Guoliang Zhu

Substitution behavior of Si in Al3Ti (D022): a first-principles study

A first-principles total-energy plane-wave pseudopotential method based on density functional theory has been used to investigate the site preference of Si in Al(3)Ti. The site preference parameter S has been calculated and the value of S decreases on increasing Si concentration, but is always greater than 1 while the Si concentration is lower than 25xa0at.%, indicating Si has a strong site preference for the Al sublattice. The physical origin of the site preference of Si in Al(3)Ti was studied by calculating the densities of states (DOSs) of the Al(3)Ti-Si systems. The heat of formation and the DOS of the position of the Fermi energy level were also studied to analyze the site preference of Si in Al(3)Ti.

Formation Mechanism of Spherical TiC in Ni-Ti-C System during Combustion Synthesis

The formation mechanism of TiC particles in a Ni-Ti-C system were revealed by using differential thermal analysis (DTA), XRD, and SEM to identify the reaction products in different temperature ranges. The results indicated that the synthesis mechanism of TiC in Ni-Ti-C system was complex; several reactions were involved in the combustion synthesis of TiC-Ni composite. The Ni-Ti intermediate phases play important roles during the formation of TiC. Moreover, the influence of heating rate on the size range of TiC was also discussed.

Prediction of Cavitation Depth in an Al-Cu Alloy Melt with Bubble Characteristics Based on Synchrotron X-ray Radiography

The size of cavitation region is a key parameter to estimate the metallurgical effect of ultrasonic melt treatment (UST) on preferential structure refinement. We present a simple numerical model to predict the characteristic length of the cavitation region, termed cavitation depth, in a metal melt. The model is based on wave propagation with acoustic attenuation caused by cavitation bubbles which are dependent on bubble characteristics and ultrasonic intensity. In situ synchrotron X-ray imaging of cavitation bubbles has been made to quantitatively measure the size of cavitation region and volume fraction and size distribution of cavitation bubbles in an Al-Cu melt. The results show that cavitation bubbles maintain a log-normal size distribution, and the volume fraction of cavitation bubbles obeys a tanh function with the applied ultrasonic intensity. Using the experimental values of bubble characteristics as input, the predicted cavitation depth agrees well with observations except for a slight deviation at higher acoustic intensities. Further analysis shows that the increase of bubble volume and bubble size both leads to higher attenuation by cavitation bubbles, and hence, smaller cavitation depth. The current model offers a guideline to implement UST, especially for structural refinement.

In Situ Observation of the Zr Poisoning Effect in Al Alloys Inoculated by Al-Ti-B

In situ synchrotron X-ray radiography observations of the Zr-poisoning phenomenon of an Al-20 wt pct Zn alloy inoculated by Al-5Ti-1B were carried out. The effects of Zr addition on heterogeneous nucleation, grain growth, and final grain size were quantitatively studied and further analyzed using the interdependence model. The experimental results show that the undercooling needed for nucleation increases and the nucleation rate decreases at the early stage of solidification with Zr addition, resulting in fast grain growth, a high solidification rate and increased severity of solute segregation in the solid–liquid coexistence regions. At the same time, the poisoning is a progressive process that is enhanced with the increasing Zr content, holding temperature, and holding time. The nucleation-free zone of the Zr-containing sample, either measured from the radiographs or calculated by the interdependency theory, is larger than that of the Zr-free sample. Our analysis shows that both the increase in the nucleation-free zone and the average interparticle spacing of the most potent available nucleation particles contribute to the increase of the grain size caused by Zr poisoning.

Effect of Ag Content on the Microstructure and Crystallization of Coupled Eutectic Growth in Directionally Solidified Al-Cu-Ag Alloys

A series of coupled eutectic growths along the univariant eutectic groove in the ternary Al-Cu-Ag alloy was studied to investigate the effect of Ag on the microstructure and crystallization of directionally solidified Al-Cu-Ag alloys. The results indicated that the eutectic morphology and orientation relationship (OR) between eutectic phases were modified as the Ag content in the Al-Cu-Ag alloys increased. At a lower growth velocity (Ru2009≤u20091xa0μm/s), a banded structure formed and the interlamellar spacing decreased with the increasing Ag content. At a higher growth velocity (Ru2009≥u20093xa0μm/s), the eutectic cell spacing decreased with increasing Ag content. Increasing the Ag content in the Al-Cu-Ag alloys enhanced the enrichment of the Ag solute in the liquid ahead of the quenched liquid/solid interface. In addition, increasing the Ag content in the Al-Cu-Ag alloys promoted the transformation from a “Beta 6” OR to an “Alpha 4” OR between eutectic phases. Modifications of the eutectic morphology and the OR during directional solidification were attributed to the enrichment of Ag content at the solid/liquid interface and the changes in the interfacial energy due to the increase in Ag solubility in the α-Al phase.

Effect of Mg on the Microstructure and Corrosion Resistance of the Continuously Hot-Dip Galvanizing Zn-Mg Coating

The microstructure of continuously hot-dip galvanizing Zn-Mg coating was investigated in order to obtain the mechanism of the effects of Mg on the corrosion resistance. In this paper, the vertical section of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner was calculated. The results indicates that the phase composition of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner is the same as Zn-Mg binary phase diagram, suggesting Al in the Zn-Mg (ZM) coatings mainly concentrates on the interfacial layer between the coating and steel substrate. The microstructure of continuously hot-dip galvanizing ZM coatings with 0.20 wt % Al containing 1.0–3.0 wt % Mg was investigated using tunneling electron microscopy (TEM). The morphology of Zn in the coating changes from bulk to strip and finally to mesh-like, and the MgZn2 changes from rod-like to mesh-like with the Mg content increasing. Al in the ZM coatings mainly segregates at the Fe2Al5 inhibition layer and the Mg added to the Zn bath makes this inhibition layer thinner and uneven. Compared to GI coating, the time of the first red rust appears increases by more than two-fold and expansion rate of red rust reduces by more than four-fold in terms of salt spray experiment. The ZM coating containing 2.0 wt % Mg has the best corrosion resistance. The enhanced corrosion resistance of ZM coatings mainly depends on different corrosion products.