Papers by Author: Zong Bin Li

Abstract: The effect of In addition on the magnetostructural transition behavior and magnetocaloric effect in the Mn-rich Heusler Mn-Ni-In melt spun ribbons have been studied in the present work. With the increasing of the In content, the crystal structure of the alloys are tetragonal NM martensite (for x=7~8.5), 7M monoclinic martensite (for x=9, 9.5) and L21 austenite (for x=10, 11), respectively. It was found that the increase of the In content results in both a progressive decrease of the martensitic transformation temperature and a continuously enhancement of the austenite magnetic transition temperature. For the ribbons with In content of 9.5 at%, it shows the combination of the structural transformation and the magnetic transition. The large magnetization difference between the two phases confirms the occurrence of the field-induced martensitic reverse transformation.

Abstract: The effect of composition on martensitic phase transformation and structure in polycrystalline NiMnGa was investigated through Ni substitution for Mn, Ga and Mn substitution for Ga. The martensitic transformation temperatures almost linerly increase with increasing Ni content in Ni_50+xMn_50-xGa₂₅, Ni_50+yMn₂₅Ga_25-y and Mn content in Ni₅₀Mn_25+zGa_25-z. The increases in rate of the martensitic transformation temperatures are different for the three conditions. It is large for Ga substituted by Ni, slow for both Mn substituted by Ni and Ga substituted by Mn. Analysis shows that electronic concentration is an important factor to influence the martensitic transformation temperature in the Ni-Mn-Ga alloys. The results show that Ni excess stabilizes the NM martensitic structure and Mn excess stabilizes the 7M martensitic strcuture in broad composition range.

Abstract: In this work, the fourth element Cu was introduced to substitute Ni in polycrystalline Ni-Mn-Sn alloys. It was shown that Cu doping did not change the crystal structure of the martensite in Ni_50-xCu_xMn₃₉Sn₁₁ (x=0, 1, ......,7) alloys, but resulted in the decrease of martensitic transformation temperatures. Due to the higher atomic radius of Cu with respect to that of Ni, the lattice volume of martensite unit cell increases with the gradual substitution of Ni by Cu. For the alloys with the Cu content of 0-4%, the martensitic transformation is from weak magnetic (paramagnetic) austenite to weak magnetic (paramagnetic) martensite. When the Cu content is higher than 4%, the paramagnetic to ferromagnetic transition of austenite was introduced. The temperature interval between magnetic transition and structural transformation was enlarged with the increase of Cu content. Due to the relatively smaller magnetization difference between austenite and martensite, the field induced inverse martensitic transformation behavior is not significant in the present Cu-doped alloys.

Abstract: In this work, NiMnGa thin film composed of non-modulated martensite (NM) and seven-layered modulated martensite (7M) was produced. The crystal structure and lattice constants were determined by X-ray diffractometer (XRD). The preferred crystallographic orientation of martensite was determined using the four-circle XRD. SEM/EBSD was employed to verify the crystal structure of the martensite and to reveal its crystallographic features correlated with the microstructure. According to the XRD patterns, the crystal structure of NM and 7M was determined as tetragonal and monoclinic crystal structure, respectively. Pole figures measured by four-circle diffractometer revealed that the NM martensite possesses (004)_NM and (220)_NM preferred plane texture close to the substrate surface, whereas the 7M martensite has (2 0 20)_7M, (2 0 )_7M and (040)_7M preferred plane texture close to the substrate surface. SEM/EBSD analysis shows that the surface layer of the film is mainly composed of NM martensite that is organized in variant groups. In each variant group, all the martensite plates consist of paired lamellar (112)_NM compound twins and there are eight orientation variants in each variant group.

Abstract: The phase transformation and magnetic properties of Heusler-type Ni₅₀Mn_50-xIn_x (x=10~16) ferromagnetic shape-memory alloys have been systemically investigated by differential scanning calorimetry and vibrating sample magnetometry. It is found that the phase transition temperatures show a linear relationship with the In concentration and the curie temperatures of austenite phases are not sensitive to the alloy composition. The existence of large magnetization change during the magneto-structural transition is the origin of magnetic-field-induced transformation effect.

Abstract: Abstract. With the advance of electron diffraction techniques in individual orientation analysis, traditional crystallographic characterization methods could be simplified, thus offering chances to develop some new approaches. In recent years, our group has devoted to working on possible extensions of the SEM and TEM based techniques for crystallographic analyses on a microstructure- and orientation-specific level. Several methods are illustrated in this paper, including the determination of dislocation type and Burgers vector without recourse to the traditional g.b invisibility condition, the identification of twinning mode and complete twinning elements for any crystal symmetry that requires minimum initial data input, and the characterization of specific interface plane or slip plane using only one sample observation plane instead of two perpendicular sample planes. These new extensions of characterization methods have proven to facilitate the related microstructural examinations.

Abstract: In the present work, we summarized two calculation methods to determine some specific crystallographic elements based on electron diffraction orientation measurements by SEM/EBSD or TEM. The first one is to determine the twin type and twinning elements of crystal twins based on the minimum shear criterion, using the experimentally determined twinning plane for Type I twin and compound twin or twinning direction for Type II twin as initial input. The method is valid for any crystal structure. The second one is one to determine the plane indices of the faceted interfaces where the orientation relationships (ORs) between the adjacent crystals are reproducible. The method requires one prepared sample surface instead of two perpendicular surfaces. These methods are expected to facilitate the related microstructural characterizations.

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: 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.