Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys

Abstract

By using isothermally forged TiAl-based intermetallic alloys, various microstructures (of ␥ -grain, duplex, dual-phase, and fully lamellar microstructures) were prepared. These TiAl-based intermetallic alloys were tensile tested in vacuum and air as functions of strain rate and temperature to investigate microstructural effects on the moisture-induced embrittlement. All the intermetallic alloys with different microstructures showed different levels of reduced tensile stress (or elongation) in air at room temperature. The reduction in tensile stress (or elongation) due to testing in air diminishes as the testing temperature (or strain-rate) increases. From the fracture stress-temperature curves, it was found that the ␥ -grain microstructure was the most resistant to the moisture-induced embrittlement, and the dual-phase microstructure was the most susceptible to the moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloys with a fully lamellar microstructure depends on the lamellar spacing and is reduced with decreasing lamellar spacing. The possible reasons for the observed microstructural effect on the moisture-induced embrittlement were discussed, in association with hydrogen behavior and properties in the constituent phases and at some interfaces.

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

“…In the previous study, which used isothermally forged TiAl-based intermetallic alloys with various microstructures [16] and other TiAl-based alloys, [12] the reduction in fracture stress or tensile elongation (i.e., the so-called moisture-induced or hydrogen-induced embrittlement) has been observed at ambient temperatures, when the alloys are deformed in air and hydrogen gas. It has been suggested that the moisture-induced (or hydrogen-induced) embrittlement of many intermetallic alloys, including TiAl, occurs because of both the decomposition of moisture (or hydrogen gas) on the alloy surface (or on the freshly exposed grain boundaries or cleavage planes), and the subsequent microprocesses, such as the permeation of the alloy by atomic hydrogen, or the migration and condensation of atomic hydrogen to the grain boundaries (or lattice planes) in front of a propagating microcrack.…”

“…[16] For example, the reduced tensile fracture stress observed in the Ti-44Al-7Nb-0.2C alloy (with a fully lamellar microstructure) did not recover up to the highest temperature tested, while that observed in the Ti-42Al-5Mn alloy (with a duplex microstructure (of ␥ and lamellar (␥/␣ 2 ) grains) mixed with an isolated ␤ phase) almost fully recovered at 1050 K. It is speculated that high-temperature environmental embrittlement cannot operate when extensive plastic deformation, promoted by the climb-and-glide motion of dislocations, occurs, the flow strength and, in turn, rendering the hydrogen ineffective.…”

“…[7][8][9][10][11] Also, it has been reported that the so-called environmental embrittlement occurs at ambient temperatures in intermetallic alloys based on gamma (␥) TiAl. [12][13][14][15][16] In these cases, hydrogen is introduced from test atmospheres, such as in a flowing hydrogen gas or in air, the moisture in which is suggested to be able to react with the alloy and to generate atomic hydrogen, resulting in reduced tensile elongation. Furthermore, similar results have been obtained under fatigue loading [17] or for polysynthetic texture (PST) TiAl crystals.…”

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

“…Research shows that the hydrogen embrittlement resistance of single γ structure is best, while duplex is the worst. For the fully lamellar microstructure, the effect of hydrogen embrittlement resistance increases with the decrease of lamellar spacing [ 21 ]. In addition to the influence of microstructure, alloying elements also play a positive role in resisting environmental embrittlement [ 22 , 23 ].…”

Effect of Microstructure on Moisture-induced Embrittlement of L12 Intermetallic Compounds

“…[8][9][10][11][12] Moisture-induced embrittlement of L1 2 -type intermetallic alloys has been considered to be caused by hydrogen released from moisture in air, 13) and to involve micro-processes such as generation of hydrogen by decomposition of moisture on the alloy surface, absorption of hydrogen into the alloy, migration and condensation of hydrogen at grain boundaries in front of a propagating micro crack introduced from alloy surface during deformation. [14][15][16] Eventually, grain boundary cohesion and associated plastic work are reduced by stress field arising in front of a propagating micro crack due to hydrogen condensation, and consequently intergranular fracture occurs, resulting in low ductility.…”