A new experimental method describing the determination of the mechanical spectra (complex Young’s modulus Y*=Y′+iY″ versus temperature) of materials from the liquid to the glassy state, including the glass transition, is reported. The conventional vibration-reed method developed for solids is extended to composite systems consisting of a reed substrate and a deposited material. Mathematical expressions for the evaluation of the mechanical spectrum of the deposited material are obtained by solving either directly the vibrating equation of the nonuniform reed, or that of an equivalent uniform reed, with new length and stiffness, using a coordinate transformation. The mechanical spectra of glycerol and 1,2-propanediol carbonate covering the liquid and the glassy state are presented as examples in this work. The glass transitions of these two kinds of materials, as well as the recrystallization, melting and, evaporation processes of 1,2-propanediol carbonate, are identified in the respective spectra.
Mg-AZ31 based composites with 10{20 vol% nano-sized ZrO 2 and 5{10 vol% nano-sized SiO 2 particles were fabricated by friction stir processing (FSP). The clusters of the nano-ZrO 2 and nano-SiO 2 particles, measuring 180-300 nm in average, were relatively uniformly dispersed. The average grain size of the Mg matrixes of the composites varied within 2-4 mm after four FSP passes. No evident interfacial product between the ZrO 2 particles and Mg matrix was found during the FSP mixing ZrO 2 into Mg-AZ31. However, significant chemical reactions occurred at the Mg/SiO 2 interface to form the Mg 2 Si phase. The mechanical responses of the resulting nano-composites in terms of hardness and tensile properties of these Mg/nano-ZrO 2 and Mg/nano-SiO 2 composites were examined and compared. The grain refinements and the corresponding hardening mechanisms are also analyzed and discussed.
The evolution of microstructure and texture in the AZ-series Mg alloys subjected to electron-beam welding and gas tungsten arc welding are examined. Electron-beam welding is demonstrated to be a promising means of welding delicate Mg plates, bars, or tubes with a thickness of up to 50 mm; gas tungsten arc welding is limited to lower-end thin Mg sheets. The grains in the fusion zone (FZ) are nearly equiaxed in shape and ϳ8 m or less in size, due to the rapid cooling rate. The as-welded FZ microhardness and tensile strength are higher than the base metals due to the smaller grain size. The weld efficiency, defined as the postweld microhardness or tensile strength at the mid-FZ region divided by that of the unwelded base metal, is around 110 to 125 pct for electron-beam welding and 97 to 110 pct for gas tungsten arc welding. There are three main texture components present in the electron-beam-welded (EBW) FZ, i.e.,, and, where TD, ND, and WD are the transverse, normal, and welding directions, respectively. The crystal growth tends to align toward the most closed-packed direction,. The texture in gas tungsten arc welded (GTAW) specimens is more diverse and complicated than the EBW counterparts, due to the limited and shallow FZ and the lower cooling rate. The cooling rates calculated by the three-dimensional (3-D) and two-dimensional (2-D) heat-transfer models are considered to be the lower and upper bounds. The cooling rate increases with decreasing Al content, increasing weld speed, and increasing distance from the weld top surface. The influences of the FZ location, welding speed, and alloy content on the resulting texture components are rationalized and discussed.
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