Microstructural evolution during creep of single-phase gamma TiAl

Abstract

Mechanical experiments and transmission electron microscope (TEM) observations indicate that single-phase ␥-TiAl does exhibit primary, secondary, and inverse creep, but not steady-state creep. Constant stress creep tests of ␥-Ti-51Al-2Mn conducted at 550 ЊC, 597 ЊC, and 703 ЊC have been interrupted at different stages in the creep process. The TEM observations of these specimens were used to document the microstructural evolution that occurs during creep. Superdislocation motion was activated and subsequently exhausted during primary creep. Ordinary dislocations were observed to be pinned during primary creep, but with time, these dislocations began to bow past their pinning points. The extended region of inverse creep has been related to the bowing and multiplication of these ordinary dislocations. Quantitative measurements of dislocation density were performed, and while the density of superdislocations remained constant, the density of ordinary dislocations increased by an order of magnitude during the life of a creep test. The acceleration in the creep rate has been related to this increase in the density of ordinary dislocations, but the change in dislocation density was not high enough to account for the increase in the creep rate. This suggests that both the mobility and density of ordinary dislocations increase as creep progresses.

Creep

“…At lower stresses (regime II), both n and Q C are nearly twice the values expected for climb -controlled creep. Primary creep is followed by a period with steadily increasing creep rate [38] . Again these fi ndings indicate that the creep rate in regime II is determined by mechanisms other than dislocation climb.…”

“…Single -phase TiAl (Ti -51Al -2Mn) examined after secondary creep at (550 to 703) ° C exhibited superdislocations, faulted dipoles, pinned ordinary dislocation, and a modest number of twins [3,38] . Single -phase TiAl (Ti -51Al -2Mn) examined after secondary creep at (550 to 703) ° C exhibited superdislocations, faulted dipoles, pinned ordinary dislocation, and a modest number of twins [3,38] .…”

“…Several authors [3, 24,35,38] have recognized that the defect structure evolves throughout the creep tests. The distinct absence of subgrains during secondary creep cast doubts on the validity of traditional creep models for single -phase TiAl alloys [3, 24,38] . Thus, secondary creep in single -phase alloys is probably accomplished by these two latter mechanisms.…”

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

“…Dislocation densities of usual metals and solid solutions that show normal primary creep behavior increase as the creep deformation proceeds within the primary creep regime [9]; single 7-TiA1 alloy also shows an increase of the dislocation density [10]. As for lamellar TiA1, the authors of the present investigation observed quantitatively that the dislocation density decreases significantly at the early stage of primary creep deformation under the conditions of 800~816/172-200MPa compared with undeformed specimens [1 l, 12].…”