The microstructure and mechanical behavior of ion-irradiated titanium aluminides

The effect of ion irradiation on the microstructure and mechanical properties of Ti-52Al (TiAl), Ti-26Al (Ti{dollar}sb3{dollar}Al), and Ti-49Al-2W (TiAl-W) were investigated. The alloys were irradiated with either 2 MeV protons, He{dollar}sp+{dollar} and Ar{dollar}sp+{dollar} ions at low temperatures ({dollar}-{dollar}175 to {dollar}-{dollar}135{dollar}spcirc{dollar}C) to maximum fluences of 9 {dollar}times{dollar} 10{dollar}sp{lcub}15{rcub}{dollar} Ar{dollar}sp+{dollar}/cm{dollar}sp2{dollar}, 2 {dollar}times{dollar} 10{dollar}sp{lcub}17{rcub}{dollar} He{dollar}sp+{dollar}/cm{dollar}sp2{dollar}, and 2 {dollar}times{dollar} 10{dollar}sp{lcub}17{rcub}{dollar} H{dollar}sp+{dollar}/cm{dollar}sp2{dollar}. The yield and fracture strengths of the unirradiated and irradiated materials were determined using a miniaturized disk-bend test. The hardness and Young's modulus of the irradiated alloys were measured using a mechanical properties microprobe. The microstructure of the irradiated alloys was observed using optical, scanning electron microscopy and transmission electron microscopy.; The yield strength of unirradiated TiAl, Ti-Al-W and the fracture strength of unirradiated Ti{dollar}sb3{dollar}Al were 367 {dollar}pm{dollar} 33 MPa, 810 MPa, and 536 {dollar}pm{dollar} 39 MPa, respectively, in excellent agreement with published data. The yield strength of TiAl and TiAl-W and fracture strength of Ti{dollar}sb3{dollar}Al increased as a result of irradiation. The strengths of irradiated TiAl and Ti{dollar}sb3{dollar}Al alloys were lowest for the Ar{dollar}sp+{dollar} irradiation at the highest peak dose. The nanohardness of all the irradiated specimens generally increased with dose, but the influence of dose on Young's modulus was erratic.; Plate-shaped defects, vacancy in character, and helical dislocations were observed in irradiated TiAl by transmission electron microscopy. The plate-shaped defects form preferentially on (200), and not at all on (020) or (002). X-ray diffraction measurements on the unirradiated TiAl alloy confirmed its tetragonal structure, thus there is no compelling explanation for the preferential formation on (200). Electron diffraction of the ion-irradiated material revealed a change in all three lattice spacings, suggesting that ion irradiation has possibly transformed the material into an orthorhombic structure, but this has not been confirmed. Proton irradiation of Ti{dollar}sb3{dollar}Al produced a faint tweed-like contrast. The Ar{dollar}sp+{dollar}-ion irradiation-induced microstructure of Ti{dollar}sb3{dollar}Al contained defects producing mottled contrast at 1 dpa and black spot contrast at 5 dpa. He{dollar}sp+{dollar}-ion irradiations of the binary alloys produced similar defects. The long-range order parameter decreased with increasing dose for all the alloys. The influence of these microstructural variables on the mechanical behavior is discussed.