II. Deformed microstructures during creep of TiAl alloys: role of mechanical twinning

Quantitative analysis of microstructures produced by creep of Ti–48Al–2Cr–2Nb–1B: Thermal and athermal mechanisms

“…[17][18][19] In conclusion, the twinning activity observed from the early stages of the creep process is due to the macroscopic applied stress which favors mechanical twinning as a major slip system. Only in the specific grain orientations for which 1͞2 ͗110͘ (001) slip has a larger Schmid factor than twinning does slip take over.…”

“…9,11 This has been interpreted as being due to a transition from diffusional creep at low stresses to dislocation glide creep at high stresses. 5,[12][13][14] Although mechanical twinning during creep deformation of TiAl alloys has been extensively reported, 5,[15][16][17] its contribution to the creep process is not easily interpreted since traditional creep theories do not include such contributions. Also it has been suggested that mechanical twinning differs between lamellar structures and equiaxed g grains.…”

“…In particular, we have examined the influence of mechanical twinning on primary creep compared to that observed in alloys with large grain size previously studied. 17 Also, we have separated the different contributions from glide of 1͞2 ͗110͘ dislocations and twinning during the secondary stage of creep.…”

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

“…Creep deformation in lamellar TiA1 alloys proceeds mainly by the slip of ordinary dislocations [2][3][4], ~=1/2<110], where ~ is Burgers vector. In addition, superdislocations of ~=1/2<112] or <101], and twinning of {111}<112]/6 also operate [5,6] since five independent slip systems must be able to operate in order to undergo general plastic shape changes [7]; there are only 2 slip systems in ordinary dislocations. However, it has been reported that superdislocations take only 10% among the total dislocation segments observed in crept specimens [5].…”

“…In addition, superdislocations of ~=1/2<112] or <101], and twinning of {111}<112]/6 also operate [5,6] since five independent slip systems must be able to operate in order to undergo general plastic shape changes [7]; there are only 2 slip systems in ordinary dislocations. However, it has been reported that superdislocations take only 10% among the total dislocation segments observed in crept specimens [5]. In addition, deformation twinning contributes significantly to creep deformation in duplex structures only at high stress levels (>350MPa) at usual creep test temperatures [5], and highly deformed states after the primary creep regime in lamellar structures [8].…”