E. Liotti

Unusual Observations in the Wear-Out of High-Purity Aluminum Wire Bonds Under Extended Range Passive Thermal Cycling

This paper reports on the reliability of ultrasonically wedge-bonded 99.99% (4N) and 99.999% (5N) pure aluminum wires under different passive thermal cycling ranges, namely, -40°C to 190°C, -60°C to 170°C, -35°C to 145 °C, and -55°C to 125°C. The rate of bond strength degradation during cycling was found to be more rapid in the wire bonds subjected to lower peak temperatures (Tjmax) and lower temperature ranges (ΔT) for both wire types. This observed effect of ΔT cannot be described by the commonly accepted empirical relationships based on damage accumulation, such as the Coffin-Manson law. In addition, the 4N wire bonds were found to degrade more rapidly than the 5N bonds under the cycling ranges investigated. Microstructural characterization and nanoindentation of the bond interfaces indicated differences in microstructural restoration in wires subjected to the different cycling ranges. These differences have been attributed to annealing phenomena occurring in the wires during the high-temperature phase of cycling, which are believed to remove some of the damage accumulated during the low-temperature phase. A model is proposed for the prediction of wire bond wear-out rate, which incorporates both damage accumulation and damage removal mechanisms. We conclude that the rate of annealing during cycling varies exponentially with temperature; the annealing effects which occur can reduce damage accumulation and therefore influence wire bond reliability.

Real-time synchrotron x-ray observations of equiaxed solidification of aluminium alloys and implications for modelling

Recently, in-situ observations were carried out by synchrotron X-ray radiography to observe the nucleation and growth in Al alloys during solidification. The nucleation and grain formation of a range of Al-Si and Al-Cu binary alloys were studied. When grain refiner was added to the alloys, the location of the nucleation events was readily observed. Once nucleation began it continued to occur in a wave of events with the movement of the temperature gradient across the field of view due to cooling. Other features observed were the settling of the primary phase grains in the Al-Si alloys and floating in the Al-Cu alloys, the effects of convection with marked fluctuation of the growth rate of the solid-liquid interface in the Al-Si alloys, and an absence of fragmentation. The microstructures are typical of those produced in the equiaxed zone of actual castings. These observations are compared with predictions arising from the Interdependence model. The results from this comparison have implications for further refinement of the model and simulation and modelling approaches in general. These implications will be discussed.

A Synchrotron X-Ray Radiography Investigation of Induced Dendrite Fragmentation in Al-15wt%Cu

The effect of a Pulsed Electro Magnetic Field (PEMF) on the solidification of an Al‑15 wt%Cu alloy was studied in situ by synchrotron X-ray radiography. Samples were solidified with and without the presence of the PEMF while recording radiographs, enabling observation and quantification of dendrite fragmentation by image analysis. Fragmentation increased with a PEMF and was attributed to induced inter-dendritic flow.

Mapping of multi-elements during melting and solidification using synchrotron X-rays and pixel-based spectroscopy.

A new synchrotron-based technique for elemental imaging that combines radiography and fluorescence spectroscopy has been developed and applied to study the spatial distribution of Ag, Zr and Mo in an Al alloy during heating and melting to 700, and then re-soldification. For the first time, multi-element distributions have been mapped independently and simultaneously, showing the dissolution of Ag- and Zr-rich particles during melting and the inter-dendritic segregation of Ag during re-solidification. The new technique is shown to have wide potential for metallurgical and materials science applications where the dynamics of elemental re-distribution and segregation in complex alloys is of importance.

Crystal nucleation in metallic alloys using x-ray radiography and machine learning

Synchrotron x-ray radiography and machine learning computer vision help explain alloy effects on metallic crystal formation. The crystallization of solidifying Al-Cu alloys over a wide range of conditions was studied in situ by synchrotron x-ray radiography, and the data were analyzed using a computer vision algorithm trained using machine learning. The effect of cooling rate and solute concentration on nucleation undercooling, crystal formation rate, and crystal growth rate was measured automatically for thousands of separate crystals, which was impossible to achieve manually. Nucleation undercooling distributions confirmed the efficiency of extrinsic grain refiners and gave support to the widely assumed free growth model of heterogeneous nucleation. We show that crystallization occurred in temporal and spatial bursts associated with a solute-suppressed nucleation zone.

Energy dispersive detector for white beam synchrotron x-ray fluorescence imaging

A novel, “single-shot” fluorescence imaging technique has been demonstrated on the B16 beamline at the Diamond Light Source synchrotron using the HEXITEC energy dispersive imaging detector. A custom made furnace with 200µm thick metal alloy samples was positioned in a white X-ray beam with a hole made in the furnace walls to allow the transmitted beam to be imaged with a conventional X-ray imaging camera consisting of a 500 µm thick single crystal LYSO scintillator, mirror and lens coupled to an AVT Manta G125B CCD sensor. The samples were positioned 45° to the incident beam to enable simultaneous transmission and fluorescence imaging. The HEXITEC detector was positioned at 90° to the sample with a 50 µm pinhole 13cm from the sample and the detector positioned 2.3m from pinhole. The geometric magnification provided a field of view of 1.1×1.1mm2 with one of the 80×80 pixels imaging an area equivalent to 13µm2. Al-Cu alloys doped with Zr, Ag and Mo were imaged in transmission and fluorescence mode. The fluo...

QUANTIFICATION STUDY ON DENDRITE FRAGMENTATION IN SOLIDIFICATION PROCESS OF ALLUMINUM ALLOYS

Alloy solidification is an important process to control the mechanical properties of engineering products. During solidification, dendrite fragmentation occurs commonly as a key phenomenon to determine the microstructure and to obtain fine grain size. Recently, in situ synchrotron X-radiography technique was developed and applied to observe thermodynamic behaviors such as dendrite growth and fragmentation during solidification. External forces such as mechanical and electromagnetic stirring, and thermal shock were added into the solidification process to investigate their effects on the fragmentation behavior. However, most work conducted in literature focused on qualitative aspects e.g. morphology transition or solute distribution and quantitative investigation such as determining the specific relationship between fragmentation and solidification conditions was rather limited. In this work, the third generation synchrotron X-radiography technique was used to observe the solidification process of an Al-15%Cu (mass fraction) alloy. Experimental conditions including the strength of the pulsed electromagnetic fields, dendrite growth direction and the temperature gradients were varied and the subsequent effect on fragmentation was studied and quantified. A computer program was developed based on Matlab to perform the image processing and measurement. The fragmentation number according to experiments was counted and correlated to the *国家自然科学基金项目51275269和51205229资助 收到初稿日期: 2014-09-10,收到修改稿日期: 2015-03-25 作者简介:毕成,男, 1988年生,博士生 DOI: 10.11900/0412.1961.2014.00501 第677-684页 pp.677-684

High-resolution synchrotron X-ray diffraction studies of size and strain effects in a complex Al–Fe–Cr–Ti alloy

We present a study of a complex ultra-high-strength Al alloy containing ~40 volume per cent of second-phase particles, ranging in size from nanometres to a few microns. The microstructure has been investigated using scanning electron microscopy and high-resolution synchrotron X-ray diffraction using the I11 beam line at the Diamond Light Source, UK. Powder diffraction was carried out to (i) determine phases present, (ii) quantify the weight per cent of each phase and (iii) quantify size and strain effects in the Al matrix. The high beam quality (i.e. low divergence and wavelength purity) and multi-analysing crystal detectors makes this an ideal instrument to resolve the high peak density and determine the contribution of sample broadening in the complex alloy. Using Pawley and Rietveld full pattern fitting, the intermetallic phases present were determined to be Al 3 Ti, Al 13 Cr 2 and Al 13 Fe 4 . The weight fraction of each phase was calculated from the Rietveld refinements and correlated well with thermodynamic calculations assuming an equilibrium microstructure. Size and strain in the Al matrix was measured from peak broadening using a Double Voigt analysis and showed significant physical broadening due to both size and strain.