Study on Fatigue Slip Bands and Nucleation of Microcracks by Using Scanning Tunneling Microscope

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A Scanning Tunneling Microscope (STM) with a maximum scanning area of 7.1mum X 7.4mum and a maximum height resolution of 0.1 nm was built in our laboratory, and fatigue slip bands of cyclically strain deformed high purity Au, Ag or Cu were observed with this STM. The observed thin plate specimens were cyclically deformed in air at a constant strain range of 1500 or 2000 muepsilon for 3000 or 5000 cycles by using the specially built 4 points cyclic bending machine.From the STH images of fatigued Au, slip bands were divided into two types. That is, the coarse slip bands with the height of 50-200 nm and the spacing of 500-2000 nm and the fine ones with the height of 50-80 nm and the spacing of 50-300 nm were observed. Furthermore, there existed a tendency that the spacing as well as the height of the coarse slip bands decreased as the number of cycles increased, while no effect was observed in the fine slip bands. The coarse slip bands could be regarded as. the aggregation of the fine slip bands. Another point of interest was that semicircular extrusion like patterns were observed along the silp lines. The STM images of the slip bands deformed by static bending revealed that their heights and spacings were 20-30 nm and 150-500 nm, respectively, and which were distributed more uniformly compared to the fatigued Au specimens' slip bands. In the case of fatigued Ag, variation in the heights was a little less compared to the Ag and their heights and spacings were 15-30 nm and 200-400 nm. Unfortunately we could not observe the STM images of Cu. This is regarded that we could not find the adequate condition for STM observations because of the formation of oxide layer. Based on the above mentioned results, discussions were made to get further achievement.