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.
Effects of （Pr＋Ce） addition on the Al-7Si-0.7Mg alloy were investigated by optical microscope （OM）, energy diffraction spectrum （EDS）, X-ray diffraction （XRD） and tensile tests. The results showed that the Al-7Si-0.7Mg alloy was modified with （Pr＋Ce） addition. The needle-like eutectic silicon phase developed into rose form and the crystalline grains decreased in size and showed a high degree of spheroidization. When the amount of the （Pr＋Ce） addition reached 0.6 wt.%, the mean diameter was 31.8μm （refined by 50%）. The aspect ratio decreased to 1.35, and the tensile strength and ductility reached 192.4 MPa and 2.18%, respectively At higher levels of addition, over-modification occurred, as indicated by increased grain size and reduced mechanical properties. The poisoning effect of the （Pr＋Ce） addition on eutectic silicon and the constitutional supercooling caused by the （Pr＋Ce） addition were the major causes of alloy modification, grain refinement, and the improvement of mechanical properties.
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-A1-Y ternary alloy were calculated with computer programming. The experimental results show that enthalpies of formation, excess free- energy 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 A1 and Y in Mg-A1-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 A1 and Y and stable compounds can be form in the Mg-A1-Y ternary alloy system.
The effects of solution treatment on the evolution of the second phases and mechanical properties of7075Al alloy werestudied with scanning electron microscopy(SEM),energy dispersive X-ray spectrometry(EDS),differential scanning calorimetry(DSC),hardness and tensile tests.The results show that Mg(Zn,Cu,Al)2phases gradually dissolve into the matrix,yet the size andmorphology of Al7Cu2Fe phase exhibit no change with the increase of the solution treatment temperature and time due to its highmelting point.When the solution treatment temperature and time continue to increase,the formation of coarse black Mg2Si particlesoccurs.Compared to the as-cast alloy,the microhardness,tensile strength,and elongation of the sample under solution heat treatmentat460°C for5h are increased by55.1%,40.9%and109.1%,respectively.This is because the eutectic Mg(Zn,Cu,Al)2phases almostcompletely dissolve and basically no coarse black Mg2Si particles are formed.
The effects of Sm additions （0, 0.5 wt.%, 1.0 wt.%and 1.5 wt.%） on the eutectic Si and β-Al5FeSi 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-likeβ-Al5FeSi phase. Whereas, Sm was less effective on modifying the needle-likeβ-iron to the Chinese script or sphericalα-iron phase. The modification mecha-nism was also discussed.
