Xiangfa Liu

Morphological evolution of TiC from octahedron to cube induced by elemental nickel

TiC particles with various morphologies from octahedron to cube were synthesized and investigated by Field emission scanning electron microscopy (FESEM) in three-dimensional space. The morphological evolution of TiC grain and its growth mechanism were also discussed. TiC particles prefer an octahedral morphology (equilibrium shape) enclosed by eight {111} facets with minimized total surface free energy in Al–Ti–C alloy, while they tend to form a cube enclosed by six {100} facets under the influence of Ni in Al–Ni–Ti–C alloy. Due to the strong interaction between Ni–3d and C–2p orbitals, Ni atoms in the melt selectively absorb on {100} faces of the growing TiC crystal rather than on the polar {111} faces and reduce the specific surface energy of {100}. According to Wuffs theorem, the growth rate of {100} is lowered correspondingly, while the relative growth rate of {111} is accelerated. Thus, the higher growth rate along direction will lead to the shrinkage of {111} faces gradually, while six {100} faces are reserved to form a TiC cube because of their lower growth rates. Furthermore, a similar morphology evolution to TiC crystals can also be found in Fe- and Co-containing melts. It is revealed that the crystal growth of TiC follows the same model under the effect of group VIII elements (Fe, Co and Ni).

Microstructures and grain refinement performance of rapidly solidified Al–Ti–C master alloys

Abstract In the present work, the microstructures and grain refining performance of melt-spun Al–3.5Ti–0.15C, Al–5Ti–0.3C and Al–10Ti–1C (in wt.%) alloys have been investigated, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and grain refining tests. All three types of ingot-like alloys contain coarse slablike TiAl 3 and fine TiC particles. The microstructures of melt-spun Al–3.5Ti–0.15C and Al–5Ti–0.3C alloys are composed of spherical or near-spherical dispersed TiC particles and α-Al supersaturated solid solution, while melt-spun Al–10Ti–1C alloy consists of small blocky TiAl 3 , spherical or near-spherical TiC particles and α-Al solid solution. Furthermore, most of the TiAl 3 and TiC particles are distributed in the edge zone of the chill surface of the rapidly solidified alloy ribbons. Rapid solidification process (RSP) does not improve the grain refinement performance of Al–Ti–C master alloys. The reasons for the formation of microstructures and the grain refining performance of melt-spun Al–Ti–C master alloys have also been discussed.

Microstructure and grain refining performance of melt-spun Al–5Ti–1B master alloy

Abstract In the present work, the microstructure and grain refining performance of the melt-spun Al–5Ti–1B (wt%) master alloy have been investigated, using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and grain refining tests. It has been found that the microstructure of the melt-spun Al–5Ti–1B master alloy is mainly composed of two phases: metastable, supersaturated α-Al solid solution and uniformly dispersed TiB 2 particles, quite different from that of the rod-like alloy consisting of three phases: α-Al, blocky TiAl 3 , and clusters of TiB 2 particles. Quenching temperatures and wheel speeds (cooling rates), however, have no obvious effect on the microstructure of the melt-spun Al–5Ti–1B alloy. Grain refining tests show that rapid solidification has a significant effect on the grain refining performance of Al–5Ti–1B alloy and leads to the great increase of nucleation rate of the alloy. Nevertheless, the melt-spun Al–5Ti–1B master alloy prepared at different wheel speeds and quenching temperatures possesses the similar grain refining performance. The reasons for the microstructure formation and the improvement of the grain refining performance of the melt-spun Al–5Ti–1B master alloy have been also discussed.

Grain refinement of AZ31 magnesium alloy by new Al-Ti-C master alloys

New Al4C3-containing Al-Ti-C master alloys (Al-0.6Ti-1C and Al-1Ti-1C) were developed by introducing Ti element into Al-C melt using melt reaction method, in which most of the TiC particles distribute around Al4C3 particles. It is believed that most of the C firstly reacts with Al melt and form Al4C3 particles by the reaction Al(l)+C(s)→Al4C3(s), and after adding Ti into the Al-C melt, the size of Al4C3 particles is decreased and the distribution of Al4C3 is improved through the reaction Ti(solute)+Al4C3(s)→TiC(s)+Al(l). With the addition of 1% Al-1Ti-1C master alloy, the average grain size of AZ31 is reduced sharply from 850 μm to 200 μm, and the grain morphology of α-Mg transits from a fully-developed equiaxed dendritic structure to a petal-like shape. Al-C-O-Mn-Fe compounds are proposed to be potent nucleating substrates for primary Mg. Appropriate addition of Ti is believed to increase the grain refinement efficiency of Al4C3-containing Al-Ti-C master alloys in AZ31 alloy.

Influence of forming process on three-dimensional morphology of TiB2 particles in Al-Ti-B alloys

Abstract A series of Al-Ti-B master alloys were prepared by different preparation routes, and the TiB 2 particles in the master alloys were extracted and analyzed. It is found that the forming process has significant influence on the three-dimensional morphology of TiB 2 particles. Different preparation routes result in different reaction forms, which accounts for the morphology variation of TiB 2 particles. When the Al-Ti-B master alloy is prepared using “halide salt” route, TiB 2 particles exhibit hexagonal platelet morphology and are independent with each other. In addition, the reaction temperature almost does not have influence on the morphology of TiB 2 particles. However, TiB 2 particles exhibit different morphologies at different reaction temperatures when the master alloys are prepared with Al-3B and Ti sponge. When the master alloy is prepared at 850 °C, a kind of TiB 2 particle agglomeration forms with a size larger than 5 μm. The TiB 2 particles change to layered stacking morphology even dendritic morphology with the reaction temperature reaching up to 1200 °C.

Unveiling the Semicoherent Interface with Definite Orientation Relationships between Reinforcements and Matrix in Novel Al3BC/Al Composites.

High-strength lightweight Al-based composites are promising materials for a wide range of applications. To provide high performance, a strong bonding interface for effective load transfer from the matrix to the reinforcement is essential. In this work, the novel Al3BC reinforced Al composites have been in situ fabricated through a liquid-solid reaction method and the bonding interface between Al3BC and Al matrix has been unveiled. The HRTEM characterizations on the Al3BC/Al interface verify it to be a semicoherent bonding structure with definite orientation relationships: (0001)Al3BC//(11̅1)Al;[112̅0]Al3BC//[011]Al. Periodic arrays of geometrical misfit dislocations are also observed along the interface at each (0001)Al3BC plane or every five (11̅1)Al planes. This kind of interface between the reinforcement and the matrix is strong enough for effective load transfer, which would lead to the evidently improved strength and stiffness of the introduced new Al3BC/Al composites.

TEM observations of a rapidly solidified Al–Ti–C alloy

Abstract In the present work, the microstructural characterization of a melt-spun Al–20 Sb alloy has been carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The phases present in the melt-spun Al–20 Sb alloy were determined to be α-Al and AlSb, identical to those in the ingot-cast alloy. The microstructure of the melt-spun Al–20 Sb alloy is dominantly composed of primary AlSb dendrites embedded in the α-Al matrix, different from that of the ingot-cast alloy composed of primary AlSb plates within an α-Al/AlSb eutectic matrix. In addition, some areas comprise primary AlSb particles within the α-Al matrix in the melt-spun alloy.

Morphological evolutions and growth patterns of Zr-containing phases in aluminum alloys

Three-dimensional morphologies of Zr-containing phases in Al–(Si–)Zr alloys were studied in detail using field emission scanning electron microscopy. Combining the analysis of crystallographic features with atomic arrangement in the crystal lattices and first-principles quantum mechanics calculations, the growth mechanisms of Zr-containing phases were discussed. As reported in previous work, the increase of Si content in Al–(Si–)Zr alloys leads to the continuous composition evolution and a structure transformation from ZrAl3 to (Al,Zr,Si) and ZrSi2. The perfect morphology of ZrAl3 is compressed cubic, whose lateral surfaces are composed of four groups of symmetric planes. (Al,Zr,Si) has similar crystal shape to ZrAl3, while the cube is much thicker and the growth process is changed. Further high Si content results in the formation of the ZrSi2 phase, presenting 18-polyhedral morphology, whose upper and lower surfaces are also covered by (0 0 1) planes, while its lateral surface is composed of eight groups of symmetric {1 1 0} and {1 1 1} planes. Similar growth mechanisms of Ti-containing phases in Al–(Si–)Ti alloys have also been found.

Influence of Si on stability of TiC in Al melts

Abstract The influence of Si on the stability of TiC in Al melts was studied. It is found that TiC particles in Al melts become unstable with the addition of Si. When the melting temperature is below 890 °C, TiC will react with Al and Si to form TiAl x Si y and Al 4 C 3 phases. But if the melt temperature is above 890 °C, TiC will react with Al and Si to form to Ti 3 SiC 2 and Al 4 C 3 . It is considered that the influence of Si on the stability of TiC in Al melts is due to its incursion into TiC crystal lattice during the holding, which will cause serious lattice distortion in TiC and then speed up the out-diffusion of surrounded C atoms.

Instability of TiC and TiAl3 compounds in Al-10Mg and Al-5Cu alloys by addition of Al-Ti-C master alloy

Abstract The performance of Al-Ti-C master alloy in refining Al-10Mg and Al-5Cu alloys was studied by using electror probe micro-analyzer (EPMA) and X-ray diffractometer (XRD) analysis. The results indicate that there are obvious fading phenomena in both Al-10Mg and Al-5Cu alloys with the addition of Al-5Ti-0.4C refiner which contains TiC and TiAl 3 compounds. Mg element has no influence on the stability of TiC and TiAl 3 , while TiC particles in Al-10Mg alloy react with Al to form Al 4 C 3 particles, resulting in the refinement fading. However, TiC particles are relatively stable in Al-5Cu alloy, while TiAl 3 phase reacts with Al 2 Cu to produce a new phase Ti(Al, Cu) 2 , which is responsible for the refinement fading in Al-5Cu alloy. These indicate that the refinement fading will not occur only when both the TiC particles and TiAl 3 compound of Al-Ti-C refiner are stable in Al alloys.