Limei Cha

Technology and mechanical properties of advanced γ-TiAl based alloys

Abstract The present paper summarizes our progress in establishing a novel production technology for -TiAl components to be used in advanced aircraft engines. In the beginning the main emphasis is put on the design of a -TiAl based alloy which exhibits excellent hot-workability. Then, the development of a “near conventional” hot-die forging route for this type of intermetallic material is described. Finally, the effect of two-step heat-treatments on the microstructure and the mechanical properties is discussed. Because of the small “deformation window” hot-working of -TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favoured. In order to increase the deformation window a novel Nb and Mo containing -TiAl based alloy (TNMTM alloy) was developed, which solidifies via the β-phase and exhibits an adjustable β/B2-phase volume fraction. Due to high volume fractions of -phase at elevated temperatures the alloy can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. Examples for the fabrication of -TiAl components employing a near conventional forging route are given. With subsequent heat-treatments balanced mechanical properties can be achieved. The results of tensile and creep tests conducted on forged and subsequently heat-treated TNMTM material are presented.

Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature

Abstract Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42–45)Al-(3–5)Nb-(0.1–2)Mo-(0.1–0.2)B (in atom percent) are termed TNMTM alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/a2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro- and micro-hardness measurements as well as room temperature tensile tests ...

Influence of Annealing Temperature on CZTS Thin Film Surface Properties

In this work, copper zinc tin sulfide (CZTS) films were deposited by direct current sputtering and the samples were annealed in different oven-set temperatures and atmosphere (Ar and H2S). The surface evolution was investigated carefully by using scanning electron microscopy (SEM), Raman spectroscopy and x-ray photoelectron spectroscopy. The surface of the as-sputtered precursor contained little Cu and large amounts of Zn and Sn. The metallic precursor was continuous and compact without pinholes or cracks. With the increase of the temperature from room temperature to 250°C, Cu atoms diffused to the film surface to form Cu1−xS and covered other compounds. Some small platelets were smaller than 500xa0nm spreading randomly in the holes of the film surfaces. When the temperature reached 350°C, Zn and Sn atoms began to diffuse to the surface and react with S or Cu1−xS. At 400°C, SEM showed the melting of large particles and small particles with a size from 100xa0nm to 200xa0nm in the background of the film surface. Excess Zn segregated towards the surface regions and formed ZnS phase on the surface. In addition, the signal of sodium in the CZTS surface was observed above 400°C. At 600°C, a large amount of regular structures with clear edges and corners were observed in the film surface in SEM images. A clear recrystallized process on the surface was assumed from those observations.

Surface evolution of sputtered Cu(In,Ga)Se2 thin films under various annealing temperatures

AbstractnIn this work, copper indium gallium selenide (CIGS) thin films were prepared by sputtering CIGS quaternary target and subsequent annealing under Se atmosphere in the 240–550xa0°C range. X-ray diffraction (XRD), Grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were performed to investigate the crystalline structures and chemical compositions of the CIGS thin films. According to XRD, GIXRD and Raman analyses, the CIGS phase was splitted into CuSe phase and In4Se3 phase at the very surface when CIGS was annealed at 240 and 270xa0°C. In addition, XRD patterns indicated a grain size growth with increasing temperature. XPS analyses contributed more information on the film surface chemistry. Indeed, O 1xa0s and Na 1xa0s signals were observed at the film surface above 380xa0°C, and were even increased from 450 to 550xa0°C. Simultaneously, the surface appeared to exhibit a Cu and Ga poor surface chemical composition. The strong correlation between O, Na and Cu atomic contents in the CIGS surface were discussed in the paper.

First observation of a hexagonal close packed metastable intermetallic phase between Cu and Al bilayer films

Abstract In this paper we describe the structure and formation of a new intermetallic phase in the Cu – Al system, which has not been reported before. The phase was found in Cu/Al bilayer films, which were deposited at room temperature on (0001) sapphire substrates using molecular beam epitaxy. The interfacial intermetallic phase is 8 nm thick, and possesses a hexagonal close-packed structure. The lattice parameters of the phase gradually increase from the near-Cu-side to the near-Al-side. In parallel with the described lattice expansion, the chemical composition of the interlayer also varies from 27 to 58 Al at.% from the near-Cu-side to the near-Al-side. The formation and microstructural characteristics of this new phase are explained by Hume-Rothery laws and Shockley partial dislocations. In addition, in-situ heating experiments were performed in a transmission electron microscope at ∼600 °C to investigate the thermodynamic stability of this new Cu – Al intermetallic phase. During annealing the intermetallic layer disappears and other known equilibrium intermetallic phases develop. This indicates that this new hexagonal close-packed Cu – Al intermetallic phase is metastable.

Investigation of Structural, Chemical, and Electrical Properties of CdTe/Back Contact Interface by TEM and XPS

CdTe solar cell back contact preparation usually includes a chemical etching process which helps to obtain a Te-rich p-doped CdTe surface. In this work we compared the influence of two different etching solutions [nitric–phosphoric (NP) and nitric–acetic acid (NA)] on the CdTe surface. Transmission electron microscopy indicated that a Te-rich layer was formed on the surface of polycrystalline CdTe films after the etching process. The layer thickness was 80Âxa0nm and 10Âxa0nm for NP and NA etching solutions, respectively. In addition, the images showed that the influence of the etching solution was preferentially along the grain boundaries. The chemical properties of the etched CdTe surface were studied by using x-ray photoelectron spectroscopy. The nitric–phosphoric acid yielded a relatively thicker Te-rich layer on the CdTe surface. On the other hand, the J–V properties of the solar cells prepared using nitric–acetic acid showed no rollover behavior, indicating improved back contact. The solar cells prepared with the NA and NP etching processes yielded >10% solar cell efficiency. The CdTe solar cell homogeneity was improved by the NA etching method.

Growth process and properties of CdS thin films prepared by chemical bath deposition at different pH values

CdS thin films were successfully fabricated on glass substrates in different pH solutions by chemical bath deposition (CBD). The influences of pH value on the thickness, growth process, structure as well as on the optical properties of CdS film were investigated. The as-deposited films exhibited pure cubic phase structure, indicating that the pH had no significant effect on the crystal structure of CdS film. The optical transmittance spectra demonstrated that the films deposited at 11.7 and 11.8 pH exhibited relatively high transmittance in the long wavelength. And the optical band gaps decreased from 2.42 to 2.31xa0eV with increasing pH values. Subsequently, the growth process of CdS films was studied by analyzing the morphologies and particles sizes at different growing stages of the deposition. It was found that the particle clusters produced by homogeneous reaction acted as nucleation sites during the deposition process, and CdS preferred to grow on their surface. As a result, the sizes of clusters expanded with deposition time until a continuous film was formed, and the duration to obtain a continuous film reduced with decreasing pH values. Hence, a growth mode of CdS film on the glass substrate was proposed, i.e. the film was formed by the connection of expanded particle clusters produced by homogeneous reaction.