Jian-Yih Wang

Crystallization behavior of r.f.-sputtered near stoichiometric Ni2MnGa thin films

Abstract Thin films of Ni51.45Mn25.30Ga23.25 alloy were deposited onto a 3-inch diameter n-type Si(100) wafer by r.f. magnetron sputtering. The observation of high resolution transmission electron microscopy shows that the as-deposited thin films were partially crystalline and dense, with a preferred growth of L21 structure nanoparticles. The crystallization activation energy Q of the as-deposited free-standing thin film was found to be 234 kJ/mol by Kissingers method. The onset crystallization temperature, Tx, was 427 °C at the 10 °C/min heating rate. Both Q and Tx are much lower than those of near-equiatomic TiNi alloy mainly due to its inherently partial crystallization in as-deposited films.

A study by high-resolution electron microscopy of an α2 + γ two-phase Ti–40 at.% Al alloy

Abstract High-resolution electron microscopy observation of a Ti-40 at.% Al alloy shows some microstructures which cannot be resolved by conventional transmission electron microscopy. The α2-γ interface with or without a ledge is found to be related to the misfit dislocations at the α2-γ interface. A transition lamella with the structure in between D019 and L10 is suggested as a way to grow the γ lamella from the α2 matrix. This lamella is formed by stacking faults occurring on every third close-packed plane. The ½[011]γ ledge dislocation can dissociate into a ⅙[112]γ partial dislocation which induces the atomic shuffle on the (111)γ plane in the [112]γ direction and causes the twinning γT to form in the γ lamella. The formation mechanisms of γT associated with a single ledge or a ‘ledge pair’ at the α2-γ interface are also proposed. γT with multilayers of (111)γ planes becomes a real twin in the γ lamella.

Interfacial microstructures of rf-sputtered TiNi shape memory alloy thin films on (100) silicon

Abstract Interfacial microstructures of TiNi thin films rf sputtered on to Si(100) and post-annealed at 400–700°C for 30mins have been investigated using analytical and high-resolution transmission electron microscopy. For annealing temperatures below 600°C, a very thin amorphous (Si, O)-rich layer is observed at the interface. Ni atoms are the primary diffusing species and NiSi2 forms triangularly and epitaxially towards the Si substrate. TiNi films initially crystallize after 30 min at 500°C. Si and Ti atoms begin to migrate in specimens annealed at 600°C for 30min. At this temperature, a near-Ti4Ni4Si7 phase in triangular NiSi2 and a near-TiNiSi phase in the TiNi film are simultaneously nucleated and grown at the interface. For the specimens annealed at 700°C for 30 min, two layers of Ti4Ni4Si7 and TiNiSi form at the interface with the sequence TiNi/TiNiSi/Ti4Ni4Si7/Si. Triangular NiSi2 islands are now embedded in the Ti4Ni4Si7 layer. A mechanism of interfacial microstructure evolution is proposed to explain the temperature effect on the interfacial reaction layers between the TiNi film and the Si(100).

Interfacial reactions of rf-sputtered TiNi thin films on (100) silicon with a SiN diffusion barrier

Silicon nitride (SiN) with a 50 nm thickness on Si(100) as a thermal barrier was obtained by plasma-enhanced chemical vapour deposition (PECVD). TiNi thin films were rf sputtered on a SiN/Si substrate and then annealed at 400–700°C for 30 min. Their interfacial reactions were studied using transmission electron microscopy, X-ray diffraction and Auger electron spectroscopy analyses. Experimental results show that the thickness of reaction layer in TiNi/SiN/Si specimens is clearly reduced, compared with that in TiNi/Si specimens under the same annealing conditions. The significant effect of the SiN layer as a diffusion barrier in TiNi/SiN/Si can be recognized. N and Si atoms diffuse from the SiN layer to react with TiNi films at 500°C and 600°C respectively. The TiN1 − x phase is formed in specimens annealed at 500°C, and mixed Ti2Ni3Si and Ti4Ni2O compounds are found at 600°C. In the specimen annealed at 700°C, the reaction layer has sublayers in the sequence TiNi/Ti4Ni2O/Ti2Ni3Si/TiN1 − x /SiN/Si. The SiN thermal barrier obtained by PECVD caused quite different diffusion species to cross the interfaces between TiNi/SiN/Si and TiNi/Si specimens during the annealing.

Microstructures and Mechanical Properties of Mg-Li-Zn Alloys as Processed by ECAE

Magnesium alloys and very light metals that can be used for structural application. A variety of Mg-Li-Zn alloys including Mg-11%Li-1%Zn, Mg-9%Li-1%Zn, Mg-9%Li-1%Zn-0.2%Mn, Mg-9%Li-1%Zn-1%Al-0.2%Mn and Mg-9%Li-3%Al-1%Zn-0.2%Mn were processed by equal channel angular extrusion (ECAE) to see how effective is this process in refining the grain structure. After 4 passes of the ECAE process, TEM studied the microstructural diversifications, especially showing the existence of compounds, MgLiZn, MgZn2 and ZnO. The room temperature and high temperature mechanical properties were also investigated. After the ECAE process, the room temperature strength was found enhanced at a modest cost of reduction in elongation.

Microstructure and Mechanical Behaviors of the New LAZ1151 Mg-Li Alloy

This research studies a brand new Magnesium Lithium Alloy, LAZ1151, with trace Sc additions. The mechanical properties and microstructures of the as-cast and alloys after three and six month aging were observed and analyzed. Microstructure and XRD confirms the existence of α phase precipitates (Mg rich) in both as-cast and aging specimens of the alloy. Grain growth was observed in the alloy after room temperature aging. The tensile strength of the as-cast LAZ1151 is 147 MPa; the value is decreased to 135 MPa after six month aging, showing typical room temperature softening. Strengthening via the use of cold rolling was tried; a maximum tensile strength of 180 MPa was achieved after 90% rolling reduction, which is mainly due to strain hardening.

Evaluation on Some Metallic Alloys for SOFC Interconnect

Five oxidized metallic alloys, namely, Crofer22, equivalent ZMG232, stainless steel SS430, SS304 and Inconel718 were subjected to oxidation treatment in hot air environment for various period of time. Then the resulted oxide scale was analyzed by scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and X-ray diffraction (XRD). All the five alloys contain comparable amount of Cr, Mn, Fe, and their surface oxides as analyzed indicate to be Cr2O3 and (Mn, Fe)Cr2O4 spinel compound.

Grain Refining of Magnesium Alloy AZ 31 by Equal Channel Angular Extrusion

Among the conventional and alleged bulk deformation processes in metalworking, equal channel angular extrusion (ECAE) is an innovative and attractive method to refine grain structures effectively. Grain refining by this process on magnesium alloy AZ31 was studied and compared to published data. An interesting and unique question is proposed and studied - that is, how dominant is the accumulated strain as obtained through multiple angular extrusion passes. Tensile tests were performed at room and high temperatures, and processed AZ31 was examined with optical and transmission electron microscopy as well as x-ray diffraction.

Oxidation Behavior of Various Metallic Alloys for Solid Oxide Fuel Cell Interconnect

Ten iron-based alloys and nickel-based alloys were subjected to oxidation treatment in a hot air environment for various periods. In this investigation, all alloys contained a certain amount of Cr, and Cr 2 O 3 and (Mn, Fe, Cr) 3 O 4 spinel compounds are generated on the surface oxides. Other spinels that contain Cr, Mn, Fe, and Ni are also formed on it; the compositions depend on the composition of the steels and any other materials that are in contact with the interconnects. Accordingly, the role of spinels in the oxidation of interconnects must be understood.

Microstructural Characterization and Mechanical Behavior of an Mg-9%Li-1%Zn Alloy

The hardening of the Mg-9%Li-1%Zn alloy does not depend strongly on precipitation through aging, but cold rolling is moderately effective. The Mg-9%Li-1 %Zn alloy had a dual phase structure with dispersed particles of ZnO and MgO oxides. The Wurtzite structure of ZnO was well oriented with respect to the Mg matrix, but the MgO was not. Peak aging at a temperature of 100°C was at 10hrs. At 50°C, the hardness reached the maximum value after an aging period of about 100 hrs. The appearance of the extra bump next to the main peak of α(002) after aging at 50°C/100 hrs and 100°C/10 hrs, was thought to correspond to a precipitate phase or spinodal decomposition.