LOW CYCLE FATIGUE BEHAVIORS AND MICROSTRUCTURES OF Ti–2Al–2.5Zr WITH FINE GRAIN AT RT AND 77 K

Ti–2Al–2.5Zr alloy with hcp structure is a kind of structural materials used under low temperature condition, e.g. pipeline system of liquid hydrogen and liquid oxygen in missile engine. It is usually serviced in condition of severe low temperature and dynamic loading. Deformation twinning is a common and important plastic deformation mode in the hexagonal close–packed alloy, but will be severely restricted as the grain is refined from tens of microns to a few microns. On the other hand, twinning has a low sensitivity to temperature, consequently becomes a favorable deformation mode at low temperature in comparison with dislocation slip. The objective of this work is to study the coupling effect of grain refinement and testing temperature on twinning behavior and the low–cycle fatigue behavior of Ti–2Al–2.5Zr. Symmetrical push–pull low–cycle fatigue (LCF) tests were performed on Ti–2Al–2.5Zr with grain size of about 5 μm at room temperature (RT) and low temperature (77 K). The results show that the alloy exhibits the higher ductility and the longer low–cycle fatigue life at 77 K than those at RT. The cyclic stress response curves show that the cyclic softening occurs at the low strain amplitudes of 0.5% and 1.0%. However, as strain amplitude increased to 1.5% and 2.0%, cyclic stress saturation appeared. When testing temperature declined to 77 K, the cyclic hardening was observed at all four strain amplitudes. The degree of cyclic  ardening increases as the strain amplitude rises. The fractography analyses suggest that transgranular fracture with well–developed fatigue striations is the predominant failure mode. The amount of secondary cracks is much higher in the alloys deformed at RT than that at 77 K. TEM examination reveals that deformation twins become more active. The primary types of twinning are {1011} and {1121}. The typical deformation microstructures consist of individual dislocation lines together with the tangled dislocation at the low strain amplitudes of 0.5% and 1.0%. As the strain amplitude increased to 1.5% and 2.0%, {1010}prismatic slip and {1121} pyramidal slip were simultaneously activated, the subgrain and dislocation cells were formed. At 77 K and strain amplitude of 2.0%, the parallel dislocation bands distribute along prismatic plane. As the strain amplitude increased to 0.5%, utual perpendicular dislocation lines appeared. The improvement of fatigue life of Ti–2Al–2.5Zr at 77 K is attributed to the constraint of inhomogeneous slip and the activation of deformation twinning.