Hengzhi Fu

Crystal Orientation Measured by XRD and Annotation of the Butterfly Diagram

Abstract An improved X-ray diffraction rotating orientation measurement method is proposed that can measure the crystal lattice orientation quickly and accurately. The method can also directly assess the quality of preferentially oriented specimens and quasi-single crystals through use of the butterfly diagram to measure the crystal orientation distribution.

Microstructure and magnetic properties developed by hot pressing of cast Pr–Fe–B magnets

The hot pressing process of cast Pr19Fe74.5B5Cu1.5 magnets were studied. Microstructure studies reveal that during the hot pressing, the crushing of Pr2Fe14B matrix grains and the relative slip and rotation between the grains take place and the Pr‐rich melt is squeezed out of the magnet, leading to fine, dense and well‐aligned Pr2Fe14B grains surrounded by a thin Pr‐rich phase layer in the fully‐deformed magnets. Magnetic properties of Br=9.9 kG, iHc=11.0 kOe and (BH)max=24.0 MG Oe are obtained. The increase of coercivity is attributed to the fine Pr2Fe14B grains and the thin Pr‐rich phase layer continuously distributed at the grain boundaries. The increasing remanence results primarily from the development of the easy‐axis alignment as well as from the densification of the Pr2Fe14B matrix. The easy‐axis alignment is developed by the relative slip and rotation of the crushed Pr2Fe14B grains. A full and slow hot pressing is necessary for developing a good easy‐axis alignment and high magnetic properties.

Preparation and characterization of Al2O3/Y3Al5O12/ZrO2 ternary hypoeutectic in situ composites by laser rapid solidification

The directionally solidified oxide eutectic in situ composite is one of the most promising high-temperature structural materials in oxidizing environments. Pore-free rods and plates of ternary Al2O3/Y3Al5O12 (YAG)/ZrO2 hypoeutectic in situ composites with hypoeutectic composition (71 mol % Al2O3, 17 mol % Y2O3, and 12 mol % ZrO2) are prepared rapidly by the laser zone remelting technique. The hypoeutectic growth and microstructure characteristic of grown crystals are studied using the scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy, aiming to understand well the rapid solidification behavior of the ternary oxide system of Al2O3–Y2O3–ZrO2. The rapidly solidified Al2O3/YAG/ZrO2 hypoeutectic shows a refined, interpenetrating, and irregular lamellar structure with a reticular distribution of coarse Al2O3 and yttrium aluminum garnet (YAG) and smaller ZrO2 phases. The fine ZrO2 phases are partially embedded at the Al2O3/YAG interfaces. Moreover, the typical dendrite microstru...

Dependence of Competitive Grain Growth on Secondary Dendrite Orientation During Directional Solidification of a Ni-based Superalloy

The competitive grain growth in bicrystal samples during unidirectional solidification of a Ni-based superalloy was found to depend on secondary dendrites perpendicular to the grain boundary of bicrystal samples, rather than primary dendrites parallel to the thermal gradient as generally recognized. The primary dendrite orientation, however, had significance for the dendrite blocking in overgrowth processes and the resultant overgrowth rate during competitive grain growth.

Microstructure evolution of Ni, Cr, Al–TaC in situ composite directionally solidified under a high temperature gradient

Abstract The effect of solidification rate on the solidified structure, solute segregation and microstructures such as TaC rods volume fraction, TaC rods average spacing, TaC average transverse area and γ′ phase were experimentally investigated under a high temperature gradient by employing a method of directional solidification with liquid metal cooling. Three types of the solidified structure were identified: planar, cellular and dendritic. Both solidified structure and solidification rate had an effect on the eutectic morphologies. The TaC rods volume fraction could be changed with increasing solidification rate. TaC average inter-rods spacing and TaC average transverse areas decreased, the PS (parameter of shape) of γ′ tends to one and the size of γ′ phase decreased with increasing solidification rate.

Investigation on the explosive welding mechanism of corrosion-resisting aluminum and stainless steel tubes through finite element simulation and experiments

To solve the difficulty in the explosive welding of corrosion-resistant aluminum and stainless steel tubes, three technologies were proposed after investigating the forming mechanism through experiments. Then, a 3D finite element model was established for systematic simulations in the parameter determination. The results show that the transition-layer approach, the coaxial initial assembly of tubes with the top-center-point the detonation, and the systematic study by numerical modeling are the key technologies to make the explosive welding of LF6 aluminum alloy and 1Cr18Ni9Ti stainless steel tubes feasible. Numerical simulation shows that radial contraction and slope collision through continuous local plastic deformation are necessary for the good bonding of tubes. Stand-off distances between tubes (D1 and D2) and explosives amount (R) have effect on the plastic deformation, moving velocity, and bonding of tubes. D1 of 1 mm, D2 of 2 mm, and R of 2/3 are suitable for the explosive welding of LF6-L2-1Cr18Ni9Ti three-layer tubes. The plastic strain and moving velocity of the flyer tubes increase with the increase of stand-off distance. More explosives (R>2/3) result in the asymmetrical distribution of plastic strain and non-bonding at the end of detonation on the tubes.

Effect of local solidification time on the dendrite-to-cell transition at high growth rates

Abstract The evolution processes of the morphological transition for an Al Cu alloy within a wide range of growth rates under different temperature gradients were observed. The results show that with increasing growth rate the interface morphology changed from plane to cell, to dendrite, and then at high enough growth rates the dendrites with sidebranches transformed into sidebranch-free cells again. The investigation also proved that at high enough temperature gradients the sidebranching process for dendrites was restricted and disappeared in a whole growth rate range. The mechanism of the cell-to-dendrite-to-cell morphological transition is discussed. There exist two factors causing the occurrence and decay of sidebranches during crystal growth. One is the intrinsic morphological instability of the dendrite tip which is the driving force of the sidebranching. The other is the local solidification time (LST) of the solidified front which is the time-and-space condition. A space-limited model for sidebranching has been constructed. The criterion calculation shows the effect of LST on the sidebranching process and the nature of the decay of sidebranches at high growth rates.

Effect of Melt Superheating Treatment on Directional Solidification Interface Morphology of Multi-component Alloy

The influence of melt superheating treatment on the solid/liquid (S/L) interface morphology of directionally solidified Ni-based superalloy DZ125 is investigated to elucidate the relationship between melt characteristic and S/L interface stability. The results indicate that the interface morphology is not only related to the withdrawal velocity ( R ) but also to the melt superheating temperature ( T s ) when the thermal gradient of solidification interface remains constant for different T s with appropriate superheating treatment regulation. The interface morphology changes from cell to plane at R of 1.1/im/s when T s increases from 1500°C to 1650°C, and maintains plane with further elevated T s of 1750°C. However, the interface morphology changes from coarse dendrite to cell and then to cellular dendrite at R of 2.25 μm/s when T s increases from 1500°C to 1650°C and then to 1750°C. It is proved that the solidification onset temperature and the solidification interval undergo the nonlinear variation when T s increases from 1500°C to 1680°C, and the turning point is 1650°C at which the solidification onset temperature and the solidification interval are all minimum. This indicates that the melt superheating treatment enhances the solidification interface stability and has important effect on the solidification characteristics.

Effect of melt heat treatment on the solid/liquid interface morphology of directional solidification

Abstract A set of quadral-electrode DC input voltage measure equipment was built up to detect the electrical resistance of Sb–Bi single-phase alloy. The results suggest that Sb–5%wt Bi undergoes structural transition in the temperature range of 730–750°C. By overheating the melt to different temperatures and then cooling to 690°C after a period of time, the samples are directionally solidified in a Bridgeman-type furnace. It is found for the first time that after heat treatment of the melt, the solid/liquid interface morphology shows significant variation depending on the speciic heat treatment. Also, in Ag–Cu, Al–Cu systems, bicrystal growth with different orientation is found to have different interface morphologies after heat treatment. It is believed that the evolution of the cluster size distribution in the melt contributes to these phenomena.

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000 h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.