Wei Jie Lu

Microstructure and tensile properties of in situ (TiB+TiC)/Ti6242 (TiB:TiC=1:1) composites prepared by common casting technique

In the present work, (TiBw+TiCp)/Ti6242 composites with TiB:TiC=1:1 were produced by common casting and hot-forging technology utilizing the SHS reactions between titanium and B4C, C powder. The microstructures of composites were examined using optical microscopy (OM) and transmission electron microscopy (TEM). The X-ray diffraction (XRD) was used to identify the phases that were present in the composites. There are three phases — TiB, TiC and titanium matrix alloy. TiB grows in short-fiber shape, whereas TiC grows in dendritic, equiaxed or near-equiaxed shape. TiB whiskers were made to align the longitudinal direction and TiC dendritic was broken up after hot-forging. The reinforcements are distributed uniformly in matrix alloy. The interfaces between reinforcements and titanium matrix alloy are very clean. The tensile strength (yield strength and ultimate tensile strength) and the Youngs modulus improve with the addition of TiB whiskers and TiC particles although some reduction in ductility is observed. (TiBw+TiCp)/Ti6242 composites with TiB:TiC=4:1 will fracture on a lower level of applied stain due to deformation restraint of TiB whiskers on titanium matrix alloy. The (TiBw+TiCp)/Ti6242 composites with TiB:TiC=1:1 show higher tensile strength and ductility. The addition of graphite not only improves the tensile strength and the Youngs modulus but also increases the ductility. The improved Youngs moduli and increased tensile strengths of the composites are explained using shear lag and rule-of mixtures theories. The Youngs moduli of the composites were found in good agreement with that calculated from Tsai–Halpin equation applied for discontinuous-reinforced composites.

Microstructure and Tensile Properties of In Situ Synthesized (TiB+TiC)/Ti-6Al-4V Composites

In this paper, Ti-6Al-4V matrix composites reinforced with 5% or 10% TiB and TiC were in situ synthesized by common casting and hot-forging technology utilizing the reaction between titanium and B4C. The phase constituents were identified by XRD while transus temperatures were determined by DSC and metallography. The evolution of microstructures was studied by optical microscopy. The effects of reinforcements on the microstructures, tensile properties and fractures at room temperature were discussed. The results show that yield strength and ultimate tensile strength increased significantly while ductility decreased with reinforcements increasing. Fracture type turned to brittle when reinforcements increased.

Weld zone characteristic and mechanical performance of in situ titanium matrix composites using gas tungsten arc welding (GTAW)

Abstract In situ TiBw and La2O3 reinforced titanium matrix composites were successfully fusion welded by the gas tungsten arc welding (GTAW) process and the weldability and feasibility for composites were studied, and uniform and defect free welds were produced with sound welding parameters. Microstructural observations showed that the joint has a distinctly identified weld zone characteristic. In addition, the distribution and size of TiBw in the weld became much more homogeneous and smaller. Moreover, the welded joints exhibited good mechanical properties at ambient temperature and the strength equal to the base metal at elevated temperature. The fracture mechanism of TiBw in the weld also was investigated.

Progress in Understanding the Fatigue Behavior of Ti Alloys

Titanium (Ti) alloys are used in critical, fatigue limited applications in aircraft and aircraft engines. Current design practices are, of necessity, conservative in order to minimize risk of unexpected failures. Among the sources of this conservatism are the inherent variations in the number of loading cycles the materials can withstand prior to fatigue crack initiation, the uncertainty in crack propagation lifetime prediction and the need to set safe minimum component life values. While the stochastic nature of fatigue is well-known, improved characterization methods have begun to provide a better understanding of the crack initiation process. This paper describes recent work designed to provide an improved understanding of the relationship between thermomechanical processing history, microstructure, texture and the fatigue behavior of α+β Ti alloys. Due to length limitations, the paper focuses on two important aspects of fatigue life variation: the effects of microstructural discontinuities on fatigue and the role of facet formation during crack initiation and the early stages of fatigue crack growth.

Improvement in Hot Workability of Titanium Matrix Composite by Thermohydrogen Treatment

Ti-6Al-4V matrix composite (TMC) reinforced with TiB plus TiC was prepared and hydrogenated. Isothermal compression tests and high temperature tensile tests were carried out to study the effect of the hydrogen on hot deformation and superplastic deformation. The flow behaviour and microstructure evaluation of hot deformation was investigated. The results show hydrogen can reduce the flow stress and decrease the deformation temperature or increase the strain rate at the same flow stress level in hot deformation. Hydrogen increasing β phase and promoting dynamic recrystallizaiton (DRX) was considered as the main reasons for hydrogen-induced plasticity in hot deformation. The results of superplastic deformation indicate hydrogen can decrease the superplastic temperature 100°C or increase strain rate one order of magnitude at the same elongation level in superplastic deformation. Hydrogen promoting DRX were considered as the main reason for improvement of superplastic elongation.

Ambient-Temperature and High-Temperature Tensile Properties of Investment Casted Titanium Matrix Composites with B4C Additions

Titanium matrix composites (TMCs) were prepared by investment casting in a consumable arc skull casting furnace. The effects of B4C additions on ambient-temperature and high-temperature tensile properties of TMCs were investigated. It has been found that with the addition of B4C, the microstructure of TMCs was refined and the strength improved. The strength enhancement of the TMCs is ascribed to the combined effects of the second-phase strengthening, grain refinement strengthening and the solution strengthening. The grain refinement and solution strengthening effects play a main role in the yield strength enhancement of TMCs at ambient temperature, and the second-phase strengthening of TiB whiskers and TiC particles plays a more important role at high temperature.

Investigation of the Effect of Argon Arc Welding Parameters on Properties of Thin Plate of In Situ Titanium Matrix Composites

The weldability of in-situ titanium matrix composites (TMCs) was studied using the gas tungsten arc welding (GTAW). The effects of GTAW on the microstructure of fusion zone and heat-affected zone were discussed, and the changes of TiB whisker reinforcements in the welded joint were investigated by optical microscope (OM), scanning electron microscopy (SEM), XRD analysis and tension testing at room temperature. Research results show that the GTAW process is a suitable welding method for in-situ TMCs. Under reasonable welding parameters, the welded joints display goo weld seam formation, and TiB whiskers show distinctly smaller sizes and uniform distribution with a special network structure. The maximum tensile strength of welded joints can reach 92% of the base metal under optimum welding parameters.

Study on In Situ Synthesized Titanium Matrix Composites

In the present work, titanium matrix composites reinforced with TiB, TiC, and Re2O3 (Nd2O3 or Y2O3) were fabricated via common casting and hot-forging technology utilizing the chemical reaction between Ti, B4C (or C), rare earth (Re) and B2O3 through homogeneously melting in a non-consumable vacuum arc remelting furnace. In this work, Nd and Y were chosen as rare earth (Re) added in the in situ reaction. The thermodynamics of in situ synthesis reactions was studied. The results of X-ray diffraction (XRD) proved that no other phases appeared except for TiB, TiC and Re2O3. The microstructures of the composites were examined by optical microscopy (OM). The results showed that there were mainly three kinds of reinforcements: TiB whiskers, TiC particles and Re2O3 particles. The reinforcements were fine and were homogeneously distributed in the matrix. The interfaces of TiB-Ti and Y2O3-Ti have been examined by high-resolution transmission electron microscopy (HREM).Transmission electron microscopy (TEM) and selected area diffraction (SAD) were used to analyze the orientation relationships of TiB-Ti, Nd2O3-Ti, and Y2O3-Ti. The mechanical properties at room temperature improved with the addition of TiB whiskers and TiC particles although some reduction in ductility was observed. The (TiB+TiC)/Ti6242 composite with TiB:TiC=1:1 shows higher tensile strength and ductility.