Research On The High Temperature Properties Of In-situ Synthesized Heat Resistant Titanium Matrix Composites

Recently, titanium matrix composites have drawn great attention due to the superior properties over titanium alloys. At present, the most important and promising application of titanium matrix composites lies in the field of aero-space, where they serve as structural materials and materials for aero-space engines. As the potential materials in place of high temperature titanium alloys, titanium matrix composites should be of high specific strength and specific modulus. Moreover, the performances at high temperatures call for high strength, excellent creep resistance, solid thermal stability and oxidation resistance, and high fatigue strength. Aiming at the above goal, fundamental research was done in the thesis. Via observing microstructures of the matrix alloy and TMCs, testing tensile properties at room and high temperatures, high temperature creep behaviors, and thermal stability of the matrix alloy and TMCs were tested, the enhancement mechanisms of TMCs at high temperatures were investigated in particular.Near-alpha based heat resistant titanium matrix composites (TMCs) reinforced with TiB, TiC and La₂O₃ were synthesized by common casting and hot-forging technology. After hot working and heat treatment, the basic microstructures of the matrix alloy and TMCs were fully lamellar. The reinforcements in the composites were TiB whiskers, TiC particles and La₂O₃ particles. TiB whiskers showed good alignment along the hot working direction. The reinforcements were uniformly distributed in the TMCs, and sizes of La₂O₃ particles were in the nano-scale.Tensile strengths of TMCs at room and high temperatures were significantly enhanced by the reinforcements. The tri-reinforced composite (TiB+TiC+La₂O₃)/Ti with 10% volume fraction of reinforcements showed the highest tensle strength. Ductilities of TMCs were deterimined by the competing effect between the refinement of matrix microstructure and the volume fraction of reinforcements. Both strength and ductility of the composite with 2.4% voume fraction of reinforcements were enhanced. Failure mechanisms at room temperature were load bearing and fracture of TiB short fibers, and the enhancement of strength was mainly determined the volume fraction of reinforcements. Fracture mechanisms of TMCs at high temperature were mainly ductile. The composite with 5% volume fraction of reinforcements showed the highest strength at 600℃. At 650℃and 700℃, the composite with 2.4% volume fraction of reinforcements showed the highest tensile strength. The fractreu mechanism of TMCs with low aspect ratios of TiB whisker was mainly the interfacial debongding between the matrix and ends of TiB whiskers. The excellent high temperature tensile properties of the composite with 2.4% volume fraction of reinforcements were attributed to the high aspect ratios of TiB whiskers.High temperature tensile properties of TMCs were quite sensitive to the strain rates. Tensile strength decreased and the fracture strains increased when the strain rate was lower. The equicohesive temperature of the matrix was around 873K at the strain rate 10?3s?1, and well below 873K at 10?5s?1. The critical aspect ratio of TiB whiskers increased when the temperature was higher or the strain rate was lower, leading to more drastic interfacial debonding and worse tensile properties of TMCs. At higher temperatures or lower strain rates, aspect ratios distribution of TiB short fibers had the dominant influence on the strength enhancement rather than the total volume fraction in TMCs.Steady state creep rates of the TMCs were 1 2 orders of magnitude lower than those of the matrix alloy and IMI 834. Creep resisitance were effectively enhanced by the reinforcements. High apparent stress exponents of TMCs were successfully explained by threshold stress theory. And after compensated with threshold stresses, the stress exponents of the TMCs and matrix alloys were the same. The decelerating creep time of the composites was much lower than the matrix alloy, and there was a long transitional period between the primary decelerating creep and the second steady state creep for the composites. The matrix alloy and composites were embrittled by further weakening of interfaces and surface oxidation under creep rupture conditions.Depending on stress levels, creep behavior of both the matrix alloy and TMCs showed two stress regions. Stress exponents of the matrix alloy were 2.0 and 4.5 in the low and high stress regions respectively. Enhancement of creep resistance in TMCs was mainly attributed to threshold stresses and stress transfer effects. The relatively high apparent stress exponents of TMCs were mainly attributed to the threshold stresses. Stress transfer effects could influence the apparent stress exponents in the high stress region, but the attribution was much lower than threshold stresses. La₂O₃ paticles were only effective to increase the threshold stresses in the high stress region. Threshold stresses were mainly dependent on the dispersion of reinforcements, while the volume fractions of reinforcements were not influential. Stress transfer effects showed a monotone decreasing relationship with the volume fractions of reinforcements, and the stress transfer effects in the low stress region were higher than those in the high stress region. The enhancement of creep resistance was mainly dependent on the morphologies of reinforcements.Thermal exposure at 600℃and 650℃led to loss of room temperature ductility of TMCs. However, thermal exposure at 700℃could bring back some ductility. The loss of room temperature ductility was mainly attributed to the ordered intermetallic Ti3Al, whose solution temperature was estimated to be between 650℃and 700℃. The reinforcements were stable during the thermal exposure, and showed no interfacial reaction with the matrix. Lathanum can absorb oxygen in the matrix to form La₂O₃ particles, which provided further dispersion strengthening and enhance the thermal stability of the composites.