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

Abstract

A g-based TiAl alloy with equiaxed microstructure and fine grain size has been studied to analyze the deformation mechanisms responsible for the creep behavior. The microstructures produced by creep and high temperature deformation have been examined by TEM to obtain information about the different aspects characterizing the primary and secondary stages of creep. Mechanical twinning has been confirmed to occur in a fraction of the grains that never exceeds 50% while 1͞2 ͗110͘ dislocations are active within all the g grains. The twins are only responsible for a small amount of strain, but they lead to a subdivision of the microstructure and determine (directly or indirectly) the hardening process observed during the primary stage of creep. We have proposed that during the secondary stage the creep rate is determined by the unblocking of pinned dislocations by processes such as a pipe diffusion or cross slip that allow thermally activated glide of 1͞2 ͗110͘ dislocations on (001) planes.

Tensile, creep, and low-cycle fatigue behavior of a cast γ-TiAl-based alloy for gas turbine applications

“…pct), which exhibits a steady-state creep regime of about 3 pct strain or more when tested at 700 ЊC to dislocation glide, i.e., power-law breakdown. It is clear from Figure 11 that the creep resistance of the PS-DP matefor stresses ranging between 260 and 360 MPa; [27] this finding was found for both the equiaxed and lamellar rial resembles that of NL material, both of which are superior to the creep behavior of the DP material. The improved microstructures.…”

Complex investigation of deformation twinning in γ-TiAl by TEM and neutron diffraction

“…Later, Appel et al (1999) presented the idea that twins would constitute a major source of hardening because the twin/matrix interface or twin intersection may serve as barriers for dislocation glide. On the other hand, Seo et al (1997) and Morris and Lipe (1997); Morris and Leboeuf (1998) argued that twin interfaces hinder dislocation motion and, therefore, harden the γ-phase. This was further followed in a detailed study (Skrotzki, 2000) where Skrotzki suggested that coherent twin boundaries seems to represent only a minor obstacle to dislocation slip, since they can pass the interface by suitable dissociation processes.…”

The role of microstructural stability on compression creep of fully lamellar γ-TiAl alloys

“…It is suggested that twinning plays a more active role in microstructure evolution than in strain contribution. While it has been proposed [30][31][32][33][34][35] that the formation of twin interfaces refines the microstructure and thus causes hardening, the coincident occurrence of the minimum creep rate and twinning leads us to believe that it is probably a flow softening process. Internal and compatibility stresses set up at lamellar and grain boundaries are probably relieved by twinning.…”