Papers by Author: Xiang Zhao

Abstract: A systematic study has been made on a Cu-40%Zn alloy treated by an electric current pulse (ECP) and by the examination of the microstructure and the crystallographic features of both the parent and the product phases. The β precipitates under ECP show a Kurdjumov Sachs Orientation Relation (K-S OR) in the vicinity of the grain boundaries (GBs), but a Nishiyama Wasserman (N-W) OR within the grains. Along the GBs the {111}_α /<110>_α dislocation arrays were spotted, whereas the {111}_α /<112>_α stacking faults were observed in the grain interiors. A closer examination of the lattice strain required for the phase transformation revealed that the maximum lattice deformation under the K-S OR is a shear on the {111}_α plane in the <110>_α direction. The dislocations arrays existing along the GBs offer the pre-strain that favors the precipitation of β particles obeying the K-S OR. Oppositely, the stacking faults within the grains provide pre-stains for the formation of the β precipitates respecting the N-W OR. This study sheds some light on the mechanisms by which crystal defects initiate phase transformation in a Cu-40%Zn alloy.

Abstract: Most of the studies on phase transformation in metallic materials have focused on transformations during cooling processes due to the easiness of the conservation of the product phase. However, for phase transformation happening during heating processes, the experimental investigations have been indirect if the product high temperature phase could not be preserved to the convenient observation temperature, for example the room temperature. The high density Electric Current Pulse (ECP) treatment allows the phase transformation during heating process and the preservation of the high temperature phase to the room temperature, offering possibilities for direct experimental examinations. Thus, in the present work, a cold-rolled Cu–40%Zn alloy was ECP treated and the microstructure of the product phase and the transformation orientation relationship were investigated. Results show that during the ECP treatment, the high temperature beta phase with BCC structure formed in the parent alpha phase with FCC structure. Especially, two kinds of orientation relationships could be detected between the parent alpha phase and the product beta precipitates. The one is the Kurdjumov-Sachs orientation relationship (K-S OR), and the other is the Nishiyama-Wasserman (N-W). In addition, the amount of beta precipitates obeying the K-S OR is more than that of precipitates obeying the N-W OR. The results of this work provide new fundamental information on phase transformation of metallic materials.

Abstract: Ni-Mn-In is a novel type of magnetic shape memory alloy, its shape memory effect has been realized through magnetic field induced reverse martensitic transformation. A variety of point defects would be generated during composition adjustment process, such as antisite defect, vacancy and exchange. The first–principles calculations within the framework of the density functional theory using the Vienna ab initio software package (VASP) have been used in this paper to investigate the defect formation energy and electronic configuration of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys. The In antisite on the X sublattice (InX) and the Ni antisite on the X sublattice (NiX) have the lowest formation energies in the investigated series. The formation energy of the Ni vacancy is the lowest, while that of the in vacancy is the highest. It is confirmed that the in constituent plays a dominant role for stabilizing the austenitic phase.

Abstract: Effect of magnetic field strength on carbide precipitation behavior in W₆Mo₅Cr₄V₃ highspeed steel during medium temperature tempering was investigated. The applied magnetic field promoted the precipitation and refinement of M₆C and MC carbides at boundaries and in the grain interior, but maximum spheroidization occurred for those M₆C carbides precipitated at boundaries, the stronger the magnetic field strength, the stronger the spheroidization effect. The high magnetic field hinders the precipitation of M₂C type carbides, and the M₂C type carbides basically disappear when applying the magnetic field.

Abstract: In this present work, TiN films with various thicknesses (from 0.3 μm to 2 μm) were deposited by DC reactive magnetron sputtering on Ti6Al4V substrates. The evolution of texture and microstructure were studied by X-ray diffraction and Scanning Electron Microscopy, respectively. The XRD characterization indicates that the preferred texture of TiN films is changed from (111) to (100) with increasing the film thickness. The microstructure characterization shows that their microstructure transform from continuous into columnar with increasing the TiN film thickness. It is considered these results are arised from the change of overall energy including surface energy and strain energy with the film thickness. The hardness of TiN film increases with increasing the film thickness.

Abstract: An Al-5.4Si-0.5Mg filler wire was developed and produced by direct chill (DC) casting, extrusion, and roll-die and hole-die drawing for the welding of newly developed Mg-containing high-Si aluminum alloys. The Al-12.7Si-0.7Mg alloy plates were butt-welded by tungsten inert gas arc welding (TIG) method using the as-produced welding wire. The microstructures and mechanical properties of these welded joints in different solid solution and artificial aging treatment conditions were studied to evaluate the weld qualities of the novel welding wire. The results showed that the Al-5.4Si-0.5Mg welding wire was an ideal special welding wire of the Al-12.7Si-0.7Mg alloy and the filler metal proved to be heat-treatable. The welded joints exhibited much higher mechanical properties after the post-weld heat treatment (PWHT). All of this provides the possibility of wide application of the novel Al-5.4Si-0.5Mg welding wire and the Al-12.7Si-0.7Mg alloy weldments.

Abstract: The strain-controlled fatigue tests on extruded AZ31B magnesium alloy were conducted under the uniaxial loading with strain ratio Rε=-∞, frequency of 0.1 Hz and strain amplitude of 2% at room temperature. The cyclic hardening behavior was investigated. It was found that, during the low cycle fatigue (LCF) process, as the number of cycles increases, the stress amplitude increases corresponding to the decrease of the plastic strain amplitude. The development of dislocation density can be described as the function of the number of fatigue cycles, and the behavior can be explained well based on the dislocation density development model.

Abstract: Ni-Mn-In is a new type of magnetic shape memory alloy, but in the stoichiometric Ni₂MnIn, the martensitic transformation cannot perform. Thus, in order to obtain the appropriate Curie temperature T_C and the martensitic transformation temperature T_M, the composition adjustment must be carried out around the stoichiometric Ni₂MnIn. The process of composition adjustment would induce various point defects. This paper scientifically studies the magnetic properties of off-stoichiometric Ni₂MnIn alloys by first-principles calculations.

Abstract: The crystallographic, magnetic and electronic structures of the magnetic shape memory alloys Ni₂XIn (X=Mn, Fe and Co) are systematically investigated by means of the ab initio calculations within the framework of density functional theory. The equilibrium lattice parameters and the bulk modulus of the austenitic phase in Ni₂XIn are systematically calculated. The formation energy of the L2₁ phase of the Ni₂XIn is estimated, and displays a destabilization tendency if Mn atom is substituted by Fe or Co. Furthermore, the magnetic properties of the Ni₂XIn have been investigated, and the essence of the variation in the magnetic properties with the X atomic number has been illustrated from the view of the electronic density of states.

Abstract: A novel Al-12.7Si-0.7Mg welding wire was successfully produced via DC casting, extrusion, roll die drawing and hole die drawing. An investigation was performed on the microstructure and mechanical properties of the welded joints using the as-produced special filler wire in different solid solution and artificial aging treatment conditions. The results showed that the as-produced Al-12.7Si-0.7Mg welding wire was an ideal special wire of the Al-12.7Si-0.7Mg wrought alloy and the homogeneous welded joints proved to be heat-treatable. Solid solution treatment and artificial aging treatment had significant influence on the microstructure, microhardness and tensile properties of the as-welded joints. A transformation of microstructures from a coarse structure to a fine structure occurred in the welded joints after the heat treatment. The microhardness and the strength of the joints increased remarkably as the solution temperature and aging time increased.