A. Wanner

Synchrotron X-ray study of bulk lattice strains in externally loaded Cu-Mo composites

A synchrotron X-ray transmission technique was applied to study the internal load transfer and micromechanical damage in molybdenum particle-reinforced copper matrix composites during plastic deformation. Mechanically loaded, 1.5-mm-thick specimens were irradiated with a monochromatic beam of 65 keV X-rays. Low-index diffraction rings of both phases were recorded with a high-resolution two-dimensional detector. By means of newly developed data processing routines, we could quantify as a function of applied stress both the ring distortion (from which the volume-averaged elastic strains in the two phases were calculated), and the ring graininess (which is related to the Bragg peak broadening). Based on this information, the deformation and damage processes in these alloys were studied in detail. As compared to conventional neutron diffraction methods, the photon transmission technique yielded similar precision but at much reduced measurement times. The main sources of experimental errors were identified and strategies to minimize these errors were developed.

Tensile testing of ultrathin polycrystalline films: A synchrotron-based technique

The mechanical properties of metallic thin films on the nanoscale acquire increasingly more importance as applications in microelectromechanical systems/NEMS as well as microelectronics have reached this size scale. Here, we present a synchrotron x-ray diffraction technique by which it is possible to characterize the evolution of mechanical stress in a metallic film thinner than 100 nm at measurement times shorter than 60 s per data point. This high data acquisition rate is achieved because no relative motions or tilting of specimen, x-ray source and detector (a large-area charge coupled device camera) are required. The technique comprises an initial “sin2 ψ” measurement to establish the absolute stress values followed by periodic “sin2 φ” measurements during straining to determine stress increments. We describe an experimental setup established at the synchrotron radiation source ANKA (Karlsruhe, Germany) which is specifically suited for monitoring the stress evolution during in situ tensile tests on thi...

On the origin of acoustic emission during room temperature compressive deformation of a γ-TiAl based alloy

Abstract In this study a two-phase γ-TiAl based alloy with near gamma microstructure was deformed in uniaxial compression at room temperature. The deformation experiments were carried out at a strain rate of 2×10−4 s−1. The acoustic emission during deformation was monitored continuously using a root mean square voltmeter. Different annealing experiments between deformation cycles combined with transmission electron microscopy investigations revealed the formation of mechanical twins as the decisive mechanism for the acoustic emission.

High-temperature, low-cycle fatigue behaviour of an aluminium alloy (Al-12Si-CuMgNi)

Abstract The low cycle fatigue properties of an aluminium alloy (Al–12Si–CuMgNi), obtained at a temperature of 623 K, are reported and discussed. The properties studied include cyclic stress response, fatigue life and nature of fracture as a function of applied strain amplitude. The alloy exhibits moderate cyclic softening from the very first cycle till failure at all the strain amplitudes tested. The variation of high strain, low cycle fatigue life (measured in terms of number of reversals to failure, 2 N f ) with plastic strain amplitude (Δ e p /2) is seen to follow a single-slope Coffin–Manson power-law relationship. The alloy is seen to fail during fatigue loading by a mixture of quasi-cleavage fracture and microdimples of high spatial density. Finally, a comparison of the fatigue life of the aluminium alloy in the over-aged (OA) condition (obtained from the present study) with that of the alloy in a highly over-aged (HOA) condition of an earlier study reveals that the effect of ageing on the low cycle fatigue resistance depends on the damage parameter considered, viz. Δ e p /2 or Δ W p , the average plastic strain energy per cycle.

Internal stress measurements by high-energy synchrotron X-ray diffraction at increased specimen-detector distance

High-energy X-ray diffraction has recently been shown to be a viable technique to measure volume-averaged lattice strains in the bulk of metallic polycrystals at increased speed compared to neutron diffraction. The established procedure is to irradiate the sample under investigation with monochromatic X-rays (∼100 keV) and to record complete diffraction rings with an area detector. The lattice strains are obtained by analyzing the minute distortions of these rings. In the present paper we present first results obtained using a setup in which two area detectors are positioned at a large distance (7 m) from the specimen. Although only segments of the rings can be recorded this way, this approach offers a number of advantages. In situ tensile tests were performed on a γ-TiAl-based alloy as an example to demonstrate the potential of the method. Both materials science aspects as well as consequences for further method development will be discussed.

Microstructural changes in the cell walls of a closed-cell aluminium foam during creep

Abstract The microstructure of an Alporas® foam was examined both before and after creep deformation. It was found that the initial microstructure of the foam consists of subgrains which are several micrometres in size. Because of the complicated distribution of the applied stress in a foam, the corresponding microstructure development is rather inhomogeneous during creep. The microstructure may change drastically by choosing different investigated areas in the same deformed foam. In most areas, no evident changes in microstructure can be seen in comparison with the initial microstructure; however, in strongly deformed areas, subgrains with a much smaller average size are found, and the shape of these subgrains is equiaxed. The stress estimated from the subgrain size reveals that locally very high stresses must have existed during creep deformation, which may reach the power-law breakdown regime. This is a reasonable explanation of the high stress exponent obtained for Alporas® foams.

On the role of twinning during room temperature deformation of γ-TiAl based alloys

The present work dealing with the micromechanical modeling of the deformation behavior of γ-TiAl based alloys is based on a strong interaction between numerical simulation facilities and detailed experimental investigations. Main attention is devoted to a near-γ microstructure which exhibits equiaxed γ-grains and α2-phase located at grain boundaries and triple junctions. The purpose is an investigation of the deformation mechanisms of polycrystalline near-γ material using both computational finite element simulations and acoustic emission (AE) measurements. A three-dimensional micromechanical model, initially developed to describe lamellar γ-TiAl, has been adjusted to simulate the deformation behavior of polycrystalline near-γ material. The contribution of deformation twinning to the total plastic deformation is computed investigating compression tests at room temperature. Concerning the onset of twinning the results of our simulation are compared with experimental data obtained from AE measurements. Investigations of the influence of various heat treatments on the twinning evolution during loading at room temperature were carried out. The deformation mechanisms were analyzed using optical microscopy, SEM and TEM.

The AEI 10 m prototype interferometer

A 10 m prototype interferometer facility is currently being set up at the AEI in Hannover, Germany. The prototype interferometer will be housed inside a 100 m 3 ultra-high vacuum envelope. Seismically isolated optical tables inside the vacuum system will be interferometrically interconnected via a suspension platform interferometer. Advanced isolation techniques will be used, such as inverted pendulums and geometrical anti-spring filters in combination with multiple-cascaded pendulum suspensions, containing an all-silica monolithic last stage. The light source is a 35 W Nd:YAG laser, geometrically filtered by passing it through a photonic crystal fibre and a rigid pre-modecleaner cavity. Laser frequency stabilisation will be achieved with the aid of a high finesse suspended reference cavity in conjunction with a molecular iodine reference. Coating thermal noise will be reduced by the use of Khalili cavities as compound end mirrors. Data acquisition and control of the experiments is based on the AdvLIGO digital control and data system. The aim of the project is to test advanced techniques for GEO 600 as well as to conduct experiments in macroscopic quantum mechanics. Reaching standard quantum-limit sensitivity for an interferometer with 100 g mirrors and subsequently breaching this limit, features most prominently among these experiments. In this paper we present the layout and current status of the AEI 10 m Prototype Interferometer project.

Size Effect on Crack Formation in Cu/Ta and Ta/Cu/Ta Thin Film Systems

Layered structures of Cu and Ta thin films on silicon are well established for many technological applications in microelectronics. Electronic circuits used for flexible displays or wearable electronics are becoming increasingly popular. For such applications, the Cu/Ta system must be transferred to flexible substrates, incorporating a design rule for several percent of total strain. We have investigated the deformation behaviour of different Cu/Ta and Ta/Cu/Ta thin film systems on a flexible polyimide substrate subjected to total strains of more than 5%. A novel synchrotron X-ray diffraction technique allowed us to characterize the evolution of mechanical stress in very thin metallic films during isothermal tensile tests. We found that samples with a Cu film thickness below 300 nm showed a sudden stress decrease at a total strain of about 2.5%. This stress drop was attributed to fracture of the entire film system, initiated by cracks in the Ta layers.

Thermal-cycling creep of γ-TiAl-based alloys

In two-phase TiAl-based alloys, the coexisting 2 and phases exhibit a thermal expansion mismatch, so that increased creep rates during thermal cycling may be expected. Creep deformation of two g-TiAl-based alloys was investigated during thermal cycles between 900 and 300 or 350C with applied tensile stresses of 32.5 or 37.0 MPa. Measured thermal cycling creep rates were compared with isothermal creep rates calculated at the eAective average temperature. No creep enhancement was measured upon thermal cycling within uncertainties of 1.610 ˇ3 % strain per cycle. # 2000 Elsevier Science Ltd. All rights reserved.