Grain Boundary Morphology and Its Effect on Creep of TiAl Alloys

Abstract

Three kinds of microstructures with different grain boundary morphologies and their creep properties of a Ti-47Al-2Nb-2Mn+0.8 vol%TiB 2 alloy are investigated. Tensile creep tests and microstructural examinations indicate that a stabilized fine-grained fully lamellar (FGFL) microstructure with relatively smooth grain boundaries shows inferior creep resistance. A stabilized fully lamellar (FL) microstructure with well-interlocked grain boundaries and wider lamellar spacing yields reduced minimum strain rate and extended creep rupture life. Furthermore, a nearly lamellar microstructure (NL) with well-interlocked grain boundaries exhibits better creep resistance than the stabilized FGFL microstructure though it has four times wider lamellar spacing and 15 vol% equiaxed grains at the grain boundaries, but worse creep resistance than the stabilized FL microstructure. Examinations to the deformed microstructures show that grain boundary instability involving spheroidization of the lamellae is a major microstructural degradation process, resulting in fine globular regions at the grain boundaries. Voids develop along the grain boundaries, particularly in the fine globular regions, leading to intergranual fracture. It is suggested that grain boundary sliding (GBS) is operating in the stabilized FGFL microstructure, and promotes mutually with the grain boundary instability during subsequent creep deformation, resulting in increased minimum strain rate and shortened tertiary stage. The well-interlocked grain boundary inhibits the onset of GBS and enhances the grain boundary stability effectively. These results demonstrated that the grain boundary stability has a great effect on creep behavior of TiAl Alloys.

Effect of initial microstructure on microstructural instability and creep resistance of XD TiAl alloys

“…Several aspects of the features and creep behavior of these microstructures have been presented previously. [5,15,[23][24][25] As expected, in the 45XD and 47XD alloys, air cooling and oil quenching from the single a-phase region produced FL (Figures 1(b) and (e)) and (Figures 1(a) and (d)) structures, respectively, whereas 1010°C/50 h/AC produced NL structures (Figures 1(c) and (f)). The NL structures of both alloys consist of a mixture of predominantly lamellar colonies and less than 15 vol pct of equiaxed g grains at the grain boundaries.…”

“…Decreasing the lamellar spacing also greatly improves the creep resistance of the FL structure, suggesting lamellar refinement is an important way to improve creep resistance. [1][2][3][4][5][6][7] In addition, the precipitation of b particles along the lamellar interfaces significantly increases the primary creep resistance, and therefore controlling the initial morphology of these precipitates may decrease the minimum creep rate. [8,9,10] Grain (or colony) boundary morphology may be another important microstructural feature influencing creep resistance.…”

Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications

“…The lamellar spacing can be refined by rapid heating from room temperature to the a single-phase region [47,48] or more commonly, by fast cooling from the a single-phase region during the annealing treatment of TiAl alloys [45,49]. However, upon fast cooling, a significant excess fraction of a 2 phase can be retained since the a 2 to c phase transformation is sluggish, decreasing the high-temperature properties.…”

Superplasticity and Superplastic Diffusion Bonding of a Fine-Grained TiAl Alloy

“…19,28) The stress exponent (n) of the investigated alloy is approximately 2 (n ¼ 1=m) at 1100 C and low strain rates, indicating that GBS is also the dominant deformation process in the present fine-grained TiAl alloy. GBS raises the stress in the grain and causes stress concentration at the grain boundaries, 29) resulting in nucleation of cavities at the grain boundaries. However, relatively coarse lamellar colonies in the duplex microstructure should restrict GBS as suggested in Ref.…”

“…6. From previous studies, it is known that the grain growth will degrade the SPF behavior, 16,35) but the coarse grain is advantageous to the creep resistance, [36][37][38] which is one important property for the TiAl alloy parts employed at elevated temperature.…”