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The powder metallurgically produced beta titanium alloys (traditional PM beta Ti-alloys) have long been plagued by high impurities contamination. For binder-based powder technologies, they originate from the sintering atmosphere, the debinding processes and the starting powders. In general, a normal carbon residual of binder-based powder technologies is capable of incurring the formation of aligned TiCx particles along beta grain boundaries (GB-TiCx) in most classes of beta Ti-alloys. Whereas, oxygen atoms are likely to deteriorate the ductility of PM Ti alloys by promoting the formation of diverse brittle phases and/or altering the deformation modes. Such materials exhibiting rather low toughness to strain ratios are not an option for critical structural applications, where catastrophic damage is completely unacceptable.
In this study, biotolerant metastable beta Ti-20Nb-10Zr alloys, containing a certain amount of carbon, oxygen residuals originated from materials processing and consequently 0.5 vol.% in situ synthesized TiCx particles, were fabricated via metal-injection-molding (MIM). With varying yttrium (Y) addition, the effects of Y-induced oxygen scavenging, beta-grain refinement and porosity increment on tensile properties were systematically investigated. To scavenge oxygen from the beta Ti-matrix, the Y elemental powder with a maximum particle size of 15 µm (e.g. <12 µm or 1200 mesh) is more appropriate than the commonly used <45 µm (i.e. 325 mesh) sized powder or larger ones and without significant detrimental effect on the as-sintered density of beta Ti-alloys.
A novel toughening strategy was proposed by regulating TiCx precipitation evolution and resultantly adjusting particles distribution pattern. Synchrotron radiation identified that two separate TiCx precipitation-type reactions occurred at the beta phase region and the alpha/beta region. In a narrow temperature range between these two precipitation reactions, dissolution of carbides was observed just below alpha/beta transus. Y addition can postpone TiCx precipitation. On the basis of those mechanisms, adjusting TiCx particle distribution was proposed for the first time, specifically a combination of yttrium addition (Y) and carbide spheroidization reprecipitation annealing (CSRA). As a result, aligned GB-TiCx particles were adjusted to dispersed intragranular TiCx particles. An apparent toughening effect (≈ 113% increment reaching elongation = 8.3%) was achieved after TiCx redistribution, while non-optimally aligned TiCx pattern seriously limited tensile toughness of materials. Here, the mechanisms of TiCx redistribution behavior and its toughening are elucidated systematically.
Internal gettering based on oxygen precipitates is a technique which is used to remove occasional metal contaminations from the active region of microelectronic devices. In order to obtain efficient internal gettering, the precipitation of oxygen in silicon must be precisely controlled. This can be done by rapid thermal annealing (RTA). RTA offers the possibility to establish well defined vacancy concentrations in silicon wafers. Since vacancies are well known to enhance the precipitation of oxygen in Czochralski silicon, RTA pre-treatments can be used to control the generation of oxygen precipitates. This work provides information about the influence of vacancies, introduced by RTA, in silicon wafers on the nucleation of oxygen precipitates during a subsequent annealing in the temperature range between 400 °C and 1000 °C. Moreover, detailed investigations show morphologies and sizes of oxygen precipitates observed in vacancy supersaturated samples subjected to an annealing in the temperature range between 700 °C and 1000 °C for different annealing times. The morphology of the oxygen precipitates was investigated by scanning transmission electron microscopy (STEM) and Fourier transform infrared (FTIR) spectroscopy. In case of FTIR spectroscopy, the absorption bands were assigned to morphologies of the oxygen precipitates observed by STEM. The final part of investigations presented in this work is dedicated to the gettering efficiency of metal impurities. Special focus is devoted towards the gettering efficiency of Cu in vacancy supersaturated samples. In order to understand the gettering of Cu in samples contaminated with high and low concentrations of Cu, two getter tests were carried out. A haze getter test was used to investigate the getter efficiency of Cu in highly contaminated samples. In this particular case, the concentration of Cu equals the concentration of Cu at its solubility limit at 900 °C. A “7 day storage getter test”, developed in this work, was used for the investigation of the getter efficiency of Cu in samples contaminated with low concentration of Cu amounting to 1×1013 cm-2. It was found that the density of oxygen precipitates increases with increasing concentration of vacancies. The nucleation curves of oxygen precipitates in the vacancy supersaturated samples consist of three maxima wherein the maximum observed at 800 °C can be found only in the vacancy rich samples. These maxima can be explained assuming the nucleation of coherent plate-like nuclei consisting of oxygen mono-layers ((Oi)2-p1) and oxygen double-layers ((Oi)2-p2) for the peaks at 450 °C and at 650 °C, respectively, and VO2 mono layers for the peak at 800 °C. The STEM investigations have shown the change of the morphology of oxygen precipitates in samples subjected to nucleation annealing at various temperatures. It was observed, that different temperatures of the RTA pre-treatment and thus different supersaturations of vacancies did not influence the morphology of oxygen precipitates in samples annealed at 800 °C. After annealing at a temperature of 800 °C three and two dimensional dendritic precipitates were found. This kind of precipitates gave rise to an absorption band at 1040 cm-1 as shown by FTIR investigations. From the results of the getter test it was deduced that secondary defects like dislocations have a strong influence on the getter efficiency in samples contaminated with high concentrations of Cu and Ni. In case of the samples contaminated with low concentration of Cu, gettering at dislocations is less important and oxygen precipitates become the main getter sink for Cu. It was also observed, that Cu aggregates at the edge of plate-like precipitates at the site of tensile strain of the silicon lattice. Moreover, the size and density of oxygen precipitates can strongly influence the getter efficiency of metal impurities. The results and observations presented in this work can be very useful for designing and fabrication of high performance silicon wafers. The results can be used for the development of a gettering simulator based on oxygen precipitation. The results of the analysis of the FTIR spectra can be helpful for the fast characterization of the morphologies of oxygen precipitates by means of FTIR.