Microstructure Evolution And Isothermal Oxidation Behavior Of Ti₂AlNb Based Alloys

Ti₂AlNb based alloys have great potential applications as high-temperature aerospace materials owing to their high specific yield,favorable creep resistance and good workability.The service properties of Ti₂ AlNb based alloys greatly depend on their phase transformation and microstructure owing to the chemical composition and thermo-mechanical process for the alloys.In this thesis,the phase transformtion and microstructure evolution during heating and isothermal holding are investigated for Ti₂ AlNb based alloys.And the influence of plan strain compression on the phase transformation and microstructure evolution is analysed.In addition,oxidation behavior and the role of alloying elements are discussed.The main contents and conclusions are obtained as follows:（1）The phase transformtion and microstructure evolution during heating are investigated for Ti-22Al-（27-x）Nb-xZr（x=0,1,6）alloy by the means of tests including dilatometry and differential scanning calorimetry combined with metallographic analysis.The effect of Nb and Zr on the phase transformation procedure is analysed.The sequence of phase transformation during heating in Ti-22Al-（27-x）Nb-xZr（x=0,1,6）alloys can be concluded as follows: B2/β→O,B2/β+α₂→O,B2/β+O→B2/β+O+α₂,B2/β+O+α₂→B2/β+α₂ and B2/β+α₂→B2/β for x=0;B2/β→a2+O,O+α₂+B2/β→B2/β+α₂,and then α₂ dissolves into B2 matrix for x=6.Partial substitution of Zr for Nb leads to the decrease in the β stability and increase in the phase transition temperature for B2/β+α₂→B2/β.And in this phase transformation stage,there is a deceleration of reduction in α₂ phase during heating.In comparison with the other experimental alloys,the dissolution of α₂ phase is accelerated in Ti-22Al-21Nb-6Zr.（2）The isothermal phase transformation and microstructure envolution at 1063¹183K are investigated through thermal dilation method.TTT diagram is estabilished based on the relationship between O phase fraction and isothermal holding time.The transformation kinetics is studied by means of JMA equation,and combining with metallographic analysis,the isothermal phase transformation and microstructure evolution are summarized.At 1183 K,O phase nucleates on grain boundaries and grows inwardly as laths.At 1103¹143K,O phase nucleates initially at grain boundaries,and then nucleates at grain inner and grows as lath.At 1063 K,O phase nucleats at small precursors and the nucleation rates decrease with the isothermal time.Along with the decrease in temperature,the nucleation rates increase followed by the acceleration of phase transformation,and the elements diffusion is slower resulting in the deceleration of phase transformation at later stage.（3）The hot deformation behavior under temperature of 1213¹273K and strain rates of 0.001¹s-1 is investigated by plane strain compression tests for Ti-22Al-25 Nb alloy.The constitutive equation for plan strain compresssion is established,and the effect of deformation on the phase transformation and microstructure is studied.At certain temperature,the deformation degree of primary a2 phase is larger under higher strain rate and the transition degree of α₂→O phase transformation is higher under lower strain rate.Under a constant strain rate,the primary α₂ particles are more elongated with increasing temperature and rod-like until 1273K;the amount of O phase precipitated during deformation increases as the temperature decreases.In contrast with undeformed specimen,the fraction of O phase increases and the elongated α₂ particles more easily transform to O phase.（4）The oxidation behavior of Ti-22Al-（27-x）Nb-x Zr（x=0,1,6）alloys are studied in the temperature range of 923¹073K.The oxidation process and mechanism are investigated through analysing oxidation kinetics,oxidation products,cross-section mophology,element distribution and oxidation-affected zones.The effect of Nb and Zr on oxidation behavior is analysed,and the quatitative relationship among width of oxygen affected zone,oxidation time and temperature is established.Ti-22Al-26Nb-1Zr alloy has higher oxidation activation energy and better oxidation resistance.Appropriate substitution of Zr for Nb is benefical to improve oxidation resistance.There are oxygen affected zone between external oxidation layer and substrate consisting of inner oxidation layer and oxygen-enriched zone.Ti-22Al-26Nb-1Zr alloy possesses denser external oxidized layer and thinner internal oxidation layer.For Ti-22Al-21Nb-6Zr alloy,the oxygen dissolves into substrate easily due to the presence of α₂ phase and enhancement of lattice parameter for O phase with 6 at.% Zr addition.In addition,ZrO2 in the oxidation products of Ti-22Al-21Nb-6Zr alloy faciliates oxygen diffusion and leads to wider oxygen-enriched zone.Therefore,Ti-22Al-21Nb-6Zr alloy has lower oxidation resistance.