AMP_06_September_2021

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 1 5 optimum condition for localization pri- or to the onset of macroscopic yielding occurs when the c-axis is ~30 o from the stress axis with an orientation spread of ~10 o . The authors further observed that MTRs impose a greater neighbor- hood effect on slip localization, rather than just allowing for slip transfer be- tween similarly oriented grains. They demonstrated that the mechanical con- straints imposed by two high-intensity MTRs promote plastic strain develop- ment in a region that would otherwise not be expected to undergo localized deformation. The results of Harr et al. illustrate experimentally several aspects of the Cappola et al. study and offer further clarity as to how these effects mani- fest in practical engineering materials with complex microstructures [25] . Spe- cifically, by observing the plastic slip activity in a large number of MTRs in Ti-6242 samples subjected to dwell fa- tigue, the authors concluded that the grains that facilitate long-range slip were subfeatures of MTRs identified using prevailing methods involving a 20° c-axis misorientation tolerance (Fig. 3). Long-range slip bands were de- veloped in grains well oriented for bas- al slip with aligned basal slip traces. No long-range slip was observed on pris- matic slip systems. The grains that be- haved cooperatively were not bound by Fig. 3 — (a) Slip traces measured via high spatial resolution digital image correlation overlaid (in black) on the inverse pole figure map for a dwell fatigue sample after 200 cycles run at 200°C; and (b) slip traces overlaid (in black) on MTR segmentation maps produced by using prevailing segmentation methods in DREAM.3D [24] using a 20°, 15°, and 10° misorientation angle between adjacent pixels. Different colors represent different MTRs. Long-range slip traces exist within MTRs, but they do not traverse across the entire MTR. Adapted fromHarr [26] . (a) (b) backscatter diffraction (EBSD) equip- ment (allowing larger scans to be col- lected) and incremental advances in quantification methods, there has been relatively little new quantitative infor- mation brought to bear on the details of MTRs that contribute to dwell fatigue crack nucleation [13,14,21,22] . The generally accepted model involves the time-de- pendent development of an intense slip band on a suitably oriented plane, which results in the formation of a dis- location pileup. This pileup induces a stress concentration that results in crack nucleation and subsequent prop- agation across the neighboring grain, which is in a difficult-to-deform orienta- tion having its [0001] axis nearly aligned with the stress axis. However, if the con- ditions to forma critical pileup occurred so readily, one would expect dwell fa- tigue failures to be far more prevalent than they are. As a result, fundamen- tal research into the impact of micro- texture on slip distributions should continue. One of the first observations re- garding the impact of MTRs on strain heterogeneity was made by Lunt et al. who studied the effects of MTRs on shear strain localization in unidirec- tionally rolled Ti-6Al-4V that was load- ed in tension to 2.5% global strain [9,23] . By comparing the strain heterogeneity of microtextured and nonmicrotextured samples, they observed that MTRs pro- mote strain heterogeneity and plastic strain localization. Plastic deformation was primarily accommodated by slip activity within the MTRs. Further, their work highlighted the importance of the orientation of the MTR relative to the loading direction. Their processing con- ditions produced long, thin MTRs that strained heterogeneously when load- ed perpendicular to the longitudinal di- rection of the MTR. The strains became almost completely homogenous when the loading direction was instead paral- lel to the longitudinal axis of the MTR. Significant slip activity in micro- textured Ti-6Al-4V under uniaxial tensile testing via scanning electron micro- scope (SEM) digital image correlation (DIC) was seen by Echlin et al. [8] . In a 500 x 500 µm field of view (FOV), they studied the strain localization over three to four sandwich style MTRs with a basal/transverse texture loaded along the rolling direction. At globally ap- plied strains of 0.71% and above, long- range prismatic slip traversed the entire length of a prismatic soft MTR, through as many as 20 grains. In a computa- tional study on similar sandwich style MTRs, Cappola et al. assessed strain localization as a function of mean ori- entation relative to the stress axis and the spread in orientations about the mean [24] . The authors report that the