Dong Qiu

A systematic study of irrational precipitation crystallography in fcc–bcc systems with an analytical O-line method

Precipitations that exhibit irrational transformation crystallography can often be explained in terms of the O-lines in the habit plane. The present work developed an analytical O-line method, which is more useful than the numerical O-line method to study the variation in the habit plane and its structure in a systematic manner. This method has an advantage that the orientation relationship (OR) is the output from the calculation, while in many other models interpreting precipitation crystallography the OR is either an input or is constrained by a prerequisite condition. This paper described the derivation of this method and its application to the study of the precipitation crystallography in fcc-bcc systems. Examples were given for three alloy systems, namely Cu-0.33wt%Cr and Ni-45wt%Cr alloys and a duplex-phase a-y stainless steel. The solutions include the OR between the precipitates and their matrix, the habit plane, the invariant line and the dislocation structure in the habit plane. The calculated results are in good agreement with the experimental observations reported in the literatures. Furthermore, the variation in the optimum O-line features with the lattice parameter ratio has also been studied systematically, providing some guidelines for estimating and understanding possible transformation crystallography from different fcc-bcc systems.

Crystallography of recently developed grain refiners for Mg–Al alloys

The grain refinement potency of some recently reported grain refiners for Mg–Al based casting alloys, including TiC, TiN, SiC and TiB2, was investigated from a crystallographic viewpoint using the edge-to-edge matching model. The results show that TiB2 is more likely to form an energetically favourable orientation relationship with the Mg matrix, and it is predicted that TiB2 should have the best grain refinement efficiency. The other three compounds may also serve as heterogeneous nucleation sites, but are not likely to be as effective as TiB2. The present analysis provides a better rationale of the superior grain refinement performance of TiB2 compared with SiC.

The Effect of Solute Elements on the Grain Refinement of Cast Zn

The effect of both peritectic-forming elements (Cu and Ag) and eutectic-forming elements (Mg and Al) on the grain refinement of cast pure Zn was investigated. It is found that these four alloying elements lead to effective grain refinement of cast pure Zn, although they have different values of growth restriction factor (Q). Mg and Al seem to have better grain refining efficiency for cast pure Zn than Cu and Ag. These results raise questions regarding the mechanisms of grain refinement in Zn-based alloys, and therefore further studies are required.

Structures in irrational singular interfaces

We consider periodic good matching bands (which are centered at the O-lines) as the characteristic feature of the structures in irrational singular interfaces in the primary preferred state. This feature is shared by various structures described by different models for irrational singular interfaces, and it can be used to consolidate different descriptions. We have made a quantitative analysis on the distribution of good matching zones (GMZs) in a relationship with plane matching geometry. This analysis emphasizes that matching of one set of principal planes does not represent good lattice matching. Good lattice matching is possible only at the locations of 0-d intersections, where three sets of nonlinearly related Moiré planes intersect. Matching of one or more sets of principal planes in an interface usually implies the possible presence of periodic GMZs in the interface. The analysis also explains why and in what condition a dislocation configuration can be described by the traces of Moiré planes. The distribution of exact 0-d intersections can be determined based on the O-lattice theory. The approximate 0-d intersections can be used for determining a possible interfacial structure when periodic O-elements do not exist.

Crystallographic study of grain refinement of Al by Nb addition

The grain refinement of Al by the addition of a small amount of peritectic-forming solute, Nb, has been studied from the crystallographic point of view. Combining the observations of optical microscopy and scanning electron microscopy with the results of energy-dispersive X-ray spectroscopy and X-ray diffraction, it is confirmed that the particles observed at or near the grain centres of refined Al alloys are pro-peritectic Al3Nb particles. The crystallographic matching between the Al3Nb particles and Al grains has also been evaluated using an edge-to-edge matching model and further verified using electron backscatter diffraction and transmission electron microscopy. It is found that there are reproducible crystallographic orientation relationships between the Al3Nb particles and Al grains, and the experimental results are consistent with the predictions of the edge-to-edge matching model. This implies that the pro-peritectic Al3Nb particles are favourable nucleation sites for Al grains from the crystallographic point of view. Furthermore, the analysis of the size distribution of Al3Nb particles reveals that the Al3Nb particles at the grain centres have relatively large particle size, which also corroborates the high potency of Al3Nb according to the free growth model. It is therefore concluded that the significant grain refinement resulting from the addition of Nb is predominantly attributed to the in situ formed Al3Nb particles which promote grain refinement via enhanced heterogeneous nucleation.

The influence of the effect of solute on the thermodynamic driving force on grain refinement of Al alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.

Grain refinement of Mg-Li-Al cast alloys by adding typical master alloys

Commercial Al-3Ti-1C and Al-5Ti-1B master alloys were added in order to refine the grains of Mg-Li-Al alloys. The effects of their addition levels on grain refinement of Mg-Li-Al cast alloy were investigated and the mechanism of the grain refinement was discussed. The results showed that the addition of Al-3Ti-1C master alloy reduced the grain size of LA141 cast alloy from 900 mu m to 400 mu m, while the addition of Al-5Ti-1B master alloy reduced the grain size of LA51 cast alloy from 500 mu m to 240 pm. The grain refining mechanism was the heterogeneity nucleation of TiC and TiB(2), because of less than 10% mismatches of TiC/beta-Li and TiB(2)/alpha-Mg.

A novel composite porous coating approach for bioactive titanium-based orthopedic implants

Surface modification of titanium-based implants is considered a highly effective solution to enhance osseointegration. This study describes a novel Ti/hydroxyapatite (HA) composite porous coating produced using a cold spraying technique. Experimental results indicate desirable open-cell structure with 50-150 μm pore size and 60-65% macroporosity. In particular, the reinforced HA particles are exposed to the surface of the coating resulting in enhanced mineralization ability in simulated body fluid. None of the coatings displayed a cytotoxic response in SaOS-2 cells cultured in vitro for up to 48 h. The bond strength between the porous coating and the Ti substrate was found to be 20 MPa. These properties are comparative to or better than products currently on the market and thus this novel coating has potential use in orthopedics.

Crystallography of self-assembled DySi2 nanowires on a Si substrate

A recently developed crystallographic model, edge-to-edge matching, has been used to interpret the crystallographic features of self-assembled DySi2 nanowires on Si substrates. All of the observed orientation relationships (ORs) and interface orientations of the DySi2 on Si(111), (001), and (110) were predicted by one criterion. The calculated results are fully consistent with the previous high-resolution transmission electron microscopy observations. The preference for each OR and interface was discussed in terms of the competition between thermodynamics and kinetic factors. This model can also be used in other epitaxy systems and has strong potential for future nanostructure design.

Crystallography of grain refinement in cast zinc-copper alloys

Adding the peritectic forming element Cu effectively reduced the average grain size of cast Zn by over 85%. At a specified cast condition, the smallest grain size was obtained at 2wt% Cu addition. A further increase in Cu content led to grain coarsening in the cast Zn-Cu alloys. Although the solute effect of Cu was predominately responsible for the grain refinement through restriction of the grain growth, it was found that the variation of grain size is also closely related to the formation of the pro-peritectic phase, -CuZn4. Crystallographic calculations using the edge-to-edge matching model showed low interatomic misfit and interplanar mismatch between Zn and the -CuZn4 phase. In addition, a reproducible h.c.p.-h.c.p. (h.c.p. denotes hexagonal close-packed) orientation relationship between Zn and the -CuZn4 particles (located within the Zn grain centres) was also experimentally determined using the electron backscattered diffraction method. This indicated the high potency of the pro-peritectic -CuZn4 particles as effective heterogeneous nucleation sites for η-Zn, which further refined the Zn grains. However, when the Cu content was over 2.0wt%, formation of large -CuZn4 particles resulted in grain coarsening of the cast alloys.