Preparation, Structure And Properties Of High Strength TiZr-based Alloy

Titanium and its alloys have been widely used in aviation and chemical industry todate, due to the exceptional strength-to-weight ratio, good corrosion resistance and theirdensity being only half of the steel and Ni-based super alloys. Meanwhile, zirconium andits alloys have been mainly used in nuclear and chemical industries because of theirunique physic-chemical properties, such as a small capture cross-section for thermalneutron, good anti-irradiation, favourable oxidation resistance and corrosion resistance,small expansion coefficient, and low density. The application of Ti and Zr alloys in spaceis also gradually increasing with the development of science and technology. At present,Ti alloys, which do not possess good irradiation resistance and have large expansioncoefficient, are not suitable to use in the extreme space environment as structural materials.Furthemore, besides the good anti-irradiation, low density and a small expansioncoefficient are also the desired properties of structural materials for space crafts, the lowstrength of Zr alloys restricts their applications as structural materials. Therefore, thisdissertation is aiming to develop new type aerospace materials by means of the alloycomposition design and optimization, forging and heat treatment process to design a seriesof TiZr-based alloys with exceptional mechanical properties. And the dissertation alsosystematically studys phase transition, microstructural evolution and strengtheningmechanism of the TiZr-based alloy.A series of TiZrAlV alloys designed are melted by utilizing a vaccumnon-consumable electro-arc furnce. Based on Ti-6Al-4V alloy, the composition of theTiZrAlV alloys are optimized according to the influence of various elements on phasecontents and mechanical properties. The strength and hardness of TiZrAlV alloysobviously increase and the densities of the alloys are around4.6g/cm~3, when the contentsof Zr is15wt.%~20wt.%. Al is the most major strengthening element in Ti alloys, andthe strength of alloys will increase distinctly when Al content is about4.5wt.%~6.9wt.%.The content of V, as a β-stabilized element, is about3.5%~4.5%. The β transustemperatures are decreased with Zr, and mechanical properties also increase. Based on the results of the composition optimization, the high-strength alloys, i.e.Ti-15Zr-6Al-4V (T15Z alloy), Ti-20Zr-6.5Al-4V (T20Z alloy) and Ti-51Zr-4.5Al-4V(T51Z alloy), are prepared by electro-magnetic induction melting and vaccum consumableelectro-arc furnace. After the break-down forging and subsequent heat treatment, themechanical properties of the alloys are obviously changed. After annealing, T15Z alloyhas good ductility (about13%). The ductility of T51Z alloy undergone water quenching atdifferent temperatures is about15%. The ultimate strength of T20Z alloy undergonesolution treatment and aging treatment reaches1740MPa with the low plasticity. Afteraging at700oC, the strength and ductility of alloy are1437MPa and6.69%respectively.The strength of the annealing T20Z alloy is about1300MPa and the ductility is greaterthan10%.Phase transition and microstructural evolution of T20Z alloy undergone different heattreating regime are investigated by means of XRD, OM, FE-SEM and TEM. FCC phasewas found after solution treatment at950oC and the lattice parameter was determined tobe a=0.4385nm. The FCC phase disappeared after aging at700oC. Annealing between550and850oC for120min, all TiZr-based alloys are mainly composed of α and β phase,and the FCC phase appeared between650and750oC. The volume fraction of β phasedecreases with anneraling temperatures below750oC, whereas the volume fraction of βphase increases with the annealing temperature over750oC. Annealing between800oCand1050oC for30min, all TiZr-based alloys are composed of α and β phase. The volumefraction of β phase is sensitive to the annealing temperature. Furthermore, at the sameannealing temperature (eg. annealing at850oC), the longer the holding time is, the higherthe volume fraction of β phase. Under the high temperatures annealing for short timeconditions, the thickness of lamellar α phase of T20Z alloy is sensitive to the annealingtemperatures. With the annealing temperature increasing, the morphology of α phase tendsto equiaxed grain, except alloy annealed at1050oC.