| Budget Amount *help |
¥3,900,000 (Direct Cost: ¥3,900,000)
Fiscal Year 2001: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2000: ¥3,400,000 (Direct Cost: ¥3,400,000)
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| Research Abstract |
Fatigue tests were performed using smooth specimens of materials subjected to two-step aging in beta Ti-22V-4Al alloy and the effect of two-step aging on fatigue strength was discussed on the basis of observation of fatigue crack initiation and small crack growth. The conclusions can be made as follows.
( 1 ) The tensile strength of the low-high step aged materials was higher than the single aged materials, but the ductility was not always improved. On the contrary, the high-low step aged materials showed lower tensile strength than the single aged materials and decreased with increasing aging temperature at the first step, but the ductility was largely increased. In particular, when the aging temperature at the first step was close to the temperature at the second step, microstructure with a good balance of strength and ductility could be established.
( 2 ) Tensile fracture mode was intergranular in the low-high step aged materials, while extensive dimples in the high-low step aged mate
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rials.
( 3 ) The step aged materials whose aging temperature at the first step was close to the aging temperature at the second step indicated slightly lower fatigue strength than, or nearly the same fatigue strength as, the single aged material. The high-low step aged materials showed no subsurface fracture in Ijng life regime, thus indicating superior fatigue strength to the single aged material in that region.
( 4 ) The fatigue strength of the high-low step aged materials characterized in terms of fatigue ratio was higher than those of the single aged material and the low-high step aged materials.
( 5 ) Fatigue cracks initiated at grain boundaries in the low-high step aged materials, while within beta grains in the high-low step aged materials.
( 6 ) The single aged material had significantly large fraction of crack initiation process in fatigue life, while the step aged materials showed almost the same fraction of both processes.
( 7 ) Subsurface fracture could be suppressed by two-step aging, particularly in the high-low step aged materials. Less
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