Takahiro Shikama

Analysis of Small Fatigue Crack in Al-Mg-Si Aluminum Alloy

In general, aluminum alloy does not exhibit distinct fatigue limit (knee point) in the S-N diagram. The growth of a small fatigue crack of precipitation-hardened Al-Mg-Si system alloy (6061-T6) was investigated to clarify the mechanism of non-appearance of distinct fatigue limit (knee point) in the S-N diagram. The small crack was analysed in detail by replica method, scanning electron microscope (SEM), and Electron Back Scatter Diffraction Patterns (EBSD). On the other hand, the existence of distinct fatigue limit (knee point) of new developed aluminum alloy by adding excess Mg to the 6061 alloy was found. In this study, the resistance of small crack growth of the developed alloy was compared with standard 6061 alloy. It was revealed that the resistance of crack growth of new developed alloy was higher than that of standard 6061 alloy in short crack region (l<1.0 mm).

Effect of Zr Addition on Recrystallization Behavior of Extruded Al-Mg-Si Alloys Containing Mn

Effects of Zr addition on recrystallized structure and texture in extruded Al-Mg-Si alloy containing Mn have been investigated. Materials were homogenized at temperatures in the range 550 °C to 590 °C and extruded at 500 °C at the speed of 10m/min. In extruded Al-Mg-Mn-Si-Zr alloy, the fraction of Cube oriented grains reached 40% as homogenization temperature raised to 590 °C. On the other hand, the fraction of Cube oriented grains in extruded Al-Mg-Mn-Si alloy was limited to 20-30 %. In addition, to clear the formation process of recrystallized grains in these alloys, observation of hot-compression deformed and recrystallized grain structures were carried out. It was suggested that moderate Zener drag promoted the preferential recrystallization of cube oriented grains

Giga-Cycle Property of a New Age-Hardened Aluminium Alloy Containing Excess Solute Magnesium

The giga-cycle property of a newly developed Al alloy, which contains 0.5wt.% excess Mg solute compared to a standard age-hardened 6061 alloy (6061-T6), was investigated by using smooth specimens subjected to ultrasonic fatigue. The fatigue strength of the new alloy was higher than that of a normal 6061 alloy particularly at relatively low stress amplitude level. Several analyses (surface crack observation, fractography, FIB cross-sectioning, etc.) were also conducted to reveal the micro-mechanism of the observed strength properties. The following results were obtained: i) No fatigue limit was confirmed for both 6061 and new alloy. ii) Total life (Nf) of 6061 and new alloys was determined by a single fatigue crack initiated from a surface PSB crack. iii) Crack initiation resistance defined by N25 (number of cycles to reach ρ = 25 mm-2, where ρ is the PSB crack number density) for new alloy was higher than that of 6061. iv) The higher fatigue strength of new alloy was explained by the effect of excess Mg solute which increased the resistance against the formation of PSB cracks.

Micro-Analyses of Small Cracks in 6061-T6 Aluminium Alloy Subjected to High-Cycle Fatigue

The growth of a small crack controlling the high-cycle fatigue life of a precipitation-strengthened 6061-T6 aluminium alloy was critically investigated. As the applied stress lowered, the small crack was arrested for a long period (over 106 cycles) at grain boundaries before regrowth, which resulted in a significantly slow growth process. The morphological and crystallographic details of the small crack were then analyzed with focused ion beam and transmission electron microscopy. It was revealed that the small crack was formed along fine persistent slip bands (PSBs) whose structure was fairly different from that reported for other metals. The concept of PSB-limited fatigue strength may be extended to include the present material type.