TiAl-based Alloys Research Articles

Effect of laser shock peening on microstructure and residual stress of Ti<sub>2</sub>AlNb alloy

Ti2AlNb-based alloys are developed from conventional titanium aluminides (TiAl-based alloys and Ti3Al-based alloys). Comparing with TiAl and Ti3Al-based alloys, Ti2AlNb-based alloys have better mechanical properties of strength density ratio, high temperature fracture toughness and creep resistance. It is expected to replace traditional high temperature materials such as nickel-based alloys for the manufacture of aero-engine components. Laser shock peening (LSP) is an advanced surface modification technology which induces high amplitude, large depth residual compressive stress on the surface of materials. It can effectively enhance mechanical properties of material such as micro-hardness, fatigue resistance, high temperature oxidation performance. Compared to other surface modification technologies such as shot peening and rolling, LSP can not only induce deeper compress residual stress (1–2 mm), which is about five to ten times deeper than that of shot peening, but it can refine the grain or even form nanocrystals which plays a significant role in enhancing properties of material. For these reasons, this surface treatment technology has been widely used in the aircraft and nuclear energy industries. In this paper, the surface modification of Ti2AlNb alloy was carried out by laser shock peening, and the microstructure evolution, residual stress and the performance at high temperature were investigated. The results show that laser shock peening can significantly reduce the grain size of the surface of Ti2AlNb alloy. Dislocation lines, dislocation tangles and dislocation walls were generated after LSP. The micro-hardness is increased from 350 HV to 395 HV and the effect layer is about 1.2 mm. The residual compressive stress of −337 MPa is generated on the surface of the sample. In addition, residual stress is released obviously at high temperature. The residual stress on the surface of sample reduces to −65 MPa after heat treatment at 720°C for 4 h. The residual stress is almost completely released after heat treatment at 720°C for 10 h. However, the release rate of residual stress is slowed down at 600°C. The effect of laser shock peening is still presence after heat treatment for 100 h.

The phase transformation behavior between γ lamellae and massive γ in a Ta containing TiAl-based alloy

The phase transformation behavior between massive γ (γm) phase and γ lamellae was studied through interrupted quenching from single α-phase region and α+γ two-phase region in a Ti–48Al–3Nb-0.5Ta alloy. Massive α→γm phase transformation temperature T0 is confirmed to be 1400 °C (in α+γ two-phase region). Combining with the thermodynamic analysis, it indicates that the formation of γm phase is gradually inhibited with the decrease of quenching temperature, instead fine γ lamellae are preferred. In this case EBSD (Electron back scattered diffraction) and HRTEM (high resolution transmission electron microscope) have shown that besides nucleating on α grain boundary, γm phase can nucleate on defects in the interior of an α grain with Blackburn orientation relationship or nucleate on fine γ lamellae along close-packed {111}γ with orientation relationship of [121‾] γlamellae//[1‾11]γm1// [101‾1‾]α2 at their interface. Furthermore, the ledge growth mechanism of γm phase is identified.

Study on High Temperature Tensile Properties and Microstructure of a Tial-Based Alloys

The high temperature tensile test of a Ti-45Al-5Nb alloy was carried out at deformation temperatures of 880 °C and 1000 °C, with strain rates of 0.001 s−1 and 0.0125 s−1, respectively. Scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD) were used to analyse the microstructure of different tensile parts. The results showed that under the deformation conditions of 880 °C / 0.001 s−1 and 880 °C / 0.0125 s−1, the alloy underwent brittle fracture, and microstructure was basically unchanged, compared with the original one. Deformed at 1000 °C/0.001s−1, the alloy exhibited plastic deformation and mixed brittle and plastic fracture. Dynamic recrystallization was the main softening mechanism, and the γ phase and B2 phase contents increased by 6.7% and 190.2%, respectively, compared with that deformed at 880 °C/0.0125s−1.

ТЕМПЕРАТУРНА ТА ШВИДКІСНА ЧУТЛИВІСТЬ МЕХАНІЧНИХ ВЛАСТИВОСТЕЙ В ПЕРСПЕКТИВНИХ СПЛАВАХ TNM


 
 
 The temperature and velocity sensitivity of the mechanical properties of TiAl-based alloys were investigated. The structure was determined by scanning electron microscopy in SEI mode. Mechanical properties were determined by the results of tensile and bending tests. It was found that the temperature and velocity dependence of the mechanical properties in alloys with 47% Al is practically absent. This makes them promising for use as structural materials, due to the stability of their properties.
 
 

Vacuum brazing of TiAl-based alloy and GH536 superalloy with a low-melting point amorphous Ti35Zr25Be30Co10 filler

The γ-TiAl-based alloy and Ni-based superalloy (GH536) were successfully brazed utilizing a low-melting-point amorphous Ti35Zr25Be30Co10 filler below 800℃. Before brazing, the wetting ability of the filler was investigated and it was found that the amorphous filler exhibited a good wetting ability with a wetting angle less than 30°. A sound joint with no cracks and holes was obtained after brazing and dendritic phases formed in the wetting interfacial regions and across the brazed joint. The structural evolution of the wetted interfaces and brazed joints were studied by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS). The results show that temperature played an important role in the formation of the dendritic phases. Coarse grains were presented and dendritic phases decreased when the joint was brazed at a higher temperature, which seriously deteriorate the shear strength of the joint. The shear tests showed that the maximum shear strength of the joint was 300 MPa when the brazing temperature and holding time were 770℃ and 20 min.

Effect of Y2O3 on physical and mechanical properties of TiAlbased alloys

Ti-45Al-5Nb alloy powders were used as the matrix, and added different contents of Y2O3. The TiAl-based alloys with excellent mechanical properties were prepared by vacuum hot pressing sintering technology. Analysed the effect about Y2O3 on the properties of the alloys. The results show that trace amounts of Y2O3 can effectively improve the physical and mechanical properties and refine the grain size of the alloys. When 0.25% Y2O3, the TiAl alloy exhibits excellent hardness, density, flexural strength and fracture toughness. TiAl-based alloys are mainly composed of brittle fracture.

Effect of TiB2 on Properties of TiAl-based Alloy

In this paper, we added particle reinforced phase TiB2 in TiAl-based composites, TiB2/TiAl matrix composites were prepared by using powder metallurgy vacuum hot pressing sintering technology, and the effects of TiB2 particle reinforced phase on the mechanical and tribological properties of TiAl-based alloy were studied. It was found that the hardness and density of TiAl-based alloy composites increased with the addition of TiB2, the friction coefficient and the wear loss decreased with the increase of TiB2 content in TiAl-based alloy. The wear resistance of TiAl-based alloy can be improved by adding TiB2 particles.

On the Microstructure and Solidification Behavior of N-Bearing Ti-46Al-8Ta (at.%) Intermetallic Alloys

Ti-46Al-8Ta (at.%) intermetallic alloys exhibit acceptable strength at low and high temperatures with good oxidation resistance. Different alloying elements are usually added to TiAl-based alloys to develop required properties. In this work, effects of N addition up to 2 at.% on microstructural features and solidification behavior of the Ti-46Al-8Ta alloy is investigated. The achieved results showed that alloying with N causes dendrite refinement and reduction of the retained metastable B2 phase in the as-cast microstructure. The dendrite morphology is also changed from fourfold to fivefold and then sixfold symmetry by increasing N content. The effect of N on interlamellar spacing, amount of constituent phases, and lattice parameters was not uniform. Increasing N content resulted in an increase in both small punch maximum load and hardness up to 1.05 at.%, whereas further N alloying, despite increasing of hardness, led to a drop in the maximum load.

Oxygen gain and aluminum loss during laser metal deposition of intermetallic TiAl

For manufacturing turbine parts for the aeronautical industry, lightweight structural materials like titanium aluminides are used. One well known example is the alloy Ti-43.5Al-4Nb-1Mo-0.1B (atomic percent) with the commercial name TNM™. In previous studies, experiments on laser metal deposition of TiAl were performed to evaluate the potential of the technology to repair defective, damaged, or worn blades. Process conditions including postprocess heat treatment were successfully adjusted. However, oxygen pick-up is still a crucial aspect in laser based additive manufacturing of Ti- and TiAl-based alloys. In this work, approaches were made to analyze the oxygen pick-up as a function of processing parameters and the powder particle size. The results show that the build rate and the powder particle size affect the oxygen content in the consolidated material.

Enhanced oxidation resistance of TiAlNbCr processed by isothermal forging

Poor ductility at room temperature and oxidation resistance at high temperature have hindered the wide usages of TiAl-based alloys, especially in advanced aero-engines. Room temperature ductility of TiAl-based alloys can be improved by grain refinement through the isothermal forging (ISF) method, whereas their oxidation behavior is unknown. In the present study, we investigate the oxidation behavior of TiAlNbCr -casting and -2nd-ISF alloys through the experiment of oxidation weight gain. Structure and phase characteristics before and after oxidation are examined through scanning electron microscopy, transmission electron microscopy and an electron probe microanalyzer. The results indicate that TiAlNbCr-2nd-ISF has higher oxidation resistance than that of TiAlNbCr-casting. The smaller grain size and more uniform structure of TiAlNbCr-2nd-ISF lead to the formation of smaller TiO2 particles on the alloy surface after oxidation, which is beneficial in hindering further diffusion of oxygen.

Effect of temperature-related factors on densification, microstructure and mechanical properties of powder metallurgy TiAl-based alloys

In the present work, the influence of temperature-related factors, including sintering temperature, heating step and temperature-control mode, on the densification, microstructure and mechanical properties of Ti-46.5Al-2.15Cr-1.90Nb-(B, Y, Mo) alloys prepared by SPS has been investigated and discussed in detail. The results obviously indicate that the sintering temperature plays a key role on densification and phase transition, when compared with the heating step and temperature-control mode. Based on the experimental results and theoretical analysis, the densification process and microstructural evolution of TiAl-based alloys during sintering are studied. Moreover, the mechanical properties of the sintered alloys are determined by the combined effects of the densification and microstructure. The obtained results will help to optimize the microstructure and properties for this kind of intermetallic alloys through controlling sintering parameters during powder metallurgy process.

Research on aluminum component change and phase transformation of TiAl-based alloy in electron beam selective melting process under multiple scan

With the developments and applications of additive manufacturing technologies, additive manufacturing of hard-workable TiAl-based alloys has been widely studied. Electron beam selective melting (EBSM) is regarded as a promising additive manufacturing process with the advantages of high energy efficiency and high vacuum environment to avoid the cracks and oxidation of the brittle TiAl-based alloys. However, element loss and phase change due to evaporation in vacuum would alters the microstructure and mechanical properties. In this paper, the influence of evaporation on microstructure is investigated with higher element evaporation than that in regular EBSM process in order to understanding the effect exhaustively. A multiple low energy scan coupled with multi-region scan strategy, instead of conventional single layer scan, is utilized to enhance the element evaporation without causing the curly surface in scan area, which could damage the powder spread rake and deposition quality of subsequent layers. The results show that the atomic ratio of aluminum decreases with increasing energy input and vacuum degree and varies little in overlap between two adjacent scan region. The new (α+β) phase was formed by high evaporation of aluminum and the (α+β+α2) phases alloy exhibited the tensile strength of 947 MPa and elongation of 2.2% at room temperature.

Microstructural Evolution and Refinement Mechanism of a Beta–Gamma TiAl-Based Alloy during Multidirectional Isothermal Forging

Multidirectional isothermal forging (MDIF) was used on a Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B (at. %) alloy to obtain a crack-free pancake. The microstructural evolution, such as dynamic recovery and recrystallization behavior, were investigated using electron backscattered diffraction and transmission electron microscopy methods. The MDIF broke down the initial near-lamellar microstructure and produced a refined and homogeneous duplex microstructure. γ grains were effectively refined from 3.6 μm to 1.6 μm after the second step of isothermal forging. The ultimate tensile strength at ambient temperature and the elongation at 800 °C increased significantly after isothermal forging. β/B2→α2 transition occurred during intermediate annealing, and α2 + γ→β/B2 transition occurred during the second step of isothermal forging. The refinement mechanism of the first-step isothermal forging process involved the conversion of the lamellar structure and discontinuous dynamic recrystallization (DDRX) of γ grains in the original mixture-phase region. The lamellar conversion included continuous dynamic recrystallization and DDRX of the γ laths and bugling of the γ phase. DDRX behavior of γ grains dominated the refinement mechanism of the second step of isothermal forging.

Non-Isothermal Oxidation Behaviors and Mechanisms of Ti-Al Intermetallic Compounds.

Non-isothermal oxidation is one of the important issues for the safe application of Ti-Al alloys, so this study aimed to illustrate the non-isothermal oxidation behaviors and the corresponding mechanisms of a TiAl-based alloy in comparison with a Ti3Al-based alloy. The non-isothermal oxidation behaviors of Ti-46Al-2Cr-5Nb and Ti-24Al-15Nb-1.5Mo alloys in pure oxygen were comparatively investigated with a thermogravimetry-differential scanning calorimetry (TGA/DSC) simultaneous thermal analyzer heating from room temperature to 1450 °C with a heating rate of 40 °C/min. When the temperature rose above 1280 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy sharply increased and exceeded that of the Ti-24Al-15Nb-1.5Mo alloy owing to the occurrence of internal oxidation. When the temperature was higher than 1350 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy decreased obviously due to the generation of an oxygen-barrier β-Al2TiO5-rich layer by a chemical reaction between Al2O3 and TiO2 in the oxide scale. Based on Wagner’s theory of internal oxidation, the reason for the occurrence of internal oxidation in the Ti-46Al-2Cr-5Nb alloy is the formation of the α phase in the subsurface, while no internal oxidation occurred in the Ti-24Al-15Nb-1.5Mo alloy due to the existence of the β phase in the subsurface with the enrichment of Nb and Mo.

Vacuum brazing of TiAl-based alloy using Ti-Zr-Fe filler metal

A Ti-Zr-Fe brazing alloy was designed and fabricated into foils by rapid solidification technique for joining of TiAl-based alloy. The phase characteristics of typical joint interface microstructure were analyzed, and the influence of process parameters on the interfacial microstructure and mechanical properties was discussed. The results showed that the joint interface consists of Ti2Al, Ti3Al (Zr, Fe), Ti-based solid solution, and (Ti, Zr)2Fe phase enriched with 10.0~15.0 at.% Al. Under the brazing condition of 1283 K/10 min, the joints exhibited a shear strength of 221.3 MPa. The joint showed a brittle fracture and the fracture propagated through the entire brazing seam.

Feasibility study of a flexible grinding method for precision machining of the TiAl-based alloy

ABSTRACTThis study presents detailed experimental investigations on precision machining of the TiAl-based alloy with an abrasive belt flexible grinding method. Subsequently, the feasibility of this precision machining method is evaluated with respect to the material removal rate, abrasive wear, machined surface roughness, and residual stress. The material removal rate and surface roughness were determined as experimental indicators and were measured via a three-coordinate measuring instrument and surface profiler, respectively. Micro-morphologies of the machined surface and worn abrasive belt were investigated via a scanning electron microscope. The residual stress distributions in the machined surface layer were detected by using an X-ray diffractometer. The experimental results revealed that the aforementioned evaluation indicators satisfied the desired requirements, thereby indicating that the abrasive belt flexible grinding technique was suitable for precision machining of the TiAl-based alloy. Additionally, the optimal combinations of grinding parameters were determined to obtain desirable material removal rate and machined surface roughness. The basic wear processes and characteristics of the abrasive belt were thoroughly examined. The formation of desirable residual compressive stresses in the machined surface layer was mainly attributed to low frequency and small amplitude vibration knocking at the grinding interface.

Hot deformation and dynamic recrystallization behavior of TiAl-based alloy

Hot compression and dynamic recrystallization (DRX) behavior of TiAl alloy (Ti-44Al-4Nb-1.0Mo-0.1B-0.01Y) were investigated by hot compression tests. The tests were conducted at temperature range of 1120–1240 °C with strain rate of 10−3∼1 s−1. Results show that the flow stress during hot compression is affected by deformation temperature and strain rate. The stress exponent n and activation energy Q are measured to be 2.711 and 524.67 kJ/mol, respectively. Critical strain for DRX initiation is measured, and the relationship between critical strain εc and peak strain εp can be expressed as: εc = (0.7–0.9) εp. It is found that crack in β phase as well as the decomposition of β phase result from inhomogeneous deformation between γ phase and β phase. With the decreasing of lnZ, volume fraction (referred to Vf) of lamellar colony decreases and α phase gradually “invade” from the periphery to the center part of lamellar colony. Two DRX models during the hot deformation of TiAl alloy are developed. Type I relies on the nucleation within lamellae. Type II is the DRX of α phase. If lnZ>39.6, only type I can be found in the microstructure. If 36.1<lnZ<39.6, the microstructure contains the mixture of type I and type II DRX grains. While lnZ<36.1, only type II DRX grains are left. The guiding principle can well predict the deformed microstructure of TiAl alloy.

Microstructures and mechanical properties of Ti3Al/Ni-based superalloy joints brazed with AuNi filler metal

For the purpose of high-temperature service and the weight reduction in aviation engineering applications, the dissimilar joining of Ti3Al-based alloy to Ni-based superalloy (GH536) was conducted using Au-17.5Ni (wt%) brazing filler metal. The microstructure and chemical composition at the interfaces were investigated by scanning electron microscope, X-ray diffraction and transmission electron microscope. The diffusion behaviors of elements were analyzed as well. The results indicated that the Ti3Al/GH536 joint microstructure was characterized by multiple layer structures. Element Ni from Au-Ni filler metal reacted with Ti3Al base metal, leading to the formation of AlNi2Ti and NiTi compounds. Element Ni from Ti3Al base metal reacted with Ni and thus Ni3Nb phase was detected in the joint central area. Due to the dissolution of Ni-based superalloy, (Ni,Au) solid solution ((Ni,Au)ss) and Ni-rich phase were visible adjacent to the superalloy side. The average tensile strength of all the joints brazed at 1253 K for 5–20 min was above 356 MPa at room-temperature. In particular, the joints brazed at 1253 K/15 min presented the maximum tensile strength of 434 MPa at room-temperature, and the strength of 314 MPa was maintained at 923 K. AlNi2Ti compound resulted in the highest hardness area and the fracture of the samples subjected to the tensile test mainly occurred in this zone.

Microstructural evolution and growth kinetics of interfacial compounds in TiAl/Ti3SiC2 diffusion bonding joints

TiAl intermetallic was direct diffusion bonded to Ti3SiC2 at 900 °C, 950 °C, and 1000 °C with a holding time ranging from 0.25 h to 64 h. The interfacial microstructural evolution, the growth kinetics and the mechanical properties of the bonded TiAl/Ti3SiC2 joints were systematically investigated. The build-up of interfacial compounds in the TiAl/Ti3SiC2 joints changed from γ+α2/γ/Ti5Si3/Ti3SiC2 to γ+α2/γ/TiAl2/Ti5Si3/Ti3SiC2, to γ+α2/γ/TiAl2/Ti5Si4/Ti5Si3/Ti3SiC2, and to γ+α2/γ/TiAl2/TiAl3+Ti5Si3/Ti5Si4/Ti5Si3/Ti3SiC2 as the holding time or bonding temperature increased. The formation sequence of interfacial phase at the joint was: Ti5Si3→Ti5Si4→TiAl2→TiAl3. The de-intercalation Si atoms from the Ti3SiC2 diffusing into the TiAl base alloy and reacting with the Ti was responsible for the formation of the interfacial reaction layers. The thicknesses of the interfacial reaction layers grew following the parabolic kinetic law, indicated their growth was controlled by the bulk diffusion. The bonding activation energy was 213 kJ/mol. The joint, bonded at 900 °C for 9 h, had the highest shear strength (about 53 MPa) with a γ+α2/γ/TiAl2/Ti5Si4/Ti5Si3/Ti3SiC2 interfacial microstructure and with a total thickness of 6 μm.

Microstructural Characterization by Automated Crystal Orientation and Phase Mapping by Precession Electron Diffraction in TEM: Application to Hot Deformation of a γ-TiAl-based Alloy.

Microstructural evolution of a hot deformed γ-TiAl-based Ti-45Al-8Nb-2Cr-0.2B (at.%) alloy has been studied using an advanced characterization technique called automated crystal orientation and phase mapping by precession electron diffraction carried out in a transmission electron microscope (with a NanoMEGAS attachment). It has been observed that the technique, having a capability to recognize diffraction patterns with improved accuracy and reliability, is particularly suitable for characterization of complex microstructural features evolved during hot deformation of multiphase (α2 + γ + β)-based TiAl alloys. Examples of coupled orientations and phase maps of the present alloy demonstrate that the accurate reproduction of the very fine lamellar structure (α2 + γ + γ) is feasible due to its inherent high-spatial resolution and absence of a pseudo-symmetry effect. It enables identification of salient features of γ-TiAl deformation behavior in terms of misorientation analyses (GAM, GOS, and KAM) and transformation characteristics of very fine lamellar constituent phases. Apart from conventional strain analyses from the orientation database, an attempt has been made to image the dislocation sub-structure of γ-phases, which supplements the deformation structure evaluation using this new technique.