Huarui Zhang

Interactions between γ-TiAl melt and Y2O3 ceramic material during directional solidification process

Abstract A γ-TiAl alloy with nominal composition of Ti-47%Al (molar fraction) was directionally solidified in an alumina mould with an Y 2 O 3 protective coating. The effects of processing parameters (melting temperature and interaction time) on the metal-coating interface, microstructure and chemical composition of the alloy were evaluated. The result shows that the Y 2 O 3 protective coating exhibits an effective barrier capability to avoid direct contact between the mould base material and the TiAl melt, although the Y 2 O 3 coating is found to suffer some erosion and be slightly dissolved by the molten TiAl due to the coating-metal interactions. The directionally solidified alloys were contaminated with Y and O, and Y 2 O 3 inclusions were dispersed in the metal matrix. The reason for this metal contamination is the Y 2 O 3 coating dissolution by the TiAl melt. One mode of the interaction between Y 2 O 3 and the TiAl melt is dissolution of yttrium and atomic oxygen in the melt by reaction Y 2 O 3 (s)=2Y (in TiAl melt)+3O (in TiAl melt). Both the extent of alloy contamination and the volume fractions of Y 2 O 3 inclusions depend on the melting temperature and the interaction time.

Interactions between Ni-44Ti-5Al-2Nb-Mo alloy and oxide ceramics during directional solidification process

Interactions between Ni-44Ti-5Al-2Nb-Mo (at.%) alloy and oxide ceramics during the directional solidification (DS) process were studied. The experiments were performed using novel Y2O3-coated Al2O3 (Y2O3/Al2O3) crucibles and conventional Al2O3 crucibles with a withdrawal rate of 1xa0×xa010−4xa0mxa0s−1 at 1550, 1650, and 1750xa0°C, respectively. The results indicated that the Al2O3 reacted with the NiTi-Al alloy strongly, leading to the formation of the TiO and (Ti,Al)4Ni2O inclusions. The extent of the reaction increased with increasing heating temperature at a constant withdrawal rate. The oxygen increase in directionally solidified ingot using the Al2O3 crucible was up to 1.366xa0wt% at 1750xa0°C. With the same DS parameters using the Y2O3/Al2O3 crucible, no chemical reaction occurred between the Y2O3 and the alloy, suggesting that the Y2O3 layer exhibited an effective barrier capability to avoid the reaction between the Al2O3 and NiTi-Al alloy. Although the alloy ingot obtained at 1750xa0°C contained a small amount of Y2O3 particles, its maximum oxygen increase was only 0.046xa0wt%.

High-Temperature Wettability and Interactions between Y-Containing Ni-Based Alloys and Various Oxide Ceramics

To obtain appropriate crucible materials for vacuum induction melting of MCrAlY alloys, four different oxide ceramics, including MgO, Y2O3, Al2O3, and ZrO2, with various microstructures were designed and characterized. The high-temperature wettability and interactions between Ni-20Co-20Cr-10Al-1.5Y alloys and oxide ceramics were studied by sessile drop experiments under vacuum. The results showed that all the systems exhibited non-wetting behavior. The contact angles were stable during the melting process of alloys and the equilibrium contact angles were 140° (MgO), 148° (Y2O3), 154° (Al2O3), and 157° (ZrO2), respectively. The interfacial reaction between the ceramic substrates and alloys occurred at high temperature. Though the ceramics had different microstructures, similar continuous Y2O3 reaction layer with thicknesses of about 25 μm at the alloy-ceramic interface in MgO, Al2O3, and ZrO2 systems formed. The average area percentage of oxides in the alloy matrices were 0.59% (MgO), 0.11% (Al2O3), 0.09% (ZrO2), and 0.02% (Y2O3), respectively. The alloys, after reacting with MgO ceramic, had the highest inclusion content, while those with the lowest content were in the Y2O3 system. Y2O3 ceramic was the most beneficial for vacuum induction melting of high-purity Y-containing Ni-based alloys.

Microstructural characteristics of directionally solidified Ni-43Ti-7Al alloys

Abstract Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s−1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s−1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti2Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti2Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti2Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s−1. However, in the sample withdrawn at 2 μm s−1, Ti2Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.

Effect of heat treatment and long-term age on microstructure of a Ni3Al-based single crystal superalloy

Abstract A new Re-containing Ni3Al-based single crystal superalloy was recently developed by our group. In order to obtain uniform microstructure and further improve mechanical properties of superalloy, the heat treatment regime (i.e. solution treatment and age treatment) was investigated by experimental analysis and dynamics calculation (by JMatPro), and the optimal heat treatment regime was 1335–1355°C/16u200ah, air cooling (AC)+1140°C/2u200ah, AC+870°C/32u200ah, AC. Meanwhile, completely heat-treated experimental alloy was long-term aged at 1150°C for 25–1000u200ah, which showed very good microstructural stability and the coarsening behaviour of γ′ phases agreed well with the typical Lifshitz–Slyozov–Wagner (LSW) theory.

Microstructure and mechanical properties of NiTi-Al based alloys with addition of Fe

Abstract The influences of Fe on the microstructure and mechanical properties of Ni–43Ti–4Al–2Nb–2Hf–xFe (xu200a=u200a0, 1, 3, 5 at-%) alloys were investigated, prepared by vacuum non-consumable arc-melting method. The results show that the microstructure of the alloys is refined gradually with increasing Fe content. The appropriate addition of Fe produces an improvement of the tensile strength and ductility of the alloy. The highest tensile strength and elongation of the study’s alloys was seen with the 3 at-%Fe addition, resulting in strength and elongation of up to 1308 MPa and 4·15%. The fracture characterisation changes from a quasi-cleavage fracture to a mixed fracture of quasi-cleavage and dimple. Further increasing the Fe content to 5 at-%, the mechanical properties deteriorated.

Microstructure evolution and solidification behaviour of Nb–16Si–22Ti–2Cr–2Al–6Hf alloy processed by directional solidification

Abstract The microstructure evolution and solidification behaviour of Nb–16Si–22Ti–2Cr–2Al–6Hf alloy processed by liquid metal cooled directional solidification were investigated. The temperatures (1850 and 1900°C) and withdrawal rates (1·2, 3, 6, 12 and 18 mm min−1) were selected. The directionally solidified alloy was composed of primary Nb solid solution dendrites, silicide laths and two types of eutectic (fish bone-like and lamellar eutectic) aligned along the growth direction. As the withdrawal rate increased, the microstructure of the quasi-steady state growth region became finer and the volume fractions of (Nb,X)5Si3 laths and the fish bone-like eutectic (Nb,Ti)SS+(Nb,X)5Si3 increased, which was contrary to lamellar eutectic (Nb,Ti)SS+(Nb,X)5Si3. The volume fraction of (Nb,X)5Si3 laths in the whole directionally solidified rod decreased as the temperature increased from 1850 to 1900°C. In addition, the solidification paths were also discussed.

Microstructure evolution of directionally solidified Ni–43Ti–7Al alloy during heat treatment

The microstructure change of directionally solidified Ni–43Ti–7Al alloy after heat treatment has been investigated by transmission electron microscopy and back scattered electron imagings in this paper. After solution and aging treatment, the NiTixa0+xa0Ti2Ni anomalous eutectic structure nearly cannot be observed and Ti2Ni phases become spheroidized. The β′-Ni2TiAl precipitates are nearly spherical at the early stage of aging at 800xa0°C (0.1–1xa0h); however, they become aligned along 〈100〉 directions and change to cubic shape after aging for 20xa0h. In the course of further aging, the coarsened semi-coherent β′ precipitates occur preferentially in intercellular regions. Then the coarsened β′ precipitates begin to occur in intracellular regions with the increasing aging time and aging temperature. These β′ precipitates change the shape to sphere and plate, accompanied with loss of their coherency by introducing interface dislocations surrounding them. Finally, the formation mechanism of β′ precipitates are discussed compared to other studies in the Ni–Ti–Al system.

Effect of process parameters on microstructures and properties of DZ125 superalloy solidified by LMC

Abstract High thermal gradient in the interface region of melting and solidified metal during the unidirectional solidification has significant impact on the microstructures and mechanical properties of the castings. In this paper, DZ125 superalloy was solidified by directional equipment associated with liquid metal cooling medium of Ga–In alloy. Several process parameters have been chosen in order to understand the effect of thermal gradient on the castings. Comparing with traditional high rate solidification prepared DZ125 superalloy, the results show that element segregation such as Al, Ti, W and Mo was decreased with high thermal gradients or high withdraw rates, the precipitation of low melting phase in the interdendritic region was suppressed and the sizes of eutectic and γ′ phase were both refined. Stress–rupture test was carried out at 980°C/235 MPa after heat treatment. Obvious stress–rupture improvement has been observed, and the stress–rupture life decrease at high withdrawal rate due to the column grain discontinuity and deflection.

Microstructure and Mechanical Property Optimization of NiTiAl-based Alloys: Directional Solidification in Novel Ceramic Crucibles

Abstract Directional solidification technique was successfully used to prepare highly reactive NiTiAl-based alloys using novel Y2O3-coated crucibles. Compared with the as-cast alloys fabricated by vacuum arc melting, the microstructure of the directionally solidified alloys was optimized and mechanical properties remarkably improved. After directional solidification, the microstructure of the alloys became much finer and preferentially grew along [001] orientation. The tensile strength and strain of the directionally solidified alloys at 1550°C were promoted to 1240 MPa and 2.1% respectively, which were 91.4% and 75% higher than that of the as-cast alloys. The strength increased up to 1328 MPa and 1419 MPa after homogenizing treating. The Y2O3-coated crucibles assured significant purification effect of the alloys. The maximum oxygen increase in the steady-state region of the ingot obtained at 1750°C was no more than 0.014 wt.%.