Abstract
The powder metallurgically produced β titanium alloys have long been plagued by high impurity contamination. One of them is the carbon contamination of binder-based powder technologies that originates from the sintering atmosphere, the debinding process 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 prior β grain boundaries (GB-TiCx) in most classes of β titanium. Premature intergranular fracture of materials invariably ensues during plastic deformation, which hinders their commercialization in structural applications. A novel toughening strategy by regulating TiCx precipitation evolution and resultantly adjusting particle distribution is suggested. In this study, biotolerant metastable β Ti-Nb-Zr alloys containing 0.05 wt% standard carbon residual and consequently 0.5 vol% in situ synthesized TiCx particles were fabricated via powder injection molding. Synchrotron radiation identified that two separate TiCx precipitation-type reactions occurred at β phase region and α/β region. In a narrow temperature range between these two precipitation reactions, dissolution of carbides is observed just below α/β transus. Yttrium addition can postpone TiCx precipitation. On the basis of those mechanisms, adjusting TiCx particle distribution is 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 εf = 8.3%) was achieved after TiCx redistribution, while non-optimally aligned TiCx pattern seriously limited tensile toughness of materials by two negative crack propagation modes. Here, the mechanisms of TiCx redistribution behavior and its toughening are elucidated systematically.
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