Quantitative Precipitate Classification and Grain Boundary Property Control in Co/Ni-Base Superalloys

A correlative approach is employed to simultaneously assess structure and chemistry of (carbide and boride) precipitates in a set of novel Co/Ni-base superalloys. Structure is derived from electron backscatter diffraction (EBSD) with pattern template matching, and chemistry obtained with energy dispersive X-ray spectroscopy (EDS). It is found that the principal carbide in these alloys is Mo and W rich with the M₆C structure. An M₂B boride also exhibiting Mo and W segregation is observed at B levels above approximately 0.085 at. pct. These phases are challenging to distinguish in an SEM with chemical information (EDS or backscatter Z-contrast) alone, without the structural information provided by EBSD. Only correlative chemical and structural fingerprinting is necessary and sufficient to fully define a phase. The identified phases are dissimilar to those predicted using ThermoCalc. We additionally perform an assessment of the grain boundary serratability in these alloys, and observe that significant amplitude is only obtained in the absence of pinning intergranular precipitates.

     

The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti–6Al–4V

Three different product forms of Ti–6Al–4V, unidirectionally rolled and cross-rolled plate and forged bar, have been cyclically loaded within the high-cycle fatigue regime to investigate fatigue crack initiation. The fracture surfaces of fatigued specimens contained large regions of neighbouring facets. The majority of facets examined had a near-basal fracture plane. It was shown that grains favouring crack initiation were primarily those with misorientations between 15° and 40° from the loading direction, whereas other orientations served as crack growth paths. This implies that the formation of a facet requires a combination of a moderately high Schmid factor for basal slip coupled with a tensile component perpendicular to the basal plane. The large regions of neighbouring facets on the fracture surface were found to be a consequence of cracking within a macrozone unfavourably oriented for slip found within the microstructure, i.e. one with its c-axis near to the loading direction.     

The role of microtexture on the faceted fracture morphology in Ti–6Al–4V subjected to high-cycle fatigue

Microtextured regions (or macrozones) are commonly reported in titanium alloys and are believed to be related to fatigue life. Here, fractographic investigations are conducted on bimodal Ti–6Al–4V plate, including transmission electron microscopy to determine the mechanism of fatigue facet formation and electron backscattered diffraction to examine the underlying macrozone structures. It is found that macrozones oriented with their c-axis close to the loading direction are responsible for facet formation, and that the facets are associated with basal slip. Microtextured regions with c-axis orientations near-perpendicular to the loading direction are believed to act as barriers to faceted crack growth, based on the change in crack morphology as the crack crosses a macrozone boundary. The variant selection occurring during the transformation of the retained beta appears to favour a common orientation with the surrounding primary alpha grains, contributing to the size of the macrozones and therefore to the extent of the observed faceted regions.     

In situ observation of texture and microstructure evolution during rolling and globularization of Ti–6Al–4V

The evolution of texture in β-annealed Ti–6Al–4V during α–β rolling and so-called recrystallization annealing has been examined using synchrotron X-ray diffraction and ex situ characterization. During rolling, the initial α (0 0 0 2) texture softens and the colony α becomes kinked. During globularization, the texture strengthens as highly strained (and hence misoriented) areas of the laths disappear and this strengthening continues once coarsening of the primary α becomes dominant. At shorter heat treatment times the α_s laths that form on cooling do so with a range of variant-related orientations to the β, but at longer annealing times this α_s takes on the orientation of the surrounding α_p. The implications for the mechanical performance of macrozone-containing bimodal Ti–6Al–4V material are discussed.