Hong Zhong

Numerical Simulation of Titanium Alloy Ingot Solidification Structure during VAR Process Based on Three-Dimensional CAFE Method

Abstract A multiscale three-dimensional (3D) mathematical model has been established for simulating the temperature field, the fluid flow and the solidification structure of Ti-6Al-4V alloy ingot during vacuum arc remelting (VAR) process, which consists of the macroscopic mass, the momentum and energy conservation equations and the macroscale model of the nucleation and growth of grains. On the basis of heat transfer and fluid flow calculation, the 3D solidification structure formation of the ingot during whole VAR process has been obtained. Comparing the simulation result with the experimental observation, a reasonably qualitative agreement is achieved on grain structure and grain growth pattern. In particular, when taking the heat radiation into consideration during the calculation, the columnar grains on the ingot top are predicted well. Furthermore, a sensitivity study of the effect of natural convection on the grain structure has been carried out. The results show that the natural convection has a great influence on columnar-to-equiaxed transition (CET) and grain size, expressing as promoting the CET and refining the grain.

Deformation Micromechanisms of a Ti-Based Metallic Glass Composite with Excellent Mechanical Properties

A Ti-based metallic glass composite with composition of Ti48Zr20Nb12Cu5Be15 exhibits good ambient plasticity in tensile and compressive loading. The macro and micro mechanisms during deformation have been investigated systematically. Obvious asymmetry between the tensile and compressive properties of the composite has been observed, indicating amorphous matrix effect on the metallic glass composite. The micro fracture mechanism of dendrites in compression can also be attributed to two mechanisms: shear induced fracture in major and tension induced fracture in local, revealing a constraint of matrix induced complex stress state in composite. Pile-ups of dislocations in dendrites cause work-hardening of composite, and the impedance of dendrites to shear bands is responsible for the improvement of plastic strain.

Microstructure and thermoelectric properties of un-doped Mg 2 Si 1−x Sn x single crystals prepared by high temperature gradient directional solidification

Un-doped single crystals of Mg2Si1−xSnx (x = 0.55, 0.65 and 0.75) were successfully prepared by high temperature gradient directional solidification (HGDS). In the Mg2Si0.45Sn0.55 crystal, Mg2Si precipitates were observed in the solidified microstructure, and no precipitates in the single crystals of Mg2Si0.35Sn0.65 and Mg2Si0.25Sn0.75. By measuring the electronic transport properties of these three single crystals, the Mg2Si0.35Sn0.65 has a largest PF value, about 2.5 times more than that of the nanocrystalline prepared by solid-state reaction methods. The corresponding ZT values of Mg2Si0.35Sn0.65 single crystal are greatly improved. It indicates that, the Mg2Si1−xSnx crystals prepared by HGDS can not only have a uniform microstructure, but also optimize the TE performance of the crystal. In addition, the first-principles calculation has been conducted to examine the intrinsic properties of Mg2Si1−xSnx single crystals, and the calculated data agree well with the experimental results.

Microstructure and fracture toughness of Cr/Cr2Nb alloys with trace Y addition

The microstructures of binary Cr/Cr2Nb alloys doped with 0.1 at-% rare earth element (REE) Y were characterised by fine interlamellar spacing, and the growth morphology of Cr2Nb primary phase was transformed from faceted to non-faceted. Moreover, the lamellar spacing was observed to decrease with the decreasing of solidification rate with the addition of REE Y. Furthermore, the mechanical tests showed that the addition of REE Y has a beneficial effect on the room temperature fracture toughness of Cr2Nb/Cr alloys. The room temperature fracture toughness of the Y doped Cr/Cr2Nb hypoeutectic and hypereutectic alloys increased by ∼130 and 140% over that of the undoped alloys respectively, which was caused by the refinement of lamellar eutectic and strengthening of lamellar eutectic bonding strength.

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

Abstract The microstructure and room temperature fracture toughness of binary Cr/Cr2Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo- and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr2Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo- and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr2Nb Laves phase (1·2 MPa m1/2). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.

Effect of a Strong Magnetic Field on Dendritic Growth of Ti-Ni Alloy

A phase field model has been developed to simulate the dendritic growth of Ti-Ni alloy subjected to a strong magnetic field. The influence of a strong magnetic field on the microstructure morphology and its evolution was successfully investigated by the model. The effect of the magnetic field intensity on the dendritic evolution has been further discussed. The simulating results revealed that with greater magnetic field intensity, the primary dendritic arms and the side branches were easier to coarsen. Besides, the dendritic tip growth rate increased with increasing magnetic field intensity, while the curvature radius had an opposite tendency. The microstructure evolution under a strong magnetic field was also studied combined with solidification thermodynamics theory. The results indicate that, the temperature of equilibrium solidification of Ti-Ni alloy changes with the presence of a strong magnetic field, and the morphology of dendritic grains will be affected eventually.