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 4 Fig. 1 — Secondary electron image of fracture surface showing faceted crack propagation through an MTR under dwell fatigue in Ti-8Al-1Mo-1V: (a) overview showing several facets in primary α grains; and (b) higher magnification of dashed region marked in (a) showing an individual facet with ridges on surface. Fig. 2 — X-ray tomography reconstruction of a dwell fatigued Ti-8Al-1Mo-1V alloy showing a sharp crack. Crack size is consistent with size and shape of MTRs in the material’s microstructure. (a) (b) M ost mechanical properties of ti- tanium alloys can be directly linked to the slip length a dislo- cation can travel before encountering a boundary [1] . Due to the similar align- ment of grain orientations in a micro- textured region (MTR), it has long been hypothesized that MTRs behave as large grains that allow for easy slip trans- fer across the entirety of the MTR [2-9] . Early attempts to quantitatively de- scribe the size of MTRs by Woodfield et al. used a point-to-point crystallograph- ic misorientation tolerance angle of 20° to define the extent of the MTRs [10,11] . This procedure produces MTR sizes that show a close correlation to the size of the faceted initiation sites observed on dwell fatigue fracture surfaces of Ti-6242 [2,12,13] . Such approaches also do a reasonable job at modeling the effect of microtexture on average material response [11,13,14] . Crack nucleation sites under cy- clic and dwell fatigue loading tend to have sharp, faceted features formed by transgranular fracture through primary α and also through suitably oriented secondary α colonies, if present (Fig. 1). The size and shape of the faceted re- gion depends on the applied waveform (cyclic vs. dwell) and on the size and shape of the underlying morphology of the MTRs. For example, Fig. 2 shows a slice through an x-ray tomographic recon- struction of a Ti-8Al-1Mo-1V alloy sub- jected to dwell fatigue. One can ob- serve a sharp, high aspect ratio crack that mirrors the size of the underly- ing MTRs. These crack morphologies are only possible because the growth rate through suitably oriented grains is so much higher than growth by oth- er mechanisms. The degree of acceler- ation can be up to 100x as shown from direct measurement on the fracture sur- faces, and also via acetate replica mea- surement of small cracks initiated from focused ion beamnotches in heavily mi- crotextured Ti-6Al-4V [15,16] . In contrast to dwell, the faceted initiation region for purely cyclic fatigue cracks is small (on the order of a few hundred µm) and occurs at the sample surface. While the MTR may influence slip systems [20] . Yet the fracture occurs in a highly brittle manner, perhaps as- sisted by the hydrogen enhanced, local- ized plasticity mechanism [12] . Beyond the extent of the faceted initiation site, MTRs have an almost negligible effect on long-crack growth rates. However, they may offer slight improvements in the long-crack threshold due to bene- fits from the development of roughness induced closure. DWELL FATIGUE, SLIP ACTIVITY While there have been great advances in the speed of electron early development of the crack as- pect ratio, only very minor changes in growth rate have been measured [17] . Ad- ditionally, there are differences in the local crystallographic orientations of the grains at the initiation sites. When loaded below the macroscopic yield strength of the material, formation of dwell facets occurs in hard MTRs that have their c-axis within ~10 o from the loading direction [12,18,19] . In addition, the crystallographic plane of fracture is about 10° misoriented from the bas- al plane [2] . This orientation results in ex- tensive dislocation activity on multiple