The influence of heat treatment(T6) on the ambient temperature dry sliding wear behavior of the extruded AZ91 alloy treated with Y using a pin-on-disc apparatus was investigated. Wear rates and friction coefficients were measured within a load range of 5–70 N at a sliding speed of 0.188 m/s over a constant sliding distance of 226.195 m. Worn surfaces and debris were examined using a scanning electron microscope equipped with an energy dispersive spectrometer. The experimental results revealed that, as applied load was increased, the alloy wear rates increased, but the friction coefficients decreased. Four chief operating wear mechanisms were observed, i.e., abrasion, oxidation, delamination and plastic deformation. The extruded AZ91 alloy treated with Y exhibited better wear resistance by adopting T6, which was mainly due to large amounts of fine Mg_(17)Al_(12) distributed in the grains and the resulting modified strength and micro-hardness.
By means of Miedema formation enthalpy model with Toop model, the excess free-energy, enthalpies of formation, excess entropies and activity values of all components of Mg-Al-Y ternary alloy were calculated with computer programming. The experimental results show that enthalpies of formation, excess freeenergy and excess entropies of the ternary alloy are negative in the whole content range, the minimum values at 1 123 K are all obtained at x Al=55%, x Y=45%, x Mg=0%, which are-37.969, –30.961 kJ/mol and-6.24 J/(mol·k) respectively. Activity curves show that the activity values of Al and Y in Mg-Al-Y ternary alloy rapidly decrease with the decrease of molar fraction, the values of which are very small when the molar fraction decreases to 0.4. It means that there is a strong interaction between Al and Y and stable compounds can be form in the Mg-Al-Y ternary alloy system.
The effects of Sm additions(0,0.5 wt.%,1.0 wt.% and 1.5 wt.%) on the eutectic Si and β-Al5 FeSi phases of ADC12 as-cast alloys were studied by optical microscopy(OM),scanning electron microscopy(SEM) and differential thermal analysis(DTA).The experimental results showed that Sm was an effective modifying agent for the eutectic Si of ADC12 alloy,when 1.0 wt.%- 1.5 wt.% Sm was added to the alloy,the coarse acicular eutectic Si was modified into fine particle or short rod structure.Moreover,the appropriate addition of Sm(about 1.0 wt.%) had marked effects on shortening the length of needle-like β-Al5 FeSi phase.Whereas,Sm was less effective on modifying the needle-like β-iron to the Chinese script or spherical α-iron phase.The modification mechanism was also discussed.
