Microstructure and mechanical properties of Ti-Al-Nb-based alloys were investigated as they are important in the development of new generation high temperature Ti alloys due to good oxidation resistance. The alloy compositions Ti-15Al-2Nb (to obtain α₂ precipitates) and Ti-15Al-2Nb-0.5Si (at%) (to obtain solid-solution hardening by Si) were selected. Heat treatment at 1000℃ after forging and rolling at 900℃ transformed the β phase to α phase and α' martensite during cooling. By heat treatment at 900℃, a single α phase was obtained in both alloys. The α₂ phase was formed by aging treatment at 600 and 700℃ of Ti-15Al-2Nb and at 600, 700, and 800℃ of Ti-15Al-2Nb-0.5Si. Compressive strength was investigated for samples heat treated at 900℃ with a single α phase and those aged at 600, 700, 800℃ with α₂ precipitates. The solid solution hardening effect of Si was found at all test temperatures. Precipitation hardening effect by α₂ precipitates was also observed in both alloys and the effect improved with Si addition. Peaks of precipitation hardening were obtained at 300℃ in both alloys; the precipitation hardening effect decreased with increase of test temperature above 300℃. It is suggested that a shearing mechanism occurs up to 300℃, which changes to a bypass mechanism above 450℃.

The recovery annealing technique was used to clarify the crystallization behavior of a rejuvenated Zr₅₀Cu₄₀Al₁₀ metallic glass. Specific heat and thermal expansion measurements showed that the glassy states of fast cooled and recovery annealed samples are almost identical from the energetic and volumetric points of view. Dynamic mechanical analysis also suggested that their internal microstructures resemble each other. These results indicate that the glassy state and the structure can be controlled according to the final cooling conditions from the supercooled liquid region. However, a clear difference of the incubation time for crystallization was observed between the fast cooled and recovery annealed samples, implying that thermal accumulation seems to mostly correlate to the crystallization rather than to atomic diffusion through the newly created excess free volume in the rejuvenated sample. In addition, a transmission electron microscopy image of the sample, performed on double sets of the recovery annealing process, shows a precipitation of the crystalline phase with approximately 50 nm in diameter in the amorphous matrix. These results suggest that the recovery annealing independently leads to two different thermal paths, one towards structural rejuvenation and the other towards crystallization. The high-resolution transmission electron microscopy result of the recovery annealed sample indicates that these two regions co-exist in the amorphous matrix.

The training effect of microstructure and shape recovery on Ti-50Pd-xZr (x = 7 and 10) at% and Ti-50Pd-xZr-(5-x)V (x = 1, 2.5, and 4) high-temperature shape memory alloys were investigated. Zr was selected as an alloying element as it is known to improve the shape recovery of TiPd. As a further alloying element, V was selected because it is effective in strengthening TiPd. The dependence of Zr content and V addition on the martensitic transformation (MT) temperature, shape recovery, and training effect were investigated. For example, M_f, decreased with increasing Zr from 480℃ in Ti-50Pd to 302℃ in Ti-50Pd-10Zr. In Ti-50Pd-xZr-(5-x)V, when the total amount of Zr and V was 5 at%, the MT temperatures did not change drastically. The MT temperatures ranged between 350 and 550℃.

Shape recovery was investigated using the thermal cyclic test under a constant applied stress in the range of 15 to 200 MPa. Perfect recovery was obtained at low stresses, while irrecoverable strain was observed at high stresses. For Ti-50Pd-2.5Zr-2.5V and Ti-50Pd-1Zr-4V, creep deformation was observed above 150 MPa. To obtain perfect recovery, training (repeated thermal cyclic tests under a constant applied stress) was performed. Perfect recovery was obtained for the alloys by training, except for Ti-50Pd-4Zr-1V. Ti-50Pd-10Zr achieved perfect recovery up to 200 MPa, while Ti-50Pd-1Zr-4V achieved perfect recovery up to 150 MPa. Other alloys achieved perfect recovery at lower stresses of 65 or 50 MPa. The microstructure changed from a random martensite variant to a specific orientation during training, to accommodate the large strain during deformation. It was found that a strong texture led to perfect shape recovery.