Preliminary Study On Microstructure And Properties Of Ti-8Si-xZr-yY₂O₃ Alloys Prepared By High-energy Milling And Cold Pressing Sintering

To prepare high-performance titanium alloy has high research value, with the excellent properties of high specific strength, high specific stiffness and good corrosion resistance. In this paper, six kinds of Ti-Si alloy（Ti-8Si, Ti-8Si-0.7Zr, Ti-8Si-1.4Z, Ti-8Si-2.8Zr,Ti-8Si-1.4Zr-0.1Y₂O₃ and Ti-8Si-1.4Zr-0.3Y₂O₃） were prepared using the “high-energy milling- cold pressed sintering” technology. The composition and organization, hardness, high temperature oxidation resistance, hot corrosion properties, wear resistance and other properties were studied. The present works and obtained innovative achievements are listed as follows:The composition and organization of Ti-Si alloy were studied. After high energy milling,the blended powders was refinement, homogenization and mechanical alloying. Microstructure of powders was consisted of Ti5Si4, Ti Si, Ti Si2 phases, and contain Ti2 Zr, Zr3Si2 phases of the formulas adding Zr and Y₂O₃. During the sintering stage, the titanium silicon compounds transform into Ti5Si3 stable phase, and the formula of Ti-8Si-1.4Zr-0.1Y₂O₃ has the high content of Ti5Si3 stable phase. The element of Zr and Y₂O₃ significantly improved the reaction between Ti and Si, enhanced the mechanical alloying effect, and improved the density obviously. Zr was functioned as solid solution strengthening element, while the principle of the alloy strengthening is due to dispersion strengthening, homogenization and fine-grain strengthening by adding Y₂O₃.The hardness and fracture toughness of the six formulations were comparatively analyzed.Functioned as dispersion strengthening or solid solution strengthening element, the microhardness of the formulas adding Zr and Y₂O₃ were improved by changing the content of Ti5Si3 and other phases and the proportion of solid solution. The formula of Ti-8Si-1.4Zr-0.1Y₂O₃ has the highest microhardness（1377 HV）, improved by 35% than Ti-8Si（1020 HV）. The fracture toughness of Ti-8Si-1.4Zr-0.3Y₂O₃（10.97 MPa/m1/2） was highest, improved by 90% than Ti-8Si（5.76 MPa/m1/2）.The oxidation resistance was studied. Oxides of six alloys at 700℃, 800℃ and 900℃ for100 h was mainly comprised of anatase or rutile Ti O2 phase, and Si O2 phase. Oxide film isbasically comprised of external Ti O2 oxide layer, middle Ti O2 and Si O2 composite oxide layer,and internal Si O2 oxide layer. Both of the thickness of oxide film and the size of oxides increased with temperature increasing. The oxidation resistance of Ti-8Si alloy significantly increased due to increasing the content of rutile Ti O2 by adding Zr and Y₂O₃, and the oxidation kinetics generally obeys parabola law. At 700℃, all of the formulas alloys reached complete antioxidant levels, and the average oxidation speed of Ti-8Si-1.4Zr alloy with the minimum K+value（0.0289 g.m-2.h-1） decreased by 70.21% than Ti-8Si alloy with the K+ value（0.097 g.m-2.h-1）. At 800℃, Ti-8Si-0.7Zr alloy and Ti-8Si-1.4Zr-0.1Y₂O₃ alloy only reached complete antioxidant levels. At 900℃, all of the formulas alloys reached antioxidant levels, and the average oxidation speed of Ti-8Si-1.4Zr-0.3Y₂O₃ alloy with the minimum K+ value（0.351 g.m-2.h-1） decreased by 61.76% than Ti-8Si alloy with the K+ value（0.918 g.m-2.h-1）. Among the alloys adding Zr element, Zr accelerated the selective oxidation of Ti element, the oxidation was controlled by the diffusion of O element at 700℃, and the diffusion of Ti, Si and O element at 800℃and 900℃. The oxidation of the alloy adding Y₂O₃ was controlled by the diffusion of Ti, Si and O element, and the formation of Si O2 oxide has significant function during the oxide stage, and Y₂O₃ element obviously decreased the thickness of the oxide film at the same temperature.Hot corrosion properties of three alloys（Ti-8Si-1.4Zr, Ti-8Si-1.4Zr-0.1Y₂O₃ and Ti-8Si-1.4Zr-0.3Y₂O₃） were Studied. The Ti-8Si-1.4Zr-0.3Y₂O₃ alloy had the best corrosion resistance in 25% Na Cl + 75% Na2SO4 molten salt at 700℃ for 30 h. The main phases of surface corrosion products were rutile Ti O2, Si O2 phase, and a small amount of Na Ti O2 and Na2Si2O5 compounds. The corrosion layer was thick and uniform on the surface without crack or desquamate, and had good metallurgical combination with the matrix. The thickness of the corrosion layer decreased with the increase of the content of Y₂O₃.The friction and wear properties of Ti-8Si, Ti-8Si-0.7Zr, Ti-8Si-1.4Zr and Ti-8Si-1.4Zr-0.1Y₂O₃ alloys were studied. The results show that the friction coefficient of Ti-8Si alloy was minimum（about 0.365）, but the wear resistance was worst with the widest wear width（about 407 μm）. Zr element increased the friction coefficient, but improved thewear resistance of Ti-8Si alloy by reducing the wear width. Adding Y₂O₃,Ti-8Si-1.4Zr-0.1Y₂O₃ alloy has the best wear resistance with small friction coefficient（0.375）and minimum wear width（255 μm）. The wear mechanism of four alloys were mainly fatigue wear, accompanied by abrasive wear and oxidation wear.In summation, Zr and Y₂O₃ improved the properties of microhardness,fracture toughness,oxidation resistance, corrosion resistance and friction and wear properties, these studied in this work provide a guideline for the design and development of Ti-Si material system.