Real components are usually subjected to variable amplitude fatigue, and yet the deformation micromechanisms that occur due to such load changes have barely been the subject of study. Here, unidirectionally rolled equiaxed Ti6Al-4V plate was subjected to mixed dwell and variable amplitude low cycle fatigue (LCF), with the finding that overloads near the yield stress were found to retard subsequent fatigue crack growth, whilst elastic underloads were found to accelerate subsequent growth. Dwell intervals were found to be especially damaging, to a far greater extent than either dwell or LCF alone. Dwell facets were found to initiate subsurface and to be smoother than LCF facets, but were otherwise similar in orientation (∼ 30 • to the loading axis) and crystallographic plane, 2-13 • from (0002). However, no alteration of the slip bands underlying striations was observed at the point of load changes using TEM. In failure investigation, striation counting is an important tool; the loading changes used were not found to affect the number of striations formed. Dislocation networks were found between similarly oriented grains in the as-received material, which disintegrated under dwell loading and at high stresses.

A significant contention in the recent literature on dwell fatigue has been the modified Stroh model, that dwell fatigue initiating facets form at the interface between 'hard' grains that are poorly oriented for slip and 'soft' grains that are well oriented for slip. It has then been suggested that regions of common orientation, termed macrozones, arising from the prior beta grains, will promote such nearest neighbour pairs and therefore promote dwell fatigue cracking. Here, by sectioning back from a dwell fatigue crack initiation site we observe just such an initiation at a macrozone boundary, whereas we do not observe low cycle fatigue crack initiation at macrozone boundaries.

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