In Situ Observations of the Deformation Behavior and Fracture Mechanisms of Ti-45Al-2Nb-2Mn + 0.8 vol pct TiB2

Abstract

The deformation and fracture mechanisms of a nearly lamellar Ti-45Al-2Nb-2Mn (at. pct) + 0.8 vol pct TiB 2 intermetallic, processed into an actual low-pressure turbine blade, were examined by means of in situ tensile and tensile-creep experiments performed inside a scanning electron microscope (SEM). Low elongation-to-failure and brittle fracture were observed at room temperature, while the larger elongations-to-failure at high temperature facilitated the observation of the onset and propagation of damage. It was found that the dominant damage mechanisms at high temperature depended on the applied stress level. Interlamellar cracking was observed only above 390 MPa, which suggests that there is a threshold below which this mechanism is inhibited. Failure during creep tests at 250 MPa was controlled by intercolony cracking. The in situ observations demonstrated that the colony boundaries are damage nucleation and propagation sites during tensile creep, and they seem to be the weakest link in the microstructure for the tertiary creep stage. Therefore, it is proposed that interlamellar areas are critical zones for fracture at higher stresses, whereas lower stress, high-temperature creep conditions lead to intercolony cracking and fracture.

Effect of Stress Level on the High Temperature Deformation and Fracture Mechanisms of Ti-45Al-2Nb-2Mn-0.8 vol. pct TiB2: An In Situ Experimental Study

“…However, their hightemperature deformation behavior must be better understood to optimize their microstructure. [4][5][6][7][8][9][10] Gamma titanium aluminides contain two phases (c-TiAl, tetragonal L1 0 structure and a 2 -Ti 3 Al, hexagonal DO 19 structure) and may exhibit various microstructures consisting of different volume fractions of equiaxed c grains and of (a 2 + c) lamellar colonies. [3,11] They are termed duplex (comprising a mixture of equiaxed grains and lamellar colonies), nearly lamellar (containing only a small fraction of equiaxed grains) and fully lamellar (no equiaxed grains).…”

“…However, the contribution from other mechanisms such as grain boundary sliding is not clear. [8][9][10]25] Further study is therefore needed to clarify the mechanisms responsible for high-temperature deformation of these intermetallic alloys.…”

“…[3] At high temperatures, colony boundaries and, in particular, triple points, are favorable sites for crack nucleation in lamellar microstructures. [10,22] Colony sliding promotes the nucleation of wedge cracks which propagate along colony boundaries. Nevertheless, further study is needed to understand better the crack nucleation and propagation mechanisms.…”

“…These disadvantages can be overcome by utilizing in situ testing methods which allow for the observation of the evolution of the surface microstructure in real time and therefore facilitate the study of the kinetics of the deformation and failure mechanisms. [10,[27][28][29][30] The primary aim of this study was to investigate the deformation and failure mechanisms operative during elevated-temperature tensile-creep deformation of a Ti-45Al-2Nb-2Mn (at. pct)-0.8 vol.…”

“…In a previous study, a transition in the dominating crack propagation path, from interlamellar to intercolonial, was observed below a threshold applied stress. [10] This investigation was intended to build on this knowledge to understand the importance of intercolonial, interlamellar and translamellar cracking as a function of the stress level.…”