The Synthesis of High Strength Titanium Alloy and Titanium Alloy Matrix Composite by Powder Compact Extrusion from Blended Elemental Powders

摘要

Powder compact extrusion (PCE) from blended elemental powders is an effective way to produce titanium alloys and titanium matrix composites with high performance. Its main advantage is cost effectiveness, where high cost pre-alloyed powders are replaced by lower cost elemental powders and a need for lengthy sintering times to achieve sufficiently high enough density is removed. However a blended elemental approach has some disadvantages, for example the ductility and strength of PCE materials are normally lower than those obtained in cast alloys. This thesis is a study of the feasibility of producing a high strength titanium alloy and titanium matrix composite using PCE from blended elemental powders. It investigates and seeks to address the effects of inhomogeneous elemental distribution on structure and properties. In this study two very different ways of producing a high strength titanium alloy using PCE through a blended elemental approach are investigated. The first approach is through the synthesis of the high strength alloy Ti-4%Al-4%Mo-4%Sn-0.5%Si and the second approach investigates the feasibility of achieving high strength by producing a titanium alloy composite and for this TiB/Ti6Al4V was chosen for this study.For synthesising the alloy Ti-4%Al-4%Mo-4%Sn-0.5%Si, the effect of extrusion temperature, different starting powders and heat treatment on the microstructure and mechanical properties were studied. The extrusion temperature has a significant effect on the degree of elemental dissolution. An increase in the extrusion temperature significantly improves the dissolution of Si and Mo. However, an extrusion temperature of 1350oC is still not enough to achieve complete dissolution of the elemental particles, especially Si and Mo particles. The results showed that by using an extrusion temperature 1350oC with a holding time of 5 min prior to extrusion, elemental dissolution was much improved and a relatively homogenous microstructure was achieved. As-extruded bars with undissolved elemental particles showed low tensile strength without any ductility, while those samples without any inhomogeneities in the microstructure caused by undissolved particles and defects showed an ultimate tensile strength and elongation to fracture of 1423MPa and 5.1%, respectively.After achieving dissolution of the elemental particles, some localized areas with an inhomogeneous elemental distribution still existed. The diffusion interactions between different elements during high temperature processing were also discussed in this thesis. The study also showed that some dendrites such as Ti5Si3 and β Ti grew during cooling due to the local elemental inhomogeneities. These dendrites are detrimental to the mechanical properties of the alloy, being the main reason for premature failure during tensile testing, leading to a large variation in tensile properties. The precipitation of dendrites was monitored.When the starting powders were dominated by gas atomized (GA) titanium powders, a good combination of tensile strength and ductility was achieved which showed a UTS of 1220-1250 MPa with an elongation to fracture of 7-11%. GA titanium powders have a beneficial effect on elemental diffusion and the achievement of more uniformity in mechanical properties.A beta transus temperature of 1075oC has been confirmed. The solutioning temperature and the concentration of beta stabilizer, especially Mo, in the beta matrix has a significant effect on martensite phase transformation. After heat treatment the microstructural homogeneity and tensile properties were significantly improved. The cooling rate after solutioning plays a key role in determining the morphology of the α lamellar structure and the α colony and beta grainsize. A faster cooling rate causes a decrease in the width of the α lamellae, the size of the α colonies and the beta grain size. After heat treatment an ultimate tensile strength of 1584MPa, a yield strength of 1505MPa and an elongation to fracture of 2.5% were obtained.The method of synthesis of this alloy by powder compact extrusion from a pre-consolidated compact was also introduced. The effect of extrusion temperature on microstructure and mechanical properties was investigated. When the extrusion temperature is increased to 1200oC a fine lamellae microstructure with good tensile properties giving a yield strength, UTS and elongation to fracture of 1278MPa, 1421MPa and 7.2%, respectively, were achieved. Good tensile properties were obtained at elevated temperature, and for testing between 300oC and 500oC the tensile strength decreased from 907MPa to 720MPa, but the elongation to fracture increased from 10% to 13%.For synthesis of TiB/Ti6Al4V composite, a Ti-6wt%Al-4wt%V alloy (Ti6Al4V) matrix composite, reinforced by in-situ synthesized TiB whiskers (TiBw) was successfully fabricated by powder compact extrusion, using a blended powder mixture. The microstructural characterization of the various extruded samples showed that the different starting powders, pre-alloyed powder plus boron powder or titanium plus Al-40V master alloy powder plus boron powder, had a significant effect on the morphology of the in-situ synthesized TiB whiskers. It is also evident that the TiB whiskers affected the microstructural evolution of the Ti6Al4V matrix. The tensile test results indicated that a composite with a dispersion of fine TiB whiskers with high aspect ratios exhibited a high ultimate tensile stress (UTS) and yield stress (YS) of 1436MPa and 1361MPa, respectively, a reasonably good tensile ductility reflected by an elongation to fracture of 5.6% was also achieved. This is a significant improvement compared with as-extruded monolithic Ti6Al4V alloy produced in this study.