Pin-on-disc apparatus was used to investigate the effect of temperature on the dry sliding friction characteristics of a Zr-based bulk metallic glass (BMG) near its glass transition temperature (T g = 625 K) (an analogous phenomenon in crystalline metals called the ductile-brittle transition). Unexpected variation in frictional behavior was observed in the temperature range of 603-643 K, suggesting that the glass transition and formation of protective oxide layers played roles in the tribological properties of BMG. An explanation of the behavior is given based on Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) analysis.
In this study, the precipitation and hardening of a commercial 6061 Al alloy during the combined natural and artificial ageing are analyzed, in order to maximize the alloy hardness by adjusting ageing time and temperature. The samples were solution-treated at 540°C for 2 hours, followed by water quenching. An artificial ageing was performed at 180, 200, and 220°C during a period from 2 min to 12 h. Natural ageing at room temperature during 12 h was performed simultaneously with the artificial one at different elevated temperatures. The Vickers hardness values of solution-treated and aged samples were measured. Precipitates were characterized via the X-ray diffraction and scanning transmission electron microscopy equipped with the energy dispersive X-ray spectroscopy. The highest hardness values were exhibited by samples that were naturally aged for 12 hours and artificially aged for 24 hours at 200 and 220°C. The hardness improvement was found to be controlled by the formation of β” phases before the peak ageing.
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