The {332}<113> Twinning Behavior of a Ti-15Mo Medical Alloy during Cyclic Deformation and Its Effect on Microstructure and Performance.

In this study, the microstructural evolution of a Ti-15Mo medical alloy was investigated, when the in situ cyclic tensile strain had 2% amplitude and the tension-compression cyclic deformation had 1%, 2%, and 3% amplitude. The Vickers hardness and wear resistance of the alloy were also optimized due to the grain-refining effect after cyclic deformation and annealing. The twinning-induced plasticity (TWIP) was considered the main deformation mechanism of the Ti-15Mo alloy during the tensile-compressive cycle deformation with suitable strain amplitude. The {332}<113> twins and boundaries were the main contributors to the grain refinement. The optimal microstructure, hardness, and wear resistance were obtained in the alloy deformed by tension-compression cyclic strain with a 3% strain amplitude. The wear resistance of the annealed alloy in Hank's solution was excellent in contrast to the original Ti-15Mo alloy due to its reasonable microstructure and hardness. It is clear that abundant twins were formed and retained in the coarse grains of the original alloy after cyclic deformation and annealing, which provided the expected refined grains and performance.

Effects of TiZn3 and TiZn16 components on the microstructure and mechanical performance of Ti-6Al-4 V alloy joints formed via ultrasonic assisted brazing using pre-galvanized workpieces.

While the use of a Zn interlayer has been demonstrated to reduce the temperature required for joining easily oxidized metal alloys in atmospheric environments, the effects of reactions between the titanium alloy workpieces and the Zn interlayer on the mechanical performance of the finished joints are poorly understood. The present work addresses this issue by evaluating the chemical compositions at the interfaces of pre-galvanized Ti-6Al-4 V alloys joined at 420 °C in an atmospheric environment by ultrasonic-assisted brazing, and relating the observed compositions to the mechanical performances of the joints. The Ti-6Al-4 V alloy workpieces are first wetted by pure Zn using an ultrasonic assisted hot dip galvanizing (U-HDG). The obtained ultrasonic excitations are demonstrated to destroy the oxide film on the surfaces of the Ti-6Al-4 V workpieces and promote reactions between Ti and Zn at the interfaces. The plating of Zn on the workpiece surfaces is demonstrated to be realized by the formation of intermetallic compounds (IMCs) comprising a uniform TiZn3 layer in contact with the Ti-6Al-4 V surface, followed by a mixed TiZn3 + TiZn16 layer and a η-Zn layer at the outer surface. Application of the ultrasonic-assisted brazing process is demonstrated to maintain uniform TiZn3 layers next to the Ti-6Al-4 V surfaces, while the concentration of the TiZn16 phase near the midpoint of the joints increases with increasing ultrasonic treatment time (UST) from 5 s to 20 s, and the corresponding concentration of the η-Zn phase decreases. The results of mechanical testing demonstrate that the shear strength of the joint obtained with a TiZn3 layer thickness of 8-12 µm and a UST of 10 s is 210 MPa, which is 3.55 times greater than that obtained for joints processed without pre-galvanization.

The Directional Solidification, Microstructural Characterization and Deformation Behavior of ß-Solidifying TiAl Alloy.

A ß-solidifying Ti-43Al-2Cr-2Mn-0.2Y alloy was directionally solidified by the optical floating zone melting method. The microstructure is mainly characterized by γ/α2 lamellae with specific orientations, which exhibits straight boundaries. The ß phase is randomly distributed in the lamellar microstructure, indicating that the ß phase cannot be directionally solidified. The directional solidification of γ/α2 lamellae was not affected by the precipitation of the ß phase. Hot compression tests show that the deformation behavior of the ß-containing lamellar microstructure also exhibits the anisotropic characteristic. The deformation resistance of the lamellae is lowest when the loading axis is aligned 45° to the lamellar interface. Microstructural observation shows that the decomposition of the lamellar microstructure tends to begin around the ß phase, which benefits from the promotion of a soft ß phase in the deformation. Moreover, the deformation mechanism of the lamellar microstructure was also studied. The bulging of the γ phase boundaries, the decomposition of α2 lamellae and the disappearance of γ/γ interfaces were considered as the main coarsening mechanisms of the lamellar microstructure.