Issue 48

A. Ghosh et alii, Frattura ed Integrità Strutturale, 48 (2019) 585-598; DOI: 10.3221/IGF-ESIS.48.57 594 O RIENTATION SPECIFIC DAMAGE EVOLUTION DURING FATIGUE he deformation microstructure of ratcheting samples show much lower volume fraction of twins due to lower applied stress as compared to tensile samples as depicted from the crystal orientation map along ND shown in Fig. 10a. Twins has been found to be prevalent in basal (B) grains (<0001> ‖ ND) and non-basal (NB) grains (<11 2 0> ‖ ND), while non-basal grains (<10 1 0> ‖ ND) are mostly twin free in all the three orientations. The characteristic feature of twin morphology and their nature could be interpreted from the band contrast maps delineated with contraction twin (blue lines) and extension twin boundaries (red line) shown in Fig. 10b. The volume fraction of contraction twin has been found to be higher compared to extension twin for 0R from Table 5. In addition, the extension twins has been found to be confined within the contraction twin as observed from Fig. 10a. Thus, extension twins are unable to reorient the parent grain orientation unfavourable for prism slip, which keeps prism slip activity high during tensile half cycle leading to slow rate of steady state creep and lower ratcheting strain accumulation. While in 45R and 90R, the volume fraction of extension twins has been found to be higher compared to contraction twins from Table 5. As indicated from Fig. 9, extension twins are formed during tensile half cycle and detwinning promote basal and pyramidal slip activity in subsequent compression and reload tension half cycle. Moreover, due to increased basal slip and pyramidal slip activity in 45R and 90R sample, cross slip becomes easy which causes rapid accumulation of higher ratcheting strain. From the above discussion it is apparent that in orientation 0R with high Schmid factor for prism slip, deformation proceeds via prism slip and formation of multi-variant contraction twins. In titanium due to low stacking fault energy of prism plane, prism slip on this plane leads to formation of twins readily [23]. Contraction twins require higher stress to nucleate and hence of dislocation pile ups are the potential nucleation site for contraction twins [13]. However, the amount of stored dislocation density in tensile specimen is much higher compared to ratcheting specimen [24]. This is due to the formation of dislocation pile up and tangles during tensile deformation while stable dislocation configuration like well organized dislocation cell structure forms during cyclic deformation. The cell walls are regions accommodating high density of dislocations surrounding dislocation free zones. The high value of low angle grain boundary (LAGB) fraction of 0R sample compared to 45R and 90R from Table 5 indicates formation of dislocation cell structures in 0R sample. Thus, cell walls could be the nucleation site for contraction twin, which helps in reducing backstress in the grain matrix during cyclic deformation. The grain boundary misorientation parameter in terms of local average misorientation angle (LAM) and grain reference orientation deviation (GROD) reported in Table 5 gives indirect measure of local and global deformation of grains. Hence, higher LAM and GROD value of sample 45R and 90R sample indicates higher overall grain deformation in 45R and 90R compared to 0R. This is due to activation of cross slip which is more feasible in orientation less favourable for primary slip system i.e, prism slip as in 45R and 90R. Similar observation has been reported in cross rolled and annealed commercially pure titanium with RD- split basal texture and hence more prone to cross slip along TD orientation [25]. The amount and rate of ratcheting strain accumulation depends on the competition between strain hardening due to primary slip and twinning and the dynamic recovery via activation of cross slip. As 45 degree orientation has strain hardening coefficient comparable to 0 degree thus 45R is less prone to cross slip and hence accumulates less ratcheting strain than expected. Cross slip activity dominates cyclic deformation with decrease in available primary slip/twin systems, which reduces geometrically necessary dislocation (GND) density required for strain compatibility between adjacent dislocation cell boundaries. The GND density map shown in Fig. 10c indicates that in 0R and 90R sample GND density is high in twin free non-basal grains compared to twinned basal grains while in 45R sample, GND density is concentrated at the grain boundaries while the grain matrix has lower GND density. This makes the crack propagation path more feasible along the grain boundaries for 45R. It is clear from the fracture surface feature at the stable fatigue crack growth region of 0R and 45R sample shown in Fig. 11. It shows that in 0R sample there are small pockets of differently oriented shallow striation marks indicating crack propagation in zig-zag path along the intersecting twin boundaries leading to higher crack propagation resistance and hence higher fatigue life of 0R. Whereas, in 45R sample due to high GND density at the grain boundaries, cracks nucleate and propagate easily along the grain boundaries forming tearing ridges while within the grain matrix, parallel arrays of shallow striation marks are present due to low GND density, thus leading to lower fatigue life of 45R. Sample %VLAGB (2-5˚) %LAGB (5-15˚) %HAGB (>15˚) %ET %CT Total twin (%) LAM (˚) GROD (˚) 0R 0.78 3.12 85.99 0.89 4.84 5.73 0.48 0.058 45R 0.14 0.89 98.95 4.41 0.82 5.23 3.01 3.72 90R 0.01 0.10 99.89 3.60 0.52 4.12 0.75 1.24 Table 5 : Grain boundary misorientation characteristics of fatigue samples T