Electron Microscopy Study of Fatigue Crack Initiation in Ni₃Al Single Crystals

Abstract

Transmission electron microscopy of single-slip oriented Ni3Al single crystals cyclically deformed at room temperature revealed a high density of dislocation dipoles and point defect clusters. The majority of the defect clusters were identified by weak-beam TEM to be of vacancy type. The observations of circular perfect dislocation loops, Frank loops, and spherical voids revealed the evidence of vacancy condensation during fatigue cycling at room temperature. Together with SEM observations of surface topography, it is suggested that fatigue crack initiation in Ni3Al single crystals can be rationalized by formation of microvoids at PSB/matrix interfaces.

Cyclic deformation, damage, and effects of environment in the Ni3Al ordered alloy at elevated temperature

“…For most experiments, significant cyclic hardening was observed. The rise in cyclic stress was dependent upon temperature, crystal orientation and strain range applied [47][48][49]. In general, cyclic hardening continued to failure except for low cyclic strain amplitudes or for temperatures beyond peak (i.e.…”

“…Cyclic deformation experiments on various single crystal L1 2 alloys [47][48][49][50] have revealed that the orientation dependent tension-compression asymmetry is stable during cyclic deformation at temperatures below peak strength [47][48][49][50]. For most experiments, significant cyclic hardening was observed.…”

“…These slip bands then initiated final fracture from the stress concentration. TEM observations of the deformed specimens revealed the presence of long straight screw dislocations [48,51], as well as dislocation debris from dislocation interactions.…”

“…Hsiung et al [48] reported a/3[111] Frank loops within slip bands presumably generated by intersection of mobile dislocations during cyclic loading. The loops were assumed to form by coalesence of defect clusters which reduced the volume decrease within slip bands.…”

Crystallographic study of fatigue cracking in Ni3Al(CrB) single crystal

“…The presence of the threshold for microcrack initiation along the slip bands at the notch root is similar to that reported for unnotched single crystal under the strain-controlled condition. [11,19] Hsiung et al [19] made a successive surface observation on specimens cycled under strain control. At an early stage of cycling, the observed (111) primary slip bands were densely and somewhat uniformly distributed.…”