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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.
TiAl alloys with a high addition of Nb have recently been applied in the aerospace and automotive fields due to their excellent high-temperature properties. The C additions to TiAl alloys have been reported to improve their strength and creep resistance through solid-solution hardening and / or precipitation hardening. In this work powder metallurgy (PM) based Ti-45Al-5Nb-xC (x=0, 0.5 0.75 and 1.0 at. %) alloys were systematically investigated after different heat treatments. The research is mainly concerned with the temperature range from 800 to 1000ºC which is interesting for processing as well as applications. The results show that the addition of C influences the phase transformations in Ti-45Al-5Nb and that the addition of Nb may influence the carbon solubility in TiAl alloys at high temperatures. The carbon solubility in Ti-45Al-5Nb is > 1.0 at. % at 1400°C, but between 0.5 and 0.75 at. % at 1000°C, and less than 0.5 at. % at 800°C. The carbide precipitation and development of carbide morphology are discussed in great detail. The thermal stability of P-Ti3AlC carbides in Ti-45Al-5Nb-xC alloys is increased, which might be attributed to the addition of high amounts of Nb, or the high amount of carbon, or a combination of both. In heat-treated Ti-45Al-5Nb-0.5C and Ti-45Al-5Nb-0.75C, H-type carbides are not detected to form during annealing. The addition of Nb may increase the formation temperature of the H-Ti2AlC precipitate phase.