Shuangjin Liu

The formation mechanism and interface structure characterization of in situ AlN/Al composites

The fabrication of AlN/Al composites through nitrogen-bearing gas (NH3) bubbling method into Al melts or Al–Mg melts was once experimentally and analytically studied. The experimental results show that Mg addition greatly enhanced the nitridation rate and volume fraction of AlN. However, a detailed characterization of these changes is still incomplete. This paper presents results of an extensive experimental study and theoretical analysis carried out to investigate the profound effect of the formation of AlN with Mg addition. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy were used to characterize the microstructure. The edge-to-edge matching crystallographic model which applied to the three common crystal structures (body-centered cubic, face-centered cubic, and hexagonal close packed) was used to analyze and investigate this notable phenomenon. The calculation and analysis of theoretical model manifested that AlN is formed by the direct interfacing of pure Al or Al–Mg and NH3 with active nitrogen, instead of forming into the precursor Mg3N2 first and then the precursor transformed to AlN.

Ti5Si3/β-Ti nano-core–shell structure toughened glassy Ti alloy matrix biocomposites

In the experiment, Ti75Zr11Si9Fe5 and Ti66Zr11Si15Fe5Mo3 ingots were prepared by vacuum arc-melting furnace. Both Ti alloy ribbons of 3–5 mm in width and about 80 μm in thickness were made from bulk samples by an as-quenched technique under an argon atmosphere. Both melt-spun glassy ribbons exhibit large supercooled liquid regions, high reduced glass transition temperatures, and good thermal stabilities. For both alloys, the stable phases after heating are a Ti glassy matrix and in situ nano-Ti5Si3 particles encircled by nano shell of β-Ti. After the Ti5Si3/β-Ti nano-core–shell structure was in situ formed, in situ Ti5Si3/β-Ti nano-core–shell structure toughened glassy Ti alloy matrix composites were prepared. For the Ti66Zr11Si15Fe5Mo3 ribbons, its high strength is attributed to both Mo solution strengthening and nano core–shell Ti5Si3/β-Ti toughening and dispersion strengthening. Observations and analysis on microstructure and fracture morphology of melt-spun glassy ribbons indicated that multi-slip bands were formed during the tensile test.