Influence of particulate size on creep properties of TiB-reinforced orthorhombic Ti-22Al-27Nb

Intermetallic titanium alloys based on the orthorhombic phase Ti2-Al-Nb exhibit a higher ductility and fracture toughness compared to other intermetallic systems such as TiAl and super-2 (Ti3Al) [1]. These are under development for applications up to 700°C in aeronautic gasturbines. The reinforcement with titanium boride (TiB) particulates leads to an improvement of the mechanical properties. TiB is chemically and physically compatible with titanium alloys, especially with respect on the coefficient of thermal expansion. The effect of the reinforcement is influenced not only by the volume content but also by the size of particulates. Subject of this study are the creep properties of Ti-22Al-27Nb alloy reinforced with 6.5% TiB. It has been reported recently, that smaller particulates enhances the tensile strength as well as the fatigue strength [2]. Reasons for the additional reinforcing effects can be found in the micromechanisms of plastic deformation such as blocking of dislocations

Creep properties of TiB Particulate-Reinforced Ti-22Al-27Nb Composites

The Ti2AlNb intermetallic alloys, based on the ordered orthorhombic (O) phase, have a higher specific strength, better creep resistance and better workability than conventional titanium aluminides, such as TiAl and Ti3Al. Typical alloy system so far studied is a Ti-22Al-27Nb (mol pct) alloy, consisting primarily of the (O+B2 (ordered BCC)) two-phase microstructure. However, the mechanical properties of these alloys must be improved before commercial applications are feasible. To further improve the mechanical properties of this Ti2AlNb alloy, a TiB particulate-reinforced Ti-22Al-27Nb matrix composite was prepared using the gas-atomized powder metallurgy method. Because of the rapid solidification during the gas atomization process, the TiB particulates dispersed in the composite were extremely fine, with an average diameter of less than 1 μm and lengths ranging up to 5 μm. This composite (PM composite) showed better creep properties at high temperatures than both an unreinforced Ti-22Al-27Nb matrix alloy and a Ti-22Al-27Nb/TiB composite produced using a conventional ingot metallurgy method (IM composite) with relatively coarse (average diameter 5 μm and average length 40 μm) TiB particulates. We have also found that the size of TiB particulate in the PM composite can be controlled by heat treatment at high temperature (around 1473 K), and investigated the effect of the size of dispersed TiB particulates on the creep properties of Ti-22Al-27Nb/TiB composites