L.J. Huang

Controllable two-scale network architecture and enhanced mechanical properties of (Ti5Si3+TiBw)/Ti6Al4V composites

Novel Ti6Al4V alloy matrix composites with a controllable two-scale network architecture were successfully fabricated by reaction hot pressing (RHP). TiB whiskers (TiBw) were in-situ synthesized around the Ti6Al4V matrix particles, and formed the first-scale network structure (FSNS). Ti5Si3 needles (Ti5Si3) precipitated in the β phase around the equiaxed α phase, and formed the secondary-scale network structure (SSNS). This resulted in increased deformation compatibility accompanied with enhanced mechanical properties. Apart from the reinforcement distribution and the volume fraction, the ratio between Ti5Si3 and TiBw fraction were controlled. The prepared (Ti5Si3 + TiBw)/Ti6Al4V composites showed higher tensile strength and ductility than the composites with a one-scale microstructure, and superior wear resistance over the Ti6Al4V alloy under dry sliding wear conditions at room temperature.

Interactive effects of cyclic oxidation and structural evolution for Ti-6Al-4V/(TiC+TiB) alloy composites at elevated temperatures

The microstructural features and high-temperature oxidation resistance of hybrid (TiC+TiB) networks reinforced Ti-6Al-4V composites were investigated after fabricated with reaction hot pressing technique. The inhomogeneous distribution of hybrid reinforcers resulted in a sort of stress-induced grain refinement for α-Ti matrix phase, which was further facilitated by heterogeneous nucleation upon additive interfaces. HRTEM analyses revealed the crystallographic orientation relation between TiB and α-Ti phases as (201)TiB//(1 — 100)α-Ti plus [11 2 — ]//[0001] α-Ti, while TiC and α-Ti phases maintained the interrelation of (2 — 00)TiC//(2 — 110) α-Ti and [001]TiC//[011 — 0] α-Ti. The hybridly reinforced Ti-6Al-4V/(TiC+TiB) composites displayed superior oxidation resistance to both the sintered matrix alloy and the two composites reinforced solely with TiC or TiB addition during the cyclic oxidation at 873, 973 and 1073 K respectively for 100 h. The hybrid reinforcers volume fraction was a more influential factor to improve oxidation resistance than the matrix alloy powder size. As temperature rose from 873 to 1073 K, the oxidation kinetics transferred from the nearly parabolic type through qusilinear tendency into the finally linear mode. This corresponded to the morphological transition of oxide scales from a continuous protective film to a partially damaged layer and ended up with the complete spallation of alternating alumina and rutile multilayers. A phenomenological model was proposed to elucidate the growth process of oxides scales. The release of thermal stress, the suppression of oxygen diffusion and the fastening of oxide adherence were found as the three major mechanisms to enhance the oxidation resistance of hybrid reinforced composites.

Cycle oxidation behavior and anti-oxidation mechanism of hot-dipped aluminum coating on TiBw/Ti6Al4V composites with network microstructure

Controlled and compacted TiAl3 coating was successfully fabricated on the network structured TiBw/Ti6Al4V composites by hot-dipping aluminum and subsequent interdiffusion treatment. The network structure of the composites was inherited to the TiAl3 coating, which effectively reduces the thermal stress and avoids the cracks appeared in the coating. Moreover, TiB reinforcements could pin the TiAl3 coating which can effectively improve the bonding strength between the coating and composite substrate. The cycle oxidation behavior of the network structured coating on 873 K, 973 K and 1073 K for 100 h were investigated. The results showed the coating can remarkably improve the high temperature oxidation resistance of the TiBw/Ti6Al4V composites. The network structure was also inherited to the Al2O3 oxide scale, which effectively decreases the tendency of cracking even spalling about the oxide scale. Certainly, no crack was observed in the coating after long-term oxidation due to the division effect of network structured coating and pinning effect of TiB reinforcements. Interfacial reaction between the coating and the composite substrate occurred and a bilayer structure of TiAl/TiAl2 formed next to the substrate after oxidation at 973 K and 1073 K. The anti-oxidation mechanism of the network structured coating was also discussed.

Study on titanium-magnesium composites with bicontinuous structure fabricated by powder metallurgy and ultrasonic infiltration

Titanium-magnesium (Ti-Mg) composites with bicontinuous structure have been successfully fabricated by powder metallurgy and ultrasonic infiltration for biomaterial potential. In the composites, Ti phase is distributed continuously by sintering necks, while Mg phase is also continuous, distributing at the interconnected pores surrounding the Ti phase. The results showed that the fabricated Ti-Mg composites exhibited low modulus and high strength, which are very suitable for load bearing biomedical materials. The composites with 100 µm and 230 µm particle sizes exhibited Youngs modulus of 37.6 GPa and 23.4 GPa, 500.7 MPa and 340 MPa of compressive strength and 631.5 MPa and 375.2 MPa of bending strength, respectively. Moreover, both of the modulus and strength of the composites increase with decreasing of Ti particle sizes. In vitro study has been done for the preliminary evaluation of the Ti-Mg composites.

Superplasitic Tensile Behavior of in Situ TiBw/Ti6Al4V Composite with Novel Network Microstructure

In situ TiB whiskers reinforced Ti6Al4V (TiBw/Ti64) composites with a novel network microstructure were successfully fabricated by reaction hot pressing. The novel composites exhibited a superior combination of mechanical properties at room temperature and a superior strengthening effect at 400–600°C. In the present work, superplastic tensile behavior of the novel composite was carried out at 900–1000°C. The tensile elongation of the novel composite is always over 100% when the tensile temperature is over 900°C. The elongation of the composite firstly increases and then decreases with increasing tensile temperatures. In particular, the tensile elongation is up to 214% at 940°C, which can be viewed as the highest ductility for the as-sintered discontinuously reinforced titanium matrix composites (DRTMCs) fabricated by powder metallurgy (PM) process up to date. The superior ductility can be attributed to the novel network microstructure including the TiBw-lean region and the TiBw-rich network region.