Basic Research On Ti-Si Eutectic Alloy

The microstructures and the compressive properties of the Ti-Si alloys prepared byarc-melting with non-consumable tungsten electrode in a water-cooled copper hearth undera purified argon atmosphere were investigated systematically. The phase composition andfracture mechanism of several Ti-Si eutectic alloys were studied. The tensile properties atroom and high temperatures of Ti-7wtï¼…Si hypoeutectic and Ti-8.5wtï¼…Si eutectic alloy incast and hot iso-static processing (HIP) were tested. The effect of solution heat treatment,cyclic heat treatment, alloying to improvement of microstructure, compressive strength andductility were investigated. The microstructure and interface of eutectic Ti-Ti₅Si₃ wereobserved by high-resolution electron microscopy (HREM).The microstructure study indicated that the microstructures of Ti-Si eutectic systemalloy appeared as hypoeutectic, eutectic and hypereutectic features depending on thecontent of Si, which was different from the "basket wave" character of traditional castingtitanium alloys. The features of eutectic structures varied from short rod, slice tochrysanthemum according to the different cooling rate. The results of X-ray diffraction(XRD) and transmission electron microscopy (TEM) showed that the Ti-Si eutectic alloyswere composed ofα-Ti and Ti₅Si₃ at room temperature, and no special orientationrelationship between two phases existed. No any reaction particles appeared in the twophases interface area, and the lattice distortion was small. The enthalpy of formation forβ-Ti and Ti₅Si₃ in eutectic reaction process were calculated to beΔH_β-Ti=-11.5kj/molandΔH_Ti5Si3=-50.1kj/mol by Miedema theory, which indicated that the Ti₅Si₃ phasewas more stable thermodynamically and precipitated easier thanβ-Ti. It was found that thetypical section of Ti₅Si₃ formed in the eutectic reaction appeared as hexagon with angle of120°in the center of theα-Ti spherical particles.The studies on tensile properties and fracture morphology of Ti-7wt%Si hypoeutecticand Ti-8.5wt%Si eutectic alloy showed that the fracture mechanism of Ti-Si eutectic alloywas brittle cleavage fracture from room temperature to high temperature of 600℃.Although the ductility of HIPed Ti-7wt%Si alloy was improved distinctly, i.e.δ₅ was10.9ï¼…at 600℃, but the fracture morphology showed that the fracture mechanism was stillnot changed.It has been found that the proper heat treatment can improve the ductility of Ti-Sieutectic alloy significantly. For example, the compressive strengths and ductilities of Ti-Sieutectic alloy reachσ=1913MPa,ε_c=20.6ï¼…andσ=1740MPa,ε_c=19.6ï¼…under thecases of solution treatments of (1150℃/lh+WQ) and ninefold cyclic heat treatment of1050℃/30min+AC respectively. No martensite and new phase were detected after the heat treatments, but the original eutectic colonies vanished and Ti₅Si₃ phases became moreuniform and finer. It has been also found that too higher treatment temperature and longerholding time will lead to the coarsening of Ti₅Si₃ particles, which will decrease themechanical properties. Fracture morphology studies showed that the second phases (Ti₅Si₃)in the matrix were pulled out and tom, and the evident macroscopic ductility appeared inthe alloy. The fracture mechanism was changed into quasi-toughness fracture. The changeof fracture mechanism radically improved the compressive properties of the alloy.The experiment results showed that compressive properties of the Ti-Si eutectic alloywere improved efficiently by small amount of low surface energy element such as Be andBi or RE (Y, Gd, Sin, Dy, Nd), and the effects of Y and Sm addition were more evident.Investigation of microstructures showed that the number of eutectic colony was increasedby addition of small amount of Y, but the size of colony and Ti₅Si₃ phases decreased.X-EDS results indicated that Y atoms distributed mostly in Ti₅Si₃ phases and it was foundthat Si atoms were likely substituted by Y atoms. By addition of small amount of Sm, theeutectic colonies almost disappeared and the percent of nanometer-level Ti₅Si₃ particlesexceeded 50ï¼….Ductility of Ti-Si eutectic alloy with addition of small amount Boron was improvedmore evident than Y or Sm. The fracture morphology were found that the interface bondsbetween ductileα-Ti matrix and the brittle Ti₅Si₃ phases were divided, which companiedwith little lancinated edge and small cleavage facets. It was proved that the fracturemechanism was quasi-brittle fracture. With more addition of boron, the silicon equivalentin alloy was increased. Thus, primary coarse Ti₅Si₃ phases appeared which generallyexisted in hypereutectic alloys. Boron can also affect the dendrite of hypoeutectic Ti-Sieutectic alloy. It was found that boron not only broke up theα-Ti dendrite, but alsoaccelerated Si atoms to spread toward the interface of dendrite of supersaturatingβ-Ti solidsolution and restrained the nucleation and growth of Ti₅Si₃ phases.The experiments revealed that proper amount of Al can strengthen the compressiveproperties. Along with A1 atoms dissolved inα-Ti matrix, the Si equivalent in the alloydecreased, which aroused the volume fraction of ductile matrix increasing and improvedthe ductility of the alloy. However, the compressive properties decreased with excessiveaddition of A1 which can make the TisSi3 phases precipitate slower and the eutectic Ti₅Si₃phases develop rougher.The microstructures and properties of Ti-Si eutectic alloy with addition of Zr or Hfwere also investigated. It was found that proper amount of Zr could improve the ductilityof the alloy while Hf play a unimportant role. With addition of Zr, the Si equivalent of thealloy increased which promoted the precipitation of the primary Ti₅Si₃ phases. Zr canpartially replace Ti in the silicides because the free energy of formation of Zr-Si bonds is greater than that of Ti-Si bonds and both Zr and Ti elements are in groupâ…£a of periodictable. Most of Zr replaced Ti in silicides, which could be quantitatively expressed as(Ti_0.6Zr_0.4)₅Si₃.