Microstructure And Mechanical Properties Of Spark Plasma Sintered Ti-22Al-25Nb Alloy

The Ti₂AlNb-based alloy has advantages of low density,great high-temperature strength,specific strength,excellent oxidation resistance and creep resistance.Therefore,the Ti₂AlNb-based alloy has become a new ideal aero-engine material with light mass and high temperature structure to replace the nickel-based superalloy.The Ti₂AlNb-based alloy prepared by conventional cast occurs microstructural defects and is of poor mechanical properties,low efficiency and high cost.In addition,Ti,Al,Nb elements differ greatly in melting point,density,and diffusion coefficients,prone to elemental segregation and burning.As a near-net shape powder metallurgy sintering technology,Spark plasma sintering has the advantages of rapid temperature rising rate,short sintering time,energy saving and environment protection.The alloy prepared by the SPS technology,is characterized by excellent mechanical properties of fine grain and uniform microstructure without macro-segregation.In this paper,Ti-22Al-25Nb（at.%）pre-alloyed powder prepared by GA technology was chosen as raw material.A series of spark plasma sintering tests were carried out on Ti-22Al-25Nb powders under the pressure of 35-80MPa in the temperature range of950-1200℃and the holding time of 10-20min to prepare the sintered alloy with compact structure and uniform composition.This paper reveals the law of different sintering parameters affecting on relative density,phase composition,microstructure and mechanical properties of sintered alloy and fracture mechanism.The experimental results show that the best microstructure and mechanical properties of alloy can be gotten when sintering parameters conclude 950℃ of sintering temperature,80MPa of sintering pressure,and 10 minutes of holding time.The relative density of the sintered alloy is as high as 99.43%.The sintered alloy consists of B2 phase,O phase andα₂ phase.The average grain size of B2 phase is 9.39μm.The room temperature elongation,yield strength,tensile strength and microhardness of the sintered alloy are 9.38%,933.57MPa,990.01MPa and 352.8HV respectively.The densification process of the pre-alloyed powders was simulated numerically by using the finite element software MSC.Marc.The effect of temperature,holding time and sintering pressure on the densification process of the Ti-22Al-25Nb powders was researched.The paper systematically analyzes variation law of powder relative density with different sintering parameters.The experiment was carried out at solution temperature of 940-1100℃ and holding time of 10-120min with 950℃/80MPa/10min sintered alloy chosen as the raw material.The results show that the microstructure and microhardness of sintered alloy after solution all change with the solution temperature and holding time.The grain size of B2 phase increases with the increase of solution temperature and the holding time.Also,the uniformity of B2 grain increases with the increase of solution temperature and the holding time.The growth kinetics model of B2 phase grain of sintered alloy was established.Under different solid solution temperature and holding time,grain growth exponents and activation energies were gotten.At the same time,the accuracy of the Beck equation and Hillert equation was verified.