Tial-based Alloys Sps And The Microstructure Characteristics And High Temperature Oxidation

TiAl-base alloys are getting more and more popular due to its low-density, high melting point and superior high-temperature intensity, It was considered one of the most widely applied new generation of high temperature structural materials in the aerospace,automobile,chemicals and other industrial areas. However, its brittleness at room temperature and lack of high-temperature oxidation resistance restrict its use and development. The objective of this experiment was to obtain TiAl alloys with good micro structure and properties by powder mechanical alloying (MA) and spark plasma sintering (SPS).Four proportions TiH2-47Al-0.2Si-5Nb (at%),TiH2-47Al-0.2Si-7Nb (at%),TiH2-47Al-0.2Si-10Nb (at%) and TiH2-47Al-0.2Si-12Nb (at%) were mixed by element mixing in this research. The mixed powders were ball milled for long time, and then park plasma sintering process and subsequently high temperature oxidation test were carried out. The morphology,composition and phase structure of milled powder were systematically investigated during high energy ball milling, as well as the influence of milled powder on the properties of sintering micrstrcture and high-temperature oxidation resistance. High-temperature oxidation tests were completed by two different methods, One is the use of NETZSCH STA 409 PC/PG cyclic oxidation, the other is the use of high-temperature furnace of discontinuous oxidation. The high temperature oxidation resistance of the TiAl-base alloys was tested under different atmosphere condation.The results show that in the early stage of ball milling, powder size decreases rapidly,and the process of particle size refinement is dominant status. However, after ball milling for 30h, change of particles size is few and mechanical alloying process is dominant status, at the same time, TiAl, Ti3Al, Ti2Al intermetallic compounds are formed. After high-energy ball milling, powder size is below 10nm. Powder size decrease with Nb increasing content. small,uniform,dense sintering microstructure are obtained by ball milling and spark plasma sintering, sintering density up to 99.03%. The granular TiAl and nanoparticles Ti3Al phase are formed. TiAl microstructure of reaction-diffusion forming show good grain interface, low porosity, high strength of the interfaces.The cyclic oxidation test shows that oxidation appear in the heating stage, isothermal high-temperature stage and cooling stage. However, the oxidation rate is the fastest in the heating stage. The thermal stress lead to crack of oxide film on surface, the part of the oxygen spread directly through the crack gap and need not through the solid oxide,so this way greatly increase speed of oxidation. However,in the cooling stage, the crack is healed again and the oxidation rate is reduced.The oxidation study of the different atmosphere show that fine granular TiAl, Ti3Al phase mictructure represente a good high-temperature oxidation resistance, the oxidation rate in air is lower than one in pure oxygen. Increasing Nb conducive to the improvement of oxidation resistance.