Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.     

Diffusion in the Ti–Al system

Many properties of industrial Ti–Al alloys, such as high-temperature stability of the lamellar structure and creep resistance, are determined by diffusion rates in the phases and along the interfaces. The knowledge of diffusion characteristics and fundamental understanding of diffusion mechanisms are of great importance to the research and design of industrial Ti–Al alloys. This paper gives an overview of recent progress in experimental and theoretical studies of diffusion behavior in the phases of the Ti–Al system. The experimental methods used in modern diffusion measurements are briefly described, and recent experimental results for Ti and Al diffusion in α-Ti(Al), β-Ti(Al), and intermetallic phases α₂-Ti₃Al and γ-TiAl, are summarized. The results for interdiffusion and impurity diffusion in these phases are also discussed in detail. The second part of the paper provides an overview of current understanding of point defects and diffusion mechanisms in Ti₃Al and TiAl. A statistical model of point-defect disorder in ordered compounds is presented and applied to Ti₃Al and TiAl using input data generated with embedded-atom potentials. Possible atomic mechanisms of diffusion in these compounds are analyzed in detail, and methods of diffusion calculations under different mechanisms are reviewed. The relative importance of different mechanisms in Ti₃Al and TiAl is evaluated by comparing their estimated activation energies. Prospective topics of further experimental and theoretical research in this area are outlined.     

Strength and fracture of two hot-rolled Ti–Al–Si–Nb dual phase alloys based on Ti3Al

Ti–Al–Si–Nb dual phase alloys are mainly composed of α₂-Ti₃Al matrix and Ti₅Si₃ silicide phases. In this paper, two alloys (402 and 405) whose Si contents are 2 and 5 at% respectively were arc melted and hot-rolled into sheets with different amounts of deformation. The silicide phase (Ti,Nb)₅(Si,Al)₃ was broken up into small pieces and redistributed in the α₂ matrix during the hot-rolling. Improved strength and ductility of the two alloys were observed after hot-rolling, which can be attributed to both the finely distributed reinforcement silicide phase and refinement of the matrix grain size. The mechanical properties of the two alloys are dependent on their volume fractions of the silicide phase: the strength of alloy 405 is higher than that of alloy 402, while alloy 402 is more ductile than alloy 405. The brittle–ductile transition temperature of the two dual phase alloys is between 600 and 800°C. The surface slip on the dual phase alloys was also observed. Obvious separation between the (Ti,Nb)₅(Si,Al)₃ particles and the α₂ matrix is found on the fracture surfaces obtained at high temperature, showing dimple-like morphology.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.     

Grain refining action of Al–5Ti–C and Al–TiC master alloys with Al–5Ti master alloy addition for commercial purity aluminium

Al–Ti–C master alloys have a great potential as efficient grain refiners for aluminium and its alloys. In the present work, the Al–5Ti–C, Al–TiC and Al–5Ti master alloys have been successfully prepared by a method of liquid solidification reactions. While the Al–5Ti–C master alloy consists of some strip- or needle-like TiAl₃, and in addition to TiC particles in the Al matrix, the Al–TiC master alloy revealed the presence of only TiC particles, and the Al–5Ti master alloy consists of only some blocky TiAl₃ particles. A united refinement technology by Al–5Ti–C+Al–5Ti and Al–TiC+Al–5Ti master alloys was put forward in this paper. The blocky TiAl₃ particles in Al–5Ti master alloy can not only improve the grain refinement efficiency of Al–5Ti–C and Al–TiC master alloys but also reduce the consumption because the blocky TiAl₃ particles improve the grain refinement efficiency of TiC particles in Al–5Ti–C and Al–TiC master alloys.     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.     

Ti/Al/Mg/Al/Ti层合板变形行为及断裂形式的原位研究

为阐明Ti/Al/Mg/Al/Ti层合板的变形行为及断裂形式,对轧制态和退火态的层合板进行原位弯曲和拉伸试验,研究组元板和金属间化合物的裂纹萌生和扩展情况.结果表明:无论Al/Mg界面有无金属间化合物,Al/Mg界面分层都是最先出现的断裂形式,因此Al/Mg界面结合强度决定层合板的力学性能.在退火态层合板中,金属间化合...     

Probing into diffusion bonding of laser surface treated γ-Ti–Al alloy/Ti–6Al–4V alloy

Abstract

In this work, a novel diffusion bonding technique combining the laser surface treatment (LST) with the diffusion bonding is used to join a γ-Ti–Al alloy with a Ti–6Al–4V alloy. By using the LST and subsequent heat treatment, a layer with a fine grain structure can be obtained on their surface of the two alloys. The diffusion bonding behaviour between γ-Ti–Al alloy and Ti–6Al–4V alloy with or without LST under the different bonding conditions is investigated. The result reveals that LST can improve the diffusion bonding behaviour of the two alloys, and the three point bending strength of the joints can be promoted. The sound bonding between the two alloys with the LST is achieved at 1173 K under 80 MPa in 2 h.     

High temperature phase equilibria near Ti–50 at% Al composition in Ti–Al system studied by directional solidification

High temperature phase diagram near the stoichiometric composition of TiAl has been established by the directional solidification and quenching technique. The quenched dendrite morphologies showed that the first solidified phase was the β phase in Ti–44, 46, 48 at% Al alloys and the α phase in Ti–50, 52 at% Al alloys. From the EDS analysis of the quenched dendrite tips and measurement of the temperature gradient directly recorded during directional solidification in Ti–(44–52 at%)Al alloys, the solid-liquid phase equilibria could be determined. The phase transformation temperatures were also confirmed by DTA. The phase equilibria established in this study agreed with the phase diagram that Okamoto proposed, while the β+γ two phase region, which Murray suggested, was not found near the TiAl composition. The lamellar orientation in TiAl alloys has been reported to be controlled in the growth direction in the presence of the primary β phase from a liquid. A composition in which the liquid phase was fully transformed to the β phase was selected, Ti–44 at% Al alloy, and directional solidification was performed at the growth rate of 45 mm/h. It was found that the lamellar orientation was aligned at nearly 0° and 45° to the growth direction. It is thought that the success in controlling the lamellar orientation is due to β solidification of the Ti–44 at% Al alloy at the beginning of liquid/solid transformation.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

Al/Ti的扩散焊工艺

根据Al合金与Ti合金在高温下具有一定的互溶性原理,采用Ti热浸Al工艺利用扩散焊接方法进行Al/Ti材料的焊接. 焊接后构件抗拉强度达到180 MPa,焊缝性能稳定,具有一定的实际应用价值.     

Ti/Al异种金属焊接研究进展

综述了Ti/Al异种金属焊接的现状,分析了钛合金与铝合金焊接缺陷产生的原因,介绍了钎焊、熔-钎焊、扩散焊、搅拌摩擦焊在钛铝异种金属焊接的应用,展望了钛铝异种金属焊接技术发展趋势。     

Investigation on TIG arc welding–brazing of Ti/Al dissimilar alloys with Al based fillers

Abstract

Dissimilar alloys of Ti–6Al–4V and 5A06 Al were butt joined by Al based fillers using a novel TIG welding process, referred to as keyhole arc welding–brazing. The flow behaviour of weld pool was introduced, which was characterised by the formation of a keyhole under the tungsten electrode. It was found that porosity tended to be produced in the middle of the fusion line, while adding elements prevented its formation. At brazing interface, interfacial reaction at root face was enhanced, and a uniform serrated layer, identified as TiAl₃, was obtained by pure Al fillers. When Al–Cu–La fillers were used, block Ti₂Al₂₀La phases appeared at the interface between the TiAl₃ layer and the brazed seam. Compared to joints brazed by pure Al fillers, the formation of Ti₂Al₂₀La reduced the hardness of the interfacial layer by more than half, while Al₂Cu increased that of the brazed seam by ～50%. The tensile strength of Ti/Al joints reached 270 MPa.     

Phase equilibria in the Ti–Al binary system

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α₂-Ti₃Al (D0₁₉) and the γ-TiAl (L1₀) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α₂/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β₂-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L1₀) and α-Ti (A3)/α₂-Ti₃Al (D0₁₉) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.     

Refining performance of Al–3Ti–0.2C–5Sr on A356 alloy and electron microanalysis of ternary Al–Ti–Sr phases

The effect of Al-3Ti-0.2C-5Sr(wt%) grain refiner on the refining performance and modification of A356 alloy was investigated using optical microscope(OM).The morphology and crystal structure of ternary Al-Ti-Sr phases in Al-3Ti-0.2C-5Sr refiner were analyzed by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The results show that the ternary Al-TiSr phases in Al-3Ti-0.2C-5Sr refiner can promote the grain refining efficiency of A356 alloy.The ternary Al-Ti-Sr phases co-exist in two morphologies,i.e.,blocky-like phase and surround-like phase,besides,which both have the same chemical composition of Al_(34)Ti_3Sr.The crystal structure of Al_(34)Ti_3Sr is face-centered cubic,and the lattice parameter is determined to be about 1.52 nm.     

Development of freeform fabrication method for Ti–Al–Ni intermetallics

A new droplet-based reactive rapid prototyping (RRP) method for making intermetallic parts is demonstrated. This method combines the processes of solid freeform fabrication (SFF) and combustion synthesis (CS). Intermetallic beads are produced by the CS reaction. These synthesized beads are assembled and joined by using the SFF technique with the aid of heat released from exothermic reactions. The intermetallic compound of Ti–Al–Ni is chosen to demonstrate this method. Ni was added to the Ti–Al system to enhance the reaction. Thermodynamics of the Ti–Al–X·NI (X=5, 10, 15 wt.%) system are calculated. The effects of the addition of Ni in powder bed and heating temperature of aluminum droplets on the width and thickness of the reacted area were investigated. As an example, a Ti–Al–Ni intermetallic bar with simple shape was fabricated as a demonstration of RRP.     

Microstructure control in two-phase Al–21Ti–23Cr alloy

To improve the mechanical properties of the Al–21Ti–23Cr two-phase alloy consisting of L1₂ matrix and 20 vol% Cr₂Al as a second phase, microstructure control was conducted through the aging treatment as a thermal process and the addition of V and Zr as conventional alloying. It was found that TiAlCr was precipitated as a third phase in the L1₂ matrix by the aging treatment at 800 and 1000°C, and its size was smaller at 800°C than at 1000°C. The yield strength of the aged alloy increased rapidly only at 800°C although the third phase was precipitated at both 800 and 1000°C. In the V-added two-phase alloys, the yield strength and the strain increased simultaneously when V was added up to 3 at%, which is attributable to the improvement in the ductility of Cr₂Al. Microstructure control conducted in this study suggests the possibility of improving the mechanical properties of L1₂ (Al,Cr)₃Ti-based two-phase alloy by precipitating the fine third phase in the L1₂ matrix and enhancing the ductility of the second phase.     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).