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

Abstract

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn 1 0.8 vol pct TiB 2 ) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed g grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed g grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.

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

“…Fine lamellar spacing has also been reported to dramatically decrease the minimum strain rate during creep and defect density during irradiation in lamellar TiAl alloys [7,37]. Fig.…”

“…This is because the above explanation does not consider the beneficial role of lamellar interfaces on creep and irradiation behavior of lamellar TiAl alloys at high temperatures. There are more a 2 /c lamellar interfaces in the 45XD than those in the 47XD [7].…”

“…A microstructure with a greater fraction of c phase or even composed completely of the c phase can be obtained by increasing the content of Al in TiAl alloys. However, it has been reported that a fully lamellar microstructure with 5% a 2 lamellae in a Ti-47 at.% Al XD (47XD) alloy exhibits a higher minimum strain rate than a fully lamellar microstructure with 15% a 2 lamellae in a 45XD alloy [7]. This is because the above explanation does not consider the beneficial role of lamellar interfaces on creep and irradiation behavior of lamellar TiAl alloys at high temperatures.…”

“…Taking b = 0.28 nm [42], the reduction in the lamellar spacing from 100 nm to 55 nm for two stabilized 45XD TiAl alloys produced by different heat treatment processes [7] results in an increase in Orowan bowing stress by as much as 61%. The higher dislocation bowing stress increases the resistance to creep deformation for the fully lamellar TiAl alloys with narrower lamellar spacing.…”

“…The typical microstructure of TiAl alloys can be classified into four types: fully-lamellar (FL), nearly-lamellar (NL), duplex, and near gamma structures [5,6]. In order to investigate the effects of various microstructural features on the creep resistance of TiAl alloys, a large amount of experimental work has been carried out in the last two decades [6][7][8][9][10][11]. Among the microstructural types, a fully lamellar microstructure was found to give a good combination of room temperature ductility and high temperature creep strength.…”