A Subsurface Fatigue Crack Generation Model in near Alpha Titanium at Low Temperature

The subsurface fatigue crack generation processes in near α type titanium alloy were divided into four steps: (1) development of a saturated dislocation structure by cyclical micro-plastic strain accumulation, (2) generation of localized slip and/or microcracking to relax the stress concentration in the vicinity of a boundary, (3) microcrack growth and transition to main crack, and (4) crack propagation. The experimentals on transgranular facets formation in Ti-Fe-O alloy were reviewed and a subsurface fatigue crack generation model was discussed. The β platelets which were aligned between the recrystallized α grain and the recovered α grain were responsible for the microcrack generation to form (0001) tansgranular facet in the recrystallized α grains. A combination of the shear stress and tensile stress normal to the basal plane may give a trigger of the (0001) microcracking in the recrystallized α grain. The localized shear stress following slip off on the basal plane was activated at the microcrack tip in the recrystallizedαgrain, and the microcrack grew into the recrystallized α grain to form (0001) transgranular facet.

The Engine Valve Umbrella Head Hot Forging Die Improvement Measures

When using monolithic hot die to forge the umbrella head of valve parts, hot die of a short life. Through the analysis of factors caused by mold, the structure design, mold material, mold manufacturing and use to take certain measures, the production practice showed that die fatigue crack generation and extended time to prolong service life, obviously improved.

Subjects on Fatigue Crack Generation in High Strength Alloys for Long-Life Design

The very localized deformation processes have been found to be decisive for subsurface fatigue crack generation at the lower stress level such as the elastic incompatibility at boundaries where only a very small fraction of plastically deformed grains was detected. The material design and its microstructure modification to achieve higher fatigue resistance in long-life range are needed for the high strength alloys, which is one of the ways developing an ecomaterial. Novel systems have employed to clarify the substance crack generation and growth mechanisms of high strength alloys. The initial crack size highly depends on the maximum cyclic stress range, which implies a threshold of stress intensity range controlling mechanism. Heterogeneous microplasticity due to planar slip and restricted system is considered to play an important role on making the subsurface crack. Then, it should be progressed in the understanding of damage stage in high-cycle fatigue fracture process.