microstructure demonstrutes excellent comprehensive mechanical properties at ambient and elevated temperature.