Ruirun Chen

Effects and mechanism of ultrasonic irradiation on solidification microstructure and mechanical properties of binary TiAl alloys

In spite of their high temperature and reactivity, the binary TiAl alloys are successfully imposed by the ultrasonic irradiation and the microstructure evolution, solidification behaviors and mechanical properties are elaborately investigated. After ultrasonic irradiation, a high quality ingot without shrinkage defects and element segregation is obtained and the coarse dendrite structure is well modified into fine non-dendrite globular grains. The coarse lamellar colony and lamellar space of Ti44Al alloy is refined from 685μm to 52μm and 1185nm to 312nm, respectively (similarly, 819μm to 102μm and 2085nm to 565nm for Ti48Al alloy). For Ti48Al alloy, the α peritectic phase is simultaneously precipitated from the melt as well as the β primary phase before the peritectic reaction and the solidification is transformed into the mixed α-solidifying and β-solidifying. Ultrasonic irradiation promotes the peritectic reaction and phase transformation completely and the phase constituent becomes more close to the equilibrium level. The compressive strength of Ti44Al and Ti48Al alloys are increased from 623MPa to 1250MPa and 980MPa to 1295MPa, respectively. The grain refinement and dendrite transformation enhance the grain boundary sliding improving the plastic deformation ability. Ultrasonic irradiation significantly accelerates the melt flow and solute redistribution and the main grain refinement mechanism is the cavitation-enhanced nucleation by inclusion activation and heightened supercooling.

Numerical simulation of intermediate phase growth in Ti/Al alternate foils

To investigate the diffusion reaction between Ti/Al solid diffusion couple, Ti/Al alternate foils formed by hot pressing were annealed at 525, 550, 575 and 600 °C for time ranging from 1 to 40 h. The experimental results show that TiAl3 was the only observed phase at Ti/Al interface. The interface thermodynamics favored the preferential formation of TiAl3 in Ti/Al couple. The growth of TiAl3 layer occurred mainly towards Al foil side and exhibited a parabolic law. Using the interdiffusion coefficients calculated based on the contribution of grain boundary diffusion, the growth of TiAl3 was simulated numerically with the finite difference method, and the simulated results were in good agreement with the experimental ones.

Temperature field calculation on cold crucible continuous melting and directional solidifying Ti50Al alloys

Abstract In order to optimize technological parameters and realize directional solidification, temperature fields of cold crucible continuous melting and directional solidifying Ti50Al (mole fraction, %) at different parameters were calculated. Continuous casting of the model is achieved by distinguishing the moving unit at different positions. The calculation results show that the feeding rod is entirely melted at 200 s, the melt of feeding rod has some superheat degree at 300 s under the conditions of 52 kW and 3.0 mm/min. Both the superheat degree and the molten zone of the feeding rod reduce, the solid-liquid interface becomes concave with increasing velocity from 1.2 mm/min to 6.0 mm/min when the power is 52 kW, and the outside layer of the rod cannot be melted at the velocity of 6.0 mm/min. Both superheat degree and the molten zone of the feeding rod increase, the solid-liquid interface descends and becomes concave with increasing power from 48 to 58 kW at velocity of 3.0 mm/min, and the rod cannot be melted entirely when the power is 48 kW. Cold crucible continuous melting and directional solidification of TiAl alloys will be achieved successfully when the pulling velocity and the power are matched appropriately.

Microstructure and microsegregation in directionally solidified Ti–46Al–8Nb alloy

Abstract Directional solidification experiments were conducted for Ti-46Al-8Nb alloy at the growth rates ranging from 3 to 70 μm/s. The microstructure evolution and microsegregation pattern were investigated. In the range of growth rate, a regular dendritic structure appears and the primary dendrite spacing decreases with increasing growth rate. The peritectic reaction is observed during the solidification and the final microstructure is composed of α 2 /γ lamellar structure and retained β( B 2) after directional solidification. The lamellar orientation is found to be parallel and 45° to the primary growth direction of β dendrite. Peritectic reaction leads to significant chemical inhomogeneity, in which aluminum is rich in interdendritic liquid and niobium is rich in the core of β dendrite during the solidification. With the nucleation and growth of α phase, the segregation amplitude of niobium increases, which promotes the formation of B 2 phase, while aluminum rich in the interdendritic becomes homogeneous gradually.

Effect of cyclic heat treatment on microstructures and mechanical properties of directionally solidified Ti–46Al–6Nb alloy

The Ti–46Al–6Nb (mole fraction, %) ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 °C in the α phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 mm, nevertheless the lamellar spacing increases from 0.71 μm to 1.38 μm. Extending the holding time and increasing the cyclic times of heat treatment eliminate the β-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.

Effect of configuration on magnetic field in cold crucible using for continuous melting and directional solidification

To improve the power efficiency and optimize the configuration of cold crucible using for continuous melting and directional solidification (DS), based on experimental verification, 3D finite element (FE) models with various configuration-elements were developed to investigate the magnetic field in cold crucible. Magnetic flux density (B) was measured and calculated under different configuration parameters. These parameters include the inner diameter (D2), the slit width (d), the thickness of crucible wall, the section shape of the slit and the shield ring. The results show that the magnetic flux density in z direction (Bz) both at the slit and at the midpoint of segment will increase with the decrease of D2 or with the increase of the width of the slit and the section area of wedge slit or removing the shield ring. In addition, there is a worst wall thickness that can induce the minimum Bz for a cold crucible with a certain outer diameter.

Numerical calculation of flow field inside TiAl melt during rectangular cold crucible directional solidification

Numerical investigations on the flow field in Ti-Al melt during rectangular cold crucible directional solidification were carried out. Combined with the experimental results, 3-D finite element models for calculating flow field inside melting pool were established, the characteristics of the flow under different power parameters were further studied. Numerical calculation results show that there is a complex circular flow in the melt, a rapid horizontal flow exists on the solid/liquid interface and those flows confluence in the center of the melting pool. The flow velocity v increases with the increase of current intensity, but the flow patterns remain unchanged. When the current is 1000 A, the vmax reaches 4 mm/s and the flow on the interface achieves 3 mm/s. Flow patterns are quite different when the frequency changes from 10 kHz to 100 kHz, the mechanism of the frequency influence on the flow pattern is analyzed, and there is an optimum frequency for cold crucible directional solidification.

Uniformity analysis of magnetic field in an electromagnetic cold crucible used for directional solidification

Purpose – The purpose of this paper is to introduce a numerical calculation method to study the uniformity of the magnetic field in a cold crucible used for directional solidification (DS) and provide information for designing a cold crucible that can induce a uniform magnetic field.Design/methodology/approach – To obtain the characteristics of the magnetic field in a cold crucible and its influence on the directional solidification processing, based on experimental verification, 3‐D finite element (FE) models with different crucible configuration‐elements and power parameters were established to study the uniformity of the magnetic field in a cold crucible. In addition, different TiAl ingots were directionally solidified with different cold crucibles, and the solid/liquid (S/L) interfaced were examined to investigate the effect of the magnetic field on the macrostructure of those ingots.Findings – The uniformity of the magnetic field in a given domain can be quantitatively analyzed by statistical methods...

Effect of growth rate and diameter on microstructure and hardness of directionally solidified Ti–46Al–8Nb alloy

Abstract Bridgman-type directional solidification experiments were conducted for Ti–46Al–8Nb (mole fraction, %) alloy. The effects of the growth rate and the diameter on the microstructure, phase transition and hardness of the alloy were investigated. The results show that with the increase of the growth rate and the decrease of the diameter, the fully β phase solidification changes to the peritectic solidification, and the final microstructure is composed of the α 2 / γ lamellar structure and a multiphase microstructure ( B 2 phase, α 2 / γ lamellar structure) respectively, which can be attributed to the solute enrichment resulting from the decreasing diffusion and convection ability. The occurrence of peritectic reaction at high growth rate promotes the solute segregation heavily and the coarse lamellar spacing in Al- and Nb-rich region, which greatly decreases the hardness values and leads to the discontinuity of the hardness curves with the increase of the growth rate. Comparatively, the Ti–46Al–8Nb alloy has lower hardness values than the other applied TiAl-based alloys in previous studies.

Design of (Nb, Mo) 40 Ti 30 Ni 30 alloy membranes for combined enhancement of hydrogen permeability and embrittlement resistance

The effect of substitution of Nb by Mo in Nb40Ti30Ni30 was investigated with respect to microstructural features and hydrogen dissolution, diffusion and permeation. As-cast Nb40−xMoxTi30Ni30 (x = 0, 5, 10) alloys consist of primary bcc-Nb phase and binary eutectic (bcc-Nb + B2-TiNi). The substitution of Nb by Mo reduces the hydrogen solubility in alloys, but may increase (x = 5) or decrease (x = 10) the apparent hydrogen diffusivity and permeability. As-cast Nb35Mo5Ti30Ni30 exhibits a combined enhancement of hydrogen permeability and embrittlement resistance as compared to Nb40Ti30Ni30. This work confirms that Mo is a desirable alloying element in Nb that can contribute to a reduction in hydrogen absorption and an increase in intrinsic hydrogen diffusion, thus improving embrittlement resistance with minimal permeability penalty.