Papers by Author: Liang Zuo

Abstract: For off-stoichiometric Ni₂MnGa ferromagnetic shape memory alloys, a large shape change could be induced through the rearrangement of martensitic variants under an external magnetic field. Insight into the orientation relationships of martensitic variants and the characteristics of variant boundaries is thus essential for understanding the magnetic shape memory performance. In this paper, a thorough crystallographic investigation was made on the incommensurate 7M modulated martensite in one polycrystalline Ni₅₀Mn₃₀Ga₂₀ alloy by means of X-ray diffraction and SEM electron backscattered diffraction (EBSD). Locally, there are four differently-oriented martensitic variants, being twin related to one another. The twin interface planes are coherent and they are in coincidence with the respective twinning planes (K₁). A primary exploration was performed to improve the microstructure by repeated magnetic field training during phase transition. The present investigation could offer useful guidance to develop specific technique for microstructure optimization.

Abstract: Al-Si alloys are widely used as cast alloys for their excellent castability, low thermal expansion coefficient, good wear resistance and corrosion resistance properties. However, the poor ductility of these alloys, caused by the presence of coarse and non-deformable Si phase in the as-cast state, has inhibited their applications as wrought materials. Recently, a process based on traditional technologies, i.e., direct chill (DC) casting followed by hot deformation and heat treatment, has been developed for potential mass production of wrought Al-Si alloys with superior mechanical properties in view of their strength and ductility. In this work, the microstructural evolutions of DC cast Al-Si alloys involved in solidification, recrystallization and precipitation during the processing are highlighted, aiming at understanding the correlations between the microstructures and the mechanical properties.

Abstract: Hot deformation behavior of Fe-3%Si steel within temperature range of 1073~1473K and strain rate range of 0.01~5s⁻¹ was investigated by isothermal compression test using thermo-simulation method. Over the applied deformation conditions, steady state flow behavior was well described by the power law relationship with dislocation climb as the rate-controlling mechanism, and the high apparent activation energy can be attributed to the high yield stress. A modified Bergström model was proposed by introducing yield stress, and consequently the whole stress-strain curves can be accurately predicted.

Abstract: The present studies are to investigate the microstructure features during transformation from austenite to ferrite without and with magnetic field on Fe-0.76%C alloy. It is found that the area fraction and numbers of proeutectoid ferrite grain as well as the lamellar spacing of pearlite in Fe-0.76%C alloy increased considerably with the increase of magnetic field intensity. The reason is that, the magnetic field increases the driving force of proeutectoid ferrite nuclei and shifts the eutectoid point to the side of high carbon content and high temperature, which increases the starting-temperature of the transformation from austenite to ferrite. The proeutectoid ferrite grains are elongated along the magnetic field direction, which can be explained as follows: the proeutectoid ferrite becomes the magnetic dipolar under high magnetic field, and then the polarized austenite atoms are much easier to diffuse into ferrite grains along the magnetic field direction. Key words: high magnetic field; Fe-0.76%C alloy; microstructure

Abstract: Our recent work on EBSD-based characterization of incommensurate 7M modulated martensite in a polycrystalline Ni₅₀Mn₃₀Ga₂₀ alloy were summarized. The 7M martensitic plates were revealed to be self-accommodated in colonies, where each colony consisted of four types of variants that were twin related. All the pairs of variants can be categorized into three twinning modes, i.e. type-I, type-II and compound twins. The twin interface planes were in coincidence with the respective twinning planes. Using the measured orientations of adjacent martensitic variants, the orientations of parent austenite grains were calculated under the assumed orientation relationships for the austenite to martensite transformation. The energetically favorable orientation relationship between austenite and martenstie for the martensitic transformation was identified to be the Pitsch relation.

Abstract: Fe₈₁Ga₁₉ sheets were produced by conventional rolling procedure. Recrystallization texture dominated with strong Goss ({110}) was successfully obtained through sheet thickness by primary recrystallization annealing. The development of Goss can be attributed to the favorable shear band morphology and deformation texture derived from the applied specific rolling process.

Abstract: The texture, microstructure, plastic strain ratio r value, elongation, strain hardening index n, the value of cupping test I_E, and the correlation between r value and the other formability indices of two automotive aluminum alloys sheet 6016 and 6181, and commercial pure aluminum sheet were investigated. The results showed that the recrystallization textures of three aluminum alloys sheet are similar to each other, which mainly contain cube component. However, the r and n value, elongation, and I_E of three aluminum alloys sheet are different from each other evidently, and there is no correlation between texture and r value, and the other formability indices except the n value. The large quantity of second-phase particles in the aluminum alloy matrix has very important effect on both the r value and the formability of aluminum alloy sheet.

Abstract: NiMnCoIn alloys are new-type magnetic shape memory alloys (MSMAs) in which a reversible magnetic-field-induced phase transformation was observed. They are ideal candidates of materials used in actuators and sensors. The polycrystalline NiMnCoIn alloys are generally brittle so that they can not be easily deformed into the shape applicable to actuators and sensors until now. In the present paper, the influence of hot deformation on the microstructure of Ni₄₅Co₅ Mn_36.7In_13.3 alloy was studied. The experimental results showed that second phase was observed after deformation at high temperature between 800~900 °C and at strain rate lower than 4×10^-3s^-1. The content of Co of second phase was higher than the matrix alloy, while the content of In was lower than the matrix alloy. It was determined by TEM measurements as γ phase with fcc structure which was popular in NiFeGa and NiMnGaCo alloys. It is possible to improve the ductility of Ni₄₅Co₅Mn_36.7In_13.3 alloy by control of amount and distribution of γ phase by hot deformation.

Abstract: The effects of the 12T magnetic field on transformed morphology and crystallographic characteristics of high purity Fe-1.1C (wt.%) alloy during diffusional decomposition of austenite are studied by SEM/EBSD. With the applied field, the lamellar spacing of pearlite is greatly enlarged and the spheroidization tendency of the pearlitic cementite is enhanced. This can be attributed to the thermodynamic effect of the field on phase equilibrium by elevating the eutectoid temperature; hence the eutectoid transformation happens at higher temperature that favors carbon diffusion. As a consequence, the eutectoid cementite that is usually in lamellar shape spheroidizes to reduce its specific surface area. In addition, three orientation relationships between ferrite and cementite in pearlite, namely Isaichev ORs and two near Pitsch-Petch ORs (P-P1 and P-P2), are found both in non-field and field treated specimens. The magnetic field favors the occurrence of P-P2 OR due to the promotion of nucleation of the pearlitic ferrite. However, there is no obvious effect of the field on the crystallographic orientation of pearlitic ferrite, and that is probably related to the quick saturation of magnetization that results in the disappearance of magnetization anisotropy.

Abstract: 4-mm thick Al-12.7Si-0.7Mg alloy plates were cut from the hot extrusion profiles. A butt-welding joint was made by friction stir welding (FSW). Optical microscopy and SEM-EBSD analysis were employed to examine the microstructure and texture evolution in the stir zone of the joint. This work provides basic information for microstructure and microtexture characteristics in the stir zone after FSW for this novel wrought Al-Si alloy.