Microstructure and Mechanical Properties of TiAl-based Alloy

Two phases gamma titanium aluminide alloy,Ti-46.5Al-2.5V-1Cr.was investigated to characterize microstructures and to define the microstructure/mechanical property relationship.Many kinds of microstructure of gamma and α_2 phases were obtained by heat treatments in the α+γ,α_2+γ and α fields.The effects of microstructure on tensile properties,fracture toughness and J-R resistance curve at room temperature,were systematically studied.The experimental results showed that the microstructure had a strong effect on mechanical properties,The duplex microstructure produced by heat treatment at 1250℃×4 h with controlled cooling resulted in the highest ductility of 4.8% tensile elongation,low fracture toughness and crack growth resistance.The fully lamellar microstructure produced by heat treatment in the α field having large grain sizes resulted in the highest fracture toughness but the lowest ductility.

Dual-Material Electron Beam Selective Melting:Hardware Development and Validation Studies

Electron beam selective melting （EBSM） is an additive manufacturing technique that directly fabricates three-dimensional parts in a layerwise fashion by using an electron beam to scan and melt metal powder. In recent years, EBSM has been successfully used in the additive manufacturing of a variety of materials. Previous research focused on the EBSM process of a single material. In this study, a novel EBSM process capable of building a gradient structure with dual metal materials was developed, and a powder-supplying method based on vibration was put forward. Two different powders can be supplied individually and then mixed. Two materials were used in this study： Ti6AI4V powder and Ti47AI2Cr2Nb powder. Ti6AI4V has excellent strength and plasticity at room temperature, while Ti47AI2Cr2Nb has excellent performance at high temperature, but is very brittle. A Ti6AI4V/Ti47AI2Cr2Nb gradient material was successfully fabricated by the developed system. The microstructures and chemical compositions were characterized by optical microscopy, scanning microscopy, and electron microprobe analysis. Results showed that the interface thickness was about 300 μm. The interface was free of cracks, and the chemical compositions exhibited a staircase-like change within the interface.

Numerical study of crucial parameters in tilt casting for titanium aluminides

Numerical modeling of the tilt casting process for TiAI alloys was investigated to achieve a tranquil mould filling and TiAI castings free of defects. Titanium alloys are very reactive in molten state, so they are widely melted in cold crucible, e.g. the Induction Skull Melting （ISM） furnace. Then the crucible holding the molten metal together with the mould is rotated to transfer the metal into the mould -- ISM＋ tilt casting. This paper emphasizes the effect of crucial parameters on mould filling and solidification of the castings during tilt casting. All crucial parameters, such as rotation rate, rotation profile, venting, initial mould temperature, casting orientation, feeder design, change of radius in ＇T＇ junction and mould insulation have been discussed using numerical modeling data. Simulations were performed using a 3D CFD code PHYSICA implemented with front tracking, heat transfer algorithms and a turbulence model （which accounts for an advancing solid front）.

Microstructure and mechanical properties of TiAl castings produced by zirconia ceramic mould

Owing to their low density and attractive high-temperature properties, gamma titanium aluminide alloys （TiAI alloys, hereafter） have significant potential application in the aerospace and automobile industries, in which these materials may replace the heavier nickel-based superalloys at service temperatures of 600 - 900℃. Investment casting of TiAI alloys has become the most promising cost-effective technique for the manufacturing of TiAI components. Ceramic moulds are fundamental to fabricating the TiAI casting components. In the present work, ceramic mould with a zirconia primary coat was designed and fabricated successfully. Investment casting of TiAI blades and tensile test of specimens was carried out to verify the correctness and feasibility of the proposed method. The tensile test results indicate that, at room temperature, the tensile strength and the elongation are about 450 MPa and 0.8%, respectively. At 700℃, the tensile strength decreases to about 410 MPa and the elongation increases to 2.7%. Microstructure and mechanical properties of investment cast TiAl alloy are discussed.

A MICROMECHANICAL MODEL FOR γ-TiAl BASE PST CRYSTALS

An analytical micromechanical method is proposed to examine the dependence of plastic deformation on the microstructure for a PST crystal. The sub-domain microstructure of the γ phase and the effect of the α2 phase are taken into account by a proper micromechanical formulation, the dislocation slip and twinning deformation mechanisms are considered in the context of crystal plasticity. The model can well predict the dependence of stress-strain relations on loading angle with respect to the microstructure. The influence of the twinning and lamellar spacing on the deformation behavior and biaxial yield surfaces for PST crystals are also examined.

A first-principles study of site occupancy and interfacial energetics of an H-doped TiAl-Ti_3 Al alloy

We investigate the site occupancy and the interfacial energetics of TiAl-Ti3Al binary-phase system with H using a first-principles method. H energetically prefers to occupy the Ti-rich octahedral interstitial site because H prefers to bond with Ti rather than with Al. The occupancy tendency of H in the binary phase TiAl-Ti3Al alloy from high to low is α2-Ti3Al to γ/α2 interface and 7-TiAl, because the decrease of the Ti local concentration is in the same order. We demonstrate that H can largely affect the mechanical properties of the TiAl-Ti3Al system. On the one hand, H at the interface reduces the interface energy with the H2 molecule as a reference, implying the TiAl/Ti3Al interface is stabilized. On the other hand, the ratio between the cleavage energy and the unstable stacking fault energy decreases after H-doping, indicating H will reduce the ductility of the TiAl/Ti3Al interface. Consequently, the mechanical property variation of TiA1 alloy due to the presence of H not only depends on the amount of TiAl/Ti3Al interfaces but also is related to the H concentration in the alloy.

Hot deformation behavior and microstructural evolution of powder metallurgical TiAl alloy

The hot deformation behavior of powder met- allurgical （PM） TiAI alloys was investigated on Gleeble- 3500 thermomechanical simulator, at a temperature range of 1050-1200 ℃ with an interval of 50℃ and a strain rate range of 0.001-1.000 s-1. The results show that the flow stress of PM TiAI alloy is sensitive to deformation tem- perature and strain rate, the peak stress decreases with the increase in deformation temperature and decrease in strain rate, and dynamic recrystallization occurs during the hot compression. The deformation active energy was calcu- lated and the flow stress model during high-temperature deformation was established based on the Arrhenius equations and Zener-Hollomon parameter. The deformed microstructure consists of refined homogeneous γ and α2/γ grains.