TiAl Specimens Research Articles

Nitrogen ion implantation into the intermetallic compound TiAl

Intermetallic compound TiAl specimens of compositions 34, 36 and 38 mass% Al, were implanted with nitrogen ions to modify the surface structure and surface properties of the specimens. It was found that the surface hardness of TiAl specimens was increased markedly by nitrogen ion implantation and reached two or three times the hardness of unimplanted specimens after a dose of 5 × 10 17 ions cm −2. X-ray photoelectron spectroscopy and X-ray diffraction studies showed that implanted nitrogen atoms react with metal atoms and form nitrides, Ti 2N and AlN. It was demonstrated that surface microcracks, which were revealed by etching the mechanically polished surfaces of unimplanted specimens, were totally eliminated by nitrogen ion implantation. The increase in hardness was attributed to hardening by solute nitrogens, including radiation damage, and by nitride precipitation. The disappearance of stress-corrosion-induced microcracks was thought to be associated with the residual compressive stresses due to solute nitrogens and the corrosion-resistant nitride formation induced in the surface layer.

Microstructure and Mechanical Properties of Sintered TiAl

Pulverised TiAl powder was cold isostatically pressed at 392 MPa and the resulting compact sintered at temperatures up to 1623 K to produce sintered TiAl with relative densities as high as 91%. This process formed a laminated structure consisting of TiAl and Ti3AI, which gave a high compressive yield strength accompanied by embrittlement of the sintered TiAl compacts, no bending plastic deformation being observed at room temperature. When centrifugally atomised powder melted in a CaO crucible was used, sintered TiAl specimens, apparently of full theoretical density, could be obtained by canned hipping at 1473 K under a pressure of 98 MPa. The hipped TiAl specimens had a grain size of ~20–30 μm, and grain coarsening was not observed, even when specimens were annealed for 720 ks at temperatures of 1273–1373 K. Hipped specimens annealed at 1273 K had a yield strength of 388 MN m −2 at room temperature, and bending ductilities > 0·4% were obtained. PM/0426

New method for electroplating aluminum: Brass World, vii, 202.

For improvement of the oxidation resistance of a TiAl alloy under high temperature conditions, aluminum chromium alloys were electroplated on the TiAl alloy in an AlCl3–NaCl–KCl molten salt containing CrCl2 at 423 K. The deposit electroplated at −0.1 V vs. Al/Al3+ consists of γ-Al8Cr5 single phase, including chromium content at 41 at.%. A mixture phase of γ-Al8Cr5 and Al is, however, formed at potential from −0.2 to −0.4 V. The oxidation resistance of the electrodeposit layer was evaluated by a high temperature oxidation test at 1173 K for 24 h. On the TiAl specimen covered with γ-Al8Cr5 which was deposited at −0.1 V, a uniform AlCr2 layer was formed during the oxidation at 1173 K. The AlCr2 layer was covered by a thin dense Al2O3. Both AlCr2 and Al2O3 layers play a role of superior protection from the high temperature oxidation of the TiAl alloy; therefore, the plating of the γ-Al8Cr5 single layer deposited at −0.1 V reduced oxide thickness by 1/40 during the oxidation at 1173 K for 24 h from that on the TiAl without any plating.