The first-principles density-functional calculation was conducted to investigate the electronic band structures of titanium dioxide with heavy nitrogen doping (TiO2-xNx).The calculation results indicate that when x≤0.25,isolated N 2p states appear above the valence-band maximum of TiO2 without a band-gap narrowing between O 2p and Ti 3d states.When x≥0.50,an obvious band gap narrowing between O 2p and Ti 3d states was observed along with the existence of isolated N 2p states above the valence-band of TiO2,indicating that the mechanism proposed by Asahi et al operates under heavy nitrogen doping condition.
A high-Mg2Si content Al alloy was extruded by equal channel angular pressing(ECAP) for 8 passes at 250 ℃ and an ultrafine-grained structure with an average grain size of about 1.5 μm was achieved.The coarse skeleton-shaped Mg2Si phase presenting in the as-cast alloy are significantly fragmented into fine rod-shaped as well as equiaxed particles mostly less than about 230 nm and become relatively dispersed.The tensile strength 192.8 MPa and the elongation up to 31.3% at ambient temperature are attained in the 8-pass ECAPed alloy versus 163.3 MPa and 9.1% in the as-cast alloy.High-temperature creep test at 250 ℃ reveals that the ECAPed sample exhibits a high elongation close to 100% at a relatively high creep rate 7.64×10-5 s-1,compared to the elongation 56% at a low strain rate 1.74×10-7 s-1 in the as-cast alloy.
