Tensile Properties of TiAl Based Alloy in a Gaseous Hydrogen Atmosphere in a Temperature Range from Room Temperature to 973 K

Abstract

Ti-48Al-2Cr-2Nb (at%) with a g phase structure was tensile-tested in a high purity hydrogen gas in a temperature range of room temperature to 973 K. The reduction of elongation was observed in hydrogen gas in the entire temperature range, although the fracture surface changed mainly from cleavage to grain boundary facets both in vacuum and in hydrogen gas as the temperature increases. From transmission electron microscopy, fractography, the content of hydrogen introduced during tensile testing, and consideration of the diffusion rate of hydrogen, the hydrogen environmental embrittlement of the TiAl based alloy is suggested to result not from the formation of hydrides but from local accumulation of solute hydrogen introduced by dislocations from the test environment during testing. The hydrogen introduced during tensile deformation mainly decreases not grain boundary strength but cleavage strength, and then the reduction of elongation in hydrogen gas is caused by the decrease of the transgranular strength in the wide temperature range, although the grain boundary strength may also decrease with increasing temperature. KEY WORDS: TiAl based alloy; hydrogen gas; environmental embrittlement; tensile property; elevated temperature.Al: 33.52 (48.2 at%), Nb: 4.71 (1.97 at%), Cr: 2.71 (2.02 at%), Si: 0.007, Mn: Ͻ0.005, Fe: 0.10, Ni: 0.02, Cu: Ͻ0.005, C: 0.008, O: 0.063, N: 0.0041, H: 0.0009, Ti: bal. The alloy ingot of about 20 kg was prepared by inductionskull-melting in an argon atmosphere, and then the alloy bars of 16 mm in diameter were prepared by hot extrusion at 1 473 K. The extrusion ratio was 7.2.Tensile specimens with a gage length of 15 mm and a gage section of 1.5ϫ4 mm were machined using an electric discharging machine (EDM). The tensile direction of the specimens was parallel to the longitudinal direction of the extruded bar. After mechanical polishing, they were heat treated at 1 313 K for 50 h in an argon atmosphere in order to obtain a g-single phase structure. After mechanical polishing followed by electro-polishing, the specimens were tensile-tested in a temperature range of room temperature to 973 K using a screw-driven testing machine at a constant crosshead speed of 0.2 mm/min (a nominal strain rate of 2ϫ10 Ϫ4 s Ϫ1). The test environments were vacuum (Ͻ4ϫ10 Ϫ4 Pa) and a flowing high purity hydrogen gas (99.99999 %) of 1 atm. The test chamber, which was attached to the testing machine, was evacuated to better vacuum than 4ϫ10 Ϫ4 Pa, and then backfilled with the high purity hydrogen gas. This process was repeated four times, before the specimens were tested in the flowing hydrogen gas. The tensile tests were carried out after the specimens were heated to a given temperature followed by holding for 15 min in each environment. In most cases, three specimens were tested in each condition, and the average values were used for the tensile properties.The fracture surfaces and microstructures of the specimens were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM)....

Precipitation sequence in niobium-alloyed ferritic stainless steel

“…As shown in figure 4, the frequency of large particles increases and that of small particles decreases after 100 h ageing. After a stasis region where the number density does not change, coarsening occurs, where the mean particle radius tends to change in proportion to 1 3 . This is described as Ostwald ripening [28].…”

“…Niobium is an important solute in the design of heat-resistant stainless steels for automotive exhaust systems [1][2][3][4][5]. When in solid solution, it improves both the high temperature strength and the resistance to thermal fatigue [5].…”

Microstructure Sensitivity on Environmental Embrittlement of a High Nb Containing TiAl Alloy under Different Atmospheres

“…In the process of plastic deformation, the hydrogen ions generated by the catalytic Al element are transported to the crack tip by moving dislocations and diffusing along the pipes of the dislocation nucleus [ 34 , 35 , 36 ]. Meanwhile, dislocations, grain boundaries, phase interfaces and crack surfaces, which connect to the free surface, can provide fast diffusion paths for hydrogen absorbed from the material surface [ 37 , 38 ]. Therefore, the local accumulation of hydrogen is considered to cause the reduction of elongation, resulting in hydrogen–induced environmental embrittlement.…”

High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys

“…pct Al alloy with a dual-phase microstructure and the Ti-48Al-2Cr-2Nb alloy with a ␥-phase microstructure, it has been reported that the presence of hydrogen, whether it exists inside or outside the material, resulted in a reduction in the tensile elongation in the range of temperatures up to 573 K [20,21] or 973 K. [22] It has been suggested that the internal hydrogen embrittlement may be attributed to the reduced cohesive strength of the lattice; the external hydrogen embrittlement, on the other hand, may be attributed to the hydrogen-enhanced localized plastic deformation. [20,21,22] The temperature range in which the high-temperature environmental embrittlement occurs in this study is similarly extended to 1000 K. At the moment, the specific mechanism by which the present high-temperature environmental embrittlement could be understood has not yet been explained. Certainly, more detailed observations of hydrogen behavior and kinetics, in association with plastic deformation and the microstructure of TiAl-based alloys, are required.…”

“…[18,19] For the Ti-49 at. pct Al alloy with a dual-phase microstructure, it has been reported that the presence of hydrogen, whether inside or outside the material, resulted in a reduction in the tensile elongation at temperatures up to 573 K. [20,21] Also, a recent study showed that a Ti-48Al-2Cr-2Nb alloy with a ␥-phase structure showed a reduction in tensile elongation in hydrogen gas, within the range of temperatures from room temperature to 973 K. [22] On the other hand, some intermetallic alloys, such as boron-doped Ni 3 Al [23] and Ni 3 (Si,Ti) [24] alloys with an L1 2 structure deformed in air, have been reported to show reduced fracture stress and elongation at high temperatures. It has been suggested that oxygen atoms released from oxygen molecules (O 2 ) in air preferentially penetrate into the grain boundaries and then promote intergranular fracture, resulting in the so-called high-temperature environmental embrittlement.…”