Zhang Tiebang

High-Temperature Oxidation Behavior of Ti-22Al-27(Nb, Zr) Alloys

Abstract The oxidation behavior of Ti-22Al-(27- x )Nb- x Zr ( x =0, 1, 6, at%) was evaluated over a temperature range of 650∼800°C in air. The phase composition and the morphology of the formed scales were investigated by X-ray diffraction and scanning electron microscopy. The results show that the alloys containing Zr exhibit better oxidation resistance than Ti-22Al-27Nb alloy. The main oxidation product is TiO 2 for all the alloys oxidized at 650°C for 100 h. A mixture of TiO 2 , Al 2 O 3 and AlNbO 4 is formed for Ti-22Al-27Nb and Ti-22Al-26Nb-1Zr oxidized at 800°C for 100 h; however, ZrO 2 is also traced in Ti-22Al-21Nb-6Zr. The improved oxidation resistance for Ti-22Al-26Nb-1Zr can be ascribed to the refinement of oxides. While in Ti-22Al-21Nb-6Zr alloy, although formation of Al 2 O 3 is obvious at 800°C, ZrO 2 in the scale can provide sites for rapid oxygen transport, leading to the high mass gain.

PHASE SELECTION AND THE SOLIDIFICATION CHARACTERISTICS OF TiAl BASE ALLOYS IN THE NONEQUILIBRIUM SOLIDIFICATION

The Al content and the β stabilizers in TiAl base alloys were investigated,which have an important effect on the solidification characteristics and phase selection.TiAl base alloys with single β-solidifying have the characteristics of low Al content and narrow crystallization temperature range.It is attributed to forming a homogeneous fine-grained microstructure.Dendrite morphology in TiAl base alloys is investigated by SEM-BSE in the nonequilibrium solidification conditions.The relationships between undercooling and nucleation of two phases are investigated systematically by using the classical nucleation theory in the undercooled Ti48A12Cr2Nb(atomic fraction,%) peritectic alloy.By calculating the interfacial critical nucleation work and steady state nucleation rates of β phase(bcc) and α phase(hcp),it is obtained that β phase is always prior to nucleate from the Ti48A12Cr2Nb undercooled melts at the cooling rate about 15 K/s in the achieved undercooling range.Due to narrow crystallization temperature range,microstructures of TiAl base alloys with high Al content as the hyper-peritectic solidification path take on the gradual solidification style and finer dendrite morphology.It is conducive to reducing and eliminating shrinkage porosity and hot cracking caused by mushy solidification.The β stabilizers have an important effect on solidification path depending on its Al equivalent and will change the solidification path of TiAl alloy from hyper-peritectic solidification to hypo-peritectic solidification.The overmuch Al content in TiAl base alloys leads to the formation of large number of massive 7 phase impacting on strength and ductility of alloys.

Hot Workability and Microstructure Evolution of TiAl Alloy in (α2+γ) Dual-phase Field

Abstract To explore the possibility of near conventional forging of TNM alloys using existing equipment for superalloy, the hot workability of a typical TNM alloy with normal composition of Ti-43Al-4Nb-1Mo-0.1B (at%) was investigated in ( α 2 + γ ) dual-phase field. Based on the hot workability maps and microstructure observation, it is found that the hot deformability of this alloy is dropped remarkably when the temperature decreases into ( α 2 + γ ) dual-phase field. The samples can be soundly compressed with strain rate of 0.001 s −1 at 1050∼1150 °C and cracks are formed when the temperature is 1000 °C. As the temperature increases and/or the strain rate decreases, cracks vanish and more dynamic recrystallized grains are formed. Furthermore, its workability can be promoted when it is packed with 304 stainless steel with a thickness of 1.5 mm and the billets can be adequately deformed at 1050∼1150 °C with strain rate of 0.1 s −1 , which is favorable to current industrial trials. By comparison, there are not cracks but fewer recrystallized grains when packed-compressed under the same conditions.

Interface Reaction between Ceramic Moulds and High Nb-TiAl Alloys

Abstract The interface reaction between Ti-44Al-8Nb-0.2W-0.1B-0.1Y (at%) alloy and Al 2 O 3 /ZrO 2 /Y 2 O 3 crucibles was investigated. The thicknesses of the interface reaction layers are about 40, 170 and 20 μm. The most interesting is the microstructure transformation of Ti-44Al-8Nb-0.2W-0.1B-0.1Y solidifying in different crucibles. After solidifying in Al 2 O 3 /ZrO 2 /Y 2 O 3 crucibles, the oxygen contents in Ti-44Al-8Nb-0.2W-0.1B-0.1Y alloy are measured to be 0.35, 0.41 and 0.11 (at%). During melting and solidification, oxygen in crucibles will diffuse into the alloy matrix. High oxygen content causes microstructure transformation. Peritectic reaction occurs in the alloys solidifying in Al 2 O 3 and ZrO 2 crucibles. For contrast, Ti-49.5Al-2Cr-2Nb alloy was chosen to make investigation. No visible microstructure difference is found in alloys solidified in various crucibles though the oxygen contents are determined as 0.40, 0.63 and 0.25 (at%).

Effect of Lanthanum on Oxidation Behavior of Ni-20Cr- 18W-1Mo Alloys at 1373 K for 100 h in Air

Abstract The isothermal oxidation experiments of Ni-20Cr-18W-1Mo superalloys with and without lanthanum element were conducted at 1373 K for 100 h in air. The oxide scales surface and the cross-section of isothermal oxidation samples were analyzed by scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray diffraction. Results show that the addition of La can significantly improve the binding ability between the matrix and the oxide scales, and consequently the oxidation resistance of Ni-20Cr-18W-1Mo alloy at 1373 K is improved. Comparing with that without La, the Ni-20Cr-18W-1Mo alloy with La has smaller parabolic rate constant κ” and weaker spalling oxidation, and the surface of oxide scales are much more compact.

Precipitation Behavior of Second Phases during Isothermal Oxidation of Hastelloy C-2000 Alloys

Abstract The precipitation behavior of the second phase in Hastelloy C-2000 alloy matrix was studied after isothermal oxidation at 800 °C for 100 h in air. Mo-rich phase was precipitated in the alloy matrix after oxidation. According to the determination Mo-rich phase with diamond cubic structure is Mo 3 Ni 3 C type carbide. Morphology characteristics of Mo-rich phase with both no-continuous and continuous irregular strip are presented along grain boundaries and in matrix, respectively. However, some variation of morphologies will take place at both grains and grain boundaries after deep etching, i.e. a large number of white flocculent structure are distributed along grain boundaries, and the corrosion pits with a diamond structure appear in intragranular. Ni-rich and Cr-rich areas are more easily corroded during deep etching due to a more negative electrode potential in the areas.

Grain Boundary Segregation and Mechanical Properties of an Aged Ni-20Cr-18W-1Mo Superalloy at Different Temperatures

Abstract The influence of aging temperature (200 to 800 °C) on grain boundary segregations in a Ni-20Cr-18W-1Mo superalloy was investigated by scanning electronic microscope (SEM), transmission electron microscopy (TEM), electron microprobe analyzer (EMPA) and a mechanical testing machine. Results indicate that the grain boundary segregation critical time of sulfur and phosphorus decreases with increasing of the aging temperature. Increasing of the aging temperature has a conspicuous effect on the concentration distribution in the grain boundary and the grain core. Grain boundary concentrations of sulfur and phosphorus increase with raising the testing temperature until a peak value is obtained at 650 and 400 °C, respectively, which is the essential reason to the declined mechanical properties from 200 to 600 °C

EFFECT OF OXYGEN ON MICROSTRUCTURE AND PHASE TRANSFORMATION OF HIGH Nb CONTAINING TiAl ALLOYS

Due to the low density, high specific strength, elastic modulus and oxidation resistance at high temperature, TiAl-based alloys have attracted much attention as a candidate of the next gen- eration high temperature materials in aerospace and automobile application. Meanwhile, the excellent properties oxidation resistance, creep strength and tensile strength at the elevated temperature make the high Nb containing TiAl alloys be one of the promising development directions of future TiAl alloys. During the studies about alloying which is an efficient way to improve theperformance of TiAl alloys, researchers have found that interstitial atoms B, C and N notably refine the grains and then improve mechanical properties including yield strength, micro-hardness, and tensile ductility of TiAl alloys. During the melting, casting, forging and the application environment, the TiAl alloys also are always inevitable to be contaminated by the O. In this work, the high Nb containing Ti-46Al-8Nb-xO alloys (atomic fraction) were prepared by non-consumable vacuum arc remelting under the protection of Ar atmosphere. The aim of the present work is to study the influence and the corresponding mechanism of oxygen atoms on the microstructure evolution and phase transformation of high Nb-TiAl alloys. The results indicate that oxygen atoms in Ti-46Al-8Nb-xO alloys remarkably increase the amount of �2 phase. The increasing oxygen content leads to the grain refinement. Meanwhile, the duplex mi- crostructures translate into fully lamellar. It indicates that the interstitial oxygen essentially reduces the kinetics of � ! . Consequently, the fully lamellar is easier formation than the duplex microstruc- tures. It is found that interstitial oxygen atoms preserve significantly influence on the microstructure of Ti-46Al-8Nb-xO alloys. With the increase of oxygen content, thesolidification translates into �