Fine Α2 Research Articles

Enhancement of Microstructure and Mechanical Properties of High Nb-TiAl Alloys Prepared Using Laser Additive Manufacturing by Annealing Treatment

The microstructure, phase composition, hardness, and tensile properties of the Ti-48Al-2Cr-5Nb alloy have been systematically investigated using laser additive manufacturing technology. Results indicate that both the as-deposited and annealed microstructures contain both the α2 (Ti3Al) and γ (TiAl) phases. As the annealing temperature increased, the structure changed significantly from a large block structure to a fine equiaxed structure and finally to a large lamellar structure. Nevertheless, the amount and distribution of precipitation of α2 phase are obviously different, especially during the annealing at 1260 °C, where the fine α2 phases are evenly distributed on the γ phase matrix. The hardness value of the as-deposited sample is the highest, with a HV value of 484 at the room temperature, while the hardness value of the annealed sample at 1260 °C is the smallest, with a HV value of 344. An annealed sample at 1260 °C exhibits the highest tensile strength and elongation at room temperature, with values of 598 MPa and 2.1%, respectively. These values are increased by 1.15 times and 1.4 times compared to the as-deposited sample (519 MPa, 1.5%).

Microstructural evolution and embrittlement of a β-solidifying γ-TiAl alloy during exposure at 700 °C

The microstructural evolution and the room-temperature tensile properties of fully lamellar Ti-43.5Al–4Nb–1Mo-0.5B alloy exposed at 700 °C for 500 h in air were investigated. In bulk microstructure, the precipitation and growth of ellipsoidal ω0 particles were found in equiaxed β0 grains, and the parallel decomposition of coarse α2 lamellae into fine α2 + γ lamellar packets was observed. The α2 lamellae near the surface decomposed first into fine α2 + γ lamellar packets and then blocky γ grains after the full consumption of the α2 phase. A mixed oxide scale with obvious stratification mainly composed of TiO2 and Al2O3 formed on the surface of the specimens. The outward diffusion of Ti atoms on the specimen surface led to the formation of an Al-rich and Ti-lean blocky γ zone between the matrix and the oxide scale. The degradation of room-temperature mechanical properties of the alloy after exposure was primarily caused by the evolution of subsurface microstructure, and the evolution of bulk microstructure had little effect on the mechanical properties at this stage. Under axial stress, microcracks nucleated from the tip of a V-shaped “notch microstructure” at the interface between the blocky γ zone and the matrix, causing premature failure. Removal of the surface layer of the thermally exposed alloy led to room temperature mechanical properties essentially similar to those before thermal exposure.

Effects of annealing on microstructure and mechanical properties of γ-TiAl alloy fabricated via laser melting deposition

The microstructure evolution and mechanical properties of the as-deposited γ-TiAl-based alloy specimen fabricated via laser melting deposition and as-annealed specimens at different temperatures were investigated. The results show that the microstructure of as-deposited specimen is composed of fine α2(Ti3Al)+γ lamellae. With the increase of annealing temperature, the bulk γm (TiAl) phase gradually changes from single γ phase to γ phase + acicular α2 phase, finally small γ phase + lamellar α2+γ phase. Compared with the mechanical properties of as-deposited γ-TiAl alloy (tensile strength 469 MPa, elongation 1.1%), after annealing at 1260 °C for 30 min followed by furnace cooling (FC), the room-temperature tensile strength of the specimen is 543.4 MPa and the elongation is 3.7%, which are obviously improved.

Influences of thermal exposure on the microstructural evolution and subsequent mechanical properties of a near-α high temperature titanium alloy

The microstructural evolution with respect to the precipitation of silicides and ordered α2 phase in near-α TA29 Ti alloy during thermal exposure at 650 °C for different time has been investigated in this work, and the subsequent mechanical properties have been evaluated by tensile and fracture toughness tests. The results indicated that the (Ti, Zr)6Si3 silicides firstly precipitated in residual β films, and then began to precipitate within α plates. A large number of fine α2 precipitates showing sphercial shape concurrently formed in the α plates at a short time. The ordered α2 precipitates asymmetrically grew up along different crystallographic directions with increasing exposure time, leading to their spherical shape change to ellipsoidal shape and finally olive shape, in which the long axis was perpendicular to [0001]α as viewed along [2‾110]α direction. The number density of ordered α2 was obviously decreased after 1000 h due to Ostwald ripening. The precipitation of silicides and ordered α2 phase improved the tensile strength but deteriorated the ductility and fracture toughness, whereas the strength was decreased and the room temperature elongation was recovered slightly after long-term exposure because of α2 ripening.

Fabrication of Metal Matrix Composite by Laser Metal Deposition-A New Process Approach by Direct Dry Injection of Nanopowders.

Laser Metal Deposition (LMD) offers new perspectives for the fabrication of metal matrix nanocomposites (MMnCs). Current methods to produce MMnCs by LMD systematically involve the premixing of the nanopowders and the micropowders or require in-situ strategies, thereby restricting the possibilities to adjust the nature, content and location of the nano-reinforcement during printing. The objective of this study is to overcome such restrictions and propose a new process approach by direct injection of nanoparticles into a metallic matrix. Alumina (n-Al2O3) nanoparticles were introduced into a titanium matrix by using two different direct dry injection modes in order to locally increase the hardness. Energy dispersive X-ray spectroscopy (EDS) analyses validate the successful incorporation of the n-Al2O3 at chosen locations. Optical and high resolution transmission electron microscopic (HR-TEM) observations as well as X-ray diffraction (XRD) analyses indicate that n-Al2O3 powders are partly or totally dissolved into the Ti melted pool leading to the in-situ formation of a composite consisting of fine α2 lamellar microstructure within a Ti matrix and a solid solution with oxygen. Mechanical tests show a significant increase in hardness with the increase of injected n-Al2O3 amount. A maximum of 620 HV was measured that is almost 4 times higher than the pure LMD-printed Ti structure.

Selective electron beam melting of TiAl alloy: Microstructure evolution, phase transformation and microhardness

In this research, a Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting (SEBM). The microstructures at different heights of SEBM-produced TiAl alloy sample and their contribution to microhardness were investigated. The results revealed that the last deposited layers displayed near lamellar structure with fine α2 lamellae. Lamellar colonies degraded at the height of 19 mm of the as-built sample, accompanied with the α2 lamellae decomposition and B2 precipitation along primary α2 lamellae. Meanwhile, equiaxed γ and blocky B2 phase dispersed on the colony boundaries. Lamellar colonies further degraded and colony boundaries further coarsen at the heights of 15 mm and 5 mm due to the in-situ heat treatment during SEBM process. Three kinds of phase transformations could be observed during SEBM process, which were α2 → γ, α2 → B2 and α2 + γ → B2. The microhardness exhibited slight fluctuation as the building height increased from 3 mm to 19 mm, and the microhardness rapidly increased with the building height increasing from 19 mm to 20 mm. Eventually, the mechanism of microstructure evolution during SEBM process was discussed in detail.

Microstructural Characterization of Melt Extracted High-Nb-Containing TiAl-Based Fiber.

The microstructure of melt extracted Ti-44Al-8Nb-0.2W-0.2B-1.5Si fiber were investigated. When the rotation speed increased from 2000 to 2600 r/min, the appearance of the wire was uniform with no Rayleigh-wave default. The structure was mainly composed of fine α2 (α) phase dendritic crystal and a second phase between dendrite arms and grain boundaries. The precipitated second phases were confirmed to be Ti5Si3 from the eutectic reaction L→Ti5Si3 + α and TiB. As the lower content of Si and higher cooling rate, a divorced eutectic microstructure was obtained. Segregation of Ti, Nb, B, Si, and Al occurred during rapid solidification.

Microstructural refinement mechanism by controlling heating process in multiphase materials with particular reference to γ-TiAl

The microstructural refinement mechanism by controlling heating rate was investigated in TiAl alloys as an example of multiphase materials. During heating in the α+γ dual phase field of TiAl alloys, the phase transformation of γ to α takes place. The increase in heating rate raises the extent of superheating, leading to increases in driving force and the nucleation rate of the α phase. At higher heating rate, numerous α nuclei form and lengthen, resulting in a fine α2∕γ lamellar microstructure after subsequent cooling. The theoretical prediction was demonstrated by the experimental results obtained on a fully lamellar Ti-48Al alloy.

Formation of Fine γ Grain Structure Through Fine α2 / γ Lamellar Structure in Ti-Rich TiAl Alloy

ABSTRACTMicrostructural development of an extremely fine α2-Ti3Al / γ-TiAl lamellar structure, which was formed by ice water quenching after solution-treatment in a high-temperature α-Ti phase field for a long period of time, was examined during isothermal aging treatment. In an as-quenched Ti-48at.%Al alloy, the massively transformed γ (γm) and untransformed (meaning massively untransformed) fine α2/γ lamellar regions were observed. Fine γ grains, which were similar to γm, were generated both within the fine α2/γ lamellae and at the boundary area between the γm and the fine α2/γ lamellar regions by aging at a low-temperature (1173K) for a short time (180s). Further aging (1.8ks) caused the coarsening of these newly generated fine γ grains. On the other hand, the coarsening of the γ grains occurred by a high-temperature (1323K) aging treatment even for 180s. Fine α2 plates and particles, which were aligned to a particular direction, were observed in the γ grain interiors, indicating that the newly generated γ grains grew at the expense of the fine α2/γ lamellae. It can be considered that the γ grain formation through the fine α2/γ lamellae is closely related to the α2→γ reaction of the α2 plates sandwiched by the γ plates, and needs the fast heating rate enough to overcome the α2/γ → γ/γ lamellae reaction.